Anatomy homework packet

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Pearson.Essentials.of.Human.Anatomy.and.Physiology.12th.Edition.B071XGCNBP.pdf

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TWELFTH EDITION

Elaine N. Marieb Suzanne M. Keller

Essentials of Human Anatomy & Physiology

GLOBAL EDITION

Learn the Essential What, How & Why of Human Anatomy & Physiology

With the Twelfth Edition of Essentials of Human Anatomy & Physiology, science educator Suzanne Keller joins bestselling author Elaine Marieb in helping learners focus on the What, How & Why of A&P, without getting sidetracked in details.

382

When most people hear the term cardio-vascular system, they immediately think of the heart. We have all felt our own heart “pound” from time to time when we are ner- vous. The crucial importance of the heart has been recognized for ages. However, the cardiovascular system is much more than just the heart, and from a scientific and medical standpoint, it is important to understand why this system is so vital to life.

Night and day, minute after minute, our tril- lions of cells take up nutrients and excrete wastes. Although the pace of these exchanges slows dur- ing sleep, they must go on continuously: when they stop, we die. Cells can make such exchanges

only with the interstitial fluid in their immediate vicinity. Thus, some means of changing and “refreshing” these fluids is necessary to renew the nutrients and prevent pollution caused by the buildup of wastes. Like a bustling factory, the body must have a transportation system to carry its various “cargoes” back and forth. Instead of roads, railway tracks, and subways, the body’s delivery routes are its hollow blood vessels.

Most simply stated, the major function of the cardiovascular system is transportation. Using blood as the transport vehicle, the system carries oxygen, nutrients, cell wastes, hormones, and many other substances vital for body homeostasis to and from the cells. The force to move the blood

The Cardiovascular System

WHAT

HoW

WHY

The cardiovascular system delivers oxygen and

nutrients to the body tissues and carries away wastes such as carbon dioxide

via blood.

The heart pumps blood throughout the body in blood vessels. Blood flow

requires both the pumping action of the heart and changes in

blood pressure.

If the cardiovascular system cannot perform its

functions, wastes build up in tissues. Body organs fail to function properly,

and then, once oxygen becomes depleted, they will die.

InSTruCTorS

New Building Vocabulary Coaching Activities for this chapter are assignable in

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NEW! What, How & Why chapter previews introduce key examples of anatomy and physiology concepts that will be covered in the chapter. This technique helps learners hone in on what they are studying, how it functions, and why it is important for them to learn.

NEW! Building Vocabulary Coaching Activities in Pearson Mastering A&P help students learn the essential language of A&P.

See p. 382.

Throughout every chapter, the text’s conversational writing style and straightforward explanations have been strengthened with familiar analogies and abundant mnemonic cues to help students learn and remember concepts.

UPDATED! Exceptionally clear photos and illustrations, including dozens of new and improved figures, present concepts and processes at the right level of detail. Many figures from the text are assignable as Art- Labeling Activities in Pearson Mastering A&P.

Unique Concept Links reinforce previously-learned concepts and help students make connec- tions across body systems while learning new material.

Focus on Essential A&P Concepts

Chapter 4: Skin and Body Membranes 137

Cutaneous membrane (skin)

Mucosa

Parietal layer

Visceral layer

Serous fluid

(a) Cutaneous membrane (the skin) covers the body surface.

(d) A fist thrust into a limp balloon demonstrates the relationship between the parietal and visceral serous membrane layers.

(b) Mucous membranes line body cavities open to the exterior.

Outer balloon wall (comparable to parietal serosa)

Air (comparable to serous cavity)

Inner balloon wall (comparable to visceral serosa)

Figure 4.1 Classes of epithelial membranes.

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282 Essentials of Human Anatomy and Physiology

Figure 7.21 Schematic of ascending (sensory) and descending (motor) pathways between the brain and the spinal cord.

Cerebral cortex (gray matter)

Thalamus

White matter

Interneuron carrying response to motor neuron

Cell body of sensory neuron in sensory ganglion

Skin

Nerve

Sensory receptors

Motor output

Muscle

Interneuron Motor neuron cell body

Gray matter White matter

Interneuron carrying sensory information to cerebral cortex

Interneuron carrying response to motor neurons

Integration (processing and interpretation of sensory input) occurs

Interneuron carrying sensory information to cerebral cortex

Brain stem

Cerebrum

Cervical spinal cord

➔ConCeptLinkThe terms for the connective tissue coverings of a nerve should seem familiar: We discussed similar struc-tures in the muscle chapter (Figure 6.1, p. 209). Names of muscle structures include the root word mys, whereas the root word neuro tells you that the struc- ture relates to a nerve. For example, the endomysium covers one individual muscle fiber, whereas the endo- neurium covers one individual neuron fiber. ➔

Like neurons, nerves are classified according to the direction in which they transmit impulses. Nerves that carry impulses only toward the CNS are called sensory (afferent) nerves, whereas those that carry only motor fibers are motor (efferent) nerves. Nerves carrying both sensory and motor fibers are called mixed nerves; all spi- nal nerves are mixed nerves.

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See p. 137.

See p. 282.

UPDATED! Homeostatic Imbalance discussions are clinical examples that revisit the text’s unique theme by describing how the loss of homeostasis leads to pathology or disease. Related assessment questions are assignable in Pearson Mastering A&P, along with Clinical Case Study coaching activities.

Explore Essential Careers and Clinical Examples

To inspire and inform students who are preparing for future healthcare careers, up-to-date clinical applications are integrated in context with discussions about the human body.

Focus on Careers essays feature conversations with working professionals and explain the relevance of anatomy and physiology course topics across a wide range of allied health careers. Featured careers include:

Ch. 2 Pharmacy Technician Ch. 4 Medical Transcriptionist Ch. 5 Radiologic Technologist Ch. 8 Physical Therapy Assistant Ch. 10 Phlebotomy Technician Ch. 15 Licensed Practical Nurse

Students can visit the Pearson Mastering A&P Study Area for more information about career options that are relevant to studying anatomy and physiology.

See p. 295.

See p. 82.

Pearson Mastering A&P improves results by engaging students before, during, and after class.

Continuous Learning Before, During, and After Class

Instructors can further encourage students to prepare for class by assigning NEW! Building Vocabulary activities, reading questions, art labeling activities, and more.

Before Class

Dynamic Study Modules enable students to study more effectively on their own. With the Dynamic Study Modules mobile app, students can quickly access and learn the concepts they need to be more successful on quizzes and exams. NEW! Instructors can now select which questions to assign to students within each module.

During Class

After Class A wide variety of interactive coaching activities can be assigned to students as homework, including Art-Labeling Activities, Interactive Physiology 2.0 tutorials, Clinical Case Studies, and activities featuring A&P Flix 3-D movie- quality animations of key physiological processes.

with Pearson Mastering A&P

NEW! Learning Catalytics is a “bring your own device” (laptop, smartphone, or tablet) engagement, assessment, and classroom intelligence system. Students use their device to respond to open-ended questions and then discuss answers in groups based on their responses. Visit learningcatalytics.com to learn more.

Media references in the text direct learners to digital resources in the Pearson Mastering A&P Study Area, including practice tests and quizzes, flashcards, a complete glossary, and more.

NEW! Interactive Physiology 2.0

Practice Anatomy Lab (PAL™ 3.0) is a virtual anatomy study and practice tool that gives students 24/7 access to the most widely used lab specimens, including the human cadaver, anatomical models, histology, cat, and fetal pig. PAL 3.0 is easy to use and includes built-in audio pronunciations, rotatable bones, and simulated fill-in-the- blank lab practical exams.

A&P concepts come to life with Pearson Mastering A&P

NEW! Interactive Physiology 2.0 helps students advance beyond memorization to a genuine understanding of complex physiological processes. Fun, interactive tutorials, games, and quizzes give students additional explanations to help them grasp difficult concepts. IP 2.0 features brand-new graphics, quicker navigation, and more robust interactivity.

Access the complete textbook online with the eText on Pearson Mastering A&P

Powerful interactive and customization functions include instructor and student note-taking, highlighting, bookmarking, search, and links to glossary terms.

Additional Support for Students and Instructors

The perfect companion to Essentials of Human Anatomy & Physiology, this engaging interactive workbook helps students get the most out of their study time. The Twelfth Edition includes NEW! crossword puzzles for every chapter, along with coloring activities, self-assessments, “At the Clinic” questions, and unique “Incredible Journey” visualization exercises that guide learners into memorable explorations of anatomical structures and physiological functions.

• All of the figures, photos, and tables from the text in JPEG and PowerPoint® formats, in labelled and unlabeled versions, and with customizable labels and leader lines

• Step-edit Powerpoint slides that present multi-step process figures step-by-step

• Clicker Questions and Quiz Show Game questions that encourage class interaction

• A&PFlix™ animations bring human anatomy and physiology concepts to life

• Customizable PowerPoint® lecture outlines save valuable class prep time

• A comprehensive Instructor’s Guide includes lecture outlines, classroom activities, and teaching demonstrations for each chapter.

• Test Bank provides a wide variety of customizable questions across Bloom’s taxonomy levels. Includes art labeling questions, and available in Microsoft® Word and TestGen® formats.

NEW! Anatomy & Physiology Coloring Workbook Twelfth Edition, Global Edition by Elaine N. Marieb and Simone Brito

The Instructor Resources Area in Pearson Mastering A&P includes the following downloadable tools:

NEW! IN FULL COLOR! Essentials of Human Anatomy & Physiology Laboratory Manual Seventh Edition by Elaine N. Marieb and Pamela B. Jackson

This popular lab manual provides 27 exercises for a wide range of hands-on laboratory experiences, designed especially for a short A&P Lab course. This edition, which includes a Histology Atlas with 55 photomicrographs, features NEW! full-color illustrations, photos, and page design that help students navigate and learn the material faster and easier than ever before. Each concise lab exercise includes a Pre-Lab Quiz, brief background information, integrated learning objectives, student-friendly review sheets, and more.

ELAINE N. MARIEB, R.N., Ph.D., hOLYOKE COMMUNITY COLLEGE

SUZANNE M. KELLER, Ph.D., INDIAN hILLS COMMUNITY COLLEGE

ESSENTIALS OF HUMAN ANATOMY

& PHYSIOLOGY

TWELFTH EDITION GLOBAL EDITION

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Authorized adaptation from the United States edition, entitled Essentials of Human Anatomy & Physiology, 12th edition, ISBN 9780134395326, by Elaine N. Marieb and Suzanne M. Keller, published by Pearson Education © 2018.

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About the Authors

Elaine Marieb After receiving her Ph.D. in zoology from the University of Massachusetts at Amherst, Elaine N. Marieb joined the faculty of the Biological Science Division of Holyoke Community College. While teaching at Holyoke Community College, where many of her students were pursu- ing nursing degrees, she developed a desire to bet- ter understand the relationship between the scientific study of the human body and the clinical aspects of the nursing practice. To that end, while continuing to teach full time, Dr. Marieb pursued her nursing education, which culminated in a Master of Science degree with a clinical specializa- tion in gerontology from the University of Massa- chusetts. It is this experience that has informed the development of the unique perspective and acces- sibility for which her publications are known.

Dr. Marieb has given generously to provide oppor- tunities for students to further their education. She funds the E. N. Marieb Science Research Awards at Mount Holyoke College, which promotes research by undergraduate science majors, and has underwritten renovation of the biology labs in Clapp Laboratory at that college. Dr. Marieb also contributes to the Univer- sity of Massachusetts at Amherst, where she gener- ously provided funding for reconstruction and instrumentation of a cutting-edge cytology research laboratory. Recognizing the severe national shortage of nursing faculty, she underwrites the Nursing Schol- ars of the Future Grant Program at the university. In January 2012, Florida Gulf Coast University named a new health professions facility in her honor. The Dr. Elaine Nicpon Marieb Hall houses several specialized laboratories for the School of Nursing, made possible by Dr. Marieb’s generous support.

Suzanne Keller Suzanne M. Keller began her teaching career while she was still in graduate school at the University of Texas Health Science Center in San Antonio, Texas. Inspired by her life- long passion for learning, Dr. Keller quickly adopted a teaching style focused on translating challenging concepts into easily understood parts using analogies and stories from her own experi- ences. An Iowa native, Dr. Keller uses her expertise to teach microbiology and anatomy and physiol- ogy at Indian Hills Community College, where most of her students are studying nursing or other health science programs.

Dr. Keller values education as a way for students to express their values through the careers they pursue. She supports those endeavors both in and out of the classroom by participating in her local Lions Club, by donating money to the Indian Hills Foundation to fund scholarships, and by financially supporting service-learning trips for students. Dr. Keller also enjoys sponsoring children in need with gifts for the holidays.

Dr. Keller is a member of the Human Anatomy and Physiology Society (HAPS) and the Iowa Acad- emy of Science. Additionally, while engaged as an author, Dr. Keller has served on multiple advisory boards for various projects at Pearson and has authored assignments for the Pearson Mastering A&P online program. When not teaching or writ- ing, Dr. Keller enjoys reading, trav eling, family gatherings, and relaxing at home under the watch- ful eyes of her two canine children.

New to the Twelfth Edition

This edition has been thoroughly updated. New “What, How, Why” art opens each chapter, highlighting key concepts relating to the chapter topic. Other specific chapter-by-chapter changes include the following:

• New colors used in Figure 3.14 DNA images to help students track new and old strands of DNA.

• New description of each chromosome being composed of two sister chromatids.

• New explanation of protein synthesis includes the role of the large ribosomal subunit in peptide bond formation.

• New Figure 3.18 descriptions of nuclei lining up in simple columnar and not lining up in pseudostratified epithelia.

• New description of cell shapes in different layers of stratified epithelia as “squished” and variable.

• New analogy for mucus produced by goblet cells as a “sticky trap” for dust and debris.

• New Figure 3.19 labels for osteocytes, the elastic and collagen fibers in areolar connective tissue, and the fluid matrix of blood; new art for dense fibrous connective tissue.

• New analogy for reticular tissue as “cellular bleachers” where other cells rest to monitor the body.

• Updated Figure 3.21 to include the term neuroglia to describe supporting cells.

• New information added to Figure 3.22: cartilage added to the connective tissue list and two major hallmarks of each of the four tissue types.

• New example of atrophy: when a broken leg is in a cast, lack of use causes muscles to atrophy during healing.

• Revised “A Closer Look: Cancer—An Intimate Enemy” and updated art. • New “Short Answer Essay” questions on the components of the

plasma membrane and their functions and on contrasting cytokinesis, interphase, and mitosis.

• New “Critical Thinking and Clinical Application” question on IV fluids and tonicity.

• New “Did You Get It?” questions throughout the chapter.

Chapter 4: Skin and Body Membranes • New Figure 4.1 on epithelial membranes. • New description of sensory receptors as part of nervous system

including a list of the stimuli detected. • New text updates on Figure 4.4 on epidermal structure; included a

new figure question on stratum lucidum. • New analogy for epidermal dendritic cells as “sentries” guarding

the skin. • New photo of stage 2 decubitus ulcer added to Homeostatic

Imbalance 4.2. • New layout for Figure 4.7 combining scanning electron micrograph

of hair shaft with existing art of the hair root and follicle. • New discussion of fourth-degree burns. • New criteria for determining whether a burn is critical, including

circumferential burns, burns of the airway, and burns to the genital area.

• New images of basal cell and squamous cell carcinoma in Figure 4.11.

• New component added to ABCDE rule: now includes “Evolution,” changes in a skin lesion over time.

• New “Short Answer Essay” questions on the risks of full-thickness burns, contrasting eccrine and apocrine sweat glands, and the relative severity of different skin infections.

• New “Critical Thinking and Clinical Application” question on burns. • New “Did You Get It?” questions throughout the chapter.

Chapter 5: The Skeletal System • Updated description of long bones. • New analogy comparing lubrication over articular cartilage at

joints to a slick marble floor.

Chapter 1: The Human Body: An Orientation • Updated description of the integumentary system to include

vitamin D production in the presence of sunlight. • Updated definition of the term crural to specify the anterior leg,

or shin. • New “Critical Thinking and Clinical Application” question on blood

clotting and feedback regulation. • New “Critical Thinking and Clinical Application” question on using

anatomical language to describe the location of a spinal injury and identifying the best medical imaging technique to diagnose a spinal problem.

• Updated “A Closer Look: Medical Imaging” with new discussion and images of mammogram and bone densitometry.

• New “Did You Get It?” questions throughout the chapter.

Chapter 2: Basic Chemistry • New example of atomic symbol and Latin derivative for potassium. • Revised discussion of hydrogen bonds to clarify that electrons are

not involved in this type of bond as they are in covalent and ionic bonds.

• New example of importance of hydrogen bond in holding DNA strands together.

• All references to hydroxyl ion have been corrected to hydroxide ion. • New sports analogies for acids and bases: putting electrons “in the

game,” to represent free protons versus taking electrons “out of the game,” to represent binding a proton so it cannot contribute to a shift in pH.

• New “Did You Get It?” question part on the difference in pH between solutions at pH 11 and pH 5.

• Introduced concepts of hydrophilic and hydrophobic in discussion of phospholipids to help explain the functions of the polar head and fatty acid tails in cell membranes.

• Updated “Did You Get It?” question on lipids to include both phospholipids and cholesterol as cell membrane components.

• New analogy comparing the alpha (α)-helix to a metal spring. • New analogy comparing a beta (β)-sheet to a pleated skirt, or

paper folded into a fan. • New analogy comparing enzymes and substrates to scissors and

paper, respectively. • New description of RNA function as DNA’s “molecular assistant.” • New shorthand symbols for messenger (mRNA), transfer (tRNA)

and ribosomal (rRNA) added. • New analogy comparing glucose and ATP to crude oil and gasoline;

energy must be in the proper form before it can be used by cells. • New explanation of why the terminal phosphate bonds in ATP are

high energy. • New “Critical Thinking and Clinical Application” question on sickle

cell anemia.

Chapter 3: Cells and Tissues • New description of the principle of complementarity. • New information about how mitochondria divide. • New “cargo” in the form of a membrane-bound receptor protein

added to pathway 2 of Figure 3.6. • New analogy for lysosomes as “cellular stomachs.” • New art of plasma membrane and new detail of mitochondrial

function including aerobic respiration in Table 3.1. • New analogy of dust “crowd surfing” on the mucus that cilia carry

from the lungs. • New description of neuron function includes production of

neurotransmitters.

• Updated descriptions of red and yellow bone marrow. • Updated descriptions of sagittal and coronal sutures. • Updated description of the capitulum of the humerus. • New analogy comparing the trochlea meeting the trochlear notch

to a curved “tongue-in-groove” joint. • Updated description of buttock injections to include the

consequences of hitting a nerve. • Updated description of a synovial membrane to include areolar

connective tissue. • Updated description of cartilaginous joints. • New description of saddle joints including a reference to

opposable thumbs. • Updated list of triggers for rheumatoid arthritis. • Discussion of the fetal skull and fontanels moved to the

Developmental Aspects section. • New analogy likening skulls of small children to “bobble heads.” • Updated review question on bones that articulate with the

sphenoid to reflect only bones shown in the figures of Chapter 5. • Updated “Short Answer Essay” question on synovial joints to

include osteoarthritis. • New “Short Answer Essay” question contrasting the foramen

magnum and obturator foramen. • New “Critical Thinking and Clinical Application” question on gouty

arthritis. • New statistics, information, and images added to “A Closer Look:

Joint Ventures.” • Updated description of comminuted fractures on Table 5.2. • Updated Figure 5.6 to include osteoblasts and osteoclasts in the

descriptions of bone addition and resorption, respectively. • Updated Systems In Sync with respect to the descriptions of

relationships of cardiovascular and muscular systems to the skeletal system.

Chapter 6: The Muscular System • Updated descriptions of tendons and aponeuroses. • New analogy about running to explain the difference between the

contraction of skeletal muscle (fast) versus smooth muscle (slow). • Updated description of a sarcomere to include its role as the

structural and functional unit of muscle. • Added discussion of titin to the description of a sarcomere as the

elastic filament that attaches myosin to the Z disc. • New Homeostatic Imbalance on ALS (amyotropic lateral sclerosis,

or Lou Gehrig’s disease). • New “Did You Get It?” question on the roles of calcium in muscle

contraction. • Updated descriptions of cross-bridge formation and the sliding

filament theory, including the role of ATP. • New link to IP Essentials for the sliding filament theory. • New description of flaccid versus spastic paralysis. • New mnemonic device for adduction: “add” back to the body by

moving toward the trunk (midline). • New descriptions of dorsiflexion and plantar flexion with respect

to the head: toes point toward the head or away, respectively. • New girdle analogy for abdominal wall muscles “holding guts in.” • New description of the consequences of an injection being too

close to, or hitting, the sciatic nerve. • New description of tailor’s muscle sitting position. • New description of myasthenia gravis as an autoimmune disease. • New “Short Answer Essay” question about the relationship

between wrist flexors and extensors, including their locations. • New figure question for Figure 6.20 on the origin(s) and

insertion(s) of the rectus femoris depending on the action being performed.

• Updated explanation of steps in Figure 6.5.

Chapter 7: The Nervous System • Updated Figure 7.13 to use superior and inferior instead of

cephalad and caudal. • Updated Figure 7.24 to clarify why there are eight cervical nerves

but only seven cervical vertebrae. • New Learning Outcome on the structures and functions of neurons

and neuroglia.

• Updated description of Nissl body function. • New description clarifying the difference between a synapse and

synaptic cleft. • New analogy for a myelin sheath as the wrapping on an electrical

cord. • New explanation clarifying the differences between myelin sheaths

in the CNS and PNS. • New explanation clarifying the “short circuit” event in multiple

sclerosis means that the signal may stop or “jump” to an unmyelinated neuron.

• New analogy for the structure of a unipolar cell body as a “cul-de- sac” off the “main road” that is the axon.

• Replaced references to the term basal ganglia with the more accurate term basal nuclei.

• Replaced the term arachnoid villi with arachnoid granulations. • New statistics on stroke as the fifth leading cause of death in the

United States (formerly identified as the third leading cause). • New statistics regarding the rate of survival after a stroke. • Replaced the phrase “mentally retarded” in the discussion of

cerebral palsy with “intellectually disabled.” • New information included in “A Closer Look: The ‘Terrible Three’”

reflecting the role of calcium in apoptosis, two new drugs for treatment of Parkinson’s disease, and the variation in dopamine levels in patients with Huntington’s disease.

• New information incorporated in “A Closer Look: Tracking Down CNS Problems” to include a new dopamine imaging technique called DaTscan.

Chapter 8: Special Senses • New description of lacrimal caruncle. • New description of optic disc and the resulting blind spot. • New analogy comparing the ability to see intermediate colors

(between the red, green, and blue cones) to mixing paint. • Updated the description of cataracts. • New example of motion detected by dynamic equilibrium: a

spinning carnival ride. • New analogy for bending of the cupula as divers’ fins in water. • New description of foliate papillae on the side of the tongue,

another location for taste buds. • New art showing the retina in Figure 8.5. • New Figure 8.6 showing the graph of rods and cones, and which

wavelengths of light are detected by each. • Updated Figure 8.12b on maculae. • New “Did You Get It?” questions throughout the chapter.

Chapter 9: The Endocrine System • Updated discussion of the mechanism of hormone action,

including Figure 9.1 and its caption, to reflect that steroid hormones can act via either second messenger or direct gene activation.

• Updated explanation of how hormones alter cell activity. • New analogy comparing second-messenger systems to delivering a

letter. • Revised coverage of endocrine glands to reflect their location in

body from superior to inferior; Table 9.1 has also been revised to reflect the new order.

• Updated description explaining why a goiter forms in the absence of iodine.

• Updated description of body proportions in cretinism. • New “Did You Get It?” question on adrenal cortex hormones.

Chapter 10: Blood • Updated explanation of why the normal temperature of blood is a

bit higher than body temperature. • Added definitions for the suffixes -cytosis and -penia. • Updated the analogy comparing the shape of the eosinophil

nucleus to earmuffs. • Updated the role of monocytes to include activation of

lymphocytes. • Updated the list of locations where red marrow is found in adults. • Updated the major anticoagulants to include warfarin. • New description of petechiae includes comparison to a skin rash.

New to the Twelfth Edition 13

14 New to the Twelfth Edition

• Updated Table 12.1 regarding the role of nasal hairs to include filtration of airborne particles.

• Updated Table 12.3 entry for “Cytokines: Perforin and granzymes” to include natural killer (NK) cells.

• New “Did You Get It?” questions throughout the chapter.

Chapter 13: The Respiratory System • New information explaining neural regulation of breathing with

respect to the dorsal and ventral respiratory groups of the medulla. • New Short Answer Essay question contrasting hyperventilation and

hyperpnea. • Updated “A Closer Look” on cleanliness and asthma. • New “Did You Get It?” questions throughout the chapter.

Chapter 14: The Digestive System • New illustration outlining the parietal and visceral layers of the

peritoneum (Figure 14.5). • New illustrations showing both deciduous and permanent teeth in

greater detail (Figure 14.9). • Updated description of circular folds to provide students with a

visual image of a corkscrew that slows progression of food and increases surface area at the same time.

• Added detail that rennin in infants is the same enzyme used to curdle milk in cheesemaking.

• Added narcotic pain medications to the list of causes of constipation, with stool softeners as a method of treatment.

• New “Did You Get It?” question on the four types of teeth and their functions.

• Added brief discussion of nucleic acid digestion, including the source of the enzymes and the reminder that nucleotides are the building blocks.

• Revised “A Closer Look” box on obesity to update references, statistics, and methods used to determine body composition, such as DEXA, the Bod Pod, and underwater weighing.

Chapter 15: The Urinary System • Updated descriptions of the arterioles that connect to the

glomerulus. • Included a new learning tool describing the internal urethral

sphincter as involuntary. • New “Short Answer Essay” question contrasting the homeostatic

imbalances oliguria, anuria, polyuria, and nocturia. • New “Critical Thinking and Clinical Application” question about

the relationship between hypertension and impaired kidney function, and tests that are used for determining impaired kidney function.

• New information included in “A Closer Look: Renal Failure and the Artificial Kidney” about a blood test to determine the creatinine level in order to estimate the rate of glomerular filtration.

• Did You Get It?

Chapter 16: The Reproductive System • New explanation of the purpose of polar bodies: to reduce the

chromosome number during oogenesis. • Update of suggested age range for women to begin having regular

mammograms: between 45 and 54. • New Concept Link on chemotaxis. • New photomicrograph showing sperm swarming an oocyte in

Figure 16.16. • New explanation of how an egg blocks additional sperm from

entering; the surface sperm receptors on an oocyte are shed after the first sperm enters the cell.

• New clarification with updated definitions of miscarriage and abortion.

• Added a learning tool about blood type reminding readers that a person does not make antibodies against their own blood type antigen(s).

• Updated discussion of lack of vitamin B12 as the cause of pernicious anemia and how this relates to intrinsic factor.

Chapter 11: The Cardiovascular System • Updated description of pericardium. • Revised discussion of the function of the atria to clarify that they

assist with ventricular filling. • Arteries and veins are now introduced in terms of the direction of

blood flow with respect to the heart. • New analogy comparing valve cusps filling with blood to a

parachute filling with air. • New analogy comparing the intrinsic conduction system setting

heart rhythm to a drummer setting the beat for a rock band playing a song.

• New discussion of AEDs (automatic external defibrillators) included in the discussion of fibrillation.

• Reorganized section on the cardiac cycle to include five stages. • New “Did You Get It?” question about isovolumetric contraction. • Updated description of the effect of congestive heart failure on

stroke volume. • Updated description of pulmonary congestion. • Updated description of pulmonary embolism. • Discussion of fetal circulation moved to the Developmental

Aspects section. • Updated description of the blood pressure gradient to include a

pressure of zero in the right atrium. • New layout of Figure 11.8 reflecting five stages of the cardiac cycle. • Updated Figure 11.9 description to clarify that any change in heart

rate or stroke volume will also cause a change in cardiac output. • Updated description of Figures 11.13 and 11.14 to include a

statement that all vessels are bilateral unless otherwise stated in the text.

• Updated “A Closer Look” box on atherosclerosis.

Chapter 12: The Lymphatic System and Body Defenses • Updated Figure 12.10 on lysis by complement to reflect water

flowing into the cell to cause lysis. • Added the role of B cells in antigen presentation to Figure 12.19. • New information added regarding discovery of lymphatics in the

central nervous system. • Updated the description of adaptive defenses as defenses that fight

antigens that get past the innate defenses. • New description of how natural killer cells kill: via perforin and

granzymes. • Updated the description of positive chemotaxis to include

movement toward the stimulus. • Revised description of interferon to clarify that interferon fights

only viral pathogens, not bacteria or fungi. • New antibody function has been listed: opsonization. • New description of Graves’ disease explaining that excess

production of thyroxine is in response to antibodies that mimic TSH (thyroid-stimulating hormone).

• New descriptions of two additional types of hypersensitivities: reactions resulting in cell lysis and those forming antigen-antibody complexes.

• New example of when epinephrine is used during acute hypersensitivity: EpiPen® injection.

• New Short Answer Essay question provided on mechanisms of killing used by the immune system, including lysozyme, perforin, and granzymes, and membrane attack complex (MAC).

• Updated “A Closer Look” box on AIDS, including new title.

Acknowledgments

University; Amy Goode, Illinois Central College; Jeannette Hafey, Springfield College; Ashley Hagler, Gaston College; Frances Miles, Lake Michigan College–Napier Avenue Campus; Margaret Ott, Tyler Junior College; Heidi Peterson, Indian Hills Commu- nity College–Ottumwa Campus; Laura Ritt, Burling- ton County College; Holly Sanders, Gwinnett Technical College; Leba Sarkis, Aims Community College; Gustavo A. Solis, Forsyth Technical Com- munity College; Ginny Stokes, Nash Community College; Robert Suddith, Cape Fear Community Col- lege; John F. Tarpey, City College of San Francisco; Deborah S. Temperly, Delta College; Claudia Wil- liams, Campbell University.

The entire group from Pearson and beyond deserves our heartfelt thanks for being the best team around! They have provided support, guid- ance, and humor throughout the writing process, which made the process fun and was most appre- ciated. Special thanks to Serina Beauparlant, Edi- tor-In-Chief, Brooke Suchomel, Sr. Acquisitions Editor, and Tiffany Mok, Program Manager who passed the reins to Lauren Harp, Sr. Portfolio Man- ager, for crossing the finish line. Thanks also to Nicky Montalvo, Editorial Coordinator. Thank you to the content development team—Suzanne Olivier, Alice Fugate, and Laura Southworth—for their attention to detail and collaborative minds. Thank you Patrice Fabel, Ziki Dekel, and Lauren Hill for supervising an impressive variety of media content that will benefit both students and instructors. A special thank-you to Gary Hespenheide for the book’s beautiful and creative new interior and cover designs. The work of Kristin Piljay, Photo Researcher, resulted in some striking new photos for this edition; and Sally Peyrefitte, our excellent and diligent copyeditor, ensured a consistent style throughout the book. Proofreader Betsy Dietrich skillfully reviewed every page proof, and Sallie Steele provided a thorough and accurate index. Our talented art house, Imagineering STA Media Services, Inc., and compositor, Aptara, worked tire- lessly to provide stunning artwork and student- friendly page layouts. Stacey Weinberger, Senior Manufacturing Buyer, Allison Rona, and Derek

Many people contributed to our efforts in the cre- ation of this twelfth edition. We offer our profound thanks to the following reviewers, whose thought- ful critiques informed and enhanced our develop- ment of this edition:

• William Brazelle, University of South Florida

• Sheree Daniel, Trinity Valley Community College

• Trevor Day, Mount Royal University

• Camille Di Scala, Chandler-Gilbert Community College

• Pamela Boyter Jackson, Piedmont Technical College

• Roop Jayaraman, Central Michigan University

• Kimberly Kushner, Pueblo Community College

• Frances Mills, Lake Michigan College

• Diane Pelletier, Green River Community College

• Heidi Peterson, Indian Hills Community College

• Kenneth Ryan, Alexandria Technical and Community College

• Holly Sanders, Gwinnett Technical College

• Scott Schaeffer, Harford Community College

• I-Chia Shih, Leeward Community College, University of Hawaii

• K. Dale Smoak, Piedmont Technical College

• Bill Snyder, Bluegrass Community and Technical College

• Greg Tefft, Northwest State Community College

• Sandra Uyeshiro, Modesto Junior College

• Khursheed Wankadiya, Central Piedmont Community College

• Carol T. Wismer, College of Lake County

Thanks are also extended to the reviewers of the Eleventh Edition: Carmen Carpenter, South Univer- sity; Steven D. Collins, Niagara College; Janie Cor- bitt, Central Georgia Technical College–Milledgeville Campus; Eric D. Forman, Sauk Valley Community College; Andrew Goliszek, North Carolina A&T State

16 Acknowledgments

Perrigo deserve special thanks for their expertise in delivering and presenting the final product to the market. A special thank you goes to David Novak, our Production and Art Coordinator, for taking on the role of two people during this edition and flaw- lessly handling every text and art-related production

detail—David made the whole process smooth and successful. And last, but not least, Michele Mangelli—a tremendous thank-you for your skillful oversight of all aspects of the 12th edition, includ- ing assisting a first-time co-author in all things pub- lishing . . . you’re an unflappable rock star!

Suzanne M. Keller

Anatomy and Physiology Pearson Education 1301 Sansome Street San Francisco, CA 94111

Elaine N. Marieb

The publishers would like to thank the following for their contribution to the Global Edition:

Contributor

Christiane Van den Branden, Vrije Universiteit Brussel

Reviewers

Hemant Kumar, MBBS Snezana Kusljic, The University of Melbourne Eva Strandell, Halmstad University

1 The Human Body: An Orientation 27 An Overview of Anatomy and Physiology 27

Anatomy 27

Physiology 28

Relationship between Anatomy and Physiology 28

Levels of Structural Organization 28 From Atoms to Organisms 28

Organ System Overview 29 Integumentary System • Skeletal System • Muscular System • Nervous System • Endocrine System • Cardiovascular System • Lymphatic System • Respiratory System • Digestive System • Urinary System • Reproductive System

Maintaining Life 33 Necessary Life Functions 33

Maintaining Boundaries • Movement • Responsiveness • Digestion • Metabolism • Excretion • Reproduction • Growth

Survival Needs 35

The Language of Anatomy 38 Anatomical Position 38

Directional Terms 38

Regional Terms 38 Anterior Body Landmarks • Posterior Body Landmarks

Body Planes and Sections 41

Body Cavities 41 Dorsal Body Cavity • Ventral Body Cavity • Other Body Cavities

Homeostasis 45 Components of Homeostatic Control Systems 45

Feedback Mechanisms 45

SUMMARy 47

REviEw QUESTiONS 48

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 49

A CLOSER LOOk Medical Imaging: Illuminating the Body 36

2 Basic Chemistry 50 Concepts of Matter and Energy 50

Matter 50

Energy 51 Forms of Energy • Energy Form Conversions

Composition of Matter 52 Elements and Atoms 52

Atomic Structure 52 The Basic Atomic Subparticles • Planetary and Orbital Models of an Atom

Identifying Elements 54 Atomic Number • Atomic Mass Number • Atomic Weight and Isotopes

Molecules and Compounds 57

Chemical Bonds and Chemical Reactions 58

Bond Formation 58 Role of Electrons • Types of Chemical Bonds

Patterns of Chemical Reactions 62 Synthesis Reactions • Decomposition Reactions • Exchange Reactions • Factors Influencing the Rate of Chemical Reactions

Biochemistry: The Chemical Composition of Living Matter 64

Inorganic Compounds 65 Water • Salts • Acids and Bases

Contents

Organic Compounds 68 Carbohydrates • Lipids • Proteins • Nucleic Acids • Adenosine Triphosphate (ATP)

SUMMARy 83

REviEw QUESTiONS 85

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 87

FOCUS ON CAREERS Pharmacy Technician 82

3 Cells and Tissues 88 PART i: CELLS 88

Overview of the Cellular Basis of Life 88

Anatomy of a generalized Cell 89 The Nucleus 89

Nuclear Envelope • Nucleolus • Chromatin

The Plasma Membrane 90 The Fluid Mosaic Model • Cell Membrane Junctions

The Cytoplasm 93 Cytosol and Inclusions • Organelles

Cell Extensions 97 Cilia and Flagella • Microvilli

Cell Diversity 100

Cell Physiology 102 Membrane Transport 102

Passive Processes: Diffusion and Filtration • Active Processes

Cell Division 108 Preparations: DNA Replication • Events of Cell Division

Protein Synthesis 111 Genes: The Blueprint for Protein Structure • The Role of RNA • The Process of Protein Synthesis

PART ii: BODy TiSSUES 114

Epithelial Tissue 114 Hallmarks of Epithelium 114

Classification of Epithelia 115 Simple Epithelia • Stratified Epithelia • Glandular Epithelium

Connective Tissue 119 Hallmarks of Connective Tissue 119

Extracellular Matrix 120

Types of Connective Tissue 120 Bone • Cartilage • Dense Connective Tissue • Loose Connective Tissue • Blood

Muscle Tissue 124 Skeletal Muscle 124

Cardiac Muscle 124

Smooth Muscle 126

Nervous Tissue 126

Tissue Repair (wound Healing) 126

PART iii: DEvELOPMENTAL ASPECTS OF CELLS AND TiSSUES 128

SUMMARy 130

REviEw QUESTiONS 133

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 134

A CLOSER LOOk IV Therapy and Cellular “Tonics” 105

A CLOSER LOOk Cancer—An Intimate Enemy 130

4 Skin and Body Membranes 135 Classification of Body Membranes 135

Epithelial Membranes 136 Cutaneous Membrane • Mucous Membranes • Serous Membranes

Connective Tissue Membranes 136

Chapter 2, continued

18 Contents

Cervical Vertebrae • Thoracic Vertebrae • Lumbar Vertebrae • Sacrum • Coccyx

Thoracic Cage 182 Sternum • Ribs

Appendicular Skeleton 184 Bones of the Shoulder Girdle 184

Bones of the Upper Limbs 184 Arm • Forearm • Hand

Bones of the Pelvic Girdle 188

Bones of the Lower Limbs 190 Thigh • Leg • Foot

Joints 192 Fibrous Joints 195

Cartilaginous Joints 195

Synovial Joints 195

Types of Synovial Joints Based on Shape 196

Developmental Aspects of the Skeleton 199

Birth to Adulthood 199

Older Adults 201

SUMMARy 203

REviEw QUESTiONS 204

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 206

FOCUS ON CAREERS Radiologic Technologist 169

A CLOSER LOOk Joint Ventures 192

SySTEMS iN SyNC 202

6 The Muscular System 207 Overview of Muscle Tissues 207

Muscle Types 207 Skeletal Muscle • Smooth Muscle • Cardiac Muscle

Muscle Functions 211 Produce Movement • Maintain Posture and Body Position • Stabilize Joints • Generate Heat • Additional Functions

The integumentary System (Skin) 138 Functions of the Integumentary System 138

Structure of the Skin 139 Epidermis • Dermis

Skin Color 144

Appendages of the Skin 145 Cutaneous Glands • Hair and Hair Follicles • Nails

Homeostatic Imbalances of Skin 149 Infections and Allergies • Burns • Skin Cancer

Developmental Aspects of Skin and Body Membranes 153

SUMMARy 156

REviEw QUESTiONS 157

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 158

A CLOSER LOOk A Wrinkle Out of Time 143

FOCUS ON CAREERS Medical Transcriptionist 154

SySTEMS iN SyNC 155

5 The Skeletal System 160 Bones: An Overview 160

Functions of the Bones 161

Classification of Bones 161

Structure of Bone 163 Gross Anatomy of a Long Bone • Microscopic Anatomy

Bone Formation, Growth, and Remodeling 167

Bone Formation and Growth • Bone Remodeling

Bone Fractures 170

Axial Skeleton 172 Skull 172

Cranium • Facial Bones • The Hyoid Bone

Vertebral Column (Spine) 178

Contents 19

Chapter 6, continued A CLOSER LOOk Anabolic Steroids: Dying to Win? 231

SySTEMS iN SyNC 246

7 The Nervous System 251 Organization of the Nervous System 252

Structural Classification 253

Functional Classification 253

Nervous Tissue: Structure and Function 253

Supporting Cells 253

Neurons 255 Anatomy • Classification • Physiology: Nerve Impulses • Physiology: Reflexes

Central Nervous System 265 Functional Anatomy of the Brain 265

Cerebral Hemispheres • Diencephalon • Brain Stem • Cerebellum

Protection of the Central Nervous System 273 Meninges • Cerebrospinal Fluid • The Blood-Brain Barrier

Brain Dysfunctions 277

Spinal Cord 278 Gray Matter of the Spinal Cord and Spinal Roots • White Matter of the Spinal Cord

Peripheral Nervous System 281 Structure of a Nerve 281

Cranial Nerves 283

Spinal Nerves and Nerve Plexuses 283

Autonomic Nervous System 290 Somatic and Autonomic Nervous Systems Compared • Anatomy of the Parasympathetic Division • Anatomy of the Sympathetic Division • Autonomic Functioning

Developmental Aspects of the Nervous System 295

SUMMARy 299

Microscopic Anatomy of Skeletal Muscle 211

Skeletal Muscle Activity 213 Stimulation and Contraction of Single Skeletal Muscle Fibers 213

The Nerve Stimulus and the Action Potential • Mechanism of Muscle Contraction: The Sliding Filament Theory

Contraction of a Skeletal Muscle as a Whole 217

Graded Responses • Providing Energy for Muscle Contraction • Muscle Fatigue and Oxygen Deficit • Types of Muscle Contractions—Isotonic and Isometric • Muscle Tone • Effect of Exercise on Muscles

Muscle Movements, Roles, and Names 222

Types of Body Movements 222 Special Movements

Interactions of Skeletal Muscles in the Body 226

Naming Skeletal Muscles 228

Arrangement of Fascicles 228

gross Anatomy of Skeletal Muscles 229

Head and Neck Muscles 229 Facial Muscles • Neck Muscles

Trunk Muscles 232 Anterior Muscles • Posterior Muscles

Muscles of the Upper Limb 235 Muscles Causing Movement at the Elbow Joint

Muscles of the Lower Limb 235 Muscles Causing Movement at the Hip Joint • Muscles Causing Movement at the Knee Joint • Muscles Causing Movement at the Ankle and Foot

Developmental Aspects of the Muscular System 244

SUMMARy 245

REviEw QUESTiONS 248

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 249

20 Contents

PART iii: CHEMiCAL SENSES: SMELL AND TASTE 324

Olfactory Receptors and the Sense of Smell 324

Taste Buds and the Sense of Taste 326

PART iv: DEvELOPMENTAL ASPECTS OF THE SPECiAL SENSES 327 SUMMARy 329

REviEw QUESTiONS 331

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 332

A CLOSER LOOk Visual Pigments—The Actual Photoreceptors 311

A CLOSER LOOk Bringing Things into Focus 315

FOCUS ON CAREERS Physical Therapy Assistant 321

9 The Endocrine System 334 The Endocrine System and Hormone Function—An Overview 335

The Chemistry of Hormones 335

Hormone Action 335 Direct Gene Activation • Second-Messenger System

Stimuli for Control of Hormone Release 337

Hormonal Stimuli • Humoral Stimuli • Neural Stimuli

The Major Endocrine Organs 338 Pituitary Gland and Hypothalamus 339

Pituitary-Hypothalamus Relationships

Pineal Gland 343

Thyroid Gland 343

Parathyroid Glands 345

Thymus 346

REviEw QUESTiONS 301

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 303

A CLOSER LOOk The “Terrible Three” 278

A CLOSER LOOk Tracking Down CNS Problems 296

SySTEMS iN SyNC 298

8 Special Senses 304 PART i: THE EyE AND viSiON 305

Anatomy of the Eye 305 External and Accessory Structures 305

Internal Structures: The Eyeball 307 Layers Forming the Wall of the Eyeball • Lens

Physiology of vision 313 Pathway of Light through the Eye and Light Refraction 313

Visual Fields and Visual Pathways to the Brain 314

Eye Reflexes 316

PART ii: THE EAR: HEARiNg AND BALANCE 316

Anatomy of the Ear 317 External (Outer) Ear 317

Middle Ear 318

Internal (Inner) Ear 318

Equilibrium 318 Static Equilibrium 319

Dynamic Equilibrium 319

Hearing 322

Hearing and Equilibrium Deficits 323

Contents 21

Chapter 9, continued Developmental Aspects of Blood 378

SUMMARy 379

REviEw QUESTiONS 380

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 381

FOCUS ON CAREERS Phlebotomy Technician 376

11 The Cardiovascular System 382 The Heart 383

Anatomy of the Heart 383 Size, Location, and Orientation • Coverings and Walls of the Heart

Chambers and Associated Great Vessels 384

Heart Valves 387 Cardiac Circulation

Physiology of the Heart 390 Intrinsic Conduction System of the Heart: Setting the Basic Rhythm • Cardiac Cycle and Heart Sounds • Cardiac Output

Blood vessels 396 Microscopic Anatomy of Blood Vessels 396

Tunics • Structural Differences in Arteries, Veins, and Capillaries

Gross Anatomy of Blood Vessels 399 Major Arteries of the Systemic Circulation • Major Veins of the Systemic Circulation • Special Circulations

Physiology of Circulation 406 Arterial Pulse • Blood Pressure • Capillary Exchange of Gases and Nutrients • Fluid Movements at Capillary Beds

Developmental Aspects of the Cardiovascular System 415

SUMMARy 418

REviEw QUESTiONS 420

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 422

A CLOSER LOOk Electrocardiography: (Don’t) Be Still My Heart 393

Adrenal Glands 346 Hormones of the Adrenal Cortex • Hormones of the Adrenal Medulla

Pancreatic Islets 349

Gonads 353 Hormones of the Ovaries • Hormones of the Testes

Other Hormone-Producing Tissues and Organs 353

Developmental Aspects of the Endocrine System 357

SUMMARy 359

REviEw QUESTiONS 361

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 362

A CLOSER LOOk Potential Uses for Growth Hormone 342

SySTEMS iN SyNC 358

10 Blood 363 Composition and Functions of Blood 363

Components 364

Physical Characteristics and Volume 364

Plasma 364

Formed Elements 366 Erythrocytes • Leukocytes • Platelets

Hematopoiesis (Blood Cell Formation) 371 Formation of Red Blood Cells • Formation of White Blood Cells and Platelets

Hemostasis 373 Phases of Hemostasis 373

Disorders of Hemostasis 374

Blood groups and Transfusions 375 Human Blood Groups 375

Blood Typing 378

22 Contents

SUMMARy 457

REviEw QUESTiONS 459

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 461

A CLOSER LOOk AIDS: An Ongoing Pandemic 454

SySTEMS iN SyNC 456

13 The Respiratory System 462 Functional Anatomy of the Respiratory System 462

The Nose 463

The Pharynx 464

The Larynx 465

The Trachea 466

The Main Bronchi 466

The Lungs 467 The Bronchial Tree • Respiratory Zone Structures and the Respiratory Membrane

Respiratory Physiology 471 Mechanics of Breathing 471

Inspiration • Expiration

Respiratory Volumes and Capacities 474

Nonrespiratory Air Movements 475

Respiratory Sounds 475

External Respiration, Gas Transport, and Internal Respiration 475

External Respiration • Gas Transport in the Blood • Internal Respiration

Control of Respiration 478 Neural Regulation: Setting the Basic Rhythm • Nonneural Factors Influencing Respiratory Rate and Depth

Respiratory Disorders 480

Developmental Aspects of the Respiratory System 483

SUMMARy 485

A CLOSER LOOk Atherosclerosis? Get Out the Cardiovascular Drāno! 412

SySTEMS iN SyNC 417

12 The Lymphatic System and Body Defenses 424

PART i: THE LyMPHATiC SySTEM 424

Lymphatic vessels 425

Lymph Nodes 426

Other Lymphoid Organs 428

PART ii: BODy DEFENSES 429

innate Body Defenses 430 Surface Membrane Barriers 430

Internal Defenses: Cells and Chemicals 432 Natural Killer Cells • Inflammatory Response • Phagocytes • Antimicrobial Proteins • Fever

Adaptive Body Defenses 436 Antigens 438

Cells of the Adaptive Defense System: An Overview 438

Lymphocytes • Antigen-Presenting Cells

Humoral (Antibody-Mediated) Immune Response 441

Active and Passive Humoral Immunity • Antibodies

Cellular (Cell-Mediated) Immune Response 446

Organ Transplants and Rejection 448

Disorders of Immunity 451

PART iii: DEvELOPMENTAL ASPECTS OF THE LyMPHATiC SySTEM AND BODy DEFENSES 455

Contents 23

Chapter 13, continued PART ii: NUTRiTiON AND METABOLiSM 513

Nutrition 513 Dietary Recommendations 513

Dietary Sources of the Major Nutrients 514 Carbohydrates • Lipids • Proteins • Vitamins • Minerals

Metabolism 516 Carbohydrate, Fat, and Protein Metabolism in Body Cells 516

Carbohydrate Metabolism • Fat Metabolism • Protein Metabolism

The Central Role of the Liver in Metabolism 520

General Metabolic Functions • Cholesterol Metabolism and Transport

Body Energy Balance 523 Regulation of Food Intake • Metabolic Rate and Body Heat Production • Body Temperature Regulation

PART iii: DEvELOPMENTAL ASPECTS OF THE DigESTivE SySTEM AND METABOLiSM 527 SUMMARy 532

REviEw QUESTiONS 534

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 536

A CLOSER LOOk Peptic Ulcers: “Something Is Eating at Me” 512

A CLOSER LOOk Obesity: Magical Solution Wanted 529

SySTEMS iN SyNC 531

15 The Urinary System 537 kidneys 538

Location and Structure 538 Kidney Structure • Blood Supply

Nephrons 539

REviEw QUESTiONS 487

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 488

A CLOSER LOOk Too Clean for Our Own Good? 482

SySTEMS iN SyNC 484

14 The Digestive System and Body Metabolism 489

PART i: ANATOMy AND PHySiOLOgy OF THE DigESTivE SySTEM 489

Anatomy of the Digestive System 489

Organs of the Alimentary Canal 490 Mouth • Pharynx • Esophagus • Stomach • Small Intestine • Large Intestine

Accessory Digestive Organs 500 Teeth • Salivary Glands • Pancreas • Liver and Gallbladder

Functions of the Digestive System 502

Overview of Gastrointestinal Processes and Controls 503

Activities Occurring in the Mouth, Pharynx, and Esophagus 506

Food Ingestion and Breakdown • Food Propulsion— Swallowing and Peristalsis

Activities of the Stomach 507 Food Breakdown • Food Propulsion

Activities of the Small Intestine 509 Chyme Breakdown and Absorption • Chyme Propulsion

Activities of the Large Intestine 511 Nutrient Breakdown and Absorption • Propulsion of Food Residue and Defecation

24 Contents

Epididymis • Ductus Deferens • Urethra

Accessory Glands and Semen 567 Seminal Vesicles • Prostate • Bulbo-urethral Glands • Semen

External Genitalia 568

Male Reproductive Functions 569 Spermatogenesis 569

Testosterone Production 571

Anatomy of the Female Reproductive System 573

Ovaries 573

Duct System 573 Uterine Tubes • Uterus • Vagina

External Genitalia and Female Perineum 576

Female Reproductive Functions and Cycles 577

Oogenesis and the Ovarian Cycle 577

Hormone Production by the Ovaries 579

Uterine (Menstrual) Cycle 580

Mammary glands 580

Pregnancy and Embryonic Development 583

Accomplishing Fertilization 584

Events of Embryonic and Fetal Development 585

Effects of Pregnancy on the Mother 589 Anatomical Changes • Physiological Changes

Childbirth 591 Initiation of Labor • Stages of Labor

Developmental Aspects of the Reproductive System 593

SUMMARy 598

REviEw QUESTiONS 600

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 602

A CLOSER LOOk Contraception: Preventing Pregnancy 594

SySTEMS iN SyNC 597

Urine Formation and Characteristics 542 Glomerular Filtration • Tubular Reabsorption • Tubular Secretion • Nitrogenous Wastes • Characteristics of Urine

Ureters, Urinary Bladder, and Urethra 546

Ureters 546

Urinary Bladder 547

Urethra 548

Micturition 548

Fluid, Electrolyte, and Acid-Base Balance 550

Maintaining Water Balance of Blood 550 Body Fluids and Fluid Compartments • The Link between Water and Electrolytes • Regulation of Water Intake and Output

Maintaining Electrolyte Balance 553

Maintaining Acid-Base Balance of Blood 555 Blood Buffers • Respiratory Mechanisms • Renal Mechanisms

Developmental Aspects of the Urinary System 557

SUMMARy 561

REviEw QUESTiONS 562

CRiTiCAL THiNkiNg AND CLiNiCAL APPLiCATiON QUESTiONS 563

A CLOSER LOOk Renal Failure and the Artificial Kidney 549

FOCUS ON CAREERS Licensed Practical Nurse (LPN) 558

SySTEMS iN SyNC 560

16 The Reproductive System 564 Anatomy of the Male Reproductive System 565

Testes 565

Duct System 565

Contents 25

Appendix D Key Information about Vitamins and Many Essential Minerals 613

Glossary 617

Credits 630

Subject Index 631

Appendixes Appendix A: Answers to Did You Get It?

Questions and Multiple Choice Review Questions 603

Appendix B: Word Roots, Prefixes, and Suffixes 610

Appendix C: Periodic Table of the Elements 612

26 Contents

the seeds. Adults become upset when their hearts pound, when they have uncontrollable hot flashes, or when they cannot keep their weight down.

Anatomy and physiology, subdivisions of biol- ogy, explore many of these topics as they describe how our bodies are put together and how they work.

Anatomy Anatomy (ah-nat′o-me) is the study of the structure and shape of the body and its parts and their rela- tionships to one another. Whenever we look at our own body or study large body structures such as the heart or bones, we are observing gross anatomy;

An Overview of Anatomy and Physiology ➔ Learning Objectives

□□ Define anatomy and physiology.

□□ Explain how anatomy and physiology are related.

Most of us are naturally curious about our bodies; we want to know what makes us tick. Infants can keep themselves happy for a long time staring at their own hands or pulling their mother’s nose. Older children wonder where food goes when they swallow it, and some believe that they will grow a watermelon in their belly if they swallow

The Human Body: An Orientation

WHAT

HOW

WHY

Anatomy is the study of the structure, or

physical form, of the body.

The function, or physiology, of each body

part and the body as a whole is dependent on the anatomy of

those parts; in other words, structure determines

function.

This text will help you learn structures and functions

so you can understand the role of individual organs and body

systems and how they interact to support life.

InsTrucTOrs

New Building Vocabulary coaching Activities for this chapter are assignable in

28 Essentials of Human Anatomy and Physiology

Levels of structural Organization ➔ Learning Objectives

□□ name the six levels of structural organization that make up the human body, and explain how they are related.

□□ name the organ systems of the body, and briefly state the major functions of each system.

□□ Identify and classify by organ system all organs discussed.

From Atoms to Organisms The human body exhibits many levels of structural complexity (Figure 1.1). The simplest level of the structural ladder is the chemical level (covered in Chapter 2). At this level, atoms, tiny building blocks of matter, combine to form molecules such as water, sugar, and proteins, like those that make up our muscles. Molecules, in turn, associate in specific ways to form microscopic cells, the small- est units of all living things. (We will examine the cellular level in Chapter 3.) All cells have some common structures and functions, but individual cells vary widely in size, shape, and their particu- lar roles in the body.

The simplest living creatures are composed of single cells, but in complex organisms such as trees or human beings, the structural ladder con- tinues on to the tissue level. Tissues consist of groups of similar cells that have a common func- tion. There are four basic tissue types, and each plays a definite but different role in the body. (We discuss tissues in Chapter 3.)

An organ is a structure composed of two or more tissue types that performs a specific function for the body. At the organ level of organization, extremely complex functions become possible. For example, the small intestine, which digests and absorbs food, is composed of all four tissue types. An organ system is a group of organs that work together to accomplish a common purpose. For example, the heart and blood vessels of the cardiovascular system circulate blood continuously to carry nutrients and oxygen to all body cells.

In all, 11 organ systems make up the living human being, or the organism, which represents the highest level of structural organization, the organismal level. The organismal level is the sum total of all structural levels working together to keep us alive. The major organs of each system

that is, we are studying large, easily observable structures. Indeed, the term anatomy, derived from the Greek words meaning to cut (tomy) apart (ana), is related most closely to gross anatomical studies because in such studies, preserved animals or their organs are dissected (cut up) to be exam- ined. Microscopic anatomy, in contrast, is the study of body structures that are too small to be seen with the naked eye. The cells and tissues of the body can only be seen through a microscope.

Physiology Physiology (fiz″e-ol′o-je) is the study of how the body and its parts work or function ( physio = nature; ology = the study of). Like anatomy, physi- ology has many subdivisions. For example, neu- rophysiology explains the workings of the nervous system, and cardiac physiology studies the func- tion of the heart.

relationship between Anatomy and Physiology Anatomy and physiology are always inseparable. The parts of your body form a well-organized unit, and each of those parts has a job to do to make the body operate as a whole. Structure determines what functions can take place. For example, the lungs are not muscular chambers like the heart and so cannot pump blood through the body, but because the walls of their air sacs are very thin, they can exchange gases and provide oxygen to the body. We stress the intimate relationship between anatomy and physiology throughout this text to make your learning meaningful.

Did You Get It? 1. Why would you have a hard time learning and

understanding physiology if you did not also understand anatomy?

2. Kidney function, bone growth, and beating of the heart are all topics of anatomy. True or false?

For answers, see Appendix A.

➔ConCeptLinkThroughout this text, Concept Links will highlight links between concepts and/or organ systems. Keep in mind that although discussions of the systems are separated into chapters for detailed study, the overall goal of this text is for you not only to gain an understanding of each individual system, but also to learn how the body systems interact to sustain life. ➔

Chapter 1: The Human Body: An Orientation 29

waterproofs the body and cushions and protects the deeper tissues from injury. With the help of sunlight, it produces vitamin D. It also excretes salts in perspiration and helps regulate body tem- perature. Sensory receptors located in the skin alert us to what is happening at the body surface.

skeletal system

The skeletal system consists of bones, cartilages, and joints (Figure 1.2b). It supports the body and

are shown in Figure 1.2 on pp. 31–32. Refer to the figure as you read through the following descrip- tions of the organ systems.

Organ system Overview Integumentary system

The integumentary (in-teg″u-men′tar-e) system is the external covering of the body, or the skin, including the hair and fingernails (Figure 1.2a). It

Molecules

Atoms

Smooth muscle tissue

Epithelial tissue

Blood vessels

Heart

Smooth muscle tissue

Connective tissue

Blood vessel (organ)

Cardio– vascular system

5 6

Chemical level Atoms combine to form molecules.

Smooth muscle cell

2 Cellular level Cells are made up of molecules.

Tissue level Tissues consist of similar types of cells.

Organ level Organs are made up of different types of tissues.Organ system level

Organ systems consist of different organs that work together closely.

Organismal level Human organisms are made up of many organ systems.

Figure 1.1 Levels of structural organization. In this diagram, components of the cardiovascular system are used to illustrate the levels of structural organization in a human being.

30 Essentials of Human Anatomy and Physiology

provides a framework that the skeletal muscles use to cause movement. It also has protective func- tions (for example, the skull encloses and protects the brain), and the cavities of the skeleton are the sites where blood cells are formed. The hard sub- stance of bones acts as a storehouse for minerals.

Muscular system

The muscles of the body have only one function— to contract, or shorten. When this happens, move- ment occurs. The mobility of the body as a whole reflects the activity of skeletal muscles, the large, fleshy muscles attached to bones (Figure 1.2c). When these contract, you are able to stand erect, walk, jump, grasp, throw a ball, or smile. The skel- etal muscles form the muscular system. These muscles are distinct from the muscles of the heart and of other hollow organs, which move fluids (such as blood or urine) or other substances (such as food) along definite pathways within the body.

nervous system

The nervous system is the body’s fast-acting con- trol system. It consists of the brain, spinal cord, nerves, and sensory receptors (Figure 1.2d). The body must be able to respond to stimuli com- ing from outside the body (such as light, sound, or changes in temperature) and from inside the body (such as decreases in oxygen or stretching of tissue). The sensory receptors detect changes in temperature, pressure, or light, and send messages (via electrical signals called nerve impulses) to the central nervous system (brain and spinal cord) so that it is constantly informed about what is going on. The central nervous system then assesses this information and responds by activating the appro- priate body effectors (muscles or glands, which are organs that produce secretions).

Endocrine system

Like the nervous system, the endocrine (en′do-krin) system controls body activities, but it acts much more slowly. Endocrine glands produce chemi- cal molecules called hormones and release them into the blood to travel to relatively distant target organs.

The endocrine glands include the pituitary, thyroid, parathyroids, adrenals, thymus, pancreas, pineal, ovaries (in the female), and testes (in the male) (Figure 1.2e). The endocrine glands are not connected anatomically in the same way that the parts of other organ systems are. What they have

in common is that they all secrete hormones, which regulate other structures. The body func- tions controlled by hormones are many and var- ied, involving every cell in the body. Growth, reproduction, and the use of nutrients by cells are all controlled (at least in part) by hormones.

cardiovascular system

The primary organs of the cardiovascular sys- tem are the heart and blood vessels (Figure 1.2f). Using blood as a carrier, the cardiovascular system delivers oxygen, nutrients, hormones, and other substances to, and picks up wastes such as carbon dioxide from, cells near sites of exchange. White blood cells and chemicals in the blood help to protect the body from such foreign invaders as bacteria, viruses, and tumor cells. The heart pro- pels blood out of its chambers into blood vessels to be transported to all body tissues.

Lymphatic system

The role of the lymphatic system comple- ments that of the cardiovascular system. Its organs include lymphatic vessels, lymph nodes, and other lymphoid organs such as the spleen and tonsils (Figure 1.2g). When fluid is leaked into tissues from the blood, lymphatic vessels return it to the bloodstream so that there is enough blood to con- tinuously circulate through the body. The lymph nodes and other lymphoid organs help to cleanse the blood and house white blood cells involved in immunity.

respiratory system

The job of the respiratory system is to keep the body supplied with oxygen and to remove car- bon dioxide. The respiratory system consists of the nasal passages, pharynx, larynx, trachea, bron- chi, and lungs (Figure 1.2h). Within the lungs are tiny air sacs. Gases are exchanged with the blood through the thin walls of these air sacs.

Digestive system

The digestive system is basically a tube running through the body from mouth to anus. The organs of the digestive system include the oral cavity (mouth), esophagus, stomach, small and large intestines, and rectum plus a number of accessory organs (liver, salivary glands, pancreas, and oth- ers) (Figure 1.2i). Their role is to break down food and deliver the resulting nutrients to the blood for dispersal to body cells. The breakdown activities

Forms the external body covering; protects deeper tissue from injury; synthesizes vitamin D; location of sensory receptors (pain, pressure, etc.) and sweat and oil glands.

(a) Integumentary System

Skin

Hair

Fingernails

Allows manipulation of the environment, locomotion, and facial expression; maintains posture; produces heat.

Skeletal muscles

Protects and supports body organs; provides a framework the muscles use to cause movement; blood cells are formed within bones; stores minerals.

(b) Skeletal System

Cartilages

Joint

Bones

Glands secrete hormones that regulate processes such as growth, reproduction, and nutrient use by body cells.

Blood vessels transport blood, which carries oxygen, nutrients, hormones, carbon dioxide, wastes, etc.; the heart pumps blood.

Fast-acting control system of the body; responds to internal and external changes by activating appropriate muscles and glands.

Heart

Blood vessels

Thyroid gland (parathyroid glands on posterior aspect)

Pineal gland

Pituitary gland

Thymus gland

Adrenal glands

Pancreas

Testis (male)

Ovary (female)

Brain

Sensory receptor

Spinal cord

Nerves

(d) Nervous System (e) Endocrine System (f) Cardiovascular System

Figure 1.2 The body’s organ systems. (Figure continues on page 32.)

Lymph nodes

Lymphatic vessels

Trachea

Larynx

Bronchi

Lungs

Nasal cavity

Pharynx Esophagus

Stomach

Small intestine

Large intestine

Rectum

Oral cavity

Ovary

Uterine tube

Uterus

Vagina

Mammary glands (in breasts)

Seminal vesicles

Prostate gland

Penis Testis

Scrotum

Ureter

Urinary bladder

Urethra

Kidney

Picks up fluid leaked from blood vessels and returns it to blood; disposes of debris in the lymphatic stream; houses white blood cells involved in immunity.

(g) Lymphatic System

Keeps blood constantly supplied with oxygen and removes carbon dioxide; the gaseous exchanges occur through the walls of the air sacs of the lungs.

(h) Respiratory System

Breaks food down into absorbable nutrients that enter the blood for distribution to body cells; indigestible foodstuffs are eliminated as feces.

(i) Digestive System

Eliminates nitrogen-containing wastes from the body; regulates water, electrolyte, and acid-base balance of the blood.

(j) Urinary System Overall function of the reproductive system is production of offspring. Testes produce sperm and male sex hormone; ducts and glands aid in delivery of viable sperm to the female reproductive tract. Ovaries produce eggs and female sex hormones; remaining structures serve as sites for fertilization and development of the fetus. Mammary glands of female breasts produce milk to nourish the newborn.

(k) Male Reproductive System (l) Female Reproductive System

Figure 1.2 (continued) The body’s organ systems.

Chapter 1: The Human Body: An Orientation 33

1Maintaining Life ➔ Learning Objectives

□□ List eight functions that humans must perform to maintain life.

□□ List the five survival needs of the human body.

necessary Life Functions Now that we have introduced the structural levels composing the human body, a question naturally follows: What does this highly organized human body do? Like all complex animals, human beings maintain their boundaries, move, respond to envi- ronmental changes, take in and digest nutrients, carry out metabolism, dispose of wastes, repro- duce themselves, and grow.

Organ systems do not work in isolation; instead, they work together to promote the well-being of the entire body (Figure 1.3). Because this theme is emphasized throughout this text, it is worthwhile to identify the most important organ systems contrib- uting to each of the necessary life functions. Also, as you study this figure, you may want to refer to the more detailed descriptions of the organ systems just provided (pp. 29–33 and in Figure 1.2).

Maintaining Boundaries

Every living organism must be able to maintain its boundaries so that its “inside” remains distinct from its “outside.” Every cell of the human body is surrounded by an external membrane that sepa- rates its contents from the outside interstitial fluid (fluid between cells) and allows entry of needed substances while generally preventing entry of potentially damaging or unnecessary substances. The body as a whole is also enclosed by the integumentary system, or skin. The integumen- tary system protects internal organs from drying out (which would be fatal), from pathogens, and from the damaging effects of heat, sunlight, and an unbelievable number of chemical substances in the external environment.

Movement

Movement includes all the activities promoted by the muscular system, such as propelling ourselves from one place to another (by walking, swim- ming, and so forth) and manipulating the exter- nal environment with our fingers. The skeletal system provides the bones that the muscles pull on as they work. Movement also occurs when substances such as blood, foodstuffs, and urine

that begin in the mouth are completed in the small intestine. From that point on, the major function of the digestive system is to reabsorb water. The undigested food that remains in the tract leaves the body through the anus as feces. The liver is considered a digestive organ because the bile it produces helps to break down fats. The pan- creas, which delivers digestive enzymes to the small intestine, has both endocrine and digestive functions.

urinary system

A normal part of healthy body function is the production of waste by-products, which must be disposed of. One type of waste contains nitro- gen (examples are urea and uric acid), which results when the body cells break down proteins and nucleic acids, which are genetic information molecules. The urinary system removes the nitrogen-containing wastes from the blood and flushes them from the body in urine. This sys- tem, often called the excretory system, is com- posed of the kidneys, ureters, bladder, and urethra (Figure 1.2j). Other important functions of this sys- tem include maintaining the body’s water and salt (electrolyte) balance, regulating the acid-base bal- ance of the blood, and helping to regulate normal blood pressure.

reproductive system

The role of the reproductive system is to pro- duce offspring. The male testes produce sperm. Other male reproductive system structures are the scrotum, penis, accessory glands, and the duct system, which carries sperm to the outside of the body (Figure 1.2k). The female ovaries produce eggs, or ova; the female duct system consists of the uterine tubes, uterus, and vagina (Figure 1.2l). The uterus provides the site for the development of the fetus (immature infant) once fertilization has occurred.

Did You Get It? 3. At which level of structural organization is the

stomach? At which level is a glucose molecule? 4. Which organ system includes the trachea, lungs,

nasal cavity, and bronchi? 5. Which system functions to remove wastes and help

regulate blood pressure?

For answers, see Appendix A.

pan). You do not need to think about it—it just happens! Likewise, when the amount of carbon dioxide in your blood rises to a dangerously high level, your breathing rate speeds up to blow off the excess carbon dioxide.

Because nerve cells are highly irritable and can communicate rapidly with each other via elec- trical impulses, the nervous system bears the major responsibility for responsiveness. However, all body cells are responsive to some extent.

are propelled through the internal organs of the cardiovascular, digestive, and urinary systems, respectively.

responsiveness

Responsiveness, or irritability, is the ability to sense changes (stimuli) in the environment and then to react to them. For example, if you acci- dentally touch a hot pan, you involuntarily pull your hand away from the painful stimulus (the

Interstitial fluid

Heart

Nutrients

Food

Feces are excreted

Urine is excreted

CO2

Digestive system Takes in nutrients, digests them (part of metabolism), and excretes unabsorbed matter (feces)

Integumentary system Protects the body as a whole from the external environment by maintaining boundaries

Urinary system Excretes nitrogen-containing wastes and excess ions

Nutrients and wastes pass between blood and cells via the interstitial fluid

Cardiovascular system Via the blood, distributes oxygen and nutrients to all body cells and delivers wastes and carbon dioxide to disposal organs

Respiratory system Takes in oxygen, which is required for metabolism, and excretes carbon dioxide

Blood

Figure 1.3 Examples of interrelationships among organ systems that illustrate life functions.

Chapter 1: The Human Body: An Orientation 35

1 by an increase in the number of cells. For growth to occur, cell-constructing activities must occur at a faster rate than cell-destroying ones. Hormones released by the endocrine system play a major role in directing growth.

survival needs The goal of nearly all body systems is to maintain life. However, life is extraordinarily fragile and requires that several factors be available. These factors, which we will call survival needs, include nutrients (food), oxygen, water, and appropriate temperature and atmospheric pressure.

Nutrients, which the body takes in through food, contain the chemicals used for energy and cell building. Carbohydrates are the major energy- providing fuel for body cells. Proteins and, to a lesser extent, fats are essential for building cell structures. Fats also cushion body organs and pro- vide reserve fuel. Minerals and vitamins are required for the chemical reactions that go on in cells and for oxygen transport in the blood.

All the nutrients in the world are useless unless oxygen is also available. Because the chemical reactions that release energy from foods require oxygen, human cells can survive for only a few minutes without it. It is made available to the blood and body cells by the cooperative efforts of the respiratory and cardiovascular systems.

Water accounts for 60 to 80 percent of body weight, depending on the age of the individual. It is the single most abundant chemical substance in the body and provides the fluid base for body secre- tions and excretions. We obtain water chiefly from ingested foods or liquids, and we lose it by evapora- tion from the lungs and skin and in body excretions.

If chemical reactions are to continue at life- sustaining levels, normal body temperature must be maintained. If body temperature drops below 37°C (98.6°F), metabolic reactions become slower and slower and finally stop. If body tem- perature is too high, chemical reactions proceed too rapidly, and body proteins begin to break down. At either extreme, death occurs. Most body heat is generated by the activity of the skeletal muscles and dissipated via blood circulating close to the skin surface or by the evaporation of sweat.

The force exerted on the surface of the body by the weight of air is referred to as atmospheric pressure. Breathing and the exchange of oxygen and carbon dioxide in the lungs depend on

Digestion

Digestion is the process of breaking down ingested food into simple molecules that can then be absorbed into the blood. The nutrient-rich blood is then distributed to all body cells by the cardiovascular system, where body cells use these simple molecules for energy and raw materials.

Metabolism

Metabolism is a broad term that refers to all chemical reactions that occur within the body and all of its cells. It includes breaking down com- plex substances into simpler building blocks (as in digestion), making larger structures from smaller ones, and using nutrients and oxygen to produce molecules of adenosine triphosphate (ATP), the energy-rich molecules that power cellular activi- ties. Metabolism depends on the digestive and respiratory systems to make nutrients and oxygen available to the blood and on the cardiovascu- lar system to distribute these needed substances throughout the body. Metabolism is regulated chiefly by hormones secreted by the glands of the endocrine system.

Excretion

Excretion is the process of removing excreta (ek-skre′tah), or wastes, from the body. Several organ systems participate in excretion. For exam- ple, the digestive system rids the body of indigest- ible food residues in feces, the urinary system disposes of nitrogen-containing metabolic wastes in urine, and the skin disposes of various waste products as components of sweat.

reproduction

Reproduction, the production of offspring, can occur on the cellular or organismal level. In cel- lular reproduction, the original cell divides, pro- ducing two identical daughter cells that may then be used for body growth or repair. Reproduction of the human organism is the task of the organs of the reproductive system, which produce sperm and eggs. When a sperm unites with an egg, a fertilized egg forms, which then develops into a baby within the mother’s body. The function of the reproductive system is regulated very precisely by hormones of the endocrine system.

Growth

Growth can be an increase in cell size or an increase in body size that is usually accomplished (Text continues on page 38.)

Currently PET can reveal signs of trou- ble in people with undiagnosed Alzheimer’s disease (AD) because regions of beta-amyloid accumulation (a defining characteristic of AD) show up in brilliant red and yellow when that molecule is tagged. By tagging glucose, we can observe that brain tissue in areas of impairment or areas with Alzheimer’s plaques use far less glucose compared to normal brain tissue as shown in photo (d).

Perhaps the best known of these newer imaging devices is computed tomography (cT), a refined version of X ray that eliminates the confusion resulting from images of overlapping structures. A CT scanner takes “pic- tures” of a thin slice of the body, about as thick as a dime. Different tis- sues absorb the radiation in varying amounts. The device’s computer translates this information into a detailed, cross-sectional picture of the body region scanned; see photo (e). CT scans are at the forefront in evalu- ating most problems that affect the brain and abdomen, and their clarity has all but eliminated exploratory sur- gery. Special ultrafast CT scanners have produced a technique called dynamic spatial reconstruction (Dsr), which provides three- dimensional images of body organs from any angle. It also allows organ movements and changes in internal volumes to be observed at normal speed, in slow motion, and at a spe- cific moment in time. The greatest value of DSR has been to visualize the heart beating and blood flowing through blood vessels. This allows medical personnel to assess heart defects, constricted blood vessels, and the status of coronary bypass grafts.

body is probed with pulses of sound waves, which cause echoes when reflected and scattered by body tis- sues. The echoes are analyzed by computer to construct visual images of body organs, much like sonar is used to map the ocean floor. Because of its safety, ultrasound is the imaging technique of choice for determining fetal age and position and locating the placenta; see photo (c). Because sound waves have very low penetrating power and are rap- idly scattered in air, sonography does not visualize air-filled structures (the lungs) or those surrounded by bone (the brain and spinal cord) well.

Though nuclear medicine, which uses radioisotopes to scan the body, was developed in the 1950s, it was not until the 1970s that a powerful technique for visualizing metabolic activities, PET, was intro- duced. The 1970s also introduced CT, an X-ray–based scanning technique, and MRI scanning techniques, which use powerful magnets.

Positron emission tomography (PET) requires an injection of short- lived radioisotopes that have been tagged to biological molecules (such as glucose) in order to view metabolic processes. The patient is positioned in the PET scanner, and as the radioiso- topes are absorbed by the most active brain cells, high-energy gamma rays are produced. A computer analyzes the gamma emissions and produces a picture of the brain’s biochemical activity in vivid colors. PET’s greatest clinical value has been its ability to provide insights into brain activity in people affected by mental illness, Alzheimer’s disease, and epilepsy.

Imaging procedures are important diagnostic tools that are either mini- mally invasive or not invasive at all. By bombarding the body with different forms of energy, medical imaging techniques can reveal the structure of internal organs, show blood flow in real time, and even determine the density of bone.

Until about 50 years ago, the magical but murky X ray was the only means of peering into a living body without performing surgery. The X-ray film is a shadowy negative image of internal structures produced by directing electromagnetic waves of very short wavelength at the body. X rays are best used to visualize hard, bony structures and locate abnor- mally dense structures (tumors, tuberculosis nodules) in the lungs and breasts.

Two examples of low-dose X-ray procedures are mammography and bone densitometry. A mammogram is used to identify changes in breast tissue, including dense masses or cal- cifications. A mammogram is per- formed by compressing the breast in a special X-ray machine because thinner tissue results in a better image; see photo (a). Bone densitometry detects the amount of calcium and minerals stored in bone and is the major diagnostic test for osteoporosis; see photo (b).

The 1950s saw the birth of ultra- sound imaging (ultrasonogra- phy), which has some distinct advantages over other imaging tech- niques. The equipment employs high-frequency sound waves (ultra- sound) as its energy source. Unlike X rays, ultrasound has no known harmful effects on living tissues. The

Medical Imaging: Illuminating the Body

A CLOSER

LOOK

the powerful magnets of the MRI can pose risks. For example, metal objects, such as implanted pacemak- ers and loose tooth fillings, can be “sucked” through the body. Also, the long-term health risks of expo- sure to strong magnetic fields are not clear.

As you can see, modern medical science enlists remarkable diagnostic tools. CT and PET scans account for about 25 percent of all imaging. Ultra- sonography, because of its safety and low cost, is the most widespread. However, conventional X rays remain the workhorse of diagnostic imaging and still account for more than half of all medical imaging.

column do not impair the view of soft tissues, such as the brain or interver- tebral discs, the cartilage pads between vertebrae. MRI is also partic- ularly good at detecting signs of degenerative disease such as multiple sclerosis plaques, which do not show up well in CT scans but are dazzlingly clear in MRI scans.

A variation of MRI called func- tional magnetic resonance imag- ing (fMrI) allows tracking of blood flow into the brain in real time. Before 1992, PET was the only way to match brain activity to disease. With no need for injections of radio- isotopes, fMRI provides a less invasive alternative. Despite its advantages,

A different technique is magnetic resonance imaging (MrI), which uses magnetic fields up to 60,000 times stronger than Earth’s to pry information from body tissues. The patient lies in a tubelike chamber within a huge magnet. Hydrogen mol- ecules spin like tops in the magnetic field, and their energy is enhanced by the presence of radio waves. When the radio waves are turned off, energy is released and translated by a computer into a visual image (see Figure 1.5, p. 42). MRI is immensely popular because it can do many things a CT scan cannot. Dense struc- tures do not show up in MRI, so bones of the skull and/or vertebral

(d) A PET scan demonstrates glucose metabolism in a normal brain (left). Glucose metabolism is decreased in a brain with mild cognitive impairment (middle), and glucose is metabolized at an even lower level in a brain with Alzheimer’s plaques (right).

CT scan showing brain tumors (indicated by black arrows).

Mammogram showing breast cancer.

(a)

(e)

(c) Sonogram of a fetus.(b) Bone densitometry scan showing osteoporosis in the small yellow box on the neck of the femur.

38 Essentials of Human Anatomy and Physiology

directional terms. To avoid confusion, we always assume that the body is in a standard position called anatomical position. It is important to understand this position because most body ter- minology used in this text refers to this body positioning regardless of the position the body happens to be in. In the anatomical position, the body is erect with the feet parallel and the arms hanging at the sides with the palms facing for- ward. Stand up now, and assume the anatomi- cal position. Notice that it is similar to “standing at attention” but is less comfortable because the palms are held unnaturally forward (with thumbs pointing away from the body) rather than hanging cupped toward the thighs.

Directional Terms Directional terms allow medical personnel and anatomists to explain exactly where one body structure is in relation to another. For example, we can describe the relationship between the ears and the nose informally by saying, “The ears are located on each side of the head to the right and left of the nose.” In anatomical terminology, this condenses to, “The ears are lateral to the nose.” Using anatomical terminology saves words and, once learned, is much clearer. Commonly used directional terms are defined and illustrated in Table 1.1. Although most of these terms are also used in everyday conversation, keep in mind that their anatomical meanings are very precise.

Before continuing, check your understanding of what you have read in the table. Give the rela- tionship between the following body parts using the correct anatomical terms.

The wrist is _____________ to the hand.

The breastbone is _____________ to the spine.

The brain is _____________ to the spinal cord.

The thumb is _____________ to the fingers. (Be careful here. Remember the anatomical position.)

regional Terms There are many visible landmarks on the surface of the body. Once you know their proper anatomi- cal names, you can be specific in referring to dif- ferent regions of the body.

Anterior Body Landmarks

Look at Figure 1.4a to find the following body regions. Once you have identified all the anterior

appropriate atmospheric pressure. At high alti- tudes, where the air is thin and atmospheric pres- sure is lower, gas exchange may be too slow to support cellular metabolism.

The mere presence of these survival factors is not sufficient to maintain life. They must be pres- ent in appropriate amounts as well; excesses and deficits may be equally harmful. For example, the food ingested must be of high quality and in proper amounts; otherwise, nutritional disease, obesity, or starvation is likely.

Did You Get It? 6. In addition to being able to metabolize, grow, digest

food, and excrete wastes, what other functions must an organism perform if it is to survive?

7. Oxygen is a survival need. Why is it so important? In which life function does oxygen participate directly?

For answers, see Appendix A.

The Language of Anatomy ➔ Learning Objectives

□□ Verbally describe or demonstrate the anatomical position.

□□ use proper anatomical terminology to describe body directions, surfaces, and body planes.

□□ Locate the major body cavities, and list the chief organs in each cavity.

Learning about the body is exciting, but it can get harder to maintain our interest when we are faced with all the new terminology of anatomy and physiology. Let’s face it: You can’t just pick up an anatomy and physiology book and read it as if it were a novel. Unfortunately, confusion is inevita- ble without specialized terminology. For example, if you are looking at a ball, “above” always means the area over the top of the ball. Other directional terms can also be used consistently because the ball is a sphere. All sides and surfaces are equal. The human body, of course, has many protrusions and bends. Thus, the question becomes: Above what? To prevent misunderstanding, anatomists use a set of terms that allow body structures to be located and identified clearly with just a few words. We present and explain this language of anatomy next.

Anatomical Position To accurately describe body parts and position, we must have an initial reference point and use

Table 1.1 Orientation and Directional Terms

Term Definition Illustration Example

Superior (cranial or cephalic)

Toward the head end or upper part of a structure or the body; above

The forehead is superior to the nose.

Inferior (caudal)* Away from the head end or toward the lower part of a structure or the body; below

The navel is inferior to the breastbone.

Anterior (ventral)† Toward or at the front of the body; in front of

The breastbone is anterior to the spine.

Posterior (dorsal)† Toward or at the backside of the body; behind

The heart is posterior to the breastbone.

Medial Toward or at the midline of the body; on the inner side of

The heart is medial to the arm.

Lateral Away from the midline of the body; on the outer side of

The arms are lateral to the chest.

Intermediate Between a more medial and a more lateral structure

The collarbone is intermediate between the breastbone and the shoulder.

Proximal Close to the origin of the body part or the point of attachment of a limb to the body trunk

The elbow is proximal to the wrist (meaning that the elbow is closer to the shoulder or attachment point of the arm than the wrist is).

Distal Farther from the origin of a body part or the point of attachment of a limb to the body trunk

The knee is distal to the thigh.

Superficial (external) Toward or at the body surface The skin is superficial to the skeleton.

Deep (internal) Away from the body surface; more internal

The lungs are deep to the rib cage.

*The term caudal, literally “toward the tail,” is synonymous with inferior only to the inferior end of the spine.

†Anterior and ventral are synonymous in humans, but not in four-legged animals. Ventral refers to an animal’s “belly,” making it the inferior surface. Likewise, posterior and dorsal surfaces are the same in humans, but dorsal refers to an animal’s back, making it the superior surface.

40 Essentials of Human Anatomy and Physiology

Study this figure for a moment to answer these two questions. Where would you hurt if you (1) pulled a groin muscle or (2) cracked a bone in your olecranal area?

(1) Your inguinal region. (2) Your posterior elbow region.A:

Cervical

Back (dorsal)

(a) Anterior/Ventral (b) Posterior/Dorsal

Pubic (genital)

Cephalic Frontal Orbital Nasal

Oral Mental

Thoracic Sternal Axillary Pectoral

Buccal

Scapular

Vertebral

Lumbar

Sacral

Gluteal

Abdominal Umbilical Pelvic Inguinal (groin)

Upper limb Acromial

Manus (hand)

Lower limb Coxal (hip)

Tarsal (ankle)

Occipital (back of head)

Deltoid

Olecranal Antecubital Brachial (arm)

Antebrachial (forearm)

Carpal (wrist)

Femoral (thigh)

Digital

Patellar

Crural (leg) Popliteal

Sural (calf) Fibular

Plantar

Calcaneal Digital

Cephalic

Pedal (foot)

KEY: Thorax

Abdomen

Back (Dorsum)

Figure 1.4 The anatomical position and regional terms. This figure shows terms used to designate specific body areas. (a) Anterior view. (b) Posterior view. The heels are raised slightly to show the inferior plantar surface (sole) of the foot, which is actually on the inferior surface of the body.

body landmarks, cover the labels that describe what the structures are. Then go through the list again, pointing out these areas on your own body.

• abdominal (ab-dom′̆ı-nal): anterior body trunk inferior to ribs

• acromial (ah-kro′me-ul): point of shoulder • antebrachial (an″te-bra′ke-al): forearm • antecubital (an″te-ku′bı̆-tal): anterior surface

of elbow

• axillary (ak′sı̆-lar″e): armpit • brachial (bra′ke-al): arm • buccal (buk′al): cheek area • carpal (kar′pal): wrist • cervical (ser′vı̆-kal): neck region • coxal (kok′sal): hip • crural (kroo′ral): anterior leg; the shin. • deltoid (del′toyd): curve of shoulder formed

by large deltoid muscle

• digital (dij′ ı̆-tal): fingers, toes • femoral (fem′or-al): thigh (applies to both

anterior and posterior)

Chapter 1: The Human Body: An Orientation 41

1 • fibular (fib′u-lar): lateral part of leg • frontal (frun′tal): forehead • inguinal (in′gwı̆-nal): area where thigh meets

body trunk; groin

• mental (men′tul): chin • nasal (na′zul): nose area • oral (o′ral): mouth • orbital (or′bı̆-tal): eye area • patellar (pah-tel′er): anterior knee • pectoral (pek′to-ral): relating to, or occurring

in or on, the chest

• pelvic (pel′vik): area overlying the pelvis anteriorly

• pubic (pyu′bik): genital region • sternal (ster′nul): breastbone area • tarsal (tar′sal): ankle region • thoracic (tho-ras′ik): area between the neck

and abdomen, supported by the ribs, sternum and costal cartilages; chest

• umbilical (um-bil′ ı̆-kal): navel

Posterior Body Landmarks

Identify the following body regions in Figure 1.4b, and then locate them on yourself without referring to this text.

• calcaneal (kal-ka′ne-ul): heel of foot • cephalic (seh-fă′lik): head • femoral (fem′or-al): thigh • gluteal (gloo′te-al): buttock • lumbar (lum′bar): area of back between ribs

and hips; the loin

• occipital (ok-sip′ ı̆-tal): posterior surface of head or base of skull

• olecranal (ol-eh-kra′nel): posterior surface of elbow

• popliteal (pop-lit′e-al): posterior knee area • sacral (sa′krul): area between hips at base of

spine

• scapular (skap′u-lar): shoulder blade region • sural (soo′ral): the posterior surface of leg;

the calf

• vertebral (ver′tĕ-bral): area of spinal column The plantar region, or the sole of the foot,

actually on the inferior body surface, is illustrated with the posterior body landmarks (see Figure 1.4b).

Did You Get It? 8. What is the anatomical position, and why is it

important that an anatomy student understand it? 9. The axillary and the acromial areas are both in the

general area of the shoulder. To what specific body area does each of these terms apply?

10. Use anatomical language to describe the location of a cut to the back of your left forearm.

For answers, see Appendix A.

Body Planes and sections When preparing to look at the internal structures of the body, medical students make a section, or cut. When the section is made through the body wall or through an organ, it is made along an imaginary line called a plane. Because the body is three-dimensional, we can refer to three types of planes or sections that lie at right angles to one another (Figure 1.5).

A sagittal (saj′ı̆-tal) section is a cut along the lengthwise, or longitudinal, plane of the body, dividing the body into right and left parts. If the cut is down the median plane of the body and the right and left parts are equal in size, it is called a median (midsagittal) section. All other sagittal sections are parasagittal sections (para = near).

A frontal section is a cut along a lengthwise plane that divides the body (or an organ) into anterior and posterior parts. It is also called a cor- onal (ko-ro9nal, “crown”) section.

A transverse section is a cut along a horizon- tal plane, dividing the body or organ into superior and inferior parts. It is also called a cross section.

Sectioning a body or one of its organs along different planes often results in very different views. For example, a transverse section of the body trunk at the level of the kidneys would show kidney structure in cross section very nicely; a frontal section of the body trunk would show a different view of kidney anatomy; and a midsagit- tal section would miss the kidneys completely. Information on body organ positioning can be gained by taking magnetic resonance imaging (MRI) scans along different body planes (see Figure 1.5). MRI scans are described further in “A Closer Look” (pp. 36–37).

Body cavities Anatomy and physiology textbooks typically describe two sets of internal body cavities, called the dorsal and ventral body cavities, that pro- vide different degrees of protection to the organs

42 Essentials of Human Anatomy and Physiology

Aorta

Spinal cord

Liver Pancreas Vertebral column

Rectum Intestines Subcutaneous fat layer

Spleen

Right lung

Left lungHeart

Liver Stomach Spleen

Figure 1.5 The planes of the body—median, frontal, and transverse—with corresponding MrI scans. Note that the planes are shown on a body in the anatomical position.

Which section type would separate the two eyes?Q:

body cavity as an internal body cavity. However, the idea of two major sets of internal body cavities is a useful learning concept, so we will continue to use it here.

Dorsal Body cavity

The dorsal body cavity has two subdivisions, which are continuous with each other. The cranial

within them (Figure 1.6). Because these cavities differ in their mode of embryological development and in their lining membranes, many anatomy ref- erence books do not identify the dorsal, or neural,

A midsagittal section would separate the two eyes.A:

Chapter 1: The Human Body: An Orientation 43

1 cavity is the space inside the bony skull. The brain is well protected because it occupies the cranial cavity. The spinal cavity extends from the cranial cavity to the end of the spinal cord. The spinal cord, which is a continuation of the brain, is protected by the bony vertebrae, which surround the spinal cavity and form the spine.

Ventral Body cavity

The ventral body cavity is much larger than the dorsal cavity. It contains all the structures within the chest and abdomen, that is, the visceral organs in those regions. Like the dorsal cavity, the ventral body cavity is subdivided. The superior thoracic cavity is separated from the rest of the ventral cavity by a dome-shaped muscle, the diaphragm (di′ah- fram). The organs in the thoracic cavity (lungs, heart, and others) are protected by the rib cage. A central region called the mediastinum (me″de-as-ti′num) separates the lungs into right and left cavities in the thoracic cavity. The mediastinum itself houses the heart, trachea, and several other visceral organs.

The cavity inferior to the diaphragm is the abdominopelvic (ab-dom″ ı̆-no-pel′vik) cavity. Some prefer to subdivide it into a superior abdomi- nal cavity containing the stomach, liver, intestines, and other organs, and an inferior pelvic cavity containing the reproductive organs, bladder, and rectum. However, there is no actual physical struc- ture dividing the abdominopelvic cavity. The pelvic cavity is not immediately inferior to the abdomi- nal cavity, but rather tips away from the abdominal cavity in the posterior direction (see Figure 1.6).

When the body is subjected to physical trauma (as often happens in an automobile accident, for example), the most vulnerable abdominopelvic organs are those within the abdominal cavity. The reason is that the abdominal cavity walls are formed only of trunk muscles and are not reinforced by bone. The pelvic organs receive some protection from the bony pelvis in which they reside.

Because the abdominopelvic cavity is quite large and contains many organs, it helps to divide it up into smaller areas for study. A scheme commonly used by medical personnel divides the abdomino- pelvic cavity into four more or less equal regions called quadrants. The quadrants are named accord- ing to their relative locations with respect to anatomi- cal position—that is, right upper quadrant (RUQ), right lower quadrant (RLQ), left upper quadrant (LUQ), and left lower quadrant (LLQ) (Figure 1.7).

Another system, used mainly by anatomists, divides the abdominopelvic cavity into nine separate regions by four planes (Figure 1.8a). As you locate these regions, notice the organs they contain (Figure 1.8b).

• The umbilical region is the centermost re- gion, deep to and surrounding the umbilicus (navel).

• The epigastric (ep″ı̆-gas′trik) region is located superior to the umbilical region (epi = upon, above; gastric = stomach).

• The hypogastric (pubic) region is inferior to the umbilical region (hypo = below).

• The right iliac (inguinal) region and left iliac (inguinal) region are lateral to the hypogastric region (iliac = superior part of the hip bone).

Cranial cavity

A bd

om in

op el

vi c

ca vi

Diaphragm

Abdominal cavity

Pelvic cavity

Spinal cavity

Thoracic cavity

KEY: Dorsal body cavity Ventral body cavity

Figure 1.6 Body cavities. Notice the angular relationship between the abdominal and pelvic cavities.

44 Essentials of Human Anatomy and Physiology

• The right lumbar region and left lumbar region lie lateral to the umbilical region ( lumbus = loins) and spinal column be- tween the bottom ribs and the hip bones; see Figure 1.4b).

• The right hypochondriac (hi″po-kon′dre-ak) region and left hypochondriac region flank the epigastric region and contain the lower ribs (chondro = cartilage).

Other Body cavities

In addition to the large closed body cavities, there are several smaller body cavities. Most are in the head and open to the body exterior.

• Oral cavity and digestive cavity. The oral cavity, or the mouth, contains the teeth and tongue. This cavity is part of and continuous with the digestive organs, which open to the exterior at the anus.

• Nasal cavity. Located within and posterior to the nose, the nasal cavity is part of the respira- tory system.

Right upper quadrant (RUQ)

Right lower quadrant (RLQ)

Left upper quadrant (LUQ)

Left lower quadrant (LLQ)

Figure 1.7 The four abdominopelvic quadrants. In this scheme used by medical personnel, the abdominopelvic cavity is divided into four quadrants by two planes.

(a) Nine regions delineated by four planes

Liver

Large intestine

Small intestine

Appendix

Diaphragm

Stomach

Large intestine

Urinary bladder

(b) Anterior view of the nine regions showing the superficial organs

Epigastric region

Umbilical region

Right lumbar region

Left lumbar region

Right hypochondriac

region

Left hypochondriac

region

Hypogastric (pubic) region

Right iliac (inguinal)

region

Left iliac (inguinal)

region

Figure 1.8 The nine abdominopelvic regions. The abdominopelvic cavity is divided into nine regions by four planes in this

scheme used mostly by anatomists. In (a), the superior transverse plane is just superior to the ribs; the inferior transverse plane is just

superior to the hip bones; and the parasagittal planes lie just medial to the nipples.

Chapter 1: The Human Body: An Orientation 45

1 • Orbital cavities. The orbital cavities (orbits)

in the skull house the eyes and present them in an anterior position.

• Middle ear cavities. The middle ear cavities carved into the skull lie just medial to the ear- drums. These cavities contain tiny bones that transmit sound vibrations to the hearing recep- tors in the inner ears.

Did You Get It? 11. If you were dissecting a cadaver and wanted to

separate the thoracic cavity from the abdominal cavity, which type of section would you make?

12. Which of the following organs are in the abdominopelvic cavity: spinal cord, small intestine, uterus, heart?

13. Joe went to the emergency room, where he complained of severe pains in the lower right quadrant of his abdomen. What might be his problem?

For answers, see Appendix A.

Homeostasis ➔ Learning Objectives

□□ Define homeostasis, and explain its importance.

□□ Define negative feedback, and describe its role in maintaining homeostasis and normal body function.

When you really think about the fact that your body contains trillions of cells in nearly constant activity, and that remarkably little usually goes wrong with it, you begin to appreciate what a marvelous organism your body really is. The word homeostasis (ho″me- o-sta′sis) describes the body’s ability to maintain relatively stable internal conditions even though the outside world is continuously changing. Although the literal translation of homeostasis is “unchanging” (homeo = the same; stasis = standing still), the term does not really mean an unchanging state. Instead, it indicates a dynamic state of equilibrium, or a bal- ance in which internal conditions change and vary but always within relatively narrow limits.

In general, the body demonstrates homeostasis when its needs are being adequately met and it is functioning smoothly. Virtually every organ system plays a role in maintaining the constancy of the internal environment. Adequate blood levels of vital nutrients must be continuously present, and heart activity and blood pressure must be con- stantly monitored and adjusted so that the blood is propelled with adequate force to reach all body tissues. Additionally, wastes must not be allowed

to accumulate, and body temperature must be pre- cisely controlled.

Communication within the body is essential for homeostasis and is accomplished chiefly by the nervous and endocrine systems, which use electri- cal signals delivered by nerves or bloodborne hor- mones, respectively, as information carriers. The details of how these two regulating systems operate are the subjects of later chapters, but we explain the basic characteristics of the neural and hormonal control systems that promote homeostasis here.

components of Homeostatic control systems Regardless of the factor or event being regulated (this is called the variable), all homeostatic con- trol mechanisms have at least three components: a receptor, control center, and effector (Figure 1.9). The receptor is a type of sensor that monitors and responds to changes in the environment. It responds to such changes, called stimuli, by sending informa- tion (input) to the second component, the control center. Information flows from the receptor to the control center along the afferent pathway. (It may help to remember that information traveling along the afferent pathway approaches the control center.)

The control center determines the level (set point) at which a variable is to be maintained. This component analyzes the information it receives and then determines the appropriate response or course of action.

The third component, the effector, provides the means for the control center’s response (out- put) to the stimulus. Information flows from the control center to the effector along the efferent pathway. (Efferent information exits from the con- trol center.) The results of the response then feed back to influence the stimulus (the original change), either by reducing the amount of change (negative feedback), so that the whole control mechanism is shut off; or by increasing the amount of change (positive feedback), so that the reaction continues at an even faster rate.

Feedback Mechanisms Most homeostatic control mechanisms are neg- ative feedback mechanisms, as indicated in Figure 1.9. In such systems, the net effect of the response to the stimulus is to either shut off the original stimulus or reduce its intensity. A good example of a nonbiological negative feedback

46 Essentials of Human Anatomy and Physiology

ting and the birth of a baby are the most familiar examples of positive feedback mechanisms.

Homeostatic Imbalance 1.1

Homeostasis is so important that most disease can be regarded as being the result of its disturbance, a condition called homeostatic imbalance. As we age, our body organs become less efficient, and our internal conditions become less and less stable. These events place us at an increasing risk for illness and produce the changes we associate with aging.

We provide examples of homeostatic imbalance throughout this text to enhance your understanding of normal physiological mechanisms. __________✚

Did You Get It? 14. When we say that the body demonstrates homeostasis,

do we mean that conditions in the body are unchanging? Explain your answer.

15. When we begin to become dehydrated, we usually get thirsty, which causes us to drink liquids. Is the thirst sensation part of a negative or a positive feedback control system? Defend your choice.

For answers, see Appendix A.

system is a home heating system connected to a thermostat. In this situation, the thermostat con- tains both the receptor and the control center. If the thermostat is set at 20°C (68°F), the heating system (effector) will be triggered ON when the house temperature drops below that setting. As the furnace produces heat, the air is warmed. When the temperature reaches 20°C or slightly higher, the thermostat sends a signal to shut off the furnace. Your body “thermostat” operates in a similar way to regulate body temperature. Other negative feedback mechanisms regulate heart rate, blood pressure, breathing rate, the release of hor- mones, and blood levels of glucose (blood sugar), oxygen, carbon dioxide, and minerals.

Positive feedback mechanisms are rare in the body because they tend to increase the original dis- turbance (stimulus) and to push the variable farther from its original value. Typically these mechanisms control infrequent events that occur explosively and do not require continuous adjustments. Blood clot-

Stimulus produces change in variable.

Receptor detects change.

Input: Information is sent along afferent pathway to control center.

Output: Information is sent along efferent pathway to effector.

Response of effector feeds back to reduce the effect of stimulus and returns variable to homeostatic level.

Receptor Effector

Control Center

3 4

VARIABLE (in homeostasis)

IMBALANCE

Afferent pathway

Efferent pathway

Figure 1.9 The elements of a homeostatic control system. Interaction between the receptor, control center, and effector is essential for normal operation of the system.

If this control system were regulating room temperature, what apparatus would be the effector?Q:

The heat-generating furnace or oil burner.A:

Chapter 1: The Human Body: An Orientation 47

summary

An Overview of Anatomy and Physiology (pp. 27–28)

1. Anatomy is the study of structure. Observation is used to see the sizes, shapes, and relationships of body parts.

2. Physiology is the study of how a structure (which may be a cell, an organ, or an organ system) func- tions or works.

3. Structure determines what functions can occur; therefore, if the structure changes, the function must also change.

Levels of structural Organization (pp. 28–33)

1. There are six levels of structural organization. Atoms (at the chemical level) combine, forming the unit of life, the cell. Cells are grouped into tissues, which in turn are arranged in specific ways to form organs. An organ system is a group of organs that performs a specific function for the body (which no other organ system can do). Together, all of the organ systems form the organism, or living body.

2. Eleven organ systems make up the human body: the integumentary, skeletal, muscular, nervous, en- docrine, cardiovascular, lymphatic, respiratory, di- gestive, urinary, and reproductive systems. (For a description of the organ systems naming the major organs and functions, see pp. 29–33).

Maintaining Life (pp. 33–36)

1. To sustain life, an organism must be able to main- tain its boundaries, move, respond to stimuli, digest nutrients and excrete wastes, carry on metabolism, reproduce itself, and grow.

2. Survival needs include food, oxygen, water, ap- propriate temperature, and normal atmospheric pressure. Extremes of any of these factors can be harmful.

The Language of Anatomy (pp. 36–45)

1. Anatomical terminology is relative and assumes that the body is in the anatomical position (stand- ing erect, with palms facing forward).

2. Directional terms

a. Superior (cranial, cephalic): above something else, toward the head.

b. Inferior (caudal): below something else, toward the tail.

c. Anterior (ventral): toward the front of the body or structure.

d. Posterior (dorsal): toward the rear or back of the body or structure.

e. Medial: toward the midline of the body.

f. Lateral: away from the midline of the body.

g. Proximal: closer to the point of attachment.

h. Distal: farther from the point of attachment.

i. Superficial (external): at or close to the body surface.

j. Deep (internal): below or away from the body surface.

3. Regional terms. Visible landmarks on the body sur- face may be used to specifically refer to a body part or area. (See pp. 38–39 for terms referring to anterior and posterior surface anatomy.)

4. Body planes and sections

a. Sagittal section: separates the body longitudi- nally into right and left parts.

b. Frontal (coronal) section: separates the body on a longitudinal plane into anterior and posterior parts.

c. Transverse (cross) section: separates the body on a horizontal plane into superior and inferior parts.

5. Body cavities

a. Dorsal: well protected by bone; has two subdivi- sions.

(1) Cranial: contains the brain.

(2) Spinal: contains the spinal cord.

b. Ventral: less protected than dorsal cavity; has two subdivisions.

(1) Thoracic: The superior cavity that extends in- feriorly to the diaphragm; contains heart and lungs, which are protected by the rib cage.

(2) Abdominopelvic: The cavity inferior to the dia- phragm that contains the digestive, urinary, and reproductive organs. The abdominal portion is vulnerable because it is protected only by the trunk muscles. The pelvic portion is somewhat protected by the bony pelvis. The abdomino- pelvic cavity is often divided into four quad- rants or nine regions (see Figures 1.8 and 1.9).

c. Smaller body cavities include the oral, nasal, orbital, and middle ear cavities. All are open to the outside of the body except the middle ear cavity.

48 Essentials of Human Anatomy and Physiology

3. Most homeostatic control systems are negative feedback systems, which act to reduce or stop the initial stimulus. Some are positive feedback sys- tems, which act to increase the initial stimulus, as in the case of blood clotting.

Homeostasis (pp. 45–46)

1. Body functions interact to maintain homeostasis, or a relatively stable internal environment within the body. Homeostasis is necessary for survival and good health; its loss results in illness.

2. All homeostatic control mechanisms have three components: (1) a receptor that responds to environmental changes (stimuli) and (2) a control center that assesses those changes and produces a response by activating (3) the effector.

review Questions

Multiple choice

More than one choice may apply.

1. Consider the following levels: (1) chemical; (2) tissue; (3) organ; (4) cellular; (5) organismal; (6) systemic. Which of the following choices has the levels listed in order of increasing complexity?

a. 1, 2, 3, 4, 5, 6

b. 1, 4, 2, 5, 3, 6

c. 3, 1, 2, 4, 6, 5

d. 1, 4, 2, 3, 6, 5

e. 4, 1, 3, 2, 6, 5

2. Which of the following is (are) essential for the maintenance of life?

a. Reproduction

b. Effectors

c. Growth

d. Irritability

e. Excretion

3. Using the terms listed below, fill in the blank with the proper term.

anterior superior medial proximal superficial

posterior inferior lateral distal deep

The heart is located _________ to the diaphragm.

The muscles are _________ to the bone.

The shoulder is _________ to the elbow.

In anatomical position, the thumb is _________ to the index finger.

The vertebral region is _________ to the scapular region.

The gluteal region is located on the _________ surface of the body.

4. Match the proper anatomical term (column B) with the common name (column A) for the body regions listed below.

column A

____ 1. buttocks ____ 2. back ____ 3. heel ____ 4. front of elbow ____ 5. toes ____ 6. groin ____ 7. forehead ____ 8. calf ____ 9. sole of foot

column B

a. inguinal b. frontal c. dorsal d. sural e. gluteal f. antecubital g. plantar h. digital i. calcaneal

5. Anatomical terms that apply to the backside of the body in the anatomical position include

a. ventral and anterior.

b. back and rear.

c. posterior and dorsal.

d. head and lateral.

6. A neurosurgeon orders a spinal tap for a patient. Into what body cavity will the needle be inserted?

a. Ventral

b. Thoracic

c. Dorsal

d. Cranial

e. Pelvic

7. Which of the following body regions are subdivisions of the abdominopelvic cavity?

a. Thoracic

b. Epigastric

c. Hypogastric

d. Right hypochondriac

e. Spinal

8. Which of the following is (are) regulated by a negative feedback mechanism?

a. Blood pressure

b. Blood clotting

c. Levels of blood sugar

d. Release of hormones

e. Heart rate

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 1: The Human Body: An Orientation 49

short Answer Essay

9. Define anatomy and physiology.

10. List the 11 organ systems of the body, briefly describe the function of each, and then name two organs in each system.

11. Many body structures are symmetrical. Are the kidneys symmetrical? What about the stomach?

12. What regions of the body do the following body landmarks refer to—acromial, crural, inguinal, popliteal, and sural?

13. Which of the following organ systems—digestive, respiratory, reproductive, circulatory, urinary, or muscular—are found in both subdivisions of the ventral body cavity? Which are found in the thoracic cavity only? In the abdominopelvic cavity only?

14. Explain the meaning of homeostasis as applied to the living organism.

15. What is the consequence of loss of homeostasis, or homeostatic imbalance?

critical Thinking and clinical Application Questions

16. A diagnostic radiographer told John that she was going to take an image of his calcaneal region. What part of his body was she referring to? Later, a nurse told him that she was going to give him an injection containing pain killers in his gluteal region. Did he take off his shirt or his pants to get the injection? When John next saw a physiothera- pist, the therapist noticed John had tenderness and swelling in the tarsal region. What part of his body was tender and swollen?

17. Jennifer fell off her motorcycle and tore a nerve in her axillary region. She also tore ligaments in her cervical and scapular regions and broke the only bone of her right brachial region. Explain where each of her injuries is located.

18. Students of an anatomy class are preparing to dis- sect a body to reach the heart. In which cavity is the heart located, and how can they gain access to it?

19. Parathyroid hormone (PTH) is secreted in response to a drop in the calcium level in the blood. The secretion of PTH is regulated by a negative feed- back mechanism. What can you expect to happen to the calcium blood level as increased amounts of PTH are secreted, and why?

20. Mr. Harvey, a computer programmer, has been complaining about numbness and pain in his right hand. His physician diagnosed carpal tunnel syn- drome and prescribed a splint. Where will Mr. Harvey apply the splint? Be sure to use correct ana- tomical terminology in your answer.

21. Dan “threw out his back” after lifting heavy furni- ture and complained of pain radiating down the back of his right leg. Use anatomical terminology to describe the body regions where the injury occurred and where Dan experienced pain. Which imaging technique would you prescribe to diag- nose a spinal problem? Explain your choice.

22. Tom likes to discuss medical matters with his doc- tor. Since Tom has fever quite often, he wants to know what might happen if a person’s body tem- perature deviates grossly from the normal. What would be the doctor’s response?

23. How is the concept of homeostasis (or its loss) related to disease and aging? Provide examples to support your reasoning.

Many short courses in anatomy and physi-ology lack the time to consider chemistry as a topic. So why include it here? The answer is simple: Your entire body is made up of chemicals— thousands of them— continuously interacting with one another at an incredible pace.

Chemical reactions underlie all body pro- cesses—movement, digestion, the pumping of your heart, and even your thoughts. In this chapter we present the basics of chemistry and biochemis- try (the chemistry of living material), providing the background you will need to understand body functions.

Concepts of Matter and Energy ➔ Learning Objectives

□□ Differentiate matter from energy.

□□ List four major energy forms, and provide one example of how each is used in the body.

Matter Matter is the “stuff” of the universe. With some exceptions, it can be seen, smelled, and felt. More precisely, matter is anything that occupies space and has mass. Weight is a measure of gravity

Basic Chemistry

WHAT HOW

WHY

Chemistry is the basis for how the body

transforms and uses energy and for how our cells use crucial

molecules such as carbohydrates, lipids, proteins, and nucleic

acids.

Everything that happens in the body, from cells

responding to their surroundings to breaking down the food we eat, requires the movement of chemicals

such as ions, carbohydrates, and lipids as they participate in

chemical reactions.

Chemistry helps us understand the interactions of

different molecules and why some interactions store energy, such as

in fat, and other interactions release energy, such as when

enzymes break down our food.

InstruCtors

New Building Vocabulary Coaching Activities for this chapter are assignable in

Chapter 2: Basic Chemistry 51

2 chemical energy harvested from the foods we eat fuels all body activities.

• Electrical energy results from the movement of charged particles. In your house, electrical energy is the flow of subatomic particles called electrons along the wiring. In your body, an electrical current is generated when charged particles (called ions) move across cell membranes. The nervous system uses electrical currents called nerve impulses to transmit mes- sages from one part of the body to another.

• Mechanical energy is energy directly in- volved in moving matter. When you pedal a bicycle, your legs provide the mechanical energy that turns the wheels. We can take this example one step further back: As the muscles in your legs shorten (contract), they pull on your bones, causing your limbs to move (so that you can pedal the bike).

• Radiant energy travels in waves; that is, it is the energy of the electromagnetic spectrum, which includes X rays, infrared radiation (heat energy), visible light, radio, and ultraviolet (UV) waves. Light energy, which stimulates the retinas of your eyes, is important in vision. UV waves cause sunburn, but they also stimulate our bodies to make vitamin D.

Energy Form Conversions

With a few exceptions, energy is easily converted from one form to another. For example, chemical energy (from gasoline) that powers the motor of a speedboat is converted into the mechanical energy of the whirling propeller that allows the boat to skim across the water. In the body, chemical energy from food is trapped in the bonds of a high-energy chemical called ATP (adenosine triphosphate), and ATP’s energy may ultimately be transformed into the electrical energy of a nerve impulse or the mechanical energy of con- tracting muscles.

Energy conversions are not very efficient, and some of the initial energy supply is always “lost” to the environment as heat (thermal energy). It is not really lost, because energy cannot be created or destroyed, but the part given off as heat is unus- able. You can easily demonstrate this principle by putting your finger close to a lightbulb that has been lit for an hour or so. Notice that some of the electrical energy reaching the bulb is producing

pulling on mass. Chemistry studies the nature of matter—how its building blocks are put together and how they interact.

Matter exists in solid, liquid, and gaseous states, all of which are found in the human body. Solids, such as bones and teeth, have a definite shape and volume. Liquids have a definite volume, but they conform to the shape of their container. Examples of body liquids are blood plasma and the interstitial fluid that bathes all body cells. Gases have neither a definite shape nor a definite volume. The air we breathe is a mixture of gases.

Matter may be changed both physically and chemically. Physical changes do not alter the basic nature of a substance. Examples include changes in state, such as ice melting to water and food being cut into smaller pieces. Chemical changes do alter the composition of the substance—often substantially. Fermenting grapes to make wine and the digestion of food in the body are examples of chemical changes.

Energy In contrast to matter, energy has no mass and does not take up space. It can be measured only by its effects on matter. We commonly define energy as the ability to do work or to put matter into motion. When energy is actually doing work (moving objects), we refer to it as kinetic (kı̆- neh′tik) energy. Kinetic energy is displayed in the constant movement of the tiniest particles of mat- ter (atoms) as well as in larger objects, such as a bouncing ball. When energy is inactive or stored (as in the batteries of an unused toy), we call it potential energy. All forms of energy exhibit both kinetic and potential work capacities.

All living things are built of matter, and to grow and function they require a continuous sup- ply of energy. Thus, matter is the substance, and energy is the mover of the substance. Because this is so, let’s take a brief detour to introduce the forms of energy the body uses as it does its work.

Forms of Energy

• Chemical energy is stored in the bonds of chemical substances. When the bonds are bro- ken, the (potential) stored energy is unleashed and becomes kinetic energy (energy in action). For example, when gasoline molecules are broken apart in your automobile engine, the energy released powers your car. Similarly, the

52 Essentials of Human Anatomy and Physiology

Each element is composed of very similar par- ticles, or building blocks, called atoms. Because all elements are unique, the atoms of each ele- ment differ from those of all other elements. We designate each element by a one- or two-letter chemical shorthand called an atomic symbol. In most cases, the atomic symbol is simply the first or first two letters of the element’s name. For exam- ple, C stands for carbon, O for oxygen, and Ca for calcium. In a few cases, the atomic symbol is taken from the Latin name for the element. For instance, Na (from the Latin word natrium) indi- cates sodium, and K (from kalium) potassium.

Atomic structure ➔ Learning Objective

□□ List the subatomic particles, and describe their relative masses, charges, and positions in the atom.

the Basic Atomic subparticles

The word atom comes from the Greek word meaning “incapable of being divided,” and histori- cally this idea of an atom was accepted as a scien- tific truth. According to this notion, you could the- oretically divide a pure element, such as a block of gold, into smaller and smaller particles until you got down to the individual atoms, at which point you could subdivide no further. We now know that atoms, although indescribably small, are clus- ters of even smaller components called subatomic particles, which include protons, neutrons, and electrons. Even so, the old idea of atomic indivisi- bility is still very useful because an atom loses the unique properties of its element when it is split into its component particles.

An atom’s subatomic particles differ in their mass, electrical charge, and position within the atom (table 2.2, p. 54). Protons (p∙) have a posi- tive charge, whereas neutrons (n0) are uncharged, or neutral. Protons and neutrons are heavy parti- cles and have approximately the same mass (1 atomic mass unit, or amu). The tiny electrons (e−) bear a negative charge equal in strength to the positive charge of the protons, but their mass is so small that it is usually designated as 0 amu.

The electrical charge of a particle is a measure of its ability to attract or repel other charged parti- cles. Particles with the same type of charge (+ to + or − to −) repel each other, but particles with opposite charges (+ to −) attract each other.

heat instead of light. Likewise, all energy conver- sions in the body liberate heat. This heat makes us warm-blooded animals and contributes to our rel- atively high body temperature, which has an important influence on body function. For exam- ple, when matter is heated, its particles begin to move more quickly; that is, their kinetic energy (energy of motion) increases. This is important to the chemical reactions that occur in the body because, up to a point, the higher the temperature, the faster those reactions occur.

Did You Get It? 1. Matter and energy—how are they interrelated? 2. What form of energy is used to transmit messages

from one part of the body to another? What form of energy fuels cellular processes?

3. What type of energy is available when we are still? When we are exercising?

4. What does it mean when we say that some energy is “lost” every time energy changes from one form to another in the body?

For answers, see Appendix A.

Composition of Matter Elements and Atoms ➔ Learning Objectives

□□ Define element, and list the four elements that form the bulk of body matter.

□□ Explain how elements and atoms are related.

All matter is composed of a limited number of sub- stances called elements, unique substances that cannot be broken down into simpler substances by ordinary chemical methods. Examples of elements include many commonly known substances, such as oxygen, silver, gold, copper, and iron.

So far, 118 elements have been identified with certainty. Ninety-two of these occur in nature; the rest are made artificially in accelerator devices. A complete listing of the elements appears in the periodic table, an odd-shaped checkerboard (see Appendix C) that appears in chemistry classrooms the world over. Four of these elements—carbon, oxygen, hydrogen, and nitrogen—make up about 96 percent of the weight of the human body. Besides these four elements, several others are present in small or trace amounts. The most abundant elements found in the body and their major roles are listed in table 2.1.

Chapter 2: Basic Chemistry 53

Element Atomic symbol

Percentage of body mass role

Major (96.1%)

Oxygen O 65.0 A major component of both organic and inorganic molecules; as a gas, essential to the oxidation of glucose and other food fuels, during which cellular energy (ATP) is produced.

Carbon C 18.5 The primary element in all organic molecules, including carbohydrates, lipids, proteins, and nucleic acids.

Hydrogen H 9.5 A component of most organic molecules; as an ion (a charged atom), it influences the pH of body fluids.

Nitrogen N 3.2 A component of proteins and nucleic acids (genetic material).

Lesser (3.9%)

Calcium Ca 1.5 Found as a salt in bones and teeth; in ionic form, required for muscle contraction, neural transmission, and blood clotting.

Phosphorus P 1.0 Present as a salt, in combination with calcium, in bones and teeth; also present in nucleic acids and many proteins; forms part of the high- energy compound ATP.

Potassium K 0.4 In its ionic form, the major intracellular cation; necessary for the conduction of nerve impulses and for muscle contraction.

Sulfur S 0.3 A component of proteins (particularly contractile proteins of muscle).

Sodium Na 0.2 As an ion, the major extracellular cation; important for water balance, conduction of nerve impulses, and muscle contraction.

Chlorine Cl 0.2 In ionic (chloride) form, the most abundant extracellular anion.

Magnesium Mg 0.1 Present in bone; also an important cofactor for enzyme activity in a number of metabolic reactions.

Iodine I 0.1 Needed to make functional thyroid hormones.

Iron Fe 0.1 A component of the functional hemoglobin molecule (which transports oxygen within red blood cells) and some enzymes.

Trace (less than 0.01%)*

Chromium (Cr), Cobalt (Co), Copper (Cu), Fluorine (F), Manganese (Mn), Molybdenum (Mo), Selenium (Se), Silicon (Si), Tin (Sn), Vanadium (V), Zinc (Zn)

*Referred to as the trace elements because they are required in very small amounts; many are found as part of enzymes or are required for enzyme activation.

Table 2.1 Common Elements Making up the Human Body

Neutral particles are neither attracted to nor repelled by charged particles.

Because all atoms are electrically neutral, the number of protons an atom has must be balanced by its number of electrons (the + and − charges will then cancel the effect of each other). Thus,

helium has two protons and two electrons, and iron has 26 protons and 26 electrons. For any atom, the number of protons and electrons is always equal. Atoms that have gained or lost elec- trons are called ions.

54 Essentials of Human Anatomy and Physiology

likely to be found. The orbital model depicts the general location of electrons outside the nucleus as a haze of negative charge referred to as the electron cloud (Figure 2.1b). Regions where electrons are most likely to be found are shown by denser shad- ing rather than by orbit lines.

Notice that in both models, the electrons have the run of nearly the entire volume of the atom. Electrons also determine an atom’s chemical behavior (that is, its ability to bond with other atoms). Though now considered outdated, the planetary model is simple and easy to understand and use. Most of the descriptions of atomic struc- ture in this text use that model.

Hydrogen is the simplest atom, with just one proton and one electron. You can visualize the hydrogen atom by imagining it as a sphere with its diameter equal to the length of a football field. The nucleus could then be represented by a lead ball the size of a gumdrop in the exact center of the sphere and the lone electron pictured as a fly buzzing about unpredictably within the sphere. This mental picture should remind you that most of the volume of an atom is empty space, and most of the mass is in the central nucleus.

Did You Get It? 5. Which four elements make up the bulk of living

matter? 6. How is an atom related to an element?

For answers, see Appendix A.

Identifying Elements ➔ Learning Objective

□□ Define radioisotope, and describe briefly how radioisotopes are used in diagnosing and treating disease.

All protons are alike, regardless of the atom being considered. The same is true of all neutrons and all electrons. So what determines the unique properties

Planetary and orbital Models of an Atom

The planetary model of an atom portrays the atom as a miniature solar system (Figure 2.1a) in which the protons and neutrons are clustered at the center of the atom in the atomic nucleus. Because the nucleus contains all the heavy particles, it is fan- tastically dense and positively charged. The tiny electrons orbit around the nucleus in fixed, gener- ally circular orbits, like planets around the sun. But we can never determine the exact location of elec- trons at a particular time because they jump around following unknown paths. So, instead of speaking of specific orbits, chemists talk about orbitals— regions around the nucleus in which electrons are

Particle Position in atom Mass (amu) Charge

Proton (p+) Nucleus 1 +

Neutron (n0) Nucleus 1 0

Electron (e−) Orbits around the nucleus 1/2000* −

*The mass of an electron is so small, that we will ignore it and assume a mass of 0 amu.

Table 2.2 subatomic Particles

Nucleus

(a) Planetary model (b) Orbital model

Helium atom

2 protons (p+) 2 neutrons (n0) 2 electrons (e−)

Helium atom

2 protons (p+) 2 neutrons (n0) 2 electrons (e−)

Nucleus

KEY: Proton

Neutron

Electron

Electron cloud

Figure 2.1 the structure of an atom. The dense central nucleus contains the protons and neutrons. (a) In the planetary model of an atom, the electrons move around the nucleus in fixed orbits. (b) In the orbital model, electrons are shown as a cloud of negative charge.

Chapter 2: Basic Chemistry 55

(Remember, the mass of an electron is so small that we ignore it.) Because hydrogen has one pro- ton and no neutrons in its nucleus, its atomic number and mass number are the same: 1. Helium, with two protons and two neutrons, has a mass number of 4. The mass number is written as a superscript to the left of the atomic symbol (see the examples in Figure 2.3 on p. 56).

Atomic Weight and Isotopes

At first glance, it seems that the atomic weight of an atom should be equal to its atomic mass. This would be so if there were only one type of atom representing each element. However, the atoms of almost all elements exhibit two or more struc- tural variations; these varieties are called isotopes (i′ s ı̆-tōps). Isotopes have the same number of protons and electrons but vary in the number of neutrons they contain. Thus, the isotopes of an element have the same atomic number but differ- ent atomic masses. Because all of an element’s isotopes have the same number of electrons (and electrons determine bonding properties), their chemical properties are exactly the same. As a general rule, the atomic weight of any element is approximately equal to the mass number of its most abundant isotope. For example, as we said before, hydrogen has an atomic number of 1, but it also has isotopes with atomic masses of 1, 2 (called deuterium), and 3 (called tritium) (see Figure 2.3). Its atomic weight is 1.008, which reveals that its lightest isotope is present in much greater amounts in our world than its 2H or 3H forms. (The atomic numbers, mass numbers, and atomic weights for elements commonly found in the body are provided in table 2.3).

of each element? The answer is that atoms of differ- ent elements are composed of different numbers of protons, neutrons, and electrons.

The simplest and smallest atom, hydrogen, has one proton, one electron, and no neutrons (Figure 2.2). Next is the helium atom, with two protons, two neutrons, and two orbiting electrons. Lithium follows with three protons, four neutrons, and three electrons. If we continue this step-by- step listing of subatomic particles, we could describe all known atoms by adding one proton and one electron at each step. The number of neu- trons is not as easy to pin down, but light atoms tend to have equal numbers of protons and neu- trons, whereas in larger atoms neutrons outnum- ber protons. However, all we really need to know to identify a particular element is its atomic num- ber, atomic mass number, and atomic weight. Taken together, these indicators provide a fairly complete picture of each element.

Atomic number

Each element is given a number, called its atomic number, that is equal to the number of protons its atoms contain. Atoms of each element contain a different number of protons from the atoms of any other element; hence, its atomic number is unique. Because the number of protons is always equal to the number of electrons, the atomic number indi- rectly also tells us the number of electrons that atom contains.

Atomic Mass number

The atomic mass number (or just mass num- ber) of any atom is the sum of the masses of all the protons and neutrons contained in its nucleus.

(b) Helium (He) (2p+; 2n0; 2e−)

(a) Hydrogen (H) (1p+; 0n0; 1e−)

KEY: Proton

Neutron

Electron

Figure 2.2 Atomic structure of the three smallest atoms.

56 Essentials of Human Anatomy and Physiology

decay, which is called radioactivity, can be com- pared to a tiny sustained explosion. All types of radioactive decay involve the ejection of particles (alpha or beta particles) or electromagnetic energy (gamma rays) from the atom’s nucleus and are damaging to living cells. Emission of alpha parti- cles has the least penetrating power; gamma

The heavier isotopes of certain atoms are unstable and tend to release energy in order to become more stable; such isotopes are called radioisotopes. Why this process occurs is very complex, but apparently the “glue” that holds the atomic nuclei together is weaker in the heavier isotopes. This process of spontaneous atomic

Table 2.3 Atomic structures of the Most Abundant Elements in the Body

Element symbol Atomic number

(# of p) Mass number (# of p + n)

Atomic weight

Electrons in valence shell

Calcium Ca 20 40 40.078 2

Carbon C 6 12 12.011 4

Chlorine Cl 17 35 35.453 7

Hydrogen H 1 1 1.008 1

Iodine I 53 127 126.905 7

Iron Fe 26 56 55.847 2

Magnesium Mg 12 24 24.305 2

Nitrogen N 7 14 14.007 5

Oxygen O 8 16 15.999 6

Phosphorus P 15 31 30.974 5

Sodium Na 11 23 22.989 1

Sulfur S 16 32 32.064 6

Tritium (3H) (1p+; 2n0; 1e−)

Deuterium (2H) (1p+; 1n0; 1e−)

Hydrogen (1H) (1p+; 0n0; 1e−)

KEY: Proton

Neutron

Electron

Figure 2.3 Isotopes of hydrogen. Isotopes differ in their numbers of neutrons.

Which of these isotopes is the heaviest?Q: Tritium.A:

Chapter 2: Basic Chemistry 57

For example, when two hydrogen atoms bond, a molecule of hydrogen gas is formed:

H (atom) + H (atom) S H2 (molecule)*

In this example of a chemical reaction, the reactants (the atoms taking part in the reaction) are indicated by their atomic symbols, and the product (the molecule formed) is indicated by a molecular formula that shows its atomic makeup. The chemical reaction is shown as a chemical equation.

When two or more different atoms bind together to form a molecule, the molecule is more specifically referred to as a molecule of a com- pound. For example, four hydrogen atoms and one carbon atom can interact chemically to form methane:

4H + C = CH4 (methane)

Thus, a molecule of methane is a compound, but a molecule of hydrogen gas is not—it is instead called molecular hydrogen.

Compounds always have properties quite dif- ferent from those of the atoms making them up, and it would be next to impossible to determine the atoms making up a compound without analyz- ing it chemically. Sodium chloride is an excellent example of the difference in properties between a compound and its constituent atoms (Figure 2.4). Sodium is a silvery white metal, and chlorine in its

radiation has the most. Contrary to what some believe, ionizing radiation does not damage the atoms in its path directly. Instead, it sends elec- trons flying, like a bowling ball through pins, all along its path. It is these electrons that do the damage.

Radioisotopes are used in minute amounts to tag biological molecules so that they can be fol- lowed, or traced, through the body and are valu- able tools for medical diagnosis and treatment. For example, PET scans, which use radioisotopes, are discussed in “A Closer Look” (pp. 36–37). A radio- isotope of iodine can be used to scan the thyroid gland of a patient suspected of having a thyroid tumor. Additionally, radium, cobalt, and certain other radioisotopes are used to destroy localized cancers.

Did You Get It? 7. An atom has five neutrons, four protons, and four

electrons. What is its atomic number? What is its atomic mass number?

8. What name is given to an unstable atom that has either more or fewer neutrons than its typical number?

For answers, see Appendix A.

Molecules and Compounds ➔ Learning Objective

□□ Define molecule, and explain how molecules are related to compounds.

When two or more atoms combine chemically, molecules are formed. If two or more atoms of the same element bond, or become chemically linked together, a molecule of that element is produced.

*Notice that when the number of atoms is written as a subscript, it indicates that the atoms are joined by a chemical bond. Thus, 2H represents two separate atoms, but H2 indicates that the two hydrogen atoms are bound together to form a molecule. The atomic symbol by itself represents one atom.

Sodium (silvery metal) Chlorine (poisonous gas) Sodium chloride (table salt)

Figure 2.4 Properties of a compound differ from those of its atoms.

58 Essentials of Human Anatomy and Physiology

strongly attracted to its positive charge, and those farther away are less securely held. As a result, the more distant electrons are likely to interact with other atoms. Put more simply, the electron shell furthest from the nucleus is also the first part of the atom any other atom will come into contact with prior to reacting or bonding.

There is an upper limit to the number of elec- trons that each electron shell can hold. Shell 1, closest to the nucleus, is small and can accommo- date only 2 electrons. Shell 2 holds a maximum of 8. Shell 3 can accommodate up to 18 electrons. Subsequent shells hold larger and larger numbers of electrons. In most (but not all) cases, the shells tend to be filled consecutively.

The only electrons that are important to bond- ing behavior are those in the atom’s outermost shell. This shell is called the valence shell, and its valence electrons determine the chemical behavior of the atom. As a general rule, the electrons of inner shells do not take part in bonding.

The key to chemical reactivity is referred to as the rule of eights: In the absence of a full valence shell, atoms interact in such a way that they will have eight electrons in their valence shell. The first electron shell represents an exception to this rule, because it is “full” when it has two electrons. When the valence shell of an atom contains eight elec- trons, the atom is completely stable and is chemi- cally inactive (inert). When the valence shell contains fewer than eight electrons, an atom will tend to gain, lose, or share electrons with other atoms to reach a stable state. When any of these events occurs, chemical bonds are formed. (Examples of chemically inert and reactive elements are shown in Figure 2.5). As you might guess, atoms must approach each other very closely for their electrons to interact in a chemical bond—in fact, their outermost electron shells must overlap.

types of Chemical Bonds

Ionic Bonds Ionic (i-on′ik) bonds form when electrons are completely transferred from one atom to another. Atoms are electrically neutral, but when they gain or lose electrons during bonding, their positive and negative charges are no longer balanced, and charged particles, called ions, result. When an atom gains an electron, it acquires a net negative charge because it now has more electrons than protons. Negatively charged ions are more specifically called anions, and they are

molecular state is a poisonous green gas used to make bleach. However, sodium chloride is table salt, a white crystalline solid that we sprinkle on our food. Notice that just as an atom is the small- est particle of an element that still retains that ele- ment’s properties, a molecule is the smallest particle of a compound that still retains the prop- erties of that compound. If you break the bonds between the atoms of the compound, properties of the atoms, rather than those of the compound, will be exhibited.

Did You Get It? 9. What is the meaning of the term molecule?

10. How does a molecular substance differ from a molecule of a compound?

For answers, see Appendix A.

Chemical Bonds and Chemical reactions ➔ Learning Objectives

□□ recognize that chemical reactions involve the interaction of electrons to make and break chemical bonds.

□□ Differentiate between ionic, polar covalent, and nonpolar covalent bonds, and describe the importance of hydrogen bonds.

□□ Contrast synthesis, decomposition, and exchange reactions.

Chemical reactions occur whenever atoms com- bine with or dissociate from other atoms. When atoms unite chemically, chemical bonds are formed.

Bond Formation A chemical bond is not an actual physical struc- ture, like a pair of handcuffs linking two people together. Instead, it is an energy relationship that involves interactions between the electrons of the reacting atoms. Let’s consider the role electrons play in forming bonds.

role of Electrons

The orbits, or generally fixed regions of space that electrons occupy around the nucleus (see Figure 2.2), are called electron shells, or energy levels. The maximum number of electron shells in any atom known so far is seven, and these are numbered 1 to 7 from the nucleus outward. The electrons closest to the nucleus are those most

Chapter 2: Basic Chemistry 59

2 a plus [+] sign). Both anions and cations result when an ionic bond is formed. Because opposite charges attract, the newly created ions tend to stay close together.

The formation of sodium chloride (NaCl), com- mon table salt, provides a good example of ionic bonding. Sodium’s valence shell contains only one electron and so is incomplete (Figure 2.6, p. 60). However, if this single electron is “lost” to another atom, shell 2, which contains eight electrons, becomes the valence shell; thus sodium becomes a cation (Na+) and achieves stability. Chlorine needs only one electron to fill its valence shell, and it is much easier to gain one electron (forming Cl−) than it is to “give away” seven. Thus, the ideal situation is for sodium to donate its valence elec- tron to chlorine, which is exactly what happens. Sodium chloride and most other compounds formed by ionic bonding fall into the general cat- egory of inorganic chemicals called salts.

Covalent Bonds Electrons do not have to be completely lost or gained for atoms to become stable. Instead, they can be shared in such a way that each atom is able to fill its valence shell at least part of the time.

Molecules in which atoms share valence elec- trons are called covalent molecules, and their bonds are covalent bonds (co = with; valent = having power). For example, hydrogen, with its single electron, can become stable if it fills its valence shell (energy level 1) by sharing a pair of electrons—its own and one from another atom. A hydrogen atom can share an electron pair with another hydrogen atom to form a molecule of hydrogen gas (Figure 2.7a). The shared electron pair orbits the whole molecule and satisfies the stability needs of both hydrogen atoms. Likewise, two oxygen atoms, each with six valence elec- trons, can share two pairs of electrons (form dou- ble bonds) with each other (Figure 2.7b) to form a molecule of oxygen gas (O2).

A hydrogen atom may also share its electron with an atom of a different element. Carbon has four valence electrons but needs eight to achieve stability. When methane gas (CH4) is formed, car- bon shares four electrons with four hydrogen atoms (each bond includes one electron from each atom to form a pair of electrons; Figure 2.7c). Because the shared electrons orbit and “belong to”

indicated by a minus sign after the atomic symbol, such as Cl− for the chloride ion. When atoms lose an electron, they become positively charged ions, cations, because they now possess more protons than electrons. Cations are represented by their atomic symbol with a plus sign, such as H+ for the hydrogen ion (it may help you to remember that a cation is positively charged by thinking of its “t” as

Helium (He) (2p+; 2n0; 2e−)

Neon (Ne) (10p+; 10n0; 10e−)

2e 2e 8e

2e 4e

2e 8e

(b) Chemically reactive elements

Outermost energy level (valence shell) incomplete

Hydrogen (H) (1p+; 0n0; 1e−)

Carbon (C) (6p+; 6n0; 6e−)

Oxygen (O) (8p+; 8n0; 8e−) Sodium (Na)

(11p+; 12n0; 11e−)

2e 6e

(a) Chemically inert elements

Outermost energy level (valence shell) complete

Figure 2.5 Chemically inert and reactive elements. To simplify the diagrams, each atomic nucleus is shown as a circle with the atom’s symbol in it; protons and neutrons are not shown.

60 Essentials of Human Anatomy and Physiology

oxygen has the stronger electron-attracting ability. But in this case, the molecule formed is V-shaped (H≈ √H). The two hydrogen atoms are located at O one end of the molecule, and the oxygen atom is at the other (Figure 2.8b); consequently, the elec- tron pairs are not shared equally. This arrange- ment allows them to spend more time in the vicinity of the oxygen atom. Because electrons are negatively charged, that end of the molecule becomes slightly more negative (indicated by δ−, the Greek letter delta with a minus sign) and the hydrogen end becomes slightly more positive (indicated by δ+). In other words, a polar mole- cule, a molecule with two charged poles, is formed.

Polar molecules orient themselves toward other polar molecules or charged particles (ions, proteins, and others), and they play an important role in chemical reactions that occur in body cells. Because body tissues are 60 to 80 percent water, the fact that water is a polar molecule is particu- larly significant.

Hydrogen Bonds Hydrogen bonds are extremely weak bonds formed when a hydrogen atom bound to one “electron-hungry” nitrogen or oxygen atom is attracted by another such atom, and the hydrogen atom forms a “bridge” between them. Electrons are not involved in hydrogen bonds, as they are in ionic and covalent bonds. Hydrogen bonding is common between water

the whole molecule, each atom has a full valence shell enough of the time to satisfy its stability needs.

In the covalent molecules described thus far, electrons have been shared equally between the atoms of the molecule. Such molecules are called nonpolar covalent molecules. However, electrons are not in all cases shared equally. When covalent bonds are made, the molecule formed always has a definite three-dimensional shape. A molecule’s shape plays a major role in determining just what other molecules (or atoms) it can interact with; the shape may also result in unequal electron-pair sharing. The following two examples illustrate this principle.

Carbon dioxide (CO2) is formed when a car- bon atom shares its four valence electrons with two oxygen atoms. Oxygen is a very “electron- hungry” atom, so it attracts the shared electrons much more strongly than does carbon. However, because the carbon dioxide molecule is linear (O�C�O), the electron-pulling power of one oxygen atom is offset by that of the other, like a tug-of-war at a standoff between equally strong teams (Figure 2.8a, p. 62). As a result, the electron pairs are shared equally and orbit the entire mole- cule, and carbon dioxide is a nonpolar molecule.

A water molecule (H2O) is formed when two hydrogen atoms bind covalently to a single oxy- gen atom. Each hydrogen atom shares an elec- tron pair with the oxygen atom, and again the

ClNa

Sodium atom (Na) (11p+; 12n0; 11e−)

Chlorine atom (Cl) (17p+; 18n0; 17e−)

ClNa

Sodium ion (Na+) Chloride ion (Cl−)

Sodium chloride (NaCl)

+ –

Figure 2.6 Formation of an ionic bond. Both sodium and chlorine atoms are chemically reactive because their valence shells are

incompletely filled. Sodium gains stability by losing one electron, whereas chlorine becomes stable by gaining one electron. After electron

transfer, sodium becomes a sodium ion (Na+), and chlorine becomes a chloride ion (Cl−). The oppositely charged ions attract each other.

Chapter 2: Basic Chemistry 61

Oxygen atom

Hydrogen atom

Molecule of hydrogen gas (H2)

Oxygen atom Molecule of oxygen gas (O2)

O O

H H

(b) Formation of a double covalent bond

(a) Formation of a single covalent bond

H H H H

Hydrogen atoms Carbon atom Molecule of methane gas (CH4)

or C

H H

O O O O

C C

Resulting moleculesReacting atoms

Figure 2.7 Formation of covalent bonds. (a) Formation of a single covalent bond between two hydrogen atoms forms a molecule of hydrogen gas. (b) Formation of a molecule of oxygen gas. Each

oxygen atom shares two electron pairs with its partner; thus, a double covalent bond is formed. (c) Formation of a molecule of methane. A carbon atom shares four electron pairs with four hydrogen

atoms. In the structural formulas of the molecules shown at the far right, each pair of shared electrons is indicated by a single line connecting the sharing atoms.

62 Essentials of Human Anatomy and Physiology

This tension allows some insects, such as water striders, to walk on water as long as they tread lightly (Figure 2.9b).

Hydrogen bonds are also important intramo- lecular bonds; that is, they help to bind different parts of the same molecule together into a special three-dimensional shape. These rather fragile bonds are very important in helping to maintain the struc- ture of both protein molecules, which are essential functional molecules and body-building materials, and DNA, the genetic information molecule.

Patterns of Chemical reactions Chemical reactions involve the making or breaking of bonds between atoms. The total number of atoms remains the same, but the atoms appear in new combinations. Most chemical reactions have one of the three recognizable patterns we describe next.

synthesis reactions

Synthesis reactions occur when two or more atoms or molecules combine to form a larger,

molecules (Figure 2.9a) and is reflected in water’s surface tension. The surface tension of water causes it to “ball up,” or form spheres at the surface.

O OC

H H

δ−

δ+ δ+

(a) Carbon dioxide (CO2)

(b) Water (H2O)

Figure 2.8 Molecular models illustrating the three-dimensional structure of carbon dioxide and water molecules.

Which molecule—(a) or (b)—is a polar molecule?Q:

Water is a polar molecule.A:

δ+

δ−

δ− δ− δ−

δ−

Hydrogen bonds

(a) (b)

Figure 2.9 Hydrogen bonding between polar water molecules. (a) The slightly positive ends of the water molecules (indicated by δ+)

become aligned with the slightly negative ends (indicated by δ−) of other water molecules. (b) Water’s high surface tension, a result of the

combined strength of its hydrogen bonds, allows a water strider to walk on a pond without breaking the surface.

Chapter 2: Basic Chemistry 63

Essentially, decomposition reactions are syn- thesis reactions in reverse. Bonds are always broken, and the products of these reactions are smaller and simpler than the original molecules. As bonds are broken, chemical energy is released.

Decomposition reactions underlie all catabolic (destructive) processes that occur in body cells; that is, they are reactions that break down mole- cules. Examples include the digestion of foods into their building blocks and the breakdown of glycogen (a large carbohydrate molecule stored in the liver) to release glucose (Figure 2.10b) when the blood sugar level starts to decline.

Exchange reactions

Exchange reactions involve simultaneous syn- thesis and decomposition reactions; in other words, bonds are both made and broken. During exchange reactions, a switch is made between molecule parts (changing partners, so to speak),

more complex molecule, which can be simply represented as

A + B S AB

Synthesis reactions always involve bond for- mation. Because energy must be absorbed to make bonds, synthesis reactions are energy-storing reactions.

Synthesis reactions underlie all anabolic (build- ing) activities that occur in body cells. They are particularly important for growth and for repair of worn-out or damaged tissues. The formation of a protein molecule by the joining of amino acids (protein building blocks) into long chains is a syn- thesis reaction (Figure 2.10a).

Decomposition reactions

Decomposition reactions occur when a mole- cule is broken down into smaller molecules, atoms, or ions and can be indicated by

AB S A + B

Example Amino acids are joined together to form a protein molecule.

Example Glycogen is broken down to release glucose units.

Example ATP transfers its terminal phosphate group to glucose to form glucose- phosphate.

Bonds are both made and broken.

(b) Decomposition reactions

Bonds are broken in larger molecules, resulting in smaller, less complex

molecules.

(a) Synthesis reactions

Smaller particles are bonded together to form

larger, more complex molecules.

Amino acid molecules

Protein molecule

Glucose molecules

Glycogen

Glucose Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

P P P

Glucose- phosphate

P P+

O O

Figure 2.10 Patterns of chemical reactions.

64 Essentials of Human Anatomy and Physiology

must overlap. In fact, for them to get close enough for this to happen, the particles must collide force- fully. Remember also that atoms move constantly because of their kinetic energy—this is what drives collisions between particles. Several factors, including temperature, concentration of the parti- cles, and size of the particles, influence the kinetic energy and, hence, the speed of the particles and the force of collisions (table 2.4).

Did You Get It? 11. How do ionic bonds differ from covalent bonds? 12. What kind of bond forms between water molecules? 13. Which reaction type (see Figure 2.10) occurs when

fats are digested in your small intestine? 14. How can you indicate that a chemical reaction is

reversible?

For answers, see Appendix A.

Biochemistry: the Chemical Composition of Living Matter ➔ Learning Objective

□□ Distinguish organic from inorganic compounds.

All chemicals found in the body fall into one of two major classes of molecules: either inorganic or organic compounds. The class of the compound is determined solely by the presence or absence of carbon. Except for a few so far unexplainable exceptions (such as carbon dioxide gas [CO2] and carbon monoxide [CO]), inorganic compounds lack carbon and tend to be small, simple mole- cules. Examples of inorganic compounds found in the body are water, salts, and many (but not all) acids and bases. Organic compounds contain

and different molecules are made. Thus, an exchange reaction can be generally indicated as

AB + C S AC + B and AB + CD S AD + CB

An exchange reaction occurs, for example, when ATP reacts with glucose and transfers its end phosphate group (its full name is adenosine tri- phosphate; tri- = three) to glucose, forming glucose-phosphate (Figure 2.10c). At the same time, the ATP becomes ADP, adenosine diphos- phate (di- = two). This important reaction, which occurs whenever glucose enters a body cell, traps glucose inside the cell.

Most chemical reactions are reversible. If chemical bonds can be made, they can be broken, and vice versa. Reversibility is indicated by a dou- ble arrow in a chemical equation. When the arrows differ in length, the longer arrow indicates the more rapid reaction or the major direction in which the reaction is proceeding. For example, in the reaction

A + B ∆ AB

the reaction going to the right is occurring more rapidly, so over time AB will accumulate while A and B will decrease in amount. If the arrows are of equal length, the reaction is at chemical equilib- rium. Thus, in

A + B ∆ AB

for each molecule of AB made, a molecule of AB is breaking down to release A and B.

Factors Influencing the rate of Chemical reactions

As mentioned earlier, for the atoms in molecules to react chemically, their outermost electron shells

Factor Mechanism to increase the number of collisions

c temperature c the kinetic energy of the molecules, which in turn move more rapidly and collide more forcefully.

c concentration of reacting particles c the number of collisions because of increased numbers of reacting particles.

T particle size Smaller particles have more kinetic energy and move faster than larger ones, hence they take part in more collisions.

Presence of catalysts T the amount of energy the molecules need to interact by holding the reactants in the proper positions to interact (see p. 77).

Table 2.4 Factors Increasing the rate of Chemical reactions

Chapter 2: Basic Chemistry 65

2 react chemically unless they are in solution, so virtually all chemical reactions that occur in the body depend on water’s solvent properties.

Because nutrients, respiratory gases (oxy- gen and carbon dioxide), and wastes can dis- solve in water, water can act as a transport and exchange medium in the body. For example, all these substances are carried around the body in blood plasma (the liquid part of blood that is mostly made up of water) and are ex- changed between the blood and body cells by passing through the water-based interstitial fluid that bathes cells. Specialized molecules that lubricate the body, such as mucus and synovial fluid, also use water as their solvent. Synovial fluid “oils” the ends of bones as they move within joint cavities.

• Chemical reactivity. Water is an important reactant in some types of chemical reactions. For example, to digest foods or break down biological molecules, water molecules are added to the bonds of the larger molecules in order to break them. Such reactions are called hydrolysis reactions, a term that specifically recognizes this role of water (hydro = water; lys = splitting).

• Cushioning. Water also serves a protective function. In cerebrospinal fluid, water forms a cushion around the brain that helps to protect it from physical trauma. Amniotic fluid, which sur- rounds a developing fetus within the mother’s body, plays a similar role in protecting the fetus.

salts

A salt is an ionic compound containing cations other than the hydrogen ion (H+) and anions other than the hydroxide ion (OH−). Salts of many metal elements are commonly found in the body, but the most plentiful salts are those containing calcium and phosphorus, found chiefly in bones and teeth. When dissolved in body fluids, salts easily sepa- rate into their ions. This process, called dissocia- tion, occurs rather easily because the ions have already been formed. All that remains is for the ions to “spread out.” This is accomplished by the polar water molecules, which orient themselves with their slightly negative ends toward the cations and their slightly positive ends toward the anions, thereby overcoming the attraction between them (Figure 2.11).

carbon. The important organic compounds in the body are carbohydrates, lipids, proteins, and nucleic acids. All organic compounds are fairly (or very) large covalent molecules.

Inorganic and organic compounds are equally essential for life. Trying to determine which is more valuable can be compared to trying to decide whether the ignition system or the engine is more essential to the operation of a car.

Inorganic Compounds ➔ Learning Objectives

□□ Explain the importance of water to body homeostasis, and provide several examples of the roles of water.

□□ List several salts (or their ions) vitally important to body functioning.

□□ Differentiate a salt, an acid, and a base.

□□ Explain the concept of pH, and state the pH of blood.

Water

Water is the most abundant inorganic compound in the body. It accounts for about two-thirds of body weight. Among the properties that make water so vital are the following:

• High heat capacity. Water has a high heat capacity; that is, it absorbs and releases large amounts of heat before its temperature changes. Thus, it prevents the sudden changes in body temperature that might otherwise result from intense sun exposure, chilling winter winds, or internal events (such as vigor- ous muscle activity) that liberate large amounts of heat.

• Polarity/solvent properties. Water is an excellent solvent because of its polarity; in- deed, it is often called the “universal solvent.” A solvent is a liquid or gas in which smaller amounts of other substances, called solutes (which may be gases, liquids, or solids), can be dissolved or suspended. The resulting mixture is called a solution when the solute particles are exceedingly tiny, and a suspension when the solute particles are fairly large. Translucent mixtures with solute particles of intermediate size are called colloids.

Small reactive chemicals—such as salts, acids, and bases—dissolve easily in water and become evenly distributed. Molecules cannot

66 Essentials of Human Anatomy and Physiology

your rug. Acids can have a devastating effect; for example, consider the damage to sea life, trees, and famous historical monuments caused by the vinegar-like acid rain. But the most useful defini- tion of an acid is that it is a substance that can release hydrogen ions (H+) in detectable amounts. Because a hydrogen ion is a hydrogen nucleus (a “naked proton”), acids are also defined as proton (H+) donors. You may find it useful to think of acids as putting protons “in the game.” As free pro- tons, hydrogen ions can influence the acidity of body fluids.

When acids are dissolved in water, they release hydrogen ions and some anions. The anions are unimportant; it is the release of protons that deter- mines an acid’s effects on the environment. The ionization of hydrochloric acid (an acid produced by stomach cells that aids digestion) is shown in the following equation:

HCl S H+ + Cl2 (hydrochloric (proton) (anion) acid)

Other acids found or produced in the body include acetic acid (the acidic component of vinegar) and carbonic acid.

Acids that ionize completely and liberate all their protons are called strong acids; an example is hydrochloric acid. Acids that ionize incom- pletely, as do acetic and carbonic acid, are called weak acids. For example, when carbonic acid dis- solves in water, only some of its molecules ionize to liberate H+.

2H2CO3 S H+ + HCO32 + H2CO3 (carbonic (proton) (anion) (carbonic acid) acid)

Characteristics of Bases Bases have a bitter taste, feel slippery, and are proton (H1) accep- tors. (You can also think of them as taking pro- tons “out of the game”; when protons are bound to a molecule, they are unable to affect the acidity of body fluids.) Hydroxides are common inorganic bases. Like acids, the hydroxides ionize and disso- ciate in water; but in this case, the hydroxide ion (OH−) and some cations are released. The ioniza- tion of sodium hydroxide (NaOH), commonly known as lye, is shown as

NaOH S Na+ + OH2 (sodium (cation) (hydroxide hydroxide) ion)

Salts, both in their ionic forms and in combina- tion with other elements, are vital to body func- tioning. For example, sodium and potassium ions are essential for nerve impulses, and iron forms part of the hemoglobin molecule that transports oxygen within red blood cells.

Because ions are charged particles, all salts are electrolytes—substances that conduct an electri- cal current in solution. When electrolyte balance is severely disturbed, virtually nothing in the body works. (The functions of the elements found in body salts are summarized in Table 2.1 on p. 53.)

Acids and Bases

Like salts, acids and bases are electrolytes. That is, they ionize, dissociate in water, and can then con- duct an electrical current.

Characteristics of Acids Acids have a sour taste and can dissolve many metals or “burn” a hole in

Water molecule

Ions in solution

Na+

Cl−Cl−

Na+

Salt crystal

δ−

δ+

Figure 2.11 Dissociation of salt in water. The slightly negative ends of the water molecules (δ−) are attracted to Na+, whereas the slightly positive ends of water molecules (δ+) orient toward Cl−, causing the ions of the salt crystal to be pulled apart.

Chapter 2: Basic Chemistry 67

Figure 2.12 the pH scale and pH values of representative substances. The pH scale is based on the number of hydrogen ions in solution. At a pH of 7, the number of H+ = the number of OH−, and the solution is neutral. A solution with a pH below 7 is acidic (more H+ than OH−); above 7, basic, or alkaline (less H+ than OH−).

[H +

]= [O

H −

]

pH Examples

Sodium hydroxide (pH 14)

Oven cleaner, lye (pH 13.5)

Household ammonia (pH 10.5−11.5)

N eu

tr al

In cr

ea si

ng ly

b as

ic In

cr ea

si ng

ly a

ci di

Household bleach (pH 9.5)

Egg white (pH 8)

Blood (pH 7.4)

Milk (pH 6.3−6.6)

Black coffee (pH 5)

Wine (pH 2.5−3.5)

Lemon juice; gastric juice (pH 2)

Hydrochloric acid (pH 0)

H+ H+

OH−

OH− OH−

OH−

OH−OH−

H+ H+

H+

H+ OH−

OH−

OH−OH−

H+ H+

H+

H+ OH−

OH−

10Basic solution

Neutral solution

Acidic solution

Which ion is responsible for increased acidity?Q: The hydrogen ion.A:

The hydroxide ion is an avid proton (H1) seeker, and any base containing this ion is consid- ered a strong base. By contrast, bicarbonate ion (HCO3

2), an important base in blood, is a fairly weak base.

Acids, Bases, and Neutralization When acids and bases are mixed, they react with each other (in an exchange reaction) to form water and a salt:

HCl + NaOH S H2O + NaCl (acid) (base) (water) (salt)

This type of exchange reaction, in which an acid and a base interact, is more specifically called a neutralization reaction.

pH: Acid-Base Concentrations The relative con- centration of hydrogen (and hydroxide) ions in various body fluids is measured in concentration units called pH (pe-āch) units. The pH scale, which was devised in 1909 by a Danish biochem- ist (and part-time beer brewer) named Sørensen, is based on the number of protons in solution.

The pH scale runs from 0 to 14 (Figure 2.12), and each successive change of 1 pH unit repre- sents a tenfold change in hydrogen ion concentra- tion. At a pH of 7, the number of hydrogen ions exactly equals the number of hydroxide ions, and the solution is neutral; that is, neither acidic nor basic. Solutions with a pH lower than 7 are acidic: The hydrogen ions outnumber the hydroxide ions. A solution with a pH of 6 has 10 times as many hydrogen ions as a solution with a pH of 7, and a pH of 3 indicates a 10,000-fold (10 × 10 × 10 × 10) increase in hydrogen ion concentration from pH 7. Solutions with a pH number higher than 7 are basic, or alkaline, and solutions with a pH of 8 and 12 have 1/10 and 1/100,000 (respectively) the number of hydrogen ions present in a solution with a pH of 7.

68 Essentials of Human Anatomy and Physiology

17. Is a pH of 11 acidic or basic? What is the difference in pH between a solution at pH 11 and a solution at pH 5?

18. Biochemistry is “wet” chemistry. What does this statement mean?

19. Salts are electrolytes. What does that mean?

For answers, see Appendix A.

organic Compounds ➔ Learning Objectives

□□ Explain the role of dehydration synthesis and hydrolysis in formation and breakdown of organic molecules.

□□ Compare and contrast carbohydrates and lipids in terms of their building blocks, structures, and functions in the body.

Most organic compounds are very large molecules, but their interactions with other molecules typi- cally involve only small, reactive parts of their structure called functional groups (acid groups, amines, and others).

Many organic compounds (carbohydrates and proteins, for example) are polymers. Polymers are chainlike molecules made of many similar or repeating units (monomers), which are joined together by dehydration synthesis (Figure 2.13a). During dehydration synthesis, a hydrogen atom is

Living cells are extraordinarily sensitive to even slight changes in pH. Acid-base balance is carefully regulated by the kidneys, lungs, and a number of chemicals called buffers that are pres- ent in body fluids. Weak acids and weak bases are important components of the body’s buffer sys- tems, which act to maintain pH stability by taking up excess hydrogen or hydroxide ions (see Chapter 15).

Because blood comes into close contact with nearly every body cell, regulation of blood pH is especially critical. Normally, blood pH varies in a narrow range, from 7.35 to 7.45. When blood pH changes more than a few tenths of a pH unit from these limits, death becomes a distinct possibility. Although we could give hundreds of examples to illustrate this point, we will provide just one very important one: When blood pH begins to dip into the acid range, the amount of life-sustaining oxy- gen that the hemoglobin in blood can carry to body cells decreases rapidly to dangerously low levels.

Did You Get It? 15. What property of water prevents rapid changes in

body temperature? 16. Which is a proton donor—an acid or a base?

+ OH HO O

H2O

Monomers linked by covalent bond

Monomers linked by covalent bond OH

(a) Dehydration synthesis

Monomers are joined by removal of OH from one monomer and removal of H from the other at the site of bond formation.

+ O

H2O

(b) Hydrolysis

Monomers are released by the addition of a water molecule, adding OH to one monomer and H to the other.

Monomer 1 Monomer 2

Figure 2.13 Dehydration synthesis and hydrolysis of biological molecules. Biological molecules are formed from their monomers (units) by dehydration synthesis and broken down to their monomers by hydrolysis.

Chapter 2: Basic Chemistry 69

word carbohydrate, which means “hydrated car- bon,” and in the molecular formulas of sugars. For example, glucose is C6H12O6, and ribose is C5H10O5.

Carbohydrates are classified according to size and solubility in water as monosaccharides, disac- charides, or polysaccharides. Because monosac- charides are joined to form the molecules of the other two groups, they are the structural units, or building blocks, of carbohydrates.

Monosaccharides Monosaccharide means one (mono) sugar (saccharide), and thus monosaccha- rides are also referred to as simple sugars. They are single-chain or single-ring structures (meaning the carbon backbone forms either a line or a cir- cle), containing from three to seven carbon atoms (Figure 2.14a).

The most important monosaccharides in the body are glucose, fructose, galactose, ribose, and deoxyribose. Glucose, also called blood sugar, is the universal cellular fuel. Fructose and galactose are converted to glucose for use by body cells.

removed from one monomer and a hydroxyl group (OH) is removed from the monomer it is to be joined with. A covalent bond then forms, unit- ing the monomers, and a water molecule is released. This removal of a water molecule (dehy- dration) at the bond site occurs each time a mono- mer is added to the growing polymer chain.

When polymers must be broken down or digested to their monomers, the reverse process, called hydrolysis, occurs (Figure 2.13b). As a water molecule is added to each bond, the bond is broken, releasing the monomers. All organic molecules covered in this chapter—carbohydrates, lipids, proteins, and nucleic acids—are formed by dehydration synthesis and broken down by hydrolysis.

Carbohydrates

Carbohydrates, which include sugars and starches, contain carbon, hydrogen, and oxygen. With slight variations, the hydrogen and oxygen atoms appear in the same ratio as in water; that is, two hydrogen atoms to one oxygen atom. This is reflected in the

(d) Starch (polysaccharide)

O O

O CH2

O O

(b) Double sugar (disaccharide)

(a) Simple sugar (monosaccharide)

H2O+

OH HO

Glucose Fructose Sucrose Water

Dehydration synthesis

Hydrolysis

Figure 2.14 Carbohydrates. (a) The generalized structure of a monosaccharide. (b) and (d) The basic structures of a disaccharide and a polysaccharide, respectively. (c) Formation and breakdown of the disaccharide sucrose by dehydration synthesis and hydrolysis, respectively.

70 Essentials of Human Anatomy and Physiology

Small amounts of carbohydrates are used for structural purposes and represent 1 to 2 percent of cell mass. Some sugars are found in our genes, and others are attached to outer surfaces of cell membranes (boundaries), where they act as road signs to guide cellular interactions.

Lipids

Lipids, or fats, are a large and diverse group of organic compounds (table 2.5, p. 73). They enter the body in the form of meats, egg yolks, dairy products, and oils. The most abundant lipids in the body are triglycerides, phospholipids, and ste- roids. Like carbohydrates, all lipids contain carbon, hydrogen, and oxygen atoms, but in lipids, carbon and hydrogen atoms far outnumber oxygen atoms, as illustrated by the formula for a typical fat named tristearin, which is C57H110O6. Lipids are insoluble in water but readily dissolve in other lipids and in organic solvents such as alcohol and acetone.

Triglycerides The triglycerides (tri-glis′er-ı̄dz), or neutral fats, are composed of two types of building blocks, fatty acids and glycerol. Their synthesis involves the attachment of three fatty acids to a single glycerol molecule. The result is an E-shaped molecule that resembles the tines of a fork (Figure 2.15a). Although the glycerol back- bone is the same in all neutral fats, the fatty acid chains vary; this variation results in different kinds of neutral fats.

The length of a triglyceride’s fatty acid chains and their type of C¬C bonds determine how solid the molecule is at a given temperature. Fatty acid chains with only single covalent bonds between carbon atoms are referred to as saturated fats. Their fatty acid chains are straight (Figure 2.16a, p. 72), and, at room temperature, the molecules of a saturated fat pack closely together, forming a solid; butter is an example. Fatty acids that contain one or more double bonds between carbon atoms are said to be unsaturated fats (monounsatu- rated and polyunsaturated, respectively). The double and triple bonds cause the fatty acid chains to kink (Figure 2.16b) so that they cannot pack closely enough to solidify. Hence, triglycerides with short fatty acid chains or unsaturated fatty acids are oils (liquid at room temperature). Plant lipids are typically oils. Examples include olive oil (rich in monounsaturated fats) and soybean and safflower oils, which contain a high percentage of

Ribose and deoxyribose form part of the structure of nucleic acids, another group of organic mole- cules responsible for genetic information.

Disaccharides Disaccharides, or double sugars (Figure 2.14b), are formed when two simple sug- ars are joined by dehydration synthesis. In this reaction, as noted earlier, a water molecule is lost as the bond forms (Figure 2.14c).

Some of the important disaccharides in the diet are sucrose (glucose-fructose), which is cane sugar; lactose (glucose-galactose), found in milk; and maltose (glucose-glucose), or malt sugar. Because the double sugars are too large to pass through cell membranes, they must be broken down (digested) to their monosaccharide units to be absorbed from the digestive tract into the blood; this is accomplished by hydrolysis (see Figures 2.14b and 2.13b).

Polysaccharides Long, branching chains of linked simple sugars are called polysaccharides (literally, “many sugars”) (Figure 2.14d). Because they are large, insoluble molecules, they are ideal storage products. Another consequence of their large size is that they lack the sweetness of simple and double sugars.

Only two polysaccharides, starch and glyco- gen, are of major importance to the body. Starch is the storage polysaccharide formed by plants. We ingest it in the form of “starchy” foods, such as grain products (corn, rice) and root vegetables (potatoes and carrots, for example). Glycogen is a slightly smaller, but similar, polysaccharide found in animal tissues (largely in the muscles and the liver). Like starch, it is a polymer of linked glu- cose units.

Carbohydrates provide a ready, easily used source of fuel for cells, and glucose is at the top of the “cellular menu.” When glucose is oxidized (“burned” by combining with oxygen) in a com- plex set of chemical reactions, it is broken down into carbon dioxide and water. Some of the energy released as the glucose bonds are broken is trapped in the bonds of high-energy ATP mole- cules, the energy “currency” of all body cells. If not immediately needed for ATP synthesis, dietary car- bohydrates are converted to glycogen or fat and stored. Those of us who have gained weight from eating too many carbohydrate-rich snacks have firsthand experience of this conversion process!

Chapter 2: Basic Chemistry 71

O C

PN+ CH2 CH2 O O

CH3

O−

CH3

Glycerol backbone

Phosphorus-containing group (polar head)

2 fatty acid chains (nonpolar tail)

CH2

CH2 CH2 CH2 C C CH 2 CH

CH2. . .

CH2 CH2 CH2 CH2 CH2 CH2 CH3CH2. . .

(b) Typical structure of a phospholipid molecule (phosphatidylcholine). Two fatty acid chains and a phosphorous-containing group are attached to a glycerol backbone.

Polar “head”

Nonpolar “tail” (schematic

phospholipid)

. . .

H H

CH3

CH3 CH3

H3C

H C

CH2 CH2 CH2 CH2 CH3

3H2O

Glycerol 3 fatty acid chains Triglyceride, or neutral fat 3 water molecules

(a) Formation of a triglyceride. Fatty acid chains are bound to glycerol by dehydration synthesis.

+ +

Figure 2.15 Lipids. (a) Triglycerides, or neutral fats, are synthesized by dehydration synthesis, and a water molecule is lost at each bond site.

(b) Structure of a typical phospholipid molecule. Notice that one fatty acid chain in (b) is unsaturated (has one or more

C“C [double] bonds). (c) The generalized structure of cholesterol (the basis for all steroids made in the body).

Triglycerides and phospholipids are similar. What is the major structural difference between them?Q:

They both have a glycerol backbone and fatty acid chains. However, triglycerides have three attached fatty acid chains, whereas phospholipids have only two; the third is replaced by a negatively charged

phosphate-containing group.

72 Essentials of Human Anatomy and Physiology

fish, appear to decrease the risk of heart disease and some inflammatory diseases.

Triglycerides represent the body’s most abun- dant and concentrated source of usable energy. When they are oxidized, they yield large amounts of energy. They are stored chiefly in fat deposits beneath the skin and around body organs, where they help insulate the body and protect deeper body tissues from heat loss and injury.

Phospholipids. Phospholipids (fos′fo-lip″idz) are similar to triglycerides. The major difference is that a phosphorus-containing group is always part of the molecule and takes the place of one of the fatty acid chains. Thus, phospholipids have two instead of three attached fatty acids (see Figure 2.15b).

Because the phosphorus-containing portion (the “head”) bears a negative charge, phospholip- ids have special chemical properties and polarity. For example, the charged region is hydrophilic (“water loving”), meaning it attracts and interacts with water and ions. The fatty acid chains (the nonpolar “tail”) are hydrophobic (“water fear- ing”) and do not interact with polar or charged molecules. The presence of phospholipids in cell membranes allows cells to be selective about what may enter or leave.

Steroids Steroids are basically flat molecules formed of four interlocking carbon rings (see Figure 2.15c); thus, their structure differs quite a bit from that of other lipids. However, like fats, ste- roids are made largely of hydrogen and carbon atoms and are fat-soluble. The single most impor- tant steroid molecule is cholesterol. We ingest cholesterol in animal products such as meat, eggs, and cheese, and some is made by the liver, regard- less of dietary intake.

Cholesterol has earned bad press because of its role in arteriosclerosis, but it is essential for human life. Cholesterol is found in cell membranes and is the raw material used to make vitamin D, steroid hormones (chemical signaling molecules in the body), and bile salts (which help break down fats during digestion). Although steroid hormones are present in the body in only small quantities, they are vital to homeostasis. Without sex hormones, reproduction would be impossible; and without the corticosteroids produced by the adrenal glands, the body would not survive.

polyunsaturated fatty acids. Longer fatty acid chains and more saturated fatty acids are common in animal fats, such as butterfat and the fat of meats, which are solid at room temperature. Of the two types of fatty acids, the unsaturated vari- ety, especially olive oil, is more “heart healthy.”

Trans fats, common in many margarines and baked products, are oils that have been solidified by the addition of hydrogen atoms at sites of dou- ble carbon bonds, which reduces those bonds to single carbon bonds. They increase the risk of heart disease by raising “bad” cholesterol and decreasing “good” cholesterol. In contrast, the omega-3 fatty acids, found naturally in cold-water

(a)

C O

Saturated fat. At room temperature, the molecules of a saturated fat such as this butter are packed closely together, forming a solid.

(b) Unsaturated fat. At room temperature, the molecules of an unsaturated fat such as this olive oil cannot pack together closely enough to solidify because of the kinks in some of their fatty acid chains.

Structural formula of a saturated fat molecule

Structural formula of an unsaturated fat molecule

Figure 2.16 Examples of saturated and unsaturated fats and fatty acids.

Chapter 2: Basic Chemistry 73

Lipid type Location/function

Triglycerides (neutral fats) Found in fat deposits (subcutaneous tissue and around organs); protect and insulate the body organs; the major source of stored energy in the body.

Phospholipids Found in cell membranes; participate in the transport of lipids in plasma; abundant in the brain and the nervous tissue in general, where they help to form insulating white matter.

Steroids

Cholesterol The basis of all body steroids.

Bile salts A breakdown product of cholesterol; released by the liver into the digestive tract, where they aid in fat digestion and absorption.

Vitamin D A fat-soluble vitamin produced in the skin on exposure to UV (ultraviolet) radiation (sunshine); necessary for normal bone growth and function.

Sex hormones Estrogen and progesterone (female hormones) and testosterone (a male hormone) produced from cholesterol; necessary for normal reproductive function; deficits result in sterility.

Corticosteroids (adrenal cortical hormones)

Cortisol, a glucocorticoid, is a long-term antistress hormone that is necessary for life; aldosterone helps regulate salt and water balance in body fluids by targeting the kidneys.

Other lipid-based substances

Fat-soluble vitamins

A Found in orange-pigmented vegetables (carrots) and fruits (tomatoes); part of the photoreceptor pigment involved in vision.

E Taken in via plant products such as wheat germ and green leafy vegetables; may promote wound healing and contribute to fertility, but not proven in humans; an antioxidant; may help to neutralize free radicals (highly reactive particles believed to be involved in triggering some types of cancers).

K Made available largely by the action of intestinal bacteria; also prevalent in a wide variety of foods; necessary for proper clotting of blood.

Prostaglandins Derivatives of fatty acids found in cell membranes; various functions depending on the specific class, including stimulation of uterine contractions (thus inducing labor and miscarriages), regulation of blood pressure, and control of motility of the gastrointestinal tract; involved in inflammation.

Lipoproteins Lipoid and protein-based substances that transport fatty acids and cholesterol in the bloodstream; major varieties are high-density lipoproteins (HDLs) and low-density lipoproteins (LDLs).

Glycolipids Component of cell membranes. Lipids associated with carbohydrate molecules that determine blood type, play a role in cell recognition or in recognition of foreign substances by immune cells.

Table 2.5 representative Lipids Found in the Body

74 Essentials of Human Anatomy and Physiology

addition, they contain nitrogen and sometimes sul- fur atoms as well.

The building blocks of proteins are small mol- ecules called amino (ah-me′no) acids. Twenty varieties of amino acids are found in human pro- teins. All amino acids have an amine group (NH2), which gives them basic properties, and an acid group (COOH), which allows them to act as acids. In fact, all amino acids are identical except for a single group of atoms called their R-group (Figure 2.17). Differences in the R-groups make each amino acid chemically unique. For example, an extra acid group (COOH) in the R-group (such as in aspartic acid) makes the amino acid more acidic, and a sulfhydryl group (—SH) in the R-group (such as in cysteine) is often involved in stabilizing protein structure by binding to another amino acid within the same protein.

Amino acids are joined together in chains to form polypeptides (fewer than 50 amino acids); proteins (more than 50 amino acids); and large, complex proteins (50 to thousands of amino acids). Because each type of amino acid has dis- tinct properties, the sequence in which they are

Did You Get It? 20. What are the structural units, or building blocks, of

carbohydrates? Of lipids? 21. Which type(s) of lipid(s) is/are found in cellular

membranes? 22. Is the formation of glycogen from glucose an example

of hydrolysis or dehydration synthesis?

For answers, see Appendix A.

Proteins

➔ Learning Objectives □□ Differentiate fibrous proteins from globular

proteins.

□□ Define enzyme, and explain the role of enzymes.

Proteins account for over 50 percent of the organic matter in the body, and they have the most varied functions of all organic molecules. Some are construction materials; others play vital roles in cell function. Like carbohydrates and lipids, all proteins contain carbon, oxygen, and hydrogen. In

C CN OH

H H

C CN OH

H H

CH2

COOH

CN OH

H H

CH2

CN OH

H H

NH2H

CH2

CN OH

H H

SH Amine (basic) group

Acid group

(a) Generalized structure of all amino acids.

(b) Glycine is the simplest amino acid.

(d) Lysine (a basic amino acid) has an amine group (—NH2) in the R group.

(e) Cysteine (a basic amino acid) has a sulfhydryl (—SH) group in the

R group.

Figure 2.17 Amino acid structures. (a) Generalized structure of amino acids. All amino acids have both an amine (NH2) group and an acid (COOH) group; they differ only in the atomic makeup of their R-groups (green). (b–e) Specific structures of four amino acids.

What is the importance of the R-groups (green here)?Q:

The R-groups make each type of amino acid functionally unique.A:

Chapter 2: Basic Chemistry 75

sequence of amino acids composing each amino acid chain is called the primary structure. This structure, which resembles a strand of amino acid “beads,” is the backbone of a protein molecule in which the chemical properties of each amino acid will affect how the protein folds (Figure 2.18a).

Most proteins do not function as simple, linear chains of amino acids. Instead, they twist or bend upon themselves to form a more complex second- ary structure. The most common secondary struc- ture is the alpha (a)-helix, which resembles a metal spring (Figure 2.18b). The α-helix is formed by coiling of the primary chain and is stabilized by hydrogen bonds. Hydrogen bonds in α-helices

bound together produces proteins that vary widely both in structure and function. To under- stand this more easily, think of the 20 amino acids as a 20-letter alphabet. The letters (amino acids) are used in specific combinations to form words (a protein). Just as a change in one letter of any word can produce a word with an entirely different meaning (flour S floor) or one that is nonsensical (flour S fluur), changes in amino acids (letters) or in their positions in the protein allow literally thousands of different protein mol- ecules to be made.

Structural Levels of Proteins Proteins can be described in terms of four structural levels. The

(c) Tertiary structure. The overall three-dimensional shape of the polypeptide or protein is called tertiary structure. It is reinforced by chemical bonds between the R-groups of amino acids in different regions of the protein chain.

(b) Secondary structure. Two types of secondary structure are the alpha- helix and beta-pleated sheet. Secondary structure is reinforced by hydrogen bonds, represented by dashed lines in this figure.

(d) Quaternary structure. Some proteins consist of two or more polypeptide chains. For example, four polypeptides construct the protein hemoglobin, a blood protein.

C C

H N

(a) Primary structure. A protein’s primary structure is the unique sequence of amino acids in the polypeptide chain.

Cys Glu Leu

Ala Ala Ala Ala Met Lys Arg His Gly Leu

Aps Hydrogen bonds

Protein (if > 50 amino acids) or polypeptide (if < 50 amino acids)

β-pleated sheet Alpha- helix

Amino acids

Complex protein with four polypeptide subunits, each with tertiary structure

C C

Figure 2.18 the four levels of protein structure.

76 Essentials of Human Anatomy and Physiology

the protein backbone determine where hydrogen bonds can form to keep water-loving (hydrophilic) amino acids near the surface and water-fearing (hydrophobic) amino acids buried in the protein’s core so the protein remains water-soluble.

Fibrous and Globular Proteins Based on their overall shape and structure, proteins are classed as either fibrous or globular proteins (Figure 2.19). The strandlike fibrous proteins, also called structural proteins, appear most often in body structures. Some exhibit only secondary structure, but most have tertiary or even quaternary struc- ture. They are very important in binding structures together and providing strength in certain body tis- sues. For example, collagen (kol′ah-jen) is found in bones, cartilage, and tendons and is the most abun- dant protein in the body (Figure 2.19a). Keratin (ker′ah-tin) is the structural protein of hair and nails and the material that makes skin tough.

Globular proteins are mobile, generally com- pact, spherical molecules that have at least tertiary structure. These water-soluble proteins play crucial roles in virtually all biological processes. Because they do things rather than just form structures, they are also called functional proteins; the scope of their activities is remarkable (Table 2.6). For exam- ple, some proteins called antibodies help to pro- vide immunity; others (hormones) help to regulate growth and development. Still others, called

always link different parts of the same chain together.

In another type of secondary structure, the beta (b)-pleated sheet, the primary polypeptide chains do not coil, but are linked side by side by hydrogen bonds to form a pleated, ribbonlike structure that resembles the pleats of a skirt or a sheet of paper folded into a fan (see Figure 2.18b). In this type of secondary structure, the hydrogen bonds may link together different polypeptide chains as well as different parts of the same chain that has folded back on itself.

Many proteins have tertiary structure (ter′she- a″re), the next higher level of complexity. Tertiary structure is achieved when α-helical or β-pleated regions of the amino acid chain fold upon one another to produce a compact ball-like, or globu- lar, protein (Figure 2.18c). The unique structure is maintained by covalent and hydrogen bonds between amino acids that are often far apart in the primary chain. Finally, when two or more amino acid chains (polypeptide chains) combine in a regular manner to form a complex protein, the protein has quaternary (kwah′ter-na″re) structure (Figure 2.18d).

Although a protein with tertiary or quaternary structure looks a bit like a crumpled ball of tin foil, the final structure of any protein is very specific and is dictated by its primary structure. In other words, the types and positions of amino acids in

(a) Triple helix of collagen (a fibrous or structural protein).

Heme group

Globin protein

(b) Hemoglobin molecule composed of the protein globin and attached heme groups. (Globin is a globular, or functional, protein.)

Figure 2.19 General structure of (a) a fibrous protein and (b) a globular protein.

Chapter 2: Basic Chemistry 77

product or being changed itself. Enzymes can accomplish this feat because they have unique regions called active sites on their surfaces. These sites “fit” and interact chemically with other mole- cules of complementary shape and charge called substrates (Figure 2.20). While the substrates are bound to the enzyme’s active site, producing a structure called an enzyme-substrate complex, they undergo structural changes that result in a new product. Whereas some enzymes build larger mol- ecules, others break things into smaller pieces or simply modify a substrate. Once the reaction has occurred, the enzyme releases the product. Because enzymes are not changed during the reaction, they are reusable, and the cells need only small amounts of each enzyme. Think of scissors cutting paper. The scissors (the enzyme) are unchanged while the paper (the substrate) is “cut” to become the product. The scissors are reusable and go on to “cut” other paper.

Enzymes are capable of catalyzing millions of reactions each minute. However, they do more than just increase the speed of chemical reactions; they also determine just which reactions are pos- sible at a particular time. No enzyme, no reaction! Enzymes can be compared to a bellows used to fan a sluggish fire into flaming activity. Without enzymes, biochemical reactions would occur far too slowly to sustain life.

enzymes (en′zı̄mz), regulate essentially every chemical reaction that goes on within the body. The oxygen-carrying protein hemoglobin is an example of a globular protein with quaternary structure (Figure 2.19b).

The fibrous structural proteins are exception- ally stable, but the globular functional proteins are quite the opposite. Hydrogen bonds are critically important in maintaining their structure, but hydro- gen bonds are fragile and are easily broken by heat and excesses of pH. When their three-dimensional structures are destroyed, the proteins are said to be denatured and can no longer perform their physiological roles. Why? Their function depends on their specific three-dimensional shapes. Hemoglobin becomes totally unable to bind and transport oxygen when blood pH becomes too acidic, as we stated earlier. Pepsin, a protein- digesting enzyme that acts in the stomach, is inactivated by alkaline pH. In each case, the improper pH has destroyed the structure required for function.

Because enzymes are important in the func- tioning of all body cells, we consider these incred- ibly complex molecules here.

Enzymes and Enzyme Activity Enzymes are functional proteins that act as biological catalysts. A catalyst is a substance that increases the rate of a chemical reaction without becoming part of the

Functional class role(s) in the body

Antibodies (immunoglobulins) Highly specialized proteins that recognize, bind with, and inactivate bacteria, toxins, and some viruses; function in the immune response, which helps protect the body from “invading” foreign substances.

Hormones Help to regulate growth and development. Examples include

• Growth hormone—an anabolic hormone necessary for optimal growth.

• Insulin—helps regulate blood sugar levels.

• Nerve growth factor—guides the growth of neurons in the development of the nervous system.

Transport proteins Hemoglobin transports oxygen in the blood; other transport proteins in the blood carry iron, cholesterol, or other substances.

Enzymes (catalysts) Essential to virtually every biochemical reaction in the body; increase the rates of chemical reactions by at least a millionfold; in their absence (or destruction), biochemical reactions cease.

Table 2.6 representative Classes of Functional Proteins

78 Essentials of Human Anatomy and Physiology

nucleic Acids

➔ Learning Objectives □□ Compare and contrast the structures and functions

of DnA and rnA.

□□ Explain the importance of AtP in the body.

The role of nucleic (nu-kle′ik) acids is funda- mental: They make up your genes, which provide the basic blueprint of life. They not only deter- mined what type of organism you would be, but also directed your growth and development—and they did this largely by dictating protein structure. (Remember that enzymes, which catalyze all the chemical reactions that occur in the body, are proteins.)

Nucleic acids, composed of carbon, oxygen, hydrogen, nitrogen, and phosphorus atoms, are the largest biological molecules in the body. Their building blocks, nucleotides (nu′kle-o-tı̄dz), are quite complex. Each consists of three basic parts: (1) a nitrogen-containing base, (2) a pentose (5-carbon) sugar, and (3) a phosphate group (Figure 2.21a and b).

The bases come in five varieties: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U). A and G are large, nitrogen-containing bases made up of two carbon rings, whereas C, T and U are smaller, single-ring structures. The nucleotides

Although there are hundreds of different kinds of enzymes in body cells, they are very specific in their activities, each controlling only one (or a small group of) chemical reaction(s) and acting only on specific molecules. Most enzymes are named according to the specific type of reaction they catalyze. For example, hydrolases add water, and oxidases cause oxidation. (In most cases, you can recognize an enzyme by the suffix -ase in its name.)

The activity of different enzymes is controlled in different ways. Many enzymes are produced in an inactive form and must be activated in some way before they can function. In other cases, enzymes are inactivated immediately after they have performed their catalytic function. Both events are true of enzymes that promote blood clotting when a blood vessel has been damaged. If this were not so, large numbers of unneeded and potentially lethal blood clots would be formed.

Did You Get It? 23. What is the primary structure of proteins? 24. Which is more important for building body structures,

fibrous or globular proteins? 25. How does an enzyme recognize its substrate(s)?

For answers, see Appendix A.

Substrates (S) e.g., simple sugars

Enzyme (E)

Enzyme-substrate complex (E-S)

Enzyme (E)

Product (P) e.g., disaccharide

Energy is absorbed; bond is formed by dehydration synthesis.

Water is released.

Active site

H2O+

1 2

Substrates bind at active site, temporarily forming an enzyme-substrate complex.

The E-S complex undergoes structural changes that form the product.

The enzyme releases the product of the reaction.

Figure 2.20 A simplified view of enzyme action.

Chapter 2: Basic Chemistry 79

(b) Adenine nucleotide (Diagrammatic representation)

Deoxyribose sugar

Hydrogen bonds (a) Adenine nucleotide (Chemical structure)

N H

H N

O–

OO O

CH2

H HH

G C

O–

(d) Diagram of a DNA molecule

Phosphate group

Pentose sugar:

Deoxyribose

Nitrogen- containing base:

Adenine (A)

Phosphate

Sugar-phosphate backbone

KEY:

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

Figure 2.21 structure of DnA. (a) The unit of DNA, the nucleotide, is composed of a deoxyribose sugar molecule linked to a phosphate group. A nitrogen-containing base is attached to the sugar. The nucleotide illustrated, both in its (a) chemical

and (b) diagrammatic structures, contains the base adenine. (c) Computer-generated image of DNA. (d) Diagrammatic structure of a DNA molecule—two nucleotide chains coiled into a double helix. The “backbones” of DNA are formed by

alternating sugar and phosphate molecules. The “rungs” are formed by complementary bases (A to T, G to C) bound by two or three hydrogen bonds, respectively.

80 Essentials of Human Anatomy and Physiology

chains are held together by hydrogen bonds between the bases, forming a ladderlike molecule. Alternating sugar and phosphate molecules form the “uprights” of the ladder, called the sugar-phosphate backbone, and each “rung” is formed by two joined bases (one base pair). Binding between the bases is very specific: A always binds to T, and G always binds to C. Thus, A and T are said to be comple- mentary bases, as are C and G. Therefore, a sequence of ACTGA on one nucleotide chain would be bound to the complementary sequence TGACT on the other nucleotide strand. The whole molecule is then coiled into a spiral-staircase-like structure called a double helix.

Whereas DNA is double-stranded, RNA mole- cules are single nucleotide strands. The RNA bases are A, G, C, and U (U replaces the T found in DNA), and its sugar is ribose instead of deoxyri- bose. Three major varieties of RNA exist— messenger, transfer, and ribosomal RNA—and each has a specific role to play in carrying out DNA’s instructions for building proteins. Messenger RNA (mRNA) carries the information for building

are named according to the base they contain: A-containing bases are adenine nucleotides, C-containing bases are cytosine nucleotides, and so on.

The two major kinds of nucleic acids are deoxy- ribonucleic (de-ok″sı̆-ri″bo-nu-kle′ik) acid (DNA) and ribonucleic acid (RNA). DNA and RNA differ in many respects. DNA is the genetic material found within the cell nucleus (the control center of the cell). It has two fundamental roles: (1) It repli- cates itself exactly before a cell can divide, thus ensuring that every body cell gets an identical copy of the genetic information; and (2) it provides the instructions for building every protein in the body. For the most part, RNA functions outside the nucleus and can be considered the “molecular assistant” of DNA; that is, RNA carries out the orders for protein synthesis issued by DNA.

Although both DNA and RNA are formed when nucleotides join together, their final structures are different. DNA is a long double chain of nucleotides (Figure 2.21c and d). Its bases are A, G, T, and C, and its sugar is deoxyribose. Its two nucleotide

ATP

EnergyP P P P ++P

H2O

Adenosine diphosphate (ADP)

Hydrolysis

(b) Hydrolysis of ATP

(a) Adenosine triphosphate (ATP)

H H

N N

Adenine base NH2

Ribose sugar

OH OH

H H

OCH2P P POO −O

O−

Phosphate groups

High-energy phosphate bonds

Figure 2.22 AtP—structure and hydrolysis. (a) The structure of ATP (adenosine triphosphate). (b) Hydrolysis of ATP to yield ADP (adenosine diphosphate) and inorganic phosphate (Pi). High- energy bonds are indicated by a red ~.

Chapter 2: Basic Chemistry 81

energy when the “catch” is released. The conse- quence of breaking its terminal phosphate bond can be represented as follows:

ATP ∆ ADP + Pi + E (adenosine (adenosine (inorganic (energy) triphosphate) diphosphate) phosphate)

Did You Get It? 26. How do DNA and RNA differ from each other in the

kinds of bases and sugars they contain? 27. What is the vital importance of ATP to body cells?

For answers, see Appendix A.

the protein from the DNA to the ribosomes, the protein-synthesizing sites. Transfer RNA (tRNA) ferries amino acids to the ribosomes. Ribosomal RNA (rRNA) forms part of the ribosomes, where it oversees the translation of the message and the binding together of amino acids to form the pro- teins. (We describe protein synthesis in greater detail in Chapter 3.)

Adenosine triphosphate (AtP)

The synthesis of adenosine triphosphate (ah- den′o-sēn tri-fos′fāt), or ATP, is all-important because it provides a form of chemical energy that all body cells can use. Without ATP, molecules cannot be made or broken down, cells cannot maintain their membrane boundaries, and all life processes grind to a halt.

Although glucose is the most important fuel for body cells, none of the chemical energy con- tained in its bonds can be used directly to power cellular work. Instead, energy is released as glu- cose is oxidized, and it is captured and stored in the bonds of ATP molecules as small packets of energy. Compare glucose and ATP to crude oil and gasoline: While oil stores lots of energy, it cannot be used directly to power a car without first being refined into gasoline.

Structurally, ATP is a modified RNA nucleotide; it consists of an adenine base, ribose sugar, and three phosphate groups instead of one (Figure 2.22a). The phosphate groups are attached by unique chemical bonds called high-energy phosphate bonds. These bonds are high energy because the phosphate groups are negatively charged and the like charges repel one another, generating tension at these sites. As ATP is used to provide cellular energy, adenosine diphosphate (ADP) accumulates (Figure 2.22b), and ATP sup- plies are replenished by oxidation of food fuels. Essentially, the same amount of energy must be captured and used to reattach a phosphate group to ADP (that is, to reverse the reaction) as is liber- ated when the terminal phosphate is removed from ATP.

When the high-energy bonds of ATP are bro- ken by hydrolysis, energy is liberated and can be used immediately by the cell to do work or power a particular activity—such as driving chemical reac- tions, transporting solutes across the membrane, or, in the case of muscle cells, contracting (Figure 2.23). ATP can be compared to a tightly coiled spring that is ready to uncoil with tremendous

Solute

Membrane protein

Relaxed muscle cell

Contracted muscle cell

ATP ADP

ATP

ADP +

ADP A

B +

Transport work. ATP drives the transport of certain solutes (amino acids, for example) across cell membranes.

Mechanical work. ATP activates contractile proteins in muscle cells so that the cells can shorten and perform mechanical work.

Chemical work. ATP provides the energy needed to drive energy-absorbing chemical reactions.

A B

(b)

(c)

(a)

Figure 2.23 three examples of how AtP drives cellular work. The high-energy bonds of ATP release energy for use by the cell when they are broken. ATP is regenerated (phosphate is again bound to ADP) as energy is released by the oxidation of food fuels and captured in the third high-energy bond.

Pharmacy Technician

Pharmacy technicians must

have a good grasp of anatomy and

physiology to understand each drug’s chemical

properties.

medication that could react badly with another medication the patient is already taking. Drug interactions hap- pen commonly when you have multi- ple doctors. “Sometimes, we’ll get two ACE inhibitors in the same cate- gory from two different doctors [pre- scribed for the same patient], and that could be lethal,” Green says.

Pharmacy technicians work in retail and mail-order pharmacies, hospitals, nursing homes, assisted living facili- ties, and anywhere else patients have high needs for medication. As the Baby Boom generation ages and the number of senior citizens grows, so does the demand for pharmacists and pharmacy technicians.

Requirements to be a pharmacy technician vary from state to state, and many aspiring technicians simply receive on-the-job training. However, some pharmacies seek out technicians with specific training requiring class- room and laboratory work in a hospi- tal, community college, or vocational program or sometimes through the military. Some of these programs also include internships in pharmacies.

For additional information on this career and others, click the Focus on Careers link at .

FOCUS ON

CAREERS

to recognize how medications affect patients, pharmacy technicians need a thorough understanding of anatomy and physiology.

When most people get a new medication, they open up the package and toss out the little pam- phlet that goes into detail about how the medication works. Not Chris Green. “I love reading the package inserts,” says Green, the lead pharmacy technician at a CVS drugstore in Birmingham, Alabama. Green’s enthusiasm for those details is a lifesaver for his customers. Phar- macy technicians are a vital link in the chain between doctor and patient.

Although pharmacy technicians are legally prohibited from talking with patients about their symptoms, they can translate medical jargon and dis- cuss a medication’s side effects and other precautions the patient may need to take. For example, doctors may recommend that patients who are on certain medications for a long time have regular tests such as eye exams, bloodwork, or tests for liver function. A pharmacy technician can convey that information to the patient—and check on subsequent visits to make sure he or she is follow- ing up.

A busy retail pharmacy has various stations: data entry, where the patient’s record is updated with a new prescription; production, where the prescription is filled; and verification, where the pharmacist reviews the pre- scription and makes sure it is filled and labeled correctly. Green’s job is to make sure the process flows smoothly from station to station.

Green started working as a cashier at a drugstore when he was in high school and gradually became inter- ested in the pharmacy itself.

“I was interested in how drugs work, how they can help people and improve their health,” he says.

Having earned a bachelor’s degree in biology, Green emphasizes that pharmacy technicians must have a good grasp of the sciences, especially basic chemistry and anatomy and phys- iology, to help them understand each drug’s chemical makeup and properties.

“When all the data is entered, we see what potential side effects there are,” he says. “It’s important to know how the medications work, how they interact with each other, how they interact with the body. I might see something and bring it to the pharma- cist’s attention.” In addition, commu- nication skills and the ability to work with people are important. Good com- munication can be the difference between life and death for a patient, particularly when a doctor prescribes a

Chapter 2: Basic Chemistry 83

protons in any atom is equal to its number of electrons.

d. The atomic mass number is equal to the sum of the protons and neutrons in the atom’s nucleus.

e. Isotopes are different atomic forms of the same element; they differ only in the number of neu- trons. Many of the heavier isotopes are unstable and decompose to a more stable form by eject- ing particles or energy from the nucleus, a phe- nomenon called radioactivity. Such radioisotopes are useful in medical diagnosis and treatment and in biochemical research.

f. The atomic weight is approximately equal to the mass number of the most abundant isotope of any element.

Molecules and Compounds (pp. 57–58)

1. A molecule is the smallest unit resulting from the bonding of two or more atoms. If the atoms are dif- ferent, a molecule of a compound is formed.

2. Compounds exhibit properties different from those of the atoms that comprise them.

Chemical Bonds and Chemical reactions (pp. 58–64)

1. Bond formation

a. Chemical bonds are energy relationships. Electrons in the outermost energy level (valence shell) of the reacting atoms are active in the bonding.

b. Atoms with a stable valence shell (two electrons in shell 1, or eight in the subsequent shells) are chemically inactive. Those with an incomplete valence shell interact by losing, gaining, or shar- ing electrons to achieve stability (that is, to either fill the valence shell or meet the rule of eight).

c. Ions are formed when valence electrons are completely transferred from one atom to another. The oppositely charged ions thus formed attract each other, forming an ionic bond. Ionic bonds are common in salts.

d. Covalent bonds involve the sharing of electron pairs between atoms. If the electrons are shared equally, the molecule is a nonpolar covalent molecule. If the electrons are not shared equally, the molecule is a polar covalent molecule. Polar molecules orient themselves toward charged particles and other molecules.

summary

Concepts of Matter and Energy (pp. 50–52)

1. Matter

a. Matter is anything that occupies space and has mass.

b. Matter exists in three states: gas, liquid, and solid.

2. Energy

a. Energy is the capacity to do work or to move matter. Energy has kinetic (active) and potential (stored) work capacities.

b. Energy forms that are important in body func- tion include chemical, electrical, mechanical, and radiant.

c. Energy forms can be converted from one form to another, but some energy is always unusable (lost as heat) in such transformations.

Composition of Matter (pp. 52–57)

1. Elements and atoms

a. Each element is a unique substance that cannot be decomposed into simpler substances by ordi- nary chemical methods. A total of 118 elements exists; they differ from one another in their chemical and physical properties.

b. Four elements (carbon, hydrogen, oxygen, and nitrogen) make up 96 percent of living matter. Several other elements are present in small or trace amounts.

c. The building blocks of elements are atoms. Each atom is designated by an atomic symbol consist- ing of one or two letters.

2. Atomic structure

a. Atoms are composed of three subatomic parti- cles: protons, electrons, and neutrons. Protons are positively charged, electrons are negatively charged, and neutrons are neutral.

b. The planetary model of the atom portrays all the mass of the atom (protons and neutrons) concentrated in a central nucleus. Electrons orbit the nucleus along specific orbits. The orbital model also locates protons and neutrons in a central nucleus, but it depicts electrons as occu- pying areas of space called orbitals and forming an electron cloud of negative charge around the nucleus.

c. Each atom can be identified by an atomic num- ber, which is equal to the number of protons contained in the atom’s nucleus. Because all atoms are electrically neutral, the number of

84 Essentials of Human Anatomy and Physiology

clotting, transport of oxygen by hemoglobin, metabolism, and many other reactions. Addition- ally, calcium salts (as bone salts) contribute to bone hardness.

d. Acids are proton donors. When dissolved in water, they release hydrogen ions. Strong acids dissociate completely; weak acids dissociate incompletely.

e. Bases are proton acceptors. The most important inorganic bases are hydroxides. Bicarbonate ions are important bases in the body that act as buf- fers. When bases and acids interact, neutraliza- tion occurs—that is, a salt and water are formed.

f. The relative concentrations of hydrogen and hydroxide ions in various body fluids are mea- sured using a pH scale. Each change of one pH unit represents a tenfold change in hydrogen ion concentration. A pH of 7 is neutral (that is, the concentrations of hydrogen and hydroxide ions are equal). A pH below 7 is acidic; a pH above 7 is alkaline (basic).

g. Normal blood pH ranges from 7.35 to 7.45. Slight deviations outside this range can be fatal.

2. Organic compounds

a. Organic compounds are the carbon-containing compounds that comprise living matter. Carbohydrates, lipids, proteins, and nucleic acids all contain carbon, oxygen, and hydrogen. Proteins and nucleic acids also contain substan- tial amounts of nitrogen, and nucleic acids also contain phosphorus.

b. Carbohydrates contain carbon, hydrogen, and oxygen in the general relationship of two hydro- gen atoms to one oxygen atom and one carbon atom. Their building blocks are monosaccharides. Monosaccharides include glucose, fructose, galac- tose, deoxyribose, and ribose. Disaccharides include sucrose, maltose, and lactose; and poly- saccharides include starch and glycogen. Carbohydrates are ingested as sugars and starches. Carbohydrates, and in particular glucose, are the major energy source for the formation of ATP.

c. Lipids include triglycerides (glycerol plus three fatty acid chains), phospholipids, and steroids (the most important of which is cholesterol). Triglycerides (neutral fats) are found primarily in fatty tissue, where they provide insulation and reserve body fuel. Phospholipids and cholesterol are found in all cell membranes. Cholesterol also forms the basis of certain hormones, bile salts, and vitamin D. Like carbohydrates, the lipids are degraded by hydrolysis and synthesized by dehydration synthesis.

e. Hydrogen bonds are fragile bonds that bind together water molecules or different parts of the same molecule (intramolecular bonds) but do not involve electrons. They are common in large, complex organic molecules, such as pro- teins and nucleic acids.

2. Patterns of chemical reactions

a. Chemical reactions involve the formation or breaking of chemical bonds. They are indicated by a chemical equation, which provides infor- mation about the atomic composition (formula) of the reactant(s) and product(s).

b. Chemical reactions that result in larger, more complex molecules are synthesis reactions; they involve storing energy in the bonds formed.

c. In decomposition reactions, larger molecules are broken down into simpler molecules or atoms. Bonds are broken, releasing energy.

d. Exchange reactions involve both the making and breaking of bonds. Atoms are replaced by other atoms.

e. Regardless of the type of reaction, most chemi- cal reactions are reversible. Reversibility is indi- cated by a double arrow.

3. Factors increasing the rate of chemical reactions

a. For atoms to interact chemically, they must col- lide forcefully.

b. Factors that affect the number or force of colli- sions include the temperature, concentration of the reactants, particle size, and catalysts (enzymes).

Biochemistry: the Chemical Composition of Living Matter (pp. 64–81)

1. Inorganic compounds

a. Inorganic compounds making up living matter do not contain carbon (exceptions include CO2 and CO). They include water, salts, and some acids and bases.

b. Water is the single most abundant compound in the body. It acts as a universal solvent in which electrolytes (salts, acids, and bases) ionize and in which chemical reactions occur, and it is the basis of transport and lubricating fluids. It slowly absorbs and releases heat, thus helping to main- tain homeostatic body temperature, and it pro- tects certain body structures (such as the brain) by forming a watery cushion. Water is also a reactant in hydrolysis reactions.

c. Salts in ionic form (electrolytes) are involved in nerve transmission, muscle contraction, blood

Chapter 2: Basic Chemistry 85

2 with the reactants and holding them in the proper position to interact. Enzymes do not become part of the product. Many enzymes are produced in an inactive form or are inactivated immediately after use.

h. Nucleic acids include deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The monomer of nucleic acids is the nucleotide; each nucleotide consists of a nitrogen-containing base, a sugar (ribose or deoxyribose), and a phosphate group. DNA (the “stuff” of the genes) maintains genetic heritage by replicating itself before cell division and specifying protein structure. RNA executes the instructions of the DNA during protein synthesis.

i. ATP (adenosine triphosphate) is the universal energy compound used by all body cells. Some of the energy liberated by the oxidation of glu- cose is captured in the high-energy phosphate bonds of ATP molecules and stored for later use. Some liberated energy is lost as heat.

d. Proteins are constructed from building blocks called amino acids; 20 amino acids are found in body proteins.

e. Levels of protein structure include the amino acid sequence (primary); the alpha helix and beta-pleated sheet (secondary); a three-dimen- sional structure superimposed on secondary structure(s) (tertiary); and a globular structure formed by two or more polypeptide chains (quaternary). Different amino acid sequences result in the construction of different proteins.

f. Fibrous, or structural, proteins are the basic structural materials of the body. Globular pro- teins are also called functional proteins; exam- ples of these include enzymes, some hormones, and hemoglobin. Functional proteins become denatured and inactivated when their hydrogen bonds are disrupted.

g. Enzymes increase the rates of chemical reac- tions by binding temporarily and specifically

review Questions

Multiple Choice

More than one choice may apply.

1. Which of the following is (are) true concerning energy?

a. Energy has mass and takes up space.

b. Energy can be kinetic and move matter.

c. Chemical energy is potential energy stored in chemical bonds.

d. Mechanical energy is indirectly involved in moving matter.

e. Radiant energy directly involves electromagnetic waves.

2. Pick out the correct match(es) of element and number of valence electrons. Draw a planetary model of each atom to help you choose the best answer.

a. Carbon—6

b. Calcium—2

c. Iodine—7

d. Oxygen—6

e. Sodium—2

3. Important functions of water include which of the following?

a. Provides cushioning

b. Acts as a transport medium

c. Participates in chemical reactions

d. Acts as a solvent for sugars, salts, and other solutes

e. Reduces temperature fluctuations

4. Acidic substances include which of the following?

a. Blood

b. Water

c. Gastric juice

d. Egg white

e. Lye

5. Glucose is to starch as

a. a steroid is to a lipid.

b. a nucleotide is to a nucleic acid.

c. an amino acid is to a protein.

d. a polypeptide is to an amino acid.

6. Which of the following is an acidic amino acid?

a. Glycine

b. Aspartic acid

c. Cysteine

d. Valine

e. Lysine

7. Which of the following molecules is (are) present in RNA but not in DNA?

a. Ribose

b. Deoxyribose

c. Phosphate group

d. Adenine

e. Uracil

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

86 Essentials of Human Anatomy and Physiology

18. Fill in the following table to fully describe an atom’s subatomic particles.

Particle Position in the atom Charge Mass

Proton

Neutron

Electron

19. Define radioactivity. If an element has three isotopes, which of them (the lightest, the one with an intermediate mass, or the heaviest) is most likely to be a radioisotope, and why?

20. Define molecule as it relates to molecular substances and compounds.

21. Explain the basis of ionic bonding.

22. What are hydrogen bonds, and how are they important in the body?

23. Being polar, water molecules play a very important role in chemical reactions. Explain the polar nature of water molecules.

24. The formation of sodium chloride, common table salt, is an example of ionic bonding. Explain why.

25. Identify each of the following reactions as a synthesis, decomposition, or exchange reaction:

2H2 + O2 S 2H2O MgCl2 S Mg2+ + Cl2

Zn2+ + CuCl2 S ZnCl2 + Cu2+

Li2CO3 S Li2O + CO2 26. Distinguish inorganic from organic compounds,

and list the major categories of each in the body.

27. Salts, acids, and bases are electrolytes. What is an electrolyte?

28. Define pH. The pH range of blood is from 7.35 to 7.45. Circle the correct answer to complete the sentence: This is slightly (acidic / basic).

29. Explain why a pH value of 6.3 is 1000 times less acidic than a pH of 3.3.

30. Define monosaccharide, disaccharide, and polysaccharide. Give at least two examples of each. What is the primary function of carbohydrates in the body?

8. ATP is not associated with

a. a basic nucleotide structure.

b. high-energy phosphate bonds.

c. deoxyribose.

d. inorganic phosphate.

e. reversible reactions.

9. The element essential for normal thyroid function is

a. iodine.

b. iron.

c. copper.

d. selenium.

e. zinc.

10. Which of the following is (are) true concerning enzymes?

a. They are very specific in their activities.

b. They cannot be reused.

c. They contain active sites.

d. They are needed in large amounts in cells.

short Answer Essay

11. Why is a study of basic chemistry essential to understanding human physiology?

12. Matter occupies space and has mass. Explain how energy must be described in terms of these two factors. Then define energy.

13. Explain the relationship between kinetic and potential energy.

14. Identify the energy form involved in each of the following examples:

a. Knee movements when kicking a ball

b. Digesting food

c. Ions moving across cell membranes

d. UV waves causing sunburn

15. According to Greek history, a Greek scientist went running through the streets announcing that he had transformed lead into gold. Both lead and gold are elements. On the basis of what you know about the nature of elements, explain why his rejoicing was short-lived.

16. Name and provide the atomic symbols of the four elements that make up the bulk of all living matter. Which of these is found primarily in proteins and nucleic acids?

17. All atoms are neutral. Explain the basis of this fact.

Chapter 2: Basic Chemistry 87

2 36. How is the secondary structure of a protein

molecule established? How does this differ from the way the tertiary structure is established?

37. What is ATP’s central role in the body?

38. Explain why you can “stack” water slightly above the rim of a glass if you pour the water in very carefully.

39. Water is a precious natural resource all over the world, and it is said that supplies are dwindling. Desalinization (salt removal) of ocean water has been recommended as a solution to the problem. Why shouldn’t we drink salt water?

40. Explain the unique chemical makeup and characteristics of phospholipids and what they have to do with the terms hydrophilic and hydrophobic.

31. What are the general structures of triglycerides, phospholipids, and steroids? Give one or two important uses of each of these lipid types in the body.

32. The building block of proteins is the amino acid. Draw a diagram of the structure of a generalized amino acid. What is the importance of the R-group?

33. Name the two protein classes based on structure and function in the body, and give two examples of each.

34. Define enzyme, and describe enzyme action.

35. Virtually no chemical reaction can occur in the body in the absence of enzymes. How might excessively high body temperature interfere with enzyme activity?

Critical thinking and Clinical Application Questions

41. Several antibiotics act by binding to certain essential enzymes in the target bacteria. How might these antibiotics influence the chemical reaction controlled by the enzyme? What might be the effect on the bacteria? On the person taking the antibiotic prescription?

42. Mrs. Roberts, who is in a diabetic coma, has just been admitted to Noble Hospital. Her blood pH indicates that she is in severe acidosis (blood pH in the acid range), and the medical staff quickly institute measures to bring her blood pH back within normal limits. Note the normal pH of blood, and discuss why severe acidosis is a problem.

43. Stephen wants to bring some changes in his diet in order to make it more heart healthy. His friend

advises him to choose the fats in his diet correctly. How can he improve his intake of fats? Why are some fats good for the heart and some are not?

44. Pediatricians become concerned about the potential for brain damage when an infant’s temperature approaches 105°F. Which class of organic molecules is most likely to be damaged by high temperature? Explain why.

45. The genetic error that causes sickle-cell anemia begins at the DNA level and results in the synthesis of hemoglobin with one amino acid difference compared to normal hemoglobin. Explain how changing one amino acid could affect hemoglobin function.

Cells and Tissues

PART I: Cells Just as bricks and lumber are used to build a house, cells are the structural units of all living things, from one-celled organisms such as amoe- bas to complex multicellular organisms such as humans, dogs, and trees. The human body con- tains 50 to 100 trillion of these tiny building blocks.

In this chapter, we focus on structures and functions shared by all cells. We consider special- ized cells and their unique functions in later chapters.

Overview of the Cellular Basis of life ➔ Learning Objectives

□□ Name and describe the four concepts of the cell theory.

□□ list four elements that make up the bulk of living matter.

In the late 1600s, Robert Hooke was looking through a crude microscope at some plant tissue—cork. He saw some cubelike structures that reminded him of the long rows of monk’s rooms (or cells) at the monastery, so he named these structures “cells.” The living cells that had formed the cork were long since

3 WHAT

HOW

WHY

The cell is the structural and functional unit of the human body.

Cells carry out the chemical activities needed to sustain life, and they divide

to form or repair tissues.

The many types of tissues formed by cells

allow for the division of labor among the body

systems.

INsTRuCTORs

New Building Vocabulary Coaching Activities for this chapter are assignable in

Chapter 3: Cells and Tissues 89

3 Although no one cell type is exactly like all others, cells do have the same basic parts, and there are certain functions common to all cells. Here we will talk about the generalized cell, which demon- strates most of these typical features.

In general, all cells have three main regions or parts—a nucleus (nu′kle-us), a plasma membrane, and the cytoplasm (si′to-plazm″) (Figure 3.1a). The nucleus is usually located near the center of the cell. It is surrounded by the semifluid cytoplasm, which in turn is enclosed by the plasma mem- brane, which forms the outer cell boundary. (If you look ahead to Figure 3.4 on p. 94, you can see a more detailed illustration of generalized cell structure.)

The Nucleus Anything that works, works best when it is con- trolled. For cells, “headquarters,” or the control center, is the nucleus (nucle = kernel). The genetic material, or deoxyribonucleic acid (DNA), is a blueprint that contains all the instructions needed for building the whole body; so, as you might expect, human DNA differs from frog DNA. More specifically, DNA has genes, which carry the instructions for building proteins. DNA is also absolutely necessary for cell reproduction. A cell that has lost or ejected its nucleus (for whatever reason) is destined to “self-destruct.”

Although the nucleus is most often oval or spherical, its shape usually conforms to the shape of the cell. For example, if the cell is elongated, the nucleus is usually elongated as well. The nucleus has three recognizable regions or structures: the nuclear envelope, nucleolus, and chromatin.

Nuclear envelope

The nuclear boundary is a double membrane bar- rier called the nuclear envelope, or nuclear membrane (Figure 3.1b). Between the two mem- branes is a fluid-filled “moat,” or space. At various points, the two layers of the nuclear envelope fuse, generating openings called nuclear pores. Like other cellular membranes, the nuclear envelope allows some but not all substances to pass through it, but substances pass through it much more freely than elsewhere because of its relatively large pores. The nuclear membrane encloses a jellylike fluid called nucleoplasm (nu′kle-o-plazm″) in which other nuclear elements are suspended.

dead; only the plant cell walls remained. However, the name stuck and is still used to describe the smallest unit of all living things.

Since the late 1800s, cell research has been exceptionally fruitful and has provided us with the following four concepts collectively known as the cell theory:

• A cell is the basic structural and functional unit of living organisms. So, when you define cell proper- ties, you are in fact defining the properties of life.

• The activity of an organism depends on the collective activities of its cells.

• According to the principle of complementarity, the activities of cells are dictated by their struc- ture (anatomy), which determines function (physiology).

• Continuity of life has a cellular basis.

We will expand on all of these concepts as we go along. Let’s begin with the idea that the cell is the smallest living unit. Whatever its form, however it behaves, the cell contains all the parts necessary to survive in a changing world. It follows, then, that loss of cell homeostasis underlies virtually every disease.

Perhaps the most striking thing about a cell is its organization. Yet if we chemically analyze cells, we find that they are made up primarily of the same four elements—carbon, oxygen, hydrogen, and nitrogen—plus much smaller amounts of several other elements. (A detailed account of body chem- istry appears in Chapter 2.)

Strange as it may seem, especially when we feel our firm muscles, living cells are about 60 per- cent water, which is one of the reasons water is essential for life.

Did You Get It? 1. Define cell. 2. According to the cell theory, what the organism can

do depends on _______. (Fill in the blank.)

For answers, see Appendix A.

Anatomy of a Generalized Cell ➔ Learning Objectives

□□ Define generalized cell.

□□ Identify on a cell model or diagram the three major cell regions (nucleus, cytoplasm, and plasma membrane).

□□ list the structures of the nucleus, and explain the function of chromatin and nucleoli.

90 Essentials of Human Anatomy and Physiology

The Plasma Membrane ➔ Learning Objectives

□□ Describe the chemical composition of the plasma membrane, and relate it to membrane functions.

□□ Compare the structure and function of tight junctions, desmosomes, and gap junctions.

The flexible plasma membrane is a fragile, transparent barrier that contains the cell contents and separates them from the surrounding envi- ronment. (The terms cell membrane or cytoplas- mic membrane are sometimes used instead, but because nearly all cellular organelles are com- posed of membranes, in this text we will always refer to the cell’s surface or outer limiting mem- brane as the plasma membrane.) Although the plasma membrane is important in defining the limits of the cell, it is much more than a passive envelope, or “baggie.” As you will see, its unique structure allows it to play a dynamic role in many cellular activities.

The Fluid Mosaic Model

The structure of the plasma membrane consists of two phospholipid (fat) layers arranged “tail to tail,” with cholesterol and floating proteins scattered among them (Figure 3.2). Some phospholipids may also have sugar groups attached, forming glycolip- ids. The proteins, some of which are free to move

Nucleolus

The nucleus contains one or more small, dark- staining, essentially round bodies called nucleoli (nu-kle′o-li; “little nuclei”). Nucleoli (plural for nucleolus) are sites where cell structures called ribosomes are assembled. Most ribosomes eventu- ally migrate into the cytoplasm, where they serve as the actual sites of protein synthesis.

Chromatin

When a cell is not dividing, its DNA is carefully wound around proteins called histones to form a loose network of “beads on a string” called chro- matin (kro′mah-tin) that is scattered throughout the nucleus. When a cell is dividing to form two daughter cells, the chromatin threads coil and con- dense to form dense, rodlike bodies called chro- mosomes (chromo = colored, soma = body)— much the way a stretched spring becomes shorter and thicker when allowed to relax. We discuss the functions of DNA and the events of cell division in the Cell Physiology section (beginning on p. 102).

Did You Get It? 3. Name the three basic parts of a cell, and give the

location of each. 4. How would you explain the meaning of a

“generalized cell” to a classmate? 5. What is the general function of nucleoli?

For answers, see Appendix A.

Chromatin

Nuclear envelope

Nucleus Nucleolus

Nuclear pores

(a) Generalized animal cell

(b) Nucleus

Nucleus

Cytoplasm

Plasma membrane

Figure 3.1 Anatomy of the generalized animal cell nucleus. (a) Orientation diagram: the three main regions of the generalized cell. (b) Structure of the nucleus surrounded by rough ER.

Chapter 3: Cells and Tissues 91

reseal themselves quickly when torn. The hydro- phobic makeup of the membrane interior makes the plasma membrane relatively impermeable to most water-soluble molecules. The cholesterol helps to both stabilize the membrane and keep it flexible.

The proteins scattered in the lipid bilayer are responsible for most of the specialized functions of the membrane. Some proteins are enzymes. Many of the proteins protruding from the cell exterior are receptors for hormones or other chemical messengers or are binding sites for anchoring the cell to fibers or to other structures inside or outside the cell. Most proteins that span the membrane are involved in transport. For exam- ple, some cluster together to form protein chan- nels (tiny pores) through which water and small water-soluble molecules or ions can move; others act as carriers that bind to a substance and move it through the membrane. Branching sugar groups are attached to most of the proteins abutting the extracellular space. Such “sugar-proteins” are called glycoproteins, and because of their pres- ence, the cell surface is a fuzzy, sticky, sugar-rich area called the glycocalyx (gli-ko-ka′liks). (You can think of your cells as being sugar coated.) Among other things, these glycoproteins determine

and bob in the lipid layer, form a constantly chang- ing pattern or mosaic, hence the name of the model that describes the plasma membrane.

Sugar group

Bimolecular lipid layer containing proteins

Polar heads of phospholipid molecules

Nonpolar tails of phospholipid molecules

Filaments of cytoskeleton

Channel

Cytoplasm (watery environment)

Glycoprotein

Cholesterol

Extracellular fluid (watery environment)

Glycolipid

Proteins

Figure 3.2 structure of the plasma membrane.

Some proteins float freely in the lipid bilayer of the membrane, whereas others are anchored in specific locations. What structure(s) could anchor the stationary proteins?Q:

Filaments of the cytoskeleton attached to membrane proteins.A:

➔ConCeptLinkRemember, phospholipids are polar molecules: The charged end interacts with water, and the fatty acid chains do not (see Chapter 2, p. 72). It is this property of polarity that makes phospholipids a good founda- tion for cell membranes. ➔

The olive oil–like phospholipid bilayer forms the basic “fabric” of the membrane. The polar “heads” of the lollipop-shaped phospholipid mol- ecules are hydrophilic (“water loving”) and are attracted to water, the main component of both the intracellular and extracellular fluids, and so they lie on both the inner and outer surfaces of the membrane. Their nonpolar fatty acid “tails,” being hydrophobic (“water fearing”), avoid water and line up in the center (interior) of the membrane. The self-orienting property of the phospholipids allows biological membranes to

92 Essentials of Human Anatomy and Physiology

Because this last factor is the most important, let us look more closely at the main types of junctions: tight junctions, desmosomes, and gap junctions.

• Tight junctions are impermeable junctions that encircle the cells and bind them together into leakproof sheets. In tight junctions, adja- cent plasma membranes fuse together tightly like a zipper and prevent substances from passing through the extracellular space be- tween cells. In the small intestine, for example, these junctions prevent digestive enzymes from seeping into the bloodstream.

• Desmosomes (dez′mo-sōmz) are anchoring junctions scattered like rivets along the sides of adjacent cells. They prevent cells sub- jected to mechanical stress (such as heart muscle cells and skin cells) from being pulled apart. Structurally, these junctions are buttonlike thickenings of adjacent plasma membranes (plaques) that are connected by fine protein filaments. Thicker protein fila- ments extend from the plaques inside the cells to the plaques on the cells’ opposite sides, thus forming an internal system of strong “guy wires.”

• Gap junctions, or communicating junc- tions, function mainly to allow communica- tion. These junctions are commonly found in the heart and between embryonic cells. In gap junctions, the neighboring cells are con- nected by hollow cylinders composed of proteins (called connexons) that span the entire width of the abutting membranes (which are therefore called transmembrane proteins). Chemical molecules, such as nutri- ents or ions, can pass directly through the water-filled connexon channels from one cell to another.

Did You Get It? 6. Why do phospholipids (which form the bulk of

plasma membranes) organize into a bilayer, tail to tail, in a watery environment?

7. The external faces of some membrane proteins have sugar groups attached to them. What are three roles these sugar-coated proteins play in the life of a cell?

8. What is the special function of gap junctions? Of tight junctions?

For answers, see Appendix A.

your blood type, act as receptors that certain bac- teria, viruses, or toxins can bind to, and play a role in cell-to-cell recognition and interactions. Definite changes in glycoproteins occur in cells that are being transformed into cancer cells. (We discuss cancer in “A Closer Look” on pp. 130–131.)

Cell Membrane Junctions

Although certain cell types—blood cells, sperm cells, and some phagocytic cells (which ingest bacteria and foreign debris)—are “footloose” in the body, many other types, particularly epithelial cells, are knit into tight communities. Typically, cells are bound together in three ways:

• Glycoproteins in the glycocalyx act as an adhe- sive or cellular glue.

• Wavy contours of the membranes of adjacent cells fit together in a tongue-and-groove fashion.

• Special cell membrane junctions are formed (Figure 3.3). These junctions vary structurally depending on their roles.

Microvilli Tight (impermeable) junction

Plasma membranes of adjacent cells

Gap (communicating) junction

Underlying basement membrane

Extracellular space between cells

Desmosome (anchoring junction)

Connexon

Figure 3.3 Cell junctions. An epithelial cell is shown joined to adjacent cells by the three common types of cell junctions: tight junctions, desmosomes, and gap junctions. Also illustrated are microvilli, which increase the cell’s surface area for absorption (seen projecting from the free cell surface).

Chapter 3: Cells and Tissues 93

3 The Cytoplasm ➔ Learning Objective

□□ Identify the organelles on a cell model or describe them, and indicate the major function of each.

The cytoplasm is the cellular material outside the nucleus and inside the plasma membrane. It is the site of most cellular activities, so you might think of the cytoplasm as the “factory floor” of the cell. Although early scientists believed that the cyto- plasm was a structureless gel, the electron micro- scope has revealed that it has three major compo- nents: the cytosol, inclusions, and organelles. Let’s take a look at each of these.

Cytosol and Inclusions

The cytosol is semitransparent fluid that suspends the other elements. Dissolved in the cytosol, which is largely water, are nutrients and a variety of other solutes (sol′yūtz; dissolved substances).

Inclusions are chemical substances that may or may not be present, depending on the specific cell type. Most inclusions are stored nutrients or cell products floating in the cytosol. They include the lipid droplets common in fat cells, glycogen gran- ules abundant in liver and muscle cells, pigments such as melanin in skin and hair cells, mucus and other secretory products, and various kinds of crys- tals. It may help to think of an inclusion as a cellular “pantry” where items are kept on hand until needed.

Organelles

The organelles (or″gah-nelz′; “little organs”) are specialized cellular compartments (Figure 3.4, p. 94) that are the metabolic machinery of the cell. Each type of organelle is specialized to carry out a specific function for the cell as a whole, much like the organs carry out specialized functions for the whole body. Some synthesize proteins, others package those proteins, and so on.

Many organelles are bounded by a membrane similar to the plasma membrane. These membrane boundaries allow organelles to maintain an inter- nal environment quite different from that of the surrounding cytosol. This compartmentalization is crucial to their ability to perform their specialized functions for the cell. Let’s consider what goes on in each of these workshops of our cellular factory.

Mitochondria Mitochondria (mi″to-kon′dre-ah; singular: mitochondrion) are usually depicted as

tiny, lozenge-like or sausage-shaped organelles (see Figure 3.4), but in living cells they lengthen and change shape almost continuously. The mitochon- drial wall consists of a double membrane, equal to two plasma membranes placed side by side. The outer membrane is smooth and featureless, but the inner membrane has shelflike protrusions called cristae (kris′te; “crests”). Enzymes dissolved in the fluid within the mitochondria, as well as enzymes that form part of the cristae membranes, carry out the reactions in which oxygen is used to break down foods. As the foods are broken down, energy is released. Much of this energy escapes as heat, but some is captured and used to form ATP molecules.

ATP provides the energy for all cellular work, and every living cell requires a constant supply of ATP for its many activities. Because the mitochon- dria supply most of this ATP, they are the “power- houses” of the cell. Metabolically “busy” cells, such as liver and muscle cells, use huge amounts of ATP and have hundreds of mitochondria, which replicate themselves by pinching in half. By con- trast, cells that are relatively inactive (an unfertil- ized egg, for instance) have fewer.

Ribosomes Ribosomes (ri′bo-sōmz) are tiny, bilobed, dark bodies made of proteins and one vari- ety of RNA called ribosomal RNA. Ribosomes are the actual sites of protein synthesis in the cell. Ribosomes that float freely in the cytoplasm manufacture pro- teins that function inside the cell, while others attach to membranes such as the rough ER, which pro- duces proteins that function outside the cell.

Endoplasmic Reticulum The endoplasmic retic- ulum (en″do-plas′mik rĕ-tik′u-lum; “network within the cytoplasm”), or ER, is a system of fluid-filled tunnels (or canals) that coil and twist through the cytoplasm. It is continuous with the nuclear enve- lope and accounts for about half of a cell’s mem- branes. It serves as a mini circulatory system for the cell because it provides a network of channels for carrying substances (primarily proteins) from one part of the cell to another. There are two forms of ER, rough and smooth (see Figure 3.4); a partic- ular cell may have both forms or only one, depend- ing on its specific functions.

The rough endoplasmic reticulum is so called because it is studded with ribosomes. Because essentially all of the building materials of cellular membranes are formed either in it or on it,

94 Essentials of Human Anatomy and Physiology

export (secrete) proteins—for example, pancreatic cells, which produce digestive enzymes to be delivered to the small intestine. The enzymes that catalyze the synthesis of membrane lipids reside on the external (cytoplasmic) face of the rough ER, where the needed building blocks are readily available.

Although the smooth endoplasmic reticu- lum communicates with the rough variety, it plays no role in protein synthesis, because it lacks ribo- somes. Instead it functions in lipid metabolism (cholesterol and fat synthesis and breakdown) and detoxification of drugs and pesticides. Hence it is

you can think of the rough ER as the cell’s mem- brane factory. The proteins made on its ribosomes migrate into the rough ER tunnels, where they fold into their functional three-dimensional shapes. These proteins are then dispatched to other areas of the cell in small “sacs” of membrane called transport vesicles (Figure 3.5) that carry sub- stances around the cell. Rough ER is especially abundant in cells that make (synthesize) and

Chromatin. Nucleoli.A:

Secretion being released from cell by exocytosis

Peroxisome

Ribosomes

Rough endoplasmic reticulum

Nucleus

Nuclear envelopeChromatin

Golgi apparatus

Nucleolus

Smooth endoplasmic reticulum

Cytosol

Lysosome

Mitochondrion

Centrioles

Microtubule

Intermediate filaments

Plasma membrane

Figure 3.4 structure of the generalized cell. No cell is exactly like this one, but this generalized cell drawing illustrates features common to many human cells.

Which nuclear component contains your genes? Which produces protein-building “factories”?Q:

Chapter 3: Cells and Tissues 95

containing proteins and phospholipids destined for a “home” in the plasma membrane (pathway 2 in Figure 3.6) or other cellular membranes. It also packages hydrolytic enzymes into membrane- bound organelles called lysosomes that remain in the cell (pathway 3 in Figure 3.6 and discussed next).

Lysosomes Lysosomes (li′so-sōmz; “breakdown bodies”), which appear in different sizes, are mem- branous “bags” containing powerful digestive enzymes. Because lysosomal enzymes are capable of digesting worn-out or nonusable cell structures and most foreign substances that enter the cell, lysosomes function as cellular “stomachs.” Lysosomes are especially abundant in white blood cells called phagocytes, the cells that dispose of bacteria and cell debris. As we mentioned, the enzymes they contain are formed by ribosomes on the rough ER and packaged by the Golgi apparatus.

Homeostatic Imbalance 3.1

The lysosomal membrane is ordinarily quite sta- ble, but it becomes fragile when the cell is injured or deprived of oxygen and when excessive amounts of vitamin A are present. When lyso- somes rupture, the cell self-digests. ____________✚

not surprising that the liver cells are chock-full of smooth ER. So too are body cells that produce steroid-based hormones—for instance, cells of the male testes that manufacture testosterone.

Golgi Apparatus The Golgi (gol′je) apparatus appears as a stack of flattened membranous sacs that are associated with swarms of tiny vesicles. It is generally found close to the ER and is the prin- cipal “traffic director” for cellular proteins. Its major function is to modify, package, and ship proteins (sent to it by the rough ER via transport vesicles) in specific ways, depending on their final destination (Figure 3.6). Initially, all proteins leav- ing the Golgi apparatus accumulate in sacs called Golgi vesicles.

As proteins “tagged” for export accumulate in the Golgi apparatus, the sacs swell. Then their swollen ends, filled with protein, pinch off and form secretory vesicles (ves′ ı̆-kulz), which travel to the plasma membrane. When the vesicles reach the plasma membrane, they fuse with it, the mem- brane ruptures, and the contents of the sac are ejected to the outside of the cell (pathway 1 in Figure 3.6). Mucus is packaged this way, as are digestive enzymes made by pancreatic cells.

In addition to its packaging-for-release functions, the Golgi apparatus pinches off sacs

Ribosome

Protein

Protein inside transport vesicle

Transport vesicle buds off

mRNA

Rough ER

1 2

As the protein is synthesized on the ribosome, it migrates into the rough ER tunnel system.

In the tunnel, the protein folds into its functional shape. Short sugar chains may be attached to the protein (forming a glycoprotein).

The transport vesicle buds from the rough ER and travels to the Golgi apparatus for further processing.

The protein is packaged in a tiny membranous sac called a transport vesicle.

Figure 3.5 synthesis and export of a protein by the rough eR.

96 Essentials of Human Anatomy and Physiology

half, like mitochondria, but most peroxisomes appear to bud directly from the ER.

Did You Get It? 9. How do the cytosol and the cytoplasm differ?

10. Which two organelles are sacs of enzymes, and what is the function of each of these organelles?

11. Which organelle is the major site of ATP synthesis? Which packages proteins?

For answers, see Appendix A.

Cytoskeleton An elaborate network of protein structures extends throughout the cytoplasm. This network, or cytoskeleton, acts as a cell’s “bones and muscles” by furnishing an internal framework that determines cell shape, supports other organ- elles, and provides the machinery for intracellular transport and various types of cellular movements. From its smallest to its largest elements, the cyto- skeleton is made up of microfilaments, intermedi- ate filaments, and microtubules (Figure 3.7). Although there is some overlap in roles, generally speaking microfilaments (such as actin and

Peroxisomes Peroxisomes (per-ok′sih-sōmz) are membranous sacs containing powerful oxidase (ok′sı̆-dāz) enzymes that use molecular oxygen (O2) to detoxify a number of harmful or poison- ous substances, including alcohol and formalde- hyde. However, their most important function is to “disarm” dangerous free radicals. Free radicals are highly reactive chemicals with unpaired elec- trons that can damage the structure of proteins and nucleic acids. Free radicals are normal by- products of cellular metabolism, but if allowed to accumulate, they can have devastating effects on cells. Peroxisomes convert free radicals to hydro- gen peroxide (H2O2), a function indicated in their naming (peroxisomes = peroxide bodies). The enzyme catalase (kat′ah-lās) then converts excess hydrogen peroxide to water. Peroxisomes are especially numerous in liver and kidney cells, which are very active in detoxification.

Although peroxisomes look like small lyso- somes (see Figure 3.4), they do not arise by bud- ding from the Golgi apparatus. Instead, one way they replicate themselves is by simply pinching in

Secretion by exocytosis Extracellular fluid

Plasma membrane

Golgi vesicle containing membrane components fuses with the plasma membrane and is incorporated into it.

Golgi vesicle containing digestive enzymes becomes a lysosome.

Proteins in tunnels

Lysosome fuses with ingested substances.

Membrane

Transport vesicle

Pathway 3

Pathway 2

Secretory vesicles Pathway 1

Golgi apparatus

Golgi vesicle containing proteins to be secreted becomes a secretory vesicle.

TunnelsRough ER

Proteins

Figure 3.6 Role of the Golgi apparatus in packaging the products of the rough eR. Protein-containing transport vesicles pinch off the rough ER and migrate

to fuse with the Golgi apparatus. As the proteins travel through the Golgi apparatus, they are sorted (and slightly modified if necessary), then packaged within vesicles. These

vesicles then leave the Golgi apparatus and head for various destinations (pathways 1–3), as shown.

Chapter 3: Cells and Tissues 97

Cell extensions In addition to the cell structures described previ- ously, some cells have obvious surface extensions. These come in two major “flavors,” or varieties, depending on whether they have a core of micro- tubules or actin filaments.

Cilia and Flagella

Cilia (sil′e-ah; “eyelashes”) are whiplike cellular extensions that move substances along the cell surface. For example, mucus is carried up and away from the lungs by “crowd surfing” on the ciliated cells lining the respiratory system. Where cilia appear, there are usually many of them pro- jecting from the exposed cell surface.

When a cell is about to make cilia, its centrioles multiply and then line up beneath the plasma membrane at the free cell surface. Microtubules then begin to “sprout” from the centrioles and put pressure on the membrane, forming the projections.

If the projections formed by the centrioles are substantially longer, they are called flagella (flah- jel′ah). The only example of a flagellated cell in the human body is the sperm, which has a single

myosin) are most involved in cell motility and in producing changes in cell shape. (You could say that cells move when they get their act(in) together.) The strong, stable, ropelike intermedi- ate filaments are made up of fibrous subunits. They help form desmosomes (see Figure 3.3) and provide internal guy wires to resist pulling forces on the cell. The tubelike microtubules are made up of repeating subunits of the protein tubulin. They determine the overall shape of a cell and the distribution of organelles. They are very important during cell division (see pp. 108–111).

Centrioles The paired centrioles (sen′tre-ōlz), collectively called the centrosome, lie close to the nucleus (see Figure 3.4). They are rod-shaped bodies that lie at right angles to each other; inter- nally they are made up of a pinwheel array of nine triplets of fine microtubules. Centrioles are best known for their role in generating microtubules and also for directing the formation of the mitotic spindle during cell division (look ahead to Figure 3.15, p. 110).

Cell parts and their structures and functions are summarized in Table 3.1 (pp. 98–100).

(a) Microfilaments (b) Intermediate filaments (c) Microtubules

Actin subunit

7 nm 10 nm 25 nm

Fibrous subunits Tubulin subunits

Microfilaments form the blue batlike network.

Intermediate filaments form the purple network surrounding the pink nucleus.

Microtubules appear as gold networks surrounding the cells’ pink nuclei.

Figure 3.7 Cytoskeletal elements support the cell and help to generate movement. Diagrammatic views (above) and photos (below). The photos are of cells with the structure of interest “tagged” by fluorescent molecules.

(Text continues on page 100.)

98 Essentials of Human Anatomy and Physiology

Cell part* structure Functions

Plasma Membrane (Figure 3.2)

Sugar group

Protein

Cholesterol

Membrane made of a double layer of lipids (phospholipids, cholesterol, and so on) with proteins embedded within. Most externally facing proteins and some lipids have attached sugar groups.

Serves as an external cell barrier and acts in transport of substances into or out of the cell. Maintains an electrical condition that is essential for functioning of excitable cells. Externally facing proteins act as receptors (for hormones, neurotransmitters, and so on), transport proteins, and in cell-to-cell recognition.

Cytoplasm Cellular region between the nuclear and plasma membranes. Consists of fluid cytosol containing dissolved solutes, organelles (the metabolic machinery of the cytoplasm), and inclusions (stored nutrients, secretory products, pigment granules).

Organelles

• Mitochondria (Figure 3.4) Rodlike, double-membrane structures; inner membrane folded into projections called cristae.

Site of aerobic respiration (the “burning” of glucose) and ATP synthesis; powerhouse of the cell.

• Ribosomes (Figures 3.4, 3.5) Dense particles consisting of two subunits, each composed of ribosomal RNA and protein. Free or attached to rough endoplasmic reticulum.

The sites of protein synthesis.

• Rough endoplasmic reticulum (Figures 3.4, 3.5)

Membranous system enclosing a cavity, the tunnel, and coiling through the cytoplasm. Externally studded with ribosomes.

Sugar groups are attached to proteins within the tunnels. Proteins are bound in vesicles for transport to the Golgi apparatus and other sites. External face synthesizes phospholipids.

• Smooth endoplasmic reticulum (Figure 3.4)

Membranous system of tunnels and sacs; free of ribosomes.

Site of lipid and steroid (cholesterol) synthesis, lipid metabolism, and drug detoxification.

• Golgi apparatus (Figures 3.4, 3.6)

A stack of flattened membranes and associated vesicles close to the ER.

Packages, modifies, and segregates proteins for secretion from the cell, inclusion in lysosomes, or incorporation into the plasma membrane.

Table 3.1 Parts of the Cell: structure and Function

*Individual cellular structures are not drawn to scale.

3 Cell part* structure Functions

• Peroxisomes (not shown) Membranous sacs of oxidase and catalase enzymes.

The enzymes detoxify a number of toxic substances such as free radicals. The most important enzyme, catalase, breaks down hydrogen peroxide.

• Lysosomes (Figures 3.4, 3.6) Membranous sacs containing acid hydrolases (powerful digestive enzymes).

Sites of intracellular digestion. The “stomach” of the cell.

• Microtubules (Figure 3.7) Cylindrical structures made of tubulin proteins.

Support the cell and give it shape. Involved in intracellular and cellular movements. Form centrioles and cilia and flagella, if present.

• Microfilaments (Figure 3.7) Fine filaments composed of the protein actin.

Involved in muscle contraction and other types of intracellular movement; help form the cell cytoskeleton and microvilli, if present.

• Intermediate filaments (Figure 3.7)

Protein fibers; composition varies. The stable cytoskeletal elements; resist mechanical forces acting on the cell.

• Centrioles (Figure 3.4) Paired cylindrical bodies, each composed of nine triplets of microtubules.

Organize a microtubule network during mitosis (cell division) to form the spindle and asters. Form the bases of cilia and flagella.

Inclusions Varied; includes stored nutrients such as lipid droplets and glycogen granules, protein crystals, pigment granules.

Storage for nutrients, wastes, and cell products.

Nucleus (Figures 3.1, 3.4)

Largest organelle. Surrounded by the nuclear envelope; contains fluid nucleoplasm, nucleoli, and chromatin.

Control center of the cell; responsible for transmitting genetic information and providing the instructions for protein synthesis.

• Nuclear envelope (Figure 3.1) Double-membrane structure pierced by large pores. Outer membrane continuous with the endoplasmic reticulum.

Separates the nucleoplasm from the cytoplasm and regulates passage of substances to and from the nucleus.

Table 3.1 (continued)

•

100 Essentials of Human Anatomy and Physiology

So far in this chapter, we have focused on a gener- alized human cell. However, the trillions of cells in the human body include over 200 different cell types that vary greatly in size, shape, and function. They include sphere-shaped fat cells, disc-shaped red blood cells, branching nerve cells, and cube- shaped cells of kidney tubules.

Depending on type, cells also vary greatly in length—ranging from 1/12,000 of an inch in the smallest cells to over a yard in the nerve cells that cause you to wiggle your toes. A cell’s shape reflects its function. For example, the flat, tilelike epithelial cells that line the inside of your cheek fit closely together, forming a living barrier that pro- tects underlying tissues from bacterial invasion.

The shapes of cells and the relative numbers of the various organelles they contain relate to specialized cell functions (Figure 3.8). Let’s take a look at some examples of specialized cells.

• Cells that connect body parts (Figure 3.8a)

• Fibroblast. This cell has an elongated shape, like the cable-like fibers that it se- cretes. It has an abundant rough ER and a large Golgi apparatus to make and secrete the protein building blocks of these fibers.

• Erythrocyte (red blood cell). This cell car- ries oxygen in the blood. Its biconcave disc shape provides extra surface area for the uptake of oxygen and streamlines the cell so it flows easily through the bloodstream. So much oxygen-carrying pigment is packed in erythrocytes that all other organ- elles have been shed to make room.

propulsive flagellum called its tail (look ahead to Figure 3.8g). Notice that cilia propel other sub- stances across a cell’s surface, whereas a flagellum propels the cell itself.

Microvilli

Microvilli (mi″kro-vil′i; “little shaggy hairs”) are tiny, fingerlike extensions of the plasma mem- brane that project from an exposed cell surface (see Figure 3.3). They increase the cell’s surface area tremendously and so are usually found on the surface of cells active in absorption such as intestinal and kidney tubule cells. Microvilli have a core of actin filaments that extend into the internal cytoskeleton of the cell and stiffen the microvillus. Note that microvilli are “alcoves” projecting off one cell surface and do not involve microtubules.

Did You Get It? 12. Which two types of cytoskeletal elements are involved

in cell mobility? 13. Which of the cytoskeletal elements is the basis of

centrioles? Of microvilli? 14. The major function of cilia is to move substances

across the free cell surface. What is the major role of microvilli?

For answers, see Appendix A.

Cell Diversity ➔ Learning Objective

□□ Apply the principle of complementarity to different cell types by comparing overall shapes, internal structures, and special functions.

Cell part* structure Functions

• Nucleolus (Figure 3.1) Dense spherical (non-membrane- bounded) bodies, composed of ribosomal RNA and proteins.

Site of ribosome subunit manufacture.

• Chromatin (Figure 3.1) Granular, threadlike material composed of DNA and histone proteins. “Beads on a string.”

DNA constitutes the genes, which carry instructions for building proteins.

Table 3.1 Parts of the Cell: structure and Function (continued)

Chapter 3: Cells and Tissues 101

Contractile filaments

Nuclei

Processes

Lipid droplet

Lysosomes

Pseudo- pods

Nucleus

Nucleus Flagellum

Nucleus

Intermediate filaments

Epithelial cells

(b) Cells that cover and line body organs

Fibroblasts No organelles

Rough ER and Golgi apparatus

Secreted fibers

Nucleus Erythrocytes

(a) Cells that connect body parts

Skeletal muscle cell

Smooth muscle cells

Rough ER

Nucleus

Nerve cell

(f) Cell that gathers information and controls body functions

(d) Cell that stores nutrients

(e) Cell that fights disease

MacrophageFat cell

Sperm

(g) Cell of reproduction

• Cells that cover and line body organs (Figure 3.8b)

• Epithelial cell. The hexagonal shape of this cell is exactly like a “cell” in a honeycomb of a beehive. This shape allows epithelial cells to pack together in sheets. An epithe- lial cell has abundant intermediate filaments and desmosomes that resist tearing when the epithelium is rubbed or pulled.

• Cells that move organs and body parts (Figure 3.8c)

• Skeletal, cardiac, and smooth muscle cells. These cells are elongated and filled with abundant contractile filaments, so they can shorten forcefully and move the bones, pump blood, or change the size of internal organs to move substances around the body.

• Cell that stores nutrients (Figure 3.8d)

• Fat cell. The huge spherical shape of a fat cell is produced by a large lipid droplet in its cytoplasm.

• Cell that fights disease (Figure 3.8e)

• White blood cells such as the macrophage (a phagocytic cell). This cell extends long pseu- dopods (“false feet”) to crawl through tissue to reach infection sites. The many lysosomes within the cell digest the infectious microor- ganisms (such as bacteria) that it “eats.”

• Cell that gathers information and controls body functions (Figure 3.8f)

• Nerve cell (neuron). This cell has long pro- cesses (extensions) for receiving messages and transmitting them to other structures in the body. The processes are covered with an extensive plasma membrane, and a plentiful rough ER synthesizes membrane components and signaling molecules called neurotransmitters.

• Cells of reproduction (Figure 3.8g)

• Oocyte (female). The largest cell in the body, this egg cell contains several cop- ies of all organelles, for distribution to the

Figure 3.8 Cell diversity. The shape of human cells and the relative abundances of their various organelles determine their function in the body. (Note that these cells are not drawn to the same scale.)

102 Essentials of Human Anatomy and Physiology

in a solution is called the solvent (or dissolving medium). Water is the body’s chief solvent. Components or substances present in smaller amounts are called solutes. The solutes in a solu- tion are so tiny that the molecules cannot be seen with the naked eye and do not settle out.

Intracellular fluid (collectively, the nucleo- plasm and the cytosol) is a solution containing small amounts of gases (oxygen and carbon diox- ide), nutrients, and salts, dissolved in water. So too is extracellular fluid, or interstitial fluid, the fluid that continuously bathes the exterior of our cells. You can think of interstitial fluid as a rich, nutritious, and rather unusual “soup.” It contains thousands of ingredients, including nutrients (amino acids, sugars, fatty acids, vitamins), regula- tory substances such as hormones and neurotrans- mitters, salts, and waste products. To remain healthy, each cell must extract from this soup the exact amounts of the substances it needs at spe- cific times and reject the rest.

The plasma membrane is a selectively perme- able barrier. Selective permeability means that a barrier allows some substances to pass through it while excluding others. Thus, it allows nutrients to enter the cell but keeps many undesirable or unnec- essary substances out. At the same time, valuable cell proteins and other substances are kept within the cell, and wastes are allowed to pass out of it.

Homeostatic Imbalance 3.2

The property of selective permeability is typical only of healthy, unharmed cells. When a cell dies or is badly damaged, its plasma membrane can no longer be selective and becomes permeable to nearly everything. We see this problem when some- one has been severely burned. Precious fluids, pro- teins, and ions “weep” (leak out) from the dead and damaged cells at the burn site. _________________✚

Substances move through the plasma mem- brane in basically two ways—passively or actively. In passive processes, substances are transported across the membrane without any energy input from the cell. In active processes, the cell provides the metabolic energy (ATP) that drives the trans- port process.

Passive Processes: Diffusion and Filtration

Diffusion (dı̆-fu′zhun) is an important means of passive membrane transport for every cell of the

daughter cells that arise when the fertilized egg divides to become an embryo.

• Sperm (male). This cell is long and stream- lined, built for swimming to the egg for fertilization. Its flagellum acts as a motile whip to propel the sperm.

Did You Get It? 15. Name the two cell types involved in connecting body

parts or regions. 16. What is the main function of a neuron?

For answers, see Appendix A.

Cell Physiology Each of the cell’s internal parts is designed to per- form a specific function for the cell. As mentioned earlier, most cells have the ability to metabolize (use nutrients to build new cell material, break down substances, and make ATP), digest foods, dispose of wastes, reproduce, grow, move, and respond to a stimulus (irritability). We consider most of these functions in detail in later chapters. (For example, we cover metabolism in Chapter 14, and the ability to react to a stimulus in Chapter 7). In this chapter, we consider only the functions of membrane transport (the means by which sub- stances get through plasma membranes), protein synthesis, and cell reproduction (cell division).

Membrane Transport ➔ Learning Objectives

□□ Define selective permeability, diffusion (including simple and facilitated diffusion and osmosis), active transport, passive transport, solute pumping, exocytosis, endocytosis, phagocytosis, pinocytosis, hypertonic, hypotonic, and isotonic.

□□ Describe plasma membrane structure, and explain how the various transport processes account for the directional movements of specific substances across the plasma membrane.

The fluid environment on both sides of the plasma membrane is an example of a solution. It is impor- tant that you really understand solutions before we dive into an explanation of membrane transport. In the most basic sense, a solution is a homoge- neous mixture of two or more components. Examples include the air we breathe (a mixture of gases), seawater (a mixture of water and salts), and rubbing alcohol (a mixture of water and alco- hol). The substance present in the largest amount

Diffusion of water through a selectively perme- able membrane such as the plasma membrane is specifically called osmosis (oz-mo′sis). Because water is highly polar, it is repelled by the (nonpo- lar) lipid core of the plasma membrane, but it can and does pass easily through special pores called aquaporins (“water pores”) created by proteins in the membrane (Figure 3.10b). Osmosis into and out of cells is occurring all the time as water moves down its concentration gradient. The movement of water across the membrane occurs quickly. Anyone administering an IV (intravenous, into the vein) solution must use the correct solution to protect the patient’s cells from life-threatening dehydration or rupture (see “A Closer Look” on p. 105).

Still another example of diffusion is facili- tated diffusion. Facilitated diffusion provides passage for certain needed substances (notably glucose) that are both lipid-insoluble and too large to pass through the membrane pores, or charged, as in the case of chloride ions passing through a membrane protein channel. Although facilitated diffusion follows the laws of diffusion—that is, the substances move down their own concentration gradients—a protein membrane protein channel is used (Figure 3.10c), or a membrane protein that acts as a carrier (Figure 3.10d) is needed to move glucose and certain other solutes passively across the membrane into the cell.

Substances that pass into and out of cells by diffusion save the cell a great deal of energy. When you consider how vitally important water, glucose, and oxygen are to cells, you can under- stand just how necessary these passive transport

body. The other passive transport process, filtra- tion, generally occurs only across capillary walls. Let’s examine how these two types of passive transport differ.

Diffusion Diffusion is the process by which molecules (and ions) move away from areas where they are more concentrated (more numerous) to areas where they are less concentrated (with fewer of them). All molecules possess kinetic energy, or energy of motion (as described in Chapter 2), and as the molecules move about randomly at high speeds, they collide and change direction with each collision. The overall effect of this erratic movement is that molecules move down their con- centration gradient (spread out). The greater the difference in concentration between the two areas, the faster diffusion occurs. Because the driving force (source of energy) is the kinetic energy of the molecules themselves, the speed of diffusion is affected by the size of the molecules (the smaller the faster) and temperature (the warmer the faster).

An example should help you understand diffu- sion. Picture yourself dropping a tea bag into a cup of boiling water, but not stirring the cup. The bag itself represents the semipermeable cell mem- brane, which lets only some molecules leave the tea bag. As the tea molecules dissolve in the hot water and collide repeatedly, they begin to “spread out” from the tea bag even though it was never stirred. Eventually, as a result of their activity, the entire cup will have the same concentration of tea molecules and will appear uniform in color. (A laboratory example that might be familiar to some students is illustrated Figure 3.9).

The hydrophobic core of the plasma mem- brane is a physical barrier to diffusion. However, molecules will diffuse through the plasma mem- brane if any of the following are true:

• The molecules are small enough to pass through the membrane’s pores (channels formed by membrane proteins).

• The molecules are lipid-soluble.

• The molecules are assisted by a membrane carrier.

The unassisted diffusion of solutes through the plasma membrane (or any selectively perme- able membrane) is called simple diffusion (Figure 3.10a). Solutes transported this way are lipid-soluble (such as fats, fat-soluble vitamins, oxygen, carbon dioxide).

Figure 3.9 Diffusion. Particles in solution move continuously and collide constantly with other particles. As a result, particles tend to move away from areas where they are most highly concentrated and to become evenly distributed, as illustrated by the diffusion of dye molecules in a beaker of water.

103

104 Essentials of Human Anatomy and Physiology

in this way eventually becomes urine. Filtration is not very selective. For the most part, only blood cells and protein molecules too large to pass through the membrane pores are held back.

Did You Get It? 17. What is the energy source for all types of diffusion? 18. What determines the net direction of any diffusion

process? 19. What are the two types of facilitated diffusion, and

how do they differ? 20. Explain the phrase “move down the concentration

gradient.”

For answers, see Appendix A.

Active Processes

Whenever a cell uses ATP to move substances across the membrane, the process is active. Substances moved actively are usually unable to pass in the desired direction by diffusion. They may be too large to pass through membrane chan- nels, the membrane may lack special protein carri- ers for their transport, they may not be able to dissolve in the fat core, or they may have to move “uphill” against their concentration gradients. The

processes really are. Glucose and oxygen continu- ally move into the cells (where they are in lower concentration because the cells keep using them up), and carbon dioxide (a waste product of cel- lular activity) continually moves out of the cells into the blood (where it is in lower concentration).

Filtration Filtration is the process by which water and solutes are forced through a membrane (or capillary wall) by fluid, or hydrostatic, pres- sure. In the body, hydrostatic pressure is usually exerted by the blood. Like diffusion, filtration is a passive process, and a gradient is involved. In fil- tration, however, the gradient is a pressure gradi- ent that actually pushes solute-containing fluid (filtrate) from the higher-pressure area through the filter to the lower-pressure area. In the kid- neys, water and small solutes filter out of the cap- illaries into the kidney tubules because the blood pressure in the capillaries is greater than the fluid pressure in the tubules. Part of the filtrate formed

Extracellular fluid

Cytoplasm

Lipid- soluble solutes

Lipid bilayer

(a) Simple diffusion of lipid-soluble solutes directly through the phospholipid bilayer

Lipid- insoluble solutes

Water molecules

Small lipid- insoluble solutes

(d) Facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes shape change in transport protein

(b) Osmosis, diffusion of water through a specific channel protein (aquaporin)

Figure 3.10 Diffusion through the plasma membrane.

What “facilitates” facilitated diffusion?Q: Carrier proteins or protein channels.A:

A CLOSER

LOOK IV Therapy and Cellular “Tonics”

Why is it essential that medical personnel give only the proper intravenous (IV), or into-the-vein, solu- tions to patients?

The tendency of a solution to hold water or “pull” water into it is called osmotic pressure. Osmotic pressure is directly related to the concentra- tion of solutes in the solution. The higher the solute concentration, the greater the osmotic pressure and the greater the tendency of water to move into the solution. Many mole- cules, particularly proteins and some ions, are prevented from diffusing through the plasma membrane. Con- sequently, any change in their con- centration on one side of the membrane forces water to move from one side of the membrane to the other, causing cells to lose or gain water. The ability of a solution to change the size and shape of cells by altering the amount of water they contain is called tonicity (ton-is′ı̆-te; ton = strength).

Isotonic (i″so-ton′ik; “same tonic- ity”) solutions (such as 5 percent glu- cose and 0.9 percent saline) have the same solute and water concentra- tions as cells do. Isotonic solutions cause no visible changes in cells, and when such solutions are infused into the bloodstream, red blood cells (RBCs) retain their normal size and disclike shape (photo a). As you might guess, interstitial fluid and most intravenous solutions are iso- tonic solutions.

If red blood cells are exposed to a hypertonic (hi″per-ton′ik) solution— a solution that contains more solutes, or dissolved substances, than there are inside the cells—the cells begin to shrink. This is because water is in higher concentration inside the cell than outside, so it travels down its concentration gradient and leaves the cell (photo b). Hypertonic solu- tions are sometimes given to patients who have edema (swelling of the feet and hands due to fluid

retention). Such solutions draw water out of the tissue spaces into the bloodstream so that the kidneys can eliminate excess fluid.

When a solution contains fewer solutes (and therefore more water) than the cell does, it is hypotonic (hi″po-ton′ik) to the cell. Cells placed in hypotonic solutions plump up rap- idly as water rushes into them (photo c). Distilled water represents the most extreme example of a hypotonic fluid. Because it contains no solutes at all, water will enter cells until they finally burst, or lyse. Hypotonic solutions are sometimes infused intravenously (slowly and with care) to rehydrate extremely dehydrated patients. In less extreme cases, drinking hypotonic fluids usu- ally does the trick.

(a) RBC in isotonic solution (b) RBC in hypertonic solution (c) RBC in hypotonic solution

105

106 Essentials of Human Anatomy and Physiology

The sodium-potassium (Na+-K+) pump alter- nately carries sodium ions (Na+) out of and potas- sium ions (K+) into the cell (Figure 3.11). This process is absolutely necessary for normal trans- mission of nerve impulses. There are more sodium ions outside the cells than inside, so those inside tend to remain in the cell unless the cell uses ATP to force, or “pump,” them out. ATP is split into ADP and Pi (inorganic phosphate), and the phosphate is then attached to the sodium-potassium pump in a process called phosphorylation. Likewise, there are more potassium ions inside cells than in the extra- cellular fluid, and potassium ions that leak out of cells must be actively pumped back inside. Because each of the pumps in the plasma membrane trans- ports only specific substances, active transport pro- vides a way for the cell to be very selective in cases where substances cannot pass by diffusion. (No pump—no transport.)

two most important active processes are active transport and vesicular transport.

Active Transport Sometimes called solute pump- ing, active transport is similar to facilitated diffu- sion in that both processes require protein carriers that interact specifically and reversibly with the substances to be transported across the mem- brane. However, facilitated diffusion is driven by the kinetic energy of the diffusing molecules, whereas active transport uses ATP to energize its protein carriers, which are called solute pumps. Amino acids, some sugars, and most ions are transported by solute pumps, and in most cases these substances move against concentration (or electrical) gradients. This is opposite to the direc- tion in which substances would naturally flow by diffusion, which explains the need for energy in the form of ATP.

Extracellular fluid

Cytoplasm

Na+

Na+ Na+

Na+

K+

Na+

Na+ Na+

ATP

ADP

Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation by ATP, which causes the pump protein to change its shape.

The shape change expels Na+ to the outside. Extracellular K+ binds, causing release of the inorganic phosphate group.

Loss of phosphate restores the original shape of the pump protein. K+ is released to the cytoplasm, and Na+ sites are ready to bind Na+ again; the cycle repeats.

Na+-K+ pump

Figure 3.11 Operation of the sodium-potassium pump, a solute pump. ATP provides the energy for a “pump” protein to move

three sodium ions out of the cell and two potassium ions into the cell. The pump moves both ions against their concentration gradients. The

numbered steps indicate the sequence of ion and phosphate binding a pump protein goes through as it expels Na+ and imports K+.

Chapter 3: Cells and Tissues 107

3 Vesicular Transport Some substances cannot get through the plasma membrane by active or passive transport. Vesicular transport, which involves help from ATP to fuse or separate membrane vesi- cles and the cell membrane, moves substances into or out of cells “in bulk” without their actually cross- ing the plasma membrane directly. The two types of vesicular transport are exocytosis and endocytosis.

Exocytosis (ek″so-si-to′sis; “out of the cell”) (Figure 3.12) is the mechanism that cells use to actively secrete hormones, mucus, and other cell products or to eject certain cellular wastes. The product to be released is first “packaged” (typically by the Golgi apparatus) into a secretory vesicle. The vesicle migrates to the plasma membrane, fuses with it, and then ruptures, spilling its contents out of the cell (also look back at pathway 1 of Figure 3.6). Exocytosis involves a “docking” process in which docking proteins on the vesicles recognize plasma membrane docking proteins and bind with them. This binding causes the membranes to “cork- screw” together and fuse (see Figure 3.12).

Endocytosis (en″do-si-to′sis; “into the cell”) includes those ATP-requiring processes that take up, or engulf, extracellular substances by enclosing them in a vesicle (Figure 3.13a, p. 108). Once the vesicle is formed, it detaches from the plasma mem- brane and moves into the cytoplasm, where it typi- cally fuses with a lysosome and its contents are digested (by lysosomal enzymes). However, in some cases, the vesicle travels to the opposite side of the cell and releases its contents by exocytosis there.

If the engulfed substances are relatively large particles, such as bacteria or dead body cells, and the cell separates them from the external environment by pseudopods, the endocytosis process is more specifically called phagocytosis (fag″o-si-to′sis), a term that means “cell eating” (Figure 3.13b). Certain white blood cells, such as the macrophage, and other “professional” phagocytes of the body act as scavenger cells that police and protect the body by ingesting bacteria and other foreign debris. Hence, phagocytosis is a protective mechanism—a way to “clean house”—not a means of getting nutrients.

Cells eat by phagocytosis and drink by a form of endocytosis called pinocytosis (pı̆″no-si-to′sis; “cell drinking”), during which the cell “gulps” droplets of extracellular fluid. (It may help you to remember the name of this process to think of your mother drink- ing a glass of pinot noir wine.) The plasma mem- brane indents to form a tiny pit, or “cup,” and then

Extracellular fluid

Plasma membrane docking protein

Vesicle docking protein

Molecule to be secreted

Secretory vesicle Cytoplasm

Fused docking proteins

Fusion pore formed

(a) The process of exocytosis

(b) Electron micrograph of a secretory vesicle in exocytosis (190,000×)

The membrane- bound vesicle migrates to the plasma membrane.

There, docking proteins on the vesicle and plasma membrane bind, the vesicle and membrane fuse, and a pore opens up.

Vesicle contents are released to the cell exterior.

Figure 3.12 exocytosis.

108 Essentials of Human Anatomy and Physiology

Did You Get It? 21. What happens when the Na+-K+ pump is

phosphorylated? When K+ binds to the pump protein?

22. Which vesicular transport process moves large particles into the cell?

23. Which process is more selective—pinocytosis or receptor-mediated endocytosis?

For answers, see Appendix A.

Cell Division ➔ Learning Objectives

□□ Briefly describe the process of DNA replication and of mitosis. explain the importance of mitotic cell division.

□□ Describe the roles of DNA and of the three major varieties of RNA in protein synthesis.

The cell life cycle is the series of changes a cell goes through from the time it is formed until it divides. The cycle has two major periods: inter- phase, in which the cell grows and carries on its

its edges fuse around the droplet of extracellular fluid containing dissolved proteins or fats (see Figure 3.13a). Unlike phagocytosis, pinocytosis is a routine activity of most cells. It is especially important in cells that function in absorption (for example, cells forming the lining of the small intestine).

Receptor-mediated endocytosis is the main cellular mechanism for taking up specific target molecules (Figure 3.13c). In this process, receptor proteins on the plasma membrane bind exclusively with certain substances. Both the receptors and high concentrations of the attached target mole- cules are internalized in a vesicle, and then the con- tents of the vesicle are dealt with in one of the ways shown in Figure 3.13a. Although phagocytosis and pinocytosis are important, they are not very selec- tive compared to receptor-mediated endocytosis. Specific substances taken in by receptor-mediated endocytosis include enzymes, some hormones, cholesterol, and iron. Unfortunately, flu viruses exploit this route to enter and attack our cells.

Extracellular fluid

Cytoplasm

Bacterium or other particle

Pseudopod

Membrane receptor

Target molecule

(b) Phagocytosis

(a) Endocytosis (pinocytosis)

Cytoplasm Extracellular fluid

Detached vesicle

Vesicle

Release of contents to cytosol

Lysosome

Plasma membrane

Ingested substance

Pit

Vesicle forms and fuses with lysosome for digestion.

Membranes and receptors (if present) recycled to plasma membrane

Transport to plasma membrane and exocytosis of vesicle contents

Figure 3.13 events and types of endocytosis. (a) Sequence of events in endocytosis. A vesicle forms by forming a pit in the membrane 1 . Once the vesicle detaches from the plasma membrane, its contents

may be digested within a lysosome 2A and then released to the cytosol.

Alternatively, the vesicle may be transported across the cell intact and then released to the cell exterior by exocytosis 2B . If present, its

membrane components and receptors are recycled to the plasma membrane 3 . The type illustrated is pinocytosis. (b) Phagocytosis. (c) Receptor-mediated endocytosis.

Chapter 3: Cells and Tissues 109

Old (template) strand

New strand forming

Old (template) strand

Newly synthesized strand

DNA of one sister chromatid

G G C

G C

T A

C G

C GG

A A

T A

A T

C G

A T

Thymine

Adenine

Cytosine

Guanine

KEY:

Figure 3.14 Replication of the DNA molecule at the end of interphase. In a process controlled by enzymes, the DNA helix unwinds (center), and its nucleotide strands are separated. Each strand then acts as a template for building a new complementary strand. As a result, two helices, each identical to the original DNA helix, are formed.

usual metabolic activities, and cell division, dur- ing which it reproduces itself. Although the term interphase might lead you to believe that it is merely a resting time between the phases of cell division, this is not the case. During interphase, which is by far the longer phase of the cell cycle, the cell is very active and is preparing for cell divi- sion. A more accurate name for interphase would be metabolic phase.

Preparations: DNA Replication

The function of cell division is to produce more cells for growth and repair processes. Because it is essential that all body cells have the same genetic material, an important event always precedes cell division: The DNA molecule (the genetic material) is duplicated exactly in a process called DNA rep- lication. This occurs toward the end of interphase.

➔ConCeptLinkRecall that DNA is a very complex molecule (see Chapter 2, p. 79). It is composed of building blocks called nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and a nitrogen-containing base. Essentially, DNA is a double helix, a ladderlike molecule that is coiled into a spiral staircase shape. The upright parts of the DNA “ladder,” or backbone, are alternating phosphate and sugar units, and the rungs of the ladder are made of pairs of nitrogen- containing bases. ➔

The precise trigger for DNA synthesis is unknown, but once it starts, it continues until all the DNA has been replicated. The process begins as the DNA helix “unzips,” gradually separating into its two nucleotide chains (Figure 3.14). Each nucleo- tide strand then serves as a template, or set of instructions, for building a new nucleotide strand.

Remember that nucleotides join in a comple- mentary way: Adenine (A) always bonds to thy- mine (T), and guanine (G) always bonds to cytosine (C). Hence, the order of the nucleotides on the template strand also determines the order on the new strand. For example, a TACTGC sequence on a template strand would generate a new strand with the order ATGACG. The end result is two DNA molecules that are identical to the original DNA helix, each consisting of one old and one newly assembled nucleotide strand.

110 Essentials of Human Anatomy and Physiology

each daughter nucleus ends up with exactly the same genetic information as the original mother cell.

The stages of mitosis include the following events (Figure 3.15):

• Prophase (pro′fāz). As cell division begins, the chromatin threads coil and shorten so that the barlike chromosomes become visible un- der a microscope. Because DNA has already been replicated, each chromosome is actually made up of two identical strands called sister chromatids (kro′mah-tidz), held together by a small buttonlike body called a centromere (sen′tro-mēr) (see Figure 3.15). The centrioles separate from each other and begin to move

events of Cell Division

In all cells other than bacteria and some cells of the reproductive system, cell division consists of two events. Mitosis (mi-to′sis), or division of the nucleus, occurs first. The second event is division of the cytoplasm, cytokinesis (si″to-kı̆-ne′sis), which begins when mitosis is nearly completed.

Mitosis Mitosis is the process of dividing a nucleus into two daughter nuclei with exactly the same genes as the “mother” nucleus. As explained previously, DNA replication precedes mitosis, so that for a short time the cell nucleus contains a double dose of genes. When the nucleus divides,

Centrioles

Plasma membrane

Interphase

Metaphase Anaphase Telophase and cytokinesis

Early prophase Late prophase

Daughter chromosomes

Nucleolus

Sister chromatids

Nuclear envelope

Chromatin Centrioles

Forming mitotic spindle

Nuclear envelope forming

Nucleolus forming

Centromere

Chromosome, consisting of two sister chromatids

Fragments of nuclear envelope

Mitotic spindle

Metaphase plate

Centromere

Cleavage furrow

Spindle microtubules

Figure 3.15 stages of mitosis.

Chapter 3: Cells and Tissues 111

3 genetically identical to the mother cell. The daugh- ter cells grow and carry out normal cell activities (interphase) until it is their turn to divide.

Mitosis and cytokinesis usually go hand in hand, but in some cases the cytoplasm is not divided. This condition leads to the formation of binucleate (two nuclei) or multinucleate cells. This is fairly common in the liver and in the for- mation of skeletal muscle.

Protein synthesis Proteins are key substances for all aspects of cell life. Fibrous (structural) proteins are the major building materials for cells (see Chapter 2). Other proteins, the globular (functional) proteins, per- form functional roles in the body. For example, all enzymes, biological catalysts that speed up every chemical reaction that occurs in cells, are func- tional proteins. It follows, then, that every cell needs to produce proteins, a process called pro- tein synthesis. This is accomplished with the DNA blueprints known as genes and with the help of the nucleic acid RNA. Let’s look at this process and its components more closely.

Genes: The Blueprint for Protein structure

In addition to replicating itself for cell division, DNA serves as the master blueprint for protein syn- thesis. Traditionally, a gene is defined as a DNA segment that carries the information for building one protein.

DNA’s information is encoded in the sequence of bases. Each sequence of three bases (a triplet) calls for a particular amino acid. (Amino acids are the building blocks of proteins and are joined dur- ing protein synthesis; see Chapter 2.) For example, a DNA base sequence of AAA specifies an amino acid called phenylalanine, and CCT calls for gly- cine. Just as different arrangements of notes on sheet music are played as different chords, varia- tions in the arrangements of A, C, T, and G in each gene allow cells to make all the different kinds of proteins needed. A single gene contains an esti- mated 300 to 3,000 base pairs in sequence.

The Role of RNA

By itself, DNA is rather like a coded message; its information is not useful until it is decoded. Furthermore, most ribosomes—the manufacturing sites for proteins—are in the cytoplasm, but DNA never leaves the nucleus during interphase. Thus,

toward opposite sides of the cell, directing the assembly of a mitotic spindle (composed of microtubules) between them as they move. The spindle provides scaffolding for the attachment and movement of the chromosomes during the later mitotic stages. By the end of prophase, the nuclear envelope and the nucleoli have broken down and temporarily disappeared, and the chromosomes have attached randomly to the spindle fibers by their centromeres.

• Metaphase (met′ah-fāz). In this short stage, the chromosomes line up at the metaphase plate (the center of the spindle midway be- tween the centrioles) so that a straight line of chromosomes is seen.

• Anaphase (an′ah-fāz). During anaphase, the centromeres that have held the chromatids to- gether split. The chromatids (now called chro- mosomes again) begin to move slowly apart, drawn toward opposite ends of the cell. The chromosomes seem to be pulled by their half- centromeres, with their “arms” dangling behind them. This careful division of sister chromatids ensures that each daughter cell gets one copy of every chromosome. Anaphase is over when the chromosomes stop moving.

• Telophase (tel′o-fāz). Telophase is essentially prophase in reverse. The chromosomes at op- posite ends of the cell uncoil to become threadlike chromatin again. The spindle breaks down and disappears, a nuclear envelope forms around each chromatin mass, and nucle- oli appear in each of the daughter nuclei.

Mitosis is basically the same in all animal cells. Depending on the type of tissue, it takes from 5 minutes to several hours to complete, but typi- cally it lasts about 2 hours. Centriole replication is deferred until late interphase of the next cell cycle, when DNA replication begins before the onset of mitosis.

Cytokinesis Cytokinesis, or the division of the cytoplasm, usually begins during late anaphase and completes during telophase. A contractile ring made of microfilaments forms a cleavage furrow over the midline of the spindle, and it eventually squeezes, or pinches, the original cytoplasmic mass into two parts. Thus, at the end of cell divi- sion, two daughter cells exist. Each is smaller with less cytoplasm than the mother cell had but is

112 Essentials of Human Anatomy and Physiology

G:C) tell us that the corresponding codons on mRNA would be UUA-GCA-AGC.

Translation A translator takes words in one lan- guage and restates them in another language. In the translation phase of protein synthesis, the language of nucleic acids (base sequence) is “translated” into the language of proteins (amino acid sequence). Translation occurs in the cyto- plasm and involves three major varieties of RNA. Translation consists of the following series of events (see Figure 3.16, steps 2–5). Once the mRNA attaches to the ribosome (

), tRNA trans- fers, or delivers, amino acids to the ribosome, where they are linked together by peptide bonds (formed by dehydration synthesis; see Chapter 2) in the exact sequence specified by the gene (and its mRNA). There are about 45 common types of tRNAs, each capable of carrying one of the 20 types of amino acids. But that is not the only job of the tRNAs. They also have to recognize the mRNA codons to “double check” that the amino acid they are toting will be added in the correct order. They can do this because they have a spe- cial three-base sequence called an anticodon on their “head” that can temporarily bind to the com- plementary codons (

). Once the first tRNA has maneuvered itself into

the correct position at the beginning of the mRNA message, the ribosome moves the mRNA strand along, bringing the next codon into position to be read by another tRNA. As amino acids are brought to their proper positions along the length of mRNA, they are joined together with peptide bonds cata- lyzed by the large ribosomal subunit (

DNA requires not only a decoder but also a trusted messenger to carry the instructions for building proteins to the ribosomes. These messenger and decoder functions are carried out by a second type of nucleic acid, called ribonucleic (ri0bo-nu- kle′ik) acid, or RNA.

RNA differs from DNA in being single-stranded, in having ribose sugar instead of deoxyribose, and in having a uracil (U) base instead of thymine (T) (recall what you learned in Chapter 2). Three vari- eties of RNA play a special role in protein synthe- sis. Ribosomal RNA (rRNA) helps form the ribosomes, where proteins are built. Messenger RNA (mRNA) molecules are long, single nucleo- tide strands that resemble half of a DNA molecule. They carry the “message” containing instructions for protein synthesis from the DNA (gene) in the nucleus to the ribosomes in the cytoplasm. Transfer RNA (tRNA) molecules are small, cloverleaf-shaped molecules that escort amino acids to the ribosome.

The Process of Protein synthesis

Protein synthesis involves two major phases: transcription, when complementary mRNA (the messenger) is made using the information in the DNA gene, and translation, when the informa- tion carried in mRNA molecules is “decoded” and translated from nucleic acids into proteins (Figure 3.16).

Transcription The word transcription often refers to one of the jobs done by a secretary— converting notes from one form (shorthand notes or an audio recording) into another form (a letter, for example). In other words, the same informa- tion is transformed from one form or format to another. In cells, transcription involves the trans- fer of information from the sequence of bases in a DNA gene into the complementary sequence of mRNA by an enzyme (see Figure 3.16, step 1

). DNA is the template for transcription, and mRNA is the product. Each three-base sequence specify- ing a particular amino acid on the DNA gene is called a triplet, and the corresponding three-base sequences on mRNA are called codons. The form is different, but the same information is being con- veyed. Thus, if the (partial) sequence of DNA trip- lets is AAT-CGT-TCG, RNA base-pairing rules (A:U,

➔ConCeptLinkRecall that the joining of amino acids by the ribosome into peptide bonds is the result of dehydration synthe-sis reactions (Chapter 2, p. 68). To make room for the new peptide bond, water (H2O) must be removed. A hydrogen atom is removed from one amino acid, and a hydroxyl group (OH) is removed from the other. ➔

As each amino acid is added to the chain, its tRNA is released and moves away from the ribo- some to pick up another amino acid ( 5 ). When the last codon (the termination, or “stop,” codon) is read, the protein is released.

Chapter 3: Cells and Tissues 113

U UA

C G

GU UU C UCC A

A G CCA U

A A G

Nucleus (site of transcription)

DNA gene

Codon

Small ribosomal subunit Portion of mRNA already translated

tRNA “head” bearing anticodon

Large ribosomal subunit

Synthetase enzyme

Amino acids

Cytoplasm (site of translation)

Growing polypeptide chain

Nuclear pore

Nuclear membrane

mRNA

Peptide bond Ala

Phe

Ser

Gly

Met

A U

U A

A A

G C

G A

A T

T T A

G C

CGT T

C G

C T

Correct amino acid attached to each type of tRNA by an enzyme

Direction of ribosome reading; ribosome moves the mRNA strand along sequentially as each codon is read.

IIe

mRNA specifying one polypeptide is made from a gene on the DNA template by an enzyme (not shown).

mRNA leaves nucleus and attaches to ribosome, and translation begins.

Incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by temporarily binding its anticodon to the codon.As the ribosome moves along

the mRNA, a new amino acid is added to the growing protein chain.

Released tRNA reenters the cytoplasmic pool, ready to be recharged with a new amino acid.

Figure 3.16 Protein synthesis. 1 Transcription. 2 – 5 Translation.

114 Essentials of Human Anatomy and Physiology

Tissues are organized into organs such as the heart, kidneys, and lungs. Most organs contain several tissue types, and the arrangement of the tissues determines each organ’s structure and what it is able to do. Thus, a study of tissues will be helpful in your later study of the body’s organs and how they work.

For now, we want to become familiar with the major similarities and differences in the primary tis- sues. Because epithelium and some types of con- nective tissue will not be considered again, they are emphasized more in this section than are muscle, nervous tissues, and bone (a connective tissue), which we cover in more depth in later chapters.

A description of the various tissue types fol- lows, and a helpful summary of the major functions and body locations of the four primary tissue types appears later (look ahead to Figure 3.22 on p. 127).

epithelial Tissue Epithelial tissue, or epithelium (ep″ ı̆-the′le-um; epithe = laid on, covering; plural epithelia), is the lining, covering, and glandular tissue of the body. Covering and lining epithelium covers all free body surfaces, both inside and out, and contains versa- tile cells. Because epithelium forms the boundaries that separate us from the outside world, nearly all substances that the body gives off or receives must pass through epithelium.

Epithelial functions include protection, absorp- tion, filtration, and secretion. For example, the epi- thelium of the skin protects against bacterial and chemical damage, and the epithelium lining the respiratory tract has cilia, which sweep dust and other debris away from the lungs. Epithelium spe- cialized to absorb substances lines some digestive system organs such as the stomach and small intes- tine, which absorb food nutrients into the body. In the kidneys, epithelium both absorbs and filters.

Glandular epithelium forms various glands in the body. Secretion is a specialty of the glands, which produce such substances as sweat, oil, digestive enzymes, and mucus.

Hallmarks of epithelium Epithelium generally has the unique characteristics listed below:

• Except for glandular epithelium (described on p. 119), epithelial cells fit closely together to

Did You Get It? 24. How do the terms template strand and complemen-

tary relate to DNA synthesis? 25. What is cytokinesis? What results if cytokinesis does

not happen? 26. What is the role of mRNA in protein synthesis? What

about tRNA? 27. What are the two stages of protein synthesis, and in

which stage are proteins actually synthesized? What occurs in the other stage?

For answers, see Appendix A.

PART II: BODY TIssues ➔ Learning Objectives

□□ Name the four major tissue types and their chief subcategories. explain how the four major tissue types differ structurally and functionally.

□□ Give the chief locations of the various tissue types in the body.

□□ Describe the process of tissue repair (wound healing).

The human body, complex as it is, starts out as a single cell, the fertilized egg, which divides almost endlessly. The millions of cells that result become specialized for particular functions. Some become muscle cells, others the transparent lens of the eye, still others skin cells, and so on. Thus, there is a division of labor in the body.

Cell specialization carries with it certain haz- ards. When a small group of cells is indispensable, its loss can disable or even destroy the body. For example, the action of the heart depends on a very specialized cell group in the heart muscle that controls its contractions. If those particular cells are damaged or stop functioning, the heart will no longer work properly, and the whole body will suffer or die from lack of oxygen.

Groups of cells that are similar in structure and function are called tissues and represent the next level of structural organization (look back at Figure 1.1). The four primary tissue types—epithe- lial, connective, nervous, and muscle tissues— interweave to form the fabric of the body. If we had to assign a single term to each primary tissue type that would best describe its overall role, the terms would most likely be covering (epithelium), support (connective), movement (muscle), and control (nervous). However, these terms reflect only a tiny fraction of the functions that each of these tissues performs.

Chapter 3: Cells and Tissues 115

3 Classification of epithelia Each epithelium is given two names. The first indi- cates the number of cell layers it has (Figure 3.17a). The classifications by cell arrangement (layers) are simple epithelium (one layer of cells) and strat- ified epithelium (more than one cell layer). The second describes the shape of its cells (Figure 3.17b). There are squamous (skwa′mus) cells, flat- tened like fish scales (squam = scale), cuboidal (ku-boi′dal) cells, which are cube-shaped like dice, and columnar cells, shaped like columns. The terms describing the shape and arrangement are then combined to describe the epithelium fully. Stratified epithelia are named for the cells at the free surface of the epithelial membrane, not those resting on the basement membrane, which can appear “squished” and variable in shape. Figure 3.17c shows the relationship between the structure of these tissues and their functions, a topic we will cover more as we look at each type.

simple epithelia

The simple epithelia are most concerned with absorption, secretion, and filtration. Because simple epithelia are usually very thin, protection is not one of their specialties.

form continuous sheets. Neighboring cells are bound together at many points by specialized cell junctions, including desmosomes and tight junctions (see p. 92).

• The membranes always have one free (unat- tached) surface or edge. This apical surface is exposed to the body’s exterior or to the cavity of an internal organ. The exposed surfaces of some epithelia are slick and smooth, but oth- ers exhibit cell surface modifications, such as microvilli or cilia.

• The anchored (basal) surface of epithelium rests on a basement membrane, a structure- less material secreted by both the epithelial cells and the connective tissue cells deep to the epithelium. Think of the basement mem- brane as the “glue” holding the epithelium in place.

• Epithelial tissues have no blood supply of their own (that is, they are avascular) and depend on diffusion from the capillaries in the under- lying connective tissue for food and oxygen.

• If well nourished, epithelial cells regenerate themselves easily.

Stratified

Simple

Apical surface

Basal surface

Apical surface

Basal surface

(a) Classification based on number of cell layers

Squamous

Cuboidal

Columnar

(b) Classification based on cell shape

Figure 3.17 Classification and functions of epithelia. (a) Classification on the basis of arrangement (layers). (b) Classification on the basis of cell shape; for each category, a whole cell is shown on the left, and a longitudinal section is shown on the right.

(Figure continues on page 116.)

116 Essentials of Human Anatomy and Physiology

Simple Squamous Epithelium Simple squamous epithelium is a single layer of thin squamous cells resting on a basement membrane. The cells fit closely together, much like floor tiles. This type of epithelium usually forms membranes where filtra- tion or exchange of substances by rapid diffusion occurs. Simple squamous epithelium is in the air sacs of the lungs (called alveoli), where oxygen and carbon dioxide are exchanged (Figure 3.18a, p. 117), and it forms the walls of capillaries, where nutrients and gases pass between the blood in the capillaries and the interstitial fluid. Simple squa- mous epithelium also forms serous membranes, or serosae (se-ro′se), the slick membranes that line the ventral body cavity and cover the organs in that cavity. (We describe the serous membranes in more detail in Chapter 4.)

Simple Cuboidal Epithelium Simple cuboidal epithelium, which is one layer of cuboidal cells resting on a basement membrane, is common in glands and their associated small tubes called ducts (for example, the salivary glands and pancreas). It also forms the walls of the kidney tubules (Figure 3.18b) and covers the surface of the ovaries.

Simple Columnar Epithelium Simple columnar epithelium is made up of a single layer of tall cells that fit closely together. Goblet cells, which

produce a lubricating mucus, are often seen in this type of epithelium. Simple columnar epithelium lines the entire length of the digestive tract from the stomach to the anus (Figure 3.18c). Epithelial membranes that line body cavities open to the body exterior are called mucosae (mu-ko′se), or mucous membranes.

Pseudostratified Columnar Epithelium All of the cells of pseudostratified (soo″do-stră′t ı̆-fı̄d) columnar epithelium rest on a basement mem- brane. However, some of its cells are shorter than others, and their nuclei appear at different heights above the basement membrane. As a result, this epithelium gives the false (pseudo) impression that it is stratified; hence its name. Like simple colum- nar epithelium, this variety mainly functions in absorption and secretion. A ciliated variety (more precisely called pseudostratified ciliated columnar epithelium) lines most of the respiratory tract (Figure 3.18d). The mucus produced by the goblet cells in this epithelium acts as a “sticky trap” to catch dust and other debris, and the cilia propel the mucus upward and away from the lungs.

stratified epithelia

Stratified epithelia consist of two or more cell lay- ers. Being considerably more durable than the simple epithelia, these epithelia function primarily in protection.

Figure 3.17 (continued) Classification and functions of epithelia. (c) Function of epithelial tissue related to tissue type.

Number of layers

Cell shape

Squamous

One layer: simple epithelial tissues More than one layer: stratified epithelial tissues

Cuboidal

Columnar

Transitional

Diffusion and filtration Secretion in serous membranes

Secretion and absorption; ciliated types propel mucus or reproductive cells

No simple transitional epithelium exists

Protection; these tissue types are rare in humans

Protection; stretching to accommodate distension of urinary structures

Secretion and absorption; ciliated types propel mucus or reproductive cells

Protection

Chapter 3: Cells and Tissues 117

(a) Diagram: Simple squamous

Air sacs of lungs

Nuclei of squamous epithelial cells

Photomicrograph: Simple squamous epithelium forming part of the alveolar (air sac) walls (275×).

Nucleus of squamous epithelial cell

Basement membrane

Nucleus of simple cuboidal epithelial cell

Photomicrograph: Simple cuboidal epithelium in kidney tubules (250×).

Simple cuboidal epithelial cells

Basement membrane

Connective tissue

(b) Diagram: Simple cuboidal

Photomicrograph: Simple columnar epithelium of the small intestine (575×).

Nuclei of simple columnar epithelial cells tend to line up

Basement membrane

Simple columnar epithelial cell

Mucus of a goblet cell

Basement membrane

Figure 3.18 Types of epithelia and examples of common locations in the body.

(Figure continues on page 118.)

View Histology

118 Essentials of Human Anatomy and Physiology

Basement membrane

Nuclei of pseudostratified cells do not line up

Pseudo- stratified epithelial layer

Cilia

Photomicrograph: Pseudostratified ciliated columnar epithelium lining the human trachea (560×).(d) Diagram: Pseudostratified (ciliated)

columnar

Pseudostratified epithelial layer

Basement membrane

Connective tissue

Stratified squamous epithelium

Basement membrane

Photomicrograph: Stratified squamous epithelium lining of the esophagus (140×).

Nuclei

Basement membrane Connective tissue

(e) Diagram: Stratified squamous

Stratified squamous epithelium

Transi- tional epithelium

Basement membrane

(f) Diagram: Transitional

Photomicrograph: Transitional epithelium lining of the bladder, relaxed state (270×); surface rounded cells flatten and elongate when the bladder fills with urine.

Basement membrane

Transitional epithelium

Connective tissue

Figure 3.18 (continued) Types of epithelia and examples of common locations in the body.

Chapter 3: Cells and Tissues 119

3 blood and use them to make their products, which they then discharge by exocytosis.

Two major types of glands develop from epi- thelial sheets: endocrine and exocrine. Endocrine (en′do-krin) glands lose their ducts; thus they are often called ductless glands. Their secretions (all hormones) diffuse directly into the blood vessels that weave through the glands. Examples of endo- crine glands include the thyroid, adrenals, and pituitary.

Exocrine (ek′so-krin) glands retain their ducts, and their secretions exit through the ducts to the epithelial surface. Exocrine glands, which include the sweat and oil glands, liver, and pan- creas, are both internal and external. We discuss them with the organ systems to which their prod- ucts are related.

Did You Get It? 28. What two criteria are used to classify epithelial

tissues? 29. How do endocrine and exocrine glands differ in

structure and function? 30. Which of the following properties apply to epithelial

tissues: has blood vessels; can repair itself; cells have specialized cell junctions?

31. Which two characteristics are hallmarks (distinguishing characteristics) of epithelial tissue?

For answers, see Appendix A.

Connective Tissue Connective tissue, as its name suggests, connects body parts. It is found everywhere in the body. It is the most abundant and widely distributed of the tissue types. Connective tissues perform many functions, but they are primarily involved in pro- tecting, supporting, and binding together other body tissues.

Hallmarks of Connective Tissue The distinguishing characteristics of connective tis- sue include the following:

• Variations in blood supply. Most connective tissues are well vascularized (that is, they have a good blood supply), but there are excep- tions. Tendons and ligaments, for example, have a poor blood supply, and cartilages are avascular. Consequently, all these structures heal very slowly when injured. (This is why some people say that, given a choice, they

Stratified Squamous Epithelium Stratified squamous epithelium is the most common strat- ified epithelium in the body. It usually consists of many cell layers. The cells at the free edge are squamous cells, whereas those close to the base- ment membrane are cuboidal or columnar. Stratified squamous epithelium is found in sites that receive a good deal of abuse or friction, such as the surface of the skin, the mouth, and the esophagus (Figure 3.18e).

Stratified Cuboidal and Stratified Columnar Epithelia Stratified cuboidal epithelium typi- cally has just two cell layers with (at least) the sur- face cells being cuboidal in shape. The surface cells of stratified columnar epithelium are columnar cells, but its basal cells vary in size and shape. Both of these epithelia are fairly rare in the body, found mainly in the ducts of large glands. (Because the distribution of these two epithelia is extremely limited, they are not illustrated in Figure 3.18. We mention them here only to provide a complete listing of the epithelial tissues.)

Transitional Epithelium Transitional epithe- lium is a highly modified, stratified squamous epi- thelium that forms the lining of only a few organs—the urinary bladder, the ureters, and part of the urethra. As part of the urinary system, all of these organs are subject to considerable stretching (Figure 3.18f). Cells of the basal layer are cuboidal or columnar; those at the free surface vary in appearance. When the organ is not stretched, the membrane is many-layered, and the superficial cells are rounded and domelike. When the organ is distended with urine, the epithelium thins like a rubber band being stretched, and the surface cells flatten and become squamouslike. This ability of transitional cells to slide past one another and change their shape (undergo “transitions”) allows the ureter wall to stretch as a greater volume of urine flows through that tubelike organ. In the bladder, it allows more urine to be stored.

Glandular epithelium

A gland consists of one or more cells that make and secrete a particular product. This product, a secretion, typically contains protein molecules in an aqueous (water-based) fluid. The term secre- tion also indicates an active process in which the glandular cells obtain needed materials from the

120 Essentials of Human Anatomy and Physiology

At one extreme, bone and cartilage have very few cells and large amounts of hard matrix, which makes them extremely strong. At the opposite extreme, fat tissue is composed mostly of cells, and the matrix is soft. From most rigid to softest or most fluid, the major connective tissue classes are bone, cartilage, dense connective tissue, loose con- nective tissue, and blood. Find the various types of connective tissues in Figure 3.19 as you read the following descriptions.

Bone

Bone, sometimes called osseous (os′e-us) tissue, is composed of osteocytes (os′te-o-sı̄tz; bone cells) sit- ting in cavities called lacunae (lah-ku′ne; “pits”). These pits are surrounded by layers of a very hard matrix that contains calcium salts in addition to large numbers of collagen fibers (Figure 3.19a). Because of its rocklike hardness, bone has an exceptional ability to protect and support other body organs (for example, the skull protects the brain).

Cartilage

Cartilage is less hard and more flexible than bone. Its major cell type is chondrocytes (cartilage cells). It is found in only a few places in the body. Most widespread is hyaline (hi′ah-lin) cartilage, which has abundant collagen fibers hidden by a rubbery matrix with a glassy (hyalin = glass), blue-white appearance (Figure 3.19b). It forms the trachea, or windpipe, attaches the ribs to the breastbone, and covers bone ends at joints. The skeleton of a fetus is made largely of hyaline cartilage; but by the time the baby is born, most of that cartilage has been replaced by bone. The exceptions include the epiphyseal, or growth, plates in long bones, which allow the bones to grow in length.

Although hyaline cartilage is the most abun- dant type of cartilage in the body, there are others. Highly compressible fibrocartilage forms the cushionlike disks between the vertebrae of the spinal column (Figure 3.19c). Elastic cartilage is found in structures with elasticity, such as the external ear. (Elastic cartilage is not illustrated in Figure 3.19.)

Dense Connective Tissue

In dense connective tissue, also called dense fibrous tissue, collagen fibers are the main matrix element (Figure 3.19d). Crowded between the colla- gen fibers are rows of fibroblasts (fiber-forming cells)

would rather have a broken bone than a torn ligament.)

• Extracellular matrix. Connective tissues are made up of many different types of cells plus varying amounts of a nonliving substance found outside the cells, called the extracellular matrix.

extracellular Matrix The extracellular matrix deserves a bit more explanation because it is what makes connective tissue so different from the other tissue types. The matrix, which is produced by the connective tissue cells and then secreted to their exterior, has two main elements, a structureless ground substance and fibers. The ground substance of the matrix is composed largely of water plus some cell adhe- sion proteins and large, charged polysaccharide molecules. The cell adhesion proteins serve as a glue that allows the connective tissue cells to attach themselves to the matrix fibers embedded in the ground substance. The charged polysaccha- ride molecules trap water as they intertwine. As these polysaccharides become more abundant, they cause the matrix to vary from fluid to gel-like to firm in its consistency.

Depending on the connective tissue type, vari- ous types and amounts of fibers contribute to the matrix. These include collagen (white) fibers dis- tinguished by their high tensile strength; elastic (yellow) fibers, which have the ability to stretch and recoil; and reticular fibers, which are fine col- lagen fibers that form the internal “skeleton” of soft organs such as the spleen. The building blocks, or monomers, of these fibers are made by the connective tissue cells and secreted into the ground substance in the extracellular space, where they join together to form the various fiber types.

Because of the variations in its extracellular matrix, connective tissue is able to form a soft packing tissue around other organs, to bear weight, and to withstand stretching and other abuses, such as abrasion, that no other tissue could endure.

Types of Connective Tissue As noted previously, all connective tissues consist of living cells surrounded by extracellular matrix. Their major differences reflect specific cell types, fiber types, and the number of fibers in the matrix.

Photomicrograph: Cross-sectional view of bone (165×).

Central canal

Lacunae

(a) Diagram: Bone

Osteocytes (bone cells) in lacunae

Lacunae

Chondrocyte (cartilage cell)

Photomicrograph: Hyaline cartilage from the trachea (400×).

Chondrocyte in lacuna

Matrix

(b) Diagram: Hyaline cartilage

Collagen fibers

Chondro- cytes in lacunae

Photomicrograph: Fibrocartilage of an intervertebral disc (150×).

Chondrocytes in lacunae

Collagen fiber

Figure 3.19 Connective tissues and their common body locations.

View Histology

Photomicrograph: Dense fibrous connective tissue from a tendon (475×).

Collagen fibers

Nuclei of fibroblasts

(d) Diagram: Dense fibrous

Tendon

Ligament

Collagen fibers

Nuclei of fibroblasts

Photomicrograph: Areolar connective tissue, a soft packaging tissue of the body (270×).

Elastic fibers

Collagen fibers

Fibroblast nuclei

(e) Diagram: Areolar

Elastic fibers of matrix

Nuclei of fibroblasts

Collagen fibers

Lamina propria

Mucosal epithelium

Photomicrograph: Adipose tissue from the subcutaneous layer beneath the skin (570×).

Vacuole containing fat droplet

Nuclei of fat cells

(f) Diagram: Adipose

Nuclei of fat cells

Vacuole containing fat droplet

Figure 3.19 (continued) Connective tissues and their common body locations. (e) and (f) are subclasses of loose connective tissues.

Chapter 3: Cells and Tissues 123

any other connective tissue type except blood. There are three main types of loose connective tis- sue: areolar, adipose, and reticular.

Areolar Connective Tissue Areolar (ah-re′o-lar) connective tissue, the most widely distributed connective tissue variety in the body, is a soft, pli- able, “cobwebby” tissue that cushions and protects the body organs it wraps (Figure 3.19e). It func- tions as a universal packing tissue and connective tissue “glue” because it helps to hold the internal organs together and in their proper positions. A soft layer of areolar connective tissue called the

that manufacture the building blocks of the fibers. Dense connective tissue forms strong, ropelike structures such as tendons and ligaments. Tendons attach skeletal muscles to bones; liga- ments connect bones to bones at joints. Ligaments are more stretchy and contain more elastic fibers than do tendons. Dense connective tissue also makes up the lower layers of the skin (dermis), where it is arranged in sheets.

loose Connective Tissue

Relatively speaking, loose connective tissues are softer and have more cells and fewer fibers than

Photomicrograph: Dark-staining network of reticular connective tissue (400×).

White blood cell (lymphocyte)

Reticular fibers

(g) Diagram: Reticular

Spleen

Reticular cell

Reticular fibers

Blood cell

(h) Diagram: Blood

White blood cell

Red blood cells

Blood cells in capillary

Photomicrograph: Smear of human blood (1290×)

Neutrophil (white blood cell)

Plasma (fluid matrix)

Red blood cells

Monocyte (white blood cell)

Figure 3.19 (continued) (g) is a subclass of loose connective tissue.

124 Essentials of Human Anatomy and Physiology

surrounded by a nonliving, fluid matrix called blood plasma (Figure 3.19h). The “fibers” of blood are soluble proteins that become visible only dur- ing blood clotting. Still, blood is quite atypical as connective tissues go. Blood is the transport vehi- cle for the cardiovascular system, carrying nutri- ents, wastes, respiratory gases, white blood cells, and many other substances throughout the body. (We consider blood in detail in Chapter 10.)

Did You Get It? 32. What are the two hallmarks of connective tissue? 33. Which fiber type contributes strength to connective

tissues?

For answers, see Appendix A.

Muscle Tissue Muscle tissues are highly specialized to contract, or shorten, which generates the force required to produce movement. There are three types of muscle tissue: skeletal, cardiac, and smooth (Figure 3.20). Notice their similarities and differ- ences as you read through the following descrip- tions.

skeletal Muscle Skeletal muscle tissue is packaged by connective tissue sheets into organs called skeletal muscles, which are attached to the skeleton. These muscles, which can be controlled voluntarily (or con- sciously), form the flesh of the body, the so-called muscular system (see Chapter 6). When the skele- tal muscles contract, they pull on bones or skin. As a result, gross body movements or changes in our facial expressions occur.

The cells of skeletal muscle are long, cylindri- cal, and multinucleate, and they have obvious stri- ations (stripes). Because skeletal muscle cells are elongated to provide a long axis for contraction, they are often called muscle fibers.

Cardiac Muscle Cardiac muscle (covered in more detail in Chapter 11) is found only in the heart wall. As it contracts, the heart acts as a pump to propel blood through the blood vessels. Like skeletal muscle, cardiac muscle has striations, but cardiac cells have only a single nucleus and are relatively short, branching cells that fit tightly together (like clasped fingers) at junctions called intercalated discs.

lamina propria (lă′mı̆-nah pro′pre-ah) underlies all mucous membranes. Its fluid matrix contains all types of fibers, which form a loose network. In fact, when viewed through a microscope, most of the matrix appears to be empty space, which explains the name of this tissue type (areola = small open space). Because of its loose and fluid nature, areolar connective tissue provides a reser- voir of water and salts for the surrounding tissues, and essentially all body cells obtain their nutrients from and release their wastes into this “tissue fluid.” When a body region is inflamed, the local areolar tissue soaks up the excess fluid like a sponge, and the area swells and becomes puffy, a condition called edema (eh-de′mah). Many types of phagocytes wander through this tissue, scav- enging for bacteria, dead cells, and other debris, which they destroy.

Adipose Connective Tissue Adipose (ad′ ı̆-pōs) tissue is commonly called fat. Basically, it is an areolar tissue in which adipose (fat) cells predomi- nate (Figure 3.19f). A glistening droplet of oil occupies most of a fat cell’s volume and com- presses the nucleus, displacing it to one side.

Adipose tissue forms the subcutaneous tissue beneath the skin, where it insulates the body and protects it from bumps and extremes of both heat and cold. Adipose tissue also protects some organs individually—the kidneys are surrounded by a capsule of fat, and adipose tissue cushions the eyeballs in their sockets. There are also fat “depots” in the body, such as the hips, breasts, and belly, where fat is stored and available for fuel if needed.

Reticular Connective Tissue Reticular connec- tive tissue consists of a delicate network of inter- woven reticular fibers associated with reticular cells, which resemble fibroblasts (Figure 3.19g). Reticular tissue is limited to certain sites: It forms the stroma (literally, “bed” or “mattress”), or internal framework of an organ. The stroma can support many free blood cells (largely white blood cells called lym- phocytes) in lymphoid organs such as lymph nodes, the spleen, and bone marrow. It may help to think of reticular tissue as “cellular bleachers” where other cells can observe their surroundings.

Blood

Blood, or vascular tissue, is considered a con- nective tissue because it consists of blood cells

Chapter 3: Cells and Tissues 125

Photomicrograph: Skeletal muscle (195×).

Multiple nuclei per fiber

Striations

Part of muscle fiber

(a) Diagram: Skeletal muscle

Photomicrograph: Cardiac muscle (475×).

Intercalated discs

Nucleus

(b) Diagram: Cardiac muscle

Photomicrograph: Sheet of smooth muscle (360×).

Smooth muscle cell

Nuclei

Figure 3.20 Types of muscle tissue and their common locations in the body. View Histology

126 Essentials of Human Anatomy and Physiology

Nervous Tissue When we think of nervous tissue, we think of cells called neurons. All neurons receive and con- duct electrochemical impulses from one part of the body to another; thus, irritability and conduc- tivity are their two major functional characteristics. The structure of neurons is unique (Figure 3.21). Their cytoplasm is drawn out into long processes (extensions), as long as 3 feet or more in the leg, which allows a single neuron to conduct an impulse to distant body locations.

However, the nervous system is more than just neurons. A special group of supporting cells called neuroglia insulate, support, and protect the delicate neurons in the structures of the nervous system—the brain, spinal cord, and nerves.

Figure 3.22 summarizes the tissue types, includ- ing their hallmarks and functions in the body.

Tissue Repair (Wound Healing) The body has many techniques for protecting itself from uninvited guests or injury. Intact physi- cal barriers such as the skin and mucous mem- branes, cilia, and the strong acid produced by

These intercalated discs contain gap junctions that allow ions to pass freely from cell to cell. This ties the cardiac cells into a functional syncytium (syn = together, cyt = cell), resulting in rapid conduction of the electrical signal to contract across the heart. Cardiac muscle is under involuntary control, which means that we cannot consciously control the activ- ity of the heart. Can you imagine having to “tell” your heart to beat like you “tell” your legs to walk?

smooth Muscle Smooth (visceral) muscle is so called because no striations are visible. The individual cells have a single nucleus and are tapered at both ends. Smooth muscle is found in the walls of hollow organs such as the stomach, uterus, and blood vessels. As smooth muscle in its walls contracts, the cavity of an organ alternately becomes smaller (constricts when smooth muscle contracts) or enlarges (dilates when smooth muscle relaxes) so that substances are mixed and/or propelled through the organ along a specific pathway. Smooth muscle contracts much more slowly than the other two muscle types, and these contractions tend to last longer. Peristalsis (per″ ı̆-stal′sis), a wavelike motion that keeps food moving through the small intestine, is typical of its activity.

Nuclei of neuroglia (supporting cells)

Diagram: Nervous tissue Photomicrograph: Neurons (320×)

Nuclei of neuroglia (supporting cells)

Neuron processes

Cell body of neuron

Brain

Neuron processes

Spinal cord

Figure 3.21 Nervous tissue. Neurons and supporting cells form the brain, spinal cord, and nerves. View Histology

Chapter 3: Cells and Tissues 127

in Chapter 12.) Here we will concentrate on the process of tissue repair itself.

Tissue repair, or wound healing, occurs in two major ways: by regeneration and by fibrosis. Regeneration is the replacement of destroyed tissue by the same kind of cells, whereas fibrosis involves repair by dense (fibrous) connective tissue, that is, by the formation of scar tissue. Which occurs depends on (1) the type of tissue damaged and (2) the severity of the injury. Generally speaking, clean

stomach glands are just three examples of body defenses exerted at the tissue level. When tissue injury does occur, it stimulates the body’s inflam- matory and immune responses, and the healing process begins almost immediately. Inflammation is a general (nonspecific) body response that attempts to prevent further injury. The immune response, in contrast, is extremely specific and mounts a vigorous attack against recognized invaders, including bacteria, viruses, and toxins. (We consider these protective responses in detail

Nervous tissue: Internal communication and control Hallmarks: irritable, conductive • Brain, spinal cord, and nerves

Muscle tissue: Contracts to cause movement Hallmarks: irritable, contractile • Muscles attached to bones (skeletal) • Muscles of heart wall (cardiac) • Muscles of walls of hollow organs (smooth)

Epithelial tissue: Forms boundaries between different environments, protects, secretes, absorbs, filters Hallmarks: one free (apical) surface, avascular • Lining of GI tract and other hollow organs • Skin surface (epidermis)

Connective tissue: Supports, protects, binds other tissues together Hallmarks: extracellular matrix, varying vascularity • Cartilage • Bones • Tendons • Fat and other soft padding tissue

Figure 3.22 summary of the major functions, characteristics, and body locations of the four tissue types: epithelial, connective, muscle, and nervous tissues.

128 Essentials of Human Anatomy and Physiology

is its inability to perform the normal functions of the tissue it replaces. Thus, if scar tissue forms in the wall of the bladder, heart, or another muscular organ, it may severely hamper the functioning of that organ.

A contracture is a permanent tightening of the skin affecting the underlying tendons or muscles. Contractures develop during the healing process as inelastic fibrous tissue replaces the normal elas- tic connective tissues. Because fibrous tissue resists stretching, movement of the affected area may be limited. _____________________________________✚

Did You Get It? 34. Which muscle type(s) is(are) injured when you pull a

muscle while exercising? 35. How does the extended length of a neuron’s

processes aid its function in the body? 36. Nervous tissue is irritable. What does this mean? 37. Which tissue type is the basis for scar tissue?

For answers, see Appendix A.

PART III: DeVelOPMeNTAl AsPeCTs OF Cells AND TIssues ➔ Learning Objectives

□□ Define neoplasm, and distinguish benign and malignant neoplasms.

□□ explain the significance of the fact that some tissue types (muscle and nerve) are largely amitotic after the growth stages are over.

Very early in embryonic development, cells begin to specialize to form the primary tissues, and by birth, most organs are well formed and function- ing. The body continues to grow and enlarge by

cuts (incisions) heal much more successfully than ragged tears of the tissue.

Tissue injury sets the following series of events into motion:

• Inflammation sets the stage. Injured tissue cells and others release inflammatory chemi- cals that make the capillaries very permeable. This allows fluid rich in clotting proteins and other substances to seep into the injured area from the bloodstream. The leaked clotting pro- teins construct a clot, which “plugs the hole” to stop blood loss and hold the edges of the wound together. The injured area becomes walled off, preventing bacteria or other harm- ful substances from spreading to surrounding tissues. Where the clot is exposed to air, it quickly dries and hardens, forming a scab.

• Granulation tissue forms. Granulation tissue is delicate pink tissue composed largely of new capillaries that grow into the damaged area from undamaged blood vessels nearby. These capillaries are fragile and bleed freely, as when a scab is picked away from a skin wound. Granulation tissue also contains phagocytes, which eventually dispose of the blood clot, and connective tissue cells (fibroblasts), which pro- duce the building blocks of collagen fibers (scar tissue) to permanently bridge the gap.

• Regeneration and fibrosis effect perma- nent repair. As the surface epithelium begins to regenerate, it makes its way between the granulation tissue and the scab. The scab soon detaches, and the final result is a fully regener- ated surface epithelium that covers an underly- ing area of fibrosis (the scar). The scar is either invisible or visible as a thin white line, de- pending on the severity of the wound.

The ability of the different tissue types to regenerate varies widely. Epithelial tissues such as the skin epidermis and mucous membranes regen- erate beautifully. So, too, do most of the fibrous connective tissues and bone. Skeletal muscle regenerates poorly, and cardiac muscle and ner- vous tissue within the brain and spinal cord are replaced largely by scar tissue.

Homeostatic Imbalance 3.3

Scar tissue is strong, but it lacks the flexibility of most normal tissues. Perhaps even more important

Photo showing post-burn contracture scars on the neck.

Chapter 3: Cells and Tissues 129

3 forming new tissue throughout childhood and adolescence.

Cell division is extremely important during the body’s growth period. Most cells (except neurons and mature red blood cells) undergo mitosis until the end of puberty, when adult body size is reached and overall body growth ends. After this time, only certain cells routinely divide (are mitotic)—for example, cells exposed to abrasion that continually wear away, such as skin and intestinal cells. Liver cells stop dividing, but they retain this ability should some of them die or become damaged and need to be replaced. Still other cell groups (for example, heart muscle and nervous tissue) almost completely lose their abil- ity to divide when they are fully mature; that is, they become amitotic (am″ ı̆-tot′ik). Amitotic tis- sues are severely handicapped by injury because the lost cells cannot be replaced by the same type of cells. This is why the heart of an individual who has had several severe heart attacks becomes weaker and weaker. Damaged cardiac muscle does not regenerate and is replaced by scar tissue that cannot contract, so the heart becomes less and less capable of acting as an efficient blood pump.

The aging process begins once maturity has been reached. (Some think it begins at birth.) No one has been able to explain just what causes aging, but there have been many suggestions. Some think it is a result of little “chemical insults” that occur continually through life—for example, the presence of toxic chemicals (such as alcohol, certain drugs, or carbon monoxide) in the blood, or the temporary absence of needed substances such as glucose or oxygen. Perhaps the effect of these chemical insults is cumulative and finally succeeds in upsetting the delicate chemical bal- ance of the body cells. Others think that external physical factors such as radiation (X rays or ultra- violet waves) contribute to the aging process. Still another theory is that the aging “clock” is built into our genes. We all know of cases like the radiant woman of 50 who looks about 35 or the young man of 24 who looks 40. It appears that such traits can run in families.

There is no question that certain events are part of the aging process. For example, with age,

epithelial membranes thin and are more easily damaged, and the skin loses its elasticity and begins to sag. The exocrine glands of the body (epithelial tissue) become less active, and we begin to “dry out” as less oil, mucus, and sweat are produced. Some endocrine glands produce decreasing amounts of hormones, and the body processes that they control (such as metabolism and reproduction) slow down or stop altogether.

Connective tissue structures also show changes with age. Bones become porous and weaken, and tissue repair slows. Muscles begin to waste away. Although a poor diet may contribute to some of these changes, there is little doubt that decreased efficiency of the circulatory system, which reduces nutrient and oxygen delivery to body tissues, is a major factor.

Besides the tissue changes associated with aging, which accelerate in the later years of life, other modifications of cells and tissues may occur at any time. For example, when cells fail to honor normal controls on cell division and multiply wildly, an abnormal mass of proliferating cells, known as a neoplasm (ne′o-plazm″; “new growth”), results. Neoplasms may be benign or malignant (cancerous). (See “A Closer Look” on pp. 130–131.)

However, not all increases in cell number involve neoplasms. Certain body tissues (or organs) may enlarge because there is some local irritant or condition that stimulates the cells. This response is called hyperplasia (hi″per-pla′ze-ah). For example, a woman’s breasts enlarge during pregnancy in response to increased hormones; this is a normal but temporary situation that doesn’t have to be treated. In contrast, atrophy (at′ro-fe), or decrease in size, can occur in an organ or body area that loses its normal stimula- tion. For example, the muscles of a broken leg atrophy while in a cast during the healing period.

Did You Get It? 38. Which of the four types of tissue is most likely to

remain mitotic throughout life? 39. What cellular error allows a neoplasm to form? 40. How does the activity of endocrine glands change as

the body ages?

For answers, see Appendix A.

What all of these factors have in com- mon is that they cause mutations— changes in DNA that alter the expression of certain genes. However, the immune system or peroxisomal and lysosomal enzymes eliminate most carcinogens. Furthermore, transforma- tion requires several genetic changes.

The discovery of oncogenes ( cancer-causing [onco = tumor] genes), and later proto-oncogenes, provided

(mĕ-tas′tă-sis). Additionally, cancer cells consume an exceptional amount of the body’s nutrients, leading to weight loss and tissue wasting.

What causes transformation—the changes that convert a normal cell into a cancerous one? Radiation, mechani- cal trauma, certain viral infections, and many chemicals (tobacco tars, organic solvents such as mineral spirits) can all act as carcinogens (cancer-causers).

When controls of the cell cycle and cell division malfunction, an abnormal cell mass called a neo- plasm (“new growth”) or tumor develops. Not all neoplasms are cancerous. Benign (be-nīn′; “kindly”) neoplasms are usually surrounded by a capsule, grow slowly, and seldom kill their hosts if they are removed before they compress vital organs. In contrast, malignant (“bad”) neo- plasms (cancers) are not encapsulated. They grow more relentlessly and may become killers. Their cells resemble immature cells, and they invade their surroundings rather than push them aside, as reflected in the name can- cer, from the Latin word for “crab.” Normal cells die when they lose con- tact with the surrounding matrix, but malignant cells tend to break away from the parent mass and spread via the blood to distant parts of the body. The formation of new masses at other body sites is called metastasis

Cancer—An Intimate Enemy A

CLOSER LOOK

Cellular changes:

Increased cell division

Growth of polyp Growth of malignant tumor (cancerous)

DNA changes:

Oncogene activated

Tumor-suppressor gene inactivated

Second tumor-suppres- sor gene inactivated

Colon wall

Stepwise development of a typical colon cancer

1 2 3

130

summary

PART I: Cells (pp. 88–114)

Overview of the Cellular Basis of life (pp. 88–89)

1. A cell is composed primarily of four elements— carbon, hydrogen, oxygen, and nitrogen—plus many trace elements. Living matter is over 60 percent water. The major building material of the cell is protein.

2. Cells vary in size from microscopic to over a meter in length. Shape reflects function. For example, muscle cells have a long axis to allow shortening.

Anatomy of a Generalized Cell (pp. 89–102)

1. Cells have three major regions—nucleus, cyto- plasm, and plasma membrane.

a. The nucleus, or control center, directs cell activ- ity and is necessary for reproduction. The nucleus contains genetic material (DNA), which carries instructions for synthesis of proteins.

b. The plasma membrane encloses the cytoplasm and acts as a semipermeable barrier to the movement of substances into and out of the cell. It is composed of a phospholipid bilayer con- taining proteins, sugars, and cholesterol. The water-impermeable lipid portion forms the basic membrane structure. The proteins (many of which are glycoproteins) act as enzymes or car- riers in membrane transport, form membrane channels, provide receptor sites for hormones and other chemicals, or play a role in cellular recognition and interactions during development and immune reactions.

effects because most target all rapidly dividing cells, including normal ones. Side effects include nausea, vomiting, and hair loss. X rays also have side effects because they kill healthy cells in their path as well as cancer cells.

Current cancer treatments—”cut, burn, and poison”—are recognized as crude and painful. Promising new treatments include:

• Delivering radiation or anticancer drugs precisely to the cancer (via monoclonal antibodies that bind specifically to a protein “target” on a cancer cell or magnetic delivery systems for chemotherapy drugs).

• Strengthening the immune system by using “handpicked” immune cells from the patient.

• Starving tumors by cutting off their blood supply.

• Destroying cancer cells with viruses. • Testing the tumor for genetic mark-

ers to customize treatment plans.

Today, about half of cancer cases are cured, and patients’ quality of life has improved.

and the inactivation of tumor- suppressor genes. The seeds of cancer appear to be in our own genes—an intimate enemy indeed.

Almost half of all Americans develop cancer in their lifetime, and a fifth of us will die of it. The most com- mon cancers originate in the skin, lung, colon, breast, and prostate.

Screening procedures, such as self- examination of the breasts or testicles for lumps and checking fecal samples for blood, aid early detection. Unfortu- nately, most cancers are diagnosed after symptoms have already appeared. A biopsy involves removing a sample of the tumor surgically and examining it microscopically for changes typical of malignant cells. Increasingly, diagnosis includes genetic or chemical analysis of tissue samples to determine which genes are switched on or off, and which drugs may be most effective. MRI and CT scans can detect large can- cers or the degree of metastasis.

Most neoplasms are removed sur- gically if possible. Radiation and drugs (chemotherapy) may also be used. Anticancer drugs have unpleasant side

clues to the role of genes in cancer. Proto-oncogenes code for proteins that are needed for normal cell division and growth. However, many have fragile sites that break when they are exposed to carcinogens, which converts them into oncogenes. This may “switch on” dormant genes, allowing cells to become invasive and metastasize. There are more than 100 known oncogenes. Another group of genes, the tumor- suppressor genes (such as p53 and p16) put the “brakes” on cell division, so inactivation of these genes leads to uncontrolled growth, allowing onco- genes to “do their thing.” Tumor-sup- pressor genes also help inactivate carcinogens, aid DNA repair, and enhance the ability of the immune sys- tem to destroy cancer cells.

Consider colorectal cancer (see the figure). One of its first signs is a polyp (a small, benign growth of apparently normal mucosa cells. As cell division continues, the polyp enlarges and may become a malignant neoplasm. In most cases, these changes parallel cel- lular changes at the DNA level and include the activation of oncogenes

131

Specializations of the plasma membrane in- clude microvilli (which increase the absorptive surface area) and cell junctions (desmosomes, tight junctions, and gap junctions).

c. The cytoplasm is where most cellular activities occur. Its fluid substance, the cytosol, contains inclusions, stored or inactive materials in the cytoplasm (fat globules, water vacuoles, crystals, and the like) and specialized bodies called organelles, each with a specific function. For example, mitochondria are sites of ATP synthe- sis, ribosomes are sites of protein synthesis, and the Golgi apparatus packages proteins for export from the cell. Lysosomes carry out intracellular digestion, and peroxisomes disarm dangerous chemicals in the cells. Cytoskeletal elements function in cellular support and motion. The centrioles play a role in cell division and form the bases of cilia and flagella.

Cell Physiology (pp. 102–114)

1. All cells exhibit irritability (the ability to respond to stimuli), digest foods, excrete wastes, and are able to reproduce, grow, move, and metabolize.

2. Transport of substances through the cell mem- brane:

a. Passive processes include diffusion and filtration.

(1) Diffusion is the movement of a substance from an area of its higher concentration to an area of its lower concentration. It occurs because of kinetic energy of the molecules themselves; no ATP is required. The diffu- sion of dissolved solutes through the plasma membrane is simple diffusion. The diffusion of water across the plasma membrane is os- mosis. Diffusion that requires a protein chan- nel or carrier is facilitated diffusion.

132 Essentials of Human Anatomy and Physiology

5. Protein synthesis involves both DNA (the genes) and RNA.

a. A gene is a segment of DNA that carries the instructions for building one protein. The infor- mation is in the sequence of bases in the nucleo- tide strands. Each three-base sequence (triplet) specifies one amino acid in the protein.

b. Messenger RNA carries the instructions for pro- tein synthesis from the DNA (gene) to the ribo- somes. Transfer RNA transports amino acids to the ribosomes. Ribosomal RNA forms part of the ribosomal structure and helps coordinate the protein building process.

PART II: BODY TIssues (pp. 114–128) 1. Epithelium is covering, lining, and glandular tissue.

Its functions include protection, absorption, and secretion. Epithelia are named according to number of layers (simple, stratified) and cell shape (squa- mous, cuboidal, columnar).

2. Connective tissue is the supportive, protective, and binding tissue. It is characterized by the presence of a nonliving, extracellular matrix (ground sub- stance plus fibers) produced and secreted by the cells; it varies in amount and consistency. Fat, liga- ments and tendons, bones, and cartilage are all connective tissues or connective tissue structures.

3. Muscle tissue is specialized to contract, or shorten, which causes movement. There are three types— skeletal (attached to the skeleton), cardiac (in the heart wall), and smooth (in the walls of hollow organs).

4. Nervous tissue is composed of irritable cells called neurons, which are highly specialized to receive and transmit nerve impulses, and supporting cells called neuroglia. Neurons are important in control of body processes. Nervous tissue is located in nervous sys- tem structures—brain, spinal cord, and nerves.

5. Tissue repair (wound healing) may involve regen- eration, fibrosis, or both. In regeneration, the in- jured tissue is replaced by the same type of cells. In fibrosis, the wound is repaired with scar tissue. Epithelia and connective tissues regenerate well. Mature cardiac muscle and nervous tissue are re- paired by fibrosis.

PART III: DeVelOPMeNTAl AsPeCTs OF Cells AND TIssues (pp. 128–131) 1. Growth by cell division continues through puberty.

Cell populations exposed to friction (such as epi- thelium) replace lost cells throughout life.

(2) Filtration is the movement of substances through a membrane from an area of high hydrostatic pressure to an area of lower fluid pressure. In the body, the driving force of fil- tration is blood pressure.

b. Active processes (active transport and vesicular transport) use energy (ATP) provided by the cell.

(1) In active transport, substances are moved across the membrane against an electrical or a concentration gradient by proteins called solute pumps. This accounts for the transport of amino acids, some sugars, and most ions.

(2) The two types of ATP-activated vesicular transport are exocytosis and endocytosis. Exocytosis moves secretions and other sub- stances out of cells; a membrane-bounded vesicle fuses with the plasma membrane, ruptures, and ejects its contents to the cell exterior. Endocytosis, in which particles are taken up by enclosure in a plasma membrane sac, includes phagocytosis (uptake of solid particles), pinocytosis (uptake of fluids), and the highly selective receptor-mediated en- docytosis. In the latter, membrane receptors bind with and internalize only selected target molecules.

3. Osmotic pressure, which reflects the solute concen- tration of a solution, determines whether cells gain or lose water. (Discussed in “A Closer Look” on p. 105.)

a. Hypertonic solutions contain more solutes (and less water) than do cells. In these solutions, cells lose water by osmosis and crenate.

b. Hypotonic solutions contain fewer solutes (and more water) than do the cells. In these solutions, cells swell and may rupture (lyse) as water rushes in by osmosis.

c. Isotonic solutions, which have the same solute- to-solvent ratio as cells, cause no changes in cell size or shape.

4. Cell division has two phases, mitosis (nuclear divi- sion) and cytokinesis (division of the cytoplasm).

a. Mitosis begins after DNA has been replicated (during interphase); it consists of four stages— prophase, metaphase, anaphase, and telophase. The result is two daughter nuclei, each identical to the mother nucleus.

b. Cytokinesis usually begins during anaphase and progressively pinches the cytoplasm in half. Cytokinesis does not always occur; in such cases, binucleate or multinucleate cells result.

c. Mitotic cell division provides an increased num- ber of cells for growth and repair.

Chapter 3: Cells and Tissues 133

3 3. Neoplasms, both benign and cancerous, represent

abnormal cell masses in which normal controls on cell division are not working. Hyperplasia (increase in size) of a tissue or organ may occur when tis- sue is strongly stimulated or irritated. Atrophy (de- crease in size) of a tissue or organ occurs when the organ is no longer stimulated normally.

Connective tissue remains mitotic and forms repair (scar) tissue. With some exceptions, muscle tissue becomes amitotic by the end of puberty, and ner- vous tissue becomes amitotic shortly after birth. Injury can severely handicap amitotic tissues.

2. The cause of aging is unknown, but chemical and physical insults, as well as genetic programming, have been proposed as possible causes.

Review Questions

Multiple Choice

More than one choice may apply.

1. Which of the following would you expect to find in or on cells whose main function is absorption?

a. Microvilli

b. Cilia

c. Gap junctions

d. Secretory vesicles

2. Which of the following cell types would not have tight junctions, desmosomes, or connexons?

a. Phagocytes

b. Epithelial cells

c. Muscle cells

d. Red blood cells

3. Which element of the plasma membrane contributes to its hydrophobic characteristics?

a. Polar heads of phospholipid molecules

b. Nonpolar tails of phospholipid molecules

c. Floating proteins

d. Cholesterol

4. A cell with abundant peroxisomes would most likely be involved in

a. secretion.

b. storage of glycogen.

c. ATP manufacture.

d. movement.

e. detoxification activities.

5. A cell stimulated to increase its steroid production will have abundant

a. ribosomes.

b. rough ER.

c. smooth ER.

d. Golgi apparatus.

e. secretory vesicles.

6. For osmosis to occur, there must be

a. a selectively permeable membrane.

b. isotonic conditions on both sides of the membrane.

c. a difference in solute concentrations.

d. a sodium/potassium ATPase.

e. a hydrostatic pressure gradient.

7. In which of the following tissue types might you expect to find goblet cells?

a. Simple cuboidal

b. Simple columnar

c. Simple squamous

d. Stratified squamous

e. Transitional

8. Which epithelium is “built” to withstand friction?

a. Simple squamous

b. Stratified squamous

c. Simple cuboidal

d. Simple columnar

e. Pseudostratified

9. What kind of connective tissue acts as a sponge, soaking up fluid when edema occurs?

a. Areolar connective

b. Adipose connective

c. Dense irregular connective

d. Reticular connective

e. Vascular

10. What type of connective tissue prevents muscles from pulling away from bones during contraction?

a. Dense connective

b. Areolar

c. Elastic connective

d. Hyaline cartilage

11. Which of the following is (are) not true of skeletal muscles?

a. They pull on bones or skin when they contract.

b. They have intercalated discs.

c. They are controlled voluntarily.

d. They have striations.

e. They contain gap junctions.

12. Cancer is the same as

a. all tumors.

b. all neoplasms.

c. all malignant neoplasms.

d. benign tumors.

e. AIDS.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

134 Essentials of Human Anatomy and Physiology

24. Why can an organ be permanently damaged if its cells are amitotic?

25. Describe the relative roles of DNA and RNA in protein synthesis.

26. Define tissue. List the four major types of tissue. Which of these types is most widely distributed in the body?

27. Describe the general characteristics of epithelial tissue. List the most important functions of epithelial tissues, and give examples of each.

28. What is the function of transitional epithelium, and how does it carry out this function?

29. What are the general structural characteristics of connective tissues? What are the functions of connective tissues? How are their functions reflected in their structures?

30. Why is blood considered to be a connective tissue?

31. What is the function of muscle tissue?

32. Tell where each of the three types of muscle tissue would be found in the body. What is meant by the statement, “Smooth muscles are involuntary in action”?

33. How does tissue healing by fibrosis differ from healing by regeneration? Which is more desirable and why?

34. Define contracture.

35. What is a neoplasm?

short Answer essay

13. Define cell and organelle.

14. Although cells have differences that reflect their special functions in the body, what functional abilities do all cells exhibit?

15. Describe the special function of DNA found in the nucleus. Which nuclear structures contain DNA? Which help to form ribosomes?

16. Describe the structures and functions of plasma membrane proteins in tight junctions, desmosomes, and gap junctions.

17. Name the cellular organelles, and explain the function of each.

18. What two structural characteristics of cell membranes determine whether substances can pass through them passively? What determines whether or not a substance can be actively transported through the membrane?

19. Explain the effect of the following solutions on living cells: hypertonic, hypotonic, and isotonic.

20. Briefly describe the process of DNA replication.

21. Define mitosis. Why is mitosis important?

22. What is the role of the spindle in mitosis?

23. Explain what the terms triplet, codon, and anticodon mean.

Critical Thinking and Clinical Application Questions

36. Two examples of chemotherapeutic drugs (used to treat cancer) and their cellular actions are given below. Explain why each drug could be fatal to a cell.

• Vincristine: damages the mitotic spindle.

• Doxorubicin: binds to DNA and blocks messen- ger RNA synthesis.

37. Hydrocortisone is an anti-inflammatory drug that stabilizes lysosomal membranes. Explain how this effect reduces cell damage and inflammation.

38. John has severely injured his knee during football practice. He is told that he has a torn knee cartilage and to expect that recovery and repair will take a long time. Why will it take a long time?

39. Three patients in an intensive care unit are examined by the resident doctor. One patient has brain damage from a stroke, another had a heart attack that severely damaged his heart muscle, and the third has a severely damaged liver (a gland) from a crushing injury in a car accident. All three

patients have stabilized and will survive, but only one will have full functional recovery through regeneration. Which one, and why?

40. Michael had a nervous habit of chewing on the inner lining of his lip with his front teeth. The lip grew thicker and thicker from years of continual irritation. Michael’s dentist noticed his greatly thickened lip, then told him to have it checked to see if the thickening was a tumor. A biopsy revealed hyperplasia, but no evidence of neoplasia. What do these terms mean? Did Michael have cancer of the mouth?

41. Think carefully about the chemistry of the plasma membrane, and then answer this question: Why is minor damage to the membrane usually not a problem?

42. Lisa, an elderly lady, is overweight and has a sedentary lifestyle. She has recently developed puffy ankles. What is this condition called, and how do you think this has developed?

135

Classification of Body Membranes ➔ Learning Objectives

□□ List the general functions of each membrane type—cutaneous, mucous, serous, and synovial— and give its location in the body.

□□ Compare the structure (tissue makeup) of the major membrane types.

The two major categories of body membranes— epithelial and connective tissue—are classified in part according to their tissue makeup.

Body membranes cover surfaces, line body cavities, and form protective (and often lubricating) sheets around organs. They fall into two major groups: (1) epithelial mem- branes, which include the cutaneous, mucous, and serous membranes; and (2) connective tissue mem- branes, represented by synovial membranes. The cutaneous membrane, generally called the skin or integumentary system, is the outer covering that we all rely on for protection. In this chapter, the skin will receive most of our attention, but first we will consider the other body membranes.

Skin and Body Membranes4 WHAT HOW

WHY

The integument includes body membranes, skin, hair, fingernails, and

sweat and oil glands.

The skin has multiple layers that protect the

body, help regulate body temperature, and help excrete

wastes via sweat.

The skin is the first barrier to keep good things

such as water in, and bad things such as harmful bacteria out.

InStruCtorS

New Building Vocabulary Coaching Activities for this chapter are assignable in

136 Essentials of Human Anatomy and Physiology

Serous Membranes

A serous membrane, or serosa, is composed of a layer of simple squamous epithelium resting on a thin layer of areolar connective tissue. In con- trast to mucous membranes, which line open body cavities, serous membranes line body cavities that are closed to the exterior (except for the dorsal body cavity and joint cavities).

Serous membranes occur in pairs (Figure 4.1c). The parietal (pah-ri9ĕ-tal: parie 5 wall) layer lines a specific portion of the wall of the ventral body cavity. It folds in on itself to form the visceral (vis9er-al) layer, which covers the outside of the organ(s) in that cavity.

You can visualize the relationship between the serosal layers by pushing your fist into a limp bal- loon only partially filled with air (Figure 4.1d). The part of the balloon that clings to your fist can be compared to the visceral serosa clinging to the organ’s external surface. The outer wall of the bal- loon represents the parietal serosa that lines the walls of the cavity and that, unlike the balloon, is never exposed but is always fused to the cavity wall.

In the body, the serous layers are separated not by air but by a scanty amount of thin, clear fluid, called serous fluid, which is secreted by both membranes. Although there is a potential space between the two membranes, they tend to lie very close to each other. The lubricating serous fluid allows the organs to slide easily across the cavity walls and one another without friction as they carry out their routine functions. This is extremely important when mobile organs such as the pumping heart and expanding lungs are involved.

The specific names of the serous membranes depend on their locations. The serosa lining the abdominal cavity and covering its organs is the peritoneum (per0ı̆ -to-ne9um). In the thorax, serous membranes isolate the lungs and heart from one another. The membranes surrounding the lungs are the pleurae (ploo9re); those around the heart are the pericardia (per0ı̆ -kar9de-ah).

Connective tissue Membranes Synovial (sı̆-no9ve-al) membranes are composed of loose areolar connective tissue and contain no epithelial cells at all. These membranes line the

Epithelial Membranes The epithelial membranes, also called covering and lining membranes, include the cutaneous membrane (skin), the mucous membranes, and the serous membranes (Figure 4.1). However, call- ing these membranes “epithelial” is misleading because it is not the whole story. Although they all do contain an epithelial layer, it is always com- bined with an underlying layer of connective tis- sue. Hence these membranes are actually simple organs. Because we will discuss the skin in detail shortly, we will list it here solely as a subcategory of the epithelial membranes.

Cutaneous Membrane

The cutaneous (ku-ta9ne-us) membrane is com- posed of two layers, the superficial epidermis and the underlying dermis. The epidermis is composed of stratified squamous epithelium, whereas the dermis is mostly dense (fibrous) connective tissue. Unlike other epithelial membranes, the cutaneous membrane is exposed to air and is a dry mem- brane (Figure 4.1a).

Mucous Membranes

A mucous (myu9kus) membrane (mucosa) is composed of epithelium (the type varies with the site) resting on a loose connective tissue mem- brane called a lamina propria (Figure 4.1b). This membrane type lines all body cavities that open to the exterior, such as those of the hollow organs of the respiratory, digestive, urinary, and repro- ductive tracts. Notice that the term mucosa refers only to the location of the epithelial membranes, not their cellular makeup, which varies. However, most mucosae contain either stratified squamous epithelium (as in the mouth and esophagus) or simple columnar epithelium (as in the rest of the digestive tract). In all cases, they are moist mem- branes that are almost continuously bathed in secretions or, in the case of the urinary mucosae, urine.

The epithelium of mucosae is often adapted for absorption or secretion. Although many muco- sae secrete mucus, not all do. The mucosae of the respiratory and digestive tracts secrete large amounts of protective, lubricating mucus; that of the urinary tract does not.

Chapter 4: Skin and Body Membranes 137

Cutaneous membrane (skin)

Mucosa

Parietal layer

Visceral layer

Serous fluid

(a) Cutaneous membrane (the skin) covers the body surface.

(d) A fist thrust into a limp balloon demonstrates the relationship between the parietal and visceral serous membrane layers.

(b) Mucous membranes line body cavities open to the exterior.

Outer balloon wall (comparable to parietal serosa)

Air (comparable to serous cavity)

Inner balloon wall (comparable to visceral serosa)

Figure 4.1 Classes of epithelial membranes.

138 Essentials of Human Anatomy and Physiology

the Integumentary System (Skin) ➔ Learning Objectives

□□ List several important functions of the integumentary system, and explain how these functions are accomplished.

□□ When provided with a model or diagram of the skin, recognize and name the following skin structures: epidermis, dermis (papillary and reticular layers), hair and hair follicle, sebaceous gland, and sweat gland.

□□ name the layers of the epidermis, and describe the characteristics of each.

Would you be enticed by an advertisement for a coat that is waterproof, stretchable, and washable; that invisibly repairs small cuts, rips, and burns; and that is guaranteed to last a lifetime with rea- sonable care? Sounds too good to be true, but you already have such a coat—your cutaneous mem- brane, or skin.

Skin is absolutely essential because it keeps water and other precious molecules in the body. It also keeps excess water (and other things) out. (This is why you can swim for hours without becoming waterlogged.) Structurally, the skin is a marvel. It is pliable yet tough, which allows it to take constant punishment from external agents. Without our skin, we would quickly fall prey to bacteria and perish from water and heat loss.

The skin and its appendages (sweat and oil glands, hair, and nails) are collectively called the integumentary (in-teg0u-men9tuh-re) system. Let’s take a look at the structure and function of this often underappreciated body system.

Functions of the Integumentary System The integumentary system, also called the integu- ment (in-teg9u-ment), which simply means “cover- ing,” performs a variety of functions; most, but not all, of which are protective (table 4.1). It insulates and cushions the deeper body organs and protects the entire body from mechanical damage (bumps and cuts), chemical damage (such as from acids and bases), thermal damage (heat and cold), ultraviolet (UV) radiation (in sunlight), and microbes. The uppermost layer of the skin is hardened, to help prevent water loss from the body surface.

The skin’s rich capillary network and sweat glands (both controlled by the nervous system)

fibrous capsules surrounding joints (Figure 4.2), where they provide a smooth surface and secrete a lubricating fluid. They also line small sacs of connective tissue called bursae (ber9se) and the tubelike tendon sheaths. Both of these structures cushion organs moving against each other during muscle activity—such as the movement of a ten- don across a bone’s surface.

Did You Get It? 1. How do the body locations of serous and mucous

membranes differ? 2. A scalpel penetrates the left lung and enters the

heart. Name the six serous membrane layers the blade passes through as it moves from the body surface into the heart.

3. Where would you find a synovial membrane?

For answers, see Appendix A.

Ligament

Joint cavity (contains synovial fluid)

Articular (hyaline) cartilage

Figure 4.2 A typical synovial joint.

Chapter 4: Skin and Body Membranes 139

Table 4.1 Functions of the Integumentary System

Functions How accomplished

Protects deeper tissues from

• Mechanical damage (bumps) Physical barrier contains keratin, which toughens cells; fat cells to cushion blows; and both pressure and pain receptors, which alert the nervous system to possible damage.

• Chemical damage (acids and bases) Has relatively impermeable keratinized cells; contains pain receptors, which alert the nervous system to possible damage.

• Microbe damage Has an unbroken surface and “acid mantle” (skin secretions are acidic and thus inhibit microbes, such as bacteria). Phagocytes ingest foreign substances and pathogens, preventing them from penetrating into deeper body tissues.

• Ultraviolet (UV) radiation (damaging effects of sunlight or tanning beds)

Melanin produced by melanocytes offers protection from UV damage.

• Thermal (heat or cold) damage Contains heat/cold/pain receptors.

• Desiccation (drying out) Contains a water-resistant glycolipid and keratin.

Aids in body heat loss or heat retention (controlled by the nervous system)

Heat loss: By activating sweat glands and by allowing blood to flush into skin capillary beds so that heat can radiate from the skin surface.

Heat retention: By not allowing blood to flush into skin capillary beds.

Aids in excretion of urea and uric acid Contained in perspiration produced by sweat glands.

Synthesizes vitamin D Modified cholesterol molecules in skin converted to vitamin D in the presence of sunlight.

play an important role in regulating heat loss from the body surface. The skin acts as a mini-excretory system; urea, salts, and water are lost when we sweat. The skin is also a chemical plant; it manu- factures several proteins important to immunity and synthesizes vitamin D. (Modified cholesterol molecules in the skin are converted to vitamin D by sunlight.) The skin also produces acidic secre- tions, called the acid mantle, that protect against bacterial invasion.

Did You Get It? 4. Explain the relationships between the words skin,

cutaneous membrane, integument, and integumentary system.

5. What are three important functions of the integumentary system?

For answers, see Appendix A.

Structure of the Skin As we mentioned, the skin is composed of two kinds of tissue (Figure 4.3). The outer epidermis

(ep0ı̆ -der9mis) is made up of stratified squamous epithelium that is capable of becoming hard and tough. The underlying dermis is made up mostly of dense connective tissue. The epidermis and dermis are firmly connected and the dermis is fairly tear resistant. However, a burn or friction (such as the rubbing of a poorly fitting shoe) may cause them to separate, allowing interstitial fluid to accumulate in the cavity between the layers, which results in a blister.

Deep to the dermis is the subcutaneous tissue, or hypodermis, which essentially is adi- pose (fat) tissue. It is not considered part of the skin, but it does anchor the skin to underlying organs and provides a site for nutrient storage. Subcutaneous tissue serves as a shock absorber and insulates the deeper tissues from extreme temperature changes occurring outside the body. It is also responsible for the curves that are more a part of a woman’s anatomy than a man’s.

140 Essentials of Human Anatomy and Physiology

to the dermis and is connected to it along a wavy border that resembles corrugated cardboard. This basal layer contains the most adequately nour- ished of the epidermal cells because nutrients dif- fusing from the dermis reach them first. Stem cells in this layer are constantly dividing, and millions of new cells are produced daily; hence its alter- nate name, stratum germinativum (jer0min-ah- tiv9um; “germinating layer”). Of the new cells produced, some become epidermal cells, and oth- ers maintain the population of stem cells by con- tinuing to divide. The daughter cells destined to become epidermal cells are pushed upward, away from the source of nutrition, to become part of the epidermal layers closer to the skin surface. As they move away from the dermis and become part of the more superficial layers, the stratum spino- sum and then the stratum granulosum, they become flatter and increasingly keratinized.

As these cells leave the stratum granulosum, they die, forming the clear stratum lucidum

Epidermis

Most cells of the epidermis are keratinocytes (ker- atin cells), which produce keratin, the fibrous pro- tein that makes the epidermis a tough protective layer in a process called keratinization. These kerat- inocytes are connected by desmosomes through- out the epidermis (refer to Figure 3.3, p. 92).

Like all other epithelial tissues, the epidermis is avascular; that is, it has no blood supply of its own. This explains why a man can shave daily and not bleed even though he cuts off many cell layers each time he shaves.

The epidermis is composed of up to five lay- ers, or strata (strah9tah). From the inside out these are the stratum basale, spinosum, granulosum, lucidum, and corneum, illustrated in Figure 4.4 (except the stratum lucidum, which is found only in thick skin).

The deepest cell layer of the epidermis, the stratum basale (stra9tum ba-sah9le), lies closest

Epidermis

Hair shaft

Dermis Reticular layer

Papillary layer

Hypodermis (subcutaneous tissue)

Dermal papillae

Pore

Appendages of skin • Eccrine sweat gland • Arrector pili muscle • Sebaceous (oil) gland • Hair follicle • Hair root (inside follicle)

Nervous structures • Sensory nerve fiber • Lamellar corpuscle • Hair follicle receptor (root hair plexus)

Cutaneous vascular plexus

Adipose tissue

Figure 4.3 Skin structure. Microscopic view of the skin and underlying subcutaneous tissue.

Chapter 4: Skin and Body Membranes 141

superficial epidermal cells because they are unable to get adequate nutrients and oxygen.

The outermost layer, the stratum corneum (kor9ne-um), is 20 to 30 cell layers thick, but it accounts for about three-quarters of the epidermal thickness. The shinglelike dead cell remnants, com- pletely filled with keratin, are referred to as corni- fied, or horny, cells (cornu 5 horn). The common saying “Beauty is only skin deep” is especially interesting in light of the fact that nearly everything we see when we look at someone is dead! The abundance of the tough keratin protein in the stra- tum corneum allows that layer to provide a durable

(lu9sid-um). This latter epidermal layer is not pres- ent in all skin regions. It occurs only where the skin is hairless and extra thick, that is, on the palms of the hands and soles of the feet. The com- bination of accumulating keratin inside them, secreting a water-repellent glycolipid into the extracellular space, and their increasing distance from the blood supply (in the dermis) effectively dooms the stratum lucidum cells and the more

Sensory nerve ending

Melanocyte

Melanin granules

Merkel cell

Epidermal dendritic cell

Dermis

Stratum corneum. Cells are dead, flat membranous sacs filled with keratin. Glycolipids in extracellular space make skin water resistant.

Stratum granulosum. Cells are flattened, organelles are deteriorating; cytoplasm full of granules.

Stratum spinosum. Cells contain thick bundles of intermediate filaments made of pre-keratin.

Stratum basale. Stem cells are actively dividing; some newly formed cells become part of the more superficial layers.

Keratinocytes

Desmosomes

Figure 4.4 the main structural features of the epidermis. Keratinocytes (tan), connected by many desmosomes, form most of the

epidermis. Melanocytes (gray) make the pigment melanin, and star- shaped epidermal dendritic cells (blue) are protective immune cells.

Occasional Merkel cells (purple), each associated with a nerve ending, act as touch receptors. (Only part of the stratum corneum is illustrated.)

If this were an image of thick skin from the bottom of your foot, where would the stratum lucidum be located?Q:

Stratum lucidum is located between stratum granulosum and stratum corneum.A:

142 Essentials of Human Anatomy and Physiology

alert and activate immune system cells to a threat such as bacterial or viral invasion. Seen here and there at the epidermal-dermal junction are Merkel cells, which are associated with sensory nerve endings and serve as touch receptors called Merkel discs.

Homeostatic Imbalance 4.1

Despite melanin’s protective effects, excessive exposure to UV light (via sunlight or tanning beds) eventually damages the skin, leading to a leathery appearance. It also depresses the immune system. This may help to explain why people infected with the human herpesvirus 1, which causes cold sores, are more likely to have an eruption after sunbathing. Overexposure to the sun can also alter the DNA of skin cells, leading to skin cancer. People with very dark skin seldom have skin cancer, attesting to melanin’s amazing effec- tiveness as a natural sunscreen. ________________✚

Did You Get It? 6. What cell type is most abundant in the epidermis? 7. Which layer of the epidermis produces new epidermal

cells? 8. Excess shedding of dead cells from the superficial layer

of the skin of the scalp causes dandruff. What is the name of that skin layer?

9. In which epidermal layer do cells begin to die?

For answers, see Appendix A.

Dermis

The dermis is your “hide.” It is a strong, stretchy envelope that helps to bind the body together. When you purchase leather goods (bags, belts, shoes, and the like), you are buying the treated dermis of animals.

The connective tissue making up the dermis consists of two major regions—the papillary and the reticular areas (Figure 4.5), which are com- posed of areolar and dense irregular connective tissue, respectively. Like the epidermis, the dermis varies in thickness. For example, it is particularly thick on the palms of the hands and soles of the feet but is quite thin on the eyelids.

The papillary layer is the superficial dermal region. It is uneven and has peglike projections from its superior surface, called dermal papillae (pah-pil9e; papill 5 nipple), which indent the epi- dermis above. Many of the dermal papillae contain capillary loops, which furnish nutrients to the

“overcoat” for the body, which protects deeper cells from the hostile external environment and from water loss, and helps the body resist biologi- cal, chemical, and physical assaults. The stratum corneum rubs and flakes off slowly and steadily as the dandruff familiar to everyone. The average person sheds about 18 kg (40 lb) of these flakes in a lifetime, providing a food source for the dust mites that inhabit our homes and bed linens. This layer is replaced by cells produced by the division of the deeper stratum basale cells. Indeed, we have a totally “new” epidermis every 25 to 45 days.

Melanin (mel9ah-nin), a pigment that ranges in color from yellow to brown to black, is pro- duced by special spider-shaped cells called mela- nocytes (mel9ah-no-sı̄ tz), found chiefly in the stratum basale. Freckles and moles are seen where melanin is concentrated in one spot.

Scattered in the epidermis are epidermal den- dritic cells, which are important “sentries” that

Dermis

Epidermis

Papillary layer

Reticular layer

Figure 4.5 Light micrograph of the two layers of the dermis (100∙). The dermal papillae are extensions of the superficial papillary layer, which consists of areolar connective tissue. The deeper reticular layer is dense irregular fibrous connective tissue.

View Histology

A CLOSER

LOOK A Wrinkle Out of Time

When it comes to preventing wrinkles, it helps to have good genes, to not smoke, to use a good sunscreen, and to think pleasant thoughts. Smoking ages the skin by increasing production of an enzyme that destroys collagen. Collagen sup- ports the skin and keeps the skin hydrated, so with less of it, wrinkles appear. Too much unprotected expo- sure to the sun or tanning beds leads to UV radiation damage, which causes elastic fibers to clump, resulting in leathery skin. For those wrinkled by years of smoking and sun damage, a surgical face-lift that removes the excess and sagging skin followed by laser resurfacing or microdermabra- sion seems to be the only way to ban- ish the wrinkles.

However, for those who sport frown lines, furrowed brows, or crow’s feet due to frequent and repetitive facial expressions, cosmetic injections of Botox may be the answer to regain-

ing younger-looking skin. Botulinum toxin type A, more familiarly called Botox Cosmetic, is a neurotoxin pro- duced by the bacterium that causes botulism, a deadly form of food poi- soning. Injected in extremely small doses, the purified toxin blocks acetyl- choline (ACh) release by nerve cells (ACh is a chemical signal from nerves telling muscles to contract), thus inhibiting the muscle’s ability to con- tract. This “induced paralysis” smooths lines in about a week.

Botox was approved in 1989 to treat two eye muscle disorders— uncontrollable blinking (blepharo- spasm) and misaligned eyes (strabismus). The discovery that Botox could be used cosmetically was pure luck—physicians using the toxin to counter abnormal eye contractions noticed that frown lines between the eyes had softened.

The process has some risks. If too much toxin is injected, a person can

end up with droopy eyelid muscles or temporary muscle weakness for weeks (the effects of Botox Cosmetic last 3 to 6 months), and the toxin can spread to nearby tissues. Still, bat- tling the signs of age in a noninvasive way is appealing to many people, and the fact that there is little or no recovery time allows treatment dur- ing a lunch hour. The attraction of Botox to physicians is both profes- sional (a new tool to fight wrinkles) and monetary (truly dedicated patients are back for injections every 3 to 6 months).

epidermis. Others house pain receptors (free nerve endings) and touch receptors. On the palms of the hands and soles of the feet, the papillae are arranged in definite patterns that form looped and whorled ridges on the epidermal surface that increase friction and enhance the gripping ability of the fingers and feet. Papillary patterns are genetically determined. The ridges of the finger- tips are well provided with sweat pores and leave unique, identifying films of sweat called finger- prints on almost anything they touch.

The reticular layer is the deepest skin layer. It contains dense irregular connective tissue, as well as blood vessels, sweat and oil glands, and deep pressure receptors called lamellar corpuscles.

Other cutaneous sensory receptors, which are actually part of the nervous system, are also located in the skin. These tiny sensors, which include

touch, pressure, temperature, and pain receptors, provide us with a great deal of information about our external environment. They alert us to sources of heat or cold, or the tickle of a bug exploring our skin. In addition to detecting stimuli from our envi- ronment, phagocytes found here (and, in fact, throughout the dermis) act to prevent microbes that have managed to get through the epidermis from penetrating any deeper into the body.

Both collagen and elastic fibers are found throughout the dermis. Collagen fibers are respon- sible for the toughness of the dermis; they also attract and bind water and thus help to keep the skin hydrated. Elastic fibers give the skin its elas- ticity when we are young. As we age, the number of collagen and elastic fibers decreases, and the subcutaneous tissue loses fat. As a result, the skin loses its elasticity and begins to sag and wrinkle.

143

144 Essentials of Human Anatomy and Physiology

Skin Color ➔ Learning Objective

□□ name the factors that determine skin color, and describe the function of melanin.

Three pigments contribute to skin color: melanin, carotene, and hemoglobin.

• The amount and kind (yellow, reddish brown, or black) of melanin in the epidermis. Skin exposure to sunlight stimulates melanocytes to produce more melanin pigment, resulting in tanning of the skin. As the melanocytes pro- duce melanin, it accumulates in their cyto- plasm in membrane-bound granules called melanosomes. These granules then move to the ends of the melanocytes’ spidery arms, where they are taken up by nearby keratino- cytes. Inside the keratinocytes, the melanin forms a pigment umbrella over the superficial, or “sunny,” side of their nuclei and shields their genetic material (DNA) from the damaging effects of UV radiation in sunlight (see Figure 4.4). People who produce a lot of mela- nin have brown-toned skin, whereas people with less melanin are light skinned.

• The amount of carotene deposited in the stra- tum corneum and subcutaneous tissue. (Carotene is an orange-yellow pigment plenti- ful in carrots and other orange, deep yellow, or leafy green vegetables.) In people who eat large amounts of carotene-rich foods, the skin tends to take on a yellow-orange cast.

• The amount of oxygen-rich hemoglobin (pig- ment in red blood cells) in the dermal blood vessels. In light-skinned people, the crimson color of oxygen-rich hemoglobin in the dermal blood supply flushes through the transparent cell layers above and gives the skin a rosy glow.

Homeostatic Imbalance 4.3

When hemoglobin is poorly oxygenated, both the blood and the skin of light-skinned people appear blue, a condition called cyanosis (si0ah-no9sis). Cyanosis is common during heart failure and severe breathing disorders. In dark-skinned peo- ple, the skin does not appear cyanotic in the same situations because of the masking effects of melanin, but cyanosis is apparent in their mucous membranes and nail beds. ____________________✚

The dermis is abundantly supplied with blood vessels that play a role in maintaining body tem- perature homeostasis. When body temperature is high, the capillaries of the dermis become engorged, or swollen, with heated blood, and the skin becomes reddened and warm. This allows body heat to radiate from the skin surface. If the environ- ment is cool and body heat must be conserved, blood bypasses the dermis capillaries temporarily, allowing internal body temperature to remain high.

Homeostatic Imbalance 4.2

Any restriction of the normal blood supply to the skin results in cell death and, if severe or prolonged enough, skin ulcers. Decubitus (de-ku9bı̆-tus) ulcers (bedsores) occur in bedrid- den patients who are not turned regularly or who are dragged or pulled across the bed repeatedly. The weight of the body puts pressure on the skin, especially over bony projections. Because this pressure restricts the blood supply, the skin becomes pale or blanched at pressure points. At first, the skin reddens when pressure is released, but if the situation is not corrected, the cells begin to die, and small cracks or breaks in the skin appear at compressed sites. Permanent damage to the superficial blood vessels and tissue eventually results in degeneration and ulceration of the skin (see the photos).

(a) Stage II decubitus ulcer

(b) Stage III decubitus ulcer

The dermis also has a rich nerve supply. As mentioned earlier, many of the nerve endings are designed to detect different types of stimuli (pres- sure, temperature, pain, etc.), then send messages to the central nervous system for interpretation. (We discuss these cutaneous receptors in more detail in Chapter 7.)

_____________________________________________✚

Chapter 4: Skin and Body Membranes 145

Emotions also influence skin color, and many alterations in skin color signal certain disease states:

• Redness, or erythema (er 0ı̆-the9mah). Reddened skin may indicate embarrassment (blushing), fever, hypertension, inflammation, or allergy.

• Pallor, or blanching. Under certain types of emotional stress (fear, anger, and others), some people become pale. Pale skin may also sig- nify anemia, low blood pressure, or impaired blood flow into the area.

• Jaundice (jon9dis), or a yellow cast. An abnor- mal yellow skin tone usually signifies a liver disorder in which excess bile pigments accu- mulate in the blood, circulate throughout the body, and become deposited in body tissues.

• Bruises. The black-and-blue marks of bruising reveal sites where blood has escaped from the circulation and has clotted in the tissue spaces. Such clotted blood masses are called hemato- mas. An unusual tendency to bruise may sig- nify a deficiency of vitamin C in the diet or he- mophilia (bleeder’s disease).

Did You Get It? 10. Which structure(s) are responsible for fingerprints? 11. You have just gotten a paper cut, but it doesn’t

bleed. Which layer(s) of the skin has/have the cut penetrated?

12. What pigments determine skin color?

For answers, see Appendix A.

Appendages of the Skin ➔ Learning Objective

□□ Describe the distribution and function of the epidermal appendages—sebaceous glands, sweat glands, hair, and nails.

The skin appendages include cutaneous glands, hair and hair follicles, and nails (see Figure 4.3). Each of these appendages arises from the epider- mis and plays a unique role in maintaining body homeostasis.

Cutaneous Glands

The cutaneous glands are all exocrine glands that release their secretions to the skin surface via ducts. They fall into two groups: sebaceous glands and sweat glands. As these glands are formed by the cells of the stratum basale, they push into the deeper skin regions and ultimately reside almost entirely in the dermis.

Sebaceous (Oil) Glands The sebaceous (se- ba9shus) glands, or oil glands, are found all over the skin, except on the palms of the hands and the soles of the feet. Their ducts usually empty into a hair follicle (Figure 4.6a, p. 146), but some open directly onto the skin surface.

The product of the sebaceous glands, sebum (se9bum; seb 5 grease), is a mixture of oily sub- stances and fragmented cells. Sebum is a lubricant that keeps the skin soft and moist and prevents the hair from becoming brittle. Sebum also con- tains chemicals that kill bacteria, so it is important in preventing bacterial infection of the skin. The sebaceous glands become very active when androgens (male sex hormones) are produced in increased amounts (in both sexes) during adoles- cence. Thus, the skin tends to become oilier dur- ing this period of life.

Homeostatic Imbalance 4.4

When sebaceous gland ducts are blocked by sebum, acne appears on the skin surface. Acne is an active infection of the sebaceous glands. If the accumulated material oxidizes and dries, it darkens, forming a blackhead. If the material does not dry or darken, a whitehead forms. Acne can be mild or extremely severe, leading to permanent scarring.

Seborrhea (seb0o-re9ah; “fast-flowing sebum”), known as “cradle cap” in infants, is caused by overactivity of the sebaceous glands. It begins on the scalp as pink, raised lesions that gradually

146 Essentials of Human Anatomy and Physiology

traces of metabolic wastes (ammonia, urea, uric acid), and lactic acid (a chemical that accumulates during vigorous muscle activity). Sweat is acidic (pH from 4 to 6), a characteristic that inhibits the growth of certain bacteria, which are always pres- ent on the skin surface. Typically, sweat reaches the skin surface via a duct that opens externally as a funnel-shaped sweat pore (Figure 4.6b). Notice, however, that the facial “pores” commonly referred to when we talk about our complexion are the external outlets of hair follicles where sebaceous ducts empty, not these sweat pores.

The eccrine sweat glands are an important and highly efficient part of the body’s heat-regulating equipment. They are supplied with nerve endings

form a yellow-to-brown crust that sloughs off oily scales and dandruff. Careful washing to remove the excessive oil often helps cradle cap in a new- born baby. ___________________________________✚

Sweat Glands Sweat glands, also called sudo- riferous (su0do-rif9er-us; sudor 5 sweat) glands, are widely distributed in the skin. Their number is staggering—more than 2.5 million per person. There are two types of sweat glands, eccrine and apocrine.

The eccrine (ek9rin) glands are far more numerous and are found all over the body. They produce sweat, a clear secretion that is primarily water plus some salts (sodium chloride), vitamin C,

Sebaceous gland

Sweat pore

Eccrine gland

(a) Photomicrograph of a sectioned sebaceous gland (100×)

(b) Photomicrograph of a sectioned eccrine gland (205×)

Sebaceous gland duct

Eccrine gland duct Hair in hair follicle

Secretory cells

Dermal connective tissue

Figure 4.6 Cutaneous glands. (Note that apocrine glands are not represented here.) View Histology

Chapter 4: Skin and Body Membranes 147

providing insulation in cold weather, but now we have other means of keeping warm.

Hairs A hair is a flexible epithelial structure. The part of the hair enclosed in the hair follicle is called the root, and the part projecting from the surface of the scalp or skin is called the shaft (Figure 4.7a, p. 148). A hair forms by division of the well-nourished stratum basale epithelial cells in the matrix (growth zone) of the hair bulb at the deep end of the follicle. As the daughter cells are pushed farther away from the growing region, they become keratinized and die. Thus the bulk of the hair shaft, like the bulk of the epidermis, is dead material and almost entirely protein.

Each hair is made up of a central core called the medulla (mĕ-dul9ah), consisting of large cells and air spaces, surrounded by a bulky cortex layer composed of several layers of flattened cells (Figure 4.7b). The cortex is, in turn, enclosed by an outermost cuticle formed by a single layer of cells that overlap one another like shingles on a roof. This arrangement of the cuticle cells helps to keep the hairs apart and keeps them from matting (Figure 4.7c). The cuticle is the most heavily kera- tinized region; it provides strength and helps keep the inner hair layers tightly compacted. Because it is most subject to abrasion, the cuticle tends to wear away at the tip of the shaft, allowing the kera- tin fibrils in the inner hair regions to frizz out, a phenomenon called “split ends.” Hair pigment is made by melanocytes in the hair bulb, and varying amounts of different types of melanin (yellow, rust, brown, and black) combine to produce all varieties of hair color from pale blond to red to pitch black.

Hairs come in a variety of sizes and shapes. They are short and stiff in the eyebrows, long and flexible on the head, and usually nearly invisible almost everywhere else. When the hair shaft is oval, hair is smooth, silky, and wavy. When the shaft is flat and ribbonlike, the hair is curly or kinky. If it is perfectly round, the hair is straight and tends to be coarse.

Hair Follicles Hair follicles (folli 5 bag) are actually compound structures. The inner epithelial root sheath is composed of epithelial tissue and forms the hair. The outer fibrous sheath is actually dermal connective tissue (Figure 4.7d). This der- mal region supplies blood vessels to the epidermal portion and reinforces it. Its nipplelike hair papilla

that cause them to secrete sweat when the exter- nal temperature or body temperature is too high. When sweat evaporates off the skin surface, it car- ries large amounts of body heat with it. On a hot day, it is possible to lose up to 7 liters of body water in this way. The heat-regulating functions of the body are important—if internal temperature changes more than a few degrees from the normal 37°C (98.6°F), life-threatening changes occur in the body. (We discuss body temperature regula- tion in more detail in Chapter 14.)

Apocrine (ap9o-krin) glands are largely con- fined to the axillary (armpit) and genital areas of the body. They are usually larger than eccrine glands, and their ducts empty into hair follicles. Their secretion contains fatty acids and proteins, as well as all the substances present in eccrine sweat; consequently, it may have a milky or yel- lowish color. The secretion is odorless, but when bacteria that live on the skin use its proteins and fats as a source of nutrients for their growth, it can take on a musky, sometimes unpleasant odor.

Apocrine glands begin to function during puberty under the influence of androgens. Although they produce their secretions almost continuously, apocrine glands play a minimal role in thermoreg- ulation. Their precise function is not yet known, but they are activated by nerve fibers during pain and stress and during sexual arousal.

Hair and Hair Follicles

Hair is an important part of our body image. Consider, for example, the spiky hair style of punk rockers and the flowing locks of high-fashion models. Millions of hairs, produced by hair folli- cles, are found all over the body surface except on the palms of the hands, soles of the feet, nipples, and lips. Humans are born with as many hair fol- licles as they will ever have, and hairs are among the fastest growing tissues in the body. Hair serves a few minor protective functions, such as guarding the head against bumps, shielding the eyes (via eyelashes), and helping to keep foreign particles out of the respiratory tract (via nose hairs). Hair may also help to attract sexual partners, as evi- denced by its place in our body image. Hormones account for the development of hairy regions—the scalp and, in the adult, the pubic and axillary areas. Despite these functions, however, our body hair has lost much of its usefulness. Hair served early humans (and still serves hairy animals) by

148 Essentials of Human Anatomy and Physiology

arrector pili (ah-rek9tor pi9li; “raiser of hair”)— connect each side of the hair follicle to the dermal tissue. When these muscles contract (as when we are cold or frightened), the hair is pulled upright, dimpling the skin surface with “goose bumps.”

provides the blood supply to the matrix in the hair bulb (the deepest part of the follicle).

If you look carefully at the structure of the hair follicle, you will notice that it is slightly slanted (see Figure 4.7a). Small bands of smooth muscle cells—

(a) Hair in hair follicle

Arrector pili muscle

Sebaceous gland

Hair root Hair follicle

Hair root

Hair bulb in follicle

(b) Hair (c) Scanning electron micrograph of hair shaft emerging from follicle (435×)

Cuticle

Cortex

Medulla

Hair shaft

Hair follicle

Hair matrix (growth zone) in hair bulb

Cuticle

Cortex

Medulla

Hair papilla containing blood vessels

Subcutaneous adipose tissue

Melanocyte

(d) Cross section of hair bulb

Fibrous sheath (dermal connective tissue)

Epithelial root sheath

Figure 4.7 Structure of a hair root and follicle. (a) Longitudinal section of a hair within its follicle. (b) Enlarged longitudinal section of a

hair. (c) Scanning electron micrograph showing a hair shaft emerging from a follicle at the skin surface. (d) Enlarged longitudinal

view of the expanded hair bulb in the follicle showing the root and matrix, the region of actively dividing epithelial cells that produces the hair.

Chapter 4: Skin and Body Membranes 149

but far more damaging to body well-being, are burns and skin cancers. We briefly summarize these homeostatic imbalances of the skin here.

Infections and Allergies

Infections and allergies cause the following com- monly occurring skin disorders:

• Athlete’s foot. An itchy, red, peeling condition of the skin between the toes, resulting from an infection with the fungus Tinea pedis.

• Boils (furuncles) and carbuncles (kar9bun- kulz). Boils are caused by inflammation of hair follicles and surrounding tissues, commonly on the dorsal neck. Carbuncles are clusters of boils often caused by the bacterium Staphylococcus aureus.

• Cold sores (fever blisters). Small fluid-filled blisters that itch and sting, caused by human herpesvirus 1 infection. The virus localizes in a cutaneous nerve, where it remains dormant until activated by emotional upset, fever, or UV

nails

A nail is a scalelike modification of the epidermis that corresponds to the hoof or claw of other ani- mals. Each nail has a free edge, a body (visible attached portion), and a root (embedded in the skin) (Figure 4.8). The borders of the nail are over- lapped by folds of skin called nail folds. The edge of the thick proximal nail fold is commonly called the cuticle.

The stratum basale of the epidermis extends beneath the nail as the nail bed. Its thickened prox- imal area, called the nail matrix, is responsible for nail growth. As the matrix produces nail cells, they become heavily keratinized and die. Thus, nails, like hairs, are mostly nonliving material.

Nails are transparent and nearly colorless, but they look pink because of the rich blood supply in the underlying dermis. The exception to this is the region over the thickened nail matrix that appears as a white crescent and is called the lunule (loo9nyul; lunul 5 crescent). As noted earlier, when the supply of oxygen in the blood is low, the nail beds take on a cyanotic (blue) cast.

Did You Get It? 13. Which of the cutaneous gland types can make your

hair limp and oily? 14. How do secretions of apocrine glands differ from

those of the eccrine sweat glands? 15. When a factory worker caught his finger in a machine,

the entire nail, plus the nail matrix and bed, was torn off. Will his nail grow back? Why or why not?

For answers, see Appendix A.

Homeostatic Imbalances of Skin ➔ Learning Objectives

□□ Differentiate among first-, second-, third, and fourth-degree burns.

□□ Explain the importance of the “rule of nines.”

□□ Summarize the characteristics of basal cell carcinoma, squamous cell carcinoma, and malignant melanoma.

Loss of homeostasis in body cells and organs reveals itself on the skin, sometimes in startling ways. The skin can develop more than 1,000 dif- ferent ailments. The most common skin disorders are infections with pathogens (path9o-jenz) such as bacteria, viruses, or fungi. Allergies, which are caused by abnormally strong immune responses, are also commonly seen in the skin. Less common,

Lateral nail fold

Lunule

(a)

Nail matrix

Root of nail

Proximal nail fold

Nail bed Bone of fingertip

CuticleBody of nail

Free edge of nail

(b)

Figure 4.8 Structure of a nail.

150 Essentials of Human Anatomy and Physiology

its precious supply of fluids containing proteins and electrolytes as these seep from the burned surfaces. Dehydration and electrolyte imbalance follow and can lead to a shutdown of the kidneys and circulatory shock (inadequate circulation of blood caused by low blood volume). To save the patient, lost fluids must be replaced immediately. The volume of fluid lost can be estimated indi- rectly by determining how much of the body sur- face is burned (extent of burns), using the rule of nines. This method divides the body into 11 areas, each accounting for 9 percent of the total body surface area, plus an additional area sur- rounding the genitals (the perineum) representing 1 percent of body surface area (Figure 4.10a).

Later, infection becomes the most important threat and is the leading cause of death in burn victims. Burned skin is sterile for about 24 hours. But after that, pathogens easily invade areas where the skin has been destroyed and multiply rapidly in the nutrient-rich environment of dead tissues. To make matters worse, the patient’s immune sys- tem becomes depressed within one to two days after severe burn injury.

Burns are classified according to their severity (depth) as first-degree (superficial), second-degree (superficial partial-thickness burns), third- degree (full-thickness burns), or fourth-degree (full-thickness burns with deep-tissue involve- ment) (Figure 4.10b). In first-degree burns, only the superficial epidermis is damaged. The area becomes red and swollen. Except for temporary discomfort, first-degree burns are not usually seri- ous and generally heal in two to three days. Sunburn without blistering is a first-degree burn.

radiation. Cold sores usually occur around the lips and in the oral mucosa of the mouth and nose (Figure 4.9a).

• Contact dermatitis. Itching, redness, and swell- ing of the skin, progressing to blistering. It is caused by exposure of the skin to chemicals (such as those in poison ivy) that provoke al- lergic responses in sensitive individuals.

• Impetigo (im-peh-ti9go; impet 5 an attack). Pink, fluid-filled, raised lesions (commonly around the mouth and nose) that develop a yellow crust and eventually rupture (Figure 4.9b). Caused by highly contagious staphylo- coccus or streptococcus infections, impetigo is common in elementary school–aged children.

• Psoriasis (so-ri9ah-sis). Characterized by red- dened epidermal lesions covered with dry, sil- very scales that itch, burn, crack, and sometimes bleed (Figure 4.9c). A chronic condition, psoria- sis is believed to be an autoimmune disorder in which the immune system attacks a person’s own tissues, leading to the rapid overproduc- tion of skin cells. Attacks are often triggered by trauma, infection, hormonal changes, or stress. When severe, psoriasis may be disfiguring.

Burns

There are few threats to life more serious than burns. A burn is tissue damage and cell death caused by intense heat, electricity, UV radiation (sunburn), or certain chemicals (such as acids), which denature proteins and cause cell death in the affected areas.

When the skin is burned and its cells are destroyed, two life-threatening problems result. First, without an intact boundary, the body loses

(a) Cold sores (b) Impetigo (c) Psoriasis

Figure 4.9 Cutaneous lesions.

Chapter 4: Skin and Body Membranes 151

Blisters are usually present, and the burned area appears blanched (gray-white) or blackened. Because the nerve endings in the area are destroyed, the burned area is not painful. In third- degree burns, regeneration is not possible, and skin grafting must be done to cover the underlying exposed tissues.

Fourth-degree burns are also full-thickness burns, but they extend into deeper tissues such as bone, muscle, or tendons. These burns appear dry and leathery, and they require surgery and grafting to cover exposed tissue. In severe cases, amputa- tion may be required to save the patient’s life.

Second-degree burns involve injury to the epidermis and the superficial part of the dermis. The skin is red, painful, and blistered. Because sufficient numbers of epithelial cells are still pres- ent, regrowth (regeneration) of the epithelium can occur. Ordinarily, no permanent scars result if care is taken to prevent infection. Third-degree burns destroy both the epidermis and the dermis and often extend into the subcutaneous tissue, reflect- ing their categorization as full thickness burns.

No. Third-degree burns destroy both the epidermis and dermis in that region, so hair follicles would have

been destroyed.

Figure 4.10 Burns. (a) Estimating the extent of burns using the rule of nines. The surface areas for the anterior body surface are indicated

on the human figure. Total surface area (anterior and posterior body surfaces) is tabulated to the right of the figure. (b) Burns of increasing

severity, from top to bottom: first- degree, second-degree, and third- degree.

Totals

Anterior and posterior head and neck, 9%

Anterior and posterior upper limbs, 18%

Anterior and posterior trunk, 36%

Anterior and posterior lower limbs, 36%

Perineum, 1%

100%

Anterior trunk, 18%

9% 9%

(a) Rule of nines (b) Examples of burns

41/2% 41/2%

41/2%

Would you expect to see hair regeneration in an area that suffers third-degree burns? Why or why not?Q:

152 Essentials of Human Anatomy and Physiology

In general, burns are considered critical if any of the following conditions exists:

• Over 30 percent of the body has second- degree burns.

• Over 10 percent of the body has third- or fourth-degree burns.

• There are third- or fourth-degree burns of the face, hands, feet, or genitals.

• Burns affect the airway.

• Circumferential (around the body or limb) burns have occurred.

Facial burns are particularly dangerous because of the possibility of burns in respiratory passageways, which can swell and cause suffoca- tion. Joint injuries are troublesome because the scar tissue that eventually forms can severely limit joint mobility. Circumferential burns can restrict movement, and depending on location, can inter- fere with normal breathing.

Skin Cancer

Numerous types of neoplasms (tumors) arise in the skin. Most skin neoplasms are benign and do not spread (metastasize) to other body areas. For example, warts are caused by human papilloma- viruses but are benign and do not spread. However, some skin neoplasms are malignant, or cancerous, and they tend to invade other body areas.

➔ConCeptLinkRecall that mitosis gone wild is the basis for cancer (Chapter 3, pp. 130–131). In malignant cancers, the stages of mitosis occur so quickly that errors are made. As a result, these cells lack normal control of cell divi- sion. Cells experiencing rapid, uncontrolled growth become cancerous. ➔

Skin cancer is the single most common type of cancer in humans. One in five Americans now develops skin cancer at some point in his or her life. The most important risk factor is overexpo- sure to UV radiation in sunlight and tanning beds. Frequent irritation of the skin by infections, chemi- cals, or physical trauma also seems to be a predis- posing factor. Let’s look in a bit more detail at the three most common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and malignant melanoma.

Basal Cell Carcinoma Basal cell carcinoma (kar0sı̆-no9mah) is the least malignant and most common skin cancer. Cells of the stratum basale, altered so that they cannot form keratin, no longer honor the boundary between epidermis and der- mis. They proliferate, invading the dermis and subcutaneous tissue. The cancerous lesions occur most often on sun-exposed areas of the face and often appear as shiny, dome-shaped nodules that later develop a central ulcer with a “pearly” beaded edge (Figure 4.11a). Basal cell carcinoma is rela- tively slow-growing, and metastasis seldom occurs before the lesion is noticed. When the lesion is

Figure 4.11 Photographs of skin cancers.

Which of these cancers arises from the most superficial epidermal cells?Q:

(a) Basal cell carcinoma (b) Squamous cell carcinoma (c) Melanoma

Squamous cell carcinoma.A:

Chapter 4: Skin and Body Membranes 153

removed surgically, 99 percent of cases are com- pletely cured.

Squamous Cell Carcinoma Squamous cell carci- noma arises from the cells of the stratum spino- sum. The lesions appear as scaly, reddened papules (small, rounded swellings) that gradually form shallow ulcers with firm, raised borders (Figure 4.11b). This variety of skin cancer appears most often on the scalp, ears, back of the hands, and lower lip, but can appear anywhere on the skin. It grows rapidly and metastasizes to adjacent lymph nodes if not removed. This epidermal can- cer is also believed to be induced by UV exposure. If it is caught early and removed surgically or by radiation therapy, the chance of complete cure is good.

Malignant Melanoma Malignant melanoma (mel0ah-no9mah) is a cancer of melanocytes. It accounts for only about 5 percent of skin cancers, but it is often deadly. Melanoma can begin wher- ever there is pigment; most such cancers appear spontaneously, but some develop from pigmented moles. It arises from accumulated DNA damage in a skin cell and usually appears as a spreading brown to black patch (Figure 4.11c) that metasta- sizes rapidly to surrounding lymph and blood ves- sels. The chance for survival is about 50 percent, and early detection helps. The American Cancer Society suggests that people who sunbathe fre- quently or attend tanning parlors examine their skin periodically for new moles or pigmented spots and apply the ABCDE rule for recognizing melanoma:

(A) Asymmetry. Any two sides of the pig- mented spot or mole do not match.

(B) Border irregularity. The borders of the lesion are not smooth but exhibit indentations.

(D) Diameter. The lesion is larger than 6 milli- meters (mm) in diameter (the size of a pencil eraser).

(E) Evolution. One or more of these character- istics (ABCD) is evolving, or changing.

The usual therapy for malignant melanoma is wide surgical excision along with immunotherapy,

a treatment that involves the patient’s immune sys- tem. Large lesions may also require radiation or chemotherapy after surgical removal.

Did You Get It? 16. What are the two life-threatening consequences of a

severe burn? 17. What are the criteria for classifying burns as first-,

second-, third-, or fourth-degree? 18. What name is given to the rule for recognizing the

signs of melanoma? 19. What is the single most common risk factor for skin

cancer? 20. Why do no skin cancers develop from stratum

corneum cells?

For answers, see Appendix A.

Developmental Aspects of Skin and Body Membranes ➔ Learning Objective

□□ List several examples of integumentary system aging.

During the fifth and sixth months of development, a fetus is covered with a downy type of hair called lanugo (lah-noo9go). By the time the infant is born, it has usually shed this hairy cloak, and instead its skin is covered with an oily coating called the vernix caseosa (ver9niks kah-se-o9sah). This white, cheesy-looking substance, produced by the sebaceous glands, protects the baby’s skin while it is floating in its water-filled sac inside the mother. The newborn’s skin is very thin, and blood vessels are easily seen through it. Commonly, there are accumulations in the sebaceous glands, which appear as small white spots called milia (mil9e-ah) on the baby’s nose and forehead. These normally disappear by the third week after birth. As the baby grows, its skin becomes thicker, and more subcutaneous fat is deposited.

During adolescence, the skin and hair become oilier as sebaceous glands are activated, and acne may appear. Acne usually subsides in early adult- hood, and the skin reaches its optimal appearance when we are in our twenties and thirties. Then visible changes in the skin begin to appear, as it is continually assaulted by abrasion, chemicals, wind, sun, and other irritants and as its pores become clogged with air pollutants and bacteria. As a result, pimples, scales, and various kinds of

(Text continues on page 156.)

154

FOCUS ON

CAREERS Medical Transcriptionist

“If you have a basic understanding of anatomy and medical terminology, you will be much more accurate at interpreting and transcribing what you hear.”

Every time you consult a doctor or are hospitalized, your medical record gets longer. Medical transcrip- tionists play a key role in creating and maintaining these vital documents.

A medical transcriptionist is a medi- cal language specialist who interprets and transcribes notes dictated by phy- sicians and other healthcare profes- sionals. These reports, which cover all aspects of a patient’s assessment, diagnosis, treatment, and outcome, become part of the person’s confiden- tial medical record. Medical transcrip- tionists work in hospitals, clinics, doctors’ offices, transcription services, insurance companies, and home healthcare agencies.

What does it take to be a transcrip- tionist? “Certainly, you need a good English background,” says Pamela Shull, an experienced transcriptionist in San Jose, California. “Strong gram- mar, spelling, and punctuation skills are crucial. Physicians often dictate these records on the go, and a good transcriptionist must be able to edit the dictated material for grammar and clarity.”

Knowledge of anatomy and physi- ology, however, is even more impor- tant. Notes Shull, “If you understand anatomy and medical terminology, you will be much more accurate at interpreting and transcribing what you

hear. A hospital transcriptionist deals with terms from a wide variety of medical specialties—one dictation might be from a gynecologist, the next from an orthopedic surgeon, and the next from a pediatrician.” This is why anatomy and physiology, medical terminology, and the study of disease processes make up most of the curric- ulum in medical transcription training programs.

Anatomy and physiology classes make up a large

part of the curriculum in

medical transcription

training programs.

All health professionals who treat a patient rely on these typed docu- ments, so accurate transcription is vital: “I see the transcriptionist as a partner with physicians. We work with them to create excellent medical records, so patients will always be assured of receiving the best and most appropriate care possible.”

Shull enjoys the variety of medical transcription work. “It’s fascinating

because you get to follow each patient’s story, from the initial prob- lem to diagnosis and treatment,” she says. “You feel like you know these people.” Of course, patient confiden- tiality is also a big responsibility.

Classes for medical transcription, which are offered through community colleges, proprietary schools, and home-study programs, vary in length from several months to two years. Accreditation procedures vary from state to state. The Association for Healthcare Documentation Integrity (AHDI) evaluates medical transcription programs and posts a list of recom- mended programs on its website.

For more information, contact:

Association for Healthcare Documen- tation Integrity

4120 Dale Rd, Suite J8-233 Modesto, CA 95356 (800) 982-2182 or (209) 527-9620 http://ahdionline.org/

For additional information on this career and others, click the Focus on Careers link at .

http://ahdionline.org

Integumentary System (Skin)

• Skin protects cardiovascular organs; prevents fluid loss from body surface; serves as blood reservoir

• Cardiovascular system transports oxygen and nutrients to skin and removes wastes from skin; provides substances needed by skin glands to make their secretions

• Skin protects urinary organs; excretes salts and some nitrogen-containing wastes in sweat

• Urinary system activates vitamin D made by keratinocytes; disposes of nitrogen-containing wastes of skin metabolism

• Skin protects muscles • Active muscles generate large

amounts of heat, which increases blood flow to the skin and may promote activation of sweat glands of skin

• Skin protects lymphoid organs; prevents pathogen invasion

• Lymphatic system prevents edema by picking up excessive leaked fluid; immune system protects skin cells

• Skin protects digestive organs; provides vitamin D needed for calcium absorption

• Digestive system provides nutrients needed by the skin

• Skin protects reproductive organs; highly modified sweat glands (mammary glands) produce milk. During pregnancy, skin stretches to accommodate growing fetus; changes in skin pigmentation may occur

• Skin protects respiratory organs • Respiratory system furnishes

oxygen to skin cells and removes carbon dioxide via gas exchange with blood

• Skin protects bones; skin synthesizes vitamin D that bones need for normal calcium absorption and deposit of bone (calcium) salts, which make bones hard

• Skeletal system provides support for the skin

• Skin protects nervous system organs; cutaneous sensory receptors located in skin

• Nervous system regulates diameter of blood vessels in skin; activates sweat glands (contributing to thermoregulation); interprets cutaneous sensation; activates arrector pili muscles

• Skin protects endocrine organs • Androgens produced by the

endocrine system activate sebaceous glands and help regulate hair growth; estrogen helps maintain skin hydration

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System

Reproductive System

Respiratory System

Skeletal System

Endocrine System

Nervous System

Homeostatic Relationships between the Integumentary System and Other Body Systems

SYSTEMS IN SYNC

155

156 Essentials of Human Anatomy and Physiology

erate, the vellus (vell 5 wool) hairs are colorless and very tiny (and may not even emerge from the follicle). Another phenomenon of aging is graying hair. Like balding, this is usually genetically con- trolled by a “delayed-action” gene. Once the gene takes effect, the amount of melanin deposited in the hair decreases or becomes entirely absent, which results in gray-to-white hair.

Homeostatic Imbalance 4.5

Certain events can cause hair to gray or fall out prematurely. For example, many people have claimed that they turned gray nearly overnight because of some emotional crisis in their life. In addition, we know that anxiety, protein-deficient diets, therapy with certain chemicals (chemother- apy), radiation, excessive vitamin A, and certain fungal diseases (ringworm) can cause both gray- ing and hair loss. However, when the cause of these conditions is not genetic, hair loss is usually not permanent. ______________________________✚

Did You Get It? 21. What change in aging skin accounts for wrinkles and

cold intolerance in older adults? 22. What is the source of the vernix caseosa that covers

the skin of the newborn baby?

For answers, see Appendix A.

dermatitis (der0mah-ti9tis), or skin inflammation, become more common.

As we get even older, the amount of subcuta- neous tissue decreases, leading to sensitivity to cold. The skin also becomes drier (because of decreased oil production), and as a result, it may become itchy and bothersome. Thinning of the skin, another result of the aging process, makes it more susceptible to bruising and other types of injuries. The decreasing elasticity of the skin, along with the loss of subcutaneous fat, allows bags to form under our eyes, and our jowls begin to sag. Smoking and sunlight speed up this loss of elastic- ity, so three of the best things you can do for your skin are to avoid smoking, shield your skin from the sun, and do not go to tanning beds. In doing so, you will also be decreasing the chance of skin cancer. There is no way to avoid aging skin, but good nutrition, plenty of fluids, and cleanliness help delay the process.

Hair loses its luster as we age, and by age 50 the number of hair follicles has dropped by one- third and continues to decline, resulting in hair thinning and some degree of baldness, or alopecia (al0o-pe9she-ah), in most people. Many men become bald as they age, a phenomenon called male pattern baldness. A bald man is not really hairless—he does have hairs in the bald area. But, because those hair follicles have begun to degen-

lating body temperature through radiation and sweating; and synthesizing defensive proteins and vitamin D. Sensory receptors are located in the skin.

2. The epidermis, the superficial part of the skin, is stratified squamous keratinized epithelium and is avascular. Its layers from superficial to deep are the stratum corneum, stratum lucidum (in thick skin only), stratum granulosum, stratum spinosum, and stratum basale. Cells at its surface are dead and continually flake off. They are replaced by dividing cells in the basal cell layer. As the cells move away from the basal layer, they accumulate keratin and die. Melanin, a pigment produced by melanocytes, protects the nuclei of epithelial cells from the dam- aging rays of the sun.

3. The dermis is dense connective tissue. It contains blood vessels, nerves, and epidermal appendages.

Summary

Classification of Body Membranes (pp. 135–138)

1. Epithelial: Simple organs, epithelium, and connec- tive tissue components.

a. Cutaneous (the skin): epidermis (stratified squa- mous epithelium) underlain by the dermis (dense connective tissue); protects body surface.

b. Mucous: epithelial sheet underlain by a lamina propria (areolar connective tissue); lines body cavities open to the exterior.

c. Serous: simple squamous epithelium resting on a scant connective tissue layer; lines the ventral body cavity.

2. Connective tissue: Synovial; lines joint cavities.

the Integumentary System (Skin) (pp. 138–153)

1. Skin functions include protecting the deeper tissue from chemicals, bacteria, bumps, and drying; regu-

Chapter 4: Skin and Body Membranes 157

a. Burns result in loss of body fluids and invasion of bacteria. The extent of burns is assessed by the rule of nines. The severity (depth) of burns is described as first-degree (epidermal damage only), second-degree (epidermal and some der- mal injury), third-degree (epidermis and dermis totally destroyed, may involve some subcutane- ous tissue) and fourth-degree (all of skin and involves deeper tissue such as bone or muscle). Third- and fourth-degree burns require skin grafting.

b. The most common cause of skin cancer is expo- sure to ultraviolet (UV) radiation. Basal cell car- cinoma and squamous cell carcinoma can be completely cured if they are removed before metastasis. Malignant melanoma, a cancer of melanocytes, is still fairly rare but metastasizes quickly and is fatal in about half the cases.

Developmental Aspects of Skin and Body Membranes (pp. 153–156)

1. The skin is thick, resilient, and well hydrated in our youth but loses elasticity and thins as we age. Skin cancer is a major threat to skin exposed to exces- sive sunlight.

2. Balding and/or graying occurs with aging. Both are genetically determined but can also be caused by other factors (drugs, emotional stress, and so on).

It has two regions, the papillary and reticular lay- ers. The papillary layer has ridges, which push outward on the epidermis to produce finger- prints.

4. Skin appendages are formed from the epidermis but reside in the dermis.

a. Sebaceous glands produce an oily product (sebum), released at a hair follicle via a duct. Sebum keeps the skin and hair soft and contains chemicals that kill bacteria.

b. Sweat (sudoriferous) glands, under the control of the nervous system, produce sweat, which is released at the epithelial surface. These glands participate in regulating body temperature. There are two types: eccrine (the most numer- ous) and apocrine (their product includes fatty acids and proteins).

c. A hair is primarily dead keratinized cells and is produced by the matrix in the root of the hair bulb. The root is enclosed in a sheath, the hair follicle.

d. Nails are hooflike derivatives of the epidermis. Like hair, nails are primarily dead keratinized cells.

5. Most minor afflictions of the skin result from infec- tions or allergic responses; more serious are burns and skin cancer. Because they interfere with skin’s protective functions, burns represent a major threat to the body.

review Questions

Multiple Choice

More than one choice may apply.

1. Select the one false statement about mucous and serous membranes.

a. The epithelial type is the same in all serous membranes, but there are different epithelial types in different mucous membranes.

b. Serous membranes line closed body cavities, whereas mucous membranes line body cavities open to the outside.

c. Serous membranes always produce serous fluid, and mucous membranes always secrete mucus.

d. Both membranes contain an epithelium plus a layer of loose connective tissue.

2. Serous membranes

a. line the mouth.

b. have parietal and visceral layers.

c. consist of epidermis and dermis.

d. have a connective tissue layer called the lamina propria.

e. secrete a lubricating fluid.

3. Which of the following is (are) true of synovial membranes?

a. They are composed of loose areolar connective tissue.

b. They line bursae.

c. They line tendon sheaths.

d. They line pericardia.

e. They line peritonea.

4. Which structure can be found in the hair bulb?

a. Hair papilla

b. Lunule

c. Melanocyte

d. Arrector pili muscle

e. Blood vessels

5. In investigating the cause of thinning hair, which of the following questions needs to be asked?

a. Is the diet deficient in proteins?

b. Is the person taking megadoses of vitamin C?

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

158 Essentials of Human Anatomy and Physiology

Short Answer Essay

10. What are freckles?

11. From what types of damage does the skin protect the body?

12. Explain why we become tanned after sitting in the sun.

13. What is jaundice? Why does it occur?

14. Name two different categories of skin secretions and the glands that manufacture them.

15. How does the skin help to regulate body temperature?

16. How does smoking contribute to skin aging and formation of wrinkles?

17. What are arrector pili? What do they do?

18. Which skin cancer arises from the youngest epidermal cells?

19. Why does hair turn gray?

20. What kind of a disorder is psoriasis? What triggers it?

21. Is a bald man really hairless? Explain.

22. Contrast the secretions of eccrine and apocrine glands.

23. Explain the difference between a blackhead, whitehead, boil, and carbuncle. Which is most serious? Why?

c. Has the person been exposed to excessive radiation?

d. Has the person recently suffered severe emotional trauma?

6. Sebaceous glands

a. are present on the soles of the feet.

b. produce sebum.

c. are present on the palms of the hands.

d. are activated by androgens.

7. Which one of the following is not associated with the production of perspiration?

a. Sweat glands

b. Sweat pores

c. Arrector pili

d. Eccrine gland

e. Apocrine gland

8. Which of the following is not a skin appendage?

a. Eccrine sweat gland

b. Lamellar corpuscle

c. Hair follicle

d. Sensory nerve fiber

9. Match the structures on the right with their function listed on the left. ____ 1. Protection from

ultraviolet radiation ____ 2. Insulation, energy

storage ____ 3. Waterproofing and

preventing water loss

____ 4. Temperature regulation

____ 5. Excretion of water, urea, and salts

____ 6. Produces the patterns for fingerprints

a. Reticular layer of the dermis

b. Dermal blood supply

c. Papillary layer of the dermis

d. Hypodermis e. Melanocytes f. Stratum corneum g. Eccrine sweat

glands

Critical thinking and Clinical Application Questions

24. A nurse tells a doctor that a patient is cyanotic. What is cyanosis? What does its presence indicate?

25. Jane’s newborn baby’s scalp is covered with a brownish crust that is full of scales and dandruff. What is the likely cause?

26. A 40-year-old beachboy is complaining to you that his suntan made him popular when he was young— but now his face is all wrinkled, and he has several darkly pigmented moles that are growing rapidly and are as big as large coins. He shows you the moles, and immediately you think “ABCDE.” What does that mean, and why should he be concerned?

27. Fred’s new cycling shorts have got special silicone grip strips on the inside of the material to hold the

shorts in place. After a day on the saddle, Fred notices that he has got more than just a tan line. Two bright red circles appeared on his thighs a few hours after he changed out of his gear. Explain what type of skin condition Fred might be experiencing.

28. The water of a swimming pool is hypotonic to our cells. Why do we not swell and pop when we go for a swim?

29. Mr. Rossi, a fisherman in his late sixties, comes to the clinic because he has small ulcers on both fore- arms as well as on his face and ears. Although he has had them for several years, he has not had any other problems. What is the likely diagnosis, and what is the likely cause?

Chapter 4: Skin and Body Membranes 159

30. Mr. Grayson is receiving a drug treatment transder- mally (through the skin). Explain why drugs deliv- ered by this route are fat-soluble rather than water-soluble.

31. Which type of injection would allow a drug to be absorbed more rapidly—intradermal or subcutane- ous (a shallow injection just deep to the epider- mis)? Why?

32. Think about the types and characteristics of cell junctions (Chapter 3). How does this help to explain why sunburned skin often peels in sheets?

33. A burn victim exhibits a red and swollen arm with blistering; a hand that has been charred black, exposing bone; and a cheek that appears blanched. How serious is each burn, and is this patient criti- cal? Explain.

160

A lthough the word skeleton comes from the Greek word meaning “dried-up body,” our internal framework is beautifully formed and proportioned. Strong yet light, it is perfectly adapted for its functions of protecting the body and allowing motion. No other animal has such long legs (compared to the arms or forelimbs) or such a strange foot, and few have grasping hands with opposable thumbs.

The bones of the skeleton are part of the skeletal system, which also includes joints, car- tilages, and ligaments (fibrous cords that bind the bones together at joints). The skeleton is divided into two parts: the axial skeleton, the bones that form the longitudinal axis of the body, and the

appendicular skeleton, the bones of the limbs and girdles that attach them to the axial skeleton. Joints give these parts of the skeleton flexibility and allow movement to occur.

Bones: An Overview ➔ Learning Objectives

□□ Identify the subdivisions of the skeleton as axial or appendicular.

□□ List at least three functions of the skeletal system.

□□ Name the four main classifications of bones.

At one time or another, all of us have heard the expressions “bone tired,” “dry as a bone,” or “bag

The Skeletal System

WHAT

HOW

WHY

The skeletal system is the internal frame of the body and includes bones,

cartilages, and joints. In addition to providing structure,

bones articulate, or come together, at joints to allow

body movement.

The skeleton is essential for protecting organs, producing

blood cells, storing essential minerals, and anchoring skeletal muscles so that their contractions

cause body movements.

INSTrucTOrS

New Building Vocabulary coaching Activities for this chapter are assignable in

Chapter 5: The Skeletal System 161

for blood to clot. Problems occur not only when there is too little calcium in the blood, but also when there is too much. Hormones control the movement of calcium to and from the bones and blood according to the needs of the body. Indeed, “deposits” and “withdrawals” of calcium (and other minerals) to and from bones go on almost all the time.

• Blood cell formation. Blood cell formation, or hematopoiesis (hem″ah-to-poi-e′sis), occurs within the marrow cavities of certain bones.

classification of Bones The adult skeleton is composed of 206 bones. There are two basic types of osseous, or bone, tis- sue: Compact bone is dense and looks smooth and homogeneous, whereas spongy bone has a spiky, open appearance like a sponge (Figure 5.1).

Additionally, bones come in many sizes and shapes. For example, the tiny pisiform bone of the wrist is the size and shape of a pea, whereas the femur, or thigh bone, is nearly 2 feet long and has a large, ball-shaped head. The unique shape of

of bones”—pretty unflattering and inaccurate images of some of our most phenomenal organs. Our brains, not our bones, convey feelings of fatigue, and living bones are far from dry. As for “bag of bones,” they are indeed more obvious in some of us, but without bones to form our internal skeleton, we would creep along the ground like slugs, lacking any definite shape or form. Let’s examine how our bones contribute to overall body homeostasis.

Functions of the Bones Besides contributing to body structure, our bones perform several important body functions:

• Support. Bones, the “steel girders” and “rein- forced concrete” of the body, form the internal framework that supports the body and cradles its soft organs. The bones of the legs act as pil- lars to support the body trunk when we stand, and the rib cage supports the thoracic wall.

• Protection. Bones protect soft body organs. For example, the fused bones of the skull pro- vide a snug enclosure for the brain, allowing someone to head a soccer ball without worry- ing about injuring the brain. The vertebrae sur- round the spinal cord, and the rib cage helps protect the vital organs of the thorax.

• Allow movement. Skeletal muscles, attached to bones by tendons, use the bones as levers to move the body and its parts. As a result, we can breathe, walk, swim, and throw a ball. Before continuing, take a moment to imagine that your bones have turned to putty. What if you were running when this change took place? Now imagine your bones forming a rigid metal framework inside your body (with- out joints). What problems could you envision with this arrangement? These images should help you understand how well our skeletal system provides support and protection while allowing movement.

• Storage. Fat is stored in the internal (marrow) cavities of bones. Bone itself serves as a store- house for minerals, the most important of which are calcium and phosphorus. Most of the body’s calcium is deposited in the bones as calcium salts, but a small amount of calcium in its ion form (Ca2+) must be present in the blood at all times for the nervous system to transmit messages, for muscles to contract, and

Compact bone

Spongy bone

Figure 5.1 Flat bones consist of a layer of spongy bone sandwiched between two thin layers of compact bone.

Explore Cadaver

162 Essentials of Human Anatomy and Physiology

them (see Figure 5.1). Most bones of the skull, the ribs, and the sternum (breastbone) are flat bones.

Short bones are generally cube-shaped and contain mostly spongy bone with an outer layer of compact bone. The bones of the wrist and ankle are short bones. Sesamoid (ses′ah-moyd) bones, which form within tendons, are a special type of short bone. The best-known example is the patella.

Bones that do not fit one of the preceding cat- egories are called irregular bones. The vertebrae, which make up the spinal column, fall into this group. Like short bones, they are mainly spongy bone with an outer layer of compact bone.

each bone fulfills a particular need. Bones are classified according to shape into four groups: long, short, flat, and irregular (Figure 5.2).

As their name suggests, long bones are typi- cally longer than they are wide. As a rule, they have a shaft with enlarged ends. Long bones are mostly compact bone but also contain spongy bone at the ends. All the bones of the limbs, except the patella (kneecap) and the wrist and ankle bones, are long bones.

Flat bones are thin, flattened, and usually curved. They have two thin layers of compact bone sandwiching a layer of spongy bone between

(a) Long bone (humerus)

(d) Irregular bone (vertebra), right lateral view (c) Short bone (talus)

(b) Flat bone (sternum)

Figure 5.2 classification of bones on the basis of shape.

Chapter 5: The Skeletal System 163

remnant of the epiphyseal plate (a flat plate of hyaline cartilage) seen in a young, growing bone. Epiphyseal plates cause the lengthwise growth of a long bone. By the end of puberty, when hor- mones inhibit long bone growth, epiphyseal plates have been completely replaced by bone, leaving only the epiphyseal lines to mark their previous location.

The inner bony surface of the shaft is covered by a delicate connective tissue called endosteum. In infants, the cavity of the shaft, called the med- ullary cavity, is a storage area for red marrow, which produces blood cells. Children’s bones con- tain red marrow until the age of 6 or 7, when it is gradually replaced by yellow marrow, which stores adipose (fat) tissue. In adult bones, red mar- row is confined to cavities in the spongy bone of the axial skeleton, the hip bones, and the epiphy- ses of long bones such as the humerus and femur.

Even when looking casually at bones, you can see that their surfaces are not smooth but scarred with bumps, holes, and ridges. These bone mark- ings (described and illustrated in Table 5.1 on p. 166) reveal where muscles, tendons, and liga- ments attach and where blood vessels and nerves pass. There are two categories of bone markings: (a) projections, or processes, which grow out from the bone surface, and (b) depressions, or cavities, which are indentations in the bone. There is a lit- tle trick for remembering some of the bone mark- ings listed in the table: All the terms beginning with T are projections, and the terms beginning with F (except facet) are depressions.

Microscopic Anatomy

The appearance of spongy bone and compact bone to the naked eye only hints at their underly- ing complexity. Under a microscope, you can see that spongy bone is composed of small needlelike pieces of bone called trabeculae and lots of “open” space filled by marrow, blood vessels and nerves (Figure 5.4a, p. 165).

In compact bone, the mature bone cells, osteocytes (os′te-o-sı̄tz″), are found within the bone matrix in tiny cavities called lacunae (lah- ku′ne). The lacunae are arranged in concentric circles called lamellae (lah-mel′e) around central canals (also called Haversian canals). Each com- plex consisting of a central canal and matrix rings is called an osteon, or Haversian system, and is the structural and functional unit of compact bone.

Did You Get It? 1. What is the relationship between muscle function and

bones? 2. What are two possible functions of a bone’s marrow

cavity? 3. Where are most long bones found in the body?

For answers, see Appendix A.

Structure of Bone ➔ Learning Objectives

□□ Identify the major anatomical areas of a long bone.

□□ Describe the microscopic structure of compact bone.

□□ Explain the role of bone salts and the organic matrix in making bone both hard and flexible.

Gross Anatomy of a Long Bone

➔ConCeptLinkAs we learn about the structure and organization of bones, remember the levels of structural organization (Figure 1.1, p. 29). Bones are organs, so they contain not only osseous tissue but also other connective tissues: fibrous tissue, cartilage, adipose tissue, and blood. ➔

In a long bone, the diaphysis (di-af′ ı̆-sis), or shaft, makes up most of the bone’s length and is composed of compact bone (Figure 5.3, p. 164). The diaphysis is covered and protected by a fibrous connective tissue membrane, the perios- teum (per-e-os′te-um). Hundreds of connective tissue fibers, called perforating fibers, or Sharpey’s fibers, secure the periosteum to the underlying bone.

The epiphyses (ĕ-pif′ ı̆-sēz) are the ends of long bones. Each epiphysis consists of a thin layer of compact bone enclosing an area filled with spongy bone. Instead of a periosteum, articular cartilage covers its external surface. Because the articular cartilage is glassy hyaline cartilage, it pro- vides a smooth surface that decreases friction at the joint when covered by lubricating fluid. Imagine how slick a marble floor (the articular cartilage) is when wet; this is analogous to the lubrication of a joint.

In adult bones, there is a thin line of bony tis- sue spanning the epiphysis that looks a bit differ- ent from the rest of the bone in that area. This is the epiphyseal line. The epiphyseal line is a

164 Essentials of Human Anatomy and Physiology

supply and waste removal services through the hard bone matrix. Because of this elaborate net- work of canals, bone cells are well nourished in spite of the hardness of the matrix, and bone inju- ries heal quickly. The communication pathway from the outside of the bone to its interior (and

Central canals run lengthwise through the bony matrix, carrying blood vessels and nerves to all areas of the bone. Tiny canals, canaliculi (kan″ah- lik′u-li), radiate outward from the central canals to all lacunae. The canaliculi form a transportation sys- tem that connects all the bone cells to the nutrient

Proximal epiphysis

(b)

(c)

(a)

Yellow bone marrow

Endosteum

Epiphyseal line

Articular cartilage

Periosteum

Spongy bone

Compact bone

Medullary cavity (lined by endosteum)

Compact bone

Periosteum

Perforating (Sharpey’s) fibers

Nutrient arteries

Diaphysis

Distal epiphysis

Figure 5.3 The structure of a long bone (humerus of arm). (a) Anterior view with longitudinal section cut away. (b) Pie-shaped,

three-dimensional view of spongy bone and compact bone of the epiphysis. (c) Cross section of the shaft (diaphysis). Note that the external

surface of the diaphysis is covered by a periosteum but that the articular surface of the epiphysis (see a and b) is covered with hyaline cartilage.

Chapter 5: The Skeletal System 165

Perforating (Sharpey’s) fibers

Periosteal blood vessel

Periosteum

Blood vessel

Compact bone

(a)

(b) (c)

Central (Haversian) canal

Trabeculae of spongy bone

Blood vessel continues into medullary cavity containing marrow

Central (Haversian) canal

Canaliculus

Lacuna

Bone matrix

Lamella

Osteocyte

Lamellae

Osteon

Interstitial lamellae

Perforating (Volkmann’s) canal

Osteon (Haversian system)

Figure 5.4 Microscopic structure of bone. (a) Diagram of a pie- shaped segment of compact and spongy bone illustrating their

structural units (osteons) and trabeculae, respectively. (b) Higher magnification view of part of one osteon. Notice the position of

osteocytes in lacunae (cavities in the matrix). (c) Photo of a cross-sectional view of an osteon.

View Histology

166 Essentials of Human Anatomy and Physiology

Name of bone marking Description Illustration

Projections that are sites of muscle and ligament attachment

Tuberosity Large, rounded projection; may be roughened

Crest Narrow ridge of bone; usually prominent

Trochanter (tro-kan′ter) Very large, blunt, irregularly shaped process (the only examples are on the femur)

Line Narrow ridge of bone; less prominent than a crest

Tubercle (too′ber-kl) Small, rounded projection or process

Epicondyle Raised area on or above a condyle

Spine Sharp, slender, often pointed projection

Process Any bony prominence

Projections that help to form joints

Head Bony expansion carried on a narrow neck

Facet Smooth, nearly flat articular surface

Condyle (kon′dı̄ l) Rounded articular projection

Ramus (ra′mus) Armlike bar of bone

Depressions and openings

For passage of blood vessels and nerves

Groove Furrow

Fissure Narrow, slitlike opening

Foramen (fo-ra′men) Round or oval opening through a bone

Notch Indentation at the edge of a structure

Others

Meatus (me-a′tus) Canal- or tunnel-like passageway

Sinus Cavity within a bone, filled with air and lined with mucous membrane

Fossa (fos′ah) Shallow, basinlike depression in a bone, often serving as an articular surface

Table 5.1 Bone Markings

Iliac crest

Ischial spine

Ischial tuberosity

Hip bone

□□□□

Trochanter

Femur of thigh

Intertrochanteric line

Adductor tubercle

Condyle

Medial epicondyle

Vertebra

Spinous process

Facet

Head

Facets

Rib

Condyle

Ramus

Mandible

Sinus

Groove Notch

Fossa

Meatus

Inferior orbital fissure

Foramen

Skull

Chapter 5: The Skeletal System 167

Bone Formation, Growth, and Remodeling ➔ Learning Objective

□□ Describe briefly the process of bone formation in the fetus, and summarize the events of bone remodeling throughout life.

Bone Formation and Growth

The skeleton is formed from two of the strongest and most supportive tissues in the body—cartilage and bone. In embryos, the skeleton is primarily made of hyaline cartilage, but in young children, most of the cartilage has been replaced by bone. Cartilage remains only in isolated areas such as the bridge of the nose, parts of the ribs, and the joints.

Except for flat bones, which form on fibrous membranes, most bones develop using hyaline car- tilage structures as their “models.” This process of bone formation, or ossification (os″ ı̆-fı̆-ka′shun), involves two major phases (Figure 5.5). First, the hyaline cartilage model is completely covered with bone matrix (a bone “collar”) by bone-building cells called osteoblasts. So, as the embryo develops into a fetus, for a short period it has cartilage “bones” enclosed by actual bone matrix. Then, in

the central canals) is completed by perforating canals (also called Volkmann’s canals), which run in the compact bone at right angles to the shaft (diaphysis) and central canals.

Bone is one of the hardest materials in the body, and although relatively light in weight, it has a remarkable ability to resist tension and other forces acting on it. Nature has given us an extremely strong and exceptionally simple sup- porting system that also allows mobility. The cal- cium salts deposited in the matrix give bone its hardness, which resists compression. The organic parts (especially the collagen fibers) provide for bone’s flexibility and great tensile strength (ability to be stretched without breaking).

Did You Get It? 4. What is the anatomical name for the shaft of a long

bone? For its ends? 5. How does the structure of compact bone differ from

the structure of spongy bone? 6. What is the importance of canaliculi?

For answers, see Appendix A.

Bone starting to replace cartilage

Epiphyseal plate cartilage

Articular cartilage

Spongy bone

In a childIn a fetusIn an embryo

New bone forming

Growth in bone width

Growth in bone length

Epiphyseal plate cartilage

New bone forming

Invading blood vessels

Hyaline cartilage

New center of bone growth

Medullary cavity

Bone collar Hyaline cartilage model

Figure 5.5 Stages of long-bone formation in an embryo, fetus, and young child.

168 Essentials of Human Anatomy and Physiology

hormone and, during puberty, the sex hormones. It ends during adolescence, when the epiphyseal plates are completely converted to bone.

Bone remodeling

Many people mistakenly think that bones are life- less structures that never change once long-bone growth has ended. Nothing could be further from the truth; bone is a dynamic and active tissue. Bones are remodeled continually in response to changes in two factors: (1) the calcium ion level in the blood and (2) the pull of gravity and muscles on the skeleton.

When the blood calcium ion level drops below its homeostatic level, the parathyroid glands (located in the throat) are stimulated to release parathyroid hormone (PTH) into the blood. PTH activates osteoclasts, giant bone-destroying cells in bones, to break down bone matrix and release calcium ions into the blood. When the blood cal- cium ion level is too high (hypercalcemia [hi″per- kal-se′me-ah]), calcium is deposited in bone matrix as hard calcium salts by osteoblasts.

Bone remodeling is essential if bones are to retain normal proportions and strength during long-bone growth as the body increases in size and weight. It also accounts for the fact that bones become thicker and form large projections to increase their strength in areas where bulky mus- cles are attached. At such sites, osteoblasts lay

the fetus, the enclosed hyaline cartilage model is replaced by bone, and the center is digested away, opening up a medullary cavity within the newly formed bone.

By birth or shortly after, most hyaline cartilage models have been converted to bone except for two regions—the articular cartilages (that cover the bone ends) and the epiphyseal plates. In order for bones to increase in length as the infant grows into a child, new cartilage is formed continuously on the external face (joint side) of the articular cartilage and on the epiphyseal plate surface that faces the bone end (is farther away from the med- ullary cavity). At the same time, the old cartilage abutting the internal face of the articular cartilage and the medullary cavity is broken down and replaced by bony matrix (Figure 5.6).

Growing bones also widen as they lengthen to maintain proper proportion. How do they widen? Simply, osteoblasts in the periosteum add bone matrix to the outside of the diaphysis as cells called osteoclasts in the endosteum remove bone from the inner face of the diaphysis wall, enlarg- ing the medullary cavity (see Figure 5.6). Because these two processes occur at about the same rate, the circumference of the long bone expands, and the bone widens. This process by which bones increase in diameter is called appositional growth, and like growth in length, is controlled by hor- mones. The most important hormones are growth

Bone growth Bone remodeling

Articular cartilage

Epiphyseal plate

Bone grows in length because:

Growing shaft is remodeled as:

Cartilage grows here.

Cartilage is replaced by bone here.

Cartilage grows here.

Bone is resorbed by osteoclasts here.

Bone is added (appositional growth) by osteoblasts here.

Bone is resorbed by osteoclasts here.

Cartilage is replaced by bone here.

Figure 5.6 Growth and remodeling of long bones. The events on the left depict the process of ossification that occurs at the

articular cartilages and epiphyseal plates as the bone grows in length. The events on the right show bone remodeling and appositional

growth during long-bone growth to maintain proper bone proportions.

169

FOCUS ON

CAREERS Radiologic Technologist

might guess, radiologic technologists, especially in hospitals, must be pre- pared to spend a lot of time on their feet and to think quickly. Regalado described one case when a two-car accident sent five children to the trauma unit. The radiologic technolo- gists had to work quickly to help the doctors see what injuries the children suffered—and equally important, to make sure not to mix up anyone’s X-ray exams.

“You don’t want to make errors, because one thing you do wrong could cost this patient his or her life,” she says. “Even though radiology can get emotional, you have to stay tech- nical with your job.”

“We can’t see your bones with our bare eyes, so we have to make sure we position you correctly. Then also, if you say, ‘It hurts here,’ I’ll call the doc- tor and see if he wants to do a differ- ent type of X-ray exam.”

Regalado enjoys working with the patients at Dell. Getting children to remain perfectly still and positioned correctly is a challenge, but the imag- ing department has toys and televi- sions to distract them. For babies who cannot easily hold still or understand why they need to, there are various devices to position them appropriately.

“We have a lot of interaction with the patients and the patient’s family; we try to joke around and make them happy,” she says. “When we make the child happy, then the parents are happy.”

In a hospital setting, radiologic technologists are needed 24 hours a day and often are required to be on- call in addition to their regular shifts. Technologists who work in clinics usu- ally have a more traditional 9-to-5

schedule. Depending on the clinic, these technologists may also special- ize in areas such as ultrasound, mam- mography, magnetic resonance imaging (MRI), or computed tomogra- phy (CT).

For more information, contact:

American Society of Radiologic Technologists 15000 Central Ave. SE Albuquerque, NM 87123-3909 (800) 444-2778 http://www.asrt.org

For additional information on this career and others, click the Focus on Careers link at .

radiologic technologists supply critical information that allows doctors to make accurate diagnoses.

You never know what’s going to walk in the door, really,” says Maggie Regalado, a radiologic tech- nologist at Dell Children’s Hospital in Austin, Texas. “In an emergency room, you see kids who swallowed something, car accident victims, all kinds of things.” Regalado and her coworkers operate X-ray equipment and must be ready to do everything from preparing patients for chest X-ray exams to MRIs.

Fortunately for Regalado, anatomy was her favorite class, because it’s an important one for radiologic technolo- gists. After getting her associate’s degree in diagnostic imaging, she completed both state and national certification. To keep her certification current, she must complete 24 hours of continuing education every 2 years.

You don’t want to make errors,

because one thing you do wrong could cost this patient his

or her life.

“I didn’t realize how big a field it was,” she says. “With X-ray exams you’re constantly moving from here to there, from surgery to the neonatal intensive care unit and so on.” As you

“

http://www.asrt.org

170 Essentials of Human Anatomy and Physiology

Did You Get It? 7. Bones don’t begin as bones. What do they begin as? 8. Which stimulus—PTH (a hormone) or mechanical

forces acting on the skeleton—is more important in maintaining the blood calcium ion level than in maintaining bone strength?

9. If osteoclasts in a long bone are more active than osteoblasts, what change in bone mass is likely to occur?

For answers, see Appendix A.

Bone Fractures ➔ Learning Objective

□□ Name and describe the various types of fractures.

Homeostatic Imbalance 5.2

For their relatively low mass, bones are amazingly strong. Consider, for example, the forces endured in football and professional hockey. Despite their remarkable strength, bones are susceptible to frac- tures, or breaks, throughout life. During youth, most fractures result from exceptional trauma that twists or smashes the bones. Sports activities such as football, skating, and skiing jeopardize the bones, and automobile accidents certainly take their toll. In old age, bones thin and weaken, and fractures occur more often.

A fracture in which the bone breaks cleanly but does not penetrate the skin is a closed (or simple) fracture. When the broken bone ends penetrate through the skin, the fracture is open (or compound). Some of the many common types of fractures are illustrated and described in Table 5.2. _______________________________ ✚

A fracture is treated by reduction, which is the realignment of the broken bone ends, followed by immobilization. In a closed reduction, the bone ends are coaxed back into their normal position by the physician’s hands. In an open reduction, sur- gery is performed, and the bone ends are secured together with pins or wires. After the broken bone is reduced, it is immobilized by a cast or traction to allow the healing process to begin. The healing time for a simple fracture is 6 to 8 weeks but is much longer for large bones and for the bones of older people (because of their poorer circulation).

The repair of bone fractures involves four major events (Figure 5.7):

1 A hematoma forms. Blood vessels are rup- tured when the bone breaks. As a result, a

down new matrix and become trapped within it. (Once they are trapped, they become osteocytes, or mature bone cells.) In contrast, the bones of bedridden or physically inactive people tend to lose mass and to atrophy because they are no lon- ger subjected to stress.

These two controlling mechanisms—calcium uptake and release as well as bone remodeling— work together. PTH determines when bone is to be broken down or formed in response to the need for more or fewer calcium ions in the blood. The stresses of muscle pull and gravity acting on the skeleton determine where bone matrix is to be broken down or formed so that the skeleton can remain as strong and vital as possible.

Homeostatic Imbalance 5.1

Rickets is a disease of children in which the bones fail to calcify. As a result, the bones soften, and the weight-bearing bones of the legs become bowed. Rickets is usually due to a lack of cal- cium in the diet or lack of vitamin D, which is needed to absorb calcium into the bloodstream. Rickets is not seen very often in the United States. Milk, bread, and other foods are fortified with vitamin D, and most children drink enough calcium-rich milk. However, it can happen in infants nursed by mothers who become vitamin D deficient, and it remains a problem in some other parts of the world.

This child suffering from rickets is a member of the el-Molo tribe in Kenya, whose diet consists primarily of fish.

___________________________________________ ✚

Chapter 5: The Skeletal System 171

dead tissue by phagocytes. As this goes on, connective tissue cells of various types form internal and external masses of repair tissue, which collectively form the fibrocartilage callus (kal′us). The internal and external masses, called calluses, originate from cells of the endosteum and periosteum, respectively,

blood-filled swelling, or bruise, called a hematoma (he-mah-to′mah) forms. Bone cells deprived of nutrition die.1

A fibrocartilage callus forms. Two early events of tissue repair are the growth of new capillaries (granulation tissue) into the clotted blood at the site of the damage and disposal of

Fracture type Illustration Description comment

Comminuted Bone breaks into three or more fragments

Particularly common in older people, whose bones are more brittle

Compression Bone is crushed Common in porous bones (i.e., osteoporotic bones of older people)

Depressed Broken bone portion is pressed inward

Typical of skull fracture

Impacted Broken bone ends are forced into each other

Commonly occurs when someone attempts to break a fall with outstretched arms

Spiral Ragged break occurs when excessive twisting forces are applied to a bone

Common sports fracture

Greenstick Bone breaks incompletely, much in the way a green twig breaks

Common in children, whose bones are more flexible than those of adults

Table 5.2 common Types of Fractures

Hematoma External callus

Bony callus of spongy bone

Healed fracture

New blood vessels

Internal callus (fibrous tissue and cartilage)

Spongy bone trabecula

Hematoma forms. 1 Bony callus

forms. 3 Bone remodeling

occurs. 4Fibrocartilage

callus forms. 2

Figure 5.7 Stages in the healing of a bone fracture.

172 Essentials of Human Anatomy and Physiology

joined together by sutures, which are interlocking, immovable joints. Only the mandible (jawbone) is attached to the rest of the skull by a freely mov- able joint.

cranium

The boxlike cranium is composed of eight large flat bones. Except for two sets of paired bones (the parietal and temporal), they are all single bones.

Frontal Bone The frontal bone forms the forehead, the bony projections under the eye- brows, and the superior part of each eye’s orbit (Figure 5.9).

Parietal Bones The paired parietal bones form most of the superior and lateral walls of the cra- nium (see Figure 5.9). The sagittal suture is formed at the midline where the two parietal bones meet (shown in Figure 5.12, p. 176), and the coro- nal suture is formed where the paired parietal bones meet the frontal bone.

Temporal Bones The temporal bones lie inferior to the parietal bones and join them at the squamous sutures. Several important bone markings appear on the temporal bones (see Figure 5.9):

• The external acoustic meatus is a canal that leads to the eardrum and the middle ear. It is the route by which sound enters the ear.

• The styloid process, a sharp, needlelike pro- jection, is just inferior to the external auditory meatus. Many neck muscles use the styloid process as an attachment point.

• The zygomatic (zi″go-mat′ik) process is a thin bridge of bone that joins with the cheek- bone (zygomatic bone) anteriorly.

• The mastoid (mas′toid) process, which is full of air cavities (the mastoid sinuses), is a rough projection posterior and inferior to the external acoustic meatus. It provides an attachment site for some muscles of the neck.

The mastoid sinuses are so close to the middle ear—a high-risk spot for infections— that they may become infected too, a condi- tion called mastoiditis. Also, this area is so close to the brain that mastoiditis may spread to the brain.

and contain several elements—cartilage matrix, bone matrix, and collagen fibers—which act to “splint” the broken bone, closing the gap.

The bony callus forms. As more osteoblasts and osteoclasts migrate into the area and multi- ply, the fibrocartilage callus is gradually replaced by the bony callus made of spongy bone.

Bone remodeling occurs. Over the next few weeks to months, depending on the bone’s size and site of the break, the bony callus is remodeled in response to the mechanical stresses placed on it, so that it forms a strong, permanent “patch” at the fracture site.

Did You Get It? 10. What is a fracture? What two fracture types are

particularly common in older people?

For the answer, see Appendix A.

Axial Skeleton As noted earlier, the skeleton is divided into two parts, the axial and appendicular skeletons. The axial skeleton forms the longitudinal axis of the body (it is shown as the green portion of Figure 5.8). It can be divided into three parts— the skull, the vertebral column, and the thoracic cage.

➔ConCeptLinkRecall the regional body terms you have already learned (look back at Figure 1.4, p. 40). Many of these terms can be associated with a bone name or group of bones. For example, the carpal region is the location of the carpals, or wrist bones. ➔

Skull ➔ Learning Objectives

□□ On a skull or diagram, identify and name the bones of the skull and the four major skull sutures.

□□ Describe how the skull of a newborn infant (or fetus) differs from that of an adult, and explain the function of fontanels.

The skull is formed by two sets of bones. The cranium encloses and protects the fragile brain tissue. The facial bones form a cradle for the eyes that is open to the anterior and allow the facial muscles to show our feelings through smiles or frowns. All but one of the bones of the skull are

Chapter 5: The Skeletal System 173

Skull

Thoracic cage (ribs and sternum)

(a) Anterior view (b) Posterior view

Facial bones

Cranium

Sacrum

Vertebral column

Clavicle

Bones of pectoral girdle

Bones of pelvic girdle

Upper limb

Scapula

Sternum

Rib

Humerus

Vertebra

Radius Ulna

Carpals

Phalanges

Metacarpals

Femur

Patella Lower limb

Tibia

Fibula

Tarsals

Metatarsals

Phalanges

Figure 5.8 The human skeleton. The bones of the axial skeleton are colored green. Bones of the appendicular skeleton are gold.

174 Essentials of Human Anatomy and Physiology

connect with the brain. Lateral to the foramen magnum on each side are the rockerlike occipital condyles (see Figure 5.11), which rest on the first vertebra of the spinal column.

Sphenoid Bone The butterfly-shaped sphe- noid (sfe′noid) bone spans the width of the skull and forms part of the floor of the cranial cavity (see Figure 5.10). In the midline of the sphenoid is a small depression, the sella turcica (sel′ah tur′sı̆-kah), or Turk’s saddle, which forms a snug enclosure for the pituitary gland. The foramen ovale, a large oval opening in line with the posterior end of the sella turcica (see Figure 5.10), allows fibers of cra- nial nerve V (the trigeminal nerve) to pass to the chewing muscles of the lower jaw (mandible). Parts of the sphenoid bone, seen exteriorly forming part of the eye orbits (Figure 5.12, p. 176), have two important openings, the optic canal, which allows the optic nerve to pass to the eye, and the slitlike superior orbital fissure, through which the cranial nerves controlling eye movements (III, IV, and VI) pass (see Figure 5.12). The central part

• The jugular foramen, at the junction of the occipital and temporal bones (Figure 5.10), al- lows passage of the jugular vein, the largest vein of the head, which drains blood from the brain. Just anterior to it in the cranial cavity is the internal acoustic meatus (see Figure 5.10), which transmits cranial nerves VII and VIII (the facial and vestibulocochlear nerves). Anterior to the jugular foramen on the skull’s inferior aspect is the carotid canal (see Figure 5.11), through which the internal ca- rotid artery runs, supplying blood to most of the brain.

Occipital Bone The occipital (ok-sip′ ı̆-tal) bone is the most posterior bone of the cranium (Figure 5.11). It forms the base and back wall of the skull. The occipital bone joins the parietal bones anteriorly at the lambdoid (lam′doyd) suture. In the base of the occipital bone is a large opening, the foramen magnum (literally, “large hole”). The foramen magnum surrounds the lower part of the brain and allows the spinal cord to

Coronal suture Frontal bone

Sphenoid bone

Ethmoid bone

Lacrimal bone

Nasal bone

Zygomatic bone

Maxilla

Alveolar processes

Mandible (body) Mental foramen

Parietal bone

Lambdoid suture

Squamous suture

Occipital bone

External acoustic meatus

Mastoid process

Styloid process

Mandibular ramus

Zygomatic process

Temporal bone

Figure 5.9 Human skull, lateral view.

Chapter 5: The Skeletal System 175

Frontal bone

Sphenoid bone

Temporal bone

Internal acoustic meatus

Parietal bone

Occipital bone

Foramen magnum

Cribriform plate Ethmoid boneCrista galli

Optic canal

Jugular foramen

Sella turcica

Foramen ovale

Figure 5.10 Human skull, superior view (top of cranium removed).

Maxilla (palatine process)Hard

palate

Zygomatic bone

Maxilla

Incisive fossa

Sphenoid bone (greater wing)

Foramen ovale

Carotid canal

Jugular foramen

Foramen magnum

Occipital condyle

Temporal bone (zygomatic process)

Mandibular fossa

Vomer

Styloid process

Mastoid process

Temporal bone

Parietal bone

Palatine bone

Occipital bone

Figure 5.11 Human skull, inferior view (mandible removed).

176 Essentials of Human Anatomy and Physiology

Parietal bone

Nasal bone

Sphenoid bone

Temporal bone Ethmoid bone

Lacrimal bone

Infraorbital foramen

Zygomatic bone

Maxilla

Mandible

Mental foramen

Frontal bone

Middle nasal concha of ethmoid bone

Inferior nasal concha

Vomer

Optic canal

Superior orbital fissure

Coronal suture

Alveolar processes

Anterior view

Posterior view

Occipital bone

Parietal bone

Mastoid process of temporal bone

Sagittal suture

Lambdoid suture

Figure 5.12 Human skull, anterior and posterior views.

What bone articulates with every other facial bone?Q: The maxilla.A:

Chapter 5: The Skeletal System 177

They form the posterior part of the hard palate (see Figure 5.11). Failure of these or the palatine processes to fuse medially results in cleft palate.

Zygomatic Bones The zygomatic bones are commonly referred to as the cheekbones. They also form a good-sized portion of the lateral walls of the orbits.

Lacrimal Bones The lacrimal (lak′rı̆-mal) bones are fingernail-sized bones forming part of the medial

of the sphenoid bone is riddled with air cavities, the sphenoidal sinuses (Figure 5.13).

Ethmoid Bone The ethmoid (eth′moid) bone is very irregularly shaped and lies anterior to the sphenoid (see Figure 5.10). It forms the roof of the nasal cavity and part of the medial walls of the orbits. Projecting from its superior surface is the crista galli (kris′tah gah′le), literally “cock’s comb” (see Figure 5.10). The outermost covering of the brain attaches to this projection. On each side of the crista galli are many small holes. These holey areas, the cribriform (krib′rı̆-form) plates, allow nerve fibers carrying impulses from the olfactory (smell) receptors of the nose to reach the brain. Extensions of the ethmoid bone, the superior nasal conchae (kong′ke) and middle nasal conchae, form part of the lateral walls of the nasal cavity (see Figure 5.12).

Facial Bones

Fourteen bones make up the face. Twelve are paired; only the mandible and vomer are single. (Figures 5.9 and 5.12 show most of the facial bones.)

Maxillae The two maxillae (mak-sı̆′le), or maxillary bones, fuse to form the upper jaw. All facial bones except the mandible join the maxillae; thus they are the main, or “keystone,” bones of the face. The maxillae carry the upper teeth in the alveolar process.

Extensions of the maxillae called the palatine (pal′ah-tı̄n) processes form the anterior part of the hard palate of the mouth (see Figure 5.11). Like many other facial bones, the maxillae contain sinuses, which drain into the nasal passages (see Figure 5.13). These paranasal sinuses, whose naming reveals their position surrounding the nasal cavity, lighten the skull bones and amplify the sounds we make as we speak.

Homeostatic Imbalance 5.3

The paranasal sinuses cause many people a great deal of misery. Because the mucosa lining these sinuses is continuous with that in the nose and throat, infections in these areas tend to migrate into the sinuses, causing sinusitis. Depending on which sinuses are infected, a headache or upper jaw pain is the usual result. ___________________✚

Palatine Bones The paired palatine bones lie posterior to the palatine processes of the maxillae.

Frontal sinus Ethmoid sinus

Maxillary sinus

Sphenoidal sinus

Frontal sinus Ethmoid sinus

Maxillary sinus

Sphenoidal sinus

(a) Anterior view

(b) Medial view

Figure 5.13 Paranasal sinuses.

178 Essentials of Human Anatomy and Physiology

The Hyoid Bone

Though not really part of the skull, the hyoid (hi′oid) bone (Figure 5.14) is closely related to the mandible and temporal bones. The hyoid bone is unique in that it is the only bone of the body that does not articulate (form a joint) with any other bone. Instead, it is suspended in the midneck region about 2 cm (1 inch) above the larynx (voicebox), where it is anchored by ligaments to the styloid pro- cesses of the temporal bones. Horseshoe-shaped, with a body and two pairs of horns, the hyoid bone serves as a movable base for the tongue and as an attachment point for neck muscles that raise and lower the larynx when we swallow and speak.

Did You Get It? 11. What are the three main parts of the axial skeleton? 12. Johnny was vigorously exercising the only joints in the

skull that are freely movable. What would you guess he was doing?

13. Which skull bone(s) form the “keystone of the face”? 14. Which bone has the cribriform plate and crista galli? 15. Which bones are connected by the coronal suture? By

the sagittal suture?

For answers, see Appendix A.

Vertebral column (Spine) ➔ Learning Objectives

□□ Name the parts of a typical vertebra, and explain in general how the cervical, thoracic, and lumbar vertebrae differ from one another.

□□ Discuss the importance of the intervertebral discs and spinal curvatures.

□□ Explain how the abnormal spinal curvatures (scoliosis, lordosis, and kyphosis) differ from one another.

Serving as the axial support of the body, the verte- bral column, or spine, extends from the skull, which it supports, to the pelvis, where it transmits the weight of the body to the lower limbs. Some people think of the vertebral column as a rigid supporting rod, but that picture is inaccurate. Instead, the spine is formed from 26 irregular bones connected and reinforced by ligaments in such a way that a flexible, curved structure results (Figure 5.15). Running through the central cavity of the vertebral column is the delicate spinal cord, which the vertebral column surrounds and protects.

Before birth, the spine consists of 33 separate bones called vertebrae, but 9 of these eventually fuse to form the two composite bones, the sacrum and the coccyx, that construct the inferior portion of

wall of each orbit. Each lacrimal bone has a groove that serves as a passageway for tears (lacrima = tear).

Nasal Bones The small rectangular bones forming the bridge of the nose are the nasal bones. (The lower part of the skeleton of the nose is made up of hyaline cartilage.)

Vomer Bone The single bone in the median line of the nasal cavity is the vomer. (Vomer means “plow,” which refers to the bone’s shape.) The vomer forms the inferior part of the bony nasal septum, which separates the two nostrils.

Inferior Nasal Conchae The inferior nasal con- chae (kon′ke) are thin, curved bones projecting medially from the lateral walls of the nasal cavity. (As mentioned earlier, the superior and middle con- chae are similar but are parts of the ethmoid bone.)

Mandible The mandible, or lower jaw, is the largest and strongest bone of the face. It joins the temporal bones on each side of the face, forming the only freely movable joints in the skull. You can find these joints on yourself by placing your fin- gers just anterior to your ears and opening and closing your mouth. The horizontal part of the mandible (the body) forms the chin. Two upright bars of bone (the rami) extend from the body to connect the mandible with the temporal bone. The lower teeth lie in alveoli (sockets) in the alve- olar process at the superior edge of the mandib- ular body.

Greater horn

Lesser horn

Body

Figure 5.14 Anatomical location and structure of the hyoid bone. Anterior view.

Remembering common meal times, 7 a.m., 12 noon, and 5 p.m., may help you to recall the number of bones in these three regions of the vertebral column.

The individual vertebrae are separated by pads of flexible fibrocartilage—intervertebral discs— that cushion the vertebrae and absorb shock while allowing the spine flexibility. In a young person, the discs have a high water content (about 90 percent) and are spongy and compressible. But as a person ages, the water content of the discs decreases (as it does in other tissues throughout the body), and the discs become harder and less compressible.

Homeostatic Imbalance 5.4

Drying of the discs, along with a weakening of the ligaments of the vertebral column, predisposes older people to herniated (“slipped”) discs. However, herniation also may result when the ver- tebral column is subjected to exceptional twisting forces. If the protruding disc presses on the spinal cord or the spinal nerves exiting from the cord, numbness and excruciating pain can result. _____✚

The discs and the S-shaped structure of the ver- tebral column work together to prevent shock to the head when we walk or run. They also make the body trunk flexible. The spinal curvatures in the thoracic and sacral regions are referred to as pri- mary curvatures because they are present when we are born. Together the two primary curvatures produce the C-shaped spine of the newborn baby (Figure 5.16). The curvatures in the cervical and lumbar regions are referred to as secondary curva- tures because they develop some time after birth. In adults, the secondary curvatures allow us to center our body weight on our lower limbs with

1st cervical vertebra (atlas)

2nd cervical vertebra (axis)

Anterior Posterior

Cervical curvature (concave) 7 vertebrae, C1 – C7

Thoracic curvature (convex) 12 vertebrae, T1 – T12

Lumbar curvature (concave) 5 vertebrae, L1 – L5

Sacral curvature (convex) 5 fused vertebrae

Coccyx 4 fused vertebrae

1st lumbar vertebra

Spinous process

Transverse process

Intervertebral disc

Intervertebral foramen

1st thoracic vertebra

Figure 5.15 The vertebral column. Thin discs between the thoracic vertebrae allow great flexibility in the thoracic region; thick discs between the lumbar vertebrae reduce flexibility. Notice that the terms convex and concave refer to the curvature of the posterior aspect of the vertebral column.

the vertebral column. Of the 24 single bones, the 7 vertebrae of the neck are cervical vertebrae, the next 12 are the thoracic vertebrae, and the remaining 5 supporting the lower back are lumbar vertebrae.

Figure 5.16 The C-shaped spine typical of a newborn.

180 Essentials of Human Anatomy and Physiology

• Transverse processes: two lateral projections from the vertebral arch.

• Spinous process: single projection arising from the posterior aspect of the vertebral arch (actually the fused laminae).

• Superior articular process and inferior articular process: paired projections lateral to the vertebral foramen, allowing a vertebra to form joints with adjacent vertebrae.

In addition to these common features, verte- brae in the different regions of the spine have very specific structural characteristics, which we describe next.

cervical Vertebrae

The seven cervical vertebrae (identified as C1 to C7) form the neck region of the spine. The first two vertebrae (atlas and axis) are different because they perform functions not shared by the other cervical vertebrae. As you can see in Figure 5.18a, the atlas (C1) has no body. The superior surfaces of its transverse processes contain large depressions that receive the occipital condyles of the skull. This joint allows you to nod “yes.” The axis (C2) acts as a pivot for the rotation of the atlas (and skull) above. It has a large upright

minimum effort. The cervical curvature appears when a baby begins to raise its head, and the lum- bar curvature develops when the baby begins to walk.

Homeostatic Imbalance 5.5

Were you ever given a “spine check” in middle school? There are several types of abnormal spinal curvatures that can be identified by simple observa- tion. Three of these are scoliosis (sko″le-o′sis), kyphosis (ki-fo′sis), and lordosis (lor-do′sis). These spinal abnormalities may be congenital (pres- ent at birth) or may result from disease, poor pos- ture, unequal muscle pull on the spine, or injury. Generally, unless there is a congenital problem, young healthy people have no skeletal problems, assuming that their diet is nutritious and they stay reasonably active. As you look at these photos, try to pinpoint how each of these conditions differs from a normal healthy spine. The usual treatments for these abnormal curvatures are braces, casts, or surgery.

Posterior

Lamina

Superior articular process and facet

Transverse process

Pedicle

Spinous process

Vertebral arch

Vertebral foramen

Body

Figure 5.17 A typical vertebra, superior view. (Inferior articulating surfaces are not shown.)

(a) Scoliosis (b) Kyphosis (c) Lordosis Abnormal spinal curvatures.

______________________________________________✚

All vertebrae have a similar structural pattern (Figure 5.17). The common features of vertebrae include the following:

• Body, or centrum: disclike, weight-bearing part of the vertebra facing anteriorly in the vertebral column.

• Vertebral arch: arch formed from the joining of all posterior extensions, the laminae and pedicles, from the vertebral body.

• Vertebral foramen: canal through which the spinal cord passes.

Chapter 5: The Skeletal System 181

Dens

Superior view of axis (C2) Body

Facet on superior articular process

Transverse process

Spinous process

Anterior arch

Posterior arch

Superior view of atlas (C1) Superior view

Right lateral view

(a) ATLAS AND AXIS

Facet on superior articular process

Spinous process

Vertebral foramen

Transverse process

(b) TYPICAL CERVICAL VERTEBRAE

Spinous process

Transverse process

Facet on superior articular process

Vertebral foramen

Body

Facet for rib

Superior view

Spinous process

Transverse process

Facet on superior articular process

Vertebral foramen

Body

Spinous process

Body

Facet on inferior articular process

Transverse process

Superior articular process

Transverse foramen

Right lateral view

Superior view

BodyFacet on superior articular process

Facet on transverse process Costal facet

for ribSpinous process

Right lateral view

Superior articular process

Facet on inferior articular process

Body

Spinous process

Figure 5.18 regional characteristics of vertebrae. (The types of vertebrae are not shown to scale with respect to one another.)

182 Essentials of Human Anatomy and Physiology

are distinguished by the fact that they are the only vertebrae to articulate with the ribs. The body is somewhat heart-shaped and has two costal facets (articulating surfaces) on each side, which receive the heads of the ribs (dotted line in Figure 5.18c). The transverse processes of each thoracic vertebra articulate with the knoblike tubercles of the ribs. The spinous process is long and hooks sharply downward, causing the vertebra to look like a giraffe’s head viewed from the side.

Lumbar Vertebrae

The five lumbar vertebrae (L1 to L5) have mas- sive, blocklike bodies that are somewhat kidney bean–shaped. Their short, hatchet-shaped spi- nous processes (dotted line in Figure 5.18d) make them look like a moose head from the lateral aspect. Because most of the stress on the verte- bral column occurs in the lumbar region, these are the sturdiest of the vertebrae.

Sacrum

The sacrum (sa′krum) is formed by the fusion of five vertebrae (Figure 5.19). Superiorly it articulates with L5, and inferiorly it connects with the coccyx. Each winglike ala articulates laterally with the hip bone, forming a sacroiliac joint. The sacrum forms the posterior wall of the pelvis. Its posterior mid- line surface is roughened by the median sacral crest, the fused spinous processes of the sacral vertebrae. This is flanked laterally by the posterior sacral foramina. The vertebral canal continues inside the sacrum as the sacral canal and termi- nates in a large inferior opening called the sacral hiatus.

coccyx

The coccyx is formed from the fusion of three to five tiny, irregularly shaped vertebrae (see Figure 5.19). It is the human “tailbone,” a remnant of the tail that other vertebrate animals have.

Thoracic cage ➔ Learning Objectives

□□ Name the components of the thoracic cage.

□□ Describe how a true rib differs from a false rib.

The sternum, ribs, and thoracic vertebrae make up the bony thorax (Figure 5.20). The bony thorax is routinely called the thoracic cage because it forms a protective cage of slender bones and

process, the dens, which acts as the pivot point. The joint between C1 and C2 allows you to rotate your head from side to side to indicate “no.”

The “typical” cervical vertebrae are C3 through C7 (Figure 5.18b). They are the smallest, lightest ver- tebrae, and most often their spinous processes are short and divided into two branches. The transverse processes of the cervical vertebrae contain foramina (openings) through which the vertebral arteries pass on their way to the brain above. Any time you see these foramina in a vertebra, you should know immediately that it is a cervical vertebra.

Thoracic Vertebrae

The 12 thoracic vertebrae (T1 to T12) are all typi- cal. They are larger than the cervical vertebrae and

Coccyx

Posterior sacral foramina

Median sacral crest

Sacral canal

Sacral hiatus

Body

Superior articular process

Auricular surfaceAla

Sacrum

Figure 5.19 Sacrum and coccyx, posterior view.

Chapter 5: The Skeletal System 183

formed at the level of the second ribs. It pro- vides a handy reference point for counting ribs to locate the second intercostal space for listening to certain heart valves. To clarify the location of these structures, you can pal- pate your own sternal angle and jugular notch.

• The xiphisternal (zi′fe-ster″nal) joint, the point where the sternal body and xiphoid pro- cess fuse, lies at the level of the ninth thoracic vertebra.

Because the sternum is so close to the body surface, it is easy to obtain samples from it of blood-forming (hematopoietic) tissue for the diag- nosis of suspected blood diseases. A needle is inserted into the marrow of the sternum, and the sample is withdrawn; this procedure is called a sternal puncture. Because the heart lies immedi- ately posterior to the sternum, the physician must

cartilages around the organs of the thoracic cavity (heart, lungs, and major blood vessels).

Sternum

The sternum (breastbone) is a typical flat bone and the result of the fusion of three bones—the manubrium (mah-nu′bre-um), body, and xiphoid (zif′oid) process. It is attached directly to the first seven pairs of ribs via costal cartilages.

The sternum has three important bony land- marks—the jugular notch, the sternal angle, and the xiphisternal joint.

• The jugular notch (concave upper border of the manubrium) can be palpated easily; generally it is at the level of the third thoracic vertebra.

• The sternal angle results where the manu- brium and body meet at a slight angle to each other, so that a transverse ridge is

Figure 5.20 The bony thorax (thoracic cage).

Intercostal spaces Xiphisternal

joint

Heart

Sternal angle

Jugular notch

True ribs (1–7)

False ribs (8–12)

Jugular notch Clavicular notch

Manubrium Sternal angle Body Xiphisternal joint

Xiphoid process

L1 vertebraFloating

ribs (11, 12)

(b)(a)

Sternum

Costal cartilage

T1 vertebra

Explore Cadaver

184 Essentials of Human Anatomy and Physiology

the sternum medially (at its sternal end) and to the scapula laterally, where it helps to form the shoulder joint. The clavicle acts as a brace to hold the arm away from the top of the thorax and helps prevent shoulder dislocation. When the clavicle is broken, the whole shoulder region caves in medially, which shows how important its bracing function is.

The paired scapulae (skap′u-le), or shoulder blades, are commonly called “wings” because they flare when we move our arms posteriorly. The scapulae are not directly attached to the axial skeleton (see Figure 5.21a); they are loosely held in place by trunk muscles. Each triangular scapula has a flattened body with three borders—supe- rior, medial (vertebral), and lateral (axillary) (Figure 5.21c). It also has three angles—superior, inferior, and lateral. The glenoid cavity, a shal- low socket that receives the head of the arm bone, is in the lateral angle (Figure 5.21d). The scapula has two important processes—the acro- mion (ah-kro′me-on), which is the enlarged lat- eral end of the spine of the scapula, and the beaklike coracoid (kor′ah-koid) process. The acromion connects with the clavicle laterally at the acromioclavicular joint. The coracoid pro- cess points laterally over the top of the shoulder and anchors some of the muscles of the arm. Just medial to the coracoid process is the large supra- scapular notch, which is a nerve passageway.

The shoulder girdle is very light and allows the upper limb exceptionally free movement. This is due to the following factors:

• Each shoulder girdle attaches to the axial skel- eton at only one point—the sternoclavicular joint.

• The loose attachment of the scapula allows it to slide back and forth against the thorax as muscles act.

• The glenoid cavity is shallow, and the shoulder joint is poorly reinforced by ligaments.

However, this exceptional flexibility also has a draw- back; the shoulder girdle is very easily dislocated.

Bones of the upper Limbs Thirty separate bones form the skeletal framework of each upper limb, including the arm, forearm, and hand.

Arm

The arm is formed by a single bone, the humerus (hu′mer-us), which is a typical long

take extreme care not to penetrate beyond the sternum during this procedure.

ribs

Twelve pairs of ribs form the walls of the bony thorax. All the ribs articulate with the vertebral column posteriorly and then curve downward and toward the anterior body surface. The true ribs, the first seven pairs, attach directly to the sternum by costal cartilages. False ribs, the next five pairs, either attach indirectly to the sternum or are not attached to the sternum at all. The last two pairs of false ribs lack the sternal attachments, so they are also called floating ribs.

The intercostal spaces (spaces between the ribs) are filled with the intercostal muscles, which aid in breathing.

Did You Get It? 16. What are the five major regions of the vertebral

column? 17. How can you distinguish a lumbar vertebra from a

cervical vertebra? 18. What is a true rib? A false rib? 19. Besides the ribs and sternum, there is a third group of

bones forming the thoracic cage. What is it? 20. What bone class do the ribs and skull bones fall into? 21. Which spinal curvatures are present at birth? 22. How does the shape of a newborn baby’s spine differ

from that of an adult?

For answers, see Appendix A.

Appendicular Skeleton ➔ Learning Objectives

□□ Identify on a skeleton or diagram the bones of the shoulder and pelvic girdles and their attached limbs.

□□ Describe important differences between a male and a female pelvis.

The appendicular skeleton (shaded gold in Figure 5.8) is composed of 126 bones of the limbs (appendages) and the pectoral and pelvic girdles, which attach the limbs to the axial skeleton.

Bones of the Shoulder Girdle Each pectoral girdle, or shoulder girdle, con- sists of two bones—a clavicle and a scapula (Figure 5.21a).

The paired clavicles (klav′ ı̆-kulz), or collar- bones, are slender, doubly curved bones (Figure 5.21b). Each clavicle attaches to the manubrium of

Chapter 5: The Skeletal System 185

Clavicle Acromio- clavicular joint

Scapula

Posterior Sternal (medial) end

AnteriorAcromial (lateral)end

Superior view

Acromial end

Posterior

Inferior view

Sternal end

(b) Right clavicle, superior and inferior views

Acromion

Coracoid process

Suprascapular notch Superior border

Superior angle

Glenoid cavity

Medial (vertebral) border

Inferior angle

Lateral (axillary) border

(d) Right scapula, anterior aspect

(a) Articulated right shoulder (pectoral) girdle showing the relationship to bones of the thorax and sternum

Coracoid process Suprascapular notch

Acromion

Glenoid cavity at lateral angle

Lateral border

Medial border

Spine

Superior angle

Lateral angle

186 Essentials of Human Anatomy and Physiology

the tubercles is the surgical neck, so named because it is the most frequently fractured part of the humerus. In the midpoint of the shaft is a roughened area called the deltoid tuberosity, where the large, fleshy deltoid muscle of the shoulder attaches. Nearby, the radial groove runs obliquely down the posterior aspect of the shaft. This groove marks the course of the radial nerve,

bone (Figure 5.22a and b). At its proximal end is a rounded head that fits into the shallow glenoid cavity of the scapula. Immediately inferior to the head is a slight constriction called the anatomical neck. Anterolateral to the head are two bony pro- jections separated by the intertubercular sul- cus—the greater tubercle and lesser tubercle, which are sites of muscle attachment. Just distal to

Greater tubercle

Lesser tubercle

Deltoid tuberosity

Coronoid fossa

Head of humerus

Intertubercular sulcus

Capitulum

Radial groove

Medial epicondyle Olecranon

fossa

Lateral epicondyleTrochlea

Radial styloid process

Ulnar styloid processDistal

radioulnar joint

Ulna

Inter- osseous membrane

Surgical neck Coronoid

process

Head

Neck

Radius

Radial tuberosity

Olecranon

Trochlear notch

Proximal radioulnar joint

(a) (b) (c)

Anatomical neck

Radial fossa

Figure 5.22 Bones of the right arm and forearm. (a) Humerus, anterior view. (b) Humerus, posterior view. (c) Anterior view of the bones of the forearm: the radius and the ulna.

Chapter 5: The Skeletal System 187

The palm of the hand consists of the meta- carpals. The metacarpals are numbered 1 to 5 from the thumb side of the hand toward the little finger. When the fist is clenched, the heads of the metacarpals become obvious as the “knuckles.” The phalanges are the bones of the fingers. Each hand contains 14 phalanges. There are three in each finger (proximal, middle, and distal), except in the thumb, which has only two (proxi- mal and distal).

Did You Get It? 23. Contrast the general function of the axial skeleton to

that of the appendicular skeleton. 24. What is the single point of attachment of the

shoulder girdle to the axial skeleton? 25. What bone forms the skeleton of the arm? 26. Where are the carpals found, and what type (long,

short, irregular, or flat) of bone are they? 27. Which bones of the upper limb have a styloid

process?

For answers, see Appendix A.

an important nerve of the upper limb. At the distal end of the humerus is the medial trochlea (trok′le-ah), which looks somewhat like a spool, and the lateral ball-like capitulum (kah-pit′u- lum), which can be “outlined” with a “C” from the anterior view. Both of these processes articulate with bones of the forearm. Above the trochlea anteriorly is a depression, the coronoid fossa; on the posterior surface is the olecranon (o-lek′rah- non) fossa. These two depressions, which are flanked by the medial epicondyle and lateral epicondyle, allow the corresponding processes of the ulna to move freely when the elbow is bent and extended.

Forearm

Two bones, the radius and the ulna, form the skeleton of the forearm (Figure 5.22c). When the body is in the anatomical position, the radius is the lateral bone; that is, it is on the thumb side of the forearm. When the hand is rotated so that the palm faces backward, the distal end of the radius crosses over and ends up medial to the ulna. Both proximally and distally the radius and ulna articu- late at small radioulnar joints, and the two bones are connected along their entire length by the flexible interosseous membrane. Both the ulna and the radius have a styloid process at their distal end.

The disc-shaped head of the radius also forms a joint with the capitulum of the humerus. Just below the head is the radial tuberosity, where the tendon of the biceps muscle attaches.

When the upper limb is in the anatomical position, the ulna is the medial bone (on the little- finger side) of the forearm. On its proximal end are the anterior coronoid process and the poste- rior olecranon, which are separated by the troch- lear notch. Together these two processes grip the trochlea of the humerus in a curved “tongue-in- groove”–like joint.

Hand

The skeleton of the hand consists of the carpals, the metacarpals, and the phalanges (fah-lan′jēz) (Figure 5.23). The eight carpal bones, arranged in two irregular rows of four bones each, form the part of the hand called the carpus, or the wrist. The carpals are bound together by ligaments that restrict movements between them.

Trapezoid

Trapezium

Scaphoid

Capitate

5 4 3 2

Radius

Triquetrum

Lunate

Pisiform

Ulna

Phalanges (fingers)

Metacarpals (palm)

Carpals (wrist)

Proximal

Hamate

Middle

Distal

Figure 5.23 Bones of the right hand, anterior view.

188 Essentials of Human Anatomy and Physiology

The pubis (pu′bis) is the most anterior and inferior part of a coxal bone. Fusion of the rami of the pubis anteriorly and the ischium posteriorly forms a bar of bone enclosing the obturator (ob′tu-ra″tor) foramen, an opening that allows blood vessels and nerves to pass into the anterior part of the thigh. The pubic bones of each hip bone articulate anteriorly to form a cartilaginous joint, the pubic symphysis (pu′bik sim′f ı̆-sis).

The ilium, ischium, and pubis fuse at the deep socket called the acetabulum (as″ĕ-tab′u-lum), which means “vinegar cup.” The acetabulum receives the head of the thigh bone.

The bony pelvis is divided into two regions. The false pelvis is superior to the true pelvis; it is the area medial to the flaring portions of the ilia. The true pelvis is surrounded by bone and lies inferior to the flaring parts of the ilia and the pel- vic brim (Figure 5.24c). The dimensions of the true pelvis of a woman are very important because they must be large enough to allow the infant’s head (the largest part of the infant) to pass during childbirth. The dimensions of the cavity, particu- larly the outlet (the inferior opening of the pelvis measured between the ischial spines) and the inlet (superior opening between the right and left sides of the pelvic brim), are critical, and they are carefully measured by the obstetrician.

Of course, individual pelvic structures vary, but there are fairly consistent differences between a male and a female pelvis, including the follow- ing characteristics (see Figure 5.24c):

• The female inlet is larger and more circular.

• The female pelvis as a whole is shallower, and the bones are lighter and thinner.

• The female ilia flare more laterally, giving women curvy hips.

• The female sacrum is shorter and less curved.

• The female ischial spines are shorter and far- ther apart; thus, the outlet is larger.

• The female pubic arch is more rounded be- cause the angle of the pubic arch is greater.

Did You Get It? 28. What three bones form the hip bone? What bones

form each pelvic girdle? 29. Describe three ways the bony pelvis of a woman

differs from that of a man?

For answers, see Appendix A.

Bones of the Pelvic Girdle The pelvic girdle is formed by two coxal (kok′sal) bones, commonly called hip bones, and the sacrum (described on p. 182). Together with the coccyx, the pelvic girdle forms the pelvis (Figure 5.24a). Note that the terms pelvic girdle (the coxal bones and the sacrum) and pelvis (the coxal bones, sacrum, and coccyx) have slightly different meanings.

The bones of the pelvic girdle are large and heavy, and they are attached securely to the axial skeleton via the sacral attachment to the L5 lumbar vertebra. The sockets, which receive the thigh bones, are deep and heavily reinforced by liga- ments that attach the limbs firmly to the girdle. Bearing weight is the most important function of this girdle because the total weight of the upper body rests on the pelvis. The reproductive organs, urinary bladder, and part of the large intestine lie within and are protected by the pelvis.

Each hip bone is formed by the fusion of three bones: the ilium, ischium, and pubis (Figure 5.24b). The ilium (il′e-um), which connects poste- riorly with the sacrum at the sacroiliac (sak″ro- il′e-ac) joint, is a large, flaring bone that forms most of the hip bone (imagine a peacock’s fanned tail feathers). When you put your hands on your hips, they are resting over the alae, or winglike portions, of the ilia. The upper edge of an ala, the iliac crest, is an important anatomical landmark that is always kept in mind by those who give intramuscular injections. The iliac crest ends ante- riorly in the anterior superior iliac spine and posteriorly in the posterior superior iliac spine. Small inferior spines are located below these.

The ischium (is′ke-um) is the “sit-down bone,” so called because it forms the most inferior part of the coxal bone. The ischial tuberosity is a roughened area that receives body weight when you are sitting. The ischial spine, superior to the tuberosity, is another important anatomical land- mark, particularly in the pregnant woman, because it narrows the outlet of the pelvis through which the baby must pass during birth. Another impor- tant structural feature of the ischium is the greater sciatic notch, which allows blood vessels and the large sciatic nerve to pass from the pelvis posteri- orly into the thigh. Injections in the buttock should always be given well away from this area to avoid possible nerve damage.

Chapter 5: The Skeletal System 189

Coxal bone (or hip bone)

llium

Sacroiliac joint

Pubis

Ischium

Sacrum

(a)

Pelvic brim

Ischial spine

Acetabulum

Pubic symphysis

Iliac crest

Coccyx

Pubic arch

Ilium Ala

Posterior superior iIiac spine

Greater sciatic notch

Posterior inferior iliac spine

Ischial body

Ischial spine

Ischial tuberosity

Ischium

Ischial ramus

Acetabulum

Body of pubis

Iliac crest

Anterior superior iliac spine

Anterior inferior iliac spine

Pubis

Inferior pubic ramus

(b) (c)

Pelvic brim

Pubic arch (more than 90°)

Pubic arch (less than 90°)

Inlet of true pelvis

Pelvic brim

Inlet of true pelvis

False pelvis

Obturator foramen

Figure 5.24 The bony pelvis. (a) Articulated pelvis. (b) Coxal (hip) bone, showing the point of fusion of the ilium, ischium, and pubic bones at the acetabulum. (c) Comparison of the pelves of the male (above) and female (below).

190 Essentials of Human Anatomy and Physiology

rated anteriorly by the intertrochanteric line and posteriorly by the intertrochanteric crest). These markings and the gluteal tuberosity, located on the proximal end of the shaft, all serve as sites for muscle attachment. The head of the femur articu- lates with the deep, secure socket of the acetabu- lum of the hip bone. However, the surgical neck of the femur is still a common fracture site, especially in old age.

The femur slants medially as it runs downward to join with the leg bones; this brings the knees in line with the body’s center of gravity. The medial course of the femur is more noticeable in women because the female pelvis is typically wider than that

Bones of the Lower Limbs The lower limbs carry our total body weight when we are erect. Hence, it is not surprising that the bones forming the three segments of the lower limbs (thigh, leg, and foot) are much thicker and stronger than the comparable bones of the upper limb.

Thigh

The femur (fe′mur), or thigh bone, is the only bone in the thigh (Figure 5.25a and b). It is the heaviest, strongest, and longest bone in the body. Its proximal end has a ball-like head, a neck, and a greater trochanter and lesser trochanter (sepa-

Surgical neck

Lateral epicondyle

Patellar surface

Greater trochanter

Inter- trochanteric crest

Lateral condyle

Head

Lesser trochanter

Gluteal tuberosity

Intercondylar fossa

Medial condyle

Medial epicondyle

(a) (b)

Intercondylar eminence

Tibial tuberosity

Medial condyle

Proximal tibiofibular joint

Lateral condyle

Head

Distal tibiofibular joint

Lateral malleolus

Medial malleolus

Tibia

Anterior border

Interosseous membrane

Fibula

(c)

Inter- trochanteric line

Figure 5.25 Bones of the right thigh and leg. Anterior view of femur (a). Posterior view of femur (b). Anterior view of leg (c).

Chapter 5: The Skeletal System 191

forms the inner bulge of the ankle. The anterior surface of the tibia is a sharp ridge, the anterior border, that is unprotected by muscles; thus, it is easily felt beneath the skin.

The fibula, which lies alongside the tibia later- ally, forms joints with it both proximally and distally, and is thin and sticklike. The fibula has no part in forming the knee joint. Its distal end, the lateral malleolus, forms the outer part of the ankle.

Foot

The foot, composed of the tarsals, metatarsals, and phalanges, has two important functions. It sup- ports our body weight and serves as a lever that allows us to propel our bodies forward when we walk and run.

The tarsus, forming the posterior half of the foot, is composed of seven tarsal bones (Figure 5.26). Body weight is carried mostly by the two largest tarsals, the calcaneus (kal-ka′ne-us), or heel bone, and the talus (ta′lus; “ankle”). The talus lies superior to the calcaneus, articulates with the tibia, and allows pointing of the toes. Five metatarsals form the sole, and 14 phalanges form the toes. Like the fingers of the hand, each toe has three phalanges, except the great toe, which has two.

The bones in the foot are arranged to form three strong arches: two longitudinal (medial and lateral) and one transverse (Figure 5.27). Ligaments, which bind the foot bones together, and tendons of the foot muscles help to hold the bones firmly in the arched position but still allow a certain

of the male. Distally on the femur are the lateral condyle and medial condyle, which articulate with the tibia below. Posteriorly these condyles are sepa- rated by the deep intercondylar fossa. Anteriorly on the distal femur is the smooth patellar surface, which forms a joint with the patella, or kneecap.

Leg

Connected along their length by an interosseous membrane, two bones, the tibia and fibula, form the skeleton of the leg (Figure 5.25c). The tibia, or shinbone, is larger and more medial. At the proxi- mal end, the medial condyle and lateral con- dyle (separated by the intercondylar eminence) articulate with the distal end of the femur to form the knee joint. The patellar (kneecap) ligament, which encloses the patella, a sesamoid bone (look ahead to Figure 6.20c and d on p. 237), attaches to the tibial tuberosity, a roughened area on the anterior tibial surface. Distally, a pro- cess called the medial malleolus (mal-le′o-lus)

Phalanges:

Metatarsals

Tarsals:

Calcaneus

Cuboid

Lateral cuneiform

Proximal

Middle Distal

Medial cuneiform

Navicular

Intermediate cuneiform

Talus

Tarsals:

Figure 5.26 Bones of the right foot, superior view.

Medial longitudinal arch

Transverse arch

Lateral longitudinal arch

Figure 5.27 Arches of the foot.

The technology for creating the prostheses (artificial joints) used in medicine today developed, in relative terms, in a flash—less than 60 years. Over 1 million knee and hip joint replacement surgeries are performed each year, mostly because of the destructive effects of osteoarthritis or rheumatoid arthritis.

The production of durable, mobile joints requires a substance that is strong, nontoxic, and resistant to the corrosive effects of organic acids in blood and that will not cause an immune response. In 1963, Sir John Charnley, an English orthopedic sur- geon, performed the first total hip replacement using a modern pros- thetic, revolutionizing the therapy of arthritic hips. His device consisted of a metal ball on a stem (the femoral head) and a cup-shaped polyethylene plastic socket (the acetabulum) anchored to the pelvis by polymethyl

methacrylate cement (bone cement). This cement proved to be exception- ally strong and relatively problem free. Hip prostheses were followed by knee prostheses (see photos a and b), and replacements are now available for many other joints, including shoul- ders, fingers, and elbows.

Total hip and knee replacements last over 20 years in elderly patients who do not excessively stress the joint but may not last as long in younger (age 45 to 60), more active people. Most such operations are

Joint Ventures A

CLOSER LOOK

amount of give, or springiness. Weak arches are referred to as “fallen arches,” or “flat feet.”

Did You Get It? 30. What two bones form the skeleton of the leg? 31. Which bone allows us to “point our toes”? 32. Which bone of the lower limb has an intertrochanteric

line and crest and an intercondylar fossa?

For answers, see Appendix A.

Joints ➔ Learning Objective

□□ Name the three major structural categories of joints, and compare the amount of movement allowed by each.

With one exception (the hyoid bone of the neck), every bone in the body forms a joint with at least

(a) A hip prosthesis. (b) X-ray image of right knee showing total knee replacement prosthesis.

one other bone. Joints, also called articula- tions, are the sites where two or more bones meet. They have two functions: They hold the bones together securely but also give the rigid skeleton mobility.

The graceful movements of a ballet dancer and the rough-and-tumble grapplings of a football player illustrate the great variety of motion that joints allow. With fewer joints, we would move like robots. Nevertheless, the bone-binding func- tion of joints is just as important as their role in mobility. The immovable joints of the skull, for instance, form a snug enclosure for the vital brain.

Joints are classified in two ways—functionally and structurally. The functional classification focuses on the amount of movement the joint

192

done to reduce pain and restore joint function.

Replacement joints are not yet strong or durable enough for young, active people. The problem is that the prostheses work loose over time. One solution is to strengthen the cement that binds the implant to the bone. Another solution is robot- assisted surgery using ROBODOC (see photo c), which uses the information from a CT scan to precisely drill a

better-fitting hole and so achieve better alignment for the femoral prosthesis in hip surgery. In unce- mented prostheses, the bone will eventually grow so that it binds strongly to the implant, but this requires extended recovery time.

Dramatic changes are also occur- ring in the way artificial joints are made. CAD/CAM (computer-aided design and computer-aided manufac- turing) techniques have significantly

reduced the time and cost of creating individualized joints. The computer draws from a database of hundreds of healthy joints, generates possible designs, and produces a program to direct the machines that shape it.

Equally exciting are techniques that call on the patient’s own tissues to regenerate, such as these three:

• Osteochondral grafting: Healthy bone and cartilage are removed from one part of the body and transplanted to the injured joint.

• Autologous chondrocyte implanta- tion: Healthy chondrocytes are removed from the body, cultivated in the lab, and implanted at the damaged joint.

• Stem cell regeneration: Undifferen- tiated stem cells are removed from bone marrow and placed in a gel, which is packed into an area of eroded cartilage.

These techniques offer hope for younger patients because they could stave off the need for a joint prosthe- sis for several years.

allows. On this basis, there are synarthroses (sin″ar-thro′sēz), or immovable joints; amphiar- throses (am″fe-ar-thro′sēz), or slightly movable joints; and diarthroses (di″ar-thro′sēz), or freely movable joints. Immovable and slightly movable joints are restricted mainly to the axial skeleton, where firm attachments and protection of internal organs are priorities. Freely movable joints pre- dominate in the limbs, where mobility is important.

Structurally, there are fibrous, cartilaginous, and synovial joints. These classifications are based on whether fibrous tissue, cartilage, or a joint cav- ity separates the bony regions at the joint. As a general rule, fibrous joints are immovable, and synovial joints are freely movable. Although carti- laginous joints have both immovable and slightly movable examples, most are amphiarthrotic.

Because the structural classification is more clear- cut, we will focus on that classification scheme here. The joint types are shown in Figure 5.28 on p. 194 and described next.

193

➔ConCeptLinkTo understand the structural classes of joints more clearly, recall the properties of tissues that form the joints. Fibrous connective tissue contains many colla- gen fibers for strength. The three types of cartilage (hyaline, fibrocartilage, elastic) provide structure with some degree of flexibility, and fibrocartilage also has the ability to absorb compressive shock (Chapter 3, pp. 120, 121). Synovial membranes contain areolar connective tissue and line the joint cavities of synovial joints (Chapter 4, pp. 136, 138). ➔

194 Essentials of Human Anatomy and Physiology

Figure 5.28 Types of joints. Joints above and below the skeleton are fibrous joints (a, b), joints to the left of the skeleton are cartilaginous joints (c–e), and joints to the right of the skeleton are synovial joints (f–h).

Cartilaginous joints Fibrous joints

(a) Suture

(f) Multiaxial joint (shoulder joint)

(g) Uniaxial joint (elbow joint)

(h) Biaxial joint (intercarpal joints of hand)

(b) Syndesmosis

(d) Symphysis

(e) Symphysis

Synovial joints

Fibrous connective tissue

Sternum

Hyaline cartilage

Fibrocartilage

Fibro- cartilage Carpals

Fibula

Tibia

Ulna

Articular capsule

Scapula

Articular (hyaline) cartilage

Articular capsule

Fibrous connective tissue

Pubis

Vertebrae

First rib

Humerus

Radius

Chapter 5: The Skeletal System 195

Figure 5.29 General structure of a synovial joint.

Acromion of scapula

Ligament

Bursa

Tendon sheath

Tendon of biceps muscle

Joint cavity containing synovial fluid

Articular (hyaline) cartilage

Synovial membrane

Fibrous layer of the articular capsule

Humerus

How does this joint type differ structurally from cartilaginous and fibrous joints?Q:

It has a joint cavity filled with lubricating fluid instead of cartilage or fibrous tissue separating the

articulating bones.

Fibrous Joints In fibrous joints, the bones are united by fibrous tissue. The best examples of this type of joint are the sutures of the skull (Figure 5.28a). In sutures, the irregular edges of the bones interlock and are bound tightly together by connective tissue fibers, allowing no movement. Gomphoses (gom-fo′sēz), are “peg-in-socket” fibrous joints that are found where the teeth meet the facial bones (not shown in Figure 5.28). In another type of fibrous joint, syndesmoses (sin-dez-mo′sēz), the connecting fibers are longer than those of sutures; thus the joint has more “give.” The joint connecting the dis- tal ends of the tibia and fibula is a syndesmosis (Figure 5.28b).

cartilaginous Joints Cartilaginous joints come in two varieties, which differ in the type of cartilage involved. Synchondroses are immoveable (synarthrotic) joints linked by

hyaline cartilage. Examples include the epiphyseal plates of growing long bones and the joints between ribs 1–7 and the sternum (Figure 5.28c). Symphyses are amphiarthrotic (slightly moveable) joints linked by discs of fibrocartilage. Examples include the intervertebral discs of the spinal column (Figure 5.28d) and the pubic symphysis of the pelvis (Figure 5.28e).

Synovial Joints Synovial joints are joints in which the articulat- ing bone ends are separated by a joint cavity con- taining synovial fluid (Figure 5.28f–h). All joints of the limbs are synovial joints.

All synovial joints have four distinguishing fea- tures (Figure 5.29):

• Articular cartilage. Articular (hyaline) carti- lage covers the ends of the bones forming the joint.

• Articular capsule. The joint surfaces are en- closed by a sleeve, or layer, of fibrous connec- tive tissue, which is lined with a smooth syno- vial membrane (the reason these joints are called synovial joints).

196 Essentials of Human Anatomy and Physiology

Types of Synovial Joints Based on Shape The shapes of the articulating bone surfaces deter- mine what movements are allowed at a joint. Based on such shapes, our synovial joints can be classified as plane, hinge, pivot, condylar (kon′dı̆- ler), saddle, or ball-and-socket joints. Examples of these types of joints are shown in Figure 5.30; the arrows indicate movement around one or more axes in each type.

• In a plane joint (Figure 5.30a), the articular surfaces are essentially flat, and only short slip- ping or gliding movements are allowed. The movements of plane joints are nonaxial; that is, gliding back and forth does not involve ro- tation around any axis. The intercarpal joints of the wrist are the best examples of plane joints.

• In a hinge joint (Figure 5.30b), the cylindrical end of one bone fits into a trough-shaped sur- face on another bone. Angular movement is al- lowed in just one plane, like a door hinge. Examples are the elbow joint, ankle joint, and the joints between the phalanges of the fin- gers. Hinge joints are classified as uniaxial (u″ne-aks′e-al; “one axis”); they allow move- ment around one axis only.

• In a pivot joint (Figure 5.30c), the rounded end of one bone fits into a sleeve or ring of bone (and possibly ligaments). Because the ro- tating bone can turn only around its long axis,

• Joint cavity. The articular capsule encloses a cavity, called the joint cavity, which contains lubricating synovial fluid secreted by the sy- novial membrane (look back at Chapter 4, pp. 136, 138).

• Reinforcing ligaments. The fibrous layer of the capsule is usually reinforced with ligaments.

Bursae and tendon sheaths are not strictly part of synovial joints, but they are often found closely associated with them (see Figure 5.29). Essentially bags of lubricant, they help cushion and act like ball bearings to reduce friction between adjacent structures during joint activity. Bursae (ber′se; “purses”) are flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid. They are common where ligaments, muscles, skin, tendons, or bones rub together. A tendon sheath is essentially an elongated bursa that wraps completely around a tendon subjected to friction, like a bun around a hot dog.

The structural joint types are summarized in Table 5.3.

Homeostatic Imbalance 5.6

A dislocation happens when a bone is forced out of its normal position in the joint cavity. The pro- cess of returning the bone to its proper position, called reduction, should be done only by a physi- cian. Attempts by an untrained person to “snap the bone back into its socket” are usually more harm- ful than helpful. _____________________________✚

Table 5.3 Summary of Joint classes

Structural class Structural characteristics Types Mobility

Fibrous Bone ends/parts united by collagenic fibers

Suture (short fibers) Immobile (synarthrosis)

Syndesmosis (longer fibers) Slightly mobile (amphiarthrosis) and immobile

Gomphosis (periodontal ligament)

Immobile

Cartilaginous Bone ends/parts united by cartilage

Synchondrosis (hyaline cartilage)

Immobile

Symphysis (fibrocartilage) Slightly movable

Synovial Bone ends/parts covered with articular cartilage and enclosed within an articular capsule lined with synovial membrane

Plane Condylar Hinge Saddle Pivot Ball and socket

Freely movable (diarthrosis; movements depend on design of joint)

Chapter 5: The Skeletal System 197

(a) Plane joint

(b) Hinge joint

Ulna Humerus

Metacarpal #1 Carpal

Radius

Ulna

(f)

(b)

(c)

(e)

(d)

(a)

(e) Saddle joint

(f) Ball-and-socket joint

Head of humerus

Scapula

(d) Condylar joint

Metacarpal Phalanx

Multiaxial Biaxial Uniaxial Nonaxial

Figure 5.30 Types of synovial joints. (a) Plane joint (intercarpal and intertarsal joints). (b) Hinge joint (elbow and interphalangeal joints). (c) Pivot joint (proximal joint between the radius and the ulna). (d) Condylar joint (knuckles). (e) Saddle joint (carpometacarpal joint of the thumb). (f) Ball-and-socket joint (shoulder and hip joints).

pivot joints are also uniaxial joints. To under- stand the motion of this type of joint, think about a basketball player pivoting around one foot that stays planted in one spot. The proxi-

mal radioulnar joint and the joint between the atlas and the dens of the axis are examples.

• In a condylar joint (“knucklelike”), the egg- shaped articular surface of one bone fits into

198 Essentials of Human Anatomy and Physiology

Acute forms of arthritis usually result from bac- terial invasion and are treated with antibiotic drugs. During such an infection, the synovial membrane thickens and fluid production decreases, leading to increased friction and pain. Chronic forms of arthritis include osteoarthritis, rheumatoid arthritis, and gouty arthritis, which differ substantially in their later symptoms and consequences. We will focus on these forms here.

Osteoarthritis (OA), the most common form of arthritis, is a chronic degenerative condition that typically affects the aged: Eighty-five percent of people in the United States eventually develop this condition. OA, also called degenerative joint dis- ease (DJD) or “wear-and-tear arthritis,” affects the articular cartilages. Over the years, the cartilage softens, frays, and eventually breaks down. As the disease progresses, the exposed bone thickens, and extra bone tissue, called bone spurs, grows around the margins of the eroded cartilage and restricts joint movement. Patients complain of stiff- ness on arising that lessens with activity, and the affected joints may make a crunching noise (crepi- tus) when moved. The joints most commonly affected are those of the fingers, the cervical and lumbar joints of the spine, and the large, weight- bearing joints of the lower limbs (knees and hips).

The course of osteoarthritis is usually slow and irreversible, but it is rarely crippling. In most cases, its symptoms are controllable with a mild analge- sic such as aspirin, moderate activity to maintain joint mobility, and rest when the joint becomes very painful. Some people with OA claim that rub- bing capsaicin (a hot pepper extract) on the skin over painful joints provides relief. Others swear to the pain-reducing ability of glucosamine sulfate, a nutritional supplement.

Rheumatoid (roo′mah-toid) arthritis (RA) is a chronic inflammatory disorder. Its onset is insidi- ous and usually occurs between the ages of 40 and 50, but it may occur at any age. It affects three times as many women as men. Many joints, par- ticularly those of the fingers, wrists, ankles, and feet, are affected at the same time and usually in a symmetrical manner. For example, if the right elbow is affected, most likely the left elbow will be affected also. The course of RA varies and is marked by remissions and flare-ups (rheumat = susceptible to change or flux).

RA is an autoimmune disease—a disorder in which the body’s immune system mistakenly attempts to destroy its own tissues. The initial trigger

an oval concavity in another (Figure 5.30d). Both of these articular surfaces are oval. Condylar joints allow the moving bone to travel (1) from side to side and (2) back and forth, but the bone cannot rotate around its long axis. Movement occurs around two axes; hence, these joints are biaxial (bi = two), as in knuckle (metacarpophalangeal) joints.

• In saddle joints, each articular surface has both convex and concave areas, like a saddle for a horse (Figure 5.30e). These biaxial joints allow essentially the same movements as condylar joints. The best examples of saddle joints are the carpometacarpal joints in the thumb, which are responsible for our opposable thumbs.

• In a ball-and-socket joint (Figure 5.30f), the spherical head of one bone fits into the round socket in another. These multiaxial joints al- low movement in all axes, including rotation, and are the most freely moving synovial joints. The shoulder and hip are examples.

Because they relate to muscle activity, we dis- cuss the various types of movements that occur at synovial joints in detail in the chapter covering the muscular system (Chapter 6).

Homeostatic Imbalance 5.7

Few of us pay attention to our joints unless we experience joint pain or inflammation. For exam- ple, falling on one’s knee can cause painful bursi- tis, called “water on the knee,” due to inflammation of bursae or a synovial membrane. Sprains and dislocations are other types of joint problems that result in swelling and pain. A sprain results when the ligaments or tendons reinforcing a joint are damaged by excessive stretching or are torn away from the bone. Both tendons and ligaments are cords of dense fibrous connective tissue with a poor blood supply; thus, sprains heal slowly and can be extremely painful.

Few inflammatory joint disorders cause more pain and suffering than arthritis. The term arthri- tis (arth = joint, itis = inflammation) describes over 100 different inflammatory or degenerative diseases that damage joints. In all its forms, arthri- tis is the most widespread, crippling disease in the United States. All forms of arthritis have the same initial symptoms: pain, stiffness, and swelling of the joint. Then, depending on the specific form, certain changes in the joint structure occur.

Chapter 5: The Skeletal System 199

_____________________________________________✚

Did You Get It? 33. What are the functions of joints? 34. What is the major difference between a fibrous joint

and a cartilaginous joint? 35. Where is synovial membrane found, and what is its role? 36. What two joints of the body are ball-and-socket

joints? What is the best example of a saddle joint?

For answers, see Appendix A.

Developmental Aspects of the Skeleton ➔ Learning Objectives

□□ Describe major changes in skeletal development at fetal, infant, youth, and adult stages of life.

□□ Identify some of the causes of bone and joint problems throughout life.

As we have seen, the skeleton goes through many developmental changes in a person’s lifetime. Let’s look in more detail at the times when it changes the most.

Birth to Adulthood As we described earlier, the first “long bones” in the very young fetus are formed of hyaline cartilage, and the earliest “flat bones” of the skull are actually fibrous membranes. As the fetus develops and grows, all of the bone models are converted to bone (Figure 5.31). At birth, the skull has fibrous regions that have yet to be converted to bone. These fibrous membranes connecting the cranial bones are called fontanels (fon″tah-nelz′) (Figure 5.32). The rhythm of the baby’s pulse can be felt in these “soft spots,”

for this reaction is unknown but could involve genetic factors, certain bacterial or viral infections, hormones of the normal stress response, or a com- bination of these.

RA begins with inflammation of the synovial membranes. The membranes thicken and the joints swell as synovial fluid accumulates. Inflammatory cells (white blood cells and others) enter the joint cavity from the blood and inappropriately release a deluge of inflammatory chemicals that lead to the destruction of body tissues. In time, the inflamed synovial membrane thickens into a pannus (“rag”), an abnormal tissue that clings to and erodes articu- lar cartilages. As the cartilage is destroyed, scar tis- sue forms and connects the bone ends. The scar tissue eventually ossifies, and the bone ends become firmly fused (ankylosis) and often deformed. Not all cases of RA progress to the severely crippling ankylosis stage, but all cases involve restricted joint movement and extreme pain.

Current therapy for RA involves many different kinds of drugs. Some, such as methotrexate, are immunosuppressants. Others, such as etanercept (Enbrel), neutralize the inflammatory chemicals in the joint space and (hopefully) prevent joint defor- mity. However, drug therapy often begins with aspirin, which in large doses is an effective anti- inflammatory agent. Exercise is recommended to maintain as much joint mobility as possible. Cold packs are used to relieve the swelling and pain, and heat helps to relieve morning stiffness. Replacement joints or bone removal are the last resort for severely crippled RA patients.

Gouty (gow′te) arthritis, or gout, is a dis- ease in which uric acid (a normal waste product of nucleic acid metabolism) accumulates in the blood and may be deposited as needle-shaped crystals in the soft tissues of joints. This leads to an agoniz- ingly painful attack that typically affects a single joint, often in the great toe. Gout is most common in men and rarely appears before the age of 30. It tends to run in families, so genetic factors are definitely implicated.

Untreated gout can be very destructive; the bone ends fuse, and the joint becomes immobi- lized. Fortunately, several drugs (colchicine, ibu- profen, and others) are successful in preventing acute gout attacks. Patients are advised to lose weight if obese; to avoid foods such as liver, kid- neys, and sardines, which are high in nucleic acids; and to avoid alcohol, which inhibits excre- tion of uric acid by the kidneys.

X-ray image of a hand deformed by rheumatoid arthritis.

200 Essentials of Human Anatomy and Physiology

which explains their name (fontanel = little foun- tain). The largest fontanel is the diamond-shaped anterior fontanel. The fontanels allow the fetal skull to be compressed slightly during birth. In addition, because they are flexible, they allow the infant’s brain to grow during the latter part of pregnancy and early infancy. This would not be possible if the cranial bones were fused by sutures, as in the adult skull. The fontanels usually fully ossify by 2 years of age.

The skeleton changes throughout life, but the changes in childhood are most dramatic. At birth and in infancy, the baby’s cranium is huge relative to its face (Figure 5.33a). This is why some young kids resemble “bobble head” dolls, because their heads are quite large compared to their body size. The rapid growth of the cranium before and after birth is related to the growth of the brain. By 2 years, the skull is three-quarters of its adult size; and, by 8 to 9 years, the skull is almost adult in size and proportion. Between the ages of 6 and

11, the head appears to enlarge substantially as the face literally grows out from the skull. The jaws increase in size, and the cheekbones and nose become more prominent as respiratory passages expand and the permanent teeth develop. During youth, growth of the skeleton not only increases overall body height and size but also changes body proportions (Figure 5.33b). At birth, the head and trunk are proportionately much lon- ger than the lower limbs. The lower limbs grow more rapidly than the trunk from this time on, and by the age of 10, the head and trunk are approxi- mately the same height as the lower limbs and change little thereafter. During puberty, the female pelvis broadens in preparation for childbearing, and the entire male skeleton becomes more

Parietal bone

Radius Ulna

Humerus

Femur

Occipital bone

Clavicle

Scapula

Ribs

Vertebra

Hip bone

Tibia

Frontal bone of skull

Mandible

Figure 5.31 Ossification centers in the skeleton of a 12-week-old fetus are indicated by the darker areas. Lighter regions are still fibrous or cartilaginous.

Occipital bone

Parietal bone

Anterior fontanel

Frontal bone

Sphenoidal fontanel

Frontal bone

(a)

(b)

Posterior fontanel

Mastoid fontanel

Parietal bone

Occipital bone

Temporal bone

Anterior fontanel

Figure 5.32 The fetal skull.

Chapter 5: The Skeletal System 201

5(a)

(b)

Human newborn

Newborn 2 yrs. 5 yrs. 15 yrs. Adult

Human adult

Figure 5.33 Differences in the growth rates for some parts of the body compared to others determine body proportions. (a) Differential growth transforms the rounded, foreshortened skull of a newborn to the sloping skull of an adult. (b) During growth of a human, the arms and legs grow faster than the head and trunk, as shown in this conceptualization of different-aged individuals all drawn at the same height.

robust. By the end of adolescence, the epiphyseal plates of long bones that provide for longitudinal growth in childhood have become fully ossified, and long-bone growth ends. Once adult height is reached, a healthy skeleton changes very little until late middle age. In old age, losses in bone mass become obvious.

Older Adults Bones have to be physically stressed to remain healthy. When we remain active physically and muscles and gravity pull on the skeleton, the bones respond by becoming stronger. By contrast, if we are totally inactive, they become thin and fragile. Osteoporosis is a bone-thinning disease that afflicts half of women over 65 and some 20 percent of men over the age of 70. The ratio of bone forma- tion to bone breakdown decreases as osteoblast

Figure 5.34 Osteoporosis. The architecture of osteoporotic bone, above, is contrasted with that of normal bone, below.

activity becomes sluggish. Osteoporosis can make the bones so fragile that even a hug or a sneeze can cause bones to fracture (Figure 5.34). The bones of the spine and the neck of the femur are particu- larly susceptible. Vertebral collapse frequently results in a hunched-over posture (kyphosis) known as dowager’s hump (Figure 5.35, p. 203).

Estrogen helps to maintain the health and normal density of a woman’s skeleton, and the estrogen deficiency that occurs after a woman goes through menopause (when menstruation ceases) is strongly implicated as a cause of osteo- porosis. Other factors that may contribute to osteoporosis are a diet poor in calcium and pro- tein, lack of vitamin D, smoking, and insufficient weight-bearing exercise to stress the bones. Sadly, many older people feel that they are helping themselves by “saving their strength” and not doing anything too physical. Their reward for this

202

SYSTEMS IN SYNC

Homeostatic Relationships between the Skeletal System and Other Body Systems

• Bone marrow cavities provide site for blood cell formation; matrix stores calcium needed for muscle, nerve, and blood- clotting activities

• Cardiovascular system delivers nutrients and oxygen to bones; carries away wastes

• Skeletal system protects pelvic organs (bladder, etc.)

• Urinary system activates vitamin D; disposes of nitrogen-containing

wastes

• Skeletal system provides levers plus calcium for muscle activity

• Muscle pull on bones increases bone strength and viability; helps determine bone shape as bone matrix is deposited at points of stress

• Skeletal system provides some protection to lymphoid organs; lymphocytes involved in immune response originate in bone marrow

• Lymphatic system drains leaked interstitial fluid; immune cells protect against pathogens

• Skeletal system provides some bony protection to intestines, pelvic organs, and liver

• Digestive system provides nutrients needed for bone health and growth

• Skeletal system protects some reproductive organs by enclosure

• Gonads produce hormones that influence form of skeleton and epiphyseal closure

• Skeletal system (rib cage) protects lungs by enclosure

• Respiratory system provides oxygen; disposes of carbon dioxide

• Skeletal system protects brain and spinal cord; provides a depot for calcium ions needed for neural function

• Nerves innervate bone and articular capsules, providing for pain and joint sense

• Skeletal system provides support for body organs including the skin

• Skin provides vitamin D needed for proper calcium absorption and use

• Skeletal system provides some bony protection

• Hormones regulate uptake and release of calcium from bone; hormones promote long-bone growth and maturation

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System

Reproductive System

Respiratory System

Nervous System

Integumentary System

Endocrine System

Skeletal System

Chapter 5: The Skeletal System 203

Figure 5.35 Vertebral collapse due to osteoporosis. Women with postmenopausal osteoporosis are at risk for vertebral fractures. Eventually these vertebrae collapse, producing spinal curvature that causes loss of height, a tilted rib cage, a dowager’s hump, and a protruding abdomen.

Age 40 Age 60 Age 70

is pathologic fractures (breaks that occur sponta- neously without apparent injury), which increase dramatically with age and are the single most common skeletal problem for this age group.

Advancing years also take their toll on joints. Weight-bearing joints in particular begin to degen- erate, and osteoarthritis is common. Such degenera- tive joint changes lead to the complaint often heard from an aging person: “My joints are getting so stiff.”

Did You Get It? 37. Ninety-year-old Mrs. Pelky is groaning in pain. Her

grandson has just given her a bear hug. What do you think might have happened to her spine, and what bone condition may she be suffering from?

38. Which two regions of the skeleton grow most rapidly during childhood?

For answers, see Appendix A.

Summary

Bones: An Overview (pp. 160–172)

1. Bones support and protect body organs; serve as levers for the muscles to pull on to cause move- ment at joints; and store calcium, fats, and other substances for the body. Some contain red marrow, the site of blood cell production.

2. Bones are classified into four groups—long, short, flat, and irregular—on the basis of their shape and the amount of compact or spongy bone they con- tain. Bone markings are important anatomical land- marks that reveal where muscles attach and where blood vessels and nerves pass.

3. A long bone is composed of a shaft (diaphysis) with two ends (epiphyses). The shaft is compact bone; its cavity contains yellow marrow. The epiphyses are covered with hyaline cartilage; they contain spongy bone (where red marrow is found).

4. The organic parts of the matrix make bone flexible; calcium salts deposited in the matrix make bone hard.

5. Bones form in the fetus on hyaline cartilage or fibrous membrane “models.” Epiphyseal plates per- sist to provide for longitudinal growth of long bones during childhood and become inactive (by calcifying) when adolescence ends.

6. Bones change in shape throughout life. This remod- eling occurs in response to hormones (for example, PTH, which regulates blood calcium levels) and mechanical stresses acting on the skeleton.

7. A fracture is a break in a bone. Common types of fractures include simple, compound, compression, comminuted, and greenstick. Bone fractures must be reduced to heal properly.

Axial Skeleton (pp. 172–184)

1. The skull is formed by cranial and facial bones. Eight cranial bones protect the brain: frontal, occipi- tal, ethmoid, and sphenoid bones, and the paired parietal and temporal bones. The 14 facial bones are all paired (maxillae, zygomatics, palatines, nasals, lacrimals, and inferior nasal conchae), except for the vomer and mandible. The hyoid bone, not really a skull bone, is supported in the neck by ligaments.

2. The vertebral column is formed from 24 vertebrae, the sacrum, and the coccyx. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar verte- brae, which have common and unique features. The vertebrae are separated by fibrocartilage discs that allow the vertebral column to be flexible. The verte- bral column is C-shaped at birth (thoracic and sacral curvatures are present); the secondary curvatures

204 Essentials of Human Anatomy and Physiology

3. Joints also can be classified structurally as fibrous, cartilaginous, or synovial joints, depending on the substance separating the articulating bones.

4. Most fibrous joints are synarthrotic, and most carti- laginous joints are amphiarthrotic. Fibrous and car- tilaginous joints occur mainly in the axial skeleton.

5. All joints of the limbs are synovial joints. In syno- vial joints, the articulating bone surfaces are cov- ered with articular cartilage and enclosed within the joint cavity by a fibrous capsule lined with a synovial membrane that secretes lubricating syno- vial fluid. All synovial joints are diarthroses.

6. The most common joint problem is arthritis, or inflammation of the joints. Osteoarthritis, or de- generative arthritis, is a result of the “wear and tear” on joints over many years and is a common affliction of the aged. Rheumatoid arthritis occurs in both young and older adults; it is believed to be an autoimmune disease. Gouty arthritis, caused by the deposit of uric acid crystals in joints, typically affects a single joint.

Developmental Aspects of the Skeleton (pp. 199–203)

1. Fontanels, which allow brain growth and ease birth passage, are present in the skull at birth. Growth of the cranium after birth is related to brain growth; the increase in size of the facial skeleton follows tooth development and enlargement of the respira- tory passageways.

2. Long bones continue to grow in length until late adolescence. By the age of 10, the head and trunk are approximately the same height as the lower limbs and change little thereafter.

3. Fractures are the most common bone problem in elderly people. Osteoporosis, a condition of bone wasting that results mainly from hormone deficit or inactivity, is also common in older individuals.

(cervical and lumbar) form when the baby begins to lift its head and walk. After infancy, the vertebral column is S-shaped to allow for upright posture.

3. The bony thorax is formed from the sternum and 12 pairs of ribs. All ribs attach posteriorly to thoracic ver- tebrae. Anteriorly, the first 7 pairs attach directly to the sternum (true ribs); the last 5 pairs attach indirectly or not at all (false ribs). The bony thorax encloses the lungs, heart, and other organs of the thoracic cavity.

Appendicular Skeleton (pp. 184–192)

1. The shoulder girdles are each composed of two bones—a scapula and a clavicle—and attach the upper limbs to the axial skeleton. They are light, poorly reinforced girdles that allow the upper limbs a great deal of freedom of movement.

2. The bones of the upper limb include the humerus of the arm, the radius and ulna of the forearm, and the carpals, metacarpals, and phalanges of the hand.

3. The pelvic girdle is formed by the two coxal bones, or hip bones, and the sacrum (part of the axial skeleton). Each hip bone is the result of fusion of the ilium, ischium, and pubis bones. The pelvic girdle is securely attached to the vertebral column, and the socket for the thigh bone is deep and heavily reinforced. This girdle receives the weight of the upper body and transfers it to the lower limbs. The female pelvis is lighter and broader than the male’s; its inlet and outlet are larger, reflecting the childbearing function.

4. The bones of the lower limb include the femur of the thigh; the tibia and fibula of the leg; and the tarsals, metatarsals, and phalanges of the foot.

Joints (pp. 192–199)

1. Joints hold bones together and allow movement of the skeleton.

2. Joints fall into three functional categories: synar- throses (immovable), amphiarthroses (slightly mov- able), and diarthroses (freely movable).

review Questions

Multiple choice

More than one choice may apply.

1. Which of the following are correctly matched?

a. Short bone—wrist

b. Long bone—leg

c. Irregular bone—sternum

d. Flat bone—cranium

2. A passageway connecting neighboring osteocytes in an osteon is a

a. central canal.

b. lamella.

c. lacuna.

d. canaliculus.

e. perforating canal.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 5: The Skeletal System 205

3. Which of the following are the elements of an osteon?

a. Haversian canals

b. Sharpey’s fibers

c. Lacunae

d. Periostea

4. Bone pain behind the external acoustic meatus probably involves the

a. maxilla.

b. ethmoid bone.

c. sphenoid bone.

d. temporal bone.

e. lacrimal bone.

5. Bones that articulate with the sphenoid include which of the following?

a. Parietal

b. Frontal

c. Occipital

d. Zygomatic

e. Ethmoid

6. The clavicle articulates with the

a. sternal manubrium.

b. sternal body.

c. acromion.

d. coracoid process.

e. head of humerus.

7. Every cervical vertebra has

a. a body.

b. a dens.

c. costal facets.

d. transverse processes with foramina for the vertebral arteries.

e. a short spinous process divided into two branches.

8. Which of the following bones are found in the feet?

a. Lunate

b. Talus

c. Metacarpal

d. Metatarsal

e. Calcaneus

9. The shoulder joint is an example of a

a. plane joint.

b. condylar joint.

c. ball-and-socket joint.

d. pivot joint.

e. hinge joint.

10. At what stage of life do the lower limbs attain the same height as the head and trunk?

a. At birth

b. By 10 years of age

c. At puberty

d. When the epiphyseal plates fuse

e. Never

11. Match the types of joints to the descriptions that apply to them. (More than one description might apply.)

a. Fibrous joints

b. Cartilaginous joints

c. Synovial joints ____ 1. Have no joint cavity ____ 2. Types are sutures and syndesmoses ____ 3. Dense connective tissue fills the space

between the bones ____ 4. Almost all joints of the skull ____ 5. Types are synchondroses and symphyses ____ 6. All are diarthroses ____ 7. The most common type of joint in the

body ____ 8. Nearly all are synarthrotic ____ 9. Shoulder, hip, knee, and elbow joints

12. Match the bone markings listed on the right with their function listed on the left. 1. Attachment site for

muscle or ligament 2. Forms a joint

surface 3. Passageway for ves-

sels or nerves

____ a. Trochanter ____ b. Condyle ____ c. Foramen ____ d. Process ____ e. Facet ____ f. Tuberosity

Short Answer Essay

13. Name three functions of the skeletal system.

14. What is yellow marrow? How do spongy and compact bone look different?

15. Why do bone injuries heal much more rapidly than injuries to cartilage?

16. Compare and contrast the role of PTH (hormone) and mechanical forces acting on the skeleton in bone remodeling.

17. What type of tissue are epiphyseal plates composed of, and what is their role in growing children?

18. Identify the four phases of fracture repair.

19. Name the skull bones that harbor paranasal sinuses.

20. What facial bone forms the chin? The cheekbone? The upper jaw? The bony eyebrow ridges?

21. Where is the jugular foramen located, and which structure passes through it?

22. What is a human tailbone?

23. Diagram the normal spinal curvatures and then the curvatures seen in scoliosis and lordosis.

206 Essentials of Human Anatomy and Physiology

24. What is the function of the intervertebral discs? What is a herniated (slipped) disc?

25. What is the function of intercostal muscles?

26. What are bursae? Where are they commonly found?

27. Name the bones that constitute the tarsus.

28. Name all the bones with which the tibia articulates.

29. What bones make up each hip bone (coxal bone)? Which of these is the largest? Which has tuberosities that we sit on? Which is the most anterior?

30. Name the long bones of the upper limb from superior to inferior.

31. Compare the amount of movement possible in syn- arthrotic, amphiarthrotic, and diarthrotic joints. Relate these terms to the structural classification of joints—that is, to fibrous, cartilaginous, and syno- vial joints.

32. Describe the structure of a synovial joint. Use this description to briefly explain what occurs in a joint with osteoarthritis.

33. Professor Rogers pointed to the foramen magnum of the skull and said, “The food passes through this hole when you swallow.” Some students believed him, but others said that this was a big mistake. What do you think? Support your answer.

34. Yolanda is asked to review a bone slide that has been set up under a microscope. She sees concentric layers surrounding a central cavity or canal. Is this bone section taken from the diaphysis or the epiphyseal plate of the bone specimen?

35. List two factors that keep bones healthy. List two factors that can cause bones to become soft or to atrophy.

36. Contrast the location and characteristics of foramen magnum and obturator foramen.

37. A 75-year-old woman and her 9-year-old grand- daughter were in a car accident in which both sus- tained trauma to the chest while seated next to each other. X-ray images showed that the grand- mother had several fractured ribs, but her grand- daughter had none. Explain these different findings.

38. The pediatrician explains to parents of a newborn that their son suffers from cleft palate. She tells them that the normal palate fuses in an anterior-to- posterior pattern. The child’s palatine processes of the maxilla have not fused. Have his palatine bones fused normally?

39. After having a severe cold accompanied by nasal congestion, Nicole complained that she had a frontal headache and that the right side of her face ached. What bony structures probably became infected by the bacteria or viruses causing the cold?

40. Deborah, a 75-year-old woman, stumbled slightly while walking, then felt a terrible pain in her left hip. At the hospital, X-ray images revealed that the hip was broken. Also, the compact bone and spongy bone throughout her spine were very thin. What was her probable condition?

41. At work, a box fell from a shelf onto Ella’s acromial region. In the emergency room, the physician felt that the head of her humerus had moved into the axilla. What had happened to Ella?

42. An X-ray image of the arm of an accident victim reveals a faint line curving around and down the shaft. What kind of fracture might this indicate?

43. Bone X-ray studies are sometimes used to determine whether a person has reached his or her final height. What are the clinicians checking out?

44. A patient complains of pain starting in the jaw and radiating down the neck. When he is questioned further, he states that when he is under stress he grinds his teeth. What joint is causing his pain?

45. 73-year-old Alice has developed an abnormal spinal curvature that has progressed into a hump. What could have led to this condition?

46. Mary, a 70-year-old woman, has been experiencing severe pain, swelling, and stiffness in her hip joint. Tests reveal Mary has osteoarthritis in her hip joint. Which treatment can help her?

critical Thinking and clinical Application Questions

207

Overview of Muscle Tissues ➔ Learning Objectives

□□ Describe similarities and differences in the structure and function of the three types of muscle tissue, and indicate where they are found in the body.

□□ Define muscular system.

□□ Define and explain the role of the following: endomysium, perimysium, epimysium, tendon, and aponeurosis.

Muscle Types There are three types of muscle tissue—skeletal, smooth, and cardiac (Table 6.1, p. 208). These differ

Because flexing muscles look like mice scur-rying beneath the skin, a scientist long ago dubbed them muscles, from the Latin word mus, meaning “little mouse.” Indeed, the rippling muscles of professional athletes often come to mind when we hear the word muscle. But muscle is also the dominant tissue in the heart and in the walls of other hollow organs of the body such as the intestines and blood vessels, and it makes up nearly half the body’s mass.

The essential function of muscle is to contract, or shorten—a unique characteristic that sets it apart from other body tissues. As a result of this ability, muscles are responsible for all body movements and can be viewed as the “machines” of the body.

The Muscular System6 WHAT

HOW

WHY

Muscles are responsible for body

movements, stabilizing joints, and generating heat. Muscles generate the

force required to cause movement by contracting, a

process in which proteins inside the muscle fibers overlap

more than when they are at rest.

In addition to whole body movements, muscles are needed to

move substances inside our bodies: for example, air into and out of our lungs, food through our digestive tracts, and

blood through our heart and blood vessels.

InSTrucTOrS

New Building Vocabulary coaching Activities for this chapter are assignable in

208 Essentials of Human Anatomy and Physiology

Table 6.1 comparison of Skeletal, cardiac, and Smooth Muscles

characteristic Skeletal cardiac Smooth

Body location Attached to bones or, for some facial muscles, to skin

Walls of the heart Mostly in walls of hollow visceral organs (other than the heart)

Cell shape and appearance

Single, very long, cylindrical, multinucleate cells with very obvious striations

Branching chains of cells; uninucleate, striations; intercalated discs

Single, fusiform, uninucleate; no striations

Connective tissue components

Epimysium, perimysium, and endomysium

Endomysium Epimysium

CellsPerimysium

Endomysium attached to the fibrous skeleton of the heart

Endomysium

Regulation of contraction

Voluntary; via nervous system controls

Involuntary; the heart has a pacemaker; also nervous system controls; hormones

Involuntary; nervous system controls; hormones, chemicals, stretch

Speed of contraction

Slow to fast Slow Very slow

Rhythmic contraction

No Yes Yes, in some

Chapter 6: The Muscular System 209

(en″do-mis′e-um). Several sheathed muscle fibers are then wrapped by a coarser fibrous membrane called perimysium to form a bundle of fibers called a fascicle (fas′ ı̆-kul). Many fascicles are bound together by an even tougher “overcoat” of connective tissue called an epimysium, which covers the entire muscle. The ends of the epimy- sium that extend beyond the muscle (like the wrapper on a piece of candy) blend either into a strong, cordlike tendon or a sheetlike aponeuro- sis (ap″o-nu-ro′sis), which indirectly attaches the muscle to bone, cartilage, or another connective tissue covering.

In addition to anchoring muscles, tendons per- form several other functions. The most important are providing durability and conserving space. Tendons are mostly tough collagen fibers, so they

in their cell structure, body location, and how they are stimulated to contract. But before we explore their differences, let’s look at how they are similar.

First, skeletal and smooth muscle cells are elongated. For this reason, these types of muscle cells (but not cardiac muscle cells) are called mus- cle fibers. Second, the ability of muscle to shorten, or contract, depends on two types of myofila- ments, the muscle cell equivalents of the microfila- ments of the cytoskeleton (studied in Chapter 3). A third similarity has to do with terminology. Whenever you see the prefixes myo- or mys- (“muscle”) or sarco- (“flesh”), you will know that muscle is being referred to. For example, in mus- cle cells, the cytoplasm is called sarcoplasm (sar′ko-plaz″um).

Skeletal Muscle

Skeletal muscle fibers are packaged into organs called skeletal muscles that attach to the skeleton. As the skeletal muscles cover our bone and carti- lage framework, they help form the smooth con- tours of the body. Skeletal muscle fibers are large, cigar-shaped, multinucleate cells. They are the largest muscle fibers—some ranging up to 30 cm (nearly 1 foot) in length. Indeed, the fibers of large, hardworking muscles, such as the antigrav- ity muscles of the hip, are so big and coarse that they can be seen with the naked eye.

Skeletal muscle is also known as striated muscle (because its fibers have obvious stripes) and as voluntary muscle (because it is the only muscle type subject to conscious control). However, it is important to recognize that skeletal muscles can be activated by reflexes (without our “willed command”) as well. Skeletal muscle tissue can contract rapidly and with great force, but it tires easily and must rest after short periods of activity. When you think of skeletal muscle tissue, the key words to remember are skeletal, striated, and voluntary.

Skeletal muscle fibers are soft and surprisingly fragile. Yet skeletal muscles can exert tremendous power—indeed, the force they generate while lifting a weight is often much greater than that required to lift the weight. The reason they are not ripped apart as they exert force is that connective tissue bundles thousands of their fibers together, which strengthens and supports the muscle as a whole (Figure 6.1).

Each muscle fiber is enclosed in a delicate connective tissue sheath called endomysium

Epi = upon, over, above; and mys = muscle. The epimysium is a sheath upon or over a muscle.A:

Figure 6.1 connective tissue wrappings of skeletal muscle.

Blood vessel

Muscle fiber (cell)

Fascicle (wrapped by perimysium)

Endomysium (between fibers)

Perimysium

Epimysium (wraps entire muscle)

Bone

Tendon

What is the meaning of epi? Of mys? How do these word roots relate to the role and position of the epimysium?Q:

210 Essentials of Human Anatomy and Physiology

can cross rough bony projections, which would tear the more delicate muscle tissues. Because of their relatively small size, more tendons than fleshy muscles can pass over a joint.

Many people think of muscles as always hav- ing an enlarged “belly” that tapers down to a ten- don at each end. However, muscles vary considerably in the way their fibers are arranged. Many are spindle-shaped as just described, but in others, the fibers are arranged in a fan shape or a circle (as described on pp. 228–229).

Smooth Muscle

Smooth muscle has no striations and is invol- untary, which means that we cannot consciously control it. Found mainly in the walls of hollow (tubelike) visceral organs such as the stomach, urinary bladder, and respiratory passages, smooth muscle propels substances along a pathway. Think of smooth muscle as visceral, nonstriated, and involuntary.

Smooth muscle fibers are spindle-shaped, uni- nucleate, and surrounded by scant endomysium (see Table 6.1). They are arranged in layers, and most often there are two such layers, one run- ning circularly and the other longitudinally (Figure 6.2a). As the two layers alternately contract and relax, they change the size and shape of the organ. Moving food through the digestive tract and emptying the bowels and bladder are examples of “housekeeping” activities normally handled by smooth muscles. Smooth muscle contraction is slow and sustained. To use a running analogy, if skeletal muscle is like a sprinter, who runs fast but tires quickly, then smooth muscle is like a mara- thoner, who runs more slowly but keeps up the pace for many miles.

cardiac Muscle

Cardiac muscle is found in only one place in the body—the heart, where it forms the bulk of the heart walls. The heart serves as a pump, propelling blood through blood vessels to all body tissues. Like skeletal muscle, cardiac muscle is striated, and like smooth muscle, it is uninucleate and its control is involuntary. Important key words for this muscle type are cardiac, striated, and involuntary.

The cardiac cells are cushioned by small amounts of endomysium and are arranged in spi- ral or figure 8–shaped bundles (Figure 6.2b). When the heart contracts, its internal chambers

Figure 6.2 Arrangement of smooth and cardiac muscle cells. (a) Diagrammatic view of a cross section of the intestine. (b) Longitudinal view of the heart showing the spiral arrangement of the cardiac muscle cells in its walls.

Mucosa

Submucosa

Cardiac muscle bundles

Circular layer of smooth muscle (longitudinal view of cells)

Longitudinal layer of smooth muscle (cross-sectional view of cells)

(a)

(b)

Chapter 6: The Muscular System 211

Stabilize Joints

As skeletal muscles pull on bones to cause move- ments, they also stabilize the joints of the skeleton. Muscles and tendons are extremely important in reinforcing and stabilizing joints that have poorly articulating surfaces, such as the shoulder and knee joints. In fact, physical therapy for knee inju- ries includes exercise to strengthen thigh muscles because they support the knee.

Generate Heat

Muscle activity generates body heat as a by-prod- uct. As ATP is used to power muscle contraction, nearly three-quarters of its energy escapes as heat. This heat is vital in maintaining normal body tem- perature. Skeletal muscle accounts for at least 40 percent of body mass, so it is the muscle type most responsible for generating heat.

Additional Functions

Muscles perform other important functions as well. Smooth muscles form valves that regulate the pas- sage of substances through internal body open- ings, dilate and constrict the pupils of our eyes, and make up the arrector pili muscles that cause our hairs to stand on end. Skeletal muscles form valves that are under voluntary control, and they enclose and protect fragile internal organs.

Did You Get It? 1. How do cells of the three types of muscle tissues differ

from one another anatomically? 2. Which muscle type has the most elaborate connective

tissue wrappings? 3. What does striated mean relative to muscle cells? 4. How do the movements promoted by skeletal muscle

differ from those promoted by smooth or cardiac muscle?

For answers, see Appendix A.

Microscopic Anatomy of Skeletal Muscle ➔ Learning Objective

□ Describe the microscopic structure of skeletal muscle, and explain the role of actin- and myosin- containing myofilaments.

As mentioned previously, skeletal muscle fibers (cells) are multinucleate (Figure 6.3a, p. 212). Many oval nuclei can be seen just beneath the plasma membrane, which is called the sarcolemma (sar″ko-lem′ah;

become smaller, forcing blood into the large arter- ies leaving the heart. Cardiac muscle fibers are branching cells joined by special gap junctions called intercalated discs (see Figure 3.20 on p. 125 and Chapter 3, p. 124). These two structural fea- tures and the spiral arrangement of the muscle bundles in the heart allow heart activity to be closely coordinated.

Cardiac muscle usually contracts at a fairly steady rate set by the heart’s “in-house” pace- maker. However, the nervous system can also stimulate the heart to shift into “high gear” for short periods, as when you run to catch a bus.

As you can see, each of the three muscle types has a structure and function well suited for its job in the body. But because the term muscular sys- tem applies specifically to skeletal muscle, we will concentrate on this muscle type in this chapter.

Muscle Functions All muscle types produce movement, but skeletal muscle plays three other important roles in the body as well: it maintains posture and body position, sta- bilizes joints, and generates heat. Let’s take a look.

Produce Movement

Skeletal muscles are responsible for our body’s mobility, including all locomotion (walking, swim- ming, and cross-country skiing, for instance) and manipulating things with your agile upper limbs. They enable us to respond quickly to changes in the external environment. For example, their speed and power enable us to jump out of the way of a runaway car and then follow its flight with our eyes. They also allow us to express our emotions with the silent language of smiles and frowns.

They are distinct from the smooth muscle of blood vessel walls and cardiac muscle of the heart, which work together to circulate blood and main- tain blood pressure, and the smooth muscle of other hollow organs, which forces fluids (urine, bile) and other substances (food, a baby) through internal body channels.

Maintain Posture and Body Position

We are rarely aware of the workings of the skeletal muscles that maintain body posture. Yet they func- tion almost continuously, making one tiny adjust- ment after another so that we maintain an erect or seated posture, even when we slouch, despite the never-ending downward pull of gravity.

212 Essentials of Human Anatomy and Physiology

a midline interruption, a darker area called the Z disc, and the dark A band has a lighter central area called the H zone (Figure 6.3b). The M line in the center of the H zone contains tiny protein rods that hold adjacent thick filaments together.

So why are we bothering with all these terms— dark this and light that? Because the banding pattern reveals the working structure of the myofi- brils. First, we find that the myofibrils are actually chains of tiny contractile units called sarcomeres

“muscle husk”) in muscle fibers. The nuclei are pushed aside by long ribbonlike organelles, the myofibrils (mi″o-fi′brilz), which nearly fill the cytoplasm. Alternating light (I) bands and dark (A) bands along the length of the perfectly aligned myofibrils give the muscle fiber its striated (banded) appearance. (Think of the second letter of light, I, and the second letter of dark, A, to help you remember which band is which.) A closer look at the banding pattern reveals that the light I band has

Figure 6.3 Anatomy of a skeletal muscle fiber (cell). (a) A portion of a muscle fiber. One myofibril has been extended. (b) Enlarged view of a section of a myofibril showing its banding pattern. (c) Enlarged view of one sarcomere (contractile unit) of a myofibril.

NucleusLight (I) band

Dark (A) band

Sarcolemma

I band

Z disc Z disc

I bandA band

H zone

(b) Myofibril or fibril (complex organelle composed of bundles of myofilaments)

Myofibril

(a) Segment of a muscle fiber (cell)

Thin (actin) myofilament

Thick (myosin) myofilament

Thin (actin) myofilament

Z disc Z disc

M line

Sarcomere

Thick (myosin) myofilament

Chapter 6: The Muscular System 213

Did You Get It? 5. Specifically, what structure(s) is/are responsible for the

banding pattern in skeletal muscle cells?

For the answer, see Appendix A.

Skeletal Muscle Activity Stimulation and contraction of Single Skeletal Muscle Fibers ➔ Learning Objective

□ Describe how an action potential is initiated in a muscle cell.

Muscle fibers have several special functional proper- ties that enable them to perform their duties. The first of these is irritability, also termed responsive- ness, which is the ability to receive and respond to a stimulus. The second, contractility, is the abil- ity to forcibly shorten when adequately stimulated. This property sets muscle apart from all other tissue types. Extensibility is the ability of muscle fibers to stretch, whereas elasticity is their ability to recoil and resume their resting length after being stretched.

The nerve Stimulus and the Action Potential

To contract, skeletal muscle fibers must be stimu- lated by nerve impulses. One motor neuron (nerve cell) may stimulate a few muscle fibers or hundreds of them, depending on the particular muscle and the work it does. A motor unit consists of one neuron and all the skeletal muscle fibers it stimu- lates (Figure 6.4, p. 214). When a long, threadlike extension of the neuron, called the axon, reaches the muscle, it branches into a number of axon terminals, each of which forms junctions with the sarcolemma of a different muscle cell (Figure 6.5, p. 215). These junctions, called neuromuscular (literally, “nerve-muscle”) junctions, contain syn- aptic vesicles filled with a chemical referred to as a neurotransmitter. The specific neurotransmitter that stimulates skeletal muscle fibers is acetylcho- line (as″e-til-ko′lēn), or ACh. Although the nerve endings and the muscle fiber membranes are very close, they never touch. The gap between them, the synaptic cleft, is filled with interstitial fluid.

Homeostatic Imbalance 6.1

In some cases, a motor nerve impulse is unable to reach the muscle. In ALS, or amyotrophic lateral

(sar′ko-mērz), which are the structural and func- tional units of skeletal muscle. The sarcomeres are aligned end to end like boxcars in a train along the length of the myofibrils. Second, it is the pre- cise arrangement of even smaller structures (myo- filaments) within sarcomeres that produces the striations in skeletal muscle fibers.

Let’s examine how the arrangement of the myofilaments leads to the banding pattern. There are two types of threadlike protein myofilaments within each sarcomere (Figure 6.3c). The thick filaments are made mostly of bundled molecules of the protein myosin, but they also contain ATPase enzymes, which split ATP to release the energy used for muscle contraction. Notice that the thick filaments extend the entire length of the dark A band. Also, notice that the midparts of the thick filaments are smooth, but their ends are studded with small projections (Figure 6.3c). These projections, or myosin heads, form cross bridges when they link the thick and thin fila- ments together during contraction. Myosin fila- ments are attached to the Z discs by titin, elastic filaments that run through the core of the thick filament.

The thin filaments are composed of the con- tractile protein called actin, plus some regulatory proteins that play a role in allowing (or prevent- ing) binding of myosin heads to actin. The thin filaments are anchored to the Z disc (a disclike membrane). Notice that the light I band includes parts of two adjacent sarcomeres and contains only the thin filaments. Although they overlap the ends of the thick filaments, the thin filaments do not extend into the middle of a relaxed sarcomere, and thus the central region (the H zone) looks a bit lighter. When the actin-containing thin fila- ments slide toward each other during contraction, the H zones disappear because the actin and myo- sin filaments completely overlap.

Another very important muscle fiber organ- elle—the sarcoplasmic reticulum (SR)—is a specialized smooth endoplasmic reticulum (not shown in Figure 6.3). The interconnecting tubules and sacs of the SR surround every myofibril just as the sleeve of a loosely crocheted sweater sur- rounds your arm. The major role of this elaborate system is to store calcium and to release it on demand when the muscle fiber is stimulated to contract. As you will see, calcium provides the final “go” signal for contraction.

214 Essentials of Human Anatomy and Physiology

Figure 6.4 Motor units. Each motor unit consists of a motor neuron and all the muscle fibers it activates. (a) Portions of two motor

units are shown. The motor neurons reside in the spinal cord, and their axons extend to the muscle. Within the muscle, each axon divides into a

number of axon terminals distributed to muscle fibers scattered throughout the muscle. (b) Photo of a portion of a motor unit (1150×).

Spinal cord Axon terminals at neuromuscular junctions

Motor neuron cell bodies

Muscle Branching axon to motor unit

Muscle fibers

Nerve

Motor unit 1

Motor unit 2

Muscle fibers

(a)

Axon of motor neuron

(b)

sclerosis (also called Lou Gehrig’s disease), motor neurons degenerate over time, resulting in paralysis that gradually worsens. The cause of ALS is unknown, but common characteristics include mal- functioning mitochondria, inflammation, and the generation of free radicals that damage DNA and tissue much like intense UV light. The prognosis for patients with ALS is generally death within three to five years because the breathing muscles will even- tually be affected, resulting in suffocation. _____ ✚

Did You Get It? 6. What two structures are closely associated at a

neuromuscular junction?

For the answer, see Appendix A.

Now that we have described the structure of the neuromuscular junction, refer to the numbered steps in Figure 6.5 as we examine what happens there. When a nerve impulse reaches the axon ter- minals 1 , calcium channels open, and calcium

(Ca2+) enters the terminal

. Calcium entry causes some of the synaptic vesicles in the axon terminal to fuse with the cell membrane and release acetylcholine

, which then diffuses across the synaptic cleft and attaches to mem- brane receptors in highly folded regions of the sarcolemma

. If enough acetylcholine is released, the sarcolemma at that point becomes temporarily even more permeable to sodium ions (Na+), which rush into the muscle fiber, and to potassium ions (K+), which diffuse out of the muscle fiber. However, more Na+ enters than K+ leaves. This imbalance gives the cell interior an excess of positive ions, which reverses the resting electrical conditions of the sarcolemma. This event, called depolarization, opens more channels that only allow Na+ entry

. This movement of ions generates an electrical current called an action potential. Once begun, the action poten- tial is unstoppable; it travels over the entire surface of the sarcolemma, conducting the electrical

Chapter 6: The Muscular System 215

Figure 6.5 Events at the neuromuscular junction.

Nucleus

Nerve impulse

Myelinated axon of motor neuron

Axon terminal of neuromuscular junction

Sarcolemma of the muscle fiber

Ion channel in sarcolemma opens; ions pass.

Na+

Ca2+ Ca2+

K+

ACh Na+

K+

Degraded ACh

Acetylcholinesterase

Ion channel closes; ions cannot pass.

Axon terminal of motor neuron

Synaptic vesicle containing ACh

Mitochondrion

Synaptic cleft

Folds of sarcolemma

Sarcolemma

Fusing synaptic vesicle

ACh receptor

ACh Sarcoplasm of muscle fiber

Nerve impulse reaches axon terminal of motor neuron.

Calcium (Ca2+) channels open, and Ca2+ enters the axon terminal.

Ca2+ entry causes some synaptic vesicles to release their contents (the neurotransmitter acetylcholine) by exocytosis.

Acetylcholine diffuses across the synaptic cleft and binds to receptors in the sarcolemma.

ACh binds and opens channels that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions leave, producing a local change in the electrical conditions of the membrane (depolarization). This eventually leads to an action potential.

The enzyme acetylcholinesterase breaks down ACh in the synaptic cleft, ending the process.

216 Essentials of Human Anatomy and Physiology

the sodium and potassium ions back to their initial positions.

Did You Get It? 7. Which ions enter the muscle cell during the

generation of an action potential? 8. What is the role of calcium ions in muscle contraction?

For answers, see Appendix A.

Mechanism of Muscle contraction: The Sliding Filament Theory

➔ Learning Objective □ Describe the events of muscle cell contraction.

What causes the filaments to slide? This question brings us back to the myosin heads that protrude all around the ends of the thick filaments.

When the nervous system activates muscle fibers as just described, the myosin heads attach to binding sites on the thin filaments, and the sliding begins. Each cross bridge attaches and detaches several times during a contraction, generating ten- sion that helps pull the thin filaments toward the center of the sarcomere. This “walking” of the myosin cross bridges, or heads, along the thin fila- ments during muscle shortening is much like a centipede’s gait. Some myosin heads (“legs”) are always in contact with actin (“the ground”), so that the thin filaments cannot slide backward, and this

impulse from one end of the cell to the other. The result is contraction of the muscle fiber.

Note that while the action potential is occur- ring, the enzyme acetylcholinesterase (AChE), present on the sarcolemma and in the synaptic cleft, breaks down acetylcholine to acetic acid and choline

6 . For this reason, a single nerve impulse produces only one contraction. This prevents con- tinued contraction of the muscle fiber in the absence of additional nerve impulses. The muscle fiber relaxes until stimulated by the next round of acetylcholine release.

Let’s compare this series of events to lighting a match under a small dry twig (Figure 6.6). The charring of the twig by the flame can be compared to the change in membrane permeability that allows sodium ions into the cell. When that part of the twig becomes hot enough (when enough sodium ions have entered the cell), the twig will suddenly burst into flame, and the flame will move along the twig (the action potential will be con- ducted along the entire length of the sarcolemma). We explain this series of events more fully in the discussion of nerve physiology (Chapter 7, pp. 260–265).

The events that return the cell to its resting state include (1) diffusion of potassium ions (K+) out of the cell and (2) operation of the sodium-potassium pump, the active transport mechanism that moves

Figure 6.6 comparing the action potential to a flame consuming a dry twig. (a) The first event in igniting a dry twig is holding the match flame under one area of the twig. The second event is the twig’s bursting into flame

when it has been heated enough and spreading of the flame to burn the entire twig. (b) The first event in exciting a muscle fiber is the rapid diffusion of sodium ions (Na+) into the cell when the permeability of the sarcolemma changes. The

second event is the spreading of the action potential along the sarcolemma when enough sodium ions have entered to upset the electrical conditions in the cell.

(b)

Small twig

Match flame

Muscle fiber (cell)

(a)

StriationsNervefiber

Neuromuscular junction

2 Flame spreads rapidly along the twig.

2 Action potential spreads rapidly along the sarcolemma.

1 Flame ignites the twig.

1 Na+ diffuses into the cell.

Chapter 6: The Muscular System 217

muscle fibers being stimulated at one time. Next, let’s describe a muscle’s response to each of these.

Muscle Response to Increasingly Rapid Stimulation Although muscle twitches (single, brief, jerky contractions) sometimes result from certain ner- vous system problems, this is not the way our muscles normally operate. In most types of muscle activity, nerve impulses are delivered to the mus- cle at a very rapid rate—so rapid that the muscle does not get a chance to relax completely between stimuli. As a result, the effects of the successive contractions are “summed” (added) together, and the contractions of the muscle get stronger and smoother. The muscle exhibits unfused tetanus (tet′ah-nus), or incomplete tetanus. When the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are com- pletely smooth and sustained, the muscle is in

cycle repeats again and again during contraction. As this event occurs simultaneously in sarcomeres throughout the muscle fiber, the cell shortens (Figure 6.7). Notice that the myofilaments them- selves do not shorten during contraction; they sim- ply slide past each other.

The formation of cross bridges—when the myosin heads attach to actin—requires calcium ions (Ca2+) and ATP (to “energize” the myosin heads). So where does the calcium come from? Action potentials pass deep into the muscle fiber along membranous tubules that fold inward from the sarcolemma. Inside the cell, the action poten- tials stimulate the sarcoplasmic reticulum to release calcium ions into the cytoplasm. The calcium ions trigger the binding of myosin to actin, initiating fila- ment sliding (Figure 6.8, p. 218). When the action potential ends, calcium ions are immediately returned to the SR storage areas, the regulatory pro- teins return to their resting shape and block myosin- binding sites, and the muscle fiber relaxes and settles back to its original length. This whole series of events takes a few thousandths of a second.

Did You Get It? 9. Which chemical—ATP or Ca2+—triggers sliding of the

muscle filaments? 10. Which is a cross-bridge attachment more similar to: a

synchronized rowing team or a person pulling a bucket on a rope out of a well?

For answers, see Appendix A.

contraction of a Skeletal Muscle as a Whole ➔ Learning Objective

□ Define graded response, tetanus, isotonic and isometric contractions, and muscle tone as these terms apply to a skeletal muscle.

Graded responses

In skeletal muscles, the “all-or-none” law of mus- cle physiology applies to the muscle fiber, not to the whole muscle. It states that a muscle fiber will contract to its fullest extent when it is stimulated adequately; it never partially contracts. However, the whole muscle reacts to stimuli with graded responses, or different degrees of shortening, which generate different amounts of force. In gen- eral, graded muscle contractions can be produced two ways: (1) by changing the frequency of mus- cle stimulation and (2) by changing the number of

Figure 6.7 Diagrammatic views of a sarcomere. Notice that in the contracted sarcomere (b), the light H zone in the center of the A band has disappeared, the Z discs are closer to the thick filaments, and the I bands have nearly disappeared. The A bands of adjacent sarcomeres move closer together but do not change in length.

(a) Relaxed sarcomere

(b) Fully contracted sarcomere

Myosin Actin

I IA

Z Z

II A

Z ZH

218 Essentials of Human Anatomy and Physiology

In a relaxed muscle fiber, the regulatory proteins forming part of the actin myofilaments prevent myosin binding (see a). When an action potential (AP) sweeps along its sarcolemma and a muscle fiber is excited, calcium ions (Ca2+) are released from intracellular storage areas (the sacs of the sarcoplasmic reticulum).

The flood of calcium acts as the final trigger for contraction, because as calcium binds to the regulatory proteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thin filaments. This action exposes myosin-binding sites on the actin, to which the myosin heads can attach (see b), and the myosin heads immediately begin seeking out binding sites.

The free myosin heads are “cocked,” much like an oar ready to be pulled on for rowing. Myosin attachment to actin causes the myosin heads to snap (pivot) toward the center of the sarcomere in a rowing motion. When this happens, the thin filaments are slightly pulled toward the center of the sarcomere (see c). ATP provides the energy needed to release and recock each myosin head so that it is ready to attach to a binding site farther along the thin filament.

(a)

(b)

(c)

Actin myofilamentMyosin myofilament

Regulatory proteins

Myosin-binding site Ca2+

Upper part of thick filament only

Figure 6.8 Schematic representation of contraction mechanism: the sliding filament theory.

fused tetanus, or complete tetanus, or in tetanic contraction* (Figure 6.9).

Muscle Response to Stronger Stimuli Tetanus produces stronger (more forceful) muscle con- tractions, but its primary role is to produce

smooth and prolonged muscle contractions. How forcefully a muscle contracts depends to a large extent on how many of its cells are stimulated. When only a few cells are stimulated, the muscle as a whole contracts only slightly. When all the motor units are active and all the muscle fibers are stimulated, the muscle contraction is as strong as it can get. Thus, muscle contractions can range from slight to vigorous depending on the work to be done. The same hand that lifts a single sheet of paper can also lift a heavy backpack full of books!

* Tetanic contraction is normal and desirable. It is quite dif- ferent from the pathological condition of tetanus (commonly called lockjaw), which is caused by a toxin made by bacteria. Lockjaw causes muscles to go into uncontrollable spasms, finally causing respiratory arrest.

Play A&P Flix Animation

Chapter 6: The Muscular System 219

6 Figure 6.9 A whole muscle’s response to different stimulation rates. A single stimulus (a) causes muscle contraction and relaxation (a twitch). As stimuli are delivered more frequently (b), the muscle does not have time to completely relax before

the next stimulus; contraction force increases because effects of the individual twitches are summed (added). Further fusion (c) of the twitches (unfused tetanus) occurs as stimuli are delivered at a still faster rate. Fused tetanus (d), a smooth continuous contraction without any

evidence of relaxation, results from a very rapid rate of stimulation. (Points at which stimuli are delivered are indicated by red arrows. Tension [measured in grams] on the vertical axis refers to the relative force of muscle contraction.)

(a) Twitch

(Stimuli)

T e

n si

o n

( g

)

(b) Summing of contractions

(d) Fused (complete) tetanus

Providing Energy for Muscle contraction

➔ Learning Objective □ Describe three pathways for ATP regeneration

during muscle activity.

As a muscle contracts, the bonds of ATP molecules are hydrolyzed to release the needed energy.

➔ConCeptLinkRecall that ATP can be compared to a tightly coiled spring that is ready to uncoil with tremendous energy when the “catch” is released (Chapter 2, p. 81). Remember that all bonds store energy and that the “catch” in this example is one of the characteristic high-energy phosphate bonds in ATP. ➔

Surprisingly, muscles store very limited sup- plies of ATP—only a few seconds’ worth, just enough to get you going. Because ATP is the only energy source that can be used directly to power muscle activity, ATP must be regenerated continu- ously if contraction is to continue.

Working muscles use three pathways to regen- erate ATP:

• Direct phosphorylation of ADP by cre- atine phosphate (Figure 6.10a, p. 220). The unique high-energy molecule creatine phos- phate (CP) is found in muscle fibers but not other cell types. As ATP is depleted, interac- tions between CP and ADP result in transfers of a high-energy phosphate group from CP

to ADP, thus regenerating more ATP in a frac- tion of a second. Although muscle fibers store perhaps five times as much CP as ATP, the CP supplies are also soon exhausted (in less than 15 seconds).

• Aerobic pathway (Figure 6.10b). At rest and during light to moderate exercise, some 95 per- cent of the ATP used for muscle activity comes from aerobic respiration. Aerobic respiration occurs in the mitochondria and involves a series of metabolic pathways that use oxygen. These pathways are collectively referred to as oxida- tive phosphorylation. During aerobic respiration, glucose is broken down completely to carbon dioxide and water, and some of the energy re- leased as the bonds are broken is captured in the bonds of ATP molecules. Although aerobic respiration provides a rich ATP harvest (about 32 ATP per 1 glucose), it is fairly slow and re- quires continuous delivery of oxygen and nutri- ent fuels to the muscle to keep it going.

• Anaerobic glycolysis and lactic acid forma- tion (Figure 6.10c). The initial steps of glu- cose breakdown occur via a pathway called glycolysis, which does not use oxygen and hence is anaerobic (literally “without oxygen”). During glycolysis, which occurs in the cyto- sol, glucose is broken down to pyruvic acid, and small amounts of energy are captured in ATP bonds (2 ATP per 1 glucose molecule). As long as enough oxygen is present, the pyruvic

220 Essentials of Human Anatomy and Physiology

Did You Get It? 11. What are the three processes used to generate energy

for skeletal muscle contraction? 12. What is the direct source of energy used by muscle

fibers for contraction?

For answers, see Appendix A.

Muscle Fatigue and Oxygen Deficit

➔ Learning Objective □ Define oxygen deficit and muscle fatigue, and list

possible causes of muscle fatigue.

If we exercise our muscles strenuously for a long time, muscle fatigue occurs. A muscle is fatigued when it is unable to contract even though it is still being stimulated. Without rest, a working muscle begins to tire and contracts more weakly until it finally ceases reacting and stops contracting.

Factors that contribute to muscle fatigue are not fully known. Suspected causes are imbalances in ions (Ca2+, K+) and problems at the neuromuscular

acid then enters the oxygen-requiring aerobic pathways that occur within the mitochondria to produce more ATP as described above. However, when muscle activity is intense, or oxygen and glucose delivery is temporar- ily inadequate to meet the needs of working muscles, the sluggish aerobic pathways can- not keep up with the demands for ATP. Under these conditions, the pyruvic acid generated during glycolysis is converted to lactic acid, and the overall process is referred to as an- aerobic glycolysis.

Anaerobic glycolysis produces only about 5 percent as much ATP from each glucose mole- cule as aerobic respiration. However, it is some 2½ times faster, and it can provide most of the ATP needed for 30 to 40 seconds of strenuous muscle activity. Anaerobic glycolysis has two main shortcomings: it uses huge amounts of glucose for a small ATP harvest, and the accu- mulating lactic acid promotes muscle soreness.

Figure 6.10 Methods of regenerating ATP during muscle activity. The fastest mechanism is (a) direct phosphorylation; the slowest is (b) aerobic respiration.

H2O

(a) Direct phosphorylation

Coupled reaction of creatine phosphate (CP) and ADP

Energy source: CP Energy source: glucoseEnergy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism

Glycolysis and lactic acid formation

(b) Aerobic pathway

Aerobic cellular respiration

Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provision: 15 seconds

Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provision: 40 seconds, or slightly more

Oxygen use: Required Products: 32 ATP per glucose, CO2, H2O Duration of energy provision: Hours

ADP

Creatine

Creatine Glucose (from glycogen breakdown or delivered from blood)

Glucose (from glycogen breakdown or delivered from blood)

Glycolysis in cytosol

Pyruvic acid

Released to blood

net gain

32 Lactic acid

CO2

Pyruvic acid Fatty acids

Amino acids

Aerobic respiration in mitochondria

ATP

net gain per glucose

Chapter 6: The Muscular System 221

movement occurs. Bending the knee, lifting weights, and smiling are all examples of isotonic contractions.

Contractions in which the muscles do not shorten are called isometric contractions (liter- ally, “same measurement” or length). In isometric contractions, the myosin filaments are “spinning their wheels,” and the tension in the muscle keeps increasing. They are trying to slide, but the muscle is pitted against some more or less immovable object. For example, when you push the palms of your hands together in front of you, your arms and chest muscles are contracting isometrically.

Muscle Tone

One aspect of skeletal muscle activity cannot be consciously controlled. Even when a muscle is voluntarily relaxed, some of its fibers are con- tracting—first one group and then another. These contractions are not visible, but thanks to them, the muscle remains firm, healthy, and constantly ready for action. This state of continuous partial contractions is called muscle tone. Muscle tone is the result of different motor units, which are scattered through the muscle, being stimulated by the nervous system in a systematic way. Think of these motor units as being “on duty” in case action is required.

Homeostatic Imbalance 6.2

If the nerve supply to a muscle is destroyed (as in an accident), the muscle is no longer stimulated in this manner, and it loses tone. Soon after, it becomes flaccid (flă′sid), or soft and flabby, and begins to atrophy (waste away). This is called flaccid paralysis. Compare this with a condition that increases muscle tone until the muscle is no longer controllable—for example, the disease teta- nus, which is caused by a bacterial toxin. This is called spastic paralysis. _____________________ ✚

Effect of Exercise on Muscles

➔ Learning Objective □ Describe the effects of aerobic and resistance

exercise on skeletal muscles and other body organs.

The amount of work a muscle does changes the muscle. Muscle inactivity (due to a loss of nerve sup- ply, immobilization, or whatever the cause) always leads to muscle weakness and wasting. Muscles

junction. However, many agree that the major factor is the oxygen deficit that occurs during prolonged muscle activity. Oxygen deficit is not a total lack of oxygen; rather, it happens when a person is not able to take in oxygen fast enough to keep the muscles supplied with all the oxygen they need when they are working vigorously. Obviously, then, the work that a muscle can do and how long it can work without becoming fatigued depend on how good its blood supply is. When muscles lack suffi- cient oxygen for aerobic respiration, lactic acid begins to accumulate in the muscle via the anaero- bic pathway. We can recognize this event by the burning sensation we experience. In addition, the muscle’s ATP supply starts to run low, and ionic imbalance tends to occur. Together these factors cause the muscle to contract less and less effec- tively and finally to stop contracting altogether.

True muscle fatigue, in which the muscle quits entirely, rarely occurs in most of us because we feel tired long before it happens and we simply slow down or stop our activity. It does happen in marathon runners. Many of them have literally col- lapsed when their muscles became fatigued and could no longer work.

Oxygen deficit, which always occurs to some extent during vigorous muscle activity, is like a loan that must be “paid back” whether fatigue occurs or not. During the recovery period after activity, the individual breathes rapidly and deeply. This continues until the muscles have received the amount of oxygen needed to get rid of the accu- mulated lactic acid and replenish ATP and creatine phosphate reserves.

Types of Muscle contractions—Isotonic and Isometric

Until now, we have been discussing contraction in terms of shortening, but muscles do not always shorten when they contract. (I can hear you say- ing, “What kind of double-talk is that?”—but pay attention.) The event that is common to all muscle contractions is that tension (force) develops in the muscle as the actin and myosin myofilaments interact and the myosin cross bridges attempt to slide the thin actin-containing filaments past the thick myosin filaments.

Isotonic contractions (literally, “same tone” or tension) are familiar to most of us. In isotonic contractions, the myofilaments are successful in their sliding movements, the muscle shortens, and

222 Essentials of Human Anatomy and Physiology

exercise, or isometric exercise (Figure 6.11b), which pit the muscles against an immovable (or dif- ficult to move) object.

Resistance exercises require very little time and little or no special equipment. A few minutes every other day is usually sufficient. You can push against a wall, and you can strongly contract buttock mus- cles even while standing in line at the grocery store. The key is forcing your muscles to contract with as much force as possible. The increased muscle size and strength that result are due mainly to enlarge- ment of individual muscle fibers (they make more contractile myofilaments) rather than to an increase in their number. The amount of connective tissue that reinforces the muscle also increases.

Because endurance and resistance exercises produce different patterns of muscle response, it is important to know what your exercise goals are. Lifting weights will not improve your endurance for a marathon. By the same token, jogging will not make you stronger for lifting furniture. Obviously, the best exercise program for most people includes both types of exercise.

Did You Get It? 13. Gary is trying with all his might to pull a tree stump

out of the ground. It does not budge. Which type of contraction are his muscles performing?

14. What is meant by the term oxygen deficit? 15. To develop big, beautiful skeletal muscles, you should

focus on which type of exercise: aerobic or resistance exercise?

For answers, see Appendix A.

Muscle Movements, roles, and names ➔ Learning Objectives

□□ Define origin, insertion, prime mover, antagonist, synergist, and fixator as they relate to muscles.

□□ Demonstrate or identify the different types of body movements.

There are five basic guidelines for understanding gross muscle activity. We refer to these as the Five Golden Rules of skeletal muscle activity because they make it easier to understand muscle movements and appreciate muscle interactions (Table 6.2).

Types of Body Movements Every one of our 600-odd skeletal muscles is attached to bone, or to other connective tissue

are no exception to the saying “Use it or lose it!” Conversely, regular exercise increases muscle size, strength, and endurance. However, not all types of exercise produce these effects—in fact, there are important differences in the benefits of exercise.

Aerobic exercise, or endurance exercise, such as participating in an aerobics class, jogging, or biking (Figure 6.11a), results in stronger, more flexible muscles with greater resistance to fatigue. These changes come about, at least partly, because the blood supply to the muscles increases, and the individual muscle fibers form more mitochondria and store more oxygen. Aerobic exercise helps us reach a steady rate of ATP production and improves the efficiency of aerobic respiration.

However, aerobic exercise benefits much more than the skeletal muscles. It makes overall body metabolism more efficient, improves digestion (and elimination), enhances neuromuscular coordina- tion, and strengthens the skeleton. The heart enlarges (hypertrophies) and pumps out more blood with each beat, helping to clear more fat deposits from the blood vessel walls. The lungs become more efficient in gas exchange. These benefits may be permanent or temporary, depending on how often and how vigorously a person exercises.

Aerobic exercise does not cause the muscles to increase much in size, even though the exercise may go on for hours. The bulging muscles of a profes- sional bodybuilder result mainly from resistance

Figure 6.11 The effects of aerobic training versus strength training. (a) A marathon runner. (b) A weight lifter.

(a) (b)

Chapter 6: The Muscular System 223

• Extension. Extension is the opposite of flex- ion, so it is a movement that increases the angle, or distance, between two bones or parts of the body (straightening the knee or elbow). Extension that is greater than 180° (as when you move your arm posteriorly beyond its normal anatomical position, or tip your head so that your chin points toward the ceiling) is called hyperextension (Figures 6.13a and 6.13b).

• Rotation. Rotation is movement of a bone around its longitudinal axis (Figure 6.13c). Rotation is a common movement of ball-and- socket joints and describes the movement of the atlas around the dens of the axis (as in shaking your head “no”).

• Abduction. Abduction is moving a limb away (generally on the frontal plane) from the mid- line, or median plane, of the body (Figure 6.13d). The terminology also applies to the fanning movement of your fingers or toes when they are spread apart.

• Adduction. Adduction is the opposite of ab- duction, so it is the movement of a limb to- ward the body midline (Figure 6.13d). Think of adduction as “adding” a body part by bringing it closer to the trunk.

structures, at no fewer than two points. One of these points, the origin, is attached to the immov- able or less movable bone (Figure 6.12). Think of the origin as the anchor, or leverage, point. Another point, the insertion, is attached to the movable bone. When the muscle contracts, the insertion moves toward the origin.

Some muscles have interchangeable origins and insertions, depending on the action being performed. For example, the rectus femoris muscle of the ante- rior thigh crosses both the hip and knee joints. Its most common action is to extend the knee, in which case the proximal pelvic attachment is the origin. However, when the knee bends (by other muscles), the rectus femoris can flex the hip, and then its distal attachment on the leg is considered the origin.

Generally speaking, body movement occurs when muscles contract across joints. The type of movement depends on the mobility of the joint and the location of the muscle in relation to the joint. The most obvious examples of the action of muscles on bones are the movements that occur at the joints of the limbs. However, less freely movable bones are also tugged into motion by the muscles, such as the vertebrae’s movements when we bend to one side.

Next we describe the most common types of body movements (Figure 6.13, pp. 224–225). Try to act out each movement as you read the following descriptions:

• Flexion. Flexion is a movement, generally in the sagittal plane, that decreases the angle of the joint and brings two bones closer together (Figures 6.13a and 6.13b). Flexion is typical of hinge joints (bending the knee or elbow), but it is also common at ball-and-socket joints (for example, bending forward at the hip).

Table 6.2 The Five Golden rules of Skeletal Muscle Activity

1. With a few exceptions, all skeletal muscles cross at least one joint.

2. Typically, the bulk of a skeletal muscle lies proximal to the joint crossed.

3. All skeletal muscles have at least two attachments: the origin and the insertion.

4. Skeletal muscles can only pull; they never push.

5. During contraction, a skeletal muscle insertion moves toward the origin.

Figure 6.12 Muscle attachments (origin and insertion). When a skeletal muscle contracts, its insertion moves toward its origin.

Origin

Brachialis

Muscle contracting

Insertion

Tendon

(Text continues on page 226.)

224 Essentials of Human Anatomy and Physiology

(b) Flexion, extension, and hyperextension

Hyperextension Extension

Flexion

Extension

Hyperextension

Extension

(a) Flexion, extension, and hyperextension of the shoulder and knee

Lateral rotation

Rotation

Medial rotation

Figure 6.13 Body movements.

Chapter 6: The Muscular System 225

6Abduction

Adduction

(d) Abduction, adduction, and circumduction

Circumduction

Dorsiflexion

Plantar flexion

Supination (radius and ulna are parallel)

Pronation (radius rotates over ulna)

EversionInversion

Opposition

P S

(f) Inversion and eversion

(g) Supination (S) and pronation (P)

(h) Opposition

(e) Dorsiflexion and plantar flexion

Figure 6.13 (continued)

226 Essentials of Human Anatomy and Physiology

the same hand. This unique action makes the human hand a fine tool for grasping and ma- nipulating objects.

Interactions of Skeletal Muscles in the Body Muscles can’t push—they can only pull as they contract—so most often body movements result from two or more muscles acting together or against each other. Muscles are arranged so that whatever one muscle (or group of muscles) can do, other muscles can reverse. In general, groups of muscles that produce opposite movements lie on opposite sides of a joint (Figure 6.14). Because of this arrangement, muscles are able to bring about an immense variety of movements.

The muscle that has the major responsibility for causing a particular movement is called the prime mover. Muscles that oppose or reverse a movement are antagonists (an-tag′o-nists). When a prime mover is active, its antagonist is stretched and relaxed. Antagonists can be prime movers in their own right. For example, the biceps brachii and brachialis muscles of the arm (prime movers of elbow flexion) are antagonized by the triceps brachii (a prime mover of elbow extension).

Synergists (sin′er-jists; syn = together, erg = work) help prime movers by producing the same movement or by reducing undesirable movements. When a muscle crosses two or more joints, its con- traction will cause movement in all the joints crossed unless synergists are there to stabilize them. For example, the flexor muscles of the fin- gers cross both the wrist and the finger joints. You can make a fist without bending your wrist because synergist muscles stabilize the wrist joints and allow the prime mover to act on your finger joints.

Fixators are specialized synergists. They hold a bone still or stabilize the origin of a prime mover so all the tension can be used to move the inser- tion bone. The postural muscles that stabilize the vertebral column are fixators, as are the muscles that anchor the scapulae to the thorax.

In summary, although prime movers seem to get all the credit for causing certain movements, the actions of antagonistic and synergistic muscles are also important in producing smooth, coordi- nated, and precise movements.

• Circumduction. Circumduction is a combina- tion of flexion, extension, abduction, and adduc- tion commonly seen in ball-and-socket joints, such as the shoulder. The proximal end of the limb is stationary, and its distal end moves in a circle. The limb as a whole outlines a cone (Figure 6.13d), as when you do big arm circles.

Special Movements

Certain movements do not fit into any of the previ- ous categories and occur at only a few joints.

• Dorsiflexion and plantar flexion. Up-and- down movements of the foot at the ankle are given special names. Lifting the foot so that its superior surface approaches the shin (pointing your toe toward your head) is dorsi- flexion, whereas pointing the toes away from your head is plantar flexion (Figure 6.13e). Dorsiflexion of the foot corresponds to exten- sion and hyperextension of the hand at the wrist, whereas plantar flexion of the foot cor- responds to flexion of the hand.

• Inversion and eversion. Inversion and ever- sion are also special movements of the foot (Figure 6.13f). To invert the foot, turn the sole medially, as if you were looking at the bottom of your foot. To evert the foot, turn the sole laterally.

• Supination and pronation. The terms supination (soo″pı̆-na′shun; “turning back- ward”) and pronation (pro-na′shun; “turning forward”) refer to movements of the radius around the ulna (Figure 6.13g). Supination occurs when the forearm rotates laterally so that the palm faces anteriorly (or up) and the radius and ulna are parallel, as in anatomical position. Pronation occurs when the forearm rotates medially so that the palm faces poste- riorly (or down). Pronation brings the radius across the ulna so that the two bones form an X. A helpful memory trick: If you lift a cup of soup up to your mouth on your palm, you are supinating (“soup”-inating).

• Opposition. In the palm of the hand, the sad- dle joint between metacarpal 1 and the carpals allows opposition of the thumb (Figure 6.13h). This is the action by which you move your thumb to touch the tips of the other fingers on

Chapter 6: The Muscular System 227

Figure 6.14 Muscle action. The action of a muscle can be inferred by the muscle’s position as it crosses a joint.

(a) A muscle that crosses on the anterior side of a joint produces flexion*

(b) A muscle that crosses on the posterior side of a joint produces extension*

(d) A muscle that crosses on the medial side of a joint produces adduction

* These generalities do not apply to the knee and ankle because the lower limb is rotated during development. The muscles that cross these joints posteriorly produce flexion, and those that cross anteriorly produce extension.

Example: Pectoralis major (anterior view)

Example: Latissimus dorsi (posterior view) The latissimus dorsi is the antagonist of the pectoralis major.

Example: Deltoid middle fibers (anterolateral view)

The teres major is the antagonist of the deltoid.

Example: Teres major (posterolateral view)

Play A&P Flix Animation

228 Essentials of Human Anatomy and Physiology

sternum (sterno) and clavicle (cleido) and in- serts on the mastoid process of the temporal bone.

• Shape of the muscle. Some muscles have a distinctive shape that helps to identify them. For example, the deltoid muscle is roughly tri- angular (deltoid means “triangular”).

• Action of the muscle. When muscles are named for their actions, terms such as flexor, extensor, and adductor appear in their names. For example, the adductor muscles of the thigh all bring about its adduction, and the extensor muscles of the wrist all extend the wrist.

Arrangement of Fascicles Skeletal muscles consist of fascicles (refer to Figure 6.1), but fascicle arrangements vary, producing mus- cles with different structures and functional proper- ties. Next, let’s look at the most common patterns of fascicle arrangement: circular, convergent, fusiform, parallel, multipennate, bipennate, and unipennate.

In a circular pattern, the fascicles are arranged in concentric rings (Figure 6.15a). Circular muscles are typically found surrounding external body openings which they close by contracting, creating a valve. A general term for such muscles is sphinc- ters (“squeezers”). Examples are the orbicularis muscles surrounding the eyes and mouth.

In a convergent muscle, the fascicles con- verge toward a single insertion tendon. A conver- gent muscle is triangular or fan-shaped, such as the pectoralis major muscle of the anterior thorax (Figure 6.15b).

In a parallel arrangement, the length of the fascicles run parallel to the long axis of the mus- cle, as in the sartorius of the anterior thigh. These muscles are straplike (Figure 6.15d). A modifica- tion of the parallel arrangement, called fusiform, results in a spindle-shaped muscle with an expanded belly (midsection); an example is the biceps brachii muscle of the arm (Figure 6.15c).

In a pennate (pen′āt; “feather”) pattern, short fascicles attach obliquely to a central tendon. In the extensor digitorum muscle of the leg, the fascicles insert into only one side of the tendon, and the muscle is unipennate (Figure 6.15g). If the fascicles insert into opposite sides of the tendon, the muscle is bipennate (Figure 6.15f). If the fascicles insert from several different sides, the muscle is multi- pennate (Figure 6.15e).

Did You Get It? 16. What action is being performed by a person who

sticks out his thumb to hitch a ride? 17. What actions take place at the neck when you nod

your head up and down as if saying “yes”? 18. In what way are fixator and synergist muscles

important?

For answers, see Appendix A.

naming Skeletal Muscles ➔ Learning Objective

□ List seven criteria used in naming muscles.

Like bones, muscles come in many shapes and sizes to suit their particular tasks in the body. Muscles are named on the basis of several criteria, each of which focuses on a particular structural or functional characteristic. Paying close attention to these cues can greatly simplify your task of learn- ing muscle names and actions:

• Direction of the muscle fibers. Some mus- cles are named in reference to some imaginary line, usually the midline of the body or the long axis of a limb bone. When a muscle’s name includes the term rectus (straight), its fi- bers run parallel to that imaginary line. For ex- ample, the rectus femoris is the straight muscle of the thigh. Similarly, the term oblique in a muscle’s name tells you that the muscle fibers run obliquely (at a slant) to the imaginary line.

• Relative size of the muscle. Such terms as maximus (largest), minimus (smallest), and lon- gus (long) are sometimes used in the names of muscles—for example, the gluteus maximus is the largest muscle of the gluteus muscle group.

• Location of the muscle. Some muscles are named for the bone with which they are asso- ciated. For example, the temporalis and fron- talis muscles overlie the temporal and frontal bones of the skull, respectively.

• Number of origins. When the term biceps, triceps, or quadriceps forms part of a muscle name, you can assume that the muscle has two, three, or four origins, respectively. For example, the biceps muscle of the arm has two heads, or origins, and the triceps muscle has three.

• Location of the muscle’s origin and inser- tion. Occasionally, muscles are named for their attachment sites. For example, the sterno- cleidomastoid muscle has its origin on the

Chapter 6: The Muscular System 229

We describe only the most important muscles here. All the superficial muscles we consider are summarized in Tables 6.3 and 6.4 and illustrated in the overall body views shown later in the chapter (Figures 6.22 and 6.23).

Head and neck Muscles The head muscles (Figure 6.16, p. 230) are an interesting group. They have many specific func- tions but are usually grouped into two large categories—facial muscles and chewing muscles. Facial muscles are unique because they insert into soft tissues, such as other muscles or skin. When they pull on the skin of the face, they permit us to express ourselves by frowning, smil- ing, and so forth. The chewing muscles begin to break down food for the body. All head and neck muscles we describe are paired except for the platysma, orbicularis oris, frontalis, and occipitalis.

Facial Muscles

Frontalis The frontalis, which covers the frontal bone, runs from the cranial aponeurosis to the skin of the eyebrows, where it inserts. This muscle allows you to raise your eyebrows, as in surprise, and to wrinkle your forehead. At the posterior end of the cranial aponeurosis is the small occipitalis

A muscle’s fascicle arrangement determines its range of motion and power. The longer and the more nearly parallel the fascicles are to a muscle’s long axis, the more the muscle can shorten, but such muscles are not usually very powerful. Muscle power depends more on the total number of mus- cle fibers in the muscle. The stocky bipennate and multipennate muscles, which pack in the most fibers, shorten very little but are very powerful.

Did You Get It? 19. Based on their names, deduce some characteristics of

the following muscles: tibialis anterior, erector spinae, rectus abdominis, extensor carpi radialis longus.

20. What is the fascicle arrangement of the orbicularis oris muscle?

For answers, see Appendix A.

Gross Anatomy of Skeletal Muscles ➔ Learning Objective

□ name and locate the major muscles of the human body (on a torso model, muscle chart, or diagram), and state the action of each.

It is beyond the scope of this book to describe the hundreds of skeletal muscles of the human body.

Figure 6.15 relationship of fascicle arrangement to muscle structure.

(f) Bipennate (rectus femoris)

(a)

(b)

(c)

(f)

(g)

(e)

(d)

(a) Circular (orbicularis oris)

(b) Convergent (pectoralis major)

(d) Parallel (sartorius)

(g) Unipennate (extensor digitorum longus)

(e) Multipennate (deltoid)

230 Essentials of Human Anatomy and Physiology

the cheek to hold food between the teeth during chewing.

Zygomaticus The zygomaticus (zi″go-mat′i-kus) extends from the corner of the mouth to the cheekbone. It is often referred to as the “smiling” muscle because it raises the corners of the mouth.

Masseter As it runs from the zygomatic process of the temporal bone to the mandible, the mas- seter (mă-se′ter) covers the angle of the lower jaw. This muscle closes the jaw by elevating the mandible.

Temporalis The temporalis is a fan-shaped mus- cle overlying the temporal bone. It inserts into the mandible and acts as a synergist of the masseter in closing the jaw.

neck Muscles

For the most part, the neck muscles, which move the head and shoulder girdle, are small

muscle, which covers the posterior aspect of the skull and pulls the scalp posteriorly.*

Orbicularis Oculi The fibers of the orbicularis oculi (or-bik″u-la′ris ok′u-li) run in circles around the eyes. It allows you to close your eyes, squint, blink, and wink.

Orbicularis Oris The orbicularis oris is the circu- lar muscle of the lips. Often called the “kissing” muscle, it closes the mouth and protrudes the lips.

Buccinator The fleshy buccinator (buck′sı̆-na″tor) muscle runs horizontally across the cheek and inserts into the orbicularis oris. It flattens the cheek (as in whistling or blowing a trumpet). It is also listed as a chewing muscle because it compresses

Figure 6.16 Superficial muscles of the head and neck.

Cranial aponeurosisFrontalis

Occipitalis

Temporalis

Masseter

Sternocleidomastoid

Trapezius

Orbicularis oculi

Zygomaticus

Buccinator

Orbicularis oris

Platysma

*Although the current references on anatomic terminology refer to the frontalis and occipitalis as the frontal and occipi- tal bellies of the epicranius (“over the cranium”) muscle, we will continue to use the terms frontalis and occipitalis here.

231

A CLOSER

LOOK Anabolic Steroids: Dying to Win?

Many people feel the pressure to look and perform their best. It is not surprising that some will do anything to accomplish this—like tak- ing performance-enhancing drugs such as anabolic steroids. These ste- roids are variants of testosterone, the hormone responsible for the changes during puberty that usher boys into manhood. The most notable effect is the increase in bone and muscle mass. Pharmaceutical companies introduced anabolic steroids in the 1950s to treat certain muscle- wasting diseases, anemia, and mus- cle atrophy in patients immobilized after surgery, and they became avail- able to the general public during the 1980s.

Convinced that “doping” could enhance muscle mass and athletic endurance, many athletes (both men and women) have turned to anabolic steroids. Well-known ath- letes such as Mark McGwire (former St. Louis Cardinals first baseman), Marion Jones (2000 track and field Olympian), Lance Armstrong (seven-time Tour de France winner), and Alex Rodriguez (New York Yankees short- stop) admitted to steroid use and were stripped of medals and titles, and

suspended from their respective sports.

Steroid use is not limited to the United States. In 2015, 11 Bulgarian weight lifters tested positive for ana- bolic steroid use prior to the Euro- pean Championships; clearly this is a widespread problem that even affects adolescents. Studies by the National Institute on Drug Abuse (NIDA), National Institutes of Health (NIH), and the Centers for Disease Control (CDC) over a 20-year period estimated that 1.5–6% of adoles- cents 14–18 years old have used steroids.

Although most international athletic competitions and leagues have banned anabolic steroids, some athletes still claim they increase muscle mass, strength, and oxygen-carrying capacity of the blood. But do the drugs do all that is claimed? Research studies have

reported increases in isomet- ric strength and body weight in steroid users. Although these are results weight lifters dream

about, there is hot dis- pute over whether the drugs also enhance fine muscle coordina- tion and endurance

(which are important to runners and other ath- letes).

Do the claimed slight advantages

conferred by steroids

outweigh their risks? Absolutely not! Side effects from steroid use vary but include increased risk of heart attack and stroke, high blood pressure, an enlarged heart, dangerous changes in cholesterol levels, fluid retention, and acne. Adolescents can experi- ence stunted growth and early onset of puberty. Gender-specific side effects can also occur: Men may experience baldness, breast enlarge- ment, and shrinking testicles, with an increased risk of prostate cancer. Women may grow facial hair but lose scalp hair, develop a deeper voice, become infertile, and have an increased risk of osteoporosis. It is also increasingly clear that steroids can be addictive and lead to serious psychological issues such as depres- sion, delusions, paranoia, and aggressive or violent behavior called “‘roid rage.”

Some athletes say they are willing to do almost anything to win, short of killing themselves. Are those who use anabolic steroids unwittingly doing this as well?

232 Essentials of Human Anatomy and Physiology

upper part of the chest. Its origin is from the ster- num, shoulder girdle, and the first six ribs. It inserts on the proximal end of the humerus. This muscle forms the anterior wall of the axilla (arm- pit) and acts to adduct and flex the arm.

Intercostal Muscles The intercostal muscles are deep muscles found between the ribs. (Although they are not shown in Figure 6.17, which shows only superficial muscles, they are illustrated in Figure 6.22, p. 240.) The external intercostals are important in breathing because they help to raise the rib cage when you inhale. The internal intercostals, which lie deep to the external intercostals, depress the rib cage, helping to move air out of the lungs when you exhale forcibly.

Muscles of the Abdominal Girdle The anterior abdominal muscles (rectus abdominis, external and internal obliques, and transversus abdominis) form a natural “girdle” that reinforces the body trunk. Taken together, they resemble the structure of plywood because the fibers of each muscle or muscle pair run in a different direction. Just as ply- wood is exceptionally strong for its thickness, the abdominal muscles form a muscular wall that is well suited for its job of containing and protecting the abdominal contents.

• Rectus abdominis. The paired straplike rec- tus abdominis muscles are the most superficial muscles of the abdomen. They run from the pubis to the rib cage, enclosed in an aponeu- rosis. Their main function is to flex the verte- bral column. They also compress the abdomi- nal contents during defecation and childbirth (they help you “push”) and are involved in forced breathing.

• External oblique. The external oblique mus- cles are paired superficial muscles that make up the lateral walls of the abdomen. Their fi- bers run downward and medially from the last eight ribs and insert into the ilium. Like the rectus abdominis, they flex the vertebral col- umn, but they also rotate the trunk and bend it laterally.

• Internal oblique. The internal oblique mus- cles are paired muscles deep to the external obliques. Their fibers run at right angles to those of the external obliques. They arise from

and straplike. We consider only two neck mus- cles here (Figure 6.16).

Platysma The platysma is a single sheetlike mus- cle that covers the anterolateral neck (see Figure 6.16). It originates from the connective tissue cov- ering of the chest muscles and inserts into the area around the mouth. Its action is to pull the corners of the mouth inferiorly, producing a downward sag of the mouth (the “sad clown” face).

Sternocleidomastoid The paired sternocleido- mastoid (ster″no-kli″do-mas′toid) muscles are two- headed muscles, one found on each side of the neck. Of the two heads of each muscle, one arises from the sternum, and the other arises from the clavicle (see Figure 6.22, p. 240). The heads fuse before inserting into the mastoid process of the temporal bone. When both sternocleidomastoid muscles contract together, they flex your neck. (It is this action of bowing the head that has led some people to call these muscles the “prayer” muscles.) If just one muscle contracts, the face is rotated toward the shoulder on the opposite side and tilts the head to its own side.

Homeostatic Imbalance 6.3

In some difficult births, one of the sternocleido- mastoid muscles may be injured and develop spasms. A baby injured in this way has torticollis (tor″ti-kol′is), or wryneck. ___________________ ✚

Did You Get It? 21. Which muscle raises your eyebrow? 22. Which two muscles are synergists in closing your jaw?

For answers, see Appendix A.

Trunk Muscles The trunk muscles include (1) muscles that move the vertebral column (most of which are posterior antigravity muscles); (2) anterior thorax muscles, which move the ribs, head, and arms; and (3) mus- cles of the abdominal wall, which “hold your guts in” by forming a natural girdle and help to move the vertebral column.

Anterior Muscles (Figure 6.17)

Pectoralis Major The pectoralis (pek″to-ra′lis) major is a large fan-shaped muscle covering the

Chapter 6: The Muscular System 233

the sternocleidomastoids). They also can elevate, depress, adduct, and stabilize the scapula.

Latissimus Dorsi The latissimus (lah-tis′ı̆-mus) dorsi muscles are the two large, flat muscles that cover the lower back. They originate on the lower spine and ilium and then sweep superiorly to insert into the proximal end of the humerus. Each latissimus dorsi extends and adducts the humerus. These are very important muscles when the arm must be brought down in a power stroke, as when swim- ming or striking a blow.

Erector Spinae The erector spinae (e-rek′tor spi′ne) group is the prime mover of back extension. These paired muscles are deep muscles of the back (Figure 6.18b). Each erector spinae is a composite muscle consisting of three muscle columns (longissimus, iliocostalis, and spinalis) that collectively span the entire length of the vertebral column. These muscles not only act as powerful back extensors (“erectors”)

the iliac crest and insert into the last three ribs. Their functions are the same as those of the external obliques.

• Transversus abdominis. The deepest muscle of the abdominal wall, the transversus abdomi- nis has fibers that run horizontally across the abdomen. It arises from the lower ribs and iliac crest and inserts into the pubis. This muscle compresses the abdominal contents.

Posterior Muscles (Figure 6.18, p. 234)

Trapezius The trapezius (trah-pe′ze-us) mus- cles are the most superficial muscles of the poste- rior neck and upper trunk. When seen together, they form a diamond- or kite-shaped muscle mass. Their origin is very broad. Each muscle runs from the occipital bone of the skull down the vertebral column to the end of the thoracic vertebrae. They then flare laterally to insert on the scapular spine and clavicle. The trapezius mus- cles extend the head (thus they are antagonists of

Figure 6.17 Muscles of the anterior trunk, shoulder, and arm. (a) Muscles crossing the shoulder joint, causing movements of the arm. The platysma of the neck is removed. (b) Muscles of the abdominal wall. Portions of the superficial muscles of the right side of the abdomen are cut away to reveal the deeper muscles.

Clavicle

Deltoid

Sternum

Pectoralis major

Biceps brachii

Brachialis

Brachio- radialis

(a) (b)

Pectoralis major

Transversus abdominis

Rectus abdominis

Internal oblique

External oblique

Aponeurosis

234 Essentials of Human Anatomy and Physiology

but also provide resistance that helps control the action of bending over at the waist. Following injury to back structures, these muscles go into spasms, a common source of lower back pain.

Quadratus Lumborum The fleshy quadratus lum- borum (qwad-ra′tus lum-bor′um) muscles form part of the posterior abdominal wall. Acting sepa- rately, each muscle of the pair flexes the spine later- ally. Acting together, they extend the lumbar spine. These muscles arise from the iliac crests and insert into the upper lumbar vertebrae (Figure 6.18b).

Deltoid The deltoids are fleshy, triangle-shaped muscles that form the rounded shape of your shoulders. Because they are so bulky, they are a favorite injection site (Figure 6.19) when rela- tively small amounts of medication (less than 5 ml) must be given intramuscularly (into mus- cle). The origin of each deltoid winds across the shoulder girdle from the spine of the scapula to the clavicle. It inserts into the proximal humerus. The deltoids are the prime movers of arm abduction.

(a) (b)

Trapezius

Deltoid

Latissimus dorsi

Sternocleidomastoid

Occipital bone

Spine of scapula Deltoid (cut)

Humerus Olecranon process of ulna (deep to tendon)

• Iliocostalis • Longissimus

Erector spinae

C7 T1

• Spinalis

Quadratus lumborum

Triceps brachii

Figure 6.18 Muscles of the posterior neck, trunk, and arm. (a) Superficial muscles. (b) The erector spinae muscles (longissimus, iliocostalis, and spinalis), deep muscles of the back.

Figure 6.19 The fleshy deltoid muscle is a favored site for administering intramuscular injections.

Deltoid muscle

Humerus

Chapter 6: The Muscular System 235

forearm. The best way to remember its actions is to think of opening a bottle of wine. The biceps supinates the forearm to turn the corkscrew and then flexes the elbow to pull the cork.

Brachialis The brachialis lies deep to the biceps brachii and, like the biceps, is a prime mover in elbow flexion. The brachialis lifts the ulna as the biceps lifts the radius.

Brachioradialis The brachioradialis is a fairly weak muscle that arises on the humerus and inserts into the distal forearm (see Figure 6.22, p. 240). Hence, it resides mainly in the forearm.

Triceps Brachii The triceps brachii is the only muscle fleshing out the posterior humerus (see Figure 6.18a). Its three heads arise from the shoul- der girdle and proximal humerus, and it inserts into the olecranon process of the ulna. Being the powerful prime mover of elbow extension, it is the antagonist of the biceps brachii and brachialis. This muscle straightens the arm—for instance, to deliver a strong jab in boxing.

Muscles of the Lower Limb Muscles that act on the lower limb cause move- ment at the hip, knee, and foot joints. They are among the largest, strongest muscles in the body and are specialized for walking and balancing the body. Because the pelvic girdle is composed of heavy, fused bones that allow little movement, no special group of muscles is necessary to stabilize it. This is very different from the shoulder girdle, which requires several fixator muscles.

Many muscles of the lower limb span two joints and can cause movement at both of them. Therefore, in reference to these muscles, the terms origin and insertion are often interchangeable depending on the action being performed.

Muscles acting on the thigh are massive mus- cles that help hold the body upright against the pull of gravity and cause various movements at the hip joint. Muscles acting on the leg form the flesh of the thigh. (In common usage, the term leg refers to the whole lower limb, but anatomically the term refers only to that part between the knee and the ankle.) The thigh muscles cross the knee and cause its flex- ion or extension. Because many of the thigh mus- cles also have attachments on the pelvic girdle, they can cause movement at the hip joint as well.

Did You Get It? 23. Which muscle group is the prime mover of back

extension? 24. What structural feature makes the abdominal

musculature especially strong for its thickness? 25. Which muscle of the posterior trunk is the synergist of

the pectoralis major muscle in arm adduction?

For answers, see Appendix A.

Muscles of the upper Limb The upper limb muscles fall into three groups. The first group includes muscles that arise from the shoulder girdle and cross the shoulder joint to insert into the humerus (see Figures 6.17 and 6.18a). We have already considered these muscles, which move the arm—they are the pectoralis major, latissimus dorsi, and deltoid.

The second group causes movement at the elbow joint. These muscles enclose the humerus and insert on the forearm bones. In this section we will focus on the muscles of this second group.

The third group of upper limb muscles includes the muscles of the forearm, which insert on the hand bones and cause their movement. The muscles of this last group are thin and spindle- shaped, and there are many of them. We will not consider them here except to mention their gen- eral naming and function. As a rule, the forearm muscles have names that reflect their activities. For example, the flexor carpi and flexor digitorum muscles, found on the anterior aspect of the fore- arm, flex the wrist and fingers, respectively. The extensor carpi and extensor digitorum muscles, found on the lateral and posterior aspect of the forearm, extend the same structures. (Table 6.4 and Figure 6.23 briefly describe some of these muscles; see pp. 241, 243).

Muscles causing Movement at the Elbow Joint

All anterior muscles of the humerus cause elbow flexion. In order of decreasing strength these are the brachialis, biceps brachii, and brachioradialis (Figures 6.17a and 6.22, p. 240).

Biceps Brachii The biceps brachii (bra′ke-i) is the most familiar muscle of the arm because it bulges when you flex your elbow (see Figure 6.17a). It originates by two heads from the shoul- der girdle and inserts into the radial tuberosity. This muscle is the powerful prime mover for flexion of the forearm and acts to supinate the

236 Essentials of Human Anatomy and Physiology

forms most of the flesh of the buttock (Figure 6.20a). It is a powerful hip extensor that acts to bring the thigh in a straight line with the pelvis. Although it is not very important in walking, it is probably the most important muscle for extending the hip when power is needed, as when climbing stairs or jump- ing. It originates from the sacrum and iliac bones and inserts on the gluteal tuberosity of the femur and into the large tendinous iliotibial tract.

Muscles originating on the leg cause assorted movements of the ankle and foot. We will consider only three muscles of this group, but there are many others that extend and flex the ankle and toe joints.

Muscles causing Movement at the Hip Joint (Figure 6.20)

Gluteus Maximus The gluteus maximus (gloo′te-us max′ ı̆-mus) is a superficial muscle of the hip that

Figure 6.20 Pelvic, hip, and thigh muscles of the right side of the body. (a) Posterior view of hip and thigh muscles. (b) Diagram showing deep structures of the gluteal region and the proper site for administering an injection into the gluteus medius muscle.

Gluteus medius

Gluteus maximus

Adductor magnus

Biceps femoris

Iliotibial tract

Semitendinosus

Semimembranosus

Gastrocnemius

Hamstring group

Posterior superior iliac spine

Iliac crest

Safe area in gluteus medius

Gluteus maximus

Sciatic nerve

(b)

(a)

Chapter 6: The Muscular System 237

Vastus lateralis

Inguinal ligament

Adductor muscles

Sartorius

12th rib

Iliac crest

Iliopsoas

Q ua

dr ic

ep s*

Psoas major Iliacus

Anterior superior iliac spine

Sartorius

Rectus femoris

Vastus lateralis

Vastus medialis

12th thoracic vertebra

5th lumbar vertebra

Adductor group

Patella

Patellar ligament

(c)

(d)

The insertion for the rectus femoris during hip flexion is the pelvis, and during knee extension, the insertion is the tibial tuberosity.

What is the insertion for the rectus femoris during hip flexion: the pelvis or the tibial tuberosity? Which is the insertion during knee extension?Q:

Figure 6.20 (continued) (c) Anterior view of pelvic and thigh muscles. (d) Diagram showing the proper site for administering an injection into the lateral thigh (vastus lateralis muscle). *Note that the fourth quadriceps muscle, the vastus intermedius, is deep to the rectus femoris.

238 Essentials of Human Anatomy and Physiology

tendons to hang hams (consisting of thigh and hip muscles) for smoking. You can feel these tendons at the back of your knee.

Sartorius Compared with other thigh muscles described here, the thin, straplike sartorius (sar- to′re-us) muscle is not too important. However, it is the most superficial muscle of the thigh and so is rather hard to miss (Figure 6.20c). It runs obliquely across the thigh from the anterior iliac crest to the medial side of the tibia. It is a weak thigh flexor. The sartorius is commonly referred to as the “tai- lor’s” muscle because it acts as a synergist to help tailors sit with both legs crossed in front of them.

Quadriceps Group The quadriceps (kwod′r ı̆- seps) group consists of four muscles—the rectus femoris and three vastus muscles—that flesh out the anterior thigh. (Only two vastus muscles are visible in Figure 6.20c. The third, the vastus inter- medius, is obscured by the rectus femoris muscle, which lies over it.) The vastus muscles originate from the femur; the rectus femoris originates on the pelvis. All four muscles insert into the tibial tuberosity via the patellar ligament. The group as a whole acts to extend the knee powerfully, as when kicking a soccer ball. Because the rectus femoris crosses two joints, the hip and knee, it can also help to flex the hip. The vastus lateralis and rectus femoris are sometimes used as intramuscular injec- tion sites (Figure 6.20d), particularly in infants, who have poorly developed gluteus muscles.

Muscles causing Movement at the Ankle and Foot (Figure 6.21)

Tibialis Anterior The tibialis anterior is a superfi- cial muscle on the anterior leg. It arises from the upper tibia and then parallels the anterior crest as it runs to the tarsal bones, where it inserts by a long tendon. It acts to dorsiflex and invert the foot.

Extensor Digitorum Longus Lateral to the tibialis anterior, the extensor digitorum longus muscle arises from the lateral tibial condyle and proximal three-quarters of the fibula and inserts into the phalanges of toes 2 to 5. It is a prime mover of toe extension.

Fibularis Muscles The three fibularis muscles— longus, brevis, and tertius—are found on the lateral part of the leg. They arise from the fibula and insert into the metatarsal bones of the foot. The

Gluteus Medius The gluteus medius runs from the ilium to the femur, beneath the gluteus maxi- mus for most of its length. The gluteus medius is a hip abductor and is important in steadying the pelvis during walking. It is also an important site for giving intramuscular injections, particularly when administering more than 5 ml (Figure 6.20b). Although it might appear that the large, fleshy glu- teus maximus that forms the bulk of the buttock mass would be a better choice, notice that the medial part of each buttock overlies the large sci- atic nerve; hence this area must be carefully avoided to prevent nerve damage. This can be accomplished by imagining the buttock is divided into four equal quadrants (shown by the division lines on Figure 6.20b). The superolateral quadrant then overlies the gluteus medius muscle, which is usually a very safe site for an intramuscular injec- tion. Injections at or near the sciatic nerve can result in physical trauma from the needle or degeneration of the nerve itself.

Iliopsoas The iliopsoas (il″e-o-so′as; the p is silent) is a fused muscle composed of two mus- cles, the iliacus and the psoas major (Figure 6.20c). It runs from the iliac bone and lower vertebrae deep inside the pelvis to insert on the lesser tro- chanter of the femur. It is a prime mover of hip flexion. It also acts to keep the upper body from falling backward when we are standing erect.

Adductor Muscles The muscles of the adductor group form the muscle mass at the medial side of each thigh (Figure 6.20c). As their name indicates, they adduct, or press, the thighs together. However, because gravity does most of the work for them, they tend to become flabby very easily. Special exercises are usually needed to keep them toned. The adductors have their origin on the pelvis and insert on the proximal aspect of the femur.

Muscles causing Movement at the Knee Joint (Figure 6.20)

Hamstring Group The muscles forming the mus- cle mass of the posterior thigh are the hamstrings (Figure 6.20a). The group consists of three mus- cles—the biceps femoris, semimembranosus, and semitendinosus—which originate on the ischial tuberosity and run down the thigh to insert on both sides of the proximal tibia. They are prime movers of thigh extension and knee flexion. Their name comes from the fact that butchers use their

Chapter 6: The Muscular System 239

group as a whole plantar flexes and everts the foot, which is antagonistic to the tibialis anterior.

Gastrocnemius The gastrocnemius (gas″trok- ne′me-us) muscle is a two-bellied muscle that forms the curved calf of the posterior leg. It arises by two heads, one from each side of the distal femur, and inserts through the large calcaneal (Achilles) tendon into the heel of the foot. It is a prime mover for plantar flexion of the foot; for this reason it is often called the “toe dancer’s” muscle. If the calcaneal tendon is severely damaged or cut, walking is very difficult. The foot drags because it is not able to “push off” the toe (raise the heel).

Figure 6.21 Superficial muscles of the right leg. (a) Anterior view. (b) Posterior view.

(b)(a)

Calcaneal (Achilles) tendon

Medial malleolus

Soleus

Fibularis longus

Fibularis brevis

Fibularis tertius

Tibia

Tibialis anterior

Extensor digitorum longus

Lateral malleolus

Gastrocnemius

Soleus Deep to the gastrocnemius is the fleshy soleus muscle. Because it arises on the tibia and fibula (rather than the femur), it does not affect knee movement, but like the gastrocnemius, it inserts into the calcaneal tendon and is a strong plantar flexor of the foot.

Figure 6.22 (p. 240) and Figure 6.23 (p. 241) show anterior and posterior views of the whole body, including most of the superficial muscles we have described. (Table 6.3) (p. 242) and (Table 6.4) (p. 243) summarize these muscles. Take time to review these muscles again before continuing with this chapter.

240 Essentials of Human Anatomy and Physiology

• Trapezius

Shoulder

• Deltoid

Arm • Triceps brachii

• Biceps brachii

• Brachialis

Forearm • Brachioradialis • Flexor carpi radialis

Pelvis/thigh • Iliopsoas

Thigh (Quadriceps) • Rectus femoris • Vastus lateralis

• Vastus medialis

• Vastus intermedius (not shown, deep to rectus femoris)

Leg • Fibularis longus

• Extensor digitorum longus

• Tibialis anterior

Facial

• Temporalis

• Frontalis

• Orbicularis oculi

• Zygomaticus

• Orbicularis oris Neck • Platysma • Sternocleidomastoid Thorax

• Pectoralis major

• Pectoralis minor

• Serratus anterior

• Intercostals

Abdomen

• External oblique

• Rectus abdominis

• Internal oblique

• Transversus abdominis

Thigh

• Sartorius

• Adductor muscles

Leg • Gastrocnemius

• Soleus

• Masseter

Figure 6.22 Major superficial muscles of the anterior surface of the body.

Chapter 6: The Muscular System 241

Arm • Triceps brachii

• Brachialis

Forearm • Brachioradialis

• Extensor carpi ulnaris • Extensor digitorum

Iliotibial tract

Calcaneal (Achilles) tendon

Leg • Gastrocnemius

• Soleus

• Fibularis longus

Neck • Occipitalis

• Sternocleidomastoid

• Trapezius

Shoulder/Back

Hip • Gluteus medius

• Gluteus maximus

Thigh

Biceps femoris

• Adductor muscle

Semitendinosus

Semimembranosus

• Hamstrings:

• Latissimus dorsi

• Deltoid

• Flexor carpi ulnaris

• Extensor carpi radialis longus

Figure 6.23 Major superficial muscles of the posterior surface of the body.

242 Essentials of Human Anatomy and Physiology

Table 6.3 Superficial Anterior Muscles of the Body (See Figure 6.22)

name Origin Insertion Primary action(s)

Head/neck muscles

Frontalis Cranial aponeurosis Skin of eyebrows Raises eyebrows

Orbicularis oculi Frontal bone and maxilla Tissue around eyes Blinks and closes eye

Orbicularis oris Mandible and maxilla Skin and muscle around mouth

Closes and protrudes lips

Temporalis Temporal bone Mandible Closes jaw

Zygomaticus Zygomatic bone Skin and muscle at corner of lips

Raises corner of mouth

Masseter Temporal bone Mandible Closes jaw

Buccinator Maxilla and mandible near molars

Orbicularis oris Compresses cheek (as in sucking); holds food between teeth during chewing

Sternocleidomastoid Sternum and clavicle Temporal bone (mastoid process)

Flexes neck; laterally rotates head

Platysma Connective tissue covering of superior chest muscles

Tissue around mouth Tenses skin of neck (as in shaving)

Trunk muscles

Pectoralis major Sternum, clavicle, and first to sixth ribs

Proximal humerus Adducts and flexes humerus

Rectus abdominis Pubis Sternum and fifth to seventh ribs

Flexes vertebral column

External oblique Lower eight ribs Iliac crest Flexes and rotates vertebral column

Arm/shoulder muscles

Biceps brachii Scapula of shoulder girdle Proximal radius Flexes elbow and supinates forearm

Brachialis Distal humerus Proximal ulna Flexes elbow

Deltoid (See Table 6.4) Abducts arm

Hip/thigh/leg muscles

Iliopsoas Ilium and lumbar vertebrae Femur (lesser trochanter) Flexes hip

Adductor muscles Pelvis Proximal femur Adduct and medially rotate thigh

Sartorius Ilium Proximal tibia Flexes thigh on hip

Quadriceps group (vastus medialis, intermedius, and lateralis; and the rectus femoris)

Vasti: femur

Rectus femoris: pelvis

Tibial tuberosity via patellar ligament

All extend knee; rectus femoris also flexes hip on thigh

Tibialis anterior Proximal tibia First cuneiform (tarsal) and first metatarsal of foot

Dorsiflexes and inverts foot

Extensor digitorum longus Proximal tibia and fibula Distal toes 2–5 Extends toes

Fibularis muscles Fibula Metatarsals of foot Plantar flex and evert foot

Chapter 6: The Muscular System 243

Table 6.4 Superficial Posterior Muscles of the Body (Some Forearm Muscles Also Shown) (See Figure 6.23)

name Origin Insertion Primary action(s)

Head/Neck/trunk/shoulder muscles

Occipitalis Occipital and temporal bones

Cranial aponeurosis Fixes aponeurosis and pulls scalp posteriorly

Trapezius Occipital bone and all cervical and thoracic vertebrae

Scapular spine and clavicle Raises, retracts, and rotates scapula

Latissimus dorsi Lower spine and iliac crest Proximal humerus Extends and adducts humerus

Erector spinae* Iliac crests, ribs 3–12, and vertebrae

Ribs, thoracic and cervical vertebrae

Extends and laterally flexes spine

Quadratus lumborum* Iliac crest, lumbar fascia Transverse processes of upper lumbar vertebrae

Flexes spine laterally; extends spine

Deltoid Scapular spine and clavicle Humerus (deltoid tuberosity) Abducts humerus

Arm/forearm muscles

Triceps brachii Shoulder girdle and proximal humerus

Olecranon process of ulna Extends elbow

Flexor carpi radialis Distal humerus Second and third metacarpals

Flexes wrist and abducts hand (see Figure 6.22)

Flexor carpi ulnaris Distal humerus and posterior ulna

Carpals of wrist and fifth metacarpal

Flexes wrist and adducts hand

Flexor digitorum superficialis†

Distal humerus, ulna, and radius

Middle phalanges of second to fifth fingers

Flexes wrist and fingers

Extensor carpi radialis Humerus Base of second and third metacarpals

Extends wrist and abducts hand

Extensor digitorum Distal humerus Distal phalanges of second to fifth fingers

Extends fingers

Hip/thigh/leg muscles

Gluteus maximus Sacrum and ilium Proximal femur (gluteal tuberosity)

Extends hip (when forceful extension is required)

Gluteus medius llium Proximal femur Abducts thigh; steadies pelvis during walking

Hamstring muscles (biceps femoris, semitendinosus, semimembranosus)

Ischial tuberosity Proximal tibia (head of fibula in the case of biceps femoris)

Flex knee and extend hip

Gastrocnemius Distal femur Calcaneus (heel via calcaneal tendon)

Plantar flexes foot and flexes knee

Soleus Proximal tibia and fibula Calcaneus Plantar flexes foot

*Erector spinae and quadratus lumborum are deep muscles (they are not shown in Figure 6.23; see Figure 6.18b). †Although its name indicates that it is a superficial muscle, the flexor digitorum superficialis lies deep to the flexor carpi radialis and is not visible in a superficial view.

244 Essentials of Human Anatomy and Physiology

dystrophin) that helps maintain the sarcolemma— a cure is still elusive. ________________________✚

After birth, a baby’s movements are all gross reflex types of movements. Because the nervous system must mature before the baby can control muscles, we can trace the increasing efficiency of the nervous system by observing a baby’s development of muscle control. This develop- ment proceeds in a superior/inferior direction, and gross muscular movements precede fine ones. Babies can raise their heads before they can sit up and can sit up before they can walk. Muscular control also proceeds in a proximal/ distal direction; that is, babies can perform the gross movements like waving “bye-bye” and pull- ing objects to themselves before they can use the pincer grasp to pick up a pin. All through child- hood, the nervous system’s control of the skele- tal muscles becomes more and more precise. By midadolescence, we have reached the peak level of development of this natural control and can simply accept it or bring it to a fine edge by ath- letic training.

Because of its rich blood supply, skeletal mus- cle is amazingly resistant to infection throughout life, and given good nutrition, relatively few prob- lems afflict skeletal muscles. We repeat, however, that muscles, like bones, will atrophy, even with normal tone, if they are not used continually. A lifelong program of regular exercise keeps the whole body operating at its best possible level.

Homeostatic Imbalance 6.5

One rare autoimmune disease that can affect muscles during adulthood is myasthenia gravis (mi″as-the′ne-ah gră′vis; asthen = weakness; gravi = heavy), a disease characterized by droop- ing upper eyelids, difficulty in swallowing and talking, and generalized muscle weakness and fatigability. The disease involves a shortage of acetylcholine receptors at neuromuscular junc- tions caused by antibodies specific for acetylcho- line receptors. Antibodies are immune molecules that will mark the receptors for destruction. Because the muscle fibers are not stimulated properly, they get progressively weaker. Death usually occurs when the respiratory muscles can no longer function, which leads to respiratory failure.

Did You Get It? 26. Which muscle is the antagonist of the biceps brachii

when the biceps flexes the elbow? 27. Which muscle group is the antagonist of the

hamstring muscles? 28. What are two good sites for intramuscular injections

in adults? 29. Which two muscles insert into the calcaneal tendon?

What movement do they effect?

For answers, see Appendix A.

Developmental Aspects of the Muscular System ➔ Learning Objectives

□□ Explain the importance of a nerve supply and exercise in keeping muscles healthy.

□□ Describe the changes that occur in aging muscles.

In the developing embryo, the muscular system is laid down in segments, and then each segment is invaded by nerves. The muscles of the thoracic and lumbar regions become very extensive because they must cover and move the bones of the limbs. The muscles and their control by the nervous system develop rather early in pregnancy. The expec tant mother is often astonished by the first movements (called the quickening) of the fetus, which usually occur by the 16th week of pregnancy.

Homeostatic Imbalance 6.4

Very few congenital muscular problems occur. The exception to this is muscular dystrophy—a group of inherited muscle-destroying diseases that affect specific muscle groups. The muscles appear to enlarge because of fat and connective tissue depos- its, but the muscle fibers degenerate and atrophy.

The most common and serious form is Duchenne’s muscular dystrophy, which is expressed almost exclusively in boys. This tragic disease is usually diagnosed between the ages of 2 and 7 years. Active, normal-appearing children become clumsy and fall frequently as their mus- cles weaken. The disease progresses from the extremities upward, finally affecting the head and chest muscles. Children with this disease rarely live beyond their early twenties and generally die of respiratory failure. Although researchers have identified the cause of muscular dystrophy— the diseased muscle fibers lack a protein (called

Chapter 6: The Muscular System 245

As we age, the amount of connective tissue in muscles increases, and the amount of muscle tis- sue decreases; thus the muscles become stringier, or more sinewy. Because skeletal muscles repre- sent so much of our body mass, body weight begins to decline in older adults as this natural loss in muscle mass occurs. Muscle strength also decreases by about 50 percent by age 80. Regular exercise can help offset the effects of aging on the muscular system, and frail older people who begin to “pump iron” (use leg and hand weights) can rebuild muscle mass and dramatically increase their strength.

Did You Get It? 30. What must happen before babies can control their

muscles? 31. How does lifelong exercise affect our skeletal muscles

and muscle mass in old age?

For answers, see Appendix A.

A female patient with myasthenia gravis. Notice the effect on the patient’s eyelid muscles.

Summary

Overview of Muscle Tissues (pp. 207–211)

1. Skeletal muscle tissue forms the muscles attached to the skeleton, which move the limbs and other body parts. Its fibers (cells) are long, striated and multinucleate, and they are subject to voluntary control. Connective tissue coverings (endomysium, perimysium, and epimysium) enclose and protect the muscle fibers and increase the strength of skel- etal muscles.

2. Smooth muscle fibers are uninucleate, spindle- shaped, and arranged in opposing layers in the walls of hollow organs. When they contract, sub- stances (food, urine, a baby) are moved along internal pathways. Smooth muscle control is invol- untary.

3. Cardiac muscle cells are striated, branching cells that fit closely together and are arranged in spiral bundles in the heart. Their contraction pumps blood through the blood vessels. Control is invol- untary.

4. The sole function of muscle tissue is to contract, or shorten, to generate tension. As it contracts, it causes movement, maintains posture, stabilizes joints, and generates heat.

Microscopic Anatomy of Skeletal Muscle (pp. 211–213)

1. The multinucleate cylindrical skeletal muscle fibers are packed with unique organelles called myofi- brils. The banding pattern (striations) of the myofi- brils and the fiber as a whole reflects the regular arrangement of thin (actin-containing) and thick (myosin) filaments within the sarcomeres, the con- tractile units (structurally and functionally) compos- ing the myofibrils.

___________________________________________✚

Complete an interactive tutorial: > Study Area > Interactive Physiology > Muscular System > Anatomy Review: Skeletal Muscle Tissue

2. Each myofibril is loosely enclosed by a specialized ER, called the sarcoplasmic reticulum (SR), which plays an important role in storing and releasing calcium ions. Calcium ions are the final trigger for muscle fiber contraction.

Skeletal Muscle Activity (pp. 213–222)

1. All skeletal muscle fibers are stimulated by motor neurons. When the neuron releases a neurotrans- mitter (acetylcholine), the permeability of the sar- colemma changes, allowing sodium ions to enter

246

SYSTEMS IN SYNC

Homeostatic Relationships between the Muscular System and Other Body Systems

• Skeletal muscle activity increases efficiency of cardiovascular functioning; helps prevent atherosclerosis and causes cardiac hypertrophy

• Cardiovascular system delivers oxygen and nutrients to muscles; carries away wastes

• Physical activity promotes normal voiding behavior; skeletal muscle forms the voluntary sphincter of the urethra

• Urinary system disposes of nitrogen-containing wastes

• Physical exercise may enhance or depress immunity depending on its intensity

• Lymphatic vessels drain leaked interstitial fluids; immune system protects muscles from disease

• Physical activity increases gastrointestinal mobility when at rest

• Digestive system provides nutrients needed for muscle health; liver metabolizes lactic acid

• Skeletal muscle helps support pelvic organs (e.g., uterus in females); assists erection of penis and clitoris

• Testicular androgen promotes increased skeletal muscle size

• Muscular exercise increases respiratory capacity

• Respiratory system provides oxygen and disposes of carbon dioxide

• Facial muscle activity allows emotions to be expressed

• Nervous system stimulates and regulates muscle activity

• Muscular exercise enhances circulation to skin and improves skin health; exercise also increases body heat, which the skin helps dissipate

• Skin protects the muscles by external enclosure

• Skeletal muscle activity maintains bone health and strength

• Bones provide levers for muscle activity

• Growth hormone and androgens influence skeletal muscle strength and mass

Cardiovascular System

Urinary System

Lymphatic System/Immunity

Digestive System

Reproductive System

Respiratory System

Nervous System

Integumentary System

Endocrine System

Muscular System

Skeletal System

Chapter 6: The Muscular System 247

regular aerobic exercise become more efficient and stronger and can work longer without tiring. Aerobic exercise also benefits other body organ systems.

Muscle Movements, roles, and names (pp. 222–229)

1. All muscles are attached to bones at two points. The origin is the immovable (or less movable) attachment; the insertion is the movable bony attachment. When contraction occurs, the insertion moves toward the origin.

2. Body movements include flexion, extension, abduc- tion, adduction, circumduction, rotation, pronation, supination, inversion, eversion, dorsiflexion, plantar flexion, and opposition.

3. On the basis of their general functions in the body, muscles are classified as prime movers, antagonists, synergists, and fixators.

4. Muscles are named according to several criteria, including muscle size, shape, number and location of origins, associated bones, and action of the muscle.

5. Muscles have several fascicle arrangements that in- fluence their force and degree of shortening.

Gross Anatomy of Skeletal Muscles (pp. 229–244)

1. Muscles of the head fall into two groups. The muscles of facial expression include the frontalis, orbicularis oris, orbicularis oculi, and zygomaticus. The chewing muscles are the masseter, temporalis, and buccinator (which is also a muscle of facial expression).

2. Muscles of the trunk and neck move the head, shoulder girdle, and trunk and form the abdominal girdle. Anterior neck and trunk muscles include the sternocleidomastoid, pectoralis major, external and internal intercostals, rectus abdominis, external and internal obliques, and transversus abdominis. Posterior trunk and neck muscles include the trape- zius, latissimus dorsi, and deltoid. Deep muscles of the back are the erector spinae muscles.

3. Muscles of the upper limb include muscles that cause movement at the shoulder joint, elbow, and hand. Muscles causing movement at the elbow include the brachialis, biceps brachii, brachioradia- lis, and triceps brachii.

4. Muscles of the lower extremity cause movement at the hip, knee, and foot. They include the iliopsoas,

the muscle fiber. This produces an electrical current (action potential), which flows across the entire sarcolemma, resulting in release of calcium ions from the SR.

2. Calcium binds to regulatory proteins on the thin filaments and exposes myosin-binding sites, allow- ing the myosin heads on the thick filaments to attach. The attached heads pivot, sliding the thin filaments toward the center of the sarcomere, and contraction occurs. ATP provides the energy for the sliding process, which continues as long as ionic calcium is present.

3. Although individual muscle fibers contract com- pletely when adequately stimulated, a muscle (which is an organ) responds to stimuli to different degrees; that is, it exhibits graded responses.

4. Most skeletal muscle contractions are tetanic (smooth and sustained) because rapid nerve impulses are reaching the muscle, and the muscle cannot relax completely between contractions. The strength of muscle contraction reflects the relative number of muscle fibers contracting (more = stronger).

5. ATP, the immediate source of energy for muscle contraction, is stored in muscle fibers in small amounts that are quickly used up. ATP is regener- ated via three routes. From the fastest to the slow- est, these are via a coupled reaction of creatine phosphate with ADP, via anaerobic glycolysis and lactic acid formation, and via aerobic respiration. Only aerobic respiration requires oxygen.

6. If muscle activity is strenuous and prolonged, muscle fatigue occurs because ionic imbalances occur, lactic acid accumulates in the muscle, and the energy (ATP) supply decreases. After exer- cise, the oxygen deficit is repaid by rapid, deep breathing.

7. Muscle contractions are isotonic (the muscle short- ens, and movement occurs) or isometric (the mus- cle does not shorten, but its tension increases).

8. Muscle tone keeps muscles healthy and ready to react. It is a result of a staggered series of nerve impulses delivered to different fibers within the muscle. If the nerve supply is destroyed, the muscle loses tone, becomes paralyzed, and atrophies.

9. Inactive muscles atrophy. Muscles challenged almost beyond their ability by resistance exercise will in- crease in size and strength. Muscles subjected to

248 Essentials of Human Anatomy and Physiology

review Questions

Multiple choice

More than one choice may apply.

1. If you compare electron micrographs of a relaxed skeletal muscle fiber and a fully contracted muscle fiber, which would you see only in the relaxed fiber?

a. Z discs

b. Sarcomeres

c. I bands

d. A bands

e. H zones

2. After ACh attaches to its receptors at the neuromuscular junction, what is the next step?

a. Sodium channels open.

b. Calcium binds to regulatory proteins on the thin filaments.

c. Cross bridges attach.

d. ATP is hydrolyzed.

3. Your ability to lift that heavy couch would be increased by which type of exercise?

a. Aerobic

b. Endurance

c. Resistance

d. Swimming

4. Which of the following can lead to muscle fatigue?

a. Imbalances in ions

b. Excessive amounts of oxygen

c. Excessive amounts of ADP

d. Oxygen deficit

5. Which are ways in which muscle names have been derived?

a. Attachments

b. Size

c. Function

d. Location

6. Which of the following muscles attach to the hip bones?

a. Rectus abdominis

b. Rectus femoris

c. Vastus medialis

d. Vastus lateralis

7. Which of these muscles is found on the posterior surface of the leg?

a. Gastrocnemius

b. Extensor digitorum longus

c. Extensor carpi ulnaris

d. Biceps femoris

8. Which of the following is (are) true concerning the pectoralis major?

a. It raises the clavicle.

b. It adducts the humerus.

c. It abducts the arm.

d. It flexes the humerus.

Short Answer Essay

9. What is the major function of muscle?

10. Compare skeletal, smooth, and cardiac muscles in regard to their microscopic anatomy, location and arrangement in body organs, and function in the body.

achieved in a superior/inferior and proximal/distal direction.

2. To remain healthy, muscles must be regularly exer- cised. Without exercise, they atrophy; with extremely vigorous exercise, they hypertrophy.

3. As we age, muscle mass decreases, and the muscles become more sinewy. Exercise helps to retain mus- cle mass and strength.

gluteus maximus, gluteus medius, adductors, quad- riceps and hamstring groups, gastrocnemius, tibi- alis anterior, fibularis muscles, soleus, and extensor digitorum longus.

Developmental Aspects of the Muscular System (pp. 244–245)

1. Increasing muscular control reflects the matura- tion of the nervous system. Muscle control is

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 6: The Muscular System 249

Critical Thinking and Clinical Application Questionscritical Thinking and clinical Application Questions

33. Name three muscles or muscle groups used as sites for intramuscular injections. Which is most often used in babies?

34. While jogging, Mr. Ahmadi was forced to jump out of the way of a speeding car. He heard a snapping sound that was immediately followed by pain in his right lower calf. A gap was visible between his swollen calf and his heel, and he was unable to plantar flex that foot. What do you think happened?

35. Susan fell off her bicycle and fractured her right clavicle. Treatment prescribed by the emergency room physician included using a sling to immobi- lize the clavicle and speed its healing. What mus-

cles are temporarily “out of business” as a result of this injury?

36. When Eric returned from jogging, he was breathing heavily and sweating profusely, and he complained that his legs ached and felt weak. On the basis of what you have learned about muscle energy metab- olism, respond to the following questions:

a. Why is Eric breathing heavily?

b. What ATP-harvesting pathway have his working muscles been using that leads to such a breathing pattern?

c. What metabolic product(s) might account for his sore muscles and his feeling of muscle weakness?

11. What two types of muscle tissue are striated?

12. Why are the connective tissue wrappings of skeletal muscles important? Name these connective tissue coverings, beginning with the finest and ending with the coarsest.

13. What is the function of tendons?

14. Define neuromuscular junction, motor unit, tetanus, graded response, aerobic respiration, anaerobic glycolysis, muscle fatigue, and neurotransmitter.

15. Describe the events that occur from the time calcium ions enter the axon terminal at the neuromuscular junction until muscle cell contraction occurs.

16. How do isotonic and isometric contractions differ?

17. How does muscle activity contribute to the maintenance of body temperature? Explain how significant this contribution is.

18. A skeletal muscle is attached to bones at two points. Name each of these attachment points, and indicate which is movable and which is immovable.

19. Circumduction is a combination of four movements. List these four movements, and describe each.

20. How is a prime mover different from a synergist muscle? How can a prime mover also be considered an antagonist?

21. Which facial muscle allows you to whistle? Why is it also considered a chewing muscle?

22. Injury to which muscle can lead to torticollis?

23. Name two muscles that reverse the movement of the deltoid muscle.

24. Name the prime mover of elbow extension. Name its antagonist(s).

25. Other than acting to flex the spine and compress the abdominal contents, the abdominal muscles are extremely important in protecting and containing the abdominal viscera. What is it about the arrangement of these muscles that makes them so well suited for their job?

26. The hamstring and quadriceps muscle groups are antagonists of each other, and each group is a prime mover in its own right. What action does each muscle group perform?

27. Which muscles insert on the transverse processes of the upper lumbar vertebrae? What are their primary functions?

28. What happens to muscles when they are exercised regularly? Exercised vigorously as in weight lifting? Not used?

29. What is the effect of aging on skeletal muscles?

30. Should a triathlete engage in aerobic or resistance training? Explain.

31. Explain the general relationship between the muscles that flex and extend the wrist, including their location on the body.

32. Harry is pondering an exam question that asks, “What muscle type has elongated cells and is found in the walls of the urinary bladder?” How should he respond?

250 Essentials of Human Anatomy and Physiology

37. Chemical A binds and blocks acetylcholine recep- tors of muscle cells. Chemical B floods the cyto- plasm of muscle cells with calcium ions. Which chemical would make the best muscle relaxant, and why?

38. Kendra’s broken leg was in a cast for 8 weeks. When the cast was removed, she was shocked that her leg was no longer muscular, but skinny. Explain what happened and why.

251

which are rapid and specific and cause almost immediate responses.

The nervous system does not work alone to regulate and maintain body homeostasis; the endocrine system is a second important regulating system. Whereas the nervous system controls with rapid electrical nerve impulses, the endocrine sys- tem produces hormones that are released into the blood. Thus, the endocrine system acts in a more leisurely way. We will discuss the endocrine sys- tem in detail in Chapter 9.

To carry out its normal role, the nervous system has three overlapping functions (Figure 7.1, p. 252):

You are driving down the freeway when a horn blares on your right, and you swerve to your left. You are with a group of friends but are not paying attention to any particular con- versation, yet when you hear your name, you immediately focus in. What do these events have in common? They are everyday examples of ner- vous system function, which has your body cells humming with activity nearly all the time.

The nervous system is the master control and communication system of the body. Every thought, action, and emotion reflects its activity. It commu- nicates with body cells using electrical impulses,

The Nervous System7 WHAT

HOW

WHY

As the primary control system of the body, the nervous

system provides for higher mental function and emotional expression,

maintains homeostasis, and regulates the activities of

muscles and glands.

Communication by the nervous system involves a combination of electrcial

and chemical signals.

All body systems are under the control or regulation

of the nervous system. If the nervous systems stops functioning, the body

can stay alive only with the assistance of life-supporting

machines.

INSTrucTorS

New Building Vocabulary coaching Activities for this chapter are assignable in

252 Essentials of Human Anatomy and Physiology

We have only one nervous system, but because of its complexity, it is difficult to consider all its parts at the same time. So, to simplify its study, we divide it in terms of its structures (structural classi- fication) or in terms of its activities (functional classification). We briefly describe each of these classification schemes next, and Figure 7.2 illus- trates their relationships. You do not need to memorize this whole scheme now, but as you are reading the descriptions, try to get a “feel” for the major parts and how they fit together. This will

(1) It uses its millions of sensory receptors to monitor changes occurring both inside and out- side the body. These changes are called stimuli, and the gathered information is called sensory input. (2) It processes and interprets the sensory input and decides what should be done at each moment—a process called integration. (3) It then causes a response, or effect, by activating muscles or glands (effectors) via motor output.

Motor output

Brain and spinal cord

Sensory input

IntegrationIntegration Sensory receptor

Effector

Figure 7.1 The nervous system’s functions.

➔ConCeptLinkThese three overlapping nervous system functions are very similar to a feedback loop (Chapter 1, p. 45). Recall that in a feedback loop, a receptor receives sensory input, which it sends to the brain (control center) for processing (integration); the brain then analyzes the information and determines the appro- priate output, which leads to a motor response. ➔

An example will illustrate how these functions work together. When you are driving and see a red light just ahead (sensory input), your nervous system integrates this information (red light means “stop”) and sends motor output to the muscles of your right leg and foot. Your foot goes for the brake pedal (the response).

organization of the Nervous System ➔ Learning Objectives

□□ List the general functions of the nervous system.

□□ Explain the structural and functional classifications of the nervous system.

□□ Define central nervous system and peripheral nervous system, and list the major parts of each.

Central Nervous System (brain and spinal cord)

Peripheral Nervous System (cranial and spinal nerves)

Somatic (voluntary)

Skeletal muscles

Autonomic (involuntary)

Cardiac and smooth muscle,

glands

Sensory (afferent)

Sense organs

Parasympathetic Sympathetic

Motor (efferent)

Figure 7.2 organization of the nervous system. As the flowchart shows, the central nervous system receives input via sensory fibers and issues commands via motor fibers. The sensory and motor fibers together form the nerves that constitute the peripheral nervous system.

Chapter 7: The Nervous System 253

• The somatic (so-mat′ik) nervous system allows us to consciously, or voluntarily, control our skeletal muscles. Hence, we often refer to this subdivision as the voluntary nervous system. However, not all skeletal muscle activ- ity controlled by this motor division is volun- tary. Skeletal muscle reflexes, such as the stretch reflex (described later in the chapter), are initiated involuntarily by these same fibers.

• The autonomic (aw″to-nom′ik) nervous sys- tem (ANS) regulates events that are automatic, or involuntary, such as the activity of smooth muscle, cardiac muscle, and glands. This subdivi- sion, commonly called the involuntary ner- vous system, itself has two parts, the sympa- thetic and parasympathetic, which typically bring about opposite effects. What one stimulates, the other inhibits. We will describe these later.

Although it is simpler to study the nervous sys- tem in terms of its subdivisions, remember that these subdivisions are made for the sake of convenience only. Remember that the nervous system acts as a coordinated unit, both structurally and functionally.

Did You Get It? 1. Name the structures that make up the CNS and those

that make up the PNS.

For the answer, see Appendix A.

Nervous Tissue: Structure and Function ➔ Learning Objective

□ Describe the structures and functions of neurons and neuroglia.

Even though it is complex, nervous tissue is made up of just two principal types of cells—supporting cells and neurons.

Supporting cells Supporting cells in the CNS are “lumped together” as neuroglia (nu-rog′le-ah), literally, “nerve glue,” also called glial cells or glia. Neuroglia include many types of cells that support, insulate, and pro- tect the delicate neurons (Figure 7.3). In addition, each of the different types of neuroglia has special functions. CNS neuroglia include the following:

• Astrocytes: abundant star-shaped cells that account for nearly half of neural tissue

make learning easier as you make your way through this chapter. Later you will see all these terms and concepts again in more detail.

Structural classification The structural classification, which includes all nervous system organs, has two subdivisions—the central nervous system and the peripheral nervous system (see Figure 7.2).

The central nervous system (CNS) consists of the brain and spinal cord, which occupy the dorsal body cavity and act as the integrating and com- mand centers of the nervous system. They interpret incoming sensory information and issue instructions based on past experience and current conditions.

The peripheral (pĕ-rif′er-al) nervous system (PNS) includes all parts of the nervous system out- side the CNS. It consists mainly of the nerves that extend from the spinal cord and brain. Spinal nerves carry impulses to and from the spinal cord. Cranial (kra′ne-al) nerves carry impulses to and from the brain. These nerves serve as communication lines. They link all parts of the body by carrying impulses from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles.

Functional classification The functional classification scheme is concerned only with PNS structures. It divides them into two principal subdivisions (see Figure 7.2).

The sensory division, or afferent (af′er-ent; literally “to go toward”) division, consists of nerves (composed of many individual nerve fibers) that convey impulses to the central nervous system from sensory receptors located in various parts of the body. The sensory division keeps the CNS con- stantly informed of events going on both inside and outside the body. Sensory fibers delivering impulses from the skin, skeletal muscles, and joints are called somatic (soma = body) sensory (afferent) fibers, whereas those transmitting impulses from the visceral organs are called vis- ceral sensory (afferent) fibers.

The motor division, or efferent (ef′er-rent) division, carries impulses from the CNS to effec- tor organs, the muscles and glands. These impulses activate muscles and glands; that is, they effect (bring about or cause) a motor response.

The motor division in turn has two subdivi- sions (see Figure 7.2):

254 Essentials of Human Anatomy and Physiology

recapturing chemicals released for communica- tion purposes.

• Microglia (mi-krog′le-ah): spiderlike phago- cytes that monitor the health of nearby neu- rons and dispose of debris, such as dead brain cells and bacteria (Figure 7.3b).

• Ependymal (ĕ-pen′dı̆-mal) cells: neuroglia that line the central cavities of the brain and the spinal cord (Figure 7.3c). The beating of their cilia helps to circulate the cerebrospinal fluid that fills those cavities and forms a pro- tective watery cushion around the CNS.

(Figure 7.3a). Their numerous projections have swollen ends that cling to neurons, bracing them and anchoring them to their nutrient sup- ply lines, the blood capillaries. Astrocytes form a living barrier between capillaries and neu- rons, help determine capillary permeability, and play a role in making exchanges between the two. In this way, they help protect the neu- rons from harmful substances that might be in the blood. Astrocytes also help control the chemical environment in the brain by “mop- ping up” leaked potassium ions, which are involved in generating a nerve impulse, and

(a) Astrocytes are the most abundant and versatile neuroglia.

(d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.

(e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.

(b) Microglial cells are phagocytes that defend CNS cells.

Schwann cells (forming myelin sheath)

Cell body of neuron

Satellite cells

Nerve fiber

Capillary

Neuron

Astrocyte

Neuron Microglial cell

Brain or spinal cord tissue

Ependymal cells

Nerve fibers

Myelin sheath

Process of oligodendrocyte

Fluid-filled cavity

Figure 7.3 Supporting cells (neuroglia) of nervous tissue.

Chapter 7: The Nervous System 255

filaments that are important in maintaining cell shape) are particularly abundant in the cell body.

Processes The armlike processes, or fibers, vary in length from microscopic to about 7 feet in the tallest humans. The longest ones in humans reach from the lumbar region of the spine to the great toe. Neuron processes that convey incoming messages (electrical signals) toward the cell body are dendrites (den′drı̄tz), whereas those that gen- erate nerve impulses and typically conduct them away from the cell body are axons (ak′sonz). Neurons may have hundreds of branching den- drites (dendr = tree), depending on the neuron type. However, each neuron has only one axon, which arises from a conelike region of the cell body called the axon hillock.

An occasional axon gives off a collateral branch along its length, but all axons branch profusely at their terminal end, forming hundreds to thousands of axon terminals. These terminals contain hun- dreds of tiny vesicles, or membranous sacs, that contain chemicals called neurotransmitters (review our discussion of events at the neuromus- cular junction in Chapter 6). As we said, axons transmit nerve impulses away from the cell body. When these impulses reach the axon terminals, they stimulate the release of neurotransmitters into the extracellular space between neurons, or between a neuron and its target cell.

Each axon terminal is separated from the next neuron by a tiny gap called the synaptic (sı̆- nap′tik) cleft. Such a functional junction, where an impulse is transmitted from one neuron to another, is called a synapse (syn = to clasp or join). Although they are close, neurons never actually touch other neurons. You will learn more about synapses and the events that occur there a bit later.

Myelin Sheaths Most long nerve fibers are cov- ered with a whitish, fatty material called myelin (mi′ĕ-lin), which has a waxy appearance. Myelin protects and insulates the fibers and increases the transmission rate of nerve impulses. Compare myelin sheaths to the many layers of insulation that cover the wires in an electrical cord; the layers keep the electricity flowing along the desired path just as myelin does for nerve fibers. Axons outside the CNS are myelinated by Schwann cells, as just noted. Many of these cells wrap themselves around the axon in a jelly-roll fashion (Figure 7.5, p. 257). Initially, the membrane coil is loose, but the

• Oligodendrocytes (ol″ ı̆-go-den′dro-sı̄tz): neuroglia that wrap their flat extensions (pro- cesses) tightly around the nerve fibers, produc- ing fatty insulating coverings called myelin sheaths (Figure 7.3d).

Although neuroglia somewhat resemble neu- rons structurally (both cell types have cell exten- sions), they are not able to transmit nerve impulses, a function that is highly developed in neurons. Another important difference is that neuroglia never lose their ability to divide, whereas most neurons do. Consequently, most brain tumors are gliomas, or tumors formed by neuroglia.

Supporting cells in the PNS come in two major varieties—Schwann cells and satellite cells (Figure 7.3e). Schwann cells form the myelin sheaths around nerve fibers in the PNS. Satellite cells act as protective, cushioning cells for peripheral neu- ron cell bodies.

Did You Get It? 2. Which neuroglia are most abundant in the body?

Which produce the insulating material called myelin? 3. Why is a brain tumor more likely to be formed from

neuroglia than from neurons?

For answers, see Appendix A.

Neurons Anatomy

➔ Learning Objectives □□ Describe the general structure of a neuron, and

name its important anatomical regions.

□□ Describe the composition of gray matter and white matter.

Neurons, also called nerve cells, are highly spe- cialized to transmit messages (nerve impulses) from one part of the body to another. Although neurons differ structurally from one another, they have many common features (Figure 7.4, p. 256). All have a cell body, which contains the nucleus and one or more slender processes extending from the cell body.

Cell Body The cell body is the metabolic center of the neuron. Its transparent nucleus contains a large nucleolus. The cytoplasm surrounding the nucleus contains the usual organelles, except that it lacks centrioles (which confirms the amitotic nature of most neurons). The rough ER, called Nissl (nis′l) bodies, and neurofibrils (intermediate

256 Essentials of Human Anatomy and Physiology

Figure 7.4 Structure of a typical motor neuron. (a) Diagrammatic view. (b) Scanning electron micrograph showing the cell body and dendrites (615*).

Cell body

Dendrite

Neurofibrils

Nissl substance

Axon hillock

Axon

Mitochondrion

Nucleus Nucleolus

Axon terminal

Collateral branch

Schwann cells, forming the myelin sheath on axon

One Schwann cell

Node of Ranvier

(a)

Dendrite

Neuron cell body

(b)

Chapter 7: The Nervous System 257

Terminology Clusters of neuron cell bodies and collections of nerve fibers are named differently in the CNS and in the PNS. For the most part, cell bodies are found in the CNS in clusters called nuclei. This well-protected location within the bony skull or vertebral column is essential to the well-being of the nervous system—remember that neurons do not routinely undergo cell division

Schwann cell cytoplasm is gradually squeezed from between the membrane layers. When the wrapping process is done, a tight coil of wrapped membranes, the myelin sheath, encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane. This part of the Schwann cell, external to the myelin sheath, is called the neurilemma (nu″rı̆-lem′mah, “neuron husk”). Because the myelin sheath is formed by many individual Schwann cells, it has gaps, or indentations, called nodes of Ranvier (rahn-vēr), at regular intervals (see Figure 7.4).

As mentioned previously, myelinated fibers are also found in the central nervous system. Oligodendrocytes form CNS myelin sheaths (see Figure 7.3d). In the PNS, it takes many Schwann cells to make a single myelin sheath; but in the CNS, the oligodendrocytes with their many flat extensions can coil around as many as 60 different fibers at the same time. Thus, in the CNS, one oligo- dendrocyte can form many myelin sheaths. Although the myelin sheaths formed by oligoden- drocytes and those formed by Schwann cells are similar, the CNS sheaths lack a neurilemma. Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the central nervous system.

Homeostatic Imbalance 7.1

The importance of myelin insulation is best illus- trated by observing what happens when myelin is not there. The disease multiple sclerosis (MS) gradually destroys the myelin sheaths around CNS fibers by converting them to hardened sheaths called scleroses. As this happens, the electrical cur- rent is short-circuited and may “jump” to another demyelinated neuron. In other words, nerve signals do not always reach the intended target. The affected person may have visual and speech distur- bances, lose the ability to control his or her muscles, and become increasingly disabled. Multiple sclero- sis is an autoimmune disease in which the person’s own immune system attacks a protein component of the sheath. As yet there is no cure, but injections of interferon (a hormonelike substance released by some immune cells) appear to hold the symptoms at bay and provide some relief. Other drugs aimed at slowing the autoimmune response are also being used, though further research is needed to deter- mine their long-term effects. ----------------------------- ✚

Figure 7.5 relationship of Schwann cells to axons in the peripheral nervous system. (a–c) As illustrated (top to bottom), a Schwann cell envelops part of an axon in a trough and then rotates around the axon. Most of the Schwann cell cytoplasm comes to lie just beneath the exposed part of its plasma membrane. The tight coil of plasma membrane material surrounding the axon is the myelin sheath. The Schwann cell cytoplasm and exposed membrane are referred to as the neurilemma.

Why does the myelin sheath that is produced by Schwann cells have gaps in it?Q:

Schwann cell cytoplasm

Axon

Neurilemma

Myelin sheath

Schwann cell nucleus

Schwann cell plasma membrane

(a)

(b)

(c)

The sheath is produced by several Schwann cells that arrange themselves end to end along the nerve fiber, each Schwann cell forming only one tiny segment of

the sheath.

258 Essentials of Human Anatomy and Physiology

Receptors

Central process (axon)

Spinal cord (central nervous system)

Interneuron (association neuron)

Afferent transmission

Efferent transmission

Sensory neuron

Ganglion

Peripheral process (axon)

Cell body

Dendrites

To effectors (muscles and glands)

Motor neuron

Peripheral nervous system

Figure 7.6 Neurons classified by function. Sensory (afferent) neurons conduct impulses from sensory receptors (in the skin, viscera, muscles) to the central nervous

system; most cell bodies are in ganglia in the PNS. Motor (efferent) neurons transmit impulses from the CNS (brain or spinal cord) to effectors in the body periphery. Interneurons

(association neurons) complete the communication pathway between sensory and motor neurons; their cell bodies reside in the CNS.

after birth. The cell body carries out most of the metabolic functions of a neuron, so if it is dam- aged, the cell dies and is not replaced. Small col- lections of cell bodies called ganglia (gang′le-ah; ganglion, singular) are found in a few sites outside the CNS in the PNS.

Bundles of nerve fibers (neuron processes) running through the CNS are called tracts, whereas in the PNS they are called nerves. The terms white matter and gray matter refer respec- tively to myelinated versus unmyelinated regions of the CNS. As a general rule, the white matter consists of dense collections of myelinated fibers (tracts), and gray matter contains mostly unmy- elinated fibers and cell bodies.

classification

➔ Learning Objectives □□ classify neurons according to structure and function.

□□ List the types of general sensory receptors and describe their functions.

Neurons may be classified based on their function or their structure.

Functional Classification Functionally, neurons are grouped according to the direction the nerve impulse travels relative to the CNS. On this basis, there are sensory, motor, and association neurons (interneurons) (Figure 7.6). Neurons carrying impulses from sensory receptors (in the internal organs or the skin) to the CNS are sensory neu- rons, or afferent neurons. (Recall that afferent means “to go toward.”) The cell bodies of sensory neurons are always found in a ganglion outside the CNS. Sensory neurons keep us informed about what is happening both inside and outside the body.

The dendrite endings of the sensory neurons are usually associated with specialized receptors that are activated by specific changes occurring nearby. (We cover the very complex receptors of the special senses—vision, hearing, equilibrium, taste, and smell—separately in Chapter 8). The simpler types of sensory receptors in the skin are cutaneous sense organs, and those in the muscles and tendons are proprioceptors (pro″pre-o-sep′torz) (shown in Figure 4.3 and Figure 7.7). The pain receptors (actually bare nerve

Chapter 7: The Nervous System 259

(a) Free nerve endings (pain and temperature receptors)

(b) Meissner’s corpuscle (touch receptor)

(d) Golgi tendon organ (proprioceptor)

(e) Muscle spindle (proprioceptor)

Figure 7.7 Types of sensory receptors. Parts (d) and (e) represent two types of proprioceptors.

endings) are the least specialized of the cutaneous receptors. They are also the most numerous, because pain warns us that some type of body damage is occurring or is about to occur. However, strong stimulation of any of the cutaneous recep- tors (for example, by searing heat, extreme cold, or excessive pressure) is also interpreted as pain.

The proprioceptors detect the amount of stretch, or tension, in skeletal muscles, their ten- dons, and joints. They send this information to the brain so that the proper adjustments can be made to maintain balance and normal posture. Propria

comes from the Latin word meaning “one’s own,” and the proprioceptors constantly advise our brain of our own movements.

Neurons carrying impulses from the CNS to the viscera and/or muscles and glands are motor neurons, or efferent neurons (see Figure 7.6). The cell bodies of motor neurons are usually located in the CNS.

The third category of neurons consists of the interneurons, or association neurons. They connect the motor and sensory neurons in neural

260 Essentials of Human Anatomy and Physiology

and distal (peripheral) processes. Unipolar neu- rons are unique in that only the small branches at the end of the peripheral process are dendrites. The remainder of the peripheral process and the central process function as the axon; thus, in this case, the axon actually conducts nerve impulses both toward and away from the cell body. Sensory neurons found in PNS ganglia are unipolar.

Did You Get It? 4. How does a tract differ from a nerve? 5. How does a ganglion differ from a nucleus? 6. Which part of a neuron conducts impulses toward the

cell body in multipolar and bipolar neurons? Which part releases neurotransmitters?

7. Your professor tells you that one neuron transmits a nerve impulse at the rate of 1 meter per second and another neuron conducts at the rate of 40 meters per second. Which neuron has the myelinated axon?

For answers, see Appendix A.

Physiology: Nerve Impulses

➔ Learning Objectives □□ Describe the events that lead to the generation of

a nerve impulse and its conduction from one neuron to another.

□□ Define reflex arc, and list its elements.

Neurons have two major functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and conductivity, the ability to transmit the impulse to other neu- rons, muscles, or glands.

Electrical Conditions of a Resting Neuron’s Membrane The plasma membrane of a resting, or inactive, neuron is polarized, which means that there are fewer positive ions sitting on the inner face of the neuron’s plasma membrane than there are on its outer face (Figure 7.9 1

). The major positive ions inside the cell are potassium (K+), whereas the major positive ions outside the cell are sodium (Na+). As long as the inside remains more negative (fewer positive ions) than the outside, the neuron will stay inactive.

Action Potential Initiation and Generation Many different types of stimuli excite neurons to become active and generate an impulse. For example, light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous

pathways. Their cell bodies are typically located in the CNS.

Structural Classification Structural classification is based on the number of processes, including both dendrites and axons, extending from the cell body (Figure 7.8). If there are several, the neuron is a multipolar neuron. Because all motor and asso- ciation neurons are multipolar, this is the most common structural type. Neurons with two pro- cesses—one axon and one dendrite—are bipolar neurons. Bipolar neurons are rare in adults, found only in some special sense organs (eye, nose), where they act in sensory processing as receptor cells. Unipolar neurons have a single process emerging from the cell body as if the cell body were on a “cul-de-sac” off the “main road” that is the axon. However, the process is very short and divides almost immediately into proximal (central)

Cell body Axon

Axon

(a) Multipolar neuron

Dendrites

Dendrite

Cell body

(b) Bipolar neuron

Axon

Short single process

Cell body Dendrites

Peripheral process

Central process

Figure 7.8 classification of neurons on the basis of structure.

Chapter 7: The Nervous System 261

+ + + + + + +

+ + + + + + + +

– – – – – – –

– – – – – – – –

– – + + + + +

+ + + + + + – –

+ + – – – – –

– – – – – – + + – – – – + + +

+ + + – – – – –

+ + + + – – –

– – – + + + + +

+ + + – – – –

– – – – + + + +

– – – + + + +

+ + + + – – – – + + + + + + +

+ + + + + + + +

– – – – – – –

– – – – – – – –

[Na+]

[K+]

Na+

Na++

K+

Na+

K+

Cell exterior

Cell interior

K+

Na+ Na+ Na+

Plasma membrane

Na+– K+

pump

N a+

D iff

us io

K +

D iff

us io

1 Resting membrane is polarized. In the resting state, the external face of the membrane is slightly positive; its internal face is slightly negative. The chief extracellular ion is sodium (Na+), whereas the chief intracellular ion is potassium (K+). The membrane is relatively impermeable to both ions.

Stimulus initiates local depolarization. A stimulus changes the permeability of a local "patch" of the membrane, and sodium ions diffuse rapidly into the cell. This changes the polarity of the membrane (the inside becomes more positive; the outside becomes more negative) at that site.

3 Depolarization and generation of an action potential. If the stimulus is strong enough, depolarization causes membrane polarity to be completely reversed, and an action potential is initiated.

4 Propagation of the action potential. Depolarization of the first membrane patch causes permeability changes in the adjacent membrane, and the events described in step are repeated. Thus, the action potential propagates rapidly along the entire length of the membrane.

5 Repolarization. Potassium ions diffuse out of the cell as the membrane permeability changes again, restoring the negative charge on the inside of the membrane and the positive charge on the outside surface. Repolarization occurs in the same direction as depolarization.

6 Initial ionic conditions restored. The ionic conditions of the resting state are restored later by the activity of the sodium-potassium pump. Three sodium ions are ejected for every two potassium ions carried back into the cell.

Figure 7.9 The nerve impulse.

262 Essentials of Human Anatomy and Physiology

faster because the nerve impulse literally jumps, or leaps, from node to node along the length of the fiber. This occurs because no electrical current can flow across the axon membrane where there is fatty myelin insulation. This faster type of electrical impulse propagation is called saltatory (sal′tah- to″re) conduction (saltare = to dance or leap).

Homeostatic Imbalance 7.2

A number of factors can impair the conduction of impulses. For example, sedatives and anesthetics block nerve impulses by altering membrane per- meability to ions, mainly sodium ions. As we have seen, no sodium entry = no action potential.

Cold and continuous pressure hinder impulse conduction because they interrupt blood circula- tion (and hence the delivery of oxygen and nutri- ents) to the neurons. For example, your fingers get numb when you hold an ice cube for more than a few seconds. Likewise, when you sit on your foot, it “goes to sleep.” When you warm your fingers or remove the pressure from your foot, the impulses begin to be transmitted again, leading to an unpleasant prickly feeling. ____________________✚

Transmission of the Signal at Synapses So far we have explained only the irritability aspect of neuronal functioning. What about conductivity— how does the electrical impulse traveling along one neuron get across the synapse to the next neuron (or effector cell) to influence its activity?

The answer is that the impulse doesn’t! Instead, a neurotransmitter chemical crosses the synapse to transmit the signal from one neuron to the next, or to the target cell (as occurred at the neuromuscular junction; see Chapter 6). When the action potential reaches an axon terminal (Figure 7.10 1

), the elec- trical change opens calcium channels. Calcium ions, in turn, cause the tiny vesicles containing neurotransmitter to fuse with the axonal membrane 1

, and porelike openings form, releasing the neu- rotransmitter into the synaptic cleft

. The neu- rotransmitter molecules diffuse across the synaptic cleft* and bind to receptors on the membrane of the next neuron

. If enough neurotransmitter is

receptors of the skin. However, most neurons in the body are excited by neurotransmitter chemicals released by other neurons, as we will describe shortly.

Regardless of the stimulus, the result is always the same—the permeability properties of the cell’s plasma membrane change for a very brief period. Normally, sodium ions cannot diffuse through the plasma membrane to any great extent, but when the neuron is adequately stimulated, the “gates” of sodium channels in the membrane open. Because sodium is in much higher concentration outside the cell, it then diffuses quickly into the neuron. (Remember the laws of diffusion?) This inward rush of sodium ions changes the polarity of the neuron’s membrane at that site, an event called depolariza- tion (Figure 7.9

). Locally, the inside is now more positive, and the outside is less positive, a local electrical situation called a graded potential. However, if the stimulus is strong enough and the sodium influx is great enough, the local depolariza- tion (graded potential) activates the neuron to initi- ate and transmit a long-distance signal called an action potential, also called a nerve impulse in neurons.

The nerve impulse is an all-or-none response, like starting a car. It is either propagated (conducted, or sent) over the entire axon

, or it doesn’t happen at all. The nerve impulse never goes partway along an axon’s length, nor does it die out with distance, as do graded potentials.

Almost immediately after the sodium ions rush into the neuron, the membrane permeability changes again, becoming impermeable to sodium ions but permeable to potassium ions. So potas- sium ions are allowed to diffuse out of the neuron into the interstitial fluid, and they do so very rap- idly. This outflow of positive ions from the cell restores the electrical conditions at the membrane to the polarized, or resting, state, an event called repolarization (Figure 7.9

). After repolarization of the electrical conditions, the sodium-potassium pump restores the initial concentrations of the sodium and potassium ions inside and outside the neuron

6 . This pump uses ATP (cellular energy) to pump excess sodium ions out of the cell and to bring potassium ions back into it. Until repolariza- tion occurs, a neuron cannot conduct another impulse. Once begun, these sequential events spread along the entire neuronal membrane.

The events just described explain propagation of a nerve impulse along unmyelinated fibers. Fibers that have myelin sheaths conduct impulses much

*Although most neurons communicate via the chemical type of synapse described above, there are some examples of electrical synapses, in which the neurons are physically joined by gap junctions and electrical currents actually flow from one neuron to the next.

Chapter 7: The Nervous System 263

released, the whole series of events described above (sodium entry 1

, depolarization, etc.) will occur, generating a graded potential and eventually a nerve impulse in the receiving neuron beyond the synapse. The electrical changes prompted by neurotransmitter binding are very brief because the neurotransmitter is quickly removed from the syn- aptic cleft

6 , either by diffusing away, by reuptake into the axon terminal, or by enzymatic break- down. This limits the effect of each nerve impulse to a period shorter than the blink of an eye.

Notice that the transmission of an impulse is an electrochemical event. Transmission down the length of the neuron’s membrane is basically electrical, but the next neuron is stimulated by a neurotransmitter, which is a chemical. Because each neuron both receives signals from and sends signals to scores of other neurons, it carries on “conversations” with many different neurons at the same time.

Physiology: reflexes

Although there are many types of communication between neurons, much of what the body must do every day is programmed as reflexes. Reflexes are rapid, predictable, and involuntary responses to stimuli. They are much like one-way streets— once a reflex begins, it always goes in the same direction. Reflexes occur over neural pathways called reflex arcs and involve both CNS and PNS structures. Think of a reflex as a preprogrammed response to a given stimulus.

The types of reflexes that occur in the body are classed as either somatic or autonomic. Somatic reflexes include all reflexes that stimulate the skel- etal muscles; these are still involuntary reflexes even though skeletal muscle normally is under voluntary control. When you quickly pull your hand away from a hot object, a somatic reflex is working. Autonomic reflexes regulate the activity of smooth muscles, the heart, and glands. Secretion of saliva (salivary reflex) and changes in the size of the eye pupils (pupillary reflex) are two such reflexes. Autonomic reflexes regulate such body functions as digestion, elimination, blood pressure, and sweating.

All reflex arcs have a minimum of five ele- ments (Figure 7.11a, p. 264): a receptor (which reacts to a stimulus), an effector (the muscle or gland eventually stimulated), and sensory and motor neurons to connect the two. The synapse or interneurons between the sensory and motor neu- rons represents the fifth element—the CNS inte- gration center.

Figure 7.10 How neurons communicate at chemical synapses. The events occurring at the synapse are numbered in order.

Receptor

Neurotransmitter

Na+ Na+

Neurotransmitter is broken down and released.

Neurotrans- mitter is released into synaptic cleft.

Neurotrans- mitter binds to receptor on receiving neuron's membrane.

Vesicle fuses with plasma membrane.

3 4

Action potential arrives.

Synaptic cleft

Neurotransmitter molecules

Ion channels

Receiving neuron

Transmitting neuron

Ion channel opens. Ion channel closes.5 6

Axon terminal

Axon of transmitting neuron

Receiving neuron

Dendrite

Synaptic cleft

Vesicles

264 Essentials of Human Anatomy and Physiology

Stimulus at distal end of neuron

Skin Spinal cord (in cross section)

Interneuron

Receptor

Effector

Sensory neuron

Motor neuron

Integration center

(a) Five basic elements of reflex arc

(b) Two-neuron reflex arc

Interneuron

Effector organ

Sensory (stretch) receptor

Sensory (afferent) neuron

Motor (efferent) neuron

Sensory receptor

2 Sensory (afferent) neuron

4 Motor (efferent) neuron

5 Effector organ

Figure 7.11 Simple reflex arcs.

Chapter 7: The Nervous System 265

central Nervous System Functional Anatomy of the Brain ➔ Learning Objective

□ Identify and indicate the functions of the major regions of the cerebral hemispheres, diencephalon, brain stem, and cerebellum on a human brain model or diagram.

The adult brain’s unimpressive appearance gives few hints of its remarkable abilities. It is about two good fistfuls of pinkish gray tissue, wrinkled like a walnut and with the texture of cold oatmeal. It weighs a little over 3 pounds. Because the brain is the largest and most complex mass of nervous tis- sue in the body, we commonly discuss it in terms of its four major regions—cerebral hemispheres, dien- cephalon (di″en-sef′ah-lon), brain stem, and cere- bellum (Figure 7.12, p. 266 and Table 7.1, p. 269).

cerebral Hemispheres

The paired cerebral (suh re′bral) hemispheres, collectively called the cerebrum, are the most superior part of the brain and together are a good deal larger than the other three brain regions com- bined. In fact, as the cerebral hemispheres develop and grow, they enclose and obscure most of the brain stem, so many brain stem structures cannot normally be seen unless a sagittal section is made. Picture how a mushroom cap covers the top of its stalk, and you have an idea of how the cerebral hemispheres cover the diencephalon and the superior part of the brain stem (see Figure 7.12).

The entire surface of the cerebrum exhibits elevated ridges of tissue called gyri (ji′re; gyrus, singular; “twisters”), separated by shallow grooves called sulci (sul′ki; sulcus, singular; “furrows”). Less numerous are the deeper grooves called fissures (Figure 7.13a, p. 267), which separate large regions of the brain. Many of the fissures and gyri are important anatomical landmarks. The cere- bral hemispheres are separated by a single deep fissure, the longitudinal fissure. Other fissures or sulci divide each cerebral hemisphere into a num- ber of lobes, named for the cranial bones that lie over them (see Figure 7.13a and b).

Each cerebral hemisphere has three basic regions: a superficial cortex of gray matter, which looks gray in fresh brain tissue; an internal area of white matter; and the basal nuclei, islands of gray

The simple patellar (pah-tel′ar), or knee-jerk, reflex is an example of a two-neuron reflex arc, the simplest type in humans (Figure 7.11b). The patellar reflex (in which the quadriceps muscle attached to the hit tendon is stretched) is familiar to most of us. It is usually tested during a physical exam to determine the general health of the motor portion of our nervous system.

Most reflexes are much more complex than the two-neuron reflex, involving synapses between one or more interneurons in the CNS (integration center). The flexor, or withdrawal, reflex is a three-neuron reflex arc in which the limb is with- drawn from a painful stimulus (see Figure 7.11c). A three-neuron reflex arc also consists of five ele- ments—receptor, sensory neuron, interneuron, motor neuron, and effector. Because there is always a delay at synapses (it takes time for neu- rotransmitter to diffuse through the synaptic cleft), the more synapses there are in a reflex pathway, the longer the reflex takes to happen.

Many spinal reflexes involve only spinal cord neurons and occur without brain involvement. As long as the spinal cord is functional, spinal reflexes, such as the flexor reflex, will work. By contrast, some reflexes require that the brain become involved because many different types of information have to be evaluated to arrive at the “right” response. The response of the pupils of the eyes to light is a reflex of this type.

As noted earlier, reflex testing is an important tool in evaluating the condition of the nervous system. Reflexes that are exaggerated, distorted, or absent indicate damage or disease in the ner- vous system. Reflex changes often occur before a pathological condition becomes obvious in other ways.

Did You Get It? 8. What is the difference between a graded potential

and an action potential? 9. Explain the difference between a synaptic cleft and a

synapse. How is a stimulus transmitted across a synapse?

10. Which portion(s) of a neuron is (are) likely to be associated with a sensory receptor or a sensory organ?

11. What is a reflex?

For answers, see Appendix A.

266 Essentials of Human Anatomy and Physiology

Midbrain

Cerebellum Cerebellum

Brain stem

Diencephalon

Brain stem

Cerebral hemisphere Cerebralhemisphere Outline of diencephalon

(a) 13 weeks (b) Adult brain

Figure 7.12 Development and regions of the human brain. The brain can be considered in terms of four main parts: cerebral hemispheres, diencephalon, brain stem, and cerebellum. (a) In the developing brain, the cerebral

hemispheres, initially smooth, are forced to grow posteriorly and laterally over the other brain regions by the bones of the skull. (b) In the adult brain, the cerebral hemispheres, now highly convoluted, enclose the diencephalon and the

superior part of the brain stem. The left cerebral hemisphere is drawn so that it looks transparent, to reveal the location of the deeply situated diencephalon and superior part of the brain stem.

matter situated deep within the white matter. We consider these regions next.

Cerebral Cortex Speech, memory, logical and emotional responses, consciousness, the interpre- tation of sensation, and voluntary movement are all functions of the cerebral cortex. Many of the functional areas of the cerebral hemispheres have been identified (Figure 7.13c). The primary somatic sensory area is located in the parietal lobe posterior to the central sulcus. Impulses traveling from the body’s sensory receptors (except for the special senses) are localized and inter- preted in this area of the brain. The primary somatic sensory area allows you to recognize pain, differences in temperature, or a light touch.

A spatial map, the sensory homunculus (ho-mung′ku-lus; “little man”), has been developed to show how much tissue in the primary somatic sensory area is devoted to various sensory func- tions. (Figure 7.14, p. 268; note that the body is represented in an upside-down manner). Body regions with the most sensory receptors—the lips and fingertips—send impulses to neurons that make up a large part of the sensory area. Furthermore, the sensory pathways are crossed pathways—meaning that the left side of the pri- mary somatic sensory area receives impulses from the right side of the body, and vice versa.

Impulses from the special sense organs are interpreted in other cortical areas (see Figure 7.13b and c). For example, the visual area is located in the posterior part of the occipital lobe, the auditory area is in the temporal lobe border- ing the lateral sulcus, and the olfactory area is deep inside the temporal lobe.

The primary motor area, which allows us to consciously move our skeletal muscles, is anterior to the central sulcus in the frontal lobe. The axons of these motor neurons form the major voluntary motor tract—the pyramidal tract, or corticospi- nal (kor″t ı̆-ko-spi′nal) tract, which descends to the cord. As in the primary somatic sensory cortex, the body is represented upside-down, and the pathways are crossed. Most of the neurons in the primary motor area control body areas having the finest motor control; that is, the face, mouth, and hands (see Figure 7.14). The body map on the motor cortex, as you might guess, is called the motor homunculus.

A specialized cortical area that is very involved in our ability to speak, Broca’s (bro′kahz) area, or motor speech area (see Figure 7.13c), is found at the base of the precentral gyrus (the gyrus anterior to the central sulcus). Damage to this area, which is located in only one cerebral hemisphere (usually the left), causes the inability to say words properly. You know what you want to say, but you can’t vocalize the words.

Chapter 7: The Nervous System 267

(b)

(c)

Primary motor area

Premotor area

Anterior association area

• Working memory and judgment

• Problem solving

• Language comprehension

Broca's area (motor speech)

Central sulcus Primary somatic sensory area

Gustatory area (taste)

Speech/language (outlined by dashes)

Posterior association area

Visual area

Auditory area

Olfactory area

Brain stem

Temporal lobe

Left cerebral hemisphere

Cerebellum

Occipital lobe

Parietal lobe

Superior

Inferior

(a)

Precentral gyrus

Central sulcus Postcentral gyrus

Parietal lobe

Parieto-occipital sulcus (deep)

Lateral sulcus

Occipital lobe

Temporal lobe

Cerebellum Pons

Medulla oblongata

Spinal cord

Frontal lobe

Cerebral white matter

Cerebral cortex (gray matter)

Fissure (a deep sulcus)

Gyrus

Sulcus

Figure 7.13 Left lateral view of the brain. (a) Diagrammatic view of major structural areas. (b) Photograph of a brain. (c) Functional areas of the cerebral hemisphere, diagrammatic view. More intense colors (red and blue) indicate primary cortical areas (motor and sensory areas). Pastel colors (pink and pale blue) represent association areas of the cerebral cortex.

268 Essentials of Human Anatomy and Physiology

Cerebral White Matter Most of the remaining cerebral hemisphere tissue—the deeper cerebral white matter (see Figures 7.13a and 7.15)—is composed of fiber tracts carrying impulses to, from, or within the cortex. One very large fiber tract, the corpus callosum (kah-lo′sum), connects the cerebral hemispheres (Figure 7.15). Such fiber tracts are called commissures. The corpus callosum arches above the structures of the brain stem and allows the cerebral hemispheres to communicate with one another. This is important because, as already noted, some of the cortical functional areas are in only one hemisphere. Association fiber tracts connect areas within a hemisphere, and projection fiber tracts connect the cerebrum with lower CNS centers, such as the brain stem.

Areas involved in higher intellectual reasoning and socially acceptable behavior are believed to be in the anterior part of the frontal lobes, the anterior association area. The frontal lobes also house areas involved with language comprehen- sion. Complex memories appear to be stored in the temporal and frontal lobes.

The posterior association area encompasses part of the posterior cortex. This area plays a role in recognizing patterns and faces, and blending several different inputs into an understanding of the whole situation. Within this area is the speech area, located at the junction of the temporal, pari- etal, and occipital lobes. The speech area allows you to sound out words. This area (like Broca’s area) is usually in only one cerebral hemisphere.

Genitals

F oot

Toes

Le g

H ip

Tr un

k N

ec k

H ea

d A

rm E

lb ow

Fo re

ar m

H an

Fi ng

er s

Th um

Ey e

No se

Fac e

Lips

Teeth Gums Jaw

Tongue

Pharynx

Intra- abdominalSwallowing

Tongue

Jaw

Lips

Face

Eye

Brow

Neck

Thumb

Fingers

H and

W rist

E lbow

A rm

S houlder

Trunk H

K nee

Primary motor cortex (precentral gyrus)

Primary somatic sensory cortex (postcentral gyrus)

Motor Motor map in precentral gyrus

Sensory Sensory map in postcentral gyrus

Posterior

Figure 7.14 Sensory and motor areas of the cerebral cortex. The relative amount of cortical tissue devoted to each function is indicated

by the amount of the gyrus occupied by the body area diagrams (homunculi). The primary motor cortex is shown on the left side of

the diagram, and the somatic sensory cortex is on the right.

Table 7.1 Functions of Major Brain regions

region Function

Cerebral hemispheres ■ cortex: Gray matter:

• Localizes and interprets sensory inputs • Controls voluntary and skilled skeletal muscle activity • Acts in intellectual and emotional processing

Basal nuclei:

• Subcortical motor centers help control skeletal muscle movements (see Figure 7.14)

Diencephalon ■ Thalamus:

• Relays sensory impulses to cerebral cortex • Relays impulses between cerebral motor cortex and lower motor centers • Involved in memory

■ Hypothalamus: • Chief integration center of autonomic (involuntary) nervous system • Regulates body temperature, food intake, water balance, and thirst • Regulates hormonal output of anterior pituitary gland and acts as an endocrine organ

(producing ADH and oxytocin)

■ Limbic system—A functional system: • Includes cerebral and diencephalon structures (e.g., hypothalamus and anterior thalamic nuclei) • Mediates emotional response; involved in memory processing

Brain stem ■ Midbrain:

• Contains visual and auditory reflex centers • Contains subcortical motor centers • Contains nuclei for cranial nerves III and IV; contains projection fibers (e.g., fibers of the pyramidal tracts)

■ Pons: • Relays information from the cerebrum to the cerebellum • Cooperates with the medullary centers to control respiratory rate and depth • Contains nuclei of cranial nerves V–VII; contains projection fibers

■ Medulla oblongata: • Relays ascending sensory pathway impulses from skin and proprioceptors • Contains nuclei controlling heart rate, blood vessel diameter, respiratory rate, vomiting, etc. • Relays sensory information to the cerebellum • Contains nuclei of cranial nerves VIII–XII; contains projection fibers • Site of crossover of pyramids

■ reticular formation—A functional system: • Maintains cerebral cortical alertness; filters out repetitive stimuli • Helps regulate skeletal and visceral muscle activity

Cerebellum ■ cerebellum:

• Processes information from cerebral motor cortex, proprioceptors, and visual and equilibrium pathways

• Provides “instructions” to cerebral motor cortex and subcortical motor centers, resulting in smooth, coordinated skeletal muscle movements

• Responsible for proper balance and posture

270 Essentials of Human Anatomy and Physiology

Did You Get It? 12. What are the three major regions of the cerebrum? 13. What is the composition of white matter of the brain?

For answers, see Appendix A.

Diencephalon

The diencephalon, or interbrain, sits atop the brain stem and is enclosed by the cerebral hemi- spheres (see Figure 7.12). The major structures of the diencephalon are the thalamus, hypothala- mus, and epithalamus (Figure 7.16). The thala- mus, which encloses the shallow third ventricle of the brain, is a relay station for sensory impulses passing upward to the sensory cortex. As impulses surge through the thalamus, we have a crude recognition of whether the sensation we are about to have is pleasant or unpleasant. It is the neurons of the sensory cortex that actually localize and interpret the sensation.

Basal Nuclei Although most of the gray matter is in the cerebral cortex, there are several “islands” of gray matter, called the basal nuclei, buried deep within the white matter of the cerebral hemi- spheres (see Figure 7.15). The basal nuclei help regulate voluntary motor activities by modifying instructions (particularly in relation to starting or stopping movement) sent to the skeletal muscles by the primary motor cortex. A tight band of pro- jection fibers, called the internal capsule, passes between the thalamus and the basal nuclei.

Homeostatic Imbalance 7.3

Individuals who have problems with their basal nuclei are often unable to walk normally or carry out other voluntary movements in a normal way. Huntington’s disease and Parkinson’s disease are two examples of such syndromes. (See “A Closer Look” on pp. 278–279. -----------------------------------------------✚

Figure 7.15 Frontal section (facing posteriorly) of the brain showing commissural, association, and projection fibers running through the cerebrum and the lower cNS. Notice the internal capsule that passes between the thalamus and the basal nuclei.

Corona radiata

Projection fibers

Longitudinal fissure Association fibers

Lateral ventricle

Fornix

Third ventricle

Thalamus

Pons

Medulla oblongata

Commissural fibers (corpus callosum)

Internal capsule

Superior

Basal nuclei

Brain Stem

The brain stem is about the size of a thumb in diameter and approximately 3 inches (approxi- mately 7.5 cm) long. Its structures are the mid- brain, pons, and medulla oblongata. In addition to providing a pathway for ascending and descend- ing tracts, the brain stem has many small gray mat- ter areas. These nuclei produce the rigidly programmed autonomic behaviors necessary for survival. In addition, some are associated with the cranial nerves and control vital activities such as breathing and blood pressure. Identify the brain stem areas (see Figure 7.16) as you read the descriptions that follow.

Midbrain A relatively small part of the brain stem, the midbrain extends from the mammil- lary bodies to the pons inferiorly. The cerebral aqueduct, a tiny canal that travels through the midbrain, connects the third ventricle of the dien- cephalon to the fourth ventricle below. Anteriorly, the midbrain is composed primarily of two bulging fiber tracts, the cerebral peduncles (pe′dun klz) (literally, “little feet of the cerebrum”), which con- vey ascending and descending impulses. Dorsally

The hypothalamus (literally, “under the thala- mus”) makes up the floor of the diencephalon. It is an important autonomic center because it plays a role in regulating body temperature, water bal- ance, and metabolism. The hypothalamus is also the center for many drives and emotions, and as such it is an important part of the so-called limbic system, or “emotional-visceral brain.” For example, thirst, appetite, sex, pain, and pleasure centers are in the hypothalamus. Additionally, the hypothalamus regulates the pituitary gland (an endocrine organ) and produces two hormones of its own. The pituitary gland hangs from the ante- rior floor of the hypothalamus by a slender stalk. (We discuss its function in Chapter 9.) The mam- millary bodies, reflex centers involved in olfac- tion (the sense of smell), bulge from the floor of the hypothalamus posterior to the pituitary gland.

The epithalamus (ep″ ı̆-thal′ah-mus) forms the roof of the third ventricle. Important parts of the epithalamus are the pineal gland (part of the endocrine system) and the choroid (ko′roid) plexus of the third ventricle. The choroid plex- uses, which are knots of capillaries within each of the four ventricles, form the cerebrospinal fluid.

Cerebral hemisphere

Corpus callosum

Choroid plexus of third ventricle Occipital lobe of cerebral hemisphere

Thalamus (encloses third ventricle) Pineal gland (part of epithalamus)

Corpora quadrigemina Cerebral aqueduct Cerebral peduncle

Fourth ventricle

Choroid plexus (part of epithalamus)

Cerebellum

Third ventricle

Anterior commissure

Hypothalamus

Optic chiasma

Pituitary gland

Mammillary body

Pons Medulla oblongata Spinal cord

Midbrain

(a)

Figure 7.16 Diencephalon and brain stem structures. (a) A midsagittal section of the brain illustrating the diencephalon (purple) and brain stem (green). (Figure continues on page 272.)

272 Essentials of Human Anatomy and Physiology

muscle in the digestive tract. A special group of reticular formation neurons, the reticular activat- ing system (RAS), plays a role in consciousness and the awake/sleep cycle (Figure 7.16b). The RAS also acts as a filter for the flood of sensory inputs that streams up the spinal cord and brain stem daily. Weak or repetitive signals are filtered out, but unusual or strong impulses do reach con- sciousness. Damage to this area can result in pro- longed unconsciousness (coma).

Cerebellum

The large, cauliflower-like cerebellum (ser″e- bel′um) projects dorsally from under the occipital lobe of the cerebrum. Like the cerebrum, the cer- ebellum has two hemispheres and a convoluted surface. The cerebellum also has an outer cortex made up of gray matter and an inner region of white matter.

The cerebellum provides the precise timing for skeletal muscle activity and controls our balance. Thanks to its activity, body movements are smooth and coordinated. It plays its role less well when it is sedated by alcohol. Fibers reach the cerebellum from the equilibrium apparatus of the inner ear, the eye, the proprioceptors of the skeletal muscles and tendons, and many other areas. The cerebellum can be compared to an automatic pilot, continu- ously comparing the brain’s “intentions” with actual body performance by monitoring body position and the amount of tension in various body parts. When needed, the cerebellum sends messages to initiate the appropriate corrective measures.

located are four rounded protrusions called the corpora quadrigemina (kor′por-ah kwah″drı̆- jem′ ı̆-nah) because they reminded some anatomist of two pairs of twins (gemini). These bulging nuclei are reflex centers involved with vision and hearing.

Pons The pons (ponz) is the rounded structure that protrudes just below the midbrain. Pons means “bridge,” and this area of the brain stem is mostly fiber tracts (bundles of nerve fibers in the CNS). However, it does have important nuclei involved in the control of breathing.

Medulla Oblongata The medulla oblongata (mĕ-dul′ah ob″long-gă′tah) is the most inferior part of the brain stem. It merges into the spinal cord below without any obvious change in struc- ture. Like the pons, the medulla is an important fiber tract area. Additionally, the medulla is the area where the important pyramidal tracts (motor fibers) cross over to the opposite side. The medulla also contains many nuclei that regulate vital vis- ceral activities. It contains centers that control heart rate, blood pressure, breathing, swallowing, and vomiting, among others. The fourth ventri- cle lies posterior to the pons and medulla and anterior to the cerebellum.

Reticular Formation Extending the entire length of the brain stem is a diffuse mass of gray matter, the reticular formation. The neurons of the retic- ular formation are involved in motor control of the visceral organs—for example, controlling smooth

Figure 7.16 (continued) (b) The reticular formation, which extends the length of the brain stem. Ascending arrows indicate sensory input to the cerebrum. Descending arrows indicate efferent output of reticular neurons.

Visual impulses

Reticular formation

Ascending general sensory tracts (touch, pain, temperature)

Descending motor projections to spinal cord

Auditory impulses

Radiations to cerebral cortex

(b)

Chapter 7: The Nervous System 273

collect venous blood, such as the superior sagittal sinus.

In several places, the inner dural membrane extends inward to form a fold that attaches the brain to the cranial cavity. Two of these folds, the falx (falks) cerebri and the tentorium cerebelli, separate the cerebellum from the cerebrum (shown in Figure 7.17).

The middle meningeal layer is the weblike arachnoid (ah-rak′noid) mater (see Figure 7.17). Arachnida means “spider,” and some think the arachnoid membrane looks like a cobweb. Its threadlike extensions span the subarachnoid space to attach it to the innermost membrane, the pia (pi′ah) mater (“gentle mother”). The delicate pia mater clings tightly to the surface of the brain and spinal cord, following every fold.

The subarachnoid space is filled with cerebro- spinal fluid. (Remember that the choroid plexuses produce CSF). Specialized projections of the arach- noid membrane, arachnoid granulations, pro- trude through the dura mater. The cerebrospinal fluid is absorbed into the venous blood in the dural sinuses through the arachnoid granulations. Next, we will discuss the production and flow of cerebrospinal fluid.

Homeostatic Imbalance 7.5

Meningitis, an inflammation of the meninges, is a serious threat to the brain because bacterial or viral meningitis may spread into the nervous tissue of the CNS. This condition of brain inflammation is called encephalitis (en-sef-ah-li′tis). Meningitis is usually diagnosed by taking a sample of cerebro- spinal fluid from the subarachnoid space sur- rounding the spinal cord. -----------------------------------------✚

cerebrospinal Fluid

Cerebrospinal (ser″e-bro-spi′nal) fluid (CSF) is a watery “broth” with components similar to blood plasma, from which it forms. However, it contains less protein and more vitamin C, and its ion com- position is different.

The choroid plexuses—clusters of capillaries hanging from the “roof” in each of the brain’s ven- tricles, or enlarged chambers—continually form CSF from blood. The CSF in and around the brain and cord forms a watery cushion that protects the fragile nervous tissue from blows and other

Homeostatic Imbalance 7.4

If the cerebellum is damaged (for example, by a blow to the head, a tumor, or a stroke), move- ments become clumsy and disorganized—a con- dition called ataxia (uh tax′e uh). Victims cannot keep their balance and may appear drunk because of the loss of muscle coordination. They are no longer able to touch their finger to their nose with eyes closed—a feat that healthy individuals accomplish easily. -------------------------------------------------------✚

Did You Get It? 14. Which brain region controls such vital activities as

breathing and blood pressure—cerebrum, brain stem, or cerebellum?

15. What is the function of the cerebellum? 16. In what major brain region are the thalamus,

hypothalamus, and pineal gland found?

For answers, see Appendix A.

Protection of the central Nervous System ➔ Learning Objectives

□□ Name the three meningeal layers, and state their functions.

□□ Discuss the formation and function of cerebrospinal fluid and the blood-brain barrier.

Nervous tissue is soft and delicate, and even slight pressure can injure the irreplaceable neurons. As we saw in Chapter 5, nature tries to protect the brain and spinal cord by enclosing them within bone (the skull and vertebral column). Now, let’s focus on three additional protections for the CNS: the menin- ges, cerebrospinal fluid, and blood-brain barrier.

Meninges

The three connective tissue membranes covering and protecting the CNS structures are meninges (mĕ-nin′jēz) (Figure 7.17, p. 274). The outermost layer, the leathery dura mater (du′rah ma′ter), meaning “tough or hard mother,” is a double-lay- ered membrane where it surrounds the brain. One of its layers is attached to the inner surface of the skull, forming the periosteum (periosteal layer). The other, called the meningeal layer, forms the outermost covering of the brain and continues as the dura mater of the spinal cord. The dural layers are fused together except in three areas where they separate to enclose dural venous sinuses that

274 Essentials of Human Anatomy and Physiology

Hydrocephalus (“water on the brain”). The ventricles would expand as cerebrospinal fluid, unable to drain

into the dural venous sinus, accumulated.

Figure 7.17 Meninges of the brain. (a) Three-dimensional frontal section showing the meninges—the dura mater, arachnoid mater, and pia mater—that surround and protect the brain. The relationship of the dura mater to the falx cerebri and the superior sagittal (dural) venous sinus is also shown. (b) Posterior view of the brain in place surrounded by the dura mater.

Superior sagittal sinus

Skin of scalp

Periosteum

Falx cerebri (in longitudinal fissure only)

Blood vessel

Arachnoid granulation

Pia mater

Arachnoid mater

Dura materMeningeal

Periosteal

Subdural space

Subarachnoid space

Bone of skull

(a)

Occipital lobe

Superior sagittal sinus

Dura mater

Transverse sinus

Temporal bone

(b)

Scalp

Skull

Tentorium cerebelli

Cerebellum

Arachnoid mater over medulla oblongata

What would be the consequence of blocked arachnoid granulations?Q:

trauma, and helps the brain “float” so it is not damaged by the pressure of its own weight.

Inside the brain, CSF is continually moving (as indicated by the numbered pathway in Figure 7.18c).

It circulates from the two lateral ventricles (in the cerebral hemispheres) into the third ventricle (in the diencephalon) and then through the cerebral aqueduct of the midbrain into the fourth ventricle dorsal to the pons and medulla oblongata. Some of the fluid reaching the fourth ventricle simply continues down into the spinal cord, but most of it

Chapter 7: The Nervous System 275

Figure 7.18 Ventricles and location of the cerebrospinal fluid. (a, b) Three-dimensional views of the ventricles of the brain.

(c) Circulatory pathway of the cerebrospinal fluid (indicated by arrows) within the central nervous system and the subarachnoid space.

(The relative position of the right lateral ventricle is indicated by the pale blue area deep to the corpus callosum.)

Anterior horn

Interventricular foramen

Inferior horn

Lateral aperture

(b) Left lateral view

Lateral ventricle

Septum pellucidum

Third ventricle

Cerebral aqueduct

(a) Anterior view

Fourth ventricle

Central canal

Inferior horn

Posterior horn

Median aperture

Lateral aperture

Superior sagittal sinus Arachnoid granulation

Subarachnoid space Arachnoid mater

Meningeal dura mater

Periosteal dura mater

Right lateral ventricle (deep to cut)

Choroid plexus of fourth ventricle

Central canal of spinal cord

Choroid plexuses of lateral and third ventricles

Interventricular foramen

Third ventricle

Cerebral aqueduct

Lateral aperture

Fourth ventricle

Median aperture

1 CSF is produced by the choroid plexus of each ventricle.

2 CSF flows through the ventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord.

3 CSF flows through the subarachnoid space.

4 CSF is absorbed into the dural venous sinuses via the arachnoid granulations.

Corpus callosum

276 Essentials of Human Anatomy and Physiology

in size. This is possible in an infant because the skull bones have not yet fused. However, in an adult this condition is likely to result in brain damage because the skull is hard, and the accu- mulating fluid creates pressure that crushes soft nervous tissue and could restrict blood flow into the brain. Today hydrocephalus is treated surgically by inserting a shunt (a plastic tube) to drain the excess fluid into a vein in the neck or abdomen. ------------------------------------------------------------------✚

The Blood-Brain Barrier

No other body organ is so absolutely dependent on a constant internal environment as is the brain. Other body tissues can withstand the rather small fluctuations in the concentrations of hormones, ions, and nutrients that continually occur, particu- larly after eating or exercising. If the brain were exposed to such chemical changes, uncontrolled neural activity might result—remember that cer- tain ions (sodium and potassium) are involved in initiating nerve impulses and that some amino acids serve as neurotransmitters.

Consequently, neurons are kept separated from bloodborne substances by the blood-brain barrier, composed of the least permeable capillaries in the whole body. These capillaries are almost seamlessly bound together by tight junctions all around. Of water-soluble substances, only water, glucose, and essential amino acids pass easily through the walls of these capillaries. Metabolic wastes, such as urea, toxins, proteins, and most drugs, are prevented from entering brain tissue. Nonessential amino acids and potassium ions not only are prevented from enter- ing the brain, but also are actively pumped from the brain into the blood across capillary walls. Although the bulbous “feet” of the astrocytes that cling to the capillaries may contribute to the barrier, the relative impermeability of the brain capillaries is most responsible for providing this protection.

The blood-brain barrier is virtually useless against fats, respiratory gases, and other fat-soluble molecules that diffuse easily through all plasma membranes. This explains why bloodborne alco- hol, nicotine, and anesthetics can affect the brain.

Did You Get It? 17. What name is given to the cerebrospinal fluid–filled

cavities within the brain? 18. What name is given to the barrier that protects the

brain from toxic chemicals?

circulates into the subarachnoid space through three openings, the paired lateral apertures and the median aperture (aper = open), in the walls of the fourth ventricle. The CSF returns to the blood in the dural venous sinuses through the arachnoid granulations. In this way, CSF is continually replaced.

Ordinarily, CSF forms and drains at a constant rate so that its normal pressure and volume (150 ml—about half a cup) are maintained. Any signifi- cant changes in CSF composition (or the appear- ance of blood cells in it) could indicate meningitis or certain other brain pathologies (such as tumors or multiple sclerosis). A procedure called a lumbar (spinal) puncture can obtain a sample of CSF for testing. Because the withdrawal of fluid decreases CSF fluid pressure, the patient must remain hori- zontal (lying down) for 6 to 12 hours after the pro- cedure to prevent an agonizingly painful “spinal headache.”

Homeostatic Imbalance 7.6

If something obstructs its drainage (for example, a tumor), CSF begins to accumulate and exert pressure on the brain. This condition is hydro- cephalus (hi-dro-sef′ah-lus), literally, “water on the brain.” Hydrocephalus in a newborn baby causes the head to enlarge as the brain increases

Hydrocephalus in an infant.

Chapter 7: The Nervous System 277

lucid following head trauma and then begin to deteriorate neurologically are most likely hem- orrhaging or suffering the delayed consequences of edema, both of which compress vital brain tissue.

Cerebrovascular Accidents Commonly called strokes, cerebrovascular (ser″e-bro-vas′ku-lar) accidents (CVAs) are the fifth leading cause of death in the United States. CVAs occur when blood circulation to a brain area is blocked, as by a blood clot or a ruptured blood vessel, and vital brain tissue dies. After a CVA, it is often possible to determine the area of brain damage by observing the patient’s symp- toms. For example, if the patient has left-sided paralysis (a one-sided paralysis is called hemi- plegia), the right motor cortex of the frontal lobe is most likely involved. Aphasias (ah-fa′ze-ahz) are a common result of damage to the left cere- bral hemisphere, where the language areas are located. There are many types of aphasias, but the most common are motor aphasia, which involves damage to Broca’s area and a loss of ability to speak, and sensory aphasia, in which a person loses the ability to understand written or spoken language. Aphasias are maddening to vic- tims because, as a rule, their intellect is unim- paired. Brain lesions can also cause marked changes in a person’s disposition (for example, a change from a sunny to a foul personality). In such cases, a tumor as well as a CVA might be suspected.

CVA mortality rates have declined over the past 15 years. More than 75 percent of patients who have a CVA survive 1 year, and more than 50 percent survive for 5 years. Some patients recover at least part of their lost faculties because undam- aged neurons spread into areas where neurons have died and take over some lost functions. Indeed, most of the recovery seen after brain injury is due to this phenomenon.

Not all strokes are “completed.” Temporary brain ischemia, or restricted blood flow, is called a transient ischemic attack (TIA). TIAs last from 5 to 50 minutes and are characterized by symp- toms such as numbness, temporary paralysis, and impaired speech. Although these defects are not permanent, they do constitute “red flags” that warn of impending, more serious CVAs. ______________ ✚

19. Which meningeal layer provides the means for draining cerebrospinal fluid back into the blood—dura mater, arachnoid mater, or pia mater?

For answers, see Appendix A.

Brain Dysfunctions ➔ Learning Objectives

□□ compare the signs of a cVA with those of Alzheimer’s disease; of a contusion with those of a concussion.

□□ Define EEG, and explain how it evaluates neural functioning.

Homeostatic Imbalance 7.7

Brain dysfunctions are unbelievably varied. We discuss some of them—the “terrible three”—in “A Closer Look” (pp. 278–279), and we will discuss developmental problems in the final section of this chapter. Here, we will focus on traumatic brain injuries and cerebrovascular accidents. We describe techniques used to diagnose many brain disorders in “A Closer Look” later in the chapter (pp. 296–297).

Traumatic Brain Injuries Head injuries are a leading cause of accidental death in the United States. Consider, for example, what happens if you crash into the rear end of another car. If you’re not wearing a seatbelt, your head will jerk forward and then violently stop as it hits the windshield. Brain trauma is caused not only by injury at the site of the blow, but also by the effect of the ricocheting brain hitting the oppo- site end of the skull.

A concussion occurs when brain injury is slight. The victim may be dizzy, “see stars,” or lose consciousness briefly, but typically little perma- nent brain damage occurs. A brain contusion results from marked tissue destruction. If the cere- bral cortex is injured, the individual may remain conscious, but severe brain stem contusions always result in a coma lasting from hours to a lifetime due to injury to the reticular activating system.

After head blows, death may result from intracranial hemorrhage (bleeding from rup- tured vessels) or cerebral edema (swelling of the brain due to inflammatory response to injury). Individuals who are initially alert and

278

Alzheimer’s (altz′hi-merz) dis-ease (AD) is a progressive degenerative disease of the brain that ultimately results in dementia (mental deterioration). Alzheimer’s patients represent between 5 and 15 percent of people over 65, and the disease is a major contributing cause in the deaths of as many as half of those over 85.

Its victims exhibit memory loss (particularly for recent events), a short attention span and disorienta- tion, and eventual language loss. Over several years, formerly good- natured people may become moody, confused, delusional, and sometimes violent.

AD is associated with a shortage of acetylcholine (ACh). AD also exhibits structural changes in the brain, partic- ularly in areas involved with thought and memory; the gyri shrink, and the brain atrophies. The precise cause of AD is unknown, but some cases appear to run in families.

Microscopic examinations of brain tissue reveal senile plaques (aggrega- tions of beta-amyloid peptide) littering the brain like shrapnel between the neurons. It has been frustratingly dif- ficult for researchers to uncover how beta-amyloid peptide acts as a neuro- toxin, particularly because it is also present in healthy brain cells (but there is less of it). Just what tips things off balance to favor production of more beta-amyloid is not understood, but it is known that this tiny peptide does its damage by enhancing cal- cium entry into certain brain neurons. High calcium levels can play a role in initiating apoptosis, or programmed cell death.

Another line of research has impli- cated a protein called tau, which appears to bind microtubule “tracks” together, much like railroad ties. In the brains of AD victims, tau grabs onto other tau molecules, forming spaghetti-like neurofibrillary tangles within neuron cell bodies. These

degenerative changes develop over several years, during which time fam- ily members watch the person they love “disappear.” It is a long and painful process. It is hoped that the lines of investigation, particularly stem cell research, will eventually merge and point to a treatment, but at present drugs that ease symptoms by inhibiting ACh breakdown are most useful.

Parkinson’s disease, an example of a basal nuclei problem, typically strikes people in their fifties and six- ties (actor Michael J. Fox is an unlucky exception who developed it around age 30). It results from a degenera- tion of specific neurons in the sub- stantia nigra of the midbrain, which normally supply dopamine to the basal nuclei. The dopamine-deprived basal nuclei, which help regulate vol- untary motor activity, become overac- tive, causing symptoms of the disease. Afflicted individuals have a persistent tremor at rest (exhibited by

The “Terrible Three” A

CLOSER LOOK

Did You Get It? 20. A person has been hit on the head with a baseball

and soon becomes comatose. Has he suffered a concussion or a contusion?

For the answer, see Appendix A.

Spinal cord ➔ Learning Objectives

□□ List two important functions of the spinal cord.

□□ Describe spinal cord structure.

The cylindrical spinal cord, which is approxi- mately 17 inches (42 cm) long, is a glistening white continuation of the brain stem. The spinal cord provides a two-way conduction pathway to

and from the brain, and it is a major reflex center (spinal reflexes are completed at this level). Enclosed within the vertebral column, the spinal cord extends from the foramen magnum of the skull to the first or second lumbar vertebra, where it ends just below the ribs (Figure 7.19, p. 280).

Like the brain, the spinal cord is cushioned and protected by meninges. Meningeal coverings do not end at the second lumbar vertebra (L2) but instead extend well beyond the end of the spinal cord in the vertebral canal. Because there is no possibility of damaging the cord beyond L3, the meningeal sac inferior to that point provides a nearly ideal spot for a lumbar puncture to remove CSF for testing.

279

movements called chorea (Greek for “dance”). Although the movements appear to be voluntary, they are not. Late in the disease, marked mental deterioration occurs, resulting in a lack of focus, fatigue, and irritability. Huntington’s disease is progressive, and the prognosis varies from 10 to 30 years from the onset of symptoms.

The signs and symptoms of early Huntington’s disease are essentially the opposite of Parkinson’s disease (overstimulation rather than inhibi- tion of the motor drive), and Hun- tington’s is usually treated with drugs that block, rather than enhance, dopamine’s effects. Late in the dis- ease, however, dopamine levels fall below normal levels, as seen in Par- kinson’s patients. As with Parkinson’s, fetal tissue or stem cell implants may help Huntington’s patients in the future.

As you can see, neurotransmitters, which are the “vocabulary” of neu- rons, can cause garbled neural lan- guage when things go wrong.

nausea, dizziness, and, in some, liver damage. In 2015, the FDA approved two new forms of L–dopa, RYTARY™ and Duopa™, which are extended- release versions of existing drugs. Both medications allow for more consistent dopamine levels and better control of symptoms.

Deep-brain (thalamic) stimulation via implanted electrodes alleviates tremors, but the procedure is risky and expensive. More promising are intrabrain transplants of embryonic substantia nigra tissue, genetically engineered adult nigral cells, or dopamine-producing cells from fetal pigs; all these have produced some regression of disease symptoms. How- ever, the use of fetal tissue is contro- versial and riddled with ethical and legal roadblocks.

Huntington’s disease is a genetic disease that strikes during middle age and leads to massive degeneration of the basal nuclei and later of the cerebral cortex. Its initial symptoms in many patients are wild, jerky, and almost continuous flapping

head nodding and a “pill-rolling” movement of the fingers), a forward- bent walking posture and shuffling gait, and a stiff facial expression. In addition, they have trouble initiating movement or getting their muscles going.

The cause of Parkinson’s disease is still unknown. The drug L-dopa, which is converted to dopamine in the brain, helps alleviate some of the symptoms. However, L-dopa is not a cure, and as more and more neurons die, it becomes ineffective. It also has undesirable side effects: severe

In humans, 31 pairs of spinal nerves arise from the cord and exit from the vertebral column to serve the body area close by. The spinal cord is about the size of a thumb for most of its length, but it is enlarged in the cervical and lumbar regions where the nerves serving the upper and lower limbs arise and leave the cord. Because the vertebral column grows faster than the spinal cord, the spinal cord does not reach the end of the ver- tebral column, and the spinal nerves leaving its inferior end must travel through the vertebral canal for some distance before exiting. This collec- tion of spinal nerves at the inferior end of the ver- tebral canal is called the cauda equina (kaw′da e-kwi′nah) because it looks so much like a horse’s tail (the literal translation of cauda equina).

Gray Matter of the Spinal cord and Spinal roots

The gray matter of the spinal cord looks like a but- terfly or the letter H in cross section (Figure 7.20, p. 281). The two posterior projections are the dor- sal horns, or posterior horns; the two anterior projections are the ventral horns, or anterior horns. The gray matter surrounds the central canal of the cord, which contains CSF.

Neurons with specific functions can be located in the gray matter. The dorsal horns contain inter- neurons. The cell bodies of the sensory neurons, whose fibers enter the cord by the dorsal root, are found in an enlarged area called the dorsal root ganglion. If the dorsal root or its ganglion is damaged, sensation from the body area served

280 Essentials of Human Anatomy and Physiology

Figure 7.19 Anatomy of the spinal cord, posterior view.

Cervical spinal nerves

Cervical enlargement

Lumbar spinal nerves

End of spinal cord

Sacral spinal nerves

Thoracic spinal nerves

Dura and arachnoid mater

Lumbar enlargement

Cauda equina

End of meningeal coverings

T12

will be lost. The ventral horns of the gray matter contain cell bodies of motor neurons of the somatic (voluntary) nervous system, which send their axons out the ventral root of the cord. The dorsal and ventral roots fuse to form the spinal nerves.

Homeostatic Imbalance 7.8

Damage to the ventral root results in flaccid paralysis of the muscles served. In flaccid paraly- sis, as described in Chapter 6, nerve impulses do not reach the muscles affected; thus, no voluntary movement of those muscles is possible. The mus- cles begin to atrophy because they are no longer stimulated. --------------------------------------------------------------✚

White Matter of the Spinal cord

White matter of the spinal cord is composed of myelinated fiber tracts—some running to higher centers, some traveling from the brain to the cord, and some conducting impulses from one side of the spinal cord to the other (Figure 7.21, p. 282).

Because of the irregular shape of gray matter, the white matter on each side of the cord is divided into three regions—the dorsal column, lateral column, and ventral column. Each of the columns contains a number of fiber tracts made up of axons with the same destination and func- tion. Tracts conducting sensory impulses to the brain are sensory, or afferent, tracts. Those carry- ing impulses from the brain to skeletal muscles are motor, or efferent, tracts. All tracts in the dorsal columns are ascending tracts that carry sensory input to the brain. The lateral and ventral columns contain both ascending and descending (motor) tracts.

Homeostatic Imbalance 7.9

If the spinal cord is transected (cut crosswise) or crushed, spastic paralysis results. The affected muscles stay healthy because they are still stimu- lated by spinal reflex arcs, and movement of those muscles does occur. However, movements are involuntary and not controllable, as described in Chapter 6. This can be as much of a problem as complete lack of mobility. In addition, because the spinal cord carries both sensory and motor impulses, a loss of feeling or sensory input occurs in the body areas below the point of cord destruction. Physicians often use a pin to see whether a person can feel pain after spinal cord injury—to find out whether regeneration is occurring. Pain is a hopeful sign in such cases. If

Chapter 7: The Nervous System 281

Figure 7.20 Spinal cord with meninges (three-dimensional, anterior view).

Dorsal root of spinal nerve

Spinal nerve

Dorsal root ganglion

Ventral root of spinal nerve

Dorsal (posterior) horn of gray matter

Lateral horn of gray matter

Central canal

Pia mater

Arachnoid mater

Dura mater

White matter

Ventral (anterior) horn of gray matter

the spinal cord injury occurs high in the cord, so that all four limbs are affected, the individual is a quadriplegic (kwod″r ı̆-ple′jik). If only the legs are paralyzed, the individual is a paraplegic (par″ă-ple′jik). ----------------------------------------------------------✚

Did You Get It? 21. What is found in the gray matter of the spinal cord? 22. Which spinal cord pathways are sensory pathways—

ascending or descending? 23. Why is the leash of nerve fibers at the end of the

spinal cord called the cauda equina?

For answers, see Appendix A.

Peripheral Nervous System The peripheral nervous system (PNS) consists of nerves and scattered ganglia (groups of neuro- nal cell bodies found outside the CNS). We have already considered one type of ganglion—the dor- sal root ganglion of the spinal cord. We will cover

others in the discussion of the autonomic nervous system. Here, we will concern ourselves only with nerves.

Structure of a Nerve ➔ Learning Objective

□ Describe the general structure of a nerve.

As noted earlier in this chapter, a nerve is a bun- dle of neuron fibers found outside the CNS. Within a nerve, neuron fibers, or processes, are wrapped in protective connective tissue coverings. Each fiber is surrounded by a delicate connective tis- sue sheath, an endoneurium (en″do-nu′re-um). Groups of fibers are bound by a coarser connec- tive tissue wrapping, the perineurium (per″ ı̆- nu′re-um), to form fiber bundles, or fascicles. Finally, all the fascicles are bound together by a tough fibrous sheath, the epineurium, to form the cordlike nerve (Figure 7.22, p. 283).

282 Essentials of Human Anatomy and Physiology

Figure 7.21 Schematic of ascending (sensory) and descending (motor) pathways between the brain and the spinal cord.

Cerebral cortex (gray matter)

Thalamus

White matter

Interneuron carrying response to motor neuron

Cell body of sensory neuron in sensory ganglion

Skin

Nerve

Sensory receptors

Motor output

Muscle

Interneuron Motor neuron cell body

Gray matter White matter

Interneuron carrying sensory information to cerebral cortex

Interneuron carrying response to motor neurons

Integration (processing and interpretation of sensory input) occurs

Interneuron carrying sensory information to cerebral cortex

Brain stem

Cerebrum

Cervical spinal cord

Chapter 7: The Nervous System 283

are tested, which is an important part of any neu- rological examination. You do not need to memo- rize these tests, but this information may help you understand cranial nerve function. As you read through the table, also look at Figure 7.23, p. 284, which shows the location of the cranial nerves on the brain’s anterior surface.

Most cranial nerves are mixed nerves; how- ever, three pairs—the optic, olfactory, and vestibu- locochlear (ves-tib″u-lo-kok′le-ar) nerves—are purely sensory in function. (The older name for the vestibulocochlear nerve is acoustic nerve, a name that reveals its role in hearing but not in equilibrium.)

This study tool may help you remember the cranial nerves in order. The first letter of each word in the saying is the first letter of the cranial nerve to be remembered: “Oh, oh, oh, to touch and feel very good velvet at home.”

Spinal Nerves and Nerve Plexuses ➔ Learning Objectives

□□ Describe the origin and fiber composition of (1) ventral and dorsal roots, (2) the spinal nerve proper, and (3) ventral and dorsal rami.

□□ Name the four major nerve plexuses, give the major nerves of each, and describe their distribution.

The 31 pairs of human spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord. Although each of the cranial nerves issuing from the brain is named specifi- cally, the spinal nerves are named for the region of the cord from which they arise. (Figure 7.24, p. 285, shows how the nerves are named in this scheme.)

Almost immediately after being formed, each spinal nerve divides into the dorsal ramus and ventral ramus (plural rami [ra′mi]), making each spinal nerve only about ½ inch long. The rami, like the spinal nerves, contain both motor and sensory fibers. Thus, damage to a spinal nerve or either of its rami results both in loss of sensation and in flaccid paralysis of the area of the body served. The smaller dorsal rami serve the skin and muscles of the posterior body trunk. The ventral rami of spinal nerves T1 through T12 form the intercostal nerves, which supply the muscles

cranial Nerves ➔ Learning Objective

□ Identify the cranial nerves by number and by name, and list the major functions of each.

The 12 pairs of cranial nerves primarily serve the head and neck. Only one pair (the vagus nerves) extends to the thoracic and abdominal cavities.

The cranial nerves are numbered in sequence, and in most cases their names reveal the most important structures they control. Table 7.2 (pp. 286–287) describes cranial nerves by name, number, course, and major function. The last column of the table describes how cranial nerves

Axon

Endoneurium

Perineurium

Epineurium

Myelin sheath

Blood vessels

Fascicle

Figure 7.22 Structure of a nerve. Three-dimensional view of a portion of a nerve, showing its connective tissue wrappings.

284 Essentials of Human Anatomy and Physiology

Figure 7.23 Distribution of cranial nerves. Sensory nerves are shown in blue, motor nerves in red. Although cranial nerves III, IV, and VI have sensory fibers, these are not shown because the sensory fibers account for only minor parts of these nerves.

I Olfactory II Optic

III Oculomotor

IV Trochlear VI Abducens

V Trigeminal

VII Facial

VIII Vestibulocochlear

IX Glossopharyngeal

X Vagus

XI AccessoryXII Hypoglossal

Vestibular branch

Cochlear branch

V Trigeminal

Chapter 7: The Nervous System 285

Figure 7.24 Spinal nerves. (a) Relationship of spinal nerves to the vertebrae. Areas of plexuses formed by the ventral rami are indicated. (b) Relative distribution of the ventral and dorsal rami of a spinal nerve (cross section of the left trunk).

*Note that the cervical nerve C 8 emerges inferior to the C

7 vertebra, while the other

seven cervical nerves emerge superior to the vertebrae for which they are named.

Ventral rami form cervical plexus (C1 – C5)

Ventral rami form brachial plexus (C5 – C8; T1)

Ventral rami form lumbar plexus (L1 – L4)

Ventral rami form sacral plexus (L4 – L5; S1 – S4)

No plexus formed (intercostal nerves) (T2 – T12 )

(b)

Dorsal root Dorsal ramusDorsal root

ganglion

Spinal cord

Ventral root

Spinal nerve

Ventral ramus

C1 2 3 4 5 6 7 8*

T1 2

S1 2

3 4

Cervical nerves

Thoracic nerves

Lumbar nerves

Sacral nerves

(a)

between the ribs and the skin and muscles of the anterior and lateral trunk. The ventral rami of all other spinal nerves form complex networks of nerves called plexuses, which serve the motor and sensory needs of the limbs. (The four nerve plexuses are described in Table 7.3, p. 288; three of the four plexuses are shown in Figure 7.25, p. 289.) (Text continues on page 290.)

286 Essentials of Human Anatomy and Physiology

Table 7.2 The cranial Nerves

Name/number origin/course Function Test

I. Olfactory Fibers arise from olfactory receptors in the nasal mucosa and synapse with the olfactory bulbs (which, in turn, send fibers to the olfactory cortex)

Purely sensory; carries impulses for the sense of smell

Subject is asked to sniff and identify aromatic substances, such as oil of cloves or vanilla

II. Optic Fibers arise from the retina of the eye and form the optic nerve. The two optic nerves form the optic chiasma by partial crossover of fibers; the fibers continue to the optic cortex as the optic tracts

Purely sensory; carries impulses for vision

Vision and visual field are tested with an eye chart and by testing the point at which the subject first sees an object (finger) moving into the visual field; eye interior is viewed with an ophthalmoscope

III. Oculomotor Fibers run from the midbrain to the eye

Supplies motor fibers to four of the six muscles (superior, inferior, and medial rectus, and inferior oblique) that direct the eyeball; to the eyelid; and to the internal eye muscles controlling lens shape and pupil size

Pupils are examined for size, shape, and size equality; pupillary reflex is tested with a penlight (pupils should constrict when illuminated); eye convergence is tested, as is the ability to follow moving objects

IV. Trochlear Fibers run from the midbrain to the eye

Supplies motor fibers for one external eye muscle (superior oblique)

Tested in common with cranial nerve III for the ability to follow moving objects

V. Trigeminal Fibers emerge from the pons and form three divisions that run to the face

Conducts sensory impulses from the skin of the face and mucosa of the nose and mouth; also contains motor fibers that activate the chewing muscles

Sensations of pain, touch, and temperature are tested with a safety pin and hot and cold objects; corneal reflex tested with a wisp of cotton; motor branch tested by asking the subject to open mouth against resistance and move jaw from side to side

VI. Abducens Fibers leave the pons and run to the eye

Supplies motor fibers to the lateral rectus muscle, which rolls the eye laterally

Tested in common with cranial nerve III for the ability to move each eye laterally

Chapter 7: The Nervous System 287

Name/number origin/course Function Test

VII. Facial Fibers leave the pons and run to the face

Activates the muscles of facial expression and the lacrimal and salivary glands; carries sensory impulses from the taste buds of anterior tongue

Anterior two-thirds of tongue is tested for ability to taste sweet, salty, sour, and bitter substances; subject is asked to close eyes, smile, whistle, etc.; tearing of eyes is tested with ammonia fumes

VIII. Vestibulocochlear Fibers run from the equilibrium and hearing receptors of the inner ear to the brain stem

Purely sensory; vestibular branch transmits impulses for the sense of balance, and cochlear branch transmits impulses for the sense of hearing

Hearing is checked by air and bone conduction, using a tuning fork

IX. Glossopharyngeal Fibers emerge from the medulla and run to the throat

Supplies motor fibers to the pharynx (throat) that promote swallowing and saliva production; carries sensory impulses from taste buds of the posterior tongue and from pressure receptors of the carotid artery

Gag and swallowing reflexes are checked; subject is asked to speak and cough; posterior tongue may be tested for taste

X. Vagus Fibers emerge from the medulla and descend into the thorax and abdominal cavity

Fibers carry sensory impulses from and motor impulses to the pharynx, larynx, and the abdominal and thoracic viscera; most motor fibers are parasympathetic fibers that promote digestive activity and help regulate heart activity

Tested in common with cranial nerve IX because they both serve muscles of the throat

XI. Accessory Fibers arise from the superior spinal cord (C1–C5)* and travel to muscles of the neck and back

Mostly motor fibers that activate the sternocleidomastoid and trapezius muscles

Sternocleidomastoid and trapezius muscles are checked for strength by asking the subject to rotate head and shrug shoulders against resistance

XII. Hypoglossal Fibers run from the medulla to the tongue

Motor fibers control tongue movements; sensory fibers carry impulses from the tongue

Subject is asked to stick out tongue, and any position abnormalities are noted

*Until recently, it was thought that the accessory nerves also received a contribution from cranial rootlets, but it has now been determined that in almost all people, these cranial rootlets are instead part of the vagus nerves. This raises an interesting question: Should the accessory nerves still be con- sidered cranial nerves? Some anatomists say yes because they pass through the cranium. Other anatomists say no because they don’t arise from the brain. Stay tuned!

Table 7.2 (continued)

288 Essentials of Human Anatomy and Physiology

Table 7.3 Spinal Nerve Plexuses

Plexus origin (from ventral rami) Important nerves Body areas served

result of damage to plexus or its nerves

Cervical C1–C5 Phrenic Diaphragm; skin and muscles of shoulder and neck

Respiratory paralysis (and death if not treated promptly)

Brachial C5–C8 and T1 Axillary Deltoid muscle and skin of shoulder; muscles and skin of superior thorax

Paralysis and atrophy of deltoid muscle

Radial Triceps and extensor muscles of the forearm; skin of posterior upper limb

Wristdrop—inability to extend hand at wrist

Median Flexor muscles and skin of forearm and some muscles of hand

Decreased ability to flex and abduct hand and flex and abduct thumb and index finger—therefore, inability to pick up small objects

Musculocutaneous Flexor muscles of arm; skin of lateral forearm

Decreased ability to flex forearm at elbow

Ulnar Some flexor muscles of forearm; wrist and many hand muscles; skin of hand

Clawhand—inability to spread fingers apart

Lumbar L1–L4 Femoral (including lateral and anterior cutaneous branches)

Lower abdomen, anterior and medial thigh muscles (hip flexors and knee extensors), and skin of anteromedial leg and thigh

Inability to extend leg and flex hip; loss of cutaneous sensation

Obturator Adductor muscles of medial thigh and small hip muscles; skin of medial thigh and hip joint

Inability to adduct thigh

Sacral L4–L5 and S1–S4

Sciatic (largest nerve in body; splits to common fibular and tibial nerves just above knee)

Lower trunk and posterior surface of thigh (hip extensors and knee flexors)

Inability to extend hip and flex knee; sciatica

• Common fibular (superficial and deep branches)

• Tibial (including sural and plantar branches)

Lateral aspect of leg and foot

Posterior aspect of leg and foot

Footdrop—inability to dorsiflex foot

Inability to plantar flex and invert foot; shuffling gait

Superior and inferior gluteal

Gluteus muscles of hip Inability to extend hip (maximus) or abduct and medially rotate thigh (medius)

Chapter 7: The Nervous System 289

Figure 7.25 Distribution of the major peripheral nerves of the upper and lower limbs.

Musculo- cutaneous nerve

Radial nerve

Humerus

Ulna

Ulnar nerve

Median nerve

Radius

(a) Brachial plexus, anterior view

Axillary nerve

Lateral femoral cutaneous nerve

Anterior femoral cutaneous nerve

Saphenous nerve

Femur

Obturator nerve

Femoral nerve

(b) Lumbar plexus, anterior view

Superior gluteal nerve

Inferior gluteal nerve

Common fibular nerve

Deep fibular nerve

Superficial fibular nerve

Plantar branches

Tibial nerve

Sural (cut) nerve

Posterior femoral cutaneous nerve

Sciatic nerve

290 Essentials of Human Anatomy and Physiology

to homeostasis, the relative stability of our inter- nal environment depends largely on the workings of the ANS. At every moment, signals flood from the visceral organs into the CNS, and the auto- nomic nervous system makes adjustments as nec- essary to best support body activities. For example, blood flow may be shunted to more “needy” areas, heart and breathing rate may be sped up or slowed down, blood pressure may be adjusted, and stomach secretions may be increased or decreased. Most of this fine-tuning occurs without our awareness or attention—few of us realize when our pupils dilate or our arteries constrict— hence the ANS is also called the involuntary nervous system, as noted at the beginning of this chapter.

Somatic and Autonomic Nervous Systems compared

Our previous discussions of nerves in the efferent (motor) division of the PNS have focused on the somatic nervous system, the PNS subdivision that controls our skeletal muscles. So, before plunging

Did You Get It? 24. Where is the epineurium located? 25. Which cranial nerve pair is the only one to serve

structures outside the head and neck? 26. What is a nerve plexus? 27. Ron has a horrible pain in his right buttock,

thigh, and leg. He is told he has sciatica. Which spinal nerve is involved, and what plexus does it belong to?

For answers, see Appendix A.

Autonomic Nervous System ➔ Learning Objective

□ Identify the site of origin, and explain the function of the sympathetic and parasympathetic divisions of the autonomic nervous system.

The autonomic nervous system (ANS) is the motor subdivision of the PNS that controls body activities automatically. It is composed of a spe- cialized group of neurons that regulate cardiac muscle (the heart), smooth muscles (found in the walls of the visceral organs and blood vessels), and glands. Although all body systems contribute

Figure 7.26 comparison of the somatic and autonomic nervous systems.

Somatic nervous system Skeletal muscle

Central nervous system Peripheral nervous system Effector organs

Acetylcholine

Smooth muscle (e.g., in stomach)

Acetylcholine

Ganglion

Adrenal medulla

GlandsBlood vessel

Cardiac muscle

Sympathetic division

Autonomic nervous system

Acetylcholine

Norepinephrine

Epinephrine and norepinephrine

Ganglion

Parasympathetic division

KEY: Preganglionic axons (sympathetic)

Postganglionic axons (sympathetic)

Myelination Preganglionic axons (parasympathetic)

Postganglionic axons (parasympathetic)

Chapter 7: The Nervous System 291

Figure 7.27 Anatomy of the autonomic nervous system. Parasympathetic fibers are shown in purple, sympathetic fibers in green. Solid lines represent preganglionic fibers; dashed lines indicate postganglionic fibers. Note the direct connection between the sympathetic nervous system and the adrenal gland.

Salivary glands

Eye

Skin

Heart

Lungs

Liver and gall- bladder

Genitals

Pancreas

Eye

Lungs

Bladder

Liver and gall- bladder

Pancreas

Stomach

Cervical

Sympathetic ganglia

Cranial

Lumbar

Thoracic

Genitals

Heart

Salivary glands

Stomach

Bladder

Adrenal gland

Parasympathetic Sympathetic

Sacral nerves (S2–S4)

Brain stem

into a description of autonomic nervous system anatomy, let’s note some important differences between the somatic and autonomic subdivisions of the PNS.

Besides differences in their effector organs and in the neurotransmitters they release, the patterns of their motor pathways differ. In the somatic divi- sion, the cell bodies of the motor neurons are inside the CNS, and their axons (in spinal nerves) extend all the way to the skeletal muscles they serve. The autonomic nervous system, however, has a chain of two motor neurons. The first motor neuron of each pair, the preganglionic neuron, is in the brain or spinal cord. Its axon, the pregan- glionic axon (literally, the “axon before the gan- glion”), leaves the CNS to form a synapse with the second motor neuron in a ganglion outside the

CNS. The axon of this ganglionic neuron, the postganglionic axon, then extends to the organ it serves. (Figure 7.26 summarizes these differ- ences.)

The autonomic nervous system has two arms, the sympathetic and the parasympathetic (Figure 7.27). Both serve the same organs but cause essentially opposite effects, counterbalancing each other’s activities to keep body systems run- ning smoothly. The sympathetic division mobilizes the body during extreme situations (such as fear, exercise, or rage), whereas the parasympathetic division allows us to “unwind” and conserve energy. We examine these differ- ences in more detail shortly, but first let’s con- sider the structural characteristics of the two arms of the ANS.

292 Essentials of Human Anatomy and Physiology

Now that we have described the anatomical details, we are ready to examine ANS functions in a little more detail.

Autonomic Functioning

➔ Learning Objective □ contrast the effect of the parasympathetic and

sympathetic divisions on the following organs: heart, lungs, digestive system, blood vessels.

Body organs served by the autonomic nervous system receive fibers from both divisions. Exceptions are most blood vessels and most struc- tures of the skin, some glands, and the adrenal medulla, all of which receive only sympathetic fibers (Table 7.4, p. 294). When both divisions serve the same organ, they cause antagonistic effects, mainly because their postganglionic axons release different neurotransmitters (see Figure 7.26). The parasympathetic fibers, called choliner- gic (ko″lin-er′jik) fibers, release acetylcholine. The sympathetic postganglionic fibers, called adrener- gic (ad″ren-er′jik) fibers, release norepinephrine (nor″ep-ı̆-nef′rin). The preganglionic axons of both divisions release acetylcholine. To emphasize the relative roles of the two arms of the ANS, we will focus briefly on situations in which each division is “in control.”

Sympathetic Division The sympathetic division is often referred to as the “fight-or-flight” system. Its activity is evident when we are excited or find ourselves in emergency or threatening situations, such as being frightened by a stranger late at night. A pounding heart; rapid, deep breathing; cold, sweaty skin; a prickly scalp; and dilated eye pupils are sure signs of sympathetic nervous sys- tem activity. Under such conditions, the sympa- thetic nervous system increases heart rate, blood pressure, and blood glucose levels; dilates the bronchioles of the lungs; and brings about many other effects that help the individual cope with the stressor. Other examples are dilation of blood ves- sels in skeletal muscles (so that we can run faster or fight better) and withdrawal of blood from the digestive organs (so that the bulk of the blood can be used to serve the heart, brain, and skeletal muscles).

The sympathetic nervous system is working at full speed not only when you are emotionally upset but also when you are physically stressed.

Anatomy of the Parasympathetic Division

The preganglionic neurons of the parasympa- thetic division are located in brain nuclei of sev- eral cranial nerves—III, VII, IX, and X (the vagus being the most important of these) and in the S2 through S4 levels of the spinal cord (see Figure 7.27). For this reason, the parasympathetic divi- sion is also called the craniosacral division. The neurons of the cranial region send their axons out in cranial nerves to serve the head and neck organs. There they synapse with the ganglionic motor neuron in a terminal ganglion. From the terminal ganglion, the postganglionic axon extends a short distance to the organ it serves. In the sacral region, the preganglionic axons leave the spinal cord and form the pelvic splanchnic (splank′nik) nerves, also called the pelvic nerves, which travel to the pelvic cavity. In the pelvic cav- ity, the preganglionic axons synapse with the sec- ond motor neurons in terminal ganglia on, or close to, the organs they serve.

Anatomy of the Sympathetic Division

The sympathetic division is also called the thora- columbar (tho″rah-ko-lum′bar) division because its preganglionic neurons are in the gray matter of the spinal cord from T1 through L2 (see Figure 7.27). The preganglionic axons leave the cord in the ventral root, enter the spinal nerve, and then pass through a ramus communicans, or small communicating branch, to enter a sympathetic trunk ganglion (Figure 7.28). The sympathetic trunk, or sympathetic chain lies alongside the vertebral column on each side. After it reaches the ganglion, the axon may synapse with the second (ganglionic) neuron in the sympathetic chain at the same level (Figure 7.28a) or a different level (Figure 7.28b), and the postganglionic axon then reenters the spinal nerve to travel to the skin. Or, the preganglionic axon may pass through the gan- glion without synapsing and form part of the splanchnic nerves (Figure 7.28c). The splanchnic nerves travel to the viscera to synapse with the ganglionic neuron, found in a collateral gan- glion anterior to the vertebral column. The major collateral ganglia—the celiac and the superior and inferior mesenteric ganglia—supply the abdominal and pelvic organs. The postganglionic axon then leaves the collateral ganglion and travels to serve a nearby visceral organ.

Chapter 7: The Nervous System 293

Homeostatic Imbalance 7.10

Some illnesses or diseases are at least aggravated, if not caused, by excessive sympathetic nervous system stimulation. Certain individuals, called type A people, always work at breakneck speed and push themselves continually. These are people who are likely to have heart disease, high blood pressure, and ulcers, all of which may be wors- ened by prolonged sympathetic nervous system activity or the rebound from it. ------------------------------✚

Parasympathetic Division The parasympathetic division is most active when the body is at rest and not threatened in any way. This division, some- times called the “rest-and-digest” system, is chiefly concerned with promoting normal digestion, with elimination of feces and urine, and with conserving body energy, particularly by decreasing demands

For example, if you have just had surgery or run a marathon, your adrenal glands (activated by the sympathetic nervous system) will be pumping out epinephrine and norepinephrine (see Figure 7.26). The effects of sympathetic nervous system activa- tion continue for several minutes until its hor- mones are destroyed by the liver. Thus, although sympathetic nerve impulses themselves may act only briefly, the hormonal effects they provoke lin- ger. The widespread and prolonged effects of sym- pathetic activation help explain why we need time to calm down after an extremely stressful situation.

The sympathetic division generates a head of steam that enables the body to cope rapidly and vigorously with situations that threaten homeosta- sis. Its function is to provide the best conditions for responding to some threat, whether the best response is to run, to see better, or to think more clearly.

Dorsal root

Ventral root

Splanchnic nerve

Sympathetic trunk ganglion

Dorsal ramus of spinal nerve

Ventral ramus of spinal nerve

To effector: blood vessels, arrector pili muscles, and sweat glands of the skin

Lateral horn of gray matter

White ramus communicans

Gray ramus communicans

Collateral ganglion (such as the celiac)

Visceral effector organ (such as small intestine)

(a)

(b)(c)

Sympathetic trunk

Spinal nerve

Figure 7.28 Sympathetic pathways. (a) Synapse in a sympathetic trunk ganglion at the same level. (b) Synapse in a sympathetic trunk ganglion at a different level. (c) Synapse in a collateral ganglion anterior to the vertebral column.

294 Essentials of Human Anatomy and Physiology

ecation, and diuresis [urination]) division and the sympathetic division as the E (exercise, excite- ment, emergency, and embarrassment) division. Remember, however, that although it is easiest to think of the sympathetic and parasympathetic divi- sions as working in an all-or-none fashion, this is rarely the case. A dynamic balance exists between the two divisions, and both are continuously mak- ing fine adjustments. Also, although we have described the parasympathetic division as the “at- rest” system, most blood vessels are controlled only by the sympathetic fibers regardless of whether the body is “on alert” or relaxing.

The major effects of each division are summa- rized in Table 7.4.

on the cardiovascular system. Its activity is best illustrated by a person who relaxes after a meal and reads the newspaper. Blood pressure and heart and respiratory rates are being regulated at low-normal levels, the digestive tract is actively digesting food, and the skin is warm (indicating that there is no need to divert blood to skeletal muscles or vital organs). The eye pupils are con- stricted to protect the retinas from excessive dam- aging light, and the lenses of the eyes are “set” for close vision. We might also consider the parasym- pathetic division as the “housekeeping” system of the body.

An easy way to remember the most important roles of the two ANS divisions is to think of the parasympathetic division as the D (digestion, def-

Table 7.4 Effects of the Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System

Target organ/system Parasympathetic effects Sympathetic effects

Digestive system Increases smooth muscle mobility (peristalsis) and amount of secretion by digestive system glands; relaxes sphincters

Decreases activity of digestive system and constricts digestive system sphincters (for example, anal sphincter)

Liver No effect Causes glucose to be released to blood

Lungs Constricts bronchioles Dilates bronchioles

Urinary bladder/urethra Relaxes sphincters (allows voiding) Constricts sphincters (prevents voiding)

Kidneys No effect Decreases urine output

Heart Decreases rate; slows and steadies Increases rate and force of heartbeat

Blood vessels No effect on most blood vessels Constricts blood vessels in viscera and skin (dilates those in skeletal muscle and heart); increases blood pressure

Glands—salivary, lacrimal, gastric

Stimulates; increases production of saliva, tears, and gastric juice

Inhibits; result is dry mouth and dry eyes

Eye (iris) Stimulates constrictor muscles; constricts pupils

Stimulates dilator muscles; dilates pupils

Eye (ciliary muscle) Stimulates to increase bulging of lens for close vision

Inhibits; decreases bulging of lens; prepares for distant vision

Adrenal medulla No effect Stimulates medulla cells to secrete epinephrine and norepinephrine

Sweat glands of skin No effect Stimulates to produce perspiration

Arrector pili muscles attached to hair follicles

No effect Stimulates; produces “goose bumps”

Penis Causes erection due to vasodilation Causes ejaculation (emission of semen)

Cellular metabolism No effect Increases metabolic rate; increases blood sugar levels; stimulates fat breakdown

Adipose tissue No effect Stimulates fat breakdown

Chapter 7: The Nervous System 295

malformations—triggered by genetic or environ- mental factors—also plague the CNS. Most serious are hydrocephalus (see p. 276), anencephaly, and spina bifida. Anencephaly is a birth defect in which the cerebrum fails to develop. Children with anencephaly cannot see, hear, or process sensory information; these babies typically die soon after birth. Spina bifida (spi′nah bı̆′f ı̆-dah; “forked spine”) results when the vertebrae form incom- pletely (typically in the lumbosacral region). There are several varieties of spina bifida. In the least serious, a dimple, and perhaps a tuft of hair, appears over the site of malformation, but no neu- rological problems occur. In the most serious, meninges, nerve roots, and even parts of the spi- nal cord protrude from the spine, rendering the lower part of the spinal cord functionless. The child is unable to control the bowels or bladder, and the lower limbs are paralyzed. -----------------------✚

One of the last areas of the CNS to mature is the hypothalamus, which contains centers for reg- ulating body temperature. For this reason, prema- ture babies usually have problems controlling their loss of body heat and must be carefully monitored. The nervous system grows and matures all through childhood, largely as a result of myelination that goes on during this period. A good indication of the degree of myelination of particular neural pathways is the level of neuromuscular control in that body area. Neuromuscular coordination

Did You Get It? 28. Which regions or organs of the body are served by

the autonomic nervous system? Which are served by the somatic nervous system?

29. How does the motor pathway of the autonomic nervous system differ from that of the somatic nervous system?

30. Which division of the autonomic nervous system is the “fight-or-flight” system?

For answers, see Appendix A.

Developmental Aspects of the Nervous System ➔ Learning Objectives

□□ List several factors that may have harmful effects on brain development.

□□ Briefly describe the cause, signs, and consequences of the following congenital disorders: spina bifida, anencephaly, and cerebral palsy.

□□ Explain the decline in brain size and weight that occurs with age.

□□ Define senility, and list some possible causes.

Because the nervous system forms during the first month of embryonic development, maternal infec- tion early in pregnancy can have extremely harm- ful effects on the fetal nervous system. For exam- ple, German measles (rubella) in the mother often causes deafness and other types of CNS damage. Also, because nervous tissue has the highest meta- bolic rate in the body, lack of oxygen for even a few minutes kills neurons. Because smoking decreases the amount of oxygen that can be car- ried in the blood, a smoking mother may be sen- tencing her infant to possible brain damage. Radiation and various drugs (alcohol, opiates, cocaine, and others) can also be very damaging if taken during early fetal development.

Homeostatic Imbalance 7.11

In difficult deliveries, temporary lack of oxygen may lead to cerebral palsy (pawl′ze), but this is only one of the suspected causes. Cerebral palsy is a neuromuscular disability in which the volun- tary muscles are poorly controlled and spastic because of brain damage. About half of its vic- tims have seizures, are intellectually disabled, and/or have impaired hearing or vision. Cerebral palsy is the largest single cause of physical dis- abilities in children. A number of other congenital

This adult patient with cerebral palsy presses a pad to communicate through a speaker.

296

When a doctor taps your patellar tendon with a reflex hammer, your thigh muscles contract, resulting in the knee-jerk response. This response shows that the spinal cord and upper brain centers are function- ing normally. When reflex tests are abnormal, more sophisticated neuro- logical tests may be ordered to try to localize and identify the problem.

An “oldie-but-goodie” procedure used to diagnose and localize many different types of brain lesions (such as epileptic lesions, tumors, and abscesses) is electroencephalography (e-lek″tro-en-sef-ah-lah′grah-fe). Normal brain function involves the continuous transmission of electrical impulses by neurons. A recording of that electrical activity, called an elec- troencephalogram (EEG), can be made by placing electrodes at vari- ous points on the scalp and connect- ing these to a recording device, as in part (a) of the figure. The patterns of electrical activity are called brain

waves. Because people differ geneti- cally, and because everything we have ever experienced has left its imprint in our brain, each of us has a brain wave pattern that is as unique as our fingerprints. The four most commonly seen brain waves are illus- trated and described in part (b) of the figure.

As might be expected, brain-wave patterns while a person is wide awake differ from those that occur during relaxation or deep sleep. Brain waves that are too fast or too slow suggest that the function of the cerebral cor- tex has been compromised, and unconsciousness occurs at both extremes. Sleep and coma result in brain-wave patterns that are slower than normal, whereas epileptic sei- zures and some kinds of drug over- dose cause abnormally fast brain waves. Because brain waves are seen even during coma, the absence of brain waves (a flat EEG) is taken as evidence of clinical death.

New imaging techniques (described in Chapter 1, pp. 43–44) have revolu- tionized the diagnosis of brain lesions. Together, computed tomography (cT) and magnetic resonance imag- ing (MrI) scans allow quick identifica- tion of most tumors, intracranial lesions, multiple sclerosis plaques, and areas of dead brain tissue (infarcts). Positron emission tomography (PET) scans can localize lesions that generate epileptic seizures.

Take, for example, a patient arriv- ing in the ER with a stroke. The first step is to image the brain, most often with CT, to determine whether the stroke is due to a clot or a bleed. If the stroke is due to a clot, then the clot-busting drug tPA can be used, but only within the first hours. cerebral angiography, which involves inject- ing a dye to make arteries in the brain stand out on X-ray imaging, might be used to visualize the clot or directly deliver tPA to the clot through a catheter. Less expensive and invasive

Tracking Down CNS Problems A

CLOSER LOOK

progresses in a superior to inferior direction and in a proximal to distal direction, and myelination occurs in the same sequence (as described in Chapter 6).

The brain reaches its maximum weight in the young adult. Over the next 60 years or so, neurons are damaged and die; and brain weight and vol- ume steadily decline. However, a nearly unlimited number of neural pathways is always available and ready to be developed, allowing us to con- tinue to learn throughout life.

As we grow older, the sympathetic nervous system gradually becomes less and less efficient, particularly in its ability to constrict blood vessels. When older people stand up quickly, they often become lightheaded or faint. The reason is that

the sympathetic nervous system is not able to react quickly enough to counteract the pull of gravity by activating the vasoconstrictor fibers, and blood pools in the feet. This condition, ortho- static hypotension, is a type of low blood pres- sure resulting from changes in body position as described. Orthostatic hypotension can be pre- vented to some degree if the person changes posi- tion slowly, giving the sympathetic nervous system time to adjust.

The usual cause of nervous system deteriora- tion is circulatory system problems. For example, arteriosclerosis (ar-ter″e-o-sklĕ-ro′sis; decreased elasticity of the arteries) and high blood pressure reduce the supply of oxygen to brain neurons. A gradual decline of oxygen due to the aging process

297

than angiography, ultrasound can be used to quickly examine the carotid arteries and even measure blood flow through them.

A new imaging procedure, DaTscan, can assist with diagnosing neurode- generative conditions involving

dopamine, such as Parkinson’s dis- ease. This imaging procedure uses a radioactive drug that binds dopamine transporters (DaT) paired with single- photon emission computed tomogra- phy (SPECT). The power of this technique is that it can image both

the amount and distribution of dopa- mine in the brain. Although DaTscan cannot diagnose Parkinson’s disease directly, it helps doctors distinguish Parkinson’s from other neurological conditions characterized by abnormal dopamine levels.

Electroencephalography and brain waves. (a) Electrodes are positioned on the scalp and attached to an electroencephalograph. (b) Typical EEGs. Alpha waves are typical of the awake, relaxed state; beta waves occur in the awake, alert state; theta waves are com- mon in children but not in normal adults; and delta waves occur during deep sleep.

alpha

beta

theta

delta

(b)(a)

1 second interval

can lead to senility, characterized by forgetful- ness, irritability, difficulty in concentrating and thinking clearly, and confusion. A sudden loss of blood and oxygen delivery to the brain results in a CVA (stroke), as described earlier. However, many people continue to enjoy intellectual lives and mentally demanding tasks their entire lives. In fact, fewer than 5 percent of people over age 65 dem- onstrate true senility.

Sadly, many cases of “reversible senility,” caused by certain drugs, low blood pressure, con- stipation, poor nutrition, depression, dehydration, and hormone imbalances, go undiagnosed. The best way to maintain your mental abilities in old age may be to seek regular medical checkups throughout life.

Although eventual shrinking of the brain is normal, some individuals (professional boxers and chronic alcoholics, for example) hasten the pro- cess. Whether a boxer wins the match or not, the likelihood of brain damage and atrophy increases with every blow. The expression “punch drunk” reflects the symptoms of slurred speech, tremors, abnormal gait, and dementia (mental illness) seen in many retired boxers.

Everyone recognizes that alcohol has a pro- found effect on the mind as well as the body. CT scans of chronic alcoholics reveal reduced brain size at a fairly early age. Like boxers, chronic alco- holics tend to exhibit signs of mental deterioration unrelated to the aging process.

298

SYSTEMS IN SYNC

Homeostatic Relationships between the Nervous System and Other Body Systems

• ANS helps regulate heart rate and blood pressure

• Cardiovascular system delivers blood containing oxygen and nutrients to the nervous system; carries away wastes

• ANS regulates bladder emptying and renal blood pressure

• Kidneys help to dispose of metabolic wastes and maintain proper electrolyte composition and pH of blood for neural functioning

• Somatic division of nervous system activates skeletal muscles; maintains muscle health

• Skeletal muscles are the effectors of the somatic division

• Nerves innervate lymphoid organs; the brain plays a role in regulating immune function

• Lymphatic vessels carry away leaked interstitial fluids from tissues surrounding nervous system structures; immune elements protect all body organs from pathogens (CNS has additional mechanisms)

• ANS (particularly the parasympathetic division) regulates digestive system activity

• Digestive system provides nutrients needed for healthy neurons

• ANS regulates sexual erection and ejaculation in males; erection of the clitoris in females

• Testosterone masculinizes the brain and underlies sex drive and aggressive behavior

• Nervous system regulates respiratory rhythm and depth

• Respiratory system provides life-sustaining oxygen; disposes of carbon dioxide

• Nerves innervate bones • Bones serve as depot for

calcium needed for neural function; protect CNS structures

• Sympathetic division of the ANS regulates sweat glands and blood vessels of skin (therefore heat loss/retention)

• Skin serves as heat loss surface

• Sympathetic division of the ANS activates the adrenal medulla; hypothalamus helps regulate the activity of the anterior pituitary gland and produces two hormones

• Hormones influence metabolism of neurons

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System Reproductive System

Respiratory System

Integumentary System

Endocrine System

Nervous System

Skeletal System

Chapter 7: The Nervous System 299

spinal cord, which acts as a reflex center, and the peripheral nerves, which provide communication links between the CNS and body periphery, are equally important to body homeostasis.

We have introduced a good deal of new termi- nology in this chapter, and, as you will see, much of it will come up again in later chapters as we study the other organ systems of the body and examine how the nervous system helps to regulate their activity. The terms are essential, so try to learn them as you go along. (Use the Glossary at the back of the book or on Mastering A&P as often as you find it helpful.)

Did You Get It? 31. Why must premature babies be placed in incubators

until their hypothalamus matures? 32. What is orthostatic hypotension? Why do many older

people suffer from this condition?

For answers, see Appendix A.

The human cerebral hemispheres—our “think- ing caps”—are awesome in their complexity. No less amazing are the brain regions that oversee all our subconscious, autonomic body functions—the diencephalon and brain stem—particularly when you consider their relatively insignificant size. The

Summary

organization of the Nervous System (pp. 252–253)

1. Structural: All nervous system structures are classi- fied as part of the CNS (brain and spinal cord) or PNS (nerves and ganglia).

2. Functional: Motor nerves of the PNS are classified on the basis of whether they stimulate skeletal muscle (somatic division) or smooth/cardiac mus- cle and glands (autonomic division).

Nervous Tissue: Structure and Function (pp. 253–265)

1. Supportive cells

a. Neuroglia support and protect neurons in the CNS. Some neuroglia are phagocytes; others myelinate neuron processes in the CNS or line cavities; others brace neurons and filter nutrients.

b. Schwann cells myelinate neuron processes in the PNS.

2. Neurons

a. Anatomy: All neurons have a cell body, which con- tains the nucleus, and processes (fibers) of two types: (1) dendrites (one to many per cell) typically carry electrical currents toward the cell body, and (2) axons (one per cell) generate and conduct impulses away from the cell body and often release a neurotransmitter. Most large fibers are myelin- ated; myelin speeds nerve impulse transmission.

b. Classification

(1) On the basis of function (direction of impulse transmission) there are sensory (afferent) and motor (efferent) neurons and interneurons (association neurons). Dendritic endings of sensory neurons are bare (pain receptors) or are associated with sensory receptors.

(2) On the basis of structure, there are unipo- lar, bipolar, and multipolar neurons; the ter- minology reveals the number of processes extending from the cell body. Motor and interneurons are multipolar; most sensory neurons are unipolar. The exceptions are sensory neurons in certain special sense or- gans (ear, eye), which are bipolar.

c. Physiology

(1) A nerve impulse is an electrochemical event (initiated by various stimuli) that causes a change in neuron plasma mem- brane permeability. This change allows sodium ions (Na+) to enter the cell, caus- ing depolarization. Once begun, the ac- tion potential, or nerve impulse, continues over the entire surface of the axon. Elec- trical conditions of the resting state are re- stored by the diffusion of potassium ions (K+) out of the cell (repolarization). Ion concentrations of the resting state are re- stored by the sodium-potassium pump.

Complete an interactive tutorial: > Study Area > Interactive Physiology > Nervous System > Generation of an Action Potential

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(2) A neuron influences other neurons or ef- fector cells by releasing neurotransmitters, chemicals that diffuse across the synaptic cleft and attach to membrane receptors on the postsynaptic cell. The result is opening of specific ion channels and activation or

300 Essentials of Human Anatomy and Physiology

d. The cerebellum is a large, cauliflower-like part of the brain posterior to the fourth ventricle. It coordinates muscle activity and body balance.

2. Protection of the CNS

a. Bones of the skull and vertebral column are the most external protective structures.

b. Meninges are three connective tissue mem- branes—dura mater (tough, outermost), arach- noid mater (middle, weblike), and pia mater (innermost, delicate). The meninges extend beyond the end of the spinal cord.

c. Cerebrospinal fluid (CSF) provides a watery cushion around the brain and cord. CSF is formed by the choroid plexuses of the brain. It is found in the subarachnoid space, ventricles, and central canal. CSF is continually formed and drained into dural venous sinuses.

d. The blood-brain barrier is composed of rela- tively impermeable capillaries.

3. Brain dysfunctions

a. Head trauma may cause concussions (reversible damage) or contusions (irreversible damage). When the brain stem is affected, unconscious- ness (temporary or permanent) occurs. Trauma- induced brain injuries may be aggravated by intracranial hemorrhage or cerebral edema, both of which compress brain tissue.

b. Cerebrovascular accidents (CVAs, or strokes) result when blood circulation to brain neurons is blocked and brain tissue dies. The result may be visual impairment, paralysis, and aphasias (prob- lems with speech and/or language).

c. Alzheimer’s disease is a degenerative brain dis- ease in which abnormal protein deposits and other structural changes appear. It results in slow, progressive loss of memory and motor control plus increasing dementia.

d. Techniques used to diagnose brain dysfunctions include the EEG, simple reflex tests, angiogra- phy, and DaTscan, CT, PET, and MRI scans.

4. The spinal cord is a reflex center and conduction pathway. Found within the vertebral canal, the cord extends from the foramen magnum to L1 or L2. The cord has a central butterfly-shaped area of gray matter surrounded by columns of white matter, which carry sensory and motor tracts to and from the brain.

Peripheral Nervous System (pp. 281–295)

1. A nerve is a bundle of neuron processes wrapped in connective tissue coverings (endoneurium, peri- neurium, and epineurium).

inhibition, depending on the neurotransmit- ter released and the target cell.

(3) A reflex is a rapid, predictable response to a stimulus. There are two types—autonomic and somatic. A reflex arc has at least five components: receptor, sensory neurons, CNS integration center, motor neurons, and effec- tor. Normal reflexes indicate normal nervous system function.

central Nervous System (pp. 265–281)

1. The brain is located within the cranial cavity of the skull and consists of the cerebral hemispheres, dien- cephalon, brain stem structures, and cerebellum.

a. The two cerebral hemispheres form the largest part of the brain. Their surface, the cortex, is gray matter, and their interior is white matter. The cor- tex is convoluted and has gyri, sulci, and fissures. The cerebral hemispheres are involved in logical reasoning, moral conduct, emotional responses, sensory interpretation, and the initiation of vol- untary muscle activity. Several functional areas of the cerebral lobes have been identified (see p. 268). The basal nuclei, regions of gray matter deep within the white matter of the cerebral hemispheres, modify voluntary motor activity. Parkinson’s disease and Huntington’s disease are disorders of the basal nuclei.

b. The diencephalon is superior to the brain stem and is enclosed by the cerebral hemispheres. The major structures include the following:

(1) The thalamus, which encloses the third ven- tricle, is the relay station for sensory impulses passing to the sensory cortex for interpretation.

(2) The hypothalamus makes up the “floor” of the third ventricle and is the most important regulatory center of the autonomic nervous system (regulates water balance, metabo- lism, thirst, temperature, and the like).

(3) The epithalamus includes the pineal gland (an endocrine gland) and the choroid plexus of the third ventricle.

c. The brain stem is the short region inferior to the hypothalamus that merges with the spinal cord.

(1) The midbrain is most superior and is primar- ily fiber tracts (bundles of nerve fibers in the CNS connecting CNS structures).

(2) The pons is inferior to the midbrain and has fiber tracts and nuclei involved in respiration.

(3) The medulla oblongata is the most inferior part of the brain stem. In addition to fiber tracts, it contains autonomic nuclei involved in the regulation of vital life activities (breath- ing, heart rate, blood pressure, etc.).

Chapter 7: The Nervous System 301

review Questionsreview Questions

Multiple choice

More than one choice may apply.

1. Which of the following is an example of integration by the nervous system?

a. The feel of a cold breeze

b. Shivering and goose bumps in response to cold

c. Perceiving the sound of rain

d. The decision to go back for an umbrella

2. Where might a gray matter nucleus be located?

a. Alongside the vertebral column

b. Within the brain

c. Within the spinal cord

d. In the sensory receptors

3. Which of the following does the blood-brain barrier prevent from entering the brain?

a. Glucose

b. Urea

c. Nicotine

d. Alcohol

4. Histological examination of a slice of neural tissue reveals a bundle of nerve fibers held together by cells whose multiple processes wrap around several fibers and form a myelin sheath. The specimen is likely to be

a. a nucleus.

b. a ganglion.

c. a nerve.

d. a tract.

5. The pineal gland is located in the

a. hypothalamus.

b. mesencephalon.

c. epithalamus.

d. corpus callosum.

6. Choose the proper term to respond to the statements describing various brain areas.

a. cerebellum

b. corpora quadrigemina

c. corpus callosum

d. hypothalamus

e. medulla

f. midbrain

g. pons

h. thalamus

with some threat. Its activation increases heart rate and blood pressure. The preganglionic neu- rons are in the gray matter of the cord. The gan- glionic sympathetic neurons are in the sympathetic trunk or in collateral ganglia. Postganglionic axons secrete norepinephrine.

Developmental Aspects of the Nervous System (pp. 295–299)

1. Maternal and environmental factors may impair embryonic brain development. Oxygen deprivation destroys brain cells. Severe congenital brain dis- eases include cerebral palsy, anencephaly, hydro- cephalus, and spina bifida.

2. Premature babies have trouble regulating body temperature because the hypothalamus is one of the last brain areas to mature prenatally.

3. Development of motor control indicates the pro- gressive myelination and maturation of a child’s nervous system. Brain growth ends in young adult- hood. Neurons die throughout life and are not replaced; thus, brain mass declines with age.

4. Healthy aged people maintain nearly optimal intel- lectual function. Disease—particularly cardiovascu- lar disease—is the major cause of declining mental function with age.

2. Cranial nerves: 12 pairs of nerves that extend from the brain to serve the head and neck region. The exception is the paired vagus nerves, which extend into the thorax and abdomen.

3. Spinal nerves: 31 pairs of nerves are formed by the union of the dorsal and ventral roots of the spinal cord on each side. The spinal nerve proper is very short and splits into dorsal and ventral rami. Dorsal rami serve the posterior body trunk; ventral rami (except T2 through T12) form plexuses (cervical, brachial, lumbar, sacral) that serve the limbs.

4. Autonomic nervous system: Part of the PNS, com- posed of neurons that regulate the activity of smooth and cardiac muscle and glands. This system differs from the somatic nervous system in that there is a chain of two motor neurons from the CNS to the effector. Two subdivisions serve the same organs with different effects.

a. The parasympathetic division is the “housekeep- ing” system and is in control most of the time. This division maintains homeostasis by seeing that normal digestion and elimination occur and that energy is conserved (“rest and digest”). The first motor neurons are in the brain or the sacral region of the cord. The second motor neurons are in the terminal ganglia close to the organ served. All parasympathetic axons secrete acetylcholine.

b. The sympathetic division is the “fight-or-flight” subdivision, which prepares the body to cope

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302 Essentials of Human Anatomy and Physiology

14. Explain both the structural and functional classifications of the nervous system. In your explanation, include the subdivisions of each.

15. What is the basis for the functional classification of neurons?

16. Two major cell groups make up the nervous system—neurons and supporting cells such as astrocytes and Schwann cells. Which are “nervous” cells? Why? What are the major functions of the other cell group?

17. Briefly explain how nerve impulses are initiated and transmitted, and why conduction at synapses is always one-way.

18. Name four types of cutaneous sensory receptors. Which of the cutaneous receptor types is most numerous? Why?

19. What is the function of Broca’s area? In which part of the brain is it found?

20. Make a rough drawing of the left cerebral hemisphere. On your drawing, locate at least five different functional areas, and then indicate their specific functions.

21. Other than serving as a conduction pathway, what is a major function of the pons? Why is the medulla the most vital part of the brain?

22. What are the basal nuclei made of, and what is their function?

23. Describe how the brain is protected by bone, membranes, fluid, and capillaries.

24. What is gray matter? White matter? How does the arrangement of gray and white matter differ in the cerebral hemispheres and the spinal cord?

25. What are two functions of the spinal cord?

26. How many pairs of cranial nerves are there? Which are purely sensory? Which activates the chewing muscles? Which helps regulate heart rate and activity of the digestive tract?

27. What general area of the body do the cranial nerves, other than the vagus nerves, serve? What do the vagus nerves serve?

28. Which glial cells promote the circulation of the cerebrospinal fluid?

29. What region of the body is served by the dorsal rami of the spinal nerves? By the ventral rami?

30. Name the four major nerve plexuses formed by the ventral rami and the body region served by each.

____ 1. Region where there is a gross crossover of fibers of descending pyramidal tracts

____ 2. Control of temperature, autonomic nervous system reflexes, hunger, and water balance

____ 3. Houses the substantia nigra and cerebral aqueduct

____ 4. Relay stations for visual and auditory stimuli input; found in midbrain

____ 5. Houses vital centers for control of the heart, respiration, and blood pressure

____ 6. Brain area through which all the sensory input is relayed to get to the cerebral cortex

____ 7. Brain area most concerned with equilibrium, body posture, and coordination of motor activity.

7. The nerves that arise from the spinal lumbar plexus include the

a. sciatic.

b. femoral.

c. obturator.

d. gluteal.

8. Which contains only motor fibers?

a. Dorsal root

b. Dorsal ramus

c. Ventral root

d. Ventral ramus

9. The cranial nerve responsible for activating the lacrimal glands is the

a. trigeminal.

b. facial.

c. hypoglossal.

d. vagus.

10. Motor functions of the extensor muscles of the arm, forearm, and fingers would be affected by damage to which one of these nerves?

a. Radial

b. Axillary

c. Ulnar

d. Median

11. Which cells are responsible for the formation of myelin in the CNS?

a. Schwann cells

b. Oligodendrocytes

c. Satellite cells

d. Astrocytes

12. Which of the following is (are) found in a three- neuron reflex arc?

a. Sensory (efferent) neuron

b. Motor (efferent) neuron

c. Sensory receptor

d. Effector organ

Short Answer Essay

13. What distinguishes spinal nerves from cranial nerves?

Chapter 7: The Nervous System 303

34. How does a Schwann cell help insulate a nerve fiber?

35. Compare CVAs and TIAs in terms of causes, symptoms, and consequences.

36. Define senility. Name possible causes of permanent and reversible senility.

31. How does the autonomic nervous system differ from the somatic nervous system?

32. What is the difference in function of the sympathetic and parasympathetic divisions of the autonomic nervous system (a) in general and (b) as specifically relates to the operation of the cardiovascular and digestive systems?

33. The sympathetic and parasympathetic fibers serve the same organs. How can their opposing effects be explained?

37. Mrs. Jones has had a progressive decline in her mental capabilities in the past five or six years. At first, her family attributed her occasional memory lapses, confusion, and agitation to grief over her husband’s death six years earlier. When examined, Mrs. Jones was aware of her cognitive problems and was shown to have an IQ score approximately 30 points lower than would be predicted by her work history. A CT scan showed diffuse cerebral atrophy. The physician prescribed a mild tranquil- izer for Mrs. Jones and told her family that there was little else he could recommend. What is Mrs. Jones’s problem?

38. Joseph, a man in his early 70s, was having prob- lems chewing his food. He was asked to stick out his tongue. It deviated to the right, and its right side was quite wasted. What cranial nerve was impaired?

39. Andy is about to take the stage to play the guitar in front of the biggest audience he has ever played for. He can feel his heart pounding in his chest and his hands are sweating. Explain Andy’s physi- ological signs in terms of the autonomic nervous system.

40. A semiconscious young woman is brought to the hospital by friends after falling from a roof. She did not lose consciousness immediately, and she was initially lucid. After a while, though, she became confused and then unresponsive. What is a likely explanation of her condition?

41. During John’s checkup, one year after an accident severed his right accessory nerve, the physician noted severe muscle atrophy. What two prominent muscles have been affected?

42. Mrs. Chen, a new mother, brings her infant to the clinic because he has suffered repeated seizures. When questioned, she states that her labor was unusually long and difficult. What condition do you suspect? Will the infant’s condition worsen?

43. Jake has been recently administered medication through an injection in his right buttock. Now he is not being able to dorsiflex his right foot. His doctor suspects that he has footdrop. What could have happened?

44. Mr. Harrison is an 82-year-old bedridden gentle- man who has discovered a new interest in learn- ing about his body. While being tended by the visiting nurse, he remarks that the supporting cells in nervous tissue (such as Schwann cells and oligodendrocytes) act like the rubber coating around household wiring. What does he mean by this analogy?

45. Maria had an automobile accident that caused injury to her head. As a result, she could not under- stand what was said to her, though she was able to pronounce words herself. What was her probable condition? What caused it?

46. Why does exposure to toxins have more devastat- ing neural effects during early pregnancy than in late pregnancy?

47. Jason is the star of his hometown ice hockey team. During a game, he is hit with a hockey stick so hard that he hits the ice. When he tries to get up, he is unable to flex his left hip or extend his left knee, but he has no pain. Which nerve has been damaged?

48. Derek got hit in the back of the neck with a base- ball, and now he can’t shrug one shoulder. Which cranial nerve is involved?

49. When Taylor begins to feel drowsy while driving, she opens her window, turns up the volume of the car stereo, and sips her ice-cold water. How do these actions keep her awake?

50. As the aroma of freshly brewed coffee drifted by Joe’s nose, his mouth began to water, and his stom- ach started to rumble. Explain these reactions in terms of ANS activity.

critical Thinking and clinical Application Questions

304

Special Senses

People are responsive creatures. Hold freshly baked bread before us, and our mouths water. Sudden thunder makes us jump. These stimuli (the bread and thunder) and many others continually greet us and are interpreted by our nervous system.

We are told that we have five senses that keep us in touch with what is going on in the external world: touch, taste, smell, sight, and hearing. Actually, touch is a mixture of the general senses that we considered in Chapter 7—the temperature, pressure, and pain receptors of the skin and the

proprioceptors of muscles and joints. The other four “traditional” senses—smell, taste, sight, and hearing—are called special senses. Receptors for a fifth special sense, equilibrium, are housed in the ear, along with the organ of hearing. In con- trast to the small and widely distributed general receptors, the special sense receptors are either large, complex sensory organs (eyes and ears) or localized clusters of receptors (taste buds and olfactory epithelium).

In this chapter we focus on the functional anatomy of each special sense organ, but keep in

WHAT

HOW

WHY

The special senses respond to stimuli involved in vision, hearing, balance,

smell, and taste. A variety of receptors, housed in special sense

organs such as the eye, ear, and nose, help detect stimuli

in your surroundings.

Without your special senses, you

could not smell or taste your favorite food, appreciate

colors, or hear your favorite song.

InStructorS

New Building Vocabulary coaching Activities for this chapter are assignable in

Chapter 8: Special Senses 305

mind that sensory inputs overlap. What we experi- ence—our “feel” of the world—is a blending of stimulus effects.

The adult eye is a sphere that measures about 1 inch (2.5 cm) in diameter. Only the anterior one- sixth of the eye’s surface is normally seen. The rest of it is enclosed and protected by a cushion of fat and the walls of the bony orbit. The acces- sory structures of the eye include the extrinsic eye muscles, eyelids, conjunctiva, and lacrimal apparatus.

Anteriorly the eyes are protected by the eye- lids, which meet at the medial and lateral corners of the eye, the medial commissure (canthus) and lateral commissure (canthus), respectively (Figure 8.1). The space between the eyelids in an open eye is called the palpebral fissure. Projecting from the border of each eyelid are the eyelashes. Modified sebaceous glands associated with the eyelid edges are the tarsal glands. These glands produce an oily secretion that lubricates the eye (Figure 8.2a, p. 306). Ciliary glands, which are modified sweat glands, lie between the eyelashes (cilium = eyelash), and their ducts open at the eyelash follicles. On the medial aspect of each eye is the lacrimal caruncle (see Figure 8.1), a raised area containing sebaceous and sweat glands that produce an oily, whitish secretion that also lubri- cates the eye.

A delicate membrane, the conjunctiva (kon- junk–ti¿vah), lines the eyelids and covers part of the outer surface of the eyeball (Figure 8.2a). It ends at the edge of the transparent cornea by fus- ing with the corneal epithelium. The conjunctiva secretes mucus, which helps to lubricate the eye- ball and keep it moist.

PArt I: tHE EYE AnD VISIon Of all the senses, vision has been studied most. Nearly 70 percent of all sensory receptors in the body are in the eyes. The optic tracts that carry information from the eyes to the brain are massive bundles, containing over a million nerve fibers. We rely heavily on our sight and often have to “see it to believe it” (Figure 8.1).

Anatomy of the Eye External and Accessory Structures ➔ Learning Objective

□ When provided with a model or diagram, identify the accessory eye structures, and list the functions of each.

Eyelashes

Sclera (covered by conjunctiva)

Site where conjunctiva merges with cornea

Lateral commissure (canthus)

Iris

Medial commissure (canthus)

Lacrimal caruncle

Eyelid

Eyebrow

PupilPalpebral fissure

Figure 8.1 Surface anatomy of the eye and accessory structures.

➔ConCeptLinkRecall the three basic functions of the nervous system (Figure 7.1, p. 252). Each of the special senses gathers unique sensory information that, once integrated, will influence motor output. For example, if you saw a ball moving toward your head, this sensory input might result in a motor output that would move your body out of the path of the ball. Additionally, recall that each type of sensory information is processed in a special- ized area of the cerebrum (Figure 7.13c, p. 268). ➔

306 Essentials of Human Anatomy and Physiology

The lacrimal apparatus (Figure 8.2b) con- sists of the lacrimal gland and a number of ducts that drain lacrimal secretions into the nasal cavity. The lacrimal glands are located above the lateral end of each eye. They continually release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts. The tears flush across the eyeball into the lacrimal canalic- uli medially, then into the lacrimal sac, and finally into the nasolacrimal duct, which empties into the inferior meatus of the nasal cavity (see Figure 8.2b). Tears also contain mucus, antibodies, and lysozyme (li¿so-zı̄m), an enzyme that destroys bacteria. Thus, they cleanse and protect the eye surface as they moisten and lubricate it. When lac- rimal secretion increases substantially, tears spill over the eyelids and fill the nasal cavities, causing congestion and the “sniffles.” This happens when the eyes are irritated by foreign objects or chemi- cals and when we are emotionally upset. In the case of irritation, the enhanced tearing acts to wash away or dilute the irritating substance. The importance of “emotional tears” is poorly under- stood, but some suspect that crying is important in reducing stress. Anyone who has had a good cry would probably agree, but this has been difficult to prove scientifically.

Homeostatic Imbalance 8.2

Because the nasal cavity mucosa is continuous with that of the lacrimal duct system, a cold or nasal inflammation often causes the lacrimal mucosa to become inflamed and swell. This impairs the drainage of tears from the eye surface, causing “watery” eyes. ______________________ ✚

Six extrinsic eye muscles (external eye muscles) are attached to the outer surface of each eye. These muscles produce gross eye movements and make it possible for the eyes to follow a mov- ing object. Figure 8.3 gives the names, locations, actions, and cranial nerve serving each of the extrinsic muscles.

Did You Get It? 1. What is the role of the eyelids? 2. Which four accessory glands or structures help

lubricate the eye? 3. What is the role of lysozyme in tears? 4. What is the visual role of the external eye muscles?

For answers, see Appendix A.

Homeostatic Imbalance 8.1

Inflammation of the conjunctiva, called conjuncti- vitis, results in reddened, irritated eyes. Pinkeye, its infectious form caused by bacteria or viruses, is highly contagious. __________________________ ✚

(b)

Excretory duct of lacrimal gland

Eyelid

Eyelashes

Lacrimal gland

(a)

Anterior aspect

Tarsal glands

Conjunctiva

Lacrimal sacLacrimal gland

Excretory ducts of lacrimal gland

Lacrimal canaliculus

Nasolacrimal duct

Inferior meatus of nasal cavity

Nostril

Figure 8.2 Accessory structures of the eye. (a) Sagittal section of the accessory structures associated with the anterior part of the eye. (b) Anterior view of the lacrimal apparatus.

Chapter 8: Special Senses 307

The eye itself, called the eyeball, is a hollow sphere (Figure 8.4, p. 308). Its wall is composed of three tunics, or layers, and its interior is filled with fluids called humors that help to maintain its shape. The lens, the main focusing apparatus of the eye, is supported upright within the eye cavity, dividing it into two chambers.

Layers Forming the Wall of the Eyeball

Now that we have covered the general anatomy of the eyeball, we are ready to get specific.

Internal Structures: the Eyeball ➔ Learning Objectives

□□ name the layers of the wall of the eye, and indicate the major function of each.

□□ Explain how the functions of rods and cones differ.

□□ Define blind spot, cataract, and glaucoma.

□□ trace the pathway of light through the eye to the retina.

□□ Discuss the importance of an ophthalmoscopic examination.

Superior oblique muscle

Trochlea

Superior oblique tendon

Superior rectus muscle

Lateral rectus muscle

Conjunctiva

Optic nerve

Inferior rectus muscle

Inferior oblique muscle

Medial rectus muscle

Inferior rectus muscle

Axis at center of eye

Lateral rectus muscle

Common tendinous ring

(a) (b)

(c)

Lateral rectus

Medial rectus

Superior rectus

Inferior rectus

Inferior oblique

Superior oblique

Name Action

Moves eye laterally

Moves eye medially

Elevates eye and turns it medially

Depresses eye and turns it medially

Elevates eye and turns it laterally

Depresses eye and turns it laterally

VI (abducens)

III (oculomotor)

IV (trochlear)

Controlling cranial nerve

Figure 8.3 Extrinsic muscles of the eye. (a) Lateral view of the right eye.(b) Superior view of the right eye. The four rectus muscles originate from the common tendinous ring, a ringlike tendon at the back of the eye socket. (c) Summary of cranial nerve supply and actions of the extrinsic eye muscles.

308 Essentials of Human Anatomy and Physiology

(a)

Ciliary body

Ciliary zonule

Cornea

Iris

Pupil

Aqueous humor (in anterior segment)

Lens

Scleral venous sinus (canal of Schlemm)

Vitreous humor (in posterior segment)

Optic nerve

Fovea centralis

Retina

Choroid

Sclera

Optic disc (blind spot)

Central artery and vein of the retina

(b)

Ciliary zonule

Cornea Lens

Aqueous humor (in anterior segment)

Margin of pupil

Iris

Ciliary body Vitreous humor in posterior segment

Choroid

Fovea centralis

Optic disc

Optic nerve

Retina

Sclera

Figure 8.4 Internal anatomy of the eye (sagittal section). (a) Diagrammatic view. (b) Photograph.

Which layer of the eye would be the first to be affected by deficient tear production?Q:

The outermost fibrous layer (the sclera and especially its cornea), which normally is continuously washed by tears.

Chapter 8: Special Senses 309

The transparent inner neural layer of the ret- ina contains millions of receptor cells, the rods and cones, which are called photoreceptors because they respond to light (Figure 8.5, p. 310). Electrical signals pass from the photoreceptors via a two-neuron chain—bipolar cells and then gan- glion cells—before leaving the retina via the optic nerve and being transmitted to, and inter- preted by, the optic cortex. The result is vision.

The photoreceptor cells are distributed over the entire retina, except where the optic nerve leaves the eyeball; this site is called the optic disc. Since there are no photoreceptors at the optic disc, it results in a blind spot in our vision. When light from an object is focused on the optic disc, the object disappears from our view and we can- not see it.

The rods and cones are not evenly distributed in the retina. The rods are densest at the periph- ery, or edge, of the retina and decrease in number as the center of the retina is approached. The rods allow us to see in gray tones in dim light, and they provide our peripheral vision.

Homeostatic Imbalance 8.3

Anything that interferes with rod function hinders our ability to see at night. This condition, called night blindness, dangerously impairs our ability to drive safely at night. Its most common cause is prolonged vitamin A deficiency, which eventually causes the neural retina to deteriorate. Vitamin A is one of the building blocks of the pigments the photoreceptor cells need to respond to light (see “A Closer Look” on p. 311). Vitamin A supple- ments will restore function if taken before degen- erative changes in the neural retina occur. _____ ✚

Cones are discriminatory receptors that allow us to see the details of our world in color under bright light conditions. They are densest in the cen- ter of the retina and decrease in number toward the retinal edge. Lateral to each blind spot is the fovea centralis (fo¿ve-ah sen-tră¿lis), a tiny pit that contains only cones (see Figure 8.4). Consequently, this is the area of greatest visual acuity, or point of sharpest vision, and anything we wish to view critically is focused on the fovea centralis.

There are three varieties of cones. Each type is most sensitive to particular wavelengths of visible light (Figure 8.6). One type responds most

Fibrous Layer The outermost layer, called the fibrous layer, consists of the protective sclera (sklĕ¿rah) and the transparent cornea (kor¿ne-ah). The sclera (thick white connective tissue) is seen anteriorly as the “white of the eye.” The central anterior portion of the fibrous layer is crystal clear. This “window” is the cornea through which light enters the eye. The cornea is well supplied with nerve endings. Most are pain fibers, and when the cornea is touched, blinking and increased tear production occur. Even so, the cornea is the most exposed part of the eye, and it is very vulnerable to damage. Luckily, its ability to repair itself is extraordinary. Furthermore, the cornea is the only tissue in the body that is transplanted from one person to another without the worry of rejection. Because the cornea has no blood vessels, it is beyond the reach of the immune system.

Vascular Layer The middle, or vascular layer, of the eyeball, has three distinguishable regions. Most posterior is the choroid (ko¿roid), a blood- rich nutritive tunic that contains a dark pigment. The pigment prevents light from scattering inside the eye. Moving anteriorly, the choroid is modified to form two smooth muscle structures, the ciliary (sil¿e-er-e) body, which is attached to the lens by a suspensory ligament called the ciliary zonule, and the iris. The pigmented iris has a rounded opening, the pupil, through which light passes. Circularly and radially arranged smooth muscle fibers form the iris, which acts like the diaphragm of a camera. That is, it regulates the amount of light entering the eye so that we can see as clearly as possible in the available light. In close vision and bright light, the circular muscles contract, and the pupil constricts, or gets smaller. In distant vision and dim light, the radial fibers contract to enlarge (dilate) the pupil, which allows more light to enter the eye. Cranial nerve III (oculomotor) controls the muscles of the iris.

Sensory Layer The innermost sensory layer of the eye is the delicate two-layered retina (ret¿ ı̆- nah), which extends anteriorly only to the ciliary body. The outer pigmented layer of the retina is composed of pigmented cells that, like those of the choroid, absorb light and prevent light from scattering inside the eye. They also act as phago- cytes to remove dead or damaged receptor cells and store vitamin A needed for vision.

vigorously to blue light, another to green light. The third cone variety responds to a range includ- ing both green and red wavelengths of light. However, this is the only cone population to respond to red light at all, so these are called the “red cones.” Impulses received at the same time from more than one type of cone by the visual cortex are interpreted as intermediate colors, simi- lar to what occurs when two colors of paint are mixed. For example, simultaneous impulses from blue and red color receptors are seen as purple or violet tones. When all three cone types are stimu- lated, we see white. If someone shines red light into one of your eyes and green into the other, you will see yellow, indicating that the “mixing” and interpretation of colors occur in the brain, not in the retina.

Homeostatic Imbalance 8.4

Lack of all three cone types results in total color blindness, whereas lack of one cone type leads to partial color blindness. Most common is the lack of red or green receptors, which leads to two varieties of red-green color blindness. Red and green are seen as the same color—either red or

Neural layer of retina

Pigmented layer of retina

Central artery and vein of retina

Optic nerve

Sclera

Choroid

Optic disc

(b)

Rod

Cone

Pigmented layer of retina

Bipolar cells

Ganglion cells

Pathway of light

(a)

Figure 8.5 The three major types of neurons composing the retina. (a) Notice that light must pass through the thickness of the retina to excite the rods and cones. Electrical signals flow in the opposite direction: from the rods and cones to the bipolar cells and finally to the ganglion cells. The ganglion cells generate the nerve impulses that leave the eye via the optic nerve. (b) Schematic view of the posterior part of the eyeball illustrating how the axons of the ganglion cells form the optic nerve.

Li gh

t a bs

or pt

io n

by c

on e

po pu

la tio

ns Wavelength (nanometers)

Visible light

420 nm (blue cones)

530 nm (green cones)

560 nm (red cones)

400 450 500 550 600 650 700

Figure 8.6 Sensitivities of the three cone types to different wavelengths of visible light.

311

A CLOSER

LOOK Visual Pigments—The Actual Photoreceptors

The names of the tiny pho-toreceptor cells of the ret- ina reflect their shapes. As shown to the left, rods are slender, elongated neurons, whereas the fatter cones taper to pointed tips. In each type of photoreceptor, there is a region called an outer seg- ment, attached to the cell body. The outer segment cor- responds to a light-trapping dendrite, in which the discs containing the visual pigments are stacked like a row of pennies.

The behavior of the visual pigments is dramatic. When light strikes them, they lose their color, or are “bleached”; shortly afterward, they regen- erate their pigment. Absorp- tion of light and pigment bleaching cause electrical changes in the photoreceptor cells that ultimately cause nerve impulses to be transmit- ted to the brain for visual interpretation. Pigment regen- eration ensures that you are

not blinded and unable to see in bright sunlight.

A good deal is known about the structure and function of rhodopsin, the purple pigment found in rods (see figure below). It is formed from the union of a protein (opsin) and a modified vitamin A product (retinal). When combined in rhodopsin, retinal has a kinked shape that allows it to bind to opsin. But when light strikes rhodopsin, retinal straightens out and releases the protein. Once straight- ened out, the retinal continues its conversion until it is once again vita- min A. As these changes occur, the purple color of rhodopsin changes to the yellow of retinal and finally becomes colorless as the change to vitamin A occurs. Thus the term “bleaching of the pigment” accurately describes the color changes that occur when light hits the pigment. Rhodop- sin is regenerated as vitamin A is again converted to the kinked form of retinal and recombined with opsin in an ATP-requiring process. The cone pigments, although similar to rhodop- sin, differ in the specific kinds of pro- teins they contain.

Process of bipolar cell

Li gh

Discs containing visual pigments

Melanin granules

Pigment cell nucleus

Inner fibers

Rod cell body

Cone cell body

Synaptic endings

Rod cell body

Nuclei

Mitochondria

Outer segment

P ig

m en

te d

la ye

Outer fiber

In ne

r se

gm en

Retinal (visual yellow)

Releases

Opsin

Light absorption causes

Rhodopsin (visual purple)

Bleaching of the pigment

the vitreous body (see Figure 8.4). Vitreous humor helps prevent the eyeball from collapsing inward by reinforcing it internally. Aqueous humor is similar to blood plasma and is continually secreted by a special area of the choroid. Like the vitreous humor, it helps maintain intraocular (in–trah-ok¿u-lar) pressure, the pressure inside the eye. It also provides nutrients for the avascular lens and cornea. Aqueous humor is reabsorbed into the venous blood through the scleral venous sinus, or canal of Schlemm (shlĕm), which is located at the junction of the sclera and cornea.

Homeostatic Imbalance 8.6

If drainage of aqueous humor is blocked, fluid backs up like a clogged sink. Pressure within the eye may increase to dangerous levels and com- press the delicate retina and optic nerve. The resulting condition, glaucoma (glaw-ko¿mah; “vision going gray”), can lead to blindness unless detected early. Glaucoma is a common cause of blindness in the elderly. Unfortunately, many forms of glaucoma progress slowly and have almost no symptoms at first. Thus, sight deteriorates slowly and painlessly until the damage is done. Signs of advanced glaucoma include seeing halos around lights, headaches, and blurred vision. A simple instrument called a tonometer (to-nom¿e-ter) is used to measure the intraocular pressure, which should be tested yearly in people over 40. Glaucoma is commonly treated with eyedrops that increase the rate of aqueous humor drainage. Laser or surgical enlargement of the drainage channel is another option. ___________________ ✚

The ophthalmoscope (of-thal¿mo-sk–op) is an instrument that illuminates the interior of the eye- ball, allowing the retina, optic disc, and internal blood vessels at the fundus, or posterior wall of the eye, to be viewed and examined (Figure 8.7). Such an examination can detect certain pathologi- cal conditions, such as diabetes, arteriosclerosis, and degeneration of the optic nerve and retina.

Did You Get It? 5. What is the meaning of the term blind spot in relation

to the eye? 6. What function does the choroid of the vascular layer

have in common with the pigmented layer of the retina?

7. How do the rods and cones differ from each other?

For answers, see Appendix A.

green, depending on the cone type present. Many color-blind people are unaware of their condition because they have learned to rely on other cues— such as differences in intensities of the same color—to distinguish green from red, for example, on traffic signals. Because the genes regulating color vision are on the X (female) sex chromo- some, color blindness is a sex-linked condition. It occurs almost exclusively in males. ___________ ✚

Lens

Light entering the eye is focused on the retina by the lens, a flexible biconvex crystal-like structure. Recall the lens is held upright in the eye by the ciliary zonule and attached to the ciliary body (see Figure 8.4).

Homeostatic Imbalance 8.5

In youth, the lens is transparent and has the consis- tency of firm jelly, but as we age it becomes increas- ingly hard and cloudy. Cataracts, the loss of lens transparency, cause vision to become hazy and dis- torted and can eventually cause blindness. Other risk factors for forming cataracts include diabetes mellitus, frequent exposure to intense sunlight, and heavy smoking. Current treatment of cataracts is either special cataract glasses or surgical removal of the lens and replacement with a lens implant.

The cataract in this photo appears as a milky structure that seems to fill the pupil.

__________________________________________✚

The lens divides the eye into two segments, or chambers. The anterior (aqueous) segment, ante- rior to the lens, contains a clear watery fluid called aqueous humor. The posterior (vitreous) seg- ment, posterior to the lens, is filled with a gel-like substance called vitreous (vit¿re-us) humor, or

Chapter 8: Special Senses 313

close object tends to scatter and diverge, or spread out, and the lens must bulge more to make close vision possible (Figure 8.8b). To achieve this, the ciliary body contracts, allowing the lens to become more convex. This ability of the eye to focus spe- cifically for close objects (those less than 20 feet away) is called accommodation. The image formed on the retina as a result of the light-bending activity of the lens is a real image—that is, it is reversed from left to right, upside down (inverted), and smaller than the object (Figure 8.9, p. 314).

The normal eye is able to accommodate properly. However, vision problems occur when a

Physiology of Vision Pathway of Light through the Eye and Light Refraction ➔ Learning Objectives

□□ Describe image formation on the retina.

□□ Define the following terms: accommodation, astigmatism, emmetropia, hyperopia, myopia, and refraction.

When light passes from one substance to another substance that has a different density, its speed changes and its rays are bent, or refracted. Light rays are bent in the eye as they encounter the cor- nea, aqueous humor, lens, and vitreous humor.

The refractive, or bending, power of the cor- nea and humors is constant. However, that of the lens can be changed by changing its shape—that is, by making it more or less convex, so that light can be properly focused on the retina. The greater the lens convexity, or bulge, the more it bends the light. The flatter the lens, the less it bends the light.

The resting eye is “set” for distant vision. In gen- eral, light from a distant source (over 20 feet away) approaches the eye as parallel rays (Figure 8.8a), and the lens does not need to change shape to focus properly on the retina. However, light from a

Lateral

Blood vessels

Macula Optic disc Retina

Medial

Fovea centralis

Figure 8.7 The posterior wall (fundus) of the retina as seen with an ophthalmoscope. Notice the optic disc, from which the blood vessels radiate.

You would be using your close vision, so your lenses would be bulged and thus relatively thick.A:

Retina

(a)

(b)

Light from distant source

Light from near source

Focal point

As you look at this figure, are your lenses relatively thick or relatively thin?Q:

Figure 8.8 Relative convexity of the lens during focusing for distant and close vision. (a) Light rays from a distant object are nearly parallel as they reach the eye and can be focused without requiring changes in lens convexity. (b) Diverging light rays from close objects require that the lens bulge more to focus the image sharply on the retina.

314 Essentials of Human Anatomy and Physiology

Homeostatic Imbalance 8.7

Hemianopia (hem–e-ah-no¿pe-ah) is the loss of the same side of the visual field of both eyes, which results from damage to the visual cortex on one side only (as occurs in some strokes). Thus, the person would not be able to see things past the middle of the visual field on either the right or left side, depending on the site of the stroke. Such individuals should be carefully attended and warned of objects in the nonfunctional side of the visual field. Their food and personal objects should always be placed on their functional side, or they might miss them. ____________________________ ✚

lens is too strong (overconverging) or too weak (underconverging) or when there are structural problems of the eyeball (as described in “A Closer Look” on near- and farsightedness on p. 315–316.

Visual Fields and Visual Pathways to the Brain ➔ Learning Objective

□ trace the visual pathway to the visual cortex.

Axons carrying impulses from the retina are bun- dled together at the posterior aspect of the eyeball and leave the back of the eye as the optic nerve. At the optic chiasma (ki-as¿mah; chiasm = cross) the fibers from the medial side of each eye cross over to the opposite side of the brain. The fiber tracts that result are the optic tracts. Each optic tract contains fibers from the lateral side of the eye on the same side and the medial side of the opposite eye. The optic tract fibers synapse with neurons in the thalamus, whose axons form the optic radiation, which runs to the occipital lobe of the brain. There they synapse with the cortical cells, and visual interpretation, or seeing, occurs. (Figure 8.10 shows the visual pathway from the eye to the brain).

Each side of the brain receives visual input from both eyes—from the lateral field of the eye on its own side and from the medial field of the other eye. Also notice that each eye “sees” a slightly different view but that their visual fields overlap quite a bit. As a result of these two phe- nomena, humans have binocular vision. Binocular vision, literally “two-eyed vision,” provides for depth perception, also called “three-dimensional” vision, as our visual cortex fuses the two slightly different images delivered by the two eyes into one “picture.”

Figure 8.9 real image (reversed left to right, and upside down) formed on the retina. Notice that the farther away the object, the smaller its image on the retina.

Fixation point

Right eye Left eye

Optic nerve

Optic tract

Optic chiasma

Occipital lobe (visual cortex)

Optic radiation

Thalamus

Figure 8.10 Visual fields of the eyes and visual pathway to the brain (inferior view). Notice that the visual fields overlap considerably (area of binocular vision). Notice also the retinal sites at which a real image would be focused when both eyes are fixed on a close, pointlike object.

315

➤

A CLOSER

LOOK Bringing Things into Focus

The eye that focuses images cor-rectly on the retina is said to have emmetropia (em–ĕ-tro¿pe-ah), liter- ally, “harmonious vision.” Part (a) of the figure shows an emmetro- pic eye.

Nearsightedness, or myopia (mi–o¿pe-ah; “short vision”), occurs when the parallel light rays from dis- tant objects fail to reach the retina and instead are focused in front of it; see part (b) in the figure. Another way to think of this is the focal point of a myopic eye falls short of the retina. Therefore, distant objects appear

blurry to myopic people. Nearby objects are in focus, however, because the lens “accommodates” (bulges) to focus the image properly on the ret- ina. Myopia results from an eyeball that is too long, a lens that is too strong, or a cornea that is too curved. Correction requires concave corrective lenses that diverge the light rays before they enter the eye, so that they converge farther back on the retinal surface. In other words, nearsighted people see near objects clearly and need corrective lenses to focus on dis- tant objects.

Farsightedness, or hyperopia (hi–per-o¿pe-ah; “far vision”), occurs when the parallel light rays from dis- tant objects are focused behind the retina—at least in the resting eye, in which the lens is flat and the ciliary muscle is relaxed; see part (c) in the figure. Hyperopia usually results from an eyeball that is too short or from a “lazy” lens. People with hyperopia see distant objects clearly because their ciliary muscles contract continu- ously to increase the light-bending power of the lens, which moves the focal point forward onto the retina.

Correction

None required

Concave lens

Focal plane

(a) Emmetropic eye

(b) Myopic eye (nearsighted)

Convex lens

316

Correction of hyperopia requires con- vex corrective lenses that converge the light rays before they enter the eye. Thus, farsighted people can see faraway objects clearly and require corrective lenses to focus on nearby objects.

Unequal curvatures in different parts of the cornea or lens cause

However, the diverging rays from nearby objects are focused so far behind the retina that even at full “bulge,” the lens cannot focus the image on the retina. Therefore, nearby objects appear blurry, and hyperopic individuals are subject to eyestrain as their endlessly contract- ing ciliary muscles tire from overwork.

astigmatism (ah-stig¿mah-tizm). In this condition, blurry images occur because points of light are focused not as points on the retina but as lines (astigma = not a point). Special cylindrically ground lenses or contacts are used to correct astigmatism.

A CLOSER LOOK Bringing Things into Focus (continued)

Eye reflexes ➔ Learning Objective

□ Discuss the importance of the convergence and pupillary reflexes.

Both the internal and external (extrinsic) eye muscles are necessary for proper eye function. The autonomic nervous system controls the inter- nal muscles. As mentioned earlier, these muscles include those of the ciliary body, which alters lens curvature, and the radial and circular muscles of the iris, which control pupil size. The external mus- cles are the rectus and oblique muscles attached to the eyeball exterior (see Figure 8.3), which con- trol eye movements and make it possible to fol- low moving objects. They are also responsible for convergence, which is the reflexive movement of the eyes medially when we view close objects. When convergence occurs, both eyes are aimed toward the near object being viewed. The extrinsic muscles are controlled by somatic fibers of cranial nerves III, IV, and VI (see Figure 8.3).

When the eyes are suddenly exposed to bright light, the pupils immediately constrict; this is the photopupillary reflex. This protective reflex pre- vents excessively bright light from damaging the delicate photoreceptors. The pupils also constrict reflexively when we view close objects; this accommodation pupillary reflex provides more acute vision.

Reading requires almost continuous work by both sets of muscles. The muscles of the ciliary body bring about the lens bulge, and the circular (or constrictor) muscles of the iris produce the accommodation pupillary reflex. In addition, the extrinsic muscles must converge the eyes as well as move them to follow the printed lines. This is why long periods of reading tire the eyes and

often result in what is commonly called eyestrain. When you read for an extended time, look up from time to time and stare into the distance. This temporarily relaxes all the eye muscles.

Did You Get It? 8. What are the refractory media of the eye? 9. What name is given to the ability of the eye to focus

on close objects? 10. What is the difference between the optic tract and

the optic nerve? 11. In what way does the photopupillary reflex protect

the eyes? 12. How is astigmatism different from myopia and

hyperopia?

For answers, see Appendix A

PArt II: tHE EAr: HEArInG AnD BALAncE At first glance, the machinery for hearing and balance appears crude. Fluids must be stirred to stimulate the receptors of the ear: sound vibra- tions move fluid to stimulate hearing receptors, whereas gross movements of the head disturb flu- ids surrounding the balance organs. Receptors that respond to such physical forces are called mecha- noreceptors (mek–ah-no-re-sep¿terz).

Our hearing apparatus allows us to hear an extraordinary range of sound, and our highly sen- sitive equilibrium receptors keep our nervous sys- tem continually up to date on the position and movements of the head. Without this information, it would be difficult if not impossible to maintain our balance or to know which way is up. Although these two sense organs are housed together in the ear, their receptors respond to different stimuli and are activated independently of one another.

Chapter 8: Special Senses 317

Anatomy of the Ear ➔ Learning Objective

□ Identify the structures of the external, middle, and internal ear, and list the functions of each.

Anatomically, the ear is divided into three major areas: the external, or outer, ear; the middle ear; and the internal, or inner, ear (Figure 8.11). The external and middle ear structures are involved with hearing only. The internal ear functions in both equilibrium and hearing.

External (outer) Ear The external ear, or outer ear, is composed of the auricle and the external acoustic meatus. The auricle (aw¿ri-kul), or pinna (pin¿nah), is what most people call the “ear”—the shell-shaped

structure surrounding the auditory canal open- ing. In many animals, the auricle collects and directs sound waves into the auditory canal, but in humans this function is largely lost.

The external acoustic meatus (or auditory canal) is a short, narrow chamber (about 1 inch long by ¼ inch wide) carved into the temporal bone of the skull. In its skin-lined walls are the ceruminous (sĕ-roo¿mı̆-nus) glands, which secrete waxy yellow cerumen, or earwax, which provides a sticky trap for foreign bodies and repels insects.

Sound waves entering the auditory canal even- tually hit the tympanic (tim-pan¿ik; tympanum = drum) membrane, or eardrum, and cause it to vibrate. The canal ends at the eardrum, which separates the external from the middle ear.

Auricle (pinna)

External (outer) ear Middle ear

Internal (inner) ear

Semicircular canals

Vestibulocochlear nerve

External acoustic meatus (auditory canal)

Pharyngotympanic (auditory) tube

Tympanic membrane (eardrum) Stirrup

(stapes)

Oval window

Round window

Cochlea

Vestibule

Anvil (incus)

Auditory ossicles

Hammer (malleus)

Figure 8.11 Anatomy of the ear. Note that the inner ear structures (vestibule, cochlea, semicircular canals) represent a cast of the cavity formed by the bony labyrinth.

318 Essentials of Human Anatomy and Physiology

(sta¿pēz). Like dominoes falling, when the eardrum moves, it moves the hammer and transfers the vibration to the anvil. The anvil, in turn, passes the vibration on to the stirrup, which presses on the oval window of the inner ear. The movement at the oval window sets the fluids of the inner ear into motion, eventually exciting the hearing receptors.

Internal (Inner) Ear The internal ear is a maze of bony chambers called the bony labyrinth, or osseous labyrinth (lab¿ ı̆-rinth; “maze”), located deep within the temporal bone behind the eye socket. The three subdivisions of the bony labyrinth are the spiraling, pea-sized cochlea (kok¿le-ah, “snail”), the vestibule (ves¿ti-būl), and the semicircular canals. The ves- tibule is situated between the semicircular canals and the cochlea. The views of the bony labyrinth typically seen in textbooks, including this one, are somewhat misleading because we are really talking about a cavity. Figure 8.11 can be compared to a cast of the bony labyrinth—that is, a labyrinth that was filled with plaster of paris and then had the bony walls removed after the plaster hardened. The shape of the plaster then reveals the shape of the cavity that worms through the temporal bone.

The bony labyrinth is filled with a plasmalike fluid called perilymph (per¿ ı̆-limf). Suspended in the perilymph is a membranous labyrinth, a sys- tem of membrane sacs that more or less follows the shape of the bony labyrinth. The membranous labyrinth itself contains a thicker fluid called endo- lymph (en¿do-limf).

Did You Get It? 13. Which region(s) of the ear (external, middle, or

internal) serve hearing only? 14. Which structures of the ear transmit sound vibrations

from the eardrum to the oval window?

For answers, see Appendix A.

Equilibrium ➔ Learning Objectives

□□ Distinguish between static and dynamic equilibrium.

□□ Describe how the equilibrium organs help maintain balance.

The equilibrium sense is not easy to describe because it does not “see,” “hear,” or “feel.” What

Middle Ear The middle ear cavity, or tympanic cavity, is a small, air-filled, mucosa-lined cavity within the tem- poral bone. It is flanked laterally by the eardrum and medially by a bony wall with two openings, the oval window and the inferior, membrane- covered round window. The pharyngotympanic (think throat-eardrum: pharynx-tympanic) tube, or auditory tube, runs obliquely downward to link the middle ear cavity with the throat, and the mucosae lining the two regions are continuous. Normally, the pharyngotympanic tube is flattened and closed, but swallowing or yawning can open it briefly to equalize the pressure in the middle ear cavity with the external, or atmospheric, pressure. This is an important function because the eardrum does not vibrate freely unless the pressure on both of its surfaces is the same. When the pressures are unequal, the eardrum bulges inward or outward, causing hearing difficulty (voices may sound far away) and sometimes earaches. The ear-popping sensation of the pressures equalizing is familiar to anyone who has flown in an airplane.

Homeostatic Imbalance 8.8

Inflammation of the middle ear, otitis media (o-ti¿tis me¿de-ah), is a fairly common result of a sore throat, especially in children, whose pharyn- gotympanic tubes run more horizontally. In otitis media, the eardrum bulges and often becomes inflamed. When large amounts of fluid or pus accumulate in the cavity, an emergency myringot- omy (lancing of the eardrum) may be required to relieve the pressure. A tiny tube is implanted in the eardrum that allows pus to drain into the external ear canal. The tube usually falls out by itself within the year. ________________________ ✚

The more horizontal course of the pharyngo- tympanic tube in infants also explains why it is never a good idea to “prop” a bottle or feed them when they are lying flat (a condition that favors the entry of the food into that tube).

The tympanic cavity is spanned by the three smallest bones in the body, the ossicles (os¿sı̆- kulz), which transmit the vibratory motion of the eardrum to the fluids of the inner ear (see Figure 8.11). These bones, named for their shape, are the hammer, or malleus (mă¿le-us); the anvil, or incus (in¿kus); and the stirrup, or stapes

Chapter 8: Special Senses 319

it does is respond (frequently without our aware- ness) to various head movements. The equilibrium receptors of the inner ear, collectively called the vestibular apparatus, can be divided into two branches—one branch is responsible for moni- toring static equilibrium, and the other monitors dynamic equilibrium.

Static Equilibrium Within the membrane sacs of the vestibule are receptors called maculae (mak¿u-le; “spots”) that are essential to our sense of static equilibrium (Figure 8.12). The maculae report on changes in the position of the head in space with respect to the pull of gravity when the body is not moving (static = at rest). Because they provide information on which way is up or down, they help us keep our head erect. The maculae are extremely impor- tant to divers swimming in the dark depths (where most other orienting cues are absent), enabling them to tell which way is up (to the surface). Each macula is a patch of receptor (hair) cells with their “hairs” embedded in the otolithic membrane, a gelatinous mass studded with otoliths (o¿tŏ-liths), tiny stones made of calcium salts. As the head moves, the otoliths roll in response to changes in the pull of gravity. This movement creates a pull on the gel, which in turn bends the hairs of the hair cells. Bending activates the hair cells, which send impulses along the vestibular nerve (a division of cranial nerve VIII) to the cerebellum of the brain, informing it of the position of the head in space.

Dynamic Equilibrium The dynamic equilibrium receptors, found in the semicircular canals, respond to angular or rotational movements of the head rather than to straight-line movements. When you twirl on the dance floor or suffer through a spinning carnival ride, these receptors are working overtime. The semicircular canals (each about ½ inch, or 1.3 cm, around) are oriented in the three planes of space. Thus, regardless of which plane you move in, there will be receptors to detect the movement.

Within the ampullae, swollen regions at the base of each membranous semicircular canal (Figure 8.13a, p. 320), are multiple receptor regions, each called a crista ampullaris (kris¿tah am–pu-lar¿is), or simply crista, which con- sists of a tuft of hair cells covered with a gelatinous cap called the cupula (ku¿pu-lah) (Figure 8.13b).

Hair cell

Head upright Head tilted

OtolithsOtolithic membrane

Force of gravity

Membranes in vestibule

Otoliths

Otolithic membrane

Hair tuft

Hair cell Supporting cell

Nerve fibers of vestibular division of cranial nerve VIII

(a)

(b)

Figure 8.12 Structure and function of maculae (static equilibrium receptors). (a) Diagrammatic view of part of a macula. (b) When the head is tipped, the otoliths in the gelatinous otolithic membrane move in the direction of gravitational pull, stimulating the maculae. This creates a pull on the hair cells, causing them to bend.

320 Essentials of Human Anatomy and Physiology

When your head moves in an arclike or angular direction, the endolymph in the canal lags behind the movement (Figure 8.13c). Then, as the cupula drags against the stationary endolymph, the cupula bends—like a diver’s fins in water—with the body’s motion. This stimulates the hair cells, and impulses are transmitted up the vestibular nerve to the cerebellum. Bending the cupula in the oppo- site direction reduces impulse generation. When you are moving at a constant rate, the receptors gradually stop sending impulses, and you no lon- ger have the sensation of motion until your speed or direction of movement changes.

(a) (b)

(c)

Semicircular canals

Vestibular nerve

Ampulla

Ampulla Endolymph

Cupula of crista ampullaris Flow of

endolymph

Cupula

Nerve fibers

Direction of body movement

Vestibule

Although the receptors of the semicircular canals and vestibule are responsible for dynamic and static equilibrium, respectively, they usually act together. Besides these equilibrium senses, sight and the proprioceptors of the muscles and tendons are also important in providing the cere- bellum with information used to control balance.

Did You Get It? 15. What sense do the vestibule and semicircular canals

serve? 16. Describe the different receptors for static and dynamic

equilibrium and their locations. 17. What are otoliths, and what is their role in equilibrium?

For answers, see Appendix A.

Figure 8.13 Structure and function of the crista ampullaris (dynamic equilibrium receptor region). (a) Arranged in the three spatial planes, the semicircular ducts in the semicircular canals each have a swelling called an ampulla at their base. (b) Each ampulla contains a crista ampullaris, a receptor that is a cluster of hair cells that project into a gelatinous cap called the cupula. (c) When head position changes during rotation or in an angular direction, inertia causes the endolymph in the semicircular ducts to lag behind; and as the cupula moves, it drags across the endolymph, which bends the hair cells in the opposite direction. The bending results in increased impulse transmission in the sensory neurons. This mechanism adjusts quickly if the angular motion (or rotation) continues at a constant speed.

321

FOCUS ON

CAREERS Physical Therapy Assistant

loose electric cords that the patient could trip on. Finally, she leaves instructions with the patient to exer- cise on his or her own.

Anatomy is an important part of physical therapy work, Burgess says. “Working with various deviations of movement, you need to know what bones and muscles are involved so that you know which bones and mus- cles to strengthen and show patients how to regain their mobility.”

In some cases, part of her job is to help her patients and their families recognize that they will not be exactly the way they were before, particularly if they have suffered a stroke or other severe injury. The fact that patients may also be coping with hearing or vision loss complicates their therapy.

“As we start to age, we begin to lose our independence,” she says. “So what can we do to change our life- style so that we can still be as inde- pendent as possible?”

The fact that patients may also be coping with

hearing or vision loss complicates

their therapy.

Patients trying to regain mobility rely on physical therapy assistants.

As the population ages, a growing number of people find them- selves needing in-home medical care as they recover from injuries or surgi- cal procedures. Many of these patients rely on physical therapy assis- tants like Leslie Burgess.

Burgess works for Amedisys Home Health Care, and 90 to 95 percent of her patients are senior citizens. Once a doctor prescribes physical therapy, a licensed physical therapist visits the patient and writes a treatment plan. Based on the nature of the problem, this regimen may incorporate strength, movement, and/or balance training, with the goal of improving mobility, reducing pain, and/or help- ing the patient function with a dis- ability. The therapist also sets goals: for example, the patient will be able to walk 300 feet with a cane after 6 weeks.

Burgess’s job is to help the patients carry out these treatment plans, visiting the patient two or three times a week, for 6 to 8 weeks or more, depending on the patient’s progress. In some cases, she will use electrical stimulation or ultrasound to stimulate nerves or muscles. If the patient has a new piece of equip- ment, such as a cane or a walker, she helps him or her learn to use it. She reviews any prescribed medica- tion to make sure the patient is tak- ing it and discusses safety concerns with the patient and family, such as

Physical therapy assistants work in hospitals, nursing homes, and clin- ics—anywhere physical therapists are found. They usually work directly with patients, putting them through exer- cises under the supervision of a physi- cal therapist. Not all patients are geriatric—some are recovering from serious injuries or have conditions such as cerebral palsy.

Many states require that physical therapy assistants complete an associ- ate’s degree and pass a board exam, in addition to completing continuing education.

For more information, contact:

American Physical Therapy Association 1111 N. Fairfax St. Alexandria, VA 22314-1488 (800) 999-2782 http://www.apta.org

For additional information on this career and others, click the Focus on Careers link at .

http://www.apta.org

322 Essentials of Human Anatomy and Physiology

Hearing ➔ Learning Objectives

□□ Explain the function of the spiral organ of corti in hearing.

□□ Define sensorineural deafness and conductive deafness, and list possible causes of each.

□□ Explain how a person is able to localize the source of a sound.

Within the cochlear duct, the endolymph-con- taining membranous labyrinth of the cochlea is the spiral organ of Corti (kor¿te), which contains the hearing receptors, or hair cells (Figure 8.14a). The chambers (scalae) above and below the cochlear duct contain perilymph. Sound waves that reach the cochlea through vibrations of the eardrum, ossicles, and oval window set the cochlear fluids into motion (Figure 8.15). As the sound waves are transmitted by the ossicles from the eardrum to the oval window, their force (amplitude) is increased by the lever activity of the ossicles. In this way, nearly the total force exerted on the much larger eardrum reaches the tiny oval window, which in turn sets the fluids of the inner ear into motion, and these pressure waves set up vibrations in the basilar mem- brane. The receptor cells, positioned on the

Basilar membrane

Supporting cells

Hair (receptor) cells of spiral organ of Corti

Tectorial membrane

Fibers of the cochlear nerve

Vestibular membrane

Temporal bone

Perilymph in scala vestibuli

Vestibular membrane

Afferent fibers of the cochlear nerve

Spiral organ of Corti

Cochlear duct (contains endolymph)

Perilymph in scala tympani

Temporal bone

(b)(a)

Figure 8.14 Anatomy of the cochlea. (a) A cross-sectional view of one turn of the cochlea, showing the position of the spiral organ of

Corti in the cochlear duct. The cavities of the bony labyrinth contain perilymph. The cochlear duct contains endolymph.

(b) Detailed structure of the spiral organ of Corti. The receptor cells (hair cells) rest on the basilar membrane.

basilar membrane in the spiral organ of Corti, are stimulated by the vibrating movement of the basilar membrane against the gel-like tectorial (tek-to¿re-al) membrane that lies over them. The “hairs” of the receptor cells are embedded in the stationary tectorial membrane such that when the basilar membrane vibrates against it, the “hairs” bend (see Figure 8.14b). The length of the fibers spanning the basilar membrane “tunes” specific regions to vibrate at specific frequen- cies. In general, high-pitched sounds disturb the shorter, stiffer fibers of the basilar membrane and stimulate receptor cells close to the oval window, whereas low-pitched sounds affect longer, more floppy fibers and activate specific hair cells fur- ther along the cochlea (Figure 8.16, p. 324).

Once stimulated, the hair cells transmit impulses along the cochlear nerve (a division of cranial nerve VIII—the vestibulocochlear nerve) to the auditory cortex in the temporal lobe, where interpretation of the sound, or hearing, occurs. Because sound usually reaches the two ears at dif- ferent times, we could say that we hear “in stereo.” Functionally, this helps us to determine where sounds are coming from in our environment.

When the same sounds, or tones, keep reach- ing the ears, the auditory receptors tend to adapt,

Chapter 8: Special Senses 323

Sensorineural deafness occurs when there is degeneration or damage to the receptor cells in the spiral organ of Corti, to the cochlear nerve, or to neurons of the auditory cortex. This often results from extended listening to excessively loud sounds. Thus, whereas conduction deafness results from mechanical factors, sensorineural deafness is a problem with nervous system structures.

A person who has a hearing loss due to con- duction deafness will still be able to hear by bone conduction, even though his or her ability to hear air-conducted sounds (the normal conduction route) is decreased or lost. In contrast, individuals with sensorineural deafness cannot hear better by either conduction route. Hearing aids, which use skull bones to conduct sound vibrations to the inner ear, are generally very successful in helping people with conduction deafness to hear. They are not helpful for sensorineural deafness.

Equilibrium problems are usually obvious. Nausea, dizziness, and problems in maintaining balance are common symptoms, particularly when impulses from the vestibular apparatus “disagree” with what we see (visual input). There also may be strange (jerky or rolling) eye movements.

A serious pathology of the inner ear is Ménière’s (mān–e-airz¿) syndrome. The exact cause of this condition is not fully known, but sus- pected causes are arteriosclerosis, degeneration of cranial nerve VIII, and increased pressure of the inner ear fluids. In Ménière’s syndrome, progressive

EXTERNAL EAR MIDDLE EAR INTERNAL EAR

Pinna Auditory canal

Ear- drum

Hammer, anvil, stirrup

Fluids in cochlear canals Upper and middle lower

P re

ss ur

Amplification in middle ear

AmplitudeOne vibration

Spiral organ of Corti

stimulated

Time

Oval window

Figure 8.15 route of sound waves through the ear. To excite the hair cells in the spiral organ of Corti in the inner ear, sound wave vibrations must pass through air, membranes, bone, and fluid.

or stop responding, to those sounds, and we are no longer aware of them. This is why the drone of a continuously running motor does not demand our attention after the first few seconds. However, hearing is the last sense to leave our awareness when we fall asleep or receive anesthesia and is the first to return as we awaken.

Hearing and Equilibrium Deficits

Homeostatic Imbalance 8.9

Children with ear problems or hearing deficits often pull on their ears or fail to respond when spoken to. Under such conditions, tuning fork or audiometry testing is done to try to diagnose the problem. Deafness is defined as hearing loss of any degree—from a slight loss to a total inability to hear sound. Generally speaking, there are two kinds of deafness, conduction and sensorineural. Temporary or permanent conduction deafness results when something interferes with the con- duction of sound vibrations to the fluids of the inner ear. Something as simple as a buildup of ear- wax may be the cause. Other causes of conduction deafness include fusion of the ossicles (a problem called otosclerosis [o–to-sklĕ-ro¿sis]), a ruptured eardrum, and otitis media (inflammation of the middle ear).

324 Essentials of Human Anatomy and Physiology

Stapes

Oval window

Scala vestibuli

Cochlear duct

Scala tympani

Basilar membrane

Round window

(a)

(b)

Perilymph

Fibers of sensory neurons

Fibers of basilar membrane

Base (short, stiff fibers)

20,000 (High notes)

20 (Low notes)

2,000 200

Frequency (Hz)

Apex (long, floppy fibers)

Figure 8.16 Activation of the cochlear hair cells. (a) The cochlea is drawn as though it were uncoiled to make the events of sound transmission occurring there easier to follow. Sound waves of low frequency, below the level of hearing, travel entirely around the cochlear duct without exciting hair cells. But sounds of higher frequency penetrate through the cochlear duct and basilar membrane to reach the scala tympani. This causes the basilar membrane to vibrate maximally in certain areas in response to certain frequencies of sound, stimulating particular hair cells and sensory neurons. The differential stimulation of hair cells is perceived in the brain as sound of a certain pitch. (b) The length and stiffness of the fibers spanning the basilar membrane tune specific regions to vibrate at specific frequencies. The higher notes—20,000 Hertz (Hz)—are detected by shorter, stiffer hair cells along the base of the basilar membrane.

deafness occurs. Affected individuals become nauseated and often have howling or ringing sounds in their ears and vertigo (a sensation of spinning) that is so severe that they cannot stand up without extreme discomfort. Anti–motion sick- ness drugs are often prescribed to decrease the discomfort. _________________________________ ✚

Did You Get It? 18. From the air outside the body, through what

substances do sound waves travel to excite the receptor cells of the cochlea?

19. Which nerve transmits impulses from the spiral organ of Corti to the brain?

20. Do high-pitched sounds peak close to or far from the oval window?

21. How do sensorineural deafness and conduction deafness differ from each other?

For answers, see Appendix A.

PArt III: cHEMIcAL SEnSES: SMELL AnD tAStE ➔ Learning Objectives

□□ Describe the location, structure, and function of the olfactory and taste receptors.

□□ name the five basic taste sensations, and list factors that modify the sense of taste.

The receptors for taste and olfaction are classified as chemoreceptors (ke–mo-re-sep¿terz) because they respond to chemicals in solution. Five types of taste receptors have been identified, but the olfactory receptors (for smell) are believed to be sensitive to a much wider range of chemicals. The receptors for smell and taste complement each other and respond to many of the same stimuli.

olfactory receptors and the Sense of Smell Even though our sense of smell is far less acute than that of many other animals, the human nose is still no slouch in picking up small differences

Chapter 8: Special Senses 325

Olfactory bulb

Cribriform plate of ethmoid bone

Olfactory tract

Supporting cell

Olfactory filaments of the olfactory nerve

Olfactory receptor cell

Olfactory hairs (cilia)

Olfactory mucosa

Mucus layer

Route of inhaled air containing odor molecules

(a)

(b)

Figure 8.17 Location and cellular makeup of the olfactory epithelium.

How does sniffing help to identify scents?Q: It brings more odor-containing air into contact with the olfactory receptors in the superior part of the nasal cavity.

in odors. Some people capitalize on this ability by becoming tea and coffee blenders, perfumers, or wine tasters.

The thousands of olfactory receptors, recep- tors for the sense of smell, occupy a postage stamp–sized area in the roof of each nasal cavity (Figure 8.17). Air entering the nasal cavities must make a 90° turn to enter the respiratory passage- way below, so sniffing, which causes more air to flow superiorly across the olfactory receptors, intensifies the sense of smell.

The olfactory receptor cells are neurons equipped with olfactory hairs, long cilia that protrude from the nasal epithelium and are con- tinuously bathed by a layer of mucus secreted by underlying glands. When the olfactory receptors located on the cilia are stimulated by chemicals dissolved in the mucus, they transmit impulses along the olfactory filaments, which are bun- dled axons of olfactory neurons that collectively make up the olfactory nerve (cranial nerve I).

The olfactory nerve conducts the impulses to the olfactory cortex of the brain. There the odor is interpreted, and an “odor snapshot” is made. The olfactory pathways are closely tied into the limbic system (emotional-visceral part of the brain). Thus, olfactory impressions are long-lasting and very much a part of our memories and emotions. For example, the smell of chocolate chip cookies may remind you of your grandmother, and the smell of a special cologne may make you think of your father. Our reactions to odors are rarely neutral. We tend to either like or dislike certain odors, and we change, avoid, or add odors according to our preferences.

The olfactory receptors are exquisitely sensi- tive—just a few molecules can activate them. Like the auditory receptors, the olfactory neurons tend to adapt rather quickly when they are exposed to an unchanging stimulus, in this case, an odor. This is why a woman stops smelling her own perfume after a while but will quickly pick up the scent of another perfume on someone else.

326 Essentials of Human Anatomy and Physiology

our environment in an intimate way, and many of us consider the sense of taste to be the most plea- surable of our special senses.

The taste buds, or receptors for the sense of taste, are widely scattered in the oral cavity. Of the 10,000 or so taste buds that we have, most are on the tongue. A few are scattered on the soft palate, superior part of the pharynx, and inner surface of the cheeks.

The dorsal tongue surface is covered with small peglike projections, or papillae (pah-pil¿e). The taste buds are found on the sides of the large round vallate papillae, or circumvallate (ser–kum-val¿at) papillae, on the tops of the more numerous fungiform (fun¿j ı̆-form) papillae and in the foliate papillae on the sides of the tongue (Figure 8.18). The specific receptor cells that respond to chemicals dissolved in the saliva are epithelial cells called gustatory cells. Their long microvilli—the gustatory hairs—protrude through the taste pore; and when they are stimulated,

Homeostatic Imbalance 8.10

Although it is possible to have either taste or smell deficits, most people seeking medical help for loss of chemical senses have olfactory disor- ders, or anosmias (ă-noz¿me-uz). Most anosmias result from head injuries, the aftereffects of nasal cavity inflammation (due to a cold, an allergy, or smoking), or aging. Some brain disorders can destroy the sense of smell or mimic it. For exam- ple, some epileptics experience olfactory auras (olfactory hallucinations) just before they go into seizures. ____________________________________ ✚

taste Buds and the Sense of taste The word taste comes from the Latin word taxare, which means “to touch, estimate, or judge.” When we taste things, we are, in fact, testing or judging

(a) (b)

Fungiform papillae

Foliate papillae

Taste buds

Surface of the tongue

Vallate papilla

Gustatory hairs (microvilli) emerging from a taste pore

(c)

Epithelium of tongue

Taste bud

Connective tissue

Gustatory (taste) cell

Basal cell

Sensory nerve fiber

Epiglottis

Palatine tonsil

Lingual tonsil

Figure 8.18 Location and structure of taste buds. (a) Taste buds on the tongue are associated with papillae, projections of the tongue mucosa. (b) A sectioned vallate papilla shows the position of the taste buds in its lateral walls. (c) An enlarged view of four taste buds.

Chapter 8: Special Senses 327

Without the sense of smell, our morning coffee would simply taste bitter. In addition, the tempera- ture and texture of food can enhance or spoil its taste for us. For example, some people will not eat foods that have a pasty texture (avocados) or that are gritty (pears), and almost everyone considers a cold greasy hamburger unfit to eat. “Hot” foods such as chili peppers actually excite pain receptors in the mouth.

Did You Get It? 22. What general name is used to describe both taste and

smell receptors? Why? 23. Where, relative to specific structures, are most taste

buds located? 24. Why does it help to sniff substances that you want to

smell?

For answers, see Appendix A.

PArt IV: Developmental Aspects of the Special Senses ➔ Learning Objective

□ Describe changes that occur with age in the special sense organs.

The special sense organs, essentially part of the nervous system, are formed very early in embry- onic development. For example, the eyes, which are literally outgrowths of the brain, are develop- ing by the fourth week. All of the special senses are functional, to a greater or lesser degree, at birth.

Homeostatic Imbalance 8.11

Congenital eye problems are relatively uncom- mon, but we can give some examples. Strabismus (strah-biz¿mus), which is commonly called “crossed eyes,” results from unequal pulls by the external eye muscles that prevent the baby from coordinat- ing movement of the two eyes. First, exercises are used to strengthen the weaker eye muscles, and/ or the stronger eye may be covered with an eye patch to force the weaker muscles to become stronger. If these measures are not successful, sur- gery is always used to correct the condition because if it is allowed to persist, the brain may stop recognizing signals from the deviating eye, causing that eye to become functionally blind.

they depolarize, and impulses are transmitted to the brain. Three cranial nerves—VII, IX, and X— carry taste impulses from the various taste buds to the gustatory cortex. The facial nerve (VII) serves the anterior part of the tongue. The other two cra- nial nerves—the glossopharyngeal nerve and vagus nerve—serve the other taste bud–containing areas. Because of their location, taste bud cells are subjected to huge amounts of friction and are rou- tinely burned by hot foods. Luckily, they are among the most dynamic cells in the body and are replaced every 7 to 10 days by basal cells (stem cells) found in the deeper regions of the taste buds.

There are five basic taste sensations, each cor- responding to stimulation of one of the five major types of taste buds. The sweet receptors respond to substances such as sugars, saccharine, some amino acids, and some lead salts (such as those found in lead paint). Sour receptors respond to hydrogen ions (H+), or the acidity of the solution; bitter receptors to alkaloids; and salty receptors to metal ions in solution. Umami (u-mah¿me; “delicious”), a taste discovered by the Japanese, is elicited by the amino acid glutamate, which appears to be responsible for the “beef taste” of steak and the flavor of monosodium glutamate, a food additive.

Historically, the tip of the tongue was believed to be most sensitive to sweet and salty substances, its sides to sour, the back of the tongue to bitter, and the pharynx to umami. Actually there are only slight differences in the locations of the taste receptors in different regions of the tongue, but the bitter receptors do seem to be clustered more at the rear of the tongue. Most taste buds respond to two, three, four, or even all five taste modalities.

Taste likes and dislikes have homeostatic value. A liking for sugar and salt will satisfy the body’s need for carbohydrates and minerals (as well as some amino acids). Many sour, naturally acidic foods (such as oranges, lemons, and toma- toes) are rich sources of vitamin C, an essential vitamin. Umami guides the intake of proteins, and because many natural poisons and spoiled foods are bitter, our dislike for bitterness is protective.

Many factors affect taste, and what is com- monly referred to as our sense of taste depends heavily on stimulation of our olfactory receptors by aromas. Think of how bland food tastes when your nasal passages are congested by a cold.

328 Essentials of Human Anatomy and Physiology

visual acuity has improved to about 20/30, and depth perception is present, providing a readiness to begin reading. By school age, the earlier hyper- opia has usually been replaced by emmetropia (normal vision). This condition continues until about age 40, when presbyopia (pres–be-o¿pe- ah) begins to set in. Presbyopia (literally, “old vision”) results from decreasing lens elasticity that accompanies aging. This condition makes it diffi- cult to focus for close vision; it is basically farsight- edness. Your grandmother holding a magazine at arm’s length to read it provides a familiar example of this developmental change in vision.

As aging occurs, the lacrimal glands become less active, and the eyes tend to become dry and more vulnerable to bacterial infection and irrita- tion. The lens loses its crystal clarity and becomes discolored. As a result, it begins to scatter light, causing a distressing glare when the person drives at night. The dilator muscles of the iris become less efficient; thus, the pupils are always some- what constricted. These last two conditions work together to decrease the amount of light reaching the retina, and visual acuity is dramatically lower by one’s seventies. In addition to these changes, older people are susceptible to certain conditions that may result in blindness, such as glaucoma, cataracts, arteriosclerosis, and diabetes.

Homeostatic Imbalance 8.12

Congenital abnormalities of the ears are fairly common. Examples include partly or completely missing pinnas and closed or absent external acoustic meatuses. Maternal infections can have a devastating effect on ear development, and mater- nal rubella during the early weeks of pregnancy results in sensorineural deafness.

A newborn infant can hear after his or her first cry, but early responses to sound are mostly reflexive—for example, crying and clenching the eyelids in response to a loud noise. By the age of 3 or 4 months, the infant is able to localize sounds and will turn to the voices of family members. The toddler listens critically as he or she begins to imi- tate sounds, and good language skills are closely tied to an ability to hear well.

Except for ear inflammations (otitis) resulting from bacterial infections or allergies, few problems affect the ears during childhood and adult life. By the sixties, however, gradual deterioration and

Maternal infections that occur during early pregnancy, particularly rubella (German measles), may lead to congenital blindness or cataracts. If the mother has a type of sexually transmitted infection called gonorrhea (gon–o-re¿ah), the bac- teria will infect the baby’s eyes during delivery. In the resulting conjunctivitis, specifically called ophthalmia neonatorum (of-thal¿me-ah ne–o-na- to¿rum), the baby’s eyelids become red, swollen, and produce pus. All states have enacted laws requiring that all newborn babies’ eyes be rou- tinely treated with silver nitrate or antibiotics shortly after birth. ___________________________ ✚

Generally speaking, vision is the only special sense that is not fully functional when the baby is born, and many years of “learning” are needed before the eyes are fully mature. The eyeballs con- tinue to enlarge until the age of 8 or 9, but the lens grows throughout life. At birth, the eyeballs are foreshortened, and all babies are hyperopic (farsighted). As the eyes grow, this condition usu- ally corrects itself. The newborn infant sees only in gray tones, makes uncoordinated eye movements, and often “sees” using only one eye at a time. Because the lacrimal glands are not fully devel- oped until about 2 weeks after birth, the baby is tearless for this period, even though he or she may cry lustily.

By 5 months, the infant is able to focus on articles within easy reach and to follow moving objects, but visual acuity is still poor. For example, an object that someone with mature vision can see clearly 200 feet away has to be a mere 20 feet away before an infant can see it clearly. (Such vision is said to be 20/200.) By the time the child is 5 years old, color vision is well developed,

This infant with strabismus is unable to focus both eyes simultaneously on the same object.

Chapter 8: Special Senses 329

of touch in guiding a newborn baby to its mother’s breast. However, very young children seem indif- ferent to odors and can play happily with their own feces. As they get older, their emotional responses to specific odors increase.

There appear to be few problems with the chemical senses throughout childhood and young adulthood. Beginning in the midforties, our ability to taste and smell diminishes, which reflects the gradual decrease in the number of these receptor cells. Almost half of people over the age of 80 can- not smell at all, and their sense of taste is poor. This may explain why older adults are inattentive to formerly disagreeable odors and why they often prefer highly seasoned foods or lose their appetite entirely.

Did You Get It? 25. Fifty-year-old Mrs. Bates is complaining that she can’t

read without holding the newspaper out at arm’s length. What is her condition, and what is its cause?

26. Which of the special senses is least mature at birth? 27. What is presbycusis?

For answers, see Appendix A.

atrophy of the spiral organ of Corti begin and lead to a loss in the ability to hear high tones and speech sounds. This condition, presbycusis (pres–bı̆-kyu¿sis), is a type of sensorineural deaf- ness. In some cases, the ear ossicles fuse (otoscle- rosis), which compounds the hearing problem by interfering with sound conduction to the inner ear. Because many elderly people refuse to accept their hearing loss and resist using hearing aids, they begin to rely more and more on their vision for clues as to what is going on around them and may be accused of ignoring people. Although presbycusis was once considered a disability of old age, it is becoming much more common in younger people as our world grows noisier day by day. The damage caused by excessively loud sounds is progressive and cumulative. Music played and heard at deafening levels definitely contributes to the deterioration of hearing receptors.

The chemical senses, taste and smell, are sharp at birth, and infants relish some food that adults consider bland or tasteless. Some researchers claim the sense of smell is just as important as the sense

Summary

PArt I: tHE EYE AnD VISIon (pp. 305–316) 1. External/accessory structures of the eye:

a. Extrinsic eye muscles aim the eyes for following moving objects and for convergence.

b. The lacrimal apparatus includes a series of ducts and the lacrimal glands that produce tears, a saline solution that washes and lubricates the eyeball.

c. Eyelids protect the eyes. Associated with the eye- lashes are the ciliary glands (modified sweat glands) and the tarsal glands (which produce an oily secretion). Both help keep the eye lubricated.

d. The conjunctiva is a mucous membrane that covers the anterior eyeball and lines the eyelids. It produces a lubricating mucus.

2. Three layers form the eyeball.

a. The sclera forms most of the outer, tough, pro- tective fibrous layer. The anterior portion is the cornea, which is transparent to allow light to enter the eye.

b. The vascular layer, or middle coat, provides nutri- tion to the internal eye structures. Its posterior portion, the choroid, is pigmented and prevents light from scattering in the eye. Anterior modifi- cations include two smooth muscle structures: the ciliary body (which changes lens shape) and the iris (which controls the size of the pupil).

c. The sensory layer consists of the two-layered retina—a pigmented epithelium and the inner- most (neural) coat, which contains the photore- ceptors. Rods are dim light receptors. Cones are receptors that provide for color vision and high visual acuity. The fovea centralis, on which acute focusing occurs, contains only cones.

3. The optic disc is the point where the optic nerve leaves the back of the eyeball. Since no photore- ceptors exist at the optic disc, it results in the blind spot in our vision.

4. The lens is the major light-bending (refractory) structure of the eye. Its convexity is increased by contraction of the ciliary body for close focus. Anterior to the lens is the aqueous humor; posterior

330 Essentials of Human Anatomy and Physiology

3. Symptoms of equilibrium apparatus problems include involuntary rolling of the eyes, nausea, ver- tigo, and an inability to stand erect.

4. Hair cells of the spiral organ of Corti (the receptor for hearing within the cochlea) are stimulated by sound vibrations transmitted through air, mem- branes, bone, and fluids.

5. Deafness is any degree of hearing loss. Conduction deafness results when the transmission of sound vibrations through the external and middle ears is hindered. Sensorineural deafness occurs when there is damage to the nervous system structures involved in hearing.

PArt III: cHEMIcAL SEnSES: SMELL AnD tAStE (pp. 324–327) 1. Chemical substances must be dissolved in aqueous

solution to excite the receptors for smell and taste.

2. The olfactory (smell) receptors are located in the superior aspect of each nasal cavity. Sniffing helps to bring more air (containing odors) over the olfac- tory mucosa.

3. Olfactory pathways are closely linked to the limbic system; odors stimulate the recall of memories and arouse emotional responses.

4. Gustatory (taste) cells are located in the taste buds, primarily on the tongue. The five major taste sensa- tions are sweet, salt, sour, bitter, and umami.

5. Taste and appreciation of foods are influenced by the sense of smell and the temperature and texture of foods.

PArt IV: DEVELoPMEntAL ASPEctS oF tHE SPEcIAL SEnSES (pp. 327–329) 1. Special sense organs are formed early in embryonic

development. Maternal infections during the first 5 or 6 months of pregnancy may cause visual abnor- malities as well as sensorineural deafness in the developing child. An important congenital eye prob- lem is strabismus. The most important congenital ear problem is lack of the external acoustic meatus.

2. Vision requires the most learning. The infant has poor visual acuity (is farsighted) and lacks color vision and depth perception at birth. The eye con- tinues to grow and mature until age 8 or 9.

3. Problems of aging associated with vision include presbyopia, glaucoma, cataracts, and arteriosclero- sis of the eye’s blood vessels.

to the lens is the vitreous humor. Both humors rein- force the eye internally. The aqueous humor also provides nutrients to the avascular lens and cornea.

5. Errors of refraction include myopia, hyperopia, and astigmatism. All are correctable with specially ground lenses.

6. The pathway of light through the eye is cornea S aqueous humor S (through pupil) S aqueous humor S lens S vitreous humor S retina.

7. Overlap of the visual fields and inputs from both eyes to each optic cortex provide for depth perception.

8. The pathway of nerve impulses from the retina of the eye is optic nerve S optic chiasma S optic tract S thalamus S optic radiation S visual cortex in occipital lobe of brain.

9. Eye reflexes include the photopupillary, accommo- dation pupillary, and convergence reflexes.

PArt II: tHE EAr: HEArInG AnD BALAncE (pp. 316–324) 1. The ear is divided into three major areas.

a. External ear structures are the auricle (pinna), external acoustic meatus, and tympanic membrane (eardrum). Sound entering the external acoustic meatus causes the eardrum to vibrate. These struc- tures are involved with sound transmission only.

b. Middle ear structures are the ossicles and pharyn- gotympanic tube within the tympanic cavity. Auditory ossicles transmit the vibratory motion from the eardrum to the oval window. The pha- ryngotympanic tube allows pressure to be equal- ized on both sides of the eardrum. These structures are involved with sound transmission only.

c. The internal ear, or bony labyrinth, consists of bony chambers (cochlea, vestibule, and semicir- cular canals) in the temporal bone. The bony labyrinth contains perilymph and membranous sacs filled with endolymph. Within the membra- nous sacs of the vestibule and semicircular canals are equilibrium receptors. Hearing recep- tors are found within the membranes of the cochlea.

2. Receptors of the semicircular canals (cristae ampul- lares) are dynamic equilibrium receptors, which respond to angular or rotational body movements. Receptors of the vestibule (maculae) are static equi- librium receptors, which respond to the pull of gravity and report on head position. Visual and proprioceptor input to the brain are also necessary for normal balance.

Chapter 8: Special Senses 331

6. Taste and smell are most acute at birth and de- crease in sensitivity after age 40 as the number of olfactory and gustatory receptors decreases.

4. The newborn infant can hear sounds, but initial responses are reflexive. By the toddler stage, the child is listening critically and beginning to imitate sounds as language development begins.

5. Sensorineural deafness (presbycusis) is a normal consequence of aging.

review Questions

Multiple choice

More than one choice may apply.

1. Gustatory cells are

a. bipolar neurons.

b. multipolar neurons.

c. unipolar neurons.

d. epithelial cells.

2. Alkaloids excite gustatory hairs mostly at the

a. tip of the tongue.

b. back of the tongue.

c. vallate papillae.

d. fungiform papillae.

3. Parts of the olfactory mucosa include the

a. mucus layer.

b. olfactory bulb.

c. olfactory hairs.

d. cribriform plate.

4. Which cranial nerve is not involved in controlling the action of the extrinsic eye muscles?

a. Abducens

b. Oculomotor

c. Trochlear

d. Trigeminal

5. The cornea is nourished by

a. corneal blood vessels.

b. aqueous humor.

c. vitreous humor.

d. scleral blood vessels.

6. When the eye focuses for far vision,

a. the lens is at its thinnest.

b. the ciliary muscles contract.

c. the light rays are nearly parallel.

d. suspensory fibers of the ciliary zonule are slack.

7. Convergence

a. requires contraction of the medial rectus muscles of both eyes.

b. is needed for near vision.

c. involves transmission of impulses along the abducens nerves.

d. can promote eyestrain.

8. Which of the following are paired incorrectly?

a. Cochlear duct—cupula

b. Vestibule—macula

c. Ampulla—otoliths

d. Semicircular duct—ampulla

9. Movement of the _______ membrane triggers bending of hairs of the hair cells in the spiral organ of Corti.

a. tympanic

b. tectorial

c. basilar

d. vestibular

10. Sounds entering the external acoustic meatus are eventually converted to nerve impulses via a chain of events including

a. vibration of the eardrum.

b. vibratory motion of the ossicles against the oval window.

c. stimulation of hair cells in the spiral organ of Corti.

d. resonance of the cupula.

Short Answer Essay

11. Distinguish between tarsal and ciliary glands.

12. Explain crying in physiological terms.

13. Diagram and label the internal structures of the eye, and give the major function of each structure.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

332 Essentials of Human Anatomy and Physiology

extrinsic and intrinsic muscles to look at close objects than at far objects?

26. Name the structures of the outer, middle, and inner ears, and give the general function of each structure and each group of structures.

27. Sound waves hitting the eardrum set it into motion. Trace the pathway of vibrations from the eardrum to the spiral organ of Corti, where the hair cells are stimulated.

28. What is the difference between the perilymph and the endolymph?

29. Normal balance depends on information transmitted from a number of sensory receptor types. Name at least three of these receptors.

30. Which cranial nerves are involved in taste registration?

31. Name the five primary taste sensations.

32. Where are the olfactory receptors located, and why is that site poorly suited for their job?

33. What is Ménière’s syndrome?

34. Which special sense requires the most learning?

35. For each of the following descriptions, indicate whether it applies to a macula or a crista ampullaris: inside a semicircular canal; contains otoliths; responds to linear acceleration and deceleration; has a cupula; responds to rotational acceleration and deceleration; inside the vestibule.

14. Name the extrinsic eye muscles that allow you to direct your eyes.

15. Locate and describe the functions of the two humors of the eye.

16. What is the blind spot, and why is it so called?

17. What name is given to the rounded opening in the pigmented iris through which light passes?

18. What is the fovea centralis, and why is it important?

19. Trace the pathway of light from the time it hits the cornea until it excites the rods and cones.

20. Trace the pathway of nerve impulses from the photoreceptors in the retina to the visual cortex of the brain.

21. Define hyperopia, myopia, and emmetropia.

22. Why do most people develop presbyopia as they age? Which of the conditions in question 21 does it most resemble?

23. There are only three types of cones. How can you explain the fact that we see many more colors?

24. A rise in intraocular pressure can be damaging for the retina and optic nerve. How is this pressure kept within the normal range?

25. Many students struggling through mountains of reading assignments are told that they need glasses for eyestrain. Why is it more of a strain on the

Critical Thinking and Clinical Application Questionscritical thinking and clinical Application Questions

36. An engineering student has been working in construction to earn money to pay for his education. After about eight months, he notices that he is having problems hearing high-pitched tones. What is the cause-and-effect relationship here?

37. Nine children attending the same day-care center developed red, inflamed eyes and eyelids. What are the most likely cause and name of this condition?

38. Dr. Nguyen used an instrument to press on Mr. Cruz’s eye during his annual physical examination on his 60th birthday. The eye deformed very little, indicating that the intraocular pressure was too high. What was Mr. Cruz’s probable condition?

39. Brandon suffered a ruptured artery in his middle cranial fossa, and a pool of blood compressed his left optic tract, destroying its axons. What part of the visual field was blinded?

40. Sylvia Marcus, age 70, recently had surgery for otosclerosis. The operation was a failure and did not improve her condition. What was the purpose of the surgery, and exactly what was it trying to accomplish?

41. Kathy visits her doctor as she has been struggling to read books, food labels, and her computer screen. An eye test reveals that Kathy is hyperopic. What sort of lenses would Kathy need? Explain.

42. Julie and her father love to find the constellations in the sky on starry nights. One evening, Julie came running into the house and whispered excitedly to her mother. “Mom, I’ve got power! When I look hard at a star it disappears!” What is happening?

43. While visiting her father’s office on the 25th floor of the Harris Building, 5-year-old Emma wandered away into the hall. Fascinated by the buttons in the

Chapter 8: Special Senses 333

she’s lost her sense of taste. How might her symptoms be explained?

45. Serge can see colors, but he confuses certain colors, especially red and green. What is the cause of his condition?

46. Explain the role of vitamin A in vision.

elevator, she entered and pressed 1, and the high- speed elevator plummeted to the first floor. Later she told her father that she felt like she “kept on going” when the elevator stopped. Explain her sensation.

44. Mrs. Garson has an immune disorder that causes dry mouth, and she complains to her doctor that

334

The Endocrine System

Your body cells have dynamic adventures on the microscopic level all the time. For instance, when insulin molecules leave the blood and bind tightly to protein receptors on nearby cells, the response is dramatic: bloodborne glucose molecules begin to enter the cells, and cellular activity accelerates. Such is the power of the second great controlling system of the body, the endocrine system. Along with the nervous system, it coordinates and directs the activity of the body’s cells.

The speed of control in these two regulating systems is very different. The nervous system is

“built for speed.” It uses nerve impulses to prod the muscles and glands into immediate action so that rapid adjustments can be made in response to changes occurring both inside and outside the body. By contrast, the endocrine system acts more slowly by using chemical messengers called hor- mones, which are released into the blood to be transported throughout the body.

Although hormones have widespread effects, the major processes they control are reproduction; growth and development; mobilizing body defenses against stressors; maintaining electrolyte, water, and nutrient balance of the blood; and regulating cellular

9 Hormones

released by endocrine glands travel through the

blood and alter the activity of target cells.

As the second control system of the body, the

endocrine system releases chemicals called hormones that

regulate complex body processes.

WHAT

HOW

WHY

The endocrine system regulates complex

processes such as growth and development, metabolism,

and reproduction.

InSTrucTorS

New Building Vocabulary coaching Activities for this chapter are assignable in

Chapter 9: The Endocrine System 335

metabolism and energy balance. As you can see, the endocrine system regulates processes that go on for relatively long periods and, in some cases, continu- ously. The scientific study of hormones and endo- crine organs is called endocrinology.

The Endocrine System and Hormone Function— An Overview ➔ Learning Objectives

□□ Define hormone and target organ.

□□ Describe how hormones bring about their effects in the body.

Compared to other organs of the body, the organs of the endocrine system are small and unimpres- sive. The endocrine system also lacks the struc- tural or anatomical continuity typical of most organ systems. Instead, bits and pieces of endo- crine tissue are tucked away in separate regions of the body (see Figure 9.3, p. 338). However, func- tionally the endocrine organs are impressive, and when their role in maintaining body homeostasis is considered, they are true giants.

The Chemistry of Hormones Hormones are chemical substances secreted by endocrine cells into the extracellular fluids that regulate the metabolic activity of other cells in the body. Although the body produces many different hormones, nearly all of them can be classified chemically as either amino acid–based mole- cules (including proteins, peptides, and amines) or steroids. Steroid hormones (made from choles- terol) include the sex hormones made by the gonads (ovaries and testes) and the hormones pro- duced by the adrenal cortex. All other hormones are nonsteroidal amino acid derivatives. If we also consider the hormones that act locally, called pros- taglandins (pros″tah-glan′dinz), we must add a third chemical class. (Prostaglandins are described later in the chapter—see Table 9.2, p. 356). The prostaglandins are made from highly active lipids released from nearly all cell membranes.

Hormone Action Although hormones circulate to all the organs of the body via blood, a given hormone affects only certain tissue cells or organs, referred to as its tar- get cells or target organs.

For a target cell to respond to a hormone, spe- cific protein receptors to which that hormone can attach must be present on the cell’s plasma mem- brane or in its interior. Only when this binding occurs can the hormone influence the workings of a cell.

The term hormone comes from a Greek word meaning “to arouse.” In fact, the body’s hormones do just that. They “arouse,” or bring about their effects on, the body’s cells primarily by altering cellular activity—that is, by increasing or decreas- ing the rate of a normal, or usual, metabolic pro- cess rather than by stimulating performance of a new one. The precise change(s) that follow hor- mone binding depend on the specific hormone and the target cell type. Hormones can:

• Change plasma membrane permeability or membrane potential (electrical state) by open- ing or closing ion channels

• Activate or inactive enzymes

• Stimulate or inhibit cell division

• Promote or inhibit secretion of a product

• Turn on or turn off transcription of certain genes (such as those encoding proteins or reg- ulatory molecules)

Direct Gene Activation

Despite the huge variety of hormones, there are really only two mechanisms by which hormones trigger changes in cells. Steroid hormones (and, strangely, thyroid hormone) can use the mecha- nism of direct gene activation (Figure 9.1a, p. 336). Because they are lipid-soluble molecules, the ste- roid hormones can diffuse through the plasma membranes of their target cells 1

. Once inside, the steroid hormone enters the nucleus

and binds to a specific hormone receptor

. The hormone-receptor complex then binds to specific sites on the cell’s DNA

, activating certain genes to transcribe messenger RNA (mRNA)

. The mRNA is translated in the cytoplasm

6 , resulting

➔ConCeptLinkA hormone’s relationship to its target cells resembles that of an enzyme to its substrate. Recall that enzymes interact very specifically with their substrates (Chapter 2, pp. 77–78). Hormone interactions with target cell receptors are also very specific and fit together like two pieces of a puzzle. ➔

336 Essentials of Human Anatomy and Physiology

cyclic AMP, also known as cAMP, or cyclic ade- nosine monophosphate) that oversee additional intracellular changes that promote the typical response of the target cell to the hormone

. Think of a postal service employee (first messen- ger) delivering a letter to campus, and a college mailroom employee (second messenger) carrying the letter from the college mailbox (cell mem- brane) to your dorm mailbox (DNA). As you might guess, the same hormone may have a variety of possible second messengers (including cyclic gua- nosine monophosphate, or cGMP, and calcium ions) and many possible target cell responses, depending on the tissue type stimulated.

Did You Get It? 1. Walking barefoot, you step on a piece of broken

glass and immediately pull your foot back. Why is it important that the signal triggering this motion come from the nervous system and not from the endocrine system?

2. What is a hormone? What does target organ mean? 3. Why is cAMP called a second messenger?

For answers, see Appendix A.

in the synthesis of new proteins. Alternatively, the steroid hormone may bind to receptors in the cyto- plasm, and then the complex moves into the nucleus to activate certain genes.

Second-Messenger System

Steroid hormones can influence cell activity either by direct gene activation or by the indirect path- way of activating a second messenger. Protein and peptide hormones, however, are not water-soluble and are unable to enter target cells directly. Instead, they bind to hormone receptors situated on the target cell’s plasma membrane and use a second-messenger system. In these cases (Figure 9.1b), the hormone (first messenger) binds to the receptor protein on the membrane 1

, and the activated receptor sets off a series of reactions (a cascade) that activates an enzyme

. The enzyme, in turn, catalyzes reactions that produce second-messenger molecules

(in this case,

What determines whether a hormone will influence a given body cell?Q:

Figure 9.1 Mechanisms of hormone action. Steroid hormones can act through either (a) direct gene activation or (b) second-messenger systems. Amino acid–based hormones typically act through second-messenger systems.

NucleusCytoplasmSteroid hormone Receptor

protein

Hormone-receptor complex

DNA

mRNA

New protein

Plasma membrane of target cell

(a) Direct gene activation

Cytoplasm Hormone (first messenger) Enzyme

Receptor protein

Second messenger

Effect on cellular function, such as glycogen breakdownPlasma

membrane of target cell

cAMP

(b) Second-messenger system

ATP

A hormone can influence a body cell only if that cell has receptors for that hormone, either on its plasma

membrane or internally.

Chapter 9: The Endocrine System 337

The stimuli that activate endocrine glands fall into three major categories—hormonal, humoral, and neural (Figure 9.2). These three mechanisms represent the most common systems that control hormone release, but they by no means explain all of them. Some endocrine organs respond to many different stimuli.

Hormonal Stimuli

The most common stimulus is a hormonal stimu- lus, in which endocrine organs are prodded into action by other hormones. For example, hormones of the hypothalamus stimulate the anterior pitu- itary gland to secrete its hormones, and many anterior pituitary hormones stimulate other endo- crine organs to release their hormones into the blood (Figure 9.2a). As the hormones produced by the final target glands increase in the blood, they “feed back” to inhibit the release of anterior pitu- itary hormones and thus their own release. Hormone release promoted by this mechanism tends to be rhythmic, with hormone blood levels rising and falling again and again.

Stimuli for Control of Hormone release ➔ Learning Objectives

□□ Explain how various endocrine glands are stimulated to release their hormonal products.

□□ Define negative feedback, and describe its role in regulating blood levels of the various hormones.

Now that we’ve discussed how hormones work, the next question is, “What prompts the endocrine glands to release or not release their hormones?” Let’s take a look.

Recall that negative feedback mechanisms are the chief means of regulating blood levels of nearly all hormones (see Chapter 1, p. 45). In such systems, some internal or external stimulus trig- gers hormone secretion; then, a rising level of the hormone inhibits further hormone release (even while promoting a response in the target organ). As a result, blood levels of many hormones vary within a very narrow range. With hormones, “a lit- tle goes a long way”!

Figure 9.2 Endocrine gland stimuli.

Capillary blood contains low concentration of Ca2+, which stimulates…

Capillary (low Ca2+ in blood)

Parathyroid glands

Thyroid gland (posterior view)

CNS (spinal cord)

Medulla of adrenal gland

Preganglionic sympathetic fiber stimulates adrenal medulla cells…

Preganglionic sympathetic fibers

PTH

Parathyroid glands

…secretion of parathyroid hormone (PTH) by parathyroid glands

…to secrete catecholamines (epinephrine and norepinephrine)

Capillary

Hypothalamus

Thyroid gland

Adrenal cortex

…stimulate the anterior pituitary gland to secrete hormones that…

Gonad (testis)

Anterior pituitary gland

…stimulate other endocrine glands to secrete hormones

The hypothalamus secretes hormones that… 1 1 1

(b) Humoral stimulus(a) Hormonal stimulus (c) Neural stimulus

338 Essentials of Human Anatomy and Physiology

Humoral Stimuli

Changing blood levels of certain ions and nutrients may also stimulate hormone release. Such stimuli are referred to as humoral (hyoo-mor′al) stimuli to distinguish them from hormonal stimuli, which are also bloodborne chemicals. The term humoral refers to the ancient use of the word humor to indicate the various body fluids (blood, bile, and others). For example, a decreasing blood calcium ion level in the capillaries serving the parathyroid glands prompts the release of parathyroid hormone (PTH). Because PTH acts by several routes to reverse that decline, the blood Ca2+ level soon rises, ending the stimulus for PTH release (Figure 9.2b). Other hormones released in response to humoral stimuli include cal- citonin, released by the thyroid gland, and insulin, produced by the pancreas.

neural Stimuli

In isolated cases, nerve fibers stimulate hormone release, and the endocrine cells are said to respond to neural stimuli. The classic example is sympathetic nervous system stimulation of the adrenal medulla to release the catecholamines norepinephrine and epinephrine during periods of stress (Figure 9.2c).

Did You Get It? 4. What are three ways in which endocrine glands are

stimulated to secrete their hormones?

For the answer, see Appendix A.

The Major Endocrine organs ➔ Learning Objectives

□□ Describe the difference between endocrine and exocrine glands.

□□ On an appropriate diagram, identify the major endocrine glands and tissues.

□□ List hormones produced by the endocrine glands, and discuss their general functions.

□□ Discuss ways in which hormones promote body homeostasis by giving examples of hormonal actions.

□□ Describe major pathological consequences of hypersecretion and hyposecretion of the hormones considered in this chapter.

The major endocrine organs of the body include the pituitary, pineal, thyroid, parathyroid, thymus and adrenal glands, pancreas, and gonads (ova- ries and testes) (Figure 9.3). The hypothalamus, which is part of the nervous system, is also recog- nized as a major endocrine organ because it pro- duces several hormones. Some hormone-produc- ing glands (the anterior pituitary, thyroid, parathyroids, and adrenals) have purely endocrine functions, but others (pancreas and gonads) have both endocrine and exocrine functions and are thus mixed glands. Both types of glands are formed from epithelial tissue, but the endocrine glands are ductless glands that produce hormones that they release into the blood or lymph. (As you might expect, the endocrine glands have a rich blood supply.) Conversely, the exocrine glands release their products at the body’s surface or into body cavities through ducts (they have an exit). We have already discussed the formation of and differences and similarities between these two types of glands (recall what you learned in Chapter 3). Here we will focus on the endocrine glands. A summary of

Figure 9.3 Location of the major endocrine organs of the body. (The parathyroid glands, which appear to be on the anterior surface of the thyroid gland in this illustration, are typically located on its posterior aspect.)

Pineal gland

Hypothalamus Pituitary gland

Parathyroid glands

Thymus

Thyroid gland

Adrenal glands

Pancreas

Ovary (female)

Testis (male)

Chapter 9: The Endocrine System 339

Posterior Pituitary and Hypothalamic Hormones The posterior pituitary is not an endocrine gland in the strict sense because it does not make the peptide hormones it releases. Instead, it acts as a storage area for hormones made by hypothalamic neurons.

Oxytocin (ok″se-to′sin) is releavsed in signifi- cant amounts only during childbirth and nursing. It stimulates powerful contractions of the uterine muscle during sexual relations, during labor, and during breastfeeding. It also causes milk ejection (the let-down reflex) in a nursing woman. Both

the endocrine organs and their hormones’ main actions and regulatory factors appears in Table 9.1, pp. 354–355.

Did You Get It? 5. What are two important differences between

endocrine and exocrine glands?

For the answer, see Appendix A.

Pituitary Gland and Hypothalamus The pituitary (pı̆-too′ ı̆-tar″e) gland is approxi- mately the size of a pea. It hangs by a stalk from the inferior surface of the hypothalamus of the brain, where it is snugly surrounded by the sella turcica (“Turk’s saddle”) of the sphenoid bone (look back at Figure 5.10, p. 175). It has two functional lobes—the anterior pituitary (glandular tissue) and the posterior pituitary (nervous tissue).

Pituitary-Hypothalamus Relationships

➔ Learning Objective □□ Describe the functional relationship between the

hypothalamus and the pituitary gland.

Despite its relatively small size, the anterior pitu- itary gland controls the activity of so many other endocrine glands that it has often been called the “master endocrine gland.” Its removal or destruc- tion has a dramatic effect on the body. The adre- nal and thyroid glands and the gonads atrophy, and results of hyposecretion by those glands quickly become obvious. However, the anterior pituitary is not as all-powerful as it might appear, because the release of each of its hormones is controlled by releasing hormones and inhibit- ing hormones produced by the hypothalamus. The hypothalamus liberates these regulatory hor- mones into the blood of the portal circulation, which connects the blood supply of the hypothal- amus with that of the anterior pituitary. (In a portal circulation, two capillary beds are connected by one or more veins; in this case, the capillaries of the hypothalamus are drained by veins that empty into the capillaries of the anterior pituitary.)

The hypothalamus also makes two additional hormones, oxytocin and antidiuretic hormone, which are transported along the axons of the hypothalamic neurosecretory cells to the poste- rior pituitary for storage (Figure 9.4). They are later released into the blood in response to nerve impulses from the hypothalamus.

Hypothalamus

Axon terminals

Optic chiasma

Hypothalamic neurosecretory cells

Posterior lobe

Anterior lobe of the pituitary

Capillary bed

Venous drainage

Arterial blood supply

ADH

Kidney tubules Mammary glands Uterine muscles

Oxytocin

Figure 9.4 Hormones released by the posterior pituitary and their target organs. Neurosecretory cells in the hypothalamus synthesize oxytocin and antidiuretic hormone (ADH) and transport them down their axons to the posterior pituitary. There, the hormones are stored until nerve impulses from the hypothalamus trigger their release.

340 Essentials of Human Anatomy and Physiology

release. In larger amounts, ADH also increases blood pressure by causing constriction of the arte- rioles (small arteries). For this reason, it is some- times referred to as vasopressin (vās″o-pres′in).

Drinking alcoholic beverages inhibits ADH secretion and results in output of large amounts of urine. The dry mouth and intense thirst experi- enced “the morning after” reflect this dehydrating effect of alcohol. Certain drugs, classed together as diuretics, antagonize the effects of ADH, causing water to be flushed from the body. These drugs are used to manage the edema (water retention in tissues) typical of congestive heart failure.

Homeostatic Imbalance 9.1

Hyposecretion of ADH leads to a condition of excessive urine output called diabetes insipi- dus (di″ah-be′tēz in-sip′ ı̆-dus). People with this It would inhibit TSH secretion by the anterior

pituitary.A:

What effect would elevated levels of thyroid hormone in the blood have on TSH secretion?Q:

HypothalamusReleasing hormones secreted into portal circulation

Posterior pituitary

Adrenal cortexThyrotropic hormone (TH)

Thyroid

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

Testes or ovaries

Prolactin (PRL)

Mammary glands

Bones and muscles

Growth hormone (GH)

Anterior pituitary

Hypophyseal portal system Adrenocorticotropic

hormone (ACTH)

Figure 9.5 Hormones of the anterior pituitary and their major target organs. Releasing hormones secreted by hypothalamic

neurons stimulate secretion of anterior pituitary hormones. The releasing hormones are secreted into a capillary network that connects via

portal veins to a second capillary bed in the anterior lobe of the pituitary gland.

natural and synthetic oxytocic drugs (Pitocin and others) are used to induce labor or to hasten labor that is progressing at a slow pace. Less often, oxy- tocics are used to stop postpartum bleeding (by causing constriction of the ruptured blood vessels at the placental site) and to stimulate the let-down reflex.

The second hormone released by the posterior pituitary is antidiuretic (an″ti-di″u-ret′ik) hor- mone (ADH). Diuresis is urine production. Thus, an antidiuretic is a chemical that inhibits or pre- vents urine production. ADH causes the kidneys to reabsorb more water from the forming urine; as a result, urine volume decreases, and blood volume increases. Water is a powerful inhibitor of ADH

Chapter 9: The Endocrine System 341

problem are continually thirsty and drink huge amounts of water. ___________________________✚

Did You Get It? 6. Both the anterior pituitary and the posterior pituitary

release hormones, but the posterior pituitary is not an endocrine gland. What is it?

7. Barry is excreting huge amounts of urine. He has an endocrine system problem, but it is not diabetes mellitus, which has a similar sign. What is his possible problem?

For answers, see Appendix A.

Anterior Pituitary Hormones The anterior pitu- itary produces several hormones that affect many body organs (Figure 9.5). Two of the six anterior pituitary hormones in the figure—growth hormone and prolactin—exert their major effects on nonendo- crine targets. The remaining four—follicle-stimulating hormone, luteinizing hormone, thyrotropic hor- mone, and adrenocorticotropic hormone—are all tropic hormones. Tropic (tro′pik = turn on) hormones stimulate their target organs, which are also endocrine glands, to secrete their hormones, which in turn exert their effects on other body organs and tissues. All anterior pituitary hor- mones (1) are proteins (or peptides), (2) act through second-messenger systems, and (3) are regulated by hormonal stimuli and, in most cases, negative feedback.

Growth hormone (GH) is a general meta- bolic hormone. However, its major effects are directed to the growth of skeletal muscles and long bones of the body, and thus it plays an important role in determining final body size. GH is a protein-sparing and anabolic hormone that causes the building of amino acids into pro- teins and stimulates most target cells to grow in size and divide. At the same time, it causes fats to be broken down and used for energy while it spares glucose, helping to maintain blood sugar homeostasis.

Homeostatic Imbalance 9.2

If untreated, both deficits and excesses of GH may result in structural abnormalities. Hyposecretion of GH during childhood leads to pituitary dwarfism. Body proportions are fairly normal, but the person as a whole is a living miniature (with a maximum adult height of 4 feet). Hypersecretion during child- hood results in gigantism. The individual becomes

extremely tall; height of 8 to 9 feet is common. Again, body proportions are fairly normal.

If hypersecretion occurs after long-bone growth has ended, acromegaly (ak″ro-meg′ah-le) results. The facial bones, particularly the lower jaw and the bony ridges underlying the eyebrows, enlarge tre- mendously, as do the feet and hands. Thickening of soft tissues leads to coarse or malformed facial features. Most cases of hypersecretion by endo- crine organs (the pituitary and the other endocrine organs) result from tumors of the affected gland. The tumor cells act in much the same way as the normal glandular cells do; that is, they produce the hormones normally made by that gland. Pharmacological doses of GH have been used to reverse some of the effects of aging. Read about this in “A Closer Look” (p. 342). ______________ ✚

Prolactin (PRL) is a protein hormone struc- turally similar to growth hormone. Its only known target in humans is the breast (pro = for; lact = milk). After childbirth, it stimulates and maintains milk production by the mother’s breasts. Its func- tion in men is not known.

Disorders of pituitary growth hormone. This individual exhibiting gigantism (right) stands 8 feet, 1 inch tall. The pituitary dwarf (left) is 2 feet, 5.4 inches tall.

Average height of 5'10''

342

rowth hormone (GH) has been used for pharmaceutical purposes

(that is, as a drug) since its discovery in the 1950s. Originally obtained from the pituitary glands of cadavers, it is now biosynthesized and administered by injection.

GH is administered legally to chil- dren who do not produce it naturally to allow these children to grow to near-normal heights. Unfortunately, some physicians succumb to parental pressures to prescribe GH to children who do produce it but are extremely short.

When GH is administered to adults with a growth hormone defi- ciency, body fat decreases, and bone density and muscle mass increase. GH also appears to increase the per- formance and muscle mass of the heart, decreases blood cholesterol, boosts the immune system, and per- haps improves a person’s psychologi- cal outlook. Such effects (particularly those involving increased muscle mass and decreased body fat) have led to abuse of GH by bodybuilders and athletes. Based on these effects, GH is a banned performrance- enhancing drug, and its use remains restricted.

Because GH may also reverse some effects of aging, anti-aging clinics using GH injections to delay aging have sprung up. Many people natu- rally stop producing GH after age 60, and this may explain why their ratio of lean-to-fat mass declines and their skin thins. GH already is the drug treatment of choice for many aging Hollywood stars who dread the loss of

their youth and vitality. Administration of GH to older patients reverses these declines. However, clinical studies reveal that the administered GH does not increase strength or exercise tol- erance in older patients, and a careful study of very sick patients in intensive care units (where GH is routinely given to restore nitrogen balance) found that high doses of GH are associated with increased mortality. For these reasons, earlier media claims that GH is a “youth potion” have proven to be dangerously misleading.

GH may help AIDS patients. Because of improved antibiotics, fewer AIDS patients are dying from opportunistic infections. The other side of this picture is that more die from the weight loss called “wast- ing.” Injections of GH can actually reverse wasting during AIDS, leading

to weight gain and gain of lean mus- cle. In 1996, the U.S. Food and Drug Administration approved the use of GH to treat such wasting.

GH is not a wonder drug, even in cases where it is clearly beneficial. GH treatment is expensive and has unde- sirable side effects. It can lead to fluid retention and edema, joint and mus- cle pain, high blood sugar, glucose intolerance, and gynecomastia (breast enlargement in men). Hypertension, heart enlargement, diabetes, and colon cancer are other possible results of high doses of GH, and edema and headaches accompany even the low- est doses.

Intensive research into the poten- tial benefits of GH should keep this hormone in the public eye for years to come. Let’s hope its uncontrolled use does not become a public health problem.

Potential Uses for Growth Hormone

A CLOSER

LOOK

Can growth hormone help older patients?

Chapter 9: The Endocrine System 343

The gonadotropic (gō″nad-o-tro′pik) hor- mones regulate the hormonal activity of the gonads (ovaries and testes). In women, the gonad- otropin follicle-stimulating hormone (FSH) stimulates follicle development in the ovaries. As the follicles mature, they produce estrogen, and eggs are readied for ovulation. In men, FSH stimu- lates sperm development by the testes. Luteinizing (lu′te-in-i″zing) hormone (LH) triggers ovulation of an egg from the ovary and causes the ruptured follicle to produce progesterone and some estro- gen. In men, LH stimulates testosterone production by the interstitial cells of the testes.

Homeostatic Imbalance 9.3

Hyposecretion of FSH or LH leads to sterility, the lack of ability to successfully reproduce, in both males and females. In general, hypersecretion does not appear to cause any problems. However, some drugs used to promote fertility stimulate the release of the gonadotropic hormones, and multiple births (indicating multiple ovulations at the same time rather than the usual single ovulation each month) are relatively common after their use. _________ ✚

Thyrotropic hormone (TH), also called thy- roid-stimulating hormone (TSH), influences the growth and activity of the thyroid gland. Adrenocorticotropic (ad-re″no-kor″tı̆-ko-tro′pik) hormone (ACTH) regulates the endocrine activ- ity of the cortex portion of the adrenal gland.

Did You Get It? 8. What are tropic hormones?

For the answer, see Appendix A.

Pineal Gland The pineal (pin′e-al) gland is a small, cone- shaped gland that hangs from the roof of the third ventricle of the brain (see Figure 9.3). The endo- crine function of this tiny gland is still somewhat of a mystery. Although many chemical substances have been identified in the pineal gland, only the hormone melatonin (mel″ah-to′nin) appears to be secreted in substantial amounts. The level of melatonin rises and falls during the course of the day and night. The peak level occurs at night and makes us drowsy; the lowest level occurs during daylight around noon. Melatonin is believed to be a “sleep trigger” that plays an important role in

establishing the body’s sleep-wake cycle. It is believed to coordinate the hormones of fertility and to inhibit the reproductive system (especially the ovaries of females) until the body matures.

Thyroid Gland The thyroid gland is located at the base of the throat, just inferior to the Adam’s apple, where it is easily palpated during a physical examination. It is a fairly large gland consisting of two lobes joined by a central mass, or isthmus (Figure 9.6a, p. 344). The thyroid gland makes two hormones, one called thyroid hormone, the other called calcitonin (pro- duced by the parafollicular cells). Internally, the thyroid gland is composed of hollow structures called follicles (Figure 9.6b), which store a sticky colloidal material. Thyroid hormone is derived from this colloid.

Thyroid hormone, often referred to as the body’s major metabolic hormone, is actually two active iodine-containing hormones, thyroxine (thi- rok′sin), or T4, and triiodothyronine (tri″i″o-do- thi′ro-nēn), or T3. Thyroxine is the major hormone secreted by the thyroid follicles. Most triiodothyro- nine is formed at the target tissues by conversion of thyroxine to triiodothyronine. These two hormones are very much alike. Each is constructed from two tyrosine amino acids linked together, but thyroxine has four bound iodine atoms, whereas triiodothyro- nine has three (thus, T4 and T3, respectively).

Thyroid hormone controls the rate at which glucose is “burned,” or oxidized, and converted to body heat and chemical energy (ATP). Because all body cells depend on a continuous supply of ATP to power their activities, every cell in the body is a target. Thyroid hormone is also important for nor- mal tissue growth and development, especially in the reproductive and nervous systems.

Homeostatic Imbalance 9.4

Without iodine, functional thyroid hormones can- not be made. The source of iodine is our diet, and foods richest in iodine are seafoods. Years ago many people who lived in the Midwest, in areas with iodine-deficient soil that were far from the sea- shore (and a supply of fresh seafood), developed goiters (goy′terz). So, that region of the country came to be known as the “goiter belt.” A goiter is an enlargement of the thyroid gland (shown in the fig- ure) that results when the diet is deficient in iodine.

344 Essentials of Human Anatomy and Physiology

TSH “calls” for the release of thyroxine, but as the peptide part of the molecule is made, the thyroid gland enlarges because the protein is nonfunctional without iodine. This inactive form of thyroxine fails to provide negative feedback to inhibit TSH release, resulting in continued production of the peptide

component, which makes the goiter larger. Simple goiter is uncommon in the United States today because most of our salt is iodized, but it is still a problem in some other areas of the world.

Hyposecretion of thyroxine may indicate problems other than iodine deficiency, such as lack of stimulation by TSH. If it occurs in early childhood, the result is cretinism (kre′tin-izm). Cretinism results in dwarfism in which adult body proportions remain childlike, with a proportion- ally longer torso and shorter legs compared to normal adults. Untreated individuals with cretin- ism are intellectually impaired. Their hair is scanty, and their skin is dry. If the hyposecretion is dis- covered early, hormone replacement will prevent mental impairment and other signs and symptoms of the deficiency. Hypothyroidism occurring in adults results in myxedema (mik″sah-de′mah), which is characterized by both physical and men- tal sluggishness (however, mental impairment does not occur). Other signs are puffiness of the face, fatigue, poor muscle tone, low body

(a) Gross anatomy of the thyroid gland, anterior view (b) Photomicrograph of thyroid gland follicles (380×)

Thyroid cartilage

Common carotid artery

Epiglottis

Isthmus of thyroid gland

Left subclavian artery

Left lobe of thyroid gland

Trachea

Aorta

Brachiocephalic artery

Colloid-filled follicles Follicle cells

Parafollicular cells

Figure 9.6 The thyroid gland.

Woman with an enlarged thyroid (goiter).

View Histology

Chapter 9: The Endocrine System 345

Did You Get It? 9. Why is iodine important for proper thyroid gland

function? 10. What hormone is called the “sleep hormone,” and

which endocrine organ produces it?

For answers, see Appendix A.

Parathyroid Glands The parathyroid glands are tiny masses of glan- dular tissue most often found on the posterior sur- face of the thyroid gland (look back at Figure 9.3). Typically, there are two parathyroid glands on each thyroid lobe, that is, a total of four parathyroids; but as many as eight have been reported, and some may be in other regions of the neck or even in the thorax. The parathyroids secrete parathyroid hor- mone (PTH), which is the most important regula- tor of calcium ion (Ca2+) homeostasis of the blood. When the blood calcium ion concentration drops below a certain level, the parathyroids release PTH, which stimulates bone destruction cells (osteo- clasts) to break down bone matrix and release cal- cium ions into the blood. Thus, PTH is a hypercal- cemic hormone (that is, it acts to increase the blood level of calcium ions), whereas calcitonin is a hypo- calcemic hormone. (The negative feedback interac- tion between these two hormones as they control the blood calcium ion level during youth is illus- trated in Figure 9.7, p. 346.) Although the skeleton is the major PTH target, PTH also stimulates the kidneys and intestine to absorb more calcium ions (from urinary filtrate and foodstuffs, respectively).

Homeostatic Imbalance 9.5

If the blood calcium ion level falls too low, neurons become extremely irritable and overactive. They deliver impulses to the muscles so rapidly that the muscles go into uncontrollable spasms (tetany), which may be fatal. Before surgeons knew the importance of the tiny parathyroid glands on the backside of the thyroid, they would remove a hyper- thyroid patient’s thyroid gland entirely. Many times this resulted in death. Once it was revealed that the parathyroids are functionally very different from the thyroid gland, surgeons began to leave at least some parathyroid-containing tissue (if at all possible) to take care of blood calcium ion homeostasis.

Severe hyperparathyroidism causes massive bone destruction—an X-ray examination of the bones shows large punched-out holes in the bony

temperature (the person is always cold), obesity, and dry skin. Oral thyroxine is prescribed to treat this condition.

Hyperthyroidism generally results from a tumor of the thyroid gland. Extreme overproduc- tion of thyroxine results in a high basal metabolic rate, intolerance of heat, rapid heartbeat, weight loss, nervous and agitated behavior, and a general inability to relax. Graves’ disease is one form of hyperthyroidism. In addition to the symptoms of hyperthyroidism described earlier, the thyroid gland enlarges, and the eyes may bulge, or pro- trude anteriorly. This condition, called exophthal- mos (ek″sof-thal′mus), is shown in the figure. Hyperthyroidism may be treated surgically by removal of part of the thyroid (and/or a tumor if present) or chemically with thyroid-blocking drugs or radioactive iodine, which destroys some of the thyroid cells. _______________________________✚

The second important hormone product of the thyroid gland, calcitonin, decreases the blood cal- cium ion level by causing calcium to be deposited in the bones. It acts antagonistically to parathyroid hormone, the hormone produced by the parathy- roid glands. Whereas thyroxine is made and stored in follicles before it is released to the blood, calci- tonin is made by the parafollicular cells found in the connective tissue between the follicles (see Figure 9.6b). It is released directly to the blood in response to an increasing level of blood calcium ions. Few effects of hypo- or hypersecretion of calcitonin are known, and calcitonin production is meager or ceases entirely in adults. This may help to explain (at least in part) the progressive decalci- fication of bones that accompanies aging.

The exophthalmos of Graves’ disease.

346 Essentials of Human Anatomy and Physiology

matrix. The bones become very fragile, and spon- taneous fractures begin to occur. _____________✚

Did You Get It? 11. How are the thyroid and parathyroid glands linked

anatomically? 12. What hormone increases the blood calcium ion level,

and which endocrine gland produces this hormone? 13. What hormone reduces the blood calcium ion level,

and which endocrine gland produces this hormone?

For answers, see Appendix A.

Thymus The thymus is located in the upper thorax, poste- rior to the sternum. Large in infants and children, it decreases in size throughout adulthood. By old age, it is composed mostly of fibrous connective tissue and fat. The thymus produces a hormone called thymosin (thi′mo-sin) and others that appear to be essential for normal development of a special group of white blood cells (T lymphocytes) and the immune response. (We describe the role of the thy- mus and its hormones in immunity in Chapter 12.)

Adrenal Glands The two adrenal glands curve over the top of the kidneys like triangular hats. Although each

Blood calcium levels would drop because the bones would no longer be exposed to the bone-digesting

stimulus of PTH.

What effect would removal of the parathyroid glands have on the blood calcium ion level?Q:

Osteoclasts degrade bone matrix and release Ca2+ into blood.

Parathyroid glands

Thyroid gland

Thyroid gland releases calcitonin.

BALANCE BALANCE

Calcitonin stimulates calcium salt deposit in bone.

Parathyroid glands release parathyroid hormone (PTH).

PTH

Calcitonin

Calcium homeostasis of blood: 9–11 mg/100 ml

IMBALANCE

Stimulus Falling blood Ca2+ levels

Stimulus Rising blood Ca2+ levels

Figure 9.7 Hormonal control of the level of calcium ions in the blood. PTH and calcitonin operate in negative feedback control systems that influence each other.

Chapter 9: The Endocrine System 347

adrenal gland looks like a single organ, it is struc- turally and functionally two endocrine organs in one. Much like the pituitary gland, it has parts made of glandular (cortex) and neural tissue (medulla). The central medulla region is enclosed by the adrenal cortex, which contains three sepa- rate layers of cells (Figure 9.8).

Hormones of the Adrenal Cortex

The adrenal cortex produces three major groups of steroid hormones, which are collectively called corticosteroids (kor″ti-ko-ster′oidz)—mineralo- corticoids, glucocorticoids, and sex hormones.

The mineralocorticoids, mainly aldosterone (al″dos-ter′ōn), are produced by the outermost adrenal cortex cell layer. As their name suggests, the mineralocorticoids are important in regulating the mineral (or salt) content of the blood, particu- larly the concentrations of sodium and potassium ions. These hormones target the kidney tubules that selectively reabsorb the minerals or allow them to be flushed out of the body in urine. When the blood level of aldosterone rises, the kidney tubule cells reabsorb increasing amounts of sodium ions and secrete more potassium ions into the urine. When sodium is reabsorbed, water follows. Thus, the mineralocorticoids help regulate both

water and electrolyte balance in body fluids. The release of aldosterone is stimulated by humoral factors, such as fewer sodium ions or more potas- sium ions in the blood (and, to a lesser degree, by ACTH, a hormonal stimulus) (Figure 9.9, p. 348). Renin, an enzyme produced by the kidneys when blood pressure drops, also causes the release of aldosterone by triggering a series of reactions that form angiotensin II, a potent stimulator of aldo- sterone release. A hormone released by the heart, atrial natriuretic (na″tre-u-ret′ik) peptide (ANP), prevents aldosterone release, its goal being to reduce blood volume and blood pressure.

The middle cortical layer mainly produces glucocorticoids, which include cortisone and cortisol. Glucocorticoids promote normal cell metabolism and help the body to resist long-term stressors, primarily by increasing the blood glucose level. When blood levels of glucocorticoids are high, fats and even proteins are broken down by body cells and converted to glucose, which is released to the blood. For this reason, glucocorti- coids are said to be hyperglycemic hormones. Glucocorticoids also seem to control the more unpleasant effects of inflammation by decreasing edema, and they reduce pain by inhibiting the pain-causing prostaglandins (see Table 9.2, p. 356).

• Cortex

Kidney

• Medulla

Adrenal gland Capsule

Mineralocorticoid- secreting area

Glucocorticoid- secreting area

C or

te x

M ed

ul la

Sex hormone– secreting area

Adrenal cortex

Adrenal medulla

Kidney

Adrenal gland

Figure 9.8 Microscopic structure of the adrenal gland. Diagram of the three regions of the adrenal cortex and part of the adrenal medulla.

348 Essentials of Human Anatomy and Physiology

hormones), but some estrogens (female sex hor- mones) are also formed.

Homeostatic Imbalance 9.6

A generalized hyposecretion of all the adrenal cor- tex hormones leads to Addison’s disease, char- acterized by a peculiar bronze tone of the skin, which can resemble a suntan. Because the aldoste- rone level is low, sodium and water are lost from the body, which leads to problems with electrolyte and water balance. This, in turn, causes the mus- cles to become weak, and shock is a possibility. Other signs and symptoms of Addison’s disease result from deficient levels of glucocorticoids, such as hypoglycemia, a lessened ability to cope with stress (burnout), and suppression of the immune system (and thereby increased susceptibility to infection). A complete lack of glucocorticoids is incompatible with life.

Hypersecretion problems may result from an ACTH-releasing tumor of the pituitary or from adrenal cortical tumors. The resulting condition depends on the cortical area involved. Hyperactivity of the outermost cortical area results in hyperaldosteronism (hi″per-al″dos′-terōn- izm). Excessive water and sodium ions are retained, leading to high blood pressure and edema. Potassium ions are lost to such an extent that the activity of the heart and nervous system may be disrupted. When the tumor is in the mid- dle cortical area or the patient has been receiving pharmacological doses (amounts higher than those released in the body) of glucocorticoids to counteract inflammatory disease, Cushing’s syn- drome occurs. Excessive glucocorticoids result in

Because of their anti-inflammatory properties, gluco- corticoids are often prescribed as drugs to suppress inflammation for patients with rheumatoid arthritis. Glucocorticoids are released from the adrenal cortex in response to a rising blood level of ACTH.

In both men and women, the adrenal cortex produces both male and female sex hormones throughout life in relatively small amounts. The bulk of the sex hormones produced by the inner- most cortex layer are androgens (male sex

Hypothalamus

Increased blood pressure or blood volume

Decreased Na+ or increased K+ in blood

Decreased blood volume and/or blood pressure

Stress

Anterior pituitary

Corticotropin- releasing hormone

Renin Indirect stimulating effect via angiotensin

Direct stimulating effect

Angiotensin II

Atrial natriuretic peptide (ANP)

Inhibitory effect

Mineralocorticoid- producing part of adrenal cortex

Kidney ACTH

Heart

Enhanced secretion of aldosterone targets kidney tubules

Increased absorption of Na+ and water; increased K+ excretion

Increased blood volume and blood pressure

Figure 9.9 Major mechanisms controlling aldosterone release from the adrenal cortex.

Lips of this individual show the hyperpigmentation of Addison’s disease.

Chapter 9: The Endocrine System 349

a swollen “moon face” and the appearance of a “buffalo hump” of fat on the upper back. Other common and undesirable effects include high blood pressure, hyperglycemia (steroid diabetes), weakening of the bones (as protein is withdrawn to be converted to glucose), and severe depres- sion of the immune system.

Hypersecretion of the sex hormones leads to masculinization, regardless of sex. In adult males these effects may be masked, but in females the results are often dramatic. A beard develops, and a masculine pattern of body hair distribution occurs, among other effects. ________________________✚

Did You Get It? 14. How do thymosin and other thymus hormones help

to protect the body? 15. What hormone stimulates the kidneys to reabsorb

more sodium ions? 16. Which group of hormones produced by the adrenal

cortex have anti-inflammatory effects and participate in the long-term stress response?

For answers, see Appendix A.

Hormones of the Adrenal Medulla

The adrenal medulla, like the posterior pitu- itary, is a knot of nervous tissue. When the medulla is stimulated by sympathetic nervous system neurons, its cells release two similar hor- mones, epinephrine (ep″ ı̆-nef′rin), also called adrenaline, and norepinephrine (noradrena- line), into the bloodstream. Collectively, these hormones are called catecholamines (kat″ĕ- kol′ah-mēnz). Because some sympathetic neu- rons also release norepinephrine as a neu- rotransmitter, the adrenal medulla is often thought of as a “misplaced sympathetic nervous system ganglion.”

When you are (or feel) threatened physically or emotionally, your sympathetic nervous system brings about the “fight-or-flight” response to help you cope with the stressful situation. One of the organs it stimulates is the adrenal medulla, which literally pumps its hormones into the bloodstream to enhance and prolong the effects of the neu- rotransmitters of the sympathetic nervous system. The catecholamines increase heart rate, blood pressure, and the blood glucose level and dilate the small passageways of the lungs. These events result in more oxygen and glucose in the blood and a faster circulation of blood to the body

organs (most importantly, to the brain, muscles, and heart). Thus, the body is better able to deal with a short-term stressor, whether the job at hand is to fight, begin the inflammatory process, or alert you so you think more clearly (Figure 9.10, p. 350).

The catecholamines of the adrenal medulla prepare the body to cope with short-term stressful situations and cause the so-called alarm stage of the stress response. Glucocorticoids, by contrast, are produced by the adrenal cortex and are impor- tant when coping with prolonged or continuing stressors, such as dealing with the death of a fam- ily member or having a major operation. Glucocorticoids operate primarily during the resis- tance stage of the stress response. If they are suc- cessful in protecting the body, the problem will eventually be resolved without lasting damage. When the stress continues on and on, the adrenal cortex may simply “burn out,” which is usually fatal. (The roles of glucocorticoids in the stress response are also shown in Figure 9.10.)

Homeostatic Imbalance 9.7

Damage to or destruction of the adrenal medulla has no major effects as long as the sympathetic nervous system neurons continue to function normally. However, hypersecretion of catechol- amines leads to symptoms such as rapid heart- beat, high blood pressure, and a tendency to perspire and be very irritable. Surgical removal of the catecholamine-secreting cells corrects this condition. _________________________________ ✚

Pancreatic Islets The pancreas, located close to the stomach in the abdominal cavity (see Figure 9.3), is a mixed gland. The pancreatic islets, also called the islets of Langerhans (lahng′er-hanz), are little masses of endocrine (hormone-producing) tissue scattered among the exocrine (enzyme-producing) tissue of the pancreas. The exocrine, or acinar, part of the pancreas acts as part of the digestive system (see Chapter 14). Here we will consider only the pancreatic islets.

Although there are more than a million islets separated by exocrine cells, each of these tiny clumps of cells works like an organ within an organ as it busily manufactures its hormones. Two

350 Essentials of Human Anatomy and Physiology

important hormones produced by the islet cells are insulin and glucagon (gloo′kah-gon).

Islet cells act as fuel sensors, secreting insulin and glucagon appropriately during fed and fasting states. A high level of glucose in the blood stimu- lates the release of insulin from the beta (ba′tah) cells (Figure 9.11) of the islets. Insulin acts on just about all body cells, increasing their ability to import glucose across their plasma membranes. Once inside the cells, glucose is oxidized for energy or converted to glycogen or fat for storage. Insulin also speeds up these “use it” or “store it” activities. Because insulin sweeps the glucose out of the blood, its effect is said to be hypoglycemic. As the blood glucose level falls, the stimulus for insulin release ends—another classic case of nega-

tive feedback control. Many hormones have hyper- glycemic effects (glucagon, glucocorticoids, and epinephrine, to name a few), but insulin is the only hormone that decreases the blood glucose level. Insulin is absolutely necessary for the use of glucose by body cells. Without it, essentially no glucose can get into the cells to be used.

Homeostatic Imbalance 9.8

Without insulin, the blood level of glucose (which normally ranges from 80 to 120 mg/100 ml of blood) rise to a dramatically high level (for exam- ple, 600 mg/100 ml of blood). In such instances, glucose begins to spill into the urine because the kidney tubule cells cannot reabsorb it fast enough.

Short term More prolongedStress

Hypothalamus

Nerve impulses

Adrenal cortex

Releasing hormones

Corticotropic cells of anterior pituitary

ACTH

Mineralocorticoids Glucocorticoids

1. Retention of sodium and water by kidneys 2. Increased blood volume and blood pressure

1. Proteins and fats converted to glucose or broken down for energy 2. Increased blood sugar 3. Suppression of immune system

Long-term stress responseShort-term stress response

Spinal cord

Adrenal medulla

Preganglionic sympathetic fibers

Catecholamines (epinephrine and norepinephrine)

1. Increased heart rate 2. Increased blood pressure 3. Liver converts glycogen to glucose and releases glucose to blood 4. Dilation of bronchioles 5. Changes in blood flow patterns, leading to increased alertness and decreased digestive and kidney activity 6. Increased metabolic rate

Figure 9.10 Roles of the hypothalamus, adrenal medulla, and adrenal cortex in the stress response. (Note that ACTH is only a weak stimulator of mineralocorticoid release under normal conditions.)

Chapter 9: The Endocrine System 351

As glucose flushes from the body, water follows, leading to dehydration. The clinical name for this condition is diabetes mellitus (mĕ-li′tus), which literally means that something sweet (mel = honey) is passing through, or siphoning (diabetes = Greek “siphon”), from the body. Because cells do not have access to glucose, fats and even proteins are broken down and used to meet the energy requirements of the body. As a result, body weight begins to decline. Loss of body proteins leads to a decreased ability to fight infections, so diabetics must maintain good hygiene and care for even small cuts and bruises. When large amounts of fats (instead of sugars) are used for energy, the blood becomes very acidic (acidosis [as″ ı̆-do′sis]) as ketones (intermediate products of fat breakdown) appear in the blood. This type of acidosis is referred to as ketosis. Unless cor- rected, coma and death result. The three cardinal signs of diabetes mellitus are (1) polyuria (pol″e- u′re-ah)—excessive urination to flush out the glu- cose and ketones; (2) polydipsia (pol″e-dip′se-ah)—excessive thirst resulting from water loss; and (3) polyphagia (pol″e-fa′je-ah)—

hunger due to inability to use sugars and the loss of fat and proteins from the body.

People with mild cases of diabetes mellitus (most cases of type 2, or adult-onset, diabetes) produce insulin, but for some reason their insulin receptors are unable to respond to it, a situation called insulin resistance. Type 2 diabetics are treated with special diets or oral hypoglycemic medications that prod the sluggish islets into action and increase the sensitivity of the target tis- sues to insulin and of beta cells to the stimulating effects of glucose. To regulate the blood glucose level in the more severe type 1 (juvenile) diabetic, insulin is infused continuously by an insulin pump worn externally or by a regimen of carefully planned insulin injections is administered through- out the day. ________________________________✚

Glucagon acts as an antagonist of insulin; that is, it helps to regulate the blood glucose level but in a way opposite that of insulin (Figure 9.12, p. 352). Its release by the alpha (al′fah) cells (see Figure 9.11c) of the islets is stimulated by a low blood level of glucose. Its action is basically

Stomach

Pancreas

(c)

Cord of beta (β) cells secreting insulin into capillaries

Pancreatic islets

Capillaries

Exocrine cells of pancreas

Alpha (α) cells

(b)

(a)

Figure 9.11 Pancreatic tissue. (a) Location of pancreas relative to the stomach and small intestine. (b) Photomicrograph of pancreas with exocrine and endocrine (islets) areas clearly visible (140*). (c) Diagrammatic view of a pancreatic islet. Beta cells produce insulin; alpha cells produce glucagon.

View Histology

352 Essentials of Human Anatomy and Physiology

BALANCE: Normal blood glucose level (about 90 mg/100 ml)

IMBALANCE

GlycogenGlucose

Liver

Tissue cells

Uptake of glucose from blood is enhanced in most body cells.

Insulin-secreting cells of the pancreas activated; release insulin into the blood.

Elevated blood sugar level

Pancreas

Low blood sugar level

Insulin

Blood glucose falls to homeostatic set point; stimulus for insulin release diminishes.

Liver takes up glucose and stores as glycogen.

Glucagon-releasing cells of pancreas activated; release glucagon into blood.

Blood glucose rises to homeostatic set point; stimulus for glucagon release diminishes.

Liver breaks down glycogen stores and releases glucose to the blood.

Glucagon

Stimulus Blood glucose level (e.g., after skipping a meal)

Stimulus Blood glucose level (e.g., after eating four jelly doughnuts)

What happens to the liver’s ability to synthesize and store glycogen when the glucagon blood level rises?Q:

Figure 9.12 Regulation of the blood glucose level by a negative feedback mechanism involving pancreatic hormones.

Glucagon inhibits those activities in the liver, so the liver’s ability to perform them decreases as the

glucagon level rises.

hyperglycemic. Its primary target organ is the liver, which it stimulates to break down stored glycogen to glucose and to release the glucose into the blood. No important disorders resulting from hypo- or hypersecretion of glucagon are known.

Did You Get It? 17. Mrs. Bellamy’s husband has suffered a heart attack

and is hospitalized. Would you expect her blood glucose level to be elevated, normal, or lower than normal? Why?

18. Insulin and glucagon are both pancreatic hormones. Which stimulates cellular uptake of glucose?

For answers, see Appendix A.

Chapter 9: The Endocrine System 353

Gonads The female and male gonads (see Figure 9.3) pro- duce sex cells (an exocrine function). They also produce sex hormones that are identical to those produced by adrenal cortex cells. The major differ- ences from the adrenal sex hormone production are the source and relative amounts of hormones produced.

Hormones of the Ovaries

The female gonads, or ovaries, are paired, slightly larger than almond-sized organs located in the pelvic cavity. Besides producing female sex cells (ova, or eggs), ovaries produce two groups of ste- roid hormones, estrogens and progesterone.

Alone, the estrogens are responsible for the development of sex characteristics in women (pri- marily growth and maturation of the reproductive organs) and the appearance of secondary sex characteristics (for example, hair in the pubic and axillary regions) at puberty. Acting with progester- one, estrogens promote breast development and cyclic changes in the uterine lining (the men- strual cycle, or menstruation).

Progesterone (pro-jes′tĕ-rōn), as already noted, acts with estrogen to bring about the menstrual cycle. During pregnancy, it quiets the muscles of the uterus so that an implanted embryo will not be aborted and helps prepare breast tissue for lactation.

Ovaries are stimulated to release their estro- gens and progesterone in a cyclic way by the ante- rior pituitary gonadotropic hormones. We give more detail on this feedback cycle and on the structure, function, and controls of the ovaries in the reproductive system chapter (Chapter 16), but keep in mind that hyposecretion of the ovarian hormones severely hampers a woman’s ability to conceive and bear children.

Hormones of the Testes

The paired oval testes of the male are suspended in a sac, the scrotum, outside the pelvic cavity. In addition to male sex cells, or sperm, the testes also produce male sex hormones, or androgens, of which testosterone (tes-tos′tĕ-rōn) is the most important. At puberty, testosterone promotes the growth and maturation of the reproductive system organs to prepare the young man for reproduc- tion. It also causes the male’s secondary sex char- acteristics (growth of facial hair, development of

heavy bones and muscles, and lowering of the voice) to appear and stimulates the male sex drive.

In adults, testosterone is necessary for continu- ous production of sperm. In cases of hyposecre- tion, the man becomes sterile; such cases are usually treated by testosterone injections. Testosterone production is specifically stimulated by LH (luteinizing hormone). (Chapter 16, which deals with the reproductive system, contains more information on the structure and exocrine function of the testes.)

Table 9.1 (pp. 354–355) summarizes the major endocrine glands and some of their hormones.

Did You Get It? 19. Which gonadal hormone causes a young girl’s

reproductive organs to mature at puberty—estrogen or progesterone?

For the answer, see Appendix A.

Other Hormone-Producing Tissues and organs ➔ Learning Objective

□□ Indicate the endocrine role of the kidneys, the stomach and intestine, the heart, and the placenta.

Besides the major endocrine organs, pockets of hormone-producing cells are found in fatty tissue and in the walls of the small intestine, stomach, kidneys, and heart—organs whose chief functions have little to do with hormone production. Because we describe most of these hormones in later chapters, we summarize only their chief char- acteristics in Table 9.2 (p. 356).

The placenta (plah-sen′tah) is a remarkable organ formed temporarily in the uterus of preg- nant women. In addition to its roles as the respi- ratory, excretory, and nutrition-delivery systems for the fetus, it also produces several protein and steroid hormones that help to maintain the pregnancy and pave the way for delivery of the baby.

During very early pregnancy, a hormone called human chorionic (ko″re-on′ik) gonadotropin (hCG) is produced by the developing embryo and then by the fetal part of the placenta. Similar to LH (luteinizing hormone), hCG stimulates the ovaries

(Text continues on p. 357.)

354 Essentials of Human Anatomy and Physiology

Table 9.1 Major Endocrine Glands and Some of Their Hormones

Gland Hormone chemical class* Major actions regulated by

Hypothalamus Hormones released by the posterior pituitary; releasing and inhibiting hormones that regulate the anterior pituitary (see below)

Pituitary gland

• Posterior lobe (releases hormones made by the hypothalamus)

Oxytocin Peptide Stimulates contraction of uterus and the milk “let-down” reflex

Nervous system (hypothalamus), in response to uterine stretching and/or suckling of a baby

Antidiuretic hormone (ADH)

Peptide Promotes retention of water by kidneys

Hypothalamus, in response to water/salt imbalance

• Anterior lobe Growth hormone (GH)

Protein Stimulates growth (especially of bones and muscles) and metabolism

Hypothalamic releasing and inhibiting hormones

Prolactin (PRL) Protein Stimulates milk production

Hypothalamic hormones

Follicle-stimulating hormone (FSH)

Protein Stimulates production of ova and sperm

Hypothalamic hormones

Luteinizing hormone (LH)

Protein Stimulates ovaries and testes

Hypothalamic hormones

Thyroid-stimulating hormone (TSH)

Protein Stimulates thyroid gland

Thyroxine in blood; hypothalamic hormones

Adrenocorticotropic hormone (ACTH)

Protein Stimulates adrenal cortex to secrete glucocorticoids

Glucocorticoids; hypothalamic hormones

Pineal gland Melatonin Amine Involved in biological rhythms (daily and seasonal)

Light/dark cycles

Thyroid gland Thyroxine (T4) and triiodothyronine (T3)

Amine Stimulates metabolism

TSH

Calcitonin Peptide Reduces blood calcium ion level

Calcium ion level in blood

Parathyroid glands Parathyroid hormone (PTH)

Peptide Raises blood calcium ion level

Calcium ion level in blood

•

Chapter 9: The Endocrine System 355

Gland Hormone chemical class* Major actions regulated by

Thymus Thymosin Peptide “Programs” T lymphocytes

Not known

Adrenal glands

• Adrenal medulla Epinephrine and norepinephrine

Amines Raise blood glucose level; increase rate of metabolism; constrict certain blood vessels

Nervous system (sympathetic division)

• Adrenal cortex Glucocorticoids Steroids Increase blood glucose level

ACTH

Mineralocorticoids Steroids Promote reabsorption of Na+ and excretion of K+ (potassium) in kidneys

Changes in blood volume or blood pressure; K+ or Na+ level in blood

Androgens and estrogens (see entry under gonads)

Pancreas Insulin Protein Reduces blood glucose level

Glucose level in blood

Glucagon Protein Raises blood glucose level

Glucose level in blood

Gonads

• Testes Androgens Steroids Support sperm formation; development and maintenance of male secondary sex characteristics

FSH and LH

• Ovaries Estrogens Steroids Stimulate uterine lining growth; development and maintenance of female secondary sex characteristics

FSH and LH

Progesterone Steroids Promotes growth of uterine lining

FSH and LH

* Any class not indicated as steroid is amino acid based.

Table 9.1 (continued)

356 Essentials of Human Anatomy and Physiology

Table 9.2 Hormones Produced by Organs Other Than the Major Endocrine Organs

Hormone chemical composition Source

Stimulus for secretion Target organ/Effects

Prostaglandins (PGs); several groups indicated by letters A–I (PGA–PGI)

Derived from fatty acid molecules

Plasma membranes of virtually all body cells

Various (local irritation, hormones, etc.)

Have many targets but act locally at site of release. Examples of effects include the following: increase blood pressure by acting as vasoconstrictors; cause constriction of respiratory passageways; stimulate muscle of the uterus, promoting menstrual pain and labor; enhance blood clotting; promote inflammation and pain; increase output of digestive secretions by stomach; cause fever.

Gastrin Peptide Stomach Food Stomach: stimulates glands to release hydrochloric acid (HCl).

Intestinal gastrin Peptide Duodenum (first segment) of small intestine

Food, especially fats

Stomach: stimulates gastric glands and motility.

Secretin Peptide Duodenum Food Pancreas: stimulates release of bicarbonate-rich juice.

Liver: increases release of bile.

Stomach: reduces secretions and motility.

Cholecystokinin (CCK)

Peptide Duodenum Food Pancreas: stimulates release of enzyme- rich juice.

Gallbladder: stimulates expulsion of stored bile.

Duodenal papilla (entry point at the small intestine): causes sphincter to relax, allowing bile and pancreatic juice to enter duodenum.

Erythropoietin Glycoprotein Kidney Hypoxia (low oxygen level)

Bone marrow: stimulates production of red blood cells.

Active vitamin D3 Steroid Kidney (activates provitamin D made by epidermal cells)

PTH Intestine: stimulates active transport of dietary calcium ions across intestinal cell membranes.

Atrial natriuretic peptide (ANP)

Peptide Heart Stretching of atria (receiving chambers) of heart

Kidney: inhibits sodium ion reabsorption and renin release.

Adrenal cortex: inhibits secretion of aldosterone, thereby decreasing blood volume and blood pressure.

Leptin Peptide Adipose tissue Fatty foods Brain: suppresses appetite and increases energy expenditure.

Resistin Peptide Adipose tissue Unknown Fat, muscle, liver: antagonizes insulin’s action on liver cells.

Chapter 9: The Endocrine System 357

to continue producing estrogen and progesterone so that the lining of the uterus is not sloughed off in menses. (The home pregnancy tests sold over the counter test for the presence of hCG in the woman’s urine.) In the third month, the placenta assumes the job of producing estrogen and pro- gesterone, and the ovaries become inactive for the rest of the pregnancy. The high estrogen and pro- gesterone blood levels maintain the lining of the uterus (thus, the pregnancy) and prepare the breasts for producing milk. Human placental lac- togen (hPL) works cooperatively with estrogen and progesterone in preparing the breasts for lac- tation. Relaxin, another placental hormone, causes the mother’s pelvic ligaments and the pubic symphysis to relax and become more flexible, which eases birth passage.

Developmental Aspects of the Endocrine System ➔ Learning Objective

□□ Describe the effect of aging on the endocrine system and body homeostasis.

The embryonic development of the endocrine glands varies. The pituitary gland is derived from epithelium of the oral cavity and a neural tissue projection of the hypothalamus. The pineal gland is entirely neural tissue. Most strictly epithelial glands develop as little saclike outpocketings of the mucosa of the digestive tract. These are the thyroid, thymus, and pancreas. Formation of the gonads and the adrenal and parathyroid glands is much more complex and is not consid- ered here.

Barring outright malfunctions of the endocrine glands, most endocrine organs seem to operate smoothly until old age. In late middle age, the efficiency of the ovaries begins to decline, caus- ing menopause (“the change of life”). During this period, a woman’s reproductive organs begin to atrophy, and the ability to bear children ends.

Problems associated with estrogen deficiency begin to occur, such as arteriosclerosis, osteopo- rosis, decreased skin elasticity, and changes in the operation of the sympathetic nervous system that result in “hot flashes.” In addition, fatigue, ner- vousness, and mood changes such as depression are common. No such dramatic changes seem to happen in men. In fact, many men remain fertile throughout their life span, indicating that testos- terone is still being produced in adequate amounts.

The efficiency of the endocrine system as a whole gradually declines in old age. Striking changes in aging women are due to decreasing levels of female hormones, and there is no ques- tion that growth hormone output by the anterior pituitary declines, which partially explains muscle atrophy in old age. Elderly persons are less able to resist stress and infection. This decreased resis- tance may result from overproduction or under- production of the defensive hormones, because both “derail” the stress defense equilibrium and alter general body metabolism. Additionally, expo- sure to pesticides, industrial chemicals, dioxin, and other soil and water pollutants diminishes endo- crine function, which may explain the higher can- cer rates among older adults in certain areas of the country. Older people are often mildly hypothy- roid. All older people have some decline in insulin production, and type 2 diabetes is most common in this age group.

Did You Get It? 20. Which two digestive system organs are important

sources of hormones associated with digestion? 21. What temporary organ produces the same hormones

as the ovaries? 22. Failure of which endocrine organ(s) leads to

menopause in women? 23. In the elderly, the decline in the level of which

hormone is associated with muscle atrophy? With osteoporosis in women?

For answers, see Appendix A.

358

SYSTEMS IN SYNC

Homeostatic Relationships between the Endocrine System and Other Body Systems

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System

Reproductive System

Respiratory System

Nervous System

Integumentary System

• Several hormones influence blood volume, blood pressure, and heart contractility; erythropoietin stimulates red blood cell production

• Blood is the main transport medium of hormones; heart produces atrial natriuretic peptide

• Aldosterone and ADH influence renal function; erythropoietin released by kidneys promotes red blood cell formation

• Kidneys activate vitamin D (considered a hormone)

• Growth hormone is essential for normal muscular development; other hormones (thyroxine and catecholamines) influence muscle metabolism

• Muscular system mechanically protects some endocrine glands; muscular activity promotes catecholamine release

• Lymphocytes “programmed” by thymic hormones seed the lymph nodes; glucocorticoids depress the immune response and inflammation

• Lymph provides a route for transport of hormones

• Local gastrointestinal (GI) hormones influence GI function; activated vitamin D necessary to absorb calcium from diet; catecholamines influence digestive system activity

• Digestive system provides nutrients to endocrine organs • Hypothalamic, anterior pituitary,

and gonadal hormones direct reproductive system development and function; oxytocin and prolactin are involved in birth and breastfeeding

• Gonadal hormones feed back to influence endocrine system function

• Epinephrine influences ventilation (dilates bronchioles)

• Respiratory system provides oxygen; disposes of carbon dioxide; converting enzyme in lungs converts angiotensin I to angiotensin II

• Many hormones (growth hormone, thyroxine, sex hormones) influence normal maturation and function of the nervous system

• Hypothalamus controls anterior pituitary function and secretes two hormones

• Androgens activate sebaceous glands; estrogen increases skin hydration

• Skin produces a precursor of vitamin D (cholecalciferol or provitamin D)

Skeletal System

• PTH is important in regulating calcium blood levels; growth hormone, T3, T4, and sex hormones are necessary for normal skeletal development

• The skeleton protects some endocrine organs, especially those in brain, chest, and pelvis

Endocrine System

Chapter 9: The Endocrine System 359

Summary

The Endocrine System and Hormone Function— An Overview (pp. 335–338)

1. The endocrine system is a major controlling system of the body. Through hormones, it stimulates such long-term processes as growth and development, metabolism, reproduction, and body defense.

2. Endocrine organs are small and widely separated in the body. Some are part of mixed glands (both endocrine and exocrine in function). Others pro- duce only hormones.

3. Nearly all hormones are amino acid–based hor- mones or steroids.

4. Endocrine organs are activated to release their hor- mones into the blood by hormonal, humoral, or neural stimuli. Negative feedback is important in regulating blood hormone levels.

5. Bloodborne hormones alter the metabolic activities of their target organs. The ability of a target organ to respond to a hormone depends on the presence of receptors in or on its cells to which the hormone binds or attaches.

6. Amino acid–based hormones act through second messengers. Steroid hormones can directly influ- ence the target cell’s DNA or act via second mes- senger.

The Major Endocrine organs (pp. 338–353)

1. Pituitary gland

a. The pituitary gland hangs from the hypothala- mus of the brain by a stalk and is enclosed by bone. It consists of a glandular portion (anterior lobe) and a neural portion (posterior lobe).

b. Except for growth hormone and prolactin, hor- mones of the anterior pituitary are all tropic hor- mones.

(1) Growth hormone (GH): An anabolic and pro- tein-conserving hormone that promotes total body growth. Its most important effect is on skeletal muscles and bones. Untreated hypo- secretion during childhood results in pituitary dwarfism; hypersecretion produces gigantism (in childhood) and acromegaly (in adulthood).

(2) Prolactin (PRL): Stimulates production of breast milk.

(3) Adrenocorticotropic hormone (ACTH): Stim- ulates the adrenal cortex to release its hor- mones, mainly glucocorticoids.

(4) Thyroid-stimulating hormone (TSH): Stimu- lates the thyroid gland to release thyroid hor- mone.

(5) Gonadotropic hormones

(a) Follicle-stimulating hormone (FSH): Be - ginning at puberty, stimulates follicle de - velopment and estrogen production by the female ovaries; promotes sperm pro- duction in the testes.

(b) Luteinizing hormone (LH): Beginning at puberty, stimulates ovulation, causes the ruptured ovarian follicle to produce pro- gesterone; stimulates testes to produce testosterone.

c. The posterior pituitary stores and releases hypo- thalamic hormones on command.

(1) Oxytocin: Stimulates powerful uterine con- tractions and causes milk ejection in nursing women.

(2) Antidiuretic hormone (ADH): Causes kidney tubule cells to reabsorb and conserve body water and increases blood pressure by con- stricting blood vessels. Hyposecretion leads to diabetes insipidus.

d. Releasing and inhibiting hormones made by the hypothalamus regulate release of hormones made by the anterior pituitary. The hypothalamus also makes two hormones that are transported to the posterior pituitary for storage and later release.

2. The pineal gland, located posterior to the third ven- tricle of the brain, releases melatonin, which affects sleep/wake cycles.

3. Thyroid gland

a. The thyroid gland is located in the anterior throat.

b. Thyroid hormone, which has two forms (thy- roxine [T4] and triiodothyronine [T3]), is released from the thyroid follicles when the blood level of TSH rises. Thyroid hormone is the body’s major metabolic hormone. It increases the rate at which cells oxidize glucose and is necessary for normal growth and development. Lack of iodine leads to goiter. Hyposecretion of thyrox- ine results in cretinism in children and myxedema in adults. Hypersecretion results from Graves’ disease or other forms of hyperthyroidism.

360 Essentials of Human Anatomy and Physiology

c. Calcitonin is released by parafollicular cells sur- rounding the thyroid follicles in response to a high blood level of calcium ions. It causes cal- cium to be deposited in bones.

4. Parathyroid glands

a. The parathyroid glands are four small glands located on the posterior aspect of the thyroid gland.

b. A low blood level of calcium stimulates the para- thyroid glands to release parathyroid hormone (PTH). It causes liberation of calcium from bone to blood. Hyposecretion of PTH results in tetany; hypersecretion leads to extreme bone wasting and fractures.

5. The thymus, located in the upper thorax, functions during youth but atrophies in old age. Its hormone, thymosin, promotes maturation of T lymphocytes, important in body defense.

6. Adrenal glands

a. The adrenal glands are paired glands perched on the kidneys. Each gland has two functional endocrine portions, the cortex and the medulla.

b. Adrenal cortex hormones include the following:

(1) Mineralocorticoids, primarily aldosterone, reg - ulate sodium ion (Na+) reabsorption and po- tassium ion (K+) secretion by the kidneys. Their release is stimulated primarily by low Na+ and/or high K+ levels in blood.

(2) Glucocorticoids enable the body to resist long-term stress by increasing the blood glu- cose level and depressing the inflammatory response.

(3) Sex hormones (mainly androgens) are pro- duced in small amounts throughout life.

c. Generalized hypoactivity of the adrenal cortex results in Addison’s disease. Hypersecretion can result in hyperaldosteronism, Cushing’s syn- drome, and/or masculinization.

d. The adrenal medulla produces catecholamines (epinephrine and norepinephrine) in response to sympathetic nervous system stimulation. Its cate- cholamines enhance and prolong the effects of the “fight-or-flight” (sympathetic nervous system) response to short-term stress. Hypersecretion leads to symptoms typical of sympathetic ner- vous system overactivity.

7. Pancreatic islets

a. Located in the abdomen close to the stomach, the pancreas is both an exocrine and endocrine

gland. The endocrine islets release insulin and glucagon to blood.

b. Insulin is released when the blood level of glu- cose is high. It increases the rate of glucose uptake and metabolism by body cells. Hyposecretion of insulin results in diabetes mel- litus, which severely disturbs body metabo - lism. Cardinal signs are polyuria, polydipsia, and polyphagia.

c. Glucagon, released when the blood level of glu- cose is low, stimulates the liver to release glu- cose to blood, thus increasing the blood glucose level.

8. Gonads

a. The ovaries of the female release:

(1) Estrogens: Release of estrogens by ovarian follicles begins at puberty under the influ- ence of FSH. Estrogens stimulate maturation of the female reproductive organs and female secondary sex characteristics. With progester- one, they cause the menstrual cycle.

(2) Progesterone: Progesterone is released from the ovary in response to a high blood level of LH. It works with estrogens in establishing the menstrual cycle.

b. The testes of the male begin to produce testos- terone at puberty in response to LH stimulation. Testosterone promotes maturation of the male reproductive organs, male secondary sex charac- teristics, and production of sperm by the testes.

c. Hyposecretion of gonadal hormones results in sterility in both females and males.

Other Hormone-Producing Tissues and Organs (pp. 353–357)

1. Several organs that are generally nonendocrine in overall function, such as the stomach, small intes- tine, kidneys, and heart, have cells that secrete hor- mones.

2. The placenta is a temporary organ formed in the uterus of pregnant women. Its primary endocrine role is to produce estrogen and progesterone, which maintain pregnancy and ready the breasts for lactation.

Developmental Aspects of the Endocrine System (p. 357)

1. Decreasing function of female ovaries at meno- pause leads to such symptoms as osteoporosis,

Chapter 9: The Endocrine System 361

increase in incidence of diabetes mellitus, immune system depression, lower metabolic rate, and, in some areas, cancer rates.

increased chance of arteriosclerosis, and possible mood changes.

2. Efficiency of all endocrine glands gradually de- creases with aging, which leads to a generalized

Review Questions

Multiple Choice

More than one choice may apply.

1. The major endocrine organs of the body

a. tend to be very large organs.

b. are closely connected with each other.

c. all contribute to the same function (digestion).

d. tend to lie near the midline of the body.

2. Which is (are) true of the hormone melatonin?

a. It is a releasing hormone.

b. Its level is the highest at night.

c. It is a sleep trigger.

d. Iodine is needed to produce it.

3. Which of the following hormones is (are) secreted by neurons?

a. Oxytocin

b. Insulin

c. ADH

d. Cortisol

4. ANP, the hormone secreted by the heart, has exactly the opposite function to this hormone secreted by the outermost zone of the adrenal cortex.

a. Epinephrine

b. Cortisol

c. Aldosterone

d. Testosterone

5. Hormones that act directly or indirectly to elevate the blood glucose level include which of the following?

a. GH

b. Cortisol

c. Insulin

d. ACTH

6. Hypertension may result from hypersecretion of

a. thyroxine.

b. cortisol.

c. aldosterone.

d. ADH.

7. Which of the following is (are) associated with the short-term stress response?

a. Increased blood sugar

b. Mineralocorticoids

c. Epinephrine

d. Increased metabolic rate

8. Which of the following is given as a drug to reduce inflammation?

a. Epinephrine

b. Cortisol

c. Aldosterone

d. ADH

9. The element needed for thyroid gland function is

a. potassium.

b. iodine.

c. calcium.

d. manganese.

Short Answer Essay

10. Explain how the nervous and endocrine systems differ in (a) the rate of their control, (b) the way in which they communicate with body cells, and (c) the types of body processes they control.

11. Which endocrine organs are actually mixed (endocrine and exocrine) glands? Which are purely endocrine?

12. Briefly describe how a steroid hormone affects the activity of the target cell.

13. Provide one example for each way endocrine glands are stimulated to release their hormones.

14. Define negative feedback.

15. Explain why not all organs are target organs for all hormones.

16. Describe the body location of each of the follow- ing endocrine organs: anterior pituitary, pineal gland, thymus, pancreas, ovaries, testes. Then, for each organ, name its hormones and their effect(s) on body processes. Finally, for each hormone, list the important results of its hypersecretion or hyposecretion.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

362 Essentials of Human Anatomy and Physiology

21. The adrenal cortex produces corticosteroids. What is the difference between mineralocorticoids and glucocorticoids with regard to their production sites? What major functions do they monitor?

22. What is insulin resistance?

23. In general, the endocrine system becomes less efficient as we age. List some examples of problems that elderly individuals have as a result of decreasing hormone production.

17. What anatomical feature makes it possible for the releasing hormones from the hypothalamus to influence the activity of the anterior pituitary?

18. What is the primary function of antidiuretic hormone? Why is it also called “vasopressin”?

19. What is the most common cause of hypersecretion by endocrine organs?

20. What is the role of alpha and beta cells in the regulation of blood glucose?

Critical Thinking and Clinical Application Questions

24. Jenny’s body mass index (BMI) is too high, and she wants to lose some weight. Is there a hormone that can suppress appetite? Explain.

25. The parents of 14-year-old Megan are concerned about her height because she is only 4 feet tall and they are both close to 6 feet tall. After tests by their doctor, certain hormones are prescribed for the girl. What is the probable diagnosis? What hormones are prescribed, and why might the girl expect to reach normal height?

26. Shannon, a 28-year-old, has been in the first stage of labor for 15 hours. Her uterine contractions are weak, and her labor is not progressing normally. Shannon and her doctor desire a vaginal delivery, so the physician orders that Pitocin (a synthetic oxytocin) be infused. What is the effect of this hor- mone?

27. Mr. Flores brings his wife to the clinic, concerned about her nervousness, heart palpitations, and excessive sweating. Tests show hyperglycemia and hypertension. What hormones are probably being

hypersecreted? What is the cause? What physical factors allow you to rule out thyroid problems?

28. What are the possible harmful effects of using ana- bolic steroids to increase muscle mass and strength?

29. Melissa, age 40, comes to the clinic, troubled by swelling in her face and unusual fat deposition on her back and abdomen. She reports that she bruises easily. Blood tests show an elevated glucose level. What is your diagnosis, and which glands might be causing the problem?

30. Multiple births are observed commonly in women treated for infertility with drugs that stimulate the release of gonadotropic hormones. What could be the reason?

31. Ryan had symptoms of excessive secretion of PTH (a high blood calcium ion level), and his physicians were certain he had a parathyroid gland tumor. Yet when surgery was performed on his neck, the sur- geon could not find the parathyroid glands at all. Where should the surgeon look next to find the tumorous parathyroid gland?

363

Composition and Functions of Blood ➔ Learning Objectives

□□ Describe the composition and volume of whole blood.

□□ Describe the composition of plasma, and discuss its importance in the body.

Blood is unique: It is the only fluid tissue in the body. Although blood appears to be a thick, homogeneous liquid, the microscope reveals that it has both solid and liquid components.

Blood is the “river of life.” Blood transports everything that must be carried from one place to another within the body— nutrients, hormones, wastes (headed for elimina- tion from the body), and body heat—through blood vessels. Long before modern medicine, blood was viewed as magical because when it drained from the body, life departed as well.

In this chapter, we consider the composition and function of this life-sustaining fluid. In the car- diovascular system chapter (Chapter 11), we dis- cuss the means by which blood is propelled throughout the body.

Blood10 WHAT

HOW

WHY

Blood transports substances such as oxygen

and nutrients throughout the body and participates in processes

such as clotting and fighting infections.

Blood is moved through blood vessels by the

pumping action of the heart. This fluid contains red blood cells to carry

oxygen, clotting proteins to stop bleeding, and white blood cells

to fight infections.

Transportation via blood is the only way substances can be moved to distant body locations. In addition, clotting

proteins are found only in blood—without them, a minor cut

could be life-threatening!

InstruCtors

New Building Vocabulary Coaching Activities for this chapter are assignable in

364 Essentials of Human Anatomy and Physiology

substances are dissolved in this straw-colored fluid. Examples of dissolved substances include nutri- ents, salts (electrolytes), respiratory gases, hor- mones, plasma proteins, and various wastes and products of cell metabolism. (Other substances found in plasma are listed in Figure 10.1).

Plasma proteins are the most abundant solutes in plasma. Except for antibodies and protein-based hormones, the liver makes most plasma proteins. The plasma proteins serve a variety of functions. For instance, albumin (al-bu′min) acts as a carrier to shuttle certain molecules through the circula- tion, is an important blood buffer, and contributes to the osmotic pressure of blood, which acts to keep water in the bloodstream. Clotting proteins help stem blood loss when a blood vessel is injured, and antibodies help protect the body from pathogens. Plasma proteins are not taken up by cells to be used as food fuels or metabolic nutri- ents, as are other solutes such as glucose, fatty acids, and oxygen.

The composition of plasma varies continu- ously as cells exchange substances with the blood. Assuming a healthy diet, however, the composi- tion of plasma is kept relatively constant by vari- ous homeostatic mechanisms of the body. For example, when blood proteins drop to undesir- able levels, the liver is stimulated to make more proteins, and when the blood starts to become too acid (acidosis) or too basic (alkalosis), both the respiratory and urinary systems are called into action to restore it to its normal, slightly alkaline pH range of 7.35 to 7.45. Various body organs make dozens of adjustments day in and day out to maintain the many plasma solutes at life-sustaining levels. Besides transporting various substances around the body, plasma helps to distribute body heat, a by-product of cellular metabolism, evenly throughout the body.

Did You Get It? 1. Which body organ plays the main role in producing

plasma proteins? 2. What are the three major categories of formed

elements? 3. What determines whether blood is bright red (scarlet)

or dull red?

For answers, see Appendix A.

Components Blood is a complex connective tissue in which liv- ing blood cells, the formed elements, are sus- pended in plasma (plaz′mah), a nonliving fluid matrix. The collagen and elastin fibers typical of other connective tissues are absent from blood; instead, dissolved proteins become visible as fibrin strands during blood clotting.

If a sample of blood is separated, the plasma rises to the top, and the formed elements, being heavier, fall to the bottom (Figure 10.1). Most of the reddish “pellet” at the bottom of the tube is erythro- cytes (ĕ-rith′ro-sı̄ts; erythro = red), or red blood cells, the formed elements that function in oxygen trans- port. There is a thin, whitish layer called the buffy coat at the junction between the erythrocytes and the plasma (just barely visible in Figure 10.1). This layer contains the remaining formed elements, leu- kocytes (lu′ko-sı̄ts; leuko = white), white blood cells that act in various ways to protect the body; and platelets, cell fragments that help stop bleeding. Erythrocytes normally account for about 45 percent of the total volume of a blood sample, a percentage known as the hematocrit (“blood fraction”). White blood cells and platelets contribute less than 1 per- cent, and plasma makes up most of the remaining 55 percent of whole blood.

Physical Characteristics and Volume Blood is a sticky, opaque fluid that is heavier than water and about five times thicker, or more vis- cous, largely because of its formed elements. Depending on the amount of oxygen it is carrying, the color of blood varies from scarlet (oxygen- rich) to a dull red or purple (oxygen-poor). Blood has a characteristic metallic, salty taste (something we often discover as children). Blood is slightly alkaline, with a pH between 7.35 and 7.45. Its tem- perature (38°C, or 100.4°F) is always slightly higher than body temperature because of the fric- tion produced as blood flows through the vessels. Blood accounts for approximately 8 percent of body weight, and its volume in healthy adults is 5 to 6 liters, or about 6 quarts.

Plasma Plasma, which is approximately 90 percent water, is the liquid part of the blood. Over 100 different

Chapter 10: Blood 365

Constituent Major Functions

Plasma 55%

Water 90% of plasma volume; solvent for carrying other substances; absorbs heat

Osmotic balance, pH buffering, regulation of membrane permeability

Clotting of blood

Osmotic balance, pH buffering

Defense (antibodies) and lipid transport

Substances transported by blood Nutrients (glucose, fatty acids, amino acids, vitamins)

Waste products of metabolism (urea, uric acid)

Respiratory gases (O2 and CO2)

Sodium Potassium Calcium Magnesium Chloride Bicarbonate

Albumin

Fibrinogen Globulins

Formed elements (cells) 45%

Cell Type

Erythrocytes (red blood cells)

Leukocytes (white blood cells)

Basophil

Neutrophil

Platelets Blood clotting250,000–400,000

Eosinophil

4–6 million

4,800–10,800

Transport oxygen and help transport carbon dioxide

Defense and immunity

Lymphocyte

Monocyte

FunctionsNumber (per mm3 of blood)

Salts (electrolytes)

Plasma proteins

Hormones (steroids and thyroid hormone are carried by plasma proteins)

Figure 10.1 the composition of blood.How would a decrease in the amount of plasma proteins affect plasma volume?Q:

Plasma proteins create the osmotic pressure that helps to maintain plasma volume and draws leaked fluid back into circulation. Hence, a decrease in the

amount of plasma proteins would result in a reduced plasma volume.

366 Essentials of Human Anatomy and Physiology

body. They are superb examples of the link between structure and function. RBCs differ from other blood cells because they are anucleate (a-nu′kle-at); that is, they lack a nucleus. They also contain very few organelles. In fact, mature RBCs circulating in the blood are literally “bags” of hemoglobin molecules. Hemoglobin (he″mo- glo′bin) (Hb), an iron-bearing protein, transports most of the oxygen that is carried in the blood. (It also binds with a small amount of carbon dioxide.)

Formed Elements ➔ Learning Objectives

□□ List the cell types making up the formed elements, and describe the major functions of each type.

□□ Define anemia, polycythemia, leukopenia, and leukocytosis, and list possible causes for each condition.

If you observe a stained smear of human blood under a light microscope, you will see disc-shaped red blood cells, a variety of gaudily stained spheri- cal white blood cells, and some scattered platelets that look like debris (Figure 10.2). However, eryth- rocytes vastly outnumber the other types of formed elements. (Look ahead to Table 10.2 on p. 370, which provides a summary of the important char- acteristics of the various formed elements.)

Erythrocytes

Erythrocytes, or red blood cells (RBCs), func- tion primarily to ferry oxygen to all cells of the

➔ConCeptLinkRecall that hemoglobin is an example of a globular protein (look back at Figure 2.19b, p. 76). Globular, or functional, proteins have tertiary structure, meaning that they are folded into a very specific shape. In this case, the folded structure of hemoglobin allows it to perform the specific function of binding and carrying oxygen. The structure of globular proteins is also very vulnerable to pH changes and can be denatured (unfolded) by a pH that is too low (acidic); denatured hemoglobin is unable to bind oxygen. ➔

Moreover, because erythrocytes lack mito- chondria and make ATP by anaerobic mecha- nisms, they do not use up any of the oxygen they are transporting, making them very efficient oxy- gen transporters indeed.

Erythrocytes are small, flexible cells shaped like biconcave discs—flattened discs with depressed centers on both sides (see the photo on p. 367). Because of their thinner centers, erythro- cytes look like miniature doughnuts when viewed with a microscope. Their small size and peculiar shape provide a large surface area relative to their volume, making them ideally suited for gas exchange.

RBCs outnumber white blood cells by about 1,000 to 1 and are the major factor contributing to blood viscosity. Although the numbers of RBCs in the circulation do vary, there are normally about 5 million cells per cubic millimeter of blood. (A cubic millimeter [mm3] is a very tiny drop of blood, almost too small to be seen.) When the number of RBC/mm3 increases, blood viscosity, or thickness, increases. Similarly, as the number of RBCs decreases, blood thins and flows more rapidly.

Although the numbers of RBCs are important, it is the amount of hemoglobin in the bloodstream

Erythrocytes Neutrophils

Platelets Lymphocyte

Figure 10.2 Photomicrograph of a blood smear. Most of the cells in this view are erythrocytes (red blood cells). Two kinds of leukocytes (white blood cells) are also present: lymphocytes and neutrophils. Also note the platelets.

View Histology

Chapter 10: Blood 367

10 parasite. Hence, the malaria-causing parasite is prevented from multiplying within the red blood cells, and individuals with the sickle cell gene have a better chance of surviving where malaria is prevalent. Only individuals carrying two copies of the defective gene have sickle cell anemia. Those carrying just one sickling gene have sickle cell trait (SCT); they generally do not display the symptoms but can pass on the sickling gene to their offspring.

An excessive or abnormal increase in the num- ber of erythrocytes is polycythemia (pol″e-si- the′me-ah). Polycythemia may result from bone marrow cancer (polycythemia vera). It may also be a normal physiologic (homeostatic) response to

at any time that really determines how well the erythrocytes are performing their role of oxygen transport. The more hemoglobin molecules the RBCs contain, the more oxygen they will be able to carry. A single red blood cell contains about 250 million hemoglobin molecules, each capable of binding 4 molecules of oxygen, so each of these tiny cells can carry about 1 billion mole- cules of oxygen! However, much more important clinically is the fact that normal blood contains 12–18 grams (g) of hemoglobin per 100 milliliters (ml) of blood. The hemoglobin content is slightly higher in men (13–18 g/ml) than in women (12–16 g/ml).

Homeostatic Imbalance 10.1

A decrease in the oxygen-carrying ability of the blood, whatever the reason, is called anemia (ah-ne′me-ah; “lacking blood”). Anemia may be the result of (1) a lower-than-normal number of RBCs or (2) abnormal or deficient hemoglobin content in the RBCs. There are several types of anemia (classified and described briefly in Table 10.1, p. 368), but one of these, sickle cell anemia, deserves a little more attention because people with this genetic disorder are frequently seen in hospital emergency rooms.

In sickle cell anemia (SCA), the body does not form normal hemoglobin (as in the RBC shown in part (a) of the figure). Instead, abnor- mal hemoglobin is formed that becomes spiky and sharp (see part (b) of the figure) when either oxygen is unloaded or the oxygen content in the blood decreases below normal. This change in hemoglobin causes the RBCs to become sickled (crescent-shaped), to rupture easily, and to dam up small blood vessels. These events interfere with oxygen delivery (leaving victims gasping for air) and cause extreme pain. It is amazing that this havoc results from a change in just one of the amino acids in two of the four polypeptide chains of the hemoglobin molecule!

Sickle cell anemia occurs chiefly in dark- skinned people who live in the malaria belt of Africa and among their descendants. Apparently, the same gene that causes sickling makes red blood cells infected by the malaria-causing para- site stick to the capillary walls and then lose potas- sium, an essential nutrient for survival of the

1 2 3 4 5 6

7...146

(a) Normal RBC and part of the amino acid sequence of its hemoglobin

(b) Sickled RBC and part of its hemoglobin sequence

1 2 3 4 5 6

7...146

Comparison of (a) a normal erythrocyte to (b) a sickled erythrocyte (6,550*).

368 Essentials of Human Anatomy and Physiology

diapedesis (di″ah-pĕ-de′sis; “leaping across”). The circulatory system is simply their means of transportation to areas of the body where their services are needed for inflammatory or immune responses (as described in Chapter 12).

In addition, WBCs can locate areas of tissue damage and infection in the body by responding to certain chemicals that diffuse from the damaged cells. This capability is called positive chemo- taxis (ke″mo-tax′is). Once they have “caught the scent,” the WBCs move through the tissue spaces by amoeboid (ah-me′boid) motion (they form flowing cytoplasmic extensions that help move them along). By following the diffusion gradient, they pinpoint areas of tissue damage and rally round in large numbers to destroy microorganisms and dispose of dead cells.

Whenever WBCs mobilize for action, the body speeds up their production, and as many as twice the normal number of WBCs may appear in the blood within a few hours. A total WBC count above 11,000 cells/mm3 is referred to as leuko- cytosis (lu″ko-si-to′sis) (cytosis = an increase in cells). Leukocytosis generally indicates that a bacterial or viral infection is stewing in the body. The opposite condition, leukopenia (lu″ko-pe′ne-ah), is an abnormally low WBC

living at high altitudes, where the air is thinner and less oxygen is available (secondary polycythe- mia). Occasionally, athletes participate in illegal “blood doping,” which is the infusion of a person’s own RBCs back into their bloodstream to artifi- cially raise oxygen-carrying capacity. The major problem that results from excessive numbers of RBCs is increased blood viscosity, which causes blood to flow sluggishly in the body and impairs circulation. __________________________________✚

Leukocytes

Although leukocytes, or white blood cells (WBCs), are far less numerous than red blood cells, they are crucial to body defense. On aver- age, there are 4,800 to 10,800 WBCs/mm3 of blood, and they account for less than 1 percent of total blood volume. White blood cells contain nuclei and the usual organelles, which makes them the only complete cells in blood.

Leukocytes form a protective, movable army that helps defend the body against damage by bacteria, viruses, parasites, and tumor cells. As such, they have some very special characteristics. Red blood cells are confined to the bloodstream. White blood cells, by contrast, are able to slip into and out of the blood vessels—a process called

Table 10.1 types of Anemia

Direct cause resulting from Leading to

Decrease in RBC number Sudden hemorrhage Hemorrhagic anemia

Lysis of RBCs as a result of bacterial infections Hemolytic (he″mo-lit′ik) anemia

Lack of vitamin B12 (usually due to lack of intrinsic factor required for absorption of the vitamin; intrinsic factor is formed by stomach mucosa cells)

Pernicious (per-nish′us) anemia

Depression/destruction of bone marrow by cancer, radiation, or certain medications

Aplastic anemia

Inadequate hemoglobin content in RBCs

Lack of iron in diet or slow/prolonged bleeding (such as heavy menstrual flow or bleeding ulcer), which depletes iron reserves needed to make hemoglobin; RBCs are small and pale because they lack hemoglobin

Iron-deficiency anemia

Abnormal hemoglobin in RBCs

Genetic defect leads to abnormal hemoglobin, which becomes sharp and sickle-shaped under conditions of increased oxygen use by body; occurs mainly in people of African descent

Sickle cell anemia

Chapter 10: Blood 369

etc.) ingested in food such as raw fish or en- tering through the skin. When eosinophils en- counter a parasitic worm, they gather around and release enzymes from their cytoplasmic granules onto the parasite’s surface, digesting it away.

• Basophils, the rarest of the WBCs, have large histamine-containing granules that stain dark blue. Histamine is an inflammatory chemical that makes blood vessels leaky and attracts other WBCs to the inflamed site.

The second group of WBCs, agranulocytes, lack visible cytoplasmic granules. Their nuclei are closer to the norm—that is, they are spheri- cal, oval, or kidney-shaped. The agranulocytes include lymphocytes (lim′fo-sı̄tz) and monocytes (mon′o-sı̄tz). • Lymphocytes have a large, dark purple nu-

cleus that occupies most of the cell volume. Only slightly larger than RBCs, lymphocytes tend to take up residence in lymphatic tissues, such as the tonsils, where they play an impor- tant role in the immune response. They are the second most numerous leukocytes in the blood.

• Monocytes are the largest of the WBCs. Except for their more abundant cytoplasm and distinctive U- or kidney-shaped nucleus, they resemble large lymphocytes. When they mi- grate into the tissues, they change into macro- phages (macro = large; phage = one that eats) with huge appetites. Macrophages are impor- tant in fighting chronic infections, such as tu- berculosis, and in activating lymphocytes.

Students are often asked to list the WBCs in order of relative abundance in the blood—from most to least. The following phrase may help you with this task: Never let monkeys eat bananas (neutrophils, lymphocytes, monocytes, eosino- phils, basophils).

Platelets

Platelets are not technically cells. They are frag- ments of bizarre multinucleate cells called mega- karyocytes (meg″ah-kar′e-o-sı̄tz), which pinch off thousands of anucleate platelet “pieces” that quickly seal themselves off from the surrounding fluids. The platelets appear as darkly staining, irregularly shaped bodies scattered among the other blood cells. The normal platelet count in

count (penia = deficiency). It is commonly caused by certain drugs, such as corticosteroids and anti- cancer agents.

Homeostatic Imbalance 10.2

Leukocytosis is a normal and desirable response to infectious threats to the body. By contrast, the excessive production of abnormal WBCs that occurs in infectious mononucleosis and leukemia is dis- tinctly pathological. In leukemia (lu-ke′me-ah), lit- erally “white blood,” the bone marrow becomes cancerous, and huge numbers of WBCs are turned out rapidly. Although this might not appear to pre- sent a problem, the “newborn” WBCs are immature and incapable of carrying out their normal protec- tive functions. Consequently, the body becomes easy prey for disease-causing bacteria and viruses. Additionally, because other blood cell lines are crowded out, severe anemia and bleeding problems result. _______________________________________✚

WBCs are classified into two major groups— granulocytes and agranulocytes—depending on whether or not they contain visible granules in their cytoplasm. Specific characteristics and micro- scopic views of the leukocytes are presented in table 10.2, p. 370.

Granulocytes (gran′u-lo-sı̄ tz″) are granule- containing WBCs. They have lobed nuclei, which typically consist of several rounded nuclear areas connected by thin strands of nuclear material. The granules in their cytoplasm stain specifically with Wright’s stain. The granulocytes include neutro- phils (nu′tro-filz), eosinophils (e″o-sin′o-filz), and basophils (ba′so-filz). • Neutrophils are the most numerous WBCs.

They have a multilobed nucleus and very fine granules that respond to both acidic and basic stains. Consequently, the cytoplasm as a whole stains pink. Neutrophils are avid phagocytes at sites of acute infection. They are particularly partial to bacteria and fungi, which they kill during a respiratory burst that deluges the phagocytized invaders with a potent brew of oxidizing substances (bleach, hydrogen perox- ide, and others).

• Eosinophils have a blue-red nucleus that re- sembles earmuffs and brick-red cytoplasmic granules. Their number increases rapidly dur- ing infections by parasitic worms (tapeworms,

370 Essentials of Human Anatomy and Physiology

Table 10.2 Characteristics of Formed Elements of the Blood

Cell type

occurrence in blood (cells per mm3) Cell anatomy* Function

Erythrocytes (red blood cells)

4–6 million Salmon-colored biconcave disks; anucleate; literally, sacs of hemoglobin; most organelles have been ejected

Transport oxygen bound to hemoglobin molecules; also transport small amount of carbon dioxide

Leukocytes (white blood cells)

4,800–10,800

Granulocytes • Neutrophils 3,000–7,000

(40–70% of WBCs)

Cytoplasm stains pale pink and contains fine granules, which are difficult to see; deep purple nucleus consists of three to seven lobes connected by thin strands of nucleoplasm

Active phagocytes; number increases rapidly during short-term or acute infections

• Eosinophils 100–400 (1–4% of WBCs)

Red coarse cytoplasmic granules; figure-8 or bilobed nucleus stains blue-red

Kill parasitic worms by deluging them with digestive enzymes; play a complex role in allergy attacks

• Basophils 20–50 (0–1% of WBCs)

Cytoplasm has a few large blue- purple granules; U- or S-shaped nucleus with constrictions, stains dark blue

Release histamine (vasodilator chemical) at sites of inflammation; contain heparin, an anticoagulant

Agranulocytes • Lymphocytes 1,500–3,000

(20–45% of WBCs)

Cytoplasm pale blue and appears as thin rim around nucleus; spherical (or slightly indented) dark purple-blue nucleus

Part of immune system; B lymphocytes produce antibodies; T lymphocytes are involved in graft rejection and in fighting tumors and viruses via direct cell attack

• Monocytes 100–700 (4–8% of WBCs)

Abundant gray-blue cytoplasm; dark blue-purple nucleus often U- or kidney-shaped

Active phagocytes that become macrophages in the tissues; long-term “cleanup team”; increase in number during chronic infections; activate lymphocytes during immune response

Platelets 150,000–400,000 Essentially irregularly shaped cell fragments; stain deep purple

Needed for normal blood clotting; initiate clotting cascade by clinging to torn area

*Appearance when stained with Wright’s stain.

Chapter 10: Blood 371

group is degraded to bilirubin, which is then secreted into the intestine by liver cells. There it becomes a brown pigment called stercobilin that leaves the body in feces. Globin is broken down to amino acids, which are released into the circulation.

Lost blood cells are replaced more or less con- tinuously by the division of hemocytoblasts in the red bone marrow. The developing RBCs divide many times and then begin synthesizing huge amounts of hemoglobin. When enough hemoglo- bin has been accumulated, the nucleus and most organelles are ejected, and the cell collapses inward. The result is the young RBC, called a retic- ulocyte (rĕ-tik′u-lo-sı̄t) because it still contains some rough endoplasmic reticulum (ER). The reticulocytes enter the bloodstream to begin their task of transporting oxygen. Within 2 days of release, they have ejected the remaining ER and have become fully functioning erythrocytes. The entire developmental process from hemocytoblast to mature RBC takes 3 to 5 days.

The rate of erythrocyte production is con- trolled by a hormone called erythropoietin (ĕ-rith″ro-po-e′tin). Normally a small amount of erythropoietin circulates in the blood at all times,

blood is about 300,000 cells per mm3. Platelets are needed for the clotting process that stops blood loss from broken blood vessels (see Table 10.2). (We explain this process on pp. 373–374.)

Did You Get It? 4. What is the role of hemoglobin in the red blood cell? 5. Which white blood cells are most important in body

immunity? 6. If you had a severe infection, would you expect your

WBC count to be closest to 5,000, 10,000, or 15,000 per mm3?

7. Little Lisa is pale and fatigued. What disorder of erythrocytes might she be suffering from?

For answers, see Appendix A.

Hematopoiesis (Blood Cell Formation) ➔ Learning Objective

□□ Explain the role of the hemocytoblast.

Blood cell formation, or hematopoiesis (hem″ah- to-po-e′sis), occurs in red bone marrow, or myeloid tissue. In adults, this tissue is found chiefly in the axial skeleton, pectoral and pelvic girdles, and proximal epiphyses of the humerus and femur. Each type of blood cell is produced in different numbers in response to changing body needs and different stimuli. After they mature, they are dis- charged into the blood vessels surrounding the area. On average, the red marrow turns out an ounce of new blood containing 100 billion new cells every day.

All the formed elements arise from a common stem cell, the hemocytoblast (he″mo-si′to-blast; “blood cell former”), which resides in red bone marrow. Their development differs, however, and once a cell is committed to a specific blood path- way, it cannot change. The hemocytoblast forms two types of descendants—the lymphoid stem cell, which produces lymphocytes, and the myeloid stem cell, which can produce all other classes of formed elements (Figure 10.3).

Formation of red Blood Cells

Because they are anucleate, RBCs are unable to synthesize proteins, grow, or divide. As they age, RBCs become rigid and begin to fall apart in 100 to 120 days. Their remains are eliminated by phago- cytes in the spleen, liver, and other body tissues. RBC components are salvaged. Iron is bound to protein as ferritin, and the balance of the heme

Hemocytoblast stem cells

Secondary stem cells

Basophils

Eosinophils

NeutrophilsMonocytesLymphocytes

Erythrocytes

Platelets

Lymphoid stem cells

Myeloid stem cells

Figure 10.3 the development of blood cells. All blood cells differentiate from hemocytoblast stem cells in red bone marrow. The population of stem cells renews itself by mitosis. Some daughter cells become lymphoid stem cells, which develop into two classes of lymphocytes that function in the immune response. All other blood cells differentiate from myeloid stem cells.

372 Essentials of Human Anatomy and Physiology

the bloodstream, depresses erythropoietin release and red blood cell production. However, RBC pro- duction is controlled not by the relative number of RBCs in the blood, but by the ability of the avail- able RBCs to transport enough oxygen to meet the body’s demands.

Formation of White Blood Cells and Platelets

Like erythrocyte production, the formation of leu- kocytes and platelets is stimulated by hormones. These colony stimulating factors (CSFs) and inter- leukins not only prompt red bone marrow to turn out leukocytes, but also enhance the ability of mature leukocytes to protect the body. Apparently, they are released in response to specific chemical signals in the environment, such as inflammatory chemicals and certain bacteria or their toxins. The hormone thrombopoietin accelerates the produc- tion of platelets from megakaryocytes, but little is known about how that process is regulated.

When bone marrow problems or a disease condition such as leukemia is suspected, a special needle is used to withdraw a small sample of red marrow from one of the flat bones (ilium or ster- num) close to the body surface. This procedure provides cells for a microscopic examination called a bone marrow biopsy.

and red blood cells are formed at a fairly constant rate. Although the liver produces some, the kid- neys play the major role in producing this hor- mone. When the blood level of oxygen begins to decline for any reason, the kidneys step up their release of erythropoietin. Erythropoietin targets the bone marrow, prodding it into “high gear” to turn out more RBCs. (Follow this sequence of events in Figure 10.4.)

➔ConCeptLinkRecall the concept of negative feedback control (see Chapter 1, p. 45). In this case, erythropoietin is released in response to a low blood oxygen level, which stimu- lates the bone marrow to produce more red blood cells. With their numbers increased, the red cells carry more oxygen, increasing the blood oxygen level and reducing the initial stimulus. ➔

The kidneys produce most of the body’s erythropoietin, which stimulates red blood cell

production by the bone marrow.

Erythropoietin stimulates red bone marrow.

Enhanced erythropoiesis increases RBC count.

O2−carrying ability of blood increases.

Homeostasis: Normal blood oxygen levels

IMBALANCE

Stimulus Low blood O2−carrying ability due to • Decreased RBC count • Decreased amount of hemoglobin • Decreased availability of O2

Kidneys (and liver, to a smaller extent) release erythropoietin.

Why do many people with advanced kidney disease become anemic?Q:

As you might expect, an overabundance of erythrocytes, or an excessive amount of oxygen in

Figure 10.4 Mechanism for regulating the rate of rBC production.

Chapter 10: Blood 373

Coagulation events occur. At the same time, the injured tissues are releasing tissue factor (TF), which interacts with PF3 (platelet factor 3), a phospholipid that coats the surfaces of the platelets. This combination interacts with other clotting factors and calcium ions (Ca2+), which are essential for many steps in the clotting pro- cess, to form an activator that leads to the for- mation of thrombin, an enzyme. Thrombin then joins soluble fibrinogen (fi-brin′o-jen) proteins into long, hairlike molecules of insoluble

Did You Get It? 8. What is the name of the stem cell that gives rise to all

formed elements? 9. What property of RBCs dooms them to a limited life

span of about 120 days? 10. How is the production of platelets different from that

of all other formed elements?

For answers, see Appendix A.

Hemostasis ➔ Learning Objectives

□□ Describe the blood-clotting process.

□□ name some factors that may inhibit or enhance the blood-clotting process.

Normally, blood flows smoothly past the intact lin- ing (endothelium) of blood vessel walls. But if a blood vessel wall breaks, a series of reactions starts the process of hemostasis (hem = blood; stasis = standing still), or stopping the bleeding. This response, which is fast and localized, involves many substances normally present in plasma, as well as some that are released by platelets and injured tissue cells.

Phases of Hemostasis Hemostasis involves three major phases, which occur in rapid sequence: vascular spasms, plate- let plug formation, and coagulation, or blood clotting. Blood loss at the site is prevented when fibrous tissue grows into the clot and seals the hole in the blood vessel.

Hemostasis occurs as follows (Figure 10.5):

Vascular spasms occur. The immediate response to blood vessel injury is vasoconstric- tion, which causes blood vessel spasms. The spasms narrow the blood vessel, decreasing blood loss until clotting can occur. (Other fac- tors causing vessel spasms include direct injury to the smooth muscle cells, stimulation of local pain receptors, and release of serotonin by anchored platelets.)1

Platelet plug forms. Platelets are repelled by an intact endothelium, but when the underly- ing collagen fibers of a broken vessel are exposed, the platelets become “sticky” and cling to the damaged site. Anchored platelets release chemicals that enhance the vascular spasms and attract more platelets to the site. As more and more platelets pile up, a platelet plug forms.

Collagen fibers

Platelets

Fibrin

Step Vascular spasms occur. • Smooth muscle contracts, causing vasoconstriction.

Step Platelet plug forms. • Injury to lining of vessel exposes collagen fibers; platelets adhere.

• Platelets release chemicals that make nearby platelets sticky; platelet plug forms.

Step Coagulation events occur. • Clotting factors present in plasma and released by injured tissue cells interact with Ca2+ to form thrombin, the enzyme that catalyzes joining of fibrinogen molecules in plasma to fibrin. • Fibrin forms a mesh that traps red blood cells and platelets, forming the clot.

Figure 10.5 Events of hemostasis.

374 Essentials of Human Anatomy and Physiology

lungs (pulmonary thrombosis, as shown in the fig- ure), the consequences may be death of lung tissue and fatal hypoxia (inadequate oxygen delivery to body tissues). If a thrombus breaks away from the vessel wall and floats freely in the bloodstream, it becomes an embolus (em′bo-lus; plural emboli). An embolus is usually no problem unless or until it lodges in a blood vessel too narrow for it to pass through. For example, a cerebral embolus may cause a stroke, in which brain tissue dies.

Undesirable clotting may be caused by any- thing that roughens the endothelium of a blood vessel and encourages clinging of platelets, such as severe burns, physical blows, or an accumula- tion of fatty material. Slowly flowing blood, or blood pooling, is another risk factor, especially in immobilized patients. In this case, clotting factors are not washed away as usual and accumulate so that clot formation becomes possible. A number of anticoagulants, the most important of which are aspirin, heparin, and warfarin, are used clinically for thrombus-prone patients.

Bleeding Disorders The most common causes of abnormal bleeding are platelet deficiency (thrombocytopenia) and deficits of some of the clotting factors, such as might result from impaired liver function or certain genetic conditions.

Thrombocytopenia results from an insuffi- cient number of circulating platelets. It can arise from any condition that suppresses the bone mar- row, such as bone marrow cancer, radiation, or certain drugs. In this disorder, even normal move- ments cause spontaneous bleeding from small blood vessels. This is evidenced by many small purplish blotches, called petechiae (pĕ-te′ke-e), that resemble a rash on the skin.

fibrin. Fibrin forms a meshwork that traps RBCs and forms the basis of the clot (Figure 10.6). Within the hour, the clot begins to retract, squeezing serum (plasma minus the clotting proteins) from the mass and pulling the rup- tured edges of the blood vessel closer together.

Normally, blood clots within 3 to 6 minutes. As a rule, once the clotting cascade has started, the triggering factors are rapidly inactivated to prevent widespread clotting. Eventually, the endothelium regenerates, and the clot is broken down. Once these events of the clotting cascade were under- stood, it became clear that placing sterile gauze over a cut or applying pressure to a wound would speed up the clotting process. The gauze provides a rough surface to which the platelets can adhere, and the pressure fractures cells, increasing the release of tissue factor locally.

Disorders of Hemostasis

Homeostatic Imbalance 10.3

The major disorders of hemostasis include unde- sirable clot formation and bleeding disorders.

Undesirable Clotting Despite the body’s safeguards against abnormal clot- ting, undesirable clots sometimes form in unbroken blood vessels, particularly in the legs. A clot that develops and persists in an unbroken blood vessel is called a thrombus (throm′bus). If the thrombus is large enough, it may prevent blood from flowing to the cells beyond the blockage. For example, if the blockage forms in the blood vessels serving the

Figure 10.6 Fibrin clot. Scanning electron micrograph (artificially colored) of red blood cells trapped in a mesh of fibrin threads.

A thrombus, or clot, occluding a small pulmonary blood vessel in a human lung.

Chapter 10: Blood 375

pallor and weakness. Loss of over 30 percent causes severe shock, which can be fatal.

Whole blood transfusions are routinely given to replace substantial blood loss and to treat severe anemia or thrombocytopenia. The usual blood bank procedure involves collecting blood from a donor and mixing it with an anticoagulant to prevent clot- ting. The treated blood can be stored (refrigerated at 4°C, or 39.2°F) until needed for about 35 days.

Human Blood Groups Although whole blood transfusions can save lives, people have different blood groups, and transfus- ing incompatible or mismatched blood can be fatal. How so? The plasma membranes of RBCs, like those of all other body cells, bear genetically deter- mined proteins (antigens), which identify each per- son as unique. An antigen (an′t ı̆-jen) is a sub- stance that the body recognizes as foreign; it stimulates the immune system to mount a defense against it. Most antigens are foreign proteins, such as those that are part of viruses or bacteria that have managed to invade the body. Although each of us tolerates our own cellular (self) antigens, one person’s RBC proteins will be recognized as for- eign if transfused into another person with differ- ent RBC antigens. The “recognizers” are antibod- ies present in plasma that attach to RBCs bearing surface antigens different from those on the patient’s (recipient’s) RBCs. Binding of the antibod- ies causes the foreign RBCs to clump, a phenome- non called agglutination* (ah-gloo″t ı̆-na′shun), which leads to the clogging of small blood vessels throughout the body. During the next few hours, the foreign RBCs are lysed (ruptured), and their hemoglobin is released into the bloodstream.

Although the transfused blood is unable to deliver the increased oxygen-carrying capacity hoped for and some tissue areas may be deprived of blood, the most devastating consequence of severe transfu- sion reactions is that the freed hemoglobin molecules may block the kidney tubules, causing kidney failure and death. Transfusion reactions can also cause fever, chills, nausea, and vomiting, but in the absence of kidney shutdown these reactions are rarely fatal. Treatment is aimed at preventing kidney damage by infusing fluids to dilute and dissolve the hemoglobin and diuretics to flush it out of the body in urine.

When the liver is unable to synthesize its usual supply of clotting factors, abnormal and often severe bleeding episodes occur. If vitamin K (needed by the liver cells to produce the clotting factors) is deficient, the problem is easily corrected with vitamin K supplements. However, when liver function is severely impaired (as in hepatitis and cirrhosis), only whole blood transfusions are help- ful. Transfusions of concentrated platelets provide temporary relief from bleeding.

The term hemophilia (he″mo-fil′e-ah) applies to several different hereditary bleeding disorders that result from a lack of any of the factors needed for clotting. The hemophilias have similar signs and symptoms that begin early in life. Even minor tis- sue trauma results in prolonged bleeding and can be life-threatening. Repeated bleeding into joints causes them to become disabled and painful.

When a bleeding episode occurs, hemophili- acs are given a transfusion of fresh plasma or injections of the purified clotting factor they lack. Because hemophiliacs are absolutely dependent on one or the other of these thera- pies, some have become the victims of blood- transmitted viral diseases such as hepatitis and AIDS. (AIDS, acquired immune deficiency syn- drome, is a condition of depressed immunity and is described in Chapter 12.) These problems have been largely resolved because of the avail- ability of genetically engineered clotting factors and hepatitis vaccines. ____________________ ✚

Did You Get It? 11. What factors enhance the risk of thrombus formation

in intact blood vessels?

For the answer, see Appendix A.

Blood Groups and Transfusions ➔ Learning Objectives

□□ Describe the ABo and rh blood groups.

□□ Explain the basis for a transfusion reaction.

As we have seen, blood is vital for transporting substances through the body. When blood is lost, the blood vessels constrict, and the bone marrow steps up blood cell formation in an attempt to maintain circulation. However, the body can com- pensate for a loss of blood volume only up to a certain limit. Losses of 15 to 30 percent lead to

*The RBC antigens that promote this clumping are sometimes called agglutinogens (ag″loo-tin′o-jenz), and the antibodies that bind them together are called agglutinins (ah-gloo′tı̆-ninz).

376

FOcus On

cAreers Phlebotomy Technician

hlebotomists must know where all the arteries and veins are located in

the body. “Phlebotomy is the most important

procedure done for a medical labora- tory,” says Michael Coté, who super- vises the phlebotomy staff at Palo Alto Veterans Administration Hospital in Cali- fornia. “To make accurate diagnosis and effective treatment possible, it’s vital to draw a good blood sample, place it in a sterile container, and process it accu- rately in the lab. Without a high-quality specimen, none of this can happen.”

Phlebotomy is not exactly a house- hold word. It derives from the Greek terms for “vein” and “to cut.” A phlebotomy technician, or phleboto- mist, is trained to collect and process blood samples that will be subjected to laboratory analysis.

Coté appreciates how important it is for phlebotomists to understand anatomy and physiology. “Anatomy is a key requirement in phlebotomy training,” he says, “because you have to learn where all the arteries and veins are located in the body. Some patients’ veins are easy to find, but others have veins that are practically invisible. You need to know the right place to insert that needle. Although 90 percent of the blood we draw comes from the antecubital region inside the elbow, we may also draw blood from the cephalic vein in the forearm, or from veins in the hands.”

Coté notes that knowledge of physiology is also important. “I have to be able to assess patients’ overall health and physical condition because this affects their ability to give an ade- quate blood sample and may demand that a different needle size be used to draw the blood sample. People who are dehydrated can be difficult

because their blood pressure is lower, and venous return is impaired. Patients with poor circulation are also harder to work with. The blood tends to stay in the body trunk rather than flowing freely into the extremities because they’re cold, and it’s difficult to get a good return from veins in the arms. Cancer patients often show increased sensitivity to pain, so we have to be very gentle and use the smallest needle possible.” Patients with a history of drug abuse pose other challenges. “Frequent ‘sticking’ with needles causes scar tissue to form. You can tell people who have used intravenous drugs—their veins feel rock-hard and are much more dif- ficult to penetrate with a needle.”

Coté says that a good phlebot- omist must also possess effective in - terpersonal skills: “People are apprehensive about being stuck with needles, so you have to be patient and be able to put them at ease. Above all, you have to be confident. If the phlebotomist is nervous, the patient will sense it and get nervous too.”

To become certified, a phlebot- omy technician must be a high school graduate, complete a phle- botomy training program or acquire equivalent experience, and pass a certification exam offered by the American Society for Clinical Pathol- ogy (ASCP).

Accreditation procedures for phle- botomists vary from state to state. For more information, contact the American Society for Clinical Pathol- ogy at

33 West Monroe Street Suite 1600 Chicago, IL 60603 (312) 541-4999 https://www.ascp.org

For additional information on this career and others, click the Focus on Careers link at .

Phlebotomy is the most important

procedure done for a medical

laboratory.

https://www.ascp.org

Chapter 10: Blood 377

same antigen was discovered in humans. Most Americans are Rh+ (“Rh positive”), meaning that their RBCs carry Rh antigen. Unlike the antibodies of the ABO system, anti-Rh antibodies are not auto- matically formed by Rh− (“Rh negative”) individuals. However, if an Rh− person receives Rh+ blood, shortly after the transfusion his or her immune sys- tem becomes sensitized and begins producing anti- Rh+ antibodies against the foreign blood type.

Hemolysis (rupture of RBCs) does not occur in an Rh− person with the first transfusion of Rh+ blood because it takes time for the body to react and start making antibodies. However, the second time and every time thereafter, a typical transfu- sion reaction occurs in which the patient’s anti- bodies attack and rupture the donor’s Rh+ RBCs.

An important Rh-related problem occurs in pregnant Rh− women who are carrying Rh+ babies. The first such pregnancy usually results in the delivery of a healthy baby. But because the mother is sensitized by Rh+ antigens that have passed through the placenta into her bloodstream, she will form anti-Rh+ antibodies unless treated with

There are over 30 common RBC antigens in humans, so each person’s blood cells can be classified into several different blood groups. However, it is the antigens of the ABO and Rh blood groups that cause the most vigorous transfusion reactions. We describe these two blood groups here.

The ABO blood groups are based on which of two antigens, type A or type B, a person inherits (table 10.3). Absence of both antigens results in type O blood, presence of both antigens leads to type AB, and the presence of either A or B antigen yields type A or B blood, respectively. In the ABO blood group, antibodies form during infancy against the ABO antigens not present on your own RBCs. As shown in the table, a baby with neither the A nor the B antigen (group O) forms both anti-A and anti-B antibodies; those with type A antigens (group A) form anti-B antibodies, and so on. To keep this idea straight, remember that antibodies against a person’s own blood type will not be produced.

The Rh blood groups are so named because one of the eight Rh antigens (agglutinogen D) was originally identified in Rhesus monkeys. Later the

Table 10.3 ABo Blood Groups

Frequency (% of u.s. population)

Blood group

rBC antigens (agglutinogens) Illustration

Plasma antibodies (agglutinins)

Blood that can be received White Black Asian

native American

AB A, B A B None A, B, AB, O

“Universal recipient”

4 4 5 <1

B B

Anti-A Anti-A (a) B, O 11 20 27 4

A A Anti-B

Anti-B (b) A, O 40 27 28 16

O None

Anti-A

Anti-B

Anti-A (a) Anti-B (b)

O “Universal donor”

45 49 40 79

378 Essentials of Human Anatomy and Physiology

Blood typing The importance of determining the blood group of both the donor and the recipient before blood is transfused is glaringly obvious. The general proce- dure for determining ABO blood type essentially involves testing the blood by mixing it with two different types of immune serum—anti-A and anti-B (Figure 10.7). Agglutination occurs when RBCs of a group A person are mixed with the anti-A serum but not when they are mixed with the anti-B serum. Likewise, RBCs of type B blood are clumped by anti-B serum but not by anti-A serum. In order to double check compatibility, cross matching is also done. Cross matching involves testing for agglutination of donor RBCs by the recipient’s serum and of the recipient’s RBCs by the donor serum. Typing for the Rh factors is done in the same manner as ABO blood typing.

Did You Get It? 12. What are the classes of human blood groups based on? 13. What is the probable result of infusing mismatched blood? 14. Cary is bleeding profusely after being hit by a truck as

he was pedaling his bike home. At the hospital, the nurse asked him whether he knew his blood type. He told her that he “had the same blood as most other people.” What is his ABO blood type?

15. What is the difference between an antigen and an antibody?

For answers, see Appendix A.

Developmental Aspects Of Blood ➔ Learning Objectives

□□ Explain the basis of physiologic jaundice seen in some newborn babies.

□□ Indicate blood disorders that increase in frequency in the aged.

In the embryo, the entire circulatory system devel- ops early. Before birth, there are many sites of blood cell formation—the fetal liver and spleen, among others—but by the seventh month of development, the fetus’s red marrow has become the chief site of hematopoiesis, and it remains so throughout life. Generally, embryonic blood cells are circulating in the newly formed blood vessels by day 28 of devel- opment. Fetal hemoglobin (HbF) differs from the hemoglobin formed after birth. It has a greater abil- ity to pick up oxygen, a characteristic that is highly desirable because fetal blood is less oxygen rich

RhoGAM in the 28th week of pregnancy and again shortly after giving birth. RhoGAM is an immune serum that prevents this sensitization and subse- quent immune response. If she is not treated and becomes pregnant again with an Rh+ baby, her anti- bodies will cross through the placenta and destroy the baby’s RBCs, producing a condition known as hemolytic disease of the newborn. The baby is ane- mic and becomes hypoxic and cyanotic (the skin takes on a blue cast). Brain damage and even death may result unless fetal transfusions are done before birth to provide more RBCs for oxygen transport.

Serum

Anti-A

Agglutinated RBCs

Anti-B

Type AB (contains antigens A and B; agglutinates with both sera)

Blood being tested

Type B (contains antigen B; agglutinates with anti-B serum)

Type A (contains antigen A; agglutinates with anti-A serum)

Type O (contains no antigens; does not agglutinate with either serum)

Figure 10.7 Blood typing of ABo blood groups. When serum containing anti-A or anti-B antibodies is added to a blood sample diluted with saline, agglutination will occur between the antibody and the corresponding antigen (if present).

Types B and O.A:

Which blood types can be transfused into a person with type B blood?Q:

Chapter 10: Blood 379

anemia is especially common in women because of their monthly blood loss during menses. The young and the old are particularly at risk for leukemia.

With increasing age, chronic types of leuke- mias, anemias, and diseases involving undesirable clot formation are more prevalent. However, these are usually secondary to disorders of the heart, blood vessels, or immune system. The elderly are particularly at risk for pernicious anemia, caused by a lack of vitamin B12, because the stomach mucosa (which produces the intrinsic factor required to absorb vitamin B12) atrophies with age. _________✚

Did You Get It? 16. How does fetal hemoglobin differ from that of the adult? 17. What blood-related disorders are particularly common

in the elderly?

For answers, see Appendix A.

than that of the mother. After birth, fetal blood cells are gradually replaced by RBCs that contain the more typical hemoglobin A (HbA). In situations in which fetal RBCs are destroyed so fast that the immature liver cannot rid the body of hemoglobin breakdown products fast enough, the infant becomes jaundiced (jawn′dist). This type of jaun- dice generally causes no major problems and is referred to as physiologic jaundice, to distinguish it from more serious disease conditions.

Homeostatic Imbalance 10.4

Various congenital diseases result from genetic fac- tors (such as hemophilia and sickle cell anemia) and from interactions with maternal blood factors (such as hemolytic disease of the newborn). Dietary factors can lead to abnormalities in blood cell for- mation and hemoglobin production. Iron-deficiency

summary

Composition and Functions of Blood (pp. 363–373)

1. Blood is composed of a nonliving fluid matrix (plasma) and formed elements. It is scarlet to dull red, depending on the amount of oxygen carried. Normal adult blood volume is 5 to 6 liters.

2. Dissolved in plasma (primarily water) are nutrients, gases, hormones, wastes, proteins, salts, and so on. Plasma composition changes as body cells remove or add substances to it, but homeostatic mecha- nisms act to keep it relatively constant. Plasma makes up 55 percent of whole blood.

3. Formed elements, the living blood cells that make up about 45 percent of whole blood, include the following:

a. Erythrocytes, or RBCs—disc-shaped, anucleate cells that transport oxygen bound to their hemo- globin molecules. Their life span is 100 to 120 days.

b. Leukocytes, or WBCs—amoeboid cells involved in protecting the body.

c. Platelets—cell fragments that act in blood clotting.

4. Anemia is a decrease in oxygen-carrying ability of blood. Possible causes are bleeding, a decrease in the number of functional RBCs, or a decrease in the amount of hemoglobin they contain. Polycythemia is an excessive number of RBCs that may result

from bone marrow cancer or a move to a location where less oxygen is available in the air (at high altitude, for example).

5. Leukocytes are nucleated cells, classed into two groups:

a. Granulocytes include neutrophils, eosinophils, and basophils.

b. Agranulocytes include monocytes and lympho- cytes.

6. When bacteria, viruses, or other foreign substances invade the body, WBCs increase in number (leuko- cytosis) and fight them in various ways.

7. An abnormal decrease in number of WBCs is leu- kopenia. An abnormal increase in WBCs is seen in infectious mononucleosis and leukemia (cancer of blood-forming bone marrow).

8. All formed elements arise in red bone marrow from a common stem cell, the hemocytoblast. However, their developmental pathways differ. The stimulus for hematopoiesis is hormonal (erythropoietin, in the case of RBCs).

Hemostasis (pp. 373–375)

1. Stoppage of blood loss from an injured blood ves- sel, or hemostasis, involves three steps: vascular spasms, platelet plug formation, and blood clot for- mation.

380 Essentials of Human Anatomy and Physiology

Multiple Choice

More than one choice may apply.

1. Which would lead to increased erythropoiesis?

a. Chronic bleeding ulcer

b. Reduction in respiratory ventilation

c. Decreased level of physical activity

d. Reduced blood flow to the kidneys

2. In a person with sickle cell anemia, sickling of RBCs can be induced by

a. blood loss.

b. vigorous exercise.

c. stress.

d. fever.

3. A child is diagnosed with sickle cell anemia. This means that

a. one parent had sickle cell anemia.

b. one parent carried the sickle cell gene.

c. both parents had sickle cell anemia.

d. both parents carried the sickle cell gene.

4. Polycythemia vera will result in

a. overproduction of WBCs.

b. exceptionally high blood volume.

c. abnormally high blood viscosity.

d. abnormally low hematocrit.

review Questions

5. Which of the following are normally associated with leukocytes?

a. Positive chemotaxis

b. Phototaxis

c. Diapedesis

d. Hemostasis

6. Which of the following cell types are granulocytes?

a. Lymphocytes

b. Platelets

c. Eosinophils

d. Basophils

7. A person with blood group A can receive blood from a person with blood group

a. B.

b. A.

c. AB.

d. O.

8. A condition resulting from thrombocytopenia is

a. thrombus formation.

b. embolus formation.

c. petechiae.

d. hemophilia.

9. Which of the following can cause problems in a transfusion reaction?

a. Donor antibodies attacking recipient RBCs

b. Clogging of small vessels by agglutinated clumps of RBCs

c. Lysis of donated RBCs

d. Blockage of kidney tubules

2. The blood group most commonly typed for is ABO. Type O is most common; least common is AB. ABO antigens are accompanied by preformed antibodies, which act against RBCs that have “foreign” antigens.

3. Rh factor is found in most Americans. Rh− people do not have preformed antibodies to Rh+ RBCs but form them once exposed (sensitized) to Rh+ blood.

Developmental Aspects of Blood (pp. 378–379)

1. Congenital blood defects include various types of hemolytic anemias and hemophilia. Incompatibility between maternal and fetal blood can lead to fetal cyanosis, resulting from destruction of fetal blood cells.

2. Fetal hemoglobin (HbF) binds more readily with oxygen than does HbA.

3. Physiologic jaundice in a newborn reflects immatu- rity of the infant’s liver.

4. Leukemias are most common in the very young and very old. Older adults are also at risk for ane- mia and clotting disorders.

2. Hemostasis is started by a tear or interruption in the blood vessel lining. Vascular spasms and accu- mulation of platelets at the site temporarily stop or slow blood loss. Platelet PF3 and tissue factor initi- ate the clotting cascade, leading to formation of fibrin threads. Fibrin traps RBCs as they flow past, forming a clot.

3. Clots are digested once a vessel has been repaired. An attached clot that forms or persists in an unbro- ken blood vessel is a thrombus; a clot traveling in the bloodstream is an embolus.

4. Abnormal bleeding may reflect a deficit of platelets (thrombocytopenia), genetic factors (hemophilia), or inability of the liver to make clotting factors.

Blood Groups and transfusions (pp. 375–378)

1. Blood groups are classified on the basis of proteins (antigens) on RBC membranes. Complementary antibodies may or may not be present in blood. Antibodies act to agglutinate (clump) and mark for- eign RBCs for lysis.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 10: Blood 381

27. Following a surgery for hip replacement, Albert has been experiencing severe chest pain. He also feels dizzy, sweats heavily, and coughs up blood often. What is the likely diagnosis?

28. A bone marrow biopsy of Mr. Lee, a man on long- term drug therapy, shows an abnormally high per- centage of nonhematopoietic connective tissue. What condition does this indicate? If the symptoms are critical, what short-term and long-term treat- ments are indicated? Which treatment is he more likely to be given: infusion of whole blood or of packed red cells?

29. A woman comes to the clinic complaining of fatigue, shortness of breath, and chills. Blood tests show anemia, and a bleeding ulcer is diagnosed. What type of anemia is this?

30. A patient is diagnosed with bone marrow cancer and has a hematocrit of 70 percent. What is this condition called?

31. A middle-aged college professor from Boston is in the Swiss Alps studying astronomy. He arrived two days ago and plans to stay the entire year. However, he notices that he is short of breath when he walks

up steps and that he tires easily with any physical activity. His symptoms gradually disappear; after two months, he feels fine. Upon returning to the United States, he has a complete physical exam and is told that his erythrocyte count is higher than nor- mal. (a) Attempt to explain this finding. (b) Will his RBC count remain at this higher-than-normal level? Why or why not?

32. Why is someone more likely to bleed to death when an artery is cleanly severed than when it is crushed and torn?

33. What could be wrong with Linda’s newborn baby whose skin has taken on a purplish-blue color?

34. Shao-Mei and Elisha, who are good friends, decide to donate blood. Shao-Mei discovers that her blood group is AB Rh− and Elisha’s blood group is O Rh−. Explain how Elisha could help Shao-Mei if she ever needed a blood transfusion. Why cannot Shao-Mei help Elisha in the same way?

35. Mr. Malone is going into shock because of blood loss, so paramedics infuse a saline solution. Why would this help?

Critical thinking and Clinical Application Questions

17. What is positive chemotaxis? What kind of motion is involved in this?

18. Name the formed elements that arise from myeloid stem cells. Name those arising from lymphoid stem cells.

19. What is the function of platelets?

20. Describe the process of hemostasis. Indicate what starts the process.

21. What is the difference between a thrombus and an embolus?

22. What are agglutinins?

23. What factors induce the red bone marrow to produce leukocytes?

24. What is a transfusion reaction? Why does it happen?

25. Explain why an Rh− person does not have a transfusion reaction on the first exposure to Rh+ blood. Why is there a transfusion reaction the second time he or she receives the Rh+ blood?

26. Why is phlebotomy often considered the most important procedure for a medical laboratory?

10. If an Rh− mother becomes pregnant, when can hemolytic disease of the newborn not possibly occur in the child?

a. If the child is Rh−

b. If the child is Rh+ c. If the father is Rh+

d. If the father is Rh−

11. Hematocrit is determined by the percentage of

a. plasma.

b. leukocytes.

c. platelets.

d. erythrocytes.

12. Clotting proteins

a. stem blood loss after injury.

b. transport certain molecules.

c. help keep water in the bloodstream.

d. protect the body from pathogens.

short Answer Essay

13. What is the blood volume of an average-sized adult?

14. Name as many different categories of substances carried in plasma as you can.

15. Define formed elements. Which category is most numerous? Which makes up the buffy coat?

16. Define anemia, and give three possible causes.

382

The Cardiovascular System

WHAT

HOW

WHY

The cardiovascular system delivers oxygen and

nutrients to the body tissues and carries away wastes such as carbon dioxide

via blood.

The heart pumps blood throughout the body in blood vessels. Blood flow

requires both the pumping action of the heart and changes in

blood pressure.

If the cardiovascular system cannot perform its

functions, wastes build up in tissues. Body organs fail to function properly,

and then, once oxygen becomes depleted, they will die.

InSTruCTorS

New Building Vocabulary Coaching Activities for this chapter are assignable in

Chapter 11: The Cardiovascular System 383

flanked on each side by the lungs (Figure 11.1). Its pointed apex is directed toward the left hip and rests on the diaphragm, approximately at the level of the fifth intercostal space. (This is exactly where one would place a stethoscope to count the heart rate for an apical pulse.) Its broad posterosuperior aspect, or base, from which the great vessels of the body emerge, points toward the right shoulder and lies beneath the second rib.

Coverings and Walls of the Heart

The heart is enclosed by a sac called the pericar- dium (per″i-kar′de-um) that is made up of three layers: an outer fibrous layer and an inner serous membrane pair. The loosely fitting superficial part of this sac is referred to as the fibrous pericar- dium. This fibrous layer helps protect the heart and anchors it to surrounding structures, such as the diaphragm and sternum. Deep to the fibrous pericardium is the slippery, two-layered serous pericardium. The parietal layer of the serous peri- cardium, or parietal pericardium, lines the inte- rior of the fibrous pericardium. At the superior aspect of the heart, this parietal layer attaches to the large arteries leaving the heart and then makes

around the body is provided by the beating heart and by blood pressure.

The cardiovascular system includes a muscular pump equipped with one-way valves and a system of large and small “plumbing” tubes within which the blood travels. (We discussed blood, the sub- stance transported, in Chapter 10.) Here we will consider the heart (the pump) and the blood ves- sels (the “plumbing”).

The Heart Anatomy of the Heart ➔ Learning Objective

□□ Describe the location of the heart in the body, and identify its major anatomical areas on an appropriate model or diagram.

Size, Location, and orientation

The modest size and weight of the heart give few hints of its incredible strength. Approximately the size of a person’s fist, the hollow, cone-shaped heart weighs less than a pound. Snugly enclosed within the inferior mediastinum (me″de-as-ti′num), the medial section of the thoracic cavity, the heart is

(a)

Superior vena cava

Left lung

Aorta

Parietal pleura (cut)

Pericardium (cut)

Pulmonary trunk

Diaphragm

Apex of heart

Figure 11.1 Location of the heart within the thorax. (a) Relationship of the heart and great vessels to the lungs. (Figure continues on page 384.)

384 Essentials of Human Anatomy and Physiology

myocardium is reinforced internally by a network of dense fibrous connective tissue called the “skeleton of the heart.” The endocardium (en″do-kar′de-um) is a thin, glistening sheet of endothelium that lines the heart chambers. It is continuous with the linings of the blood vessels leaving and entering the heart. (Figure 11.3 shows two views of the heart—an exter- nal anterior view and a frontal section. As the ana- tomical areas of the heart are described in the next section, keep referring to Figure 11.3 to locate each of the heart structures or regions.)

Chambers and Associated Great Vessels ➔ Learning Objectives

□□ Trace the pathway of blood through the heart.

□□ Compare the pulmonary and systemic circuits.

The heart has four hollow cavities, or chambers— two atria (a′tre-ah; singular atrium) and two ven- tricles (ven′trı̆-kulz). Each of these chambers is lined with endocardium, which helps blood flow smoothly through the heart. The superior atria are primarily receiving chambers. As a rule, they are not important in the pumping activity of the heart. Instead, they assist with filling the ventricles. Blood flows into the atria under low pressure from the veins of the body and then continues on to fill the ventricles. The inferior, thick-walled ventricles are the discharging chambers, or actual pumps of the heart. When they contract, blood is propelled out of the heart and into circulation. The right ven- tricle forms most of the heart’s anterior surface; the left ventricle forms its apex (Figure 11.3a). The

a U-turn and continues inferiorly over the heart surface. The visceral layer of the serous pericar- dium, or visceral pericardium, also called the epicardium, is part of the heart wall (Figure 11.2). In other words, the epicardium is the innermost layer of the pericardium and the outermost layer of the heart wall. Lubricating serous fluid is produced by the serous pericardial membranes and collects in the pericardial cavity between these serous lay- ers. This fluid allows the heart to beat easily in a relatively frictionless environment as the serous pericardial layers slide smoothly across each other.

Homeostatic Imbalance 11.1

Inflammation of the pericardium, pericarditis (per″ ı̆-kar-di′tis), often results in a decrease in the already small amount of serous fluid. This causes the pericardial layers to rub, bind, and stick to each other, forming painful adhesions that inter- fere with heart movements. ___________________✚

The heart walls are composed of three layers: the outer epicardium (the visceral pericardium just described), the myocardium, and the innermost endocardium (see Figure 11.2). The myocardium (mi″o-kar′de-um) consists of thick bundles of cardiac muscle twisted and whorled into ringlike arrange- ments (see Figure 6.2b, p. 210). It is the layer that actually contracts. Myocardial cells are linked together by intercalated discs, which contain both desmo- somes and gap junctions. The gap junctions at the intercalated discs allow ions to flow from cell to cell, carrying a wave of excitement across the heart. The

Heart

Posterior

Right lung

Point of maximal intensity (PMI)

Diaphragm

(b) (c)

Sternum

2nd rib

Midsternal line Mediastinum

Figure 11.1 (continued) Location of the heart within the thorax. (b) Relationship of the heart to the sternum and ribs. (c) Cross-sectional view showing relative position of the heart in the thorax.

Chapter 11: The Cardiovascular System 385

Left subclavian artery

Aortic arch

Left common carotid artery

Ligamentum arteriosum

Left pulmonary veins

Left atrium

Circumflex artery

Left coronary artery in coronary sulcus (left atrioventricular groove)

Left ventricle

Great cardiac vein

Left pulmonary artery

Apex

Anterior interventricular artery (in anterior interventricular sulcus)

(a) Anterior view of heart showing major vessels

Superior vena cava

Right pulmonary artery

Brachiocephalic trunk

Ascending aorta

Pulmonary trunk

Right pulmonary veins

Right atrium

Right coronary artery in coronary sulcus (right atrioventricular groove)

Anterior cardiac vein

Right ventricle

Marginal artery

Inferior vena cava

Small cardiac vein

Fibrous pericardium

Parietal layer of serous pericardium

Pericardial cavity

Visceral layer of serous pericardium

Epicardium

Myocardium

Endocardium

Pulmonary trunk

Heart chamber

Heart wall

Pericardium

Figure 11.2 Heart wall and coverings. Note that the visceral layer of the pericardium and the epicardium of the heart wall are the same structure.

Figure 11.3 Gross anatomy of the heart. (Figure continues on page 386.)

386 Essentials of Human Anatomy and Physiology

(oxygen enters the blood and carbon dioxide enters the lungs) and then return it to the heart.

Oxygen-rich blood returned to the left atrium flows into the left ventricle and is pumped out into the aorta (a-or′tah), from which the systemic arter- ies branch to supply essentially all body tissues. After oxygen is delivered to tissues, oxygen-poor blood circulates from the tissues back to the right atrium via the systemic veins, which finally empty their cargo into either the superior or inferior vena cava. This second circuit, from the left ventricle through the body tissues and back to the right atrium, is called the systemic circulation (see Figure 11.4). It supplies oxygen- and nutrient-rich blood to all body organs. Because the left ventricle pumps blood over the much longer systemic path- way through the body, its walls are substantially thicker than those of the right ventricle (Figure 11.5), and it is a much more powerful pump.

Did You Get It? 1. What is the location of the heart in the thorax?

septum that divides the heart longitudinally is referred to as the interatrial septum where it divides the atria and the interventricular septum where it divides the ventricles.

Although it is a single organ, the heart functions as a double pump, with arteries carrying blood away from and veins carrying blood toward the heart. The right side works as the pulmonary circuit pump. It receives oxygen-poor blood from the veins of the body through the large superior vena cava and inferior vena cava (plural venae cavae; ka′ve) and pumps it out through the pulmonary trunk. The pulmonary trunk splits into the right and left pulmo- nary arteries, which carry blood to the lungs, where oxygen is picked up and carbon dioxide is unloaded. Oxygen-rich blood drains from the lungs and is returned to the left side of the heart through the four pulmonary veins. This circuit, from the right ventricle (the pump) to the lungs and back to the left atrium (receiving chamber), is called the pul- monary circulation (Figure 11.4). Its only function is to carry blood to the lungs for gas exchange

Left pulmonary artery

Left atrium

Aorta

Left pulmonary veins

Left atrioventricular valve (bicuspid valve)

Aortic semilunar valve

Left ventricle

Interventricular septum

Pulmonary semilunar valve

Visceral pericardium (epicardium)

Myocardium

(b) Frontal section showing interior chambers and valves

Right pulmonary artery

Superior vena cava

Right atrium

Right pulmonary veins

Fossa ovalis

Right atrioventricular valve (tricuspid valve)

Right ventricle

Chordae tendineae

Inferior vena cava

Figure 11.3 (continued) Gross anatomy of the heart.

Chapter 11: The Cardiovascular System 387

Heart Valves ➔ Learning Objective

□□ Explain the operation of the heart valves.

The heart is equipped with four valves, which allow blood to flow in only one direction through the heart chambers—from the atria through the ventricles and out the great arteries leaving the heart (see Figure 11.3b). The atrioventricular (AV) valves (a″tre-o-ven-trik′u-lar) are located between the atria and ventricles on each side. These valves prevent backflow into the atria when the ventricles contract. The left AV valve—the bicuspid valve, also called the mitral (mi′tral) valve—consists of two flaps, or cusps, of endocar- dium. The right AV valve, the tricuspid valve, has three cusps. Tiny white cords, the chordae ten- dineae (kor′de ten-din′e)—literally, “tendinous cords” (think of them as “heart strings”)—anchor the cusps to the walls of the ventricles. When the heart is relaxed and blood is passively filling its chambers, the AV valve cusps hang limply into the ventricles (Figure 11. 6a, p. 388).

As the ventricles contract, they press on the blood in their chambers, and the pressure inside the ventricles (intraventricular pressure) begins to rise. This forces the AV valve cusps upward, clos- ing the valves. At this point the chordae tendineae tighten and anchor the cusps in a closed position.

2. Which heart chamber has the thickest walls? What is the functional significance of this structural difference?

3. How does the function of the systemic circulation differ from that of the pulmonary circulation?

For answers, see Appendix A.

Oxygen-rich, CO2-poor blood Oxygen-poor, CO2-rich blood

Capillary beds of lungs where gas exchange occurs

Capillary beds of all body tissues where gas exchange occurs

Pulmonary veins

Pulmonary arteries

Pulmonary Circuit

Systemic Circuit

Aorta and branches

Left atrium

Heart

Left ventricleRight

atrium Right ventricle

Venae cavae

KEY:

Figure 11.4 The systemic and pulmonary circulations. The left side of the heart is the systemic pump; the right side is the pulmonary circuit pump. (Although there are two pulmonary arteries, one each to the right and left lung, for simplicity only one is shown.)

Right ventricle

Left ventricle

Muscular interventricular septum

Figure 11.5 Anatomical differences in right and left ventricles. The left ventricle has a thicker wall, and its cavity is basically circular. The right ventricle cavity is crescent-shaped and wraps around the left ventricle.

388 Essentials of Human Anatomy and Physiology

Ventricles contract, forcing blood against AV valve cusps.

AV valves close.

Chordae tendineae tighten, preventing valve cusps from everting into atria.

Blood returning to the atria puts pressure against AV valves; the AV valves are forced open.

As the ventricles fill, AV valve cusps hang limply into ventricles.

Atria contract, forcing additional blood into ventricles.

Ventricles

AV valves open; atrial pressure greater than ventricular pressure

AV valves closed; atrial pressure less than ventricular pressure

As ventricles contract and intraventricular pressure rises, blood is pushed up against semilunar valves, forcing them open.

Aorta Pulmonary trunk

Semilunar valves open Semilunar valves closed

As ventricles relax and intraventricular pressure falls, blood flows back from arteries, filling the cusps of semilunar valves and forcing them to close.

(a) Operation of the AV valves

(b) Operation of the semilunar valves

1 2

Figure 11.6 operation of the heart valves. (a) Atrioventricular (AV) valves. (b) Semilunar valves.

Chapter 11: The Cardiovascular System 389

Prosthetic aortic heart valve. _____________________________________________✚

Cardiac Circulation

➔ Learning Objective □□ name the functional blood supply of the heart.

Although the heart chambers are bathed with blood almost continuously, the blood contained in the heart does not nourish the myocardium. The func- tional blood supply that oxygenates and nourishes the myocardium is provided by the right and left coronary arteries. The coronary arteries branch from the base of the aorta and encircle the heart in the coronary sulcus (atrioventricular groove) at the junction of the atria and ventricles (see Figure 11.3a). The coronary arteries and their major branches (the anterior interventricular artery and circumflex artery on the left, and the poste- rior interventricular artery and marginal artery on the right) are compressed (flow is inhibited, not stopped completely) when the ventricles are con- tracting and fill when the heart is relaxed. The myo- cardium is drained by several cardiac veins, which empty into an enlarged vessel on the posterior of the heart called the coronary sinus. The coronary sinus, in turn, empties into the right atrium.

Homeostatic Imbalance 11.3

When the heart beats at a very rapid rate, the myocardium may receive an inadequate blood supply because the relaxation periods (when the blood is able to flow to the heart tissue) are short- ened. Situations in which the myocardium is deprived of oxygen often result in crushing chest pain called angina pectoris (an-ji′nah pek′tor-is). This pain is a warning that should never be ignored, because if angina is prolonged, the oxygen- deprived heart cells may die, forming an area called an infarct. The resulting myocardial infarction (in-fark′shun), or MI, is commonly called a “heart attack” or a “coronary.” _________✚

If the cusps were unanchored, they would blow upward into the atria like an umbrella being turned inside out by a gusty wind. In this manner, the AV valves prevent backflow into the atria when the ventricles are contracting.

The second set of valves, the semilunar (sem″ ı̆-lu′nar) valves, guards the bases of the two large arteries leaving the ventricular chambers. Thus, they are known as the pulmonary semilu- nar valve and aortic semilunar valve (see Figure 11.3b). Each semilunar valve has three cusps that fit tightly together when the valves are closed. When the ventricles are contracting and forcing blood out of the heart, the cusps are forced open and flattened against the walls of the arteries by the tremendous force of rushing blood (Figure 11.6b). Then, when the ventricles relax, the blood begins to flow backward toward the heart, and the cusps fill with blood like a para- chute filling with air, closing the valves. This pre- vents arterial blood from reentering the heart.

Each set of valves operates at a different time. The AV valves are open during heart relaxation and closed when the ventricles are contracting. The semilunar valves are closed during heart relaxation and are forced open when the ventri- cles contract. The valves force blood to continually move forward through the heart by opening and closing in response to pressure changes in the heart.

Homeostatic Imbalance 11.2

Heart valves are simple devices, and the heart— like any mechanical pump—can function with “leaky” valves as long as the damage is not too great. However, severely deformed valves can seri- ously hamper cardiac function. For example, an incompetent valve forces the heart to pump and repump the same blood because the valve does not close properly, so blood backflows. In valvu- lar stenosis, the valve cusps become stiff, often because of repeated bacterial infection of the endocardium (endocarditis). This forces the heart to contract more vigorously than normal to create enough pressure to drive blood through the nar- rowed valve. In each case, the heart’s workload increases, and ultimately the heart weakens and may fail. Under such conditions, the faulty valve is replaced with a synthetic valve (see photo), a cryopreserved human valve, or a chemically treated valve taken from a pig heart.

390 Essentials of Human Anatomy and Physiology

before they will contract, cardiac muscle cells can and do contract spontaneously and independently, even if all nervous connections are severed. Moreover, these spontaneous contractions occur in a regular and continuous way. Although cardiac muscle can beat independently, the muscle cells in different areas of the heart have different rhythms. Atrial cells beat about 60 times per min- ute, but ventricular cells contract more slowly (20–40 times per minute). Therefore, without some type of unifying control system, the heart would be an uncoordinated and inefficient pump.

Two systems act to regulate heart activity. One of these involves the nerves of the auto- nomic nervous system, which act like brakes and gas pedals to decrease or increase the heart rate, depending on which division is activated. We consider this topic later (see p. 394). The second system is the intrinsic conduction system, or nodal system, that is built into the heart tissue (Figure 11.7) and sets its basic rhythm like a drummer sets the beat for a rock band playing a song. The intrinsic conduction system is com- posed of a special tissue found nowhere else in the body; it is much like a cross between muscle and nervous tissue. This system causes heart muscle depolarization in only one direction— from the atria to the ventricles.

Did You Get It? 4. Why are the heart valves important? 5. Why might a thrombus in a coronary artery cause

sudden death?

For answers, see Appendix A.

Physiology of the Heart As the heart beats, or contracts, the blood makes continuous round-trips—into and out of the heart, through the rest of the body, and then back to the heart—only to be sent out again. The amount of work that a heart does is almost too incredible to believe. In one day it pushes the body’s supply of 6 quarts or so of blood (6 liters [L]) through the blood vessels over 1,000 times, meaning that it actually pumps about 6,000 quarts of blood (1500 gallons) in a single day!

Intrinsic Conduction System of the Heart: Setting the Basic rhythm

➔ Learning Objectives □□ name the elements of the intrinsic conduction

system of the heart, and describe the pathway of impulses through this system.

□□ Explain what information can be gained from an electrocardiogram.

What makes the heart beat? Unlike skeletal muscle cells, which must be stimulated by nerve impulses

Sinoatrial (SA) node (pacemaker)

Atrioventricular (AV) node

Atrioventricular (AV) bundle (bundle of His)

Bundle branches

Purkinje fibers

Superior vena cava

Right atrium

Left atrium

Purkinje fibers

Interventricular septum

Figure 11.7 The intrinsic conduction system of the heart. The depolarization wave initiated by the sinoatrial (SA) node passes successively through the atrial myocardium to the atrioventricular (AV) node, the AV bundle, the right and left bundle branches, and the Purkinje fibers in the ven- tricular walls.

Chapter 11: The Cardiovascular System 391

There are other conditions that can interfere with the regular conduction of impulses across the heart—for example, damage to the SA node results in a slower heart rate. When this is a problem, arti- ficial pacemakers are usually installed surgically.

Ischemia (is-ke′me-ah), or lack of an ade- quate blood supply to the heart muscle, may lead to fibrillation—a rapid, uncoordinated quivering of the ventricles (it looks like a bag of wiggling worms). Fibrillation makes the heart unable to pump any blood and so is a major cause of death from heart attacks in adults. Many businesses train their employees in the use of AEDs (automatic external defibrillators), which has proven to be lifesaving in many cases. _____________________✚

Tachycardia (tak″e-kar′de-ah) is a rapid heart rate (over 100 beats per minute). Bradycardia (brad″e-kar′de-ah) is a heart rate that is substan- tially slower than normal (less than 60 beats per minute). Neither condition is pathological, but prolonged tachycardia may progress to fibrillation.

Cardiac Cycle and Heart Sounds

➔ Learning Objective □□ Define systole, diastole, stroke volume, cardiac

cycle, heart sounds, and murmur.

In a healthy heart, the atria contract simultaneously. Then, as they start to relax, the ventricles begin to contract. Systole (sis′to-le) and diastole (di-as′to-le) mean heart contraction and relaxation, respectively. Because most of the pumping work is done by the ventricles, these terms refer to the contraction and relaxation of the ventricles unless otherwise stated.

The term cardiac cycle refers to the events of one complete heartbeat, during which both atria and ventricles contract and then relax. The aver- age heart beats approximately 75 times per min- ute, so the length of the cardiac cycle is normally about 0.8 second. We will consider the cardiac cycle in terms of events occurring during five peri- ods (Figure 11.8, p. 392).

Atrial diastole (ventricular filling). Our dis- cussion begins with the heart completely relaxed. Pressure in the heart is low, the AV valves are open, and blood is flowing pas- sively through the atria into the ventricles. The semilunar valves are closed.

Atrial systole. The ventricles remain in dias- tole as the atria contract, forcing blood into the ventricles to complete ventricular filling.

➔ConCeptLinkThis is very similar to the one-way generation of an action potential as it travels down the axon of a neuron like a wave (Chapter 7, pp. 260–262). The signals that stimulate cardiac muscle contraction also travel one way throughout the intrinsic conduction system. ➔

In addition, the intrinsic conduction system enforces a contraction rate of approximately 75 beats per minute on the heart; thus, the heart beats as a coordinated unit.

One of the most important parts of the intrin- sic conduction system is a crescent-shaped node of tissue called the sinoatrial (si″no-a′tre-al) (SA) node, located in the right atrium. Other compo- nents include the atrioventricular (AV) node at the junction of the atria and ventricles, the atrio- ventricular (AV) bundle (bundle of His) and the right and left bundle branches located in the interventricular septum, and finally the Purkinje (pur-kin′je) fibers, which spread within the myo- cardium of the ventricle walls.

The SA node is a tiny cell mass with a mammoth job. Because it has the highest rate of depolarization in the whole system, it starts each heartbeat and sets the pace for the whole heart. Consequently, the SA node is often called the pacemaker. From the SA node, the impulse spreads through the atria to the AV node, and then the atria contract. At the AV node, the impulse is delayed briefly to give the atria time to finish contracting. It then passes rapidly through the AV bundle, the bundle branches, and the Purkinje fibers, resulting in a “wringing” contrac- tion of the ventricles that begins at the heart apex and moves toward the atria. This contraction effec- tively ejects blood superiorly into the large arteries leaving the heart. “A Closer Look” (on p. 393) describes electrocardiography, the clinical proce- dure for mapping the electrical activity of the heart.

Homeostatic Imbalance 11.4

Because the atria and ventricles are separated from one another by “insulating” connective tissue, which is part of the fibrous skeleton of the heart, depolarization waves can reach the ventricles only by traveling through the AV node. Thus, any dam- age to the AV node can partially or totally block the ventricles from the control of the SA node. When this occurs, the ventricles begin to beat at their own rate, which is much slower, some or all of the time. This condition is called heart block.

392 Essentials of Human Anatomy and Physiology

dup, pause, lub-dup, pause, and so on. The first heart sound (lub) is caused by the closing of the AV valves. The second heart sound (dup) occurs when the semilunar valves close at the end of ven- tricular systole. The first heart sound is longer and louder than the second heart sound, which tends to be short and sharp.

Homeostatic Imbalance 11.5

Abnormal or unusual heart sounds are called heart murmurs. Blood flows silently as long as the flow is smooth and uninterrupted. If it strikes obstructions, its flow becomes turbulent and gen- erates sounds that can be heard with a stetho- scope. Heart murmurs are fairly common in young children (and some elderly people) with perfectly healthy hearts, probably because their heart walls are relatively thin and vibrate with rushing blood. However, murmurs in patients who do not fall into either of these groups most often indicate valve problems. For example, if a valve does not close tightly (is incompetent), a swishing sound will be heard after that valve has (supposedly) closed, as the blood flows back through the partially open valve. Distinct sounds also can be heard when blood flows turbulently through stenosed (nar- rowed) valves. ______________________________✚

Did You Get It? 6. What is the function of the intrinsic conduction

system of the heart? 7. To which heart chambers do the terms systole and

diastole usually apply?

Isovolumetric contraction. Atrial systole ends, and ventricular systole begins. The initial rise in intraventricular pressure closes the AV valves, preventing backflow of blood into the atria. For a moment, the ventricles are com- pletely closed chambers.

Ventricular systole (ejection phase). The ven- tricles continue to contract, causing the intra- ventricular pressure to surpass the pressure in the major arteries leaving the heart. This causes the semilunar valves to open and blood to be ejected from the ventricles. During this phase, the atria are again relaxed and filling with blood.

Isovolumetric relaxation. As ventricular dias- tole begins, the pressure in the ventricles falls below that in the major arteries, and the semi- lunar valves close to prevent backflow into the ventricles. For another moment, the ventricles are completely closed chambers and intraven- tricular pressure continues to decrease. Meanwhile, the atria have been in diastole, fill- ing with blood. When atrial pressure increases above intraventricular pressure, the AV valves open, and the cycle repeats.

When using a stethoscope, you can hear two distinct sounds during each cardiac cycle. These heart sounds are often described by the two syllables “lub” and “dup,” and the sequence is lub-

Right atrium

Left ventricle Right ventricle

Left atrium

1 Atrial diastole (ventricular filling)

2 Atrial systole

3 Isovolumetric contraction

4 Ventricular systole (ejection phase)

5 Isovolumetric relaxation

The cells contract isometrically until they have enough force to overcome the back pressure of the blood against the semilunar valves, at which point

their contraction becomes isotonic.

Are the ventricular cardiac cells contracting isometrically or isotonically during phase 3?Q:

Figure 11.8 Summary of events occurring during the cardiac cycle. Small black arrows indicate the regions of the heart that are

contracting; thick red and blue arrows indicate direction of blood flow. During the isovolumetric (literally “same volume

measurement”) phases in periods 3 and 5, the ventricles are closed chambers, and the volume of blood they contain is unchanging.

393

A CLOSER

LOOK

When impulses pass through the heart, electrical currents are generated that spread throughout the body. These impulses can be detected on the body surface and recorded with an electrocardiograph. The recording that is made, the electro- cardiogram (ECG), traces the flow of current through the heart. The illustra- tion shows a normal ECG tracing.

The typical ECG has three recogniz- able waves. The first wave, which fol- lows the firing of the SA node, is the P wave. The P wave is small and signals the depolarization of the atria immedi- ately before they contract. The large QrS complex, which results from the depolarization of the ventricles, has a complicated shape. It precedes the con- traction of the ventricles. The T wave results from currents flowing during the repolarization of the ventricles. (Atrial repolarization is generally hidden by the large QRS complex, which is being recorded at the same time.)

Abnormalities in the shape of the waves and changes in their timing

signal that something may be wrong with the nodal system, or they may indicate a myocardial infarct (present or past). A myocardial infarct (heart attack) is an area of heart tissue in which the cardiac cells have died; it is

Electrocardiography: (Don’t) Be Still My Heart

generally a result of ischemia (inadequate blood flow). During fibrillation (quivering of the heart), the normal pattern of the ECG is totally lost, and the heart ceases to pump blood.

heart rate (HR) and the stroke volume (SV). Stroke volume is the volume of blood pumped out by a ventricle with each heartbeat. In general, stroke volume increases as the force of ventricular contraction increases. If we use the normal resting values for heart rate (75 beats per minute) and stroke volume (70 ml per beat), the average adult cardiac output can be easily calculated:

CO = HR (75 beats/min) × SV (70 ml/beat) CO = 5250 ml/min = 5.25 L/min

The normal adult blood volume is about 6,000 ml, so nearly the entire blood supply passes through the body once each minute. Cardiac out- put varies with the demands of the body. It rises

8. During isovolumetric contraction of the cardiac cycle, which chambers are relaxing, and which are contracting?

9. What causes the lub-dup sounds heard with a stethoscope?

For answers, see Appendix A.

Cardiac output

➔ Learning Objective □□ Describe the effect of each of the following on

heart rate: stimulation by the vagus nerve, exercise, epinephrine, and various ions.

Cardiac output (CO) is the amount of blood pumped out by each side of the heart (actually each ventricle) in 1 minute. It is the product of the

Sinoatrial (SA) node

Atrioventricular (AV) node

Time(s) 0 0.2 0.4 0.6 0.8

Atrial depolarization

QRS complex

Ventricular depolarization

Ventricular repolarization

An electrocardiogram tracing showing the three normally recognizable deflection waves—P, QRS, and T.

394 Essentials of Human Anatomy and Physiology

1. Neural (ANS) controls. During times of physical or emotional stress, the nerves of the sympathetic division of the autonomic nervous system more strongly stimulate the SA and AV nodes and the cardiac muscle itself. As a result, the heart beats more rap- idly. This is a familiar phenomenon to any- one who has ever been frightened or has had to run to catch a bus. As fast as the heart pumps under ordinary conditions, it really speeds up when special demands are placed on it. Because a faster blood flow increases the rate at which fresh blood reaches body cells, more oxygen and glucose are made available to them during periods of stress. When demand declines, the heart adjusts accordingly. Parasympathetic nerves, primar- ily vagus nerve fibers, slow and steady the heart, giving it more time to rest during non- crisis times.

2. Hormones and ions. Various hormones and ions can have a dramatic effect on heart activ- ity. Both epinephrine, which mimics sympa- thetic nerves and is released in response to sympathetic nerve stimulation, and thyroxine, a thyroid hormone, increase heart rate. Electrolyte imbalances pose a real threat to the heart. For example, recall that calcium ions are required for muscle contraction. A reduced level of ionic calcium in the blood depresses the heartbeat, whereas an excessive level of blood calcium ions causes such pro- longed contractions that the heart may stop entirely. Either excess or lack of needed ions such as sodium and potassium also modifies heart activity. A deficit of potassium ions in the blood, for example, causes the heart to beat feebly, and abnormal heart rhythms appear.

3. Physical factors. A number of physical fac- tors, including age, gender, exercise, and body temperature, influence heart rate. Resting heart rate is fastest in the fetus (140–160 beats per minute) and then gradually decreases throughout life. The average adult heart rate is faster in females (72–80 beats per minute) than in males (64–72 beats per minute). Heat increases heart rate by boosting the metabolic rate of heart cells. This explains the rapid, pounding heartbeat you feel when you have a

when the stroke volume is increased or the heart beats faster or both; it drops when either or both of these factors decrease. Let’s take a look at how stroke volume and heart rate are regulated.

Regulation of Stroke Volume A healthy heart pumps out about 60 percent of the blood present in its ventricles. As noted previously, this is approx- imately 70 ml (about 2 ounces) with each heart- beat. According to Starling’s law of the heart, the critical factor controlling stroke volume is how much the cardiac muscle cells are stretched by the filling of the chambers just before they contract. The more they are stretched, the stronger the con- traction will be. The important factor stretching the heart muscle is venous return, the amount of blood entering the heart and distending its ventri- cles. If one side of the heart suddenly begins to pump more blood than the other, the increased venous return to the opposite ventricle will force it to pump out an equal amount, thus preventing backup of blood in the circulation.

Anything that increases the volume or speed of venous return also increases stroke volume and force of contraction (Figure 11.9). For example, a slow heartbeat allows more time for the ventricles to fill. Exercise speeds venous return because it results in increased heart rate and force; the enhanced squeezing action of active skeletal mus- cles on the veins helps return blood to the heart. This so-called muscular pump also plays a major role in increasing the venous return. In contrast, low venous return, such as might result from severe blood loss or an abnormally rapid heart rate, decreases stroke volume, causing the heart to beat less forcefully.

Factors Modifying Basic Heart Rate In healthy people, stroke volume tends to be relatively con- stant. However, when blood volume drops sud- denly or when the heart has been seriously weakened, stroke volume declines, and cardiac output is maintained by a faster heartbeat. Although heart contraction does not depend on the nervous system, its rate can be changed tem- porarily by the autonomic nerves. Indeed, the most important external influence on heart rate is the activity of the autonomic nervous system. Several chemicals, hormones, and ions also mod- ify heart rate. Some of these factors are discussed next (see also Figure 11.9).

Chapter 11: The Cardiovascular System 395

progressive condition that reflects weakening of the heart by coronary atherosclerosis (clogging of the coronary vessels with fatty buildup), hyperten- sive heart disease, or multiple myocardial infarc- tions (repaired with noncontracting scar tissue). In these patients, the heart pumps weakly and is nearly “worn out.” The weak contractions of a heart in CHF result in a lower stroke volume. For those patients, the drug digitalis is routinely pre- scribed. It enhances contractile force and stroke volume of the heart, resulting in greater cardiac output.

Because the heart is a double pump, each side can fail independently of the other. If the left heart fails, pulmonary congestion occurs. The right side of the heart continues to propel blood to the lungs, but the left side is unable to eject the returning blood into the systemic circulation. As

high fever and accounts in part for the effect of exercise on heart rate (remember, working muscles generate heat). Cold has the opposite effect; it directly decreases heart rate. As noted previously, exercise acts through ner- vous system controls (sympathetic division) to increase heart rate (and also, through the action of the muscular pump, to increase stroke volume).

Homeostatic Imbalance 11.6

The pumping action of the healthy heart maintains a balance between cardiac output and venous return. But when the pumping efficiency of the heart is reduced so that circulation is inadequate to meet tissue needs, congestive heart failure (CHF) occurs. Congestive heart failure is usually a

Low blood pressure

Hormones: epinephrine, thyroxine

Crisis has passed

High blood pressure or blood volume

Exercise Decreased blood volume (hemorrhage)

Crisis stressors (physical or emotional trauma; increased body temperature; exercise)

Sympathetic nervous system activity

Activation of skeletal muscle and respiratory “pumps”

Parasympathetic nervous system controls (via vagus nerves)

Increased contractile force of cardiac muscle

Increased venous return

Stroke volume (ml/beat)Heart rate (beats/min)

Cardiac output (ml/min)

Decreased venous return

KEY: Increases, stimulates Reduces, inhibits Initial stimulus Physiological response End result

Figure 11.9 Influence of selected factors on cardiac output. Note that any decrease in heart rate or stroke volume will result in a corresponding decrease in cardiac output.

396 Essentials of Human Anatomy and Physiology

which carry blood away from the heart, and veins, which drain the tissues and return the blood to the heart, are simply conducting vessels—the free- ways and secondary roads. Only the tiny hairlike capillaries, which extend and branch through the tissues and connect the smallest arteries (arteri- oles) to the smallest veins (venules), directly serve the needs of the body cells. The capillaries are the side streets or alleys that intimately intertwine among the body cells and provide access to individual “homes.” It is only through their walls that exchanges between tissue cells and the blood can occur.

Notice that we routinely depict arteries in red and veins in blue. By convention, red indicates oxygen-rich blood, the normal status of blood in most of the body’s arteries, and blue indicates rela- tively oxygen-depleted, carbon dioxide–rich blood, the normal status of blood in most of the veins. However, there are exceptions. For instance, we have seen that oxygen-poor blood is carried in the pulmonary trunk, an artery, while oxygen- rich blood is transported back to the heart in pulmonary veins. An easy way to remember this difference is the following: Arteries are red and veins are blue, but for the lungs there’s an exception of two.

Microscopic Anatomy of Blood Vessels Tunics

Except for the microscopic capillaries (which have only one layer), the walls of blood vessels have three layers, or tunics (Figure 11.10). The tunica intima (tu′nı̆-kah in-tim′ah), which lines the lumen, or interior, of the vessels, is a thin layer of endo- thelium (squamous epithelial cells) resting on a basement membrane. Its cells fit closely together and form a slick surface that decreases friction as blood flows through the vessel lumen.

The tunica media (me′de-ah) is the bulky middle layer, made up mostly of smooth muscle and elastic fibers. Some of the larger arteries have elastic laminae, sheets of elastic tissue, in addi- tion to the scattered elastic fibers. The smooth muscle, which is controlled by the sympathetic nervous system, is active in changing the diame- ter of the vessels. As the vessels constrict or dilate, blood pressure increases or decreases, respectively.

blood “backs up” in the lungs, they become swol- len with blood, the pressure within them increases, and fluid leaks into the lung tissue, causing pul- monary edema. If untreated, the person “drowns” in these fluids.

If the right side of the heart fails, peripheral congestion occurs as blood backs up in the sys- temic circulation. Edema is most noticeable in the distal parts of the body: The feet, ankles, and fin- gers become swollen and puffy. Failure of one side of the heart puts a greater strain on the opposite side, and eventually the whole heart fails. _______________________________________ ✚

Did You Get It? 10. What does the term cardiac output mean? 11. What would you expect to happen to the heart rate

of an individual with a fever? Why? 12. What is the most important factor affecting stroke

volume?

For answers, see Appendix A.

Blood Vessels ➔ Learning Objective

□□ Compare and contrast the structure and function of arteries, veins, and capillaries.

Blood circulates inside the blood vessels, which form a closed transport system called the vascu- lar system. The idea that blood circulates, or “makes rounds,” through the body is only about 300 years old. The ancient Greeks believed that blood moved through the body like an ocean tide, first moving out from the heart and then ebbing back to it in the same vessels to get rid of its impurities in the lungs. It was not until the sev- enteenth century that William Harvey, an English physician, proved that blood did, in fact, move in circles.

Like a system of roads, the vascular system has its freeways, secondary roads, and alleys. As the heart beats, it propels blood into the large arter- ies leaving the heart. As the large arteries branch, blood moves into successively smaller and smaller arteries and then into the arterioles (ar-ter′e-ōlz), which feed the capillary (kap′ ı̆-lar″e) beds in the tissues. Capillary beds are drained by venules (ven′ulz), which in turn empty into veins that merge and finally empty into the great veins (venae cavae) entering the heart. Thus arteries,

Chapter 11: The Cardiovascular System 397

particular, tends to be much heavier. This struc- tural difference is related to a difference in func- tion of these two types of vessels. Arteries, which carry blood away from the heart, must be able to expand as blood is forced into them and then recoil passively as the blood flows off into the cir- culation during diastole. Their walls must be strong and stretchy enough to take these continuous

The tunica externa (eks′tern-ah) is the outer- most tunic. This layer is composed largely of fibrous connective tissue, and its function is to support and protect the vessels.

Structural Differences in Arteries, Veins, and Capillaries

The walls of arteries are usually much thicker than those of veins. The arterial tunica media, in

Tunica intima • Endothelium • Loose connective tissue

• Smooth muscle • Elastic fibers

Tunica media

• Collagen fibers Tunica externa

Artery Vein

Lumen Lumen

Internal elastic lamina

External elastic lamina

Valve

Artery Vein(a)

(b)

Endothelial cells

Basement membrane

Capillary bed

Capillary

Arteriole Venule

Figure 11.10 Structure of blood vessels. (a) Light photomicrograph of a muscular artery and the corresponding vein in cross section (85*). (b) The walls of arteries and veins are composed of three tunics: the tunica intima, tunica media, and tunica externa. Capillaries— between arteries and veins in the circulatory pathway—are composed only of the tunica intima. Notice that the tunica media is thick in arteries and relatively thin in veins.

View Histology

398 Essentials of Human Anatomy and Physiology

• To see the effect of venous valves, perform the following simple experiment on yourself: Allow one hand to hang by your side for a minute or two, until the blood vessels on its dorsal aspect become distended (swollen) with blood. Place two fingertips side by side on top of and per- pendicular to one of the distended veins. Then, pressing firmly, move your proximal finger along the vein toward your heart. Now release that finger. As you can see, the vein remains collapsed in spite of gravity because your prox- imal finger pushed the blood past a valve. Now remove your distal finger, and watch the vein fill rapidly with blood.

Skeletal muscle activity, known as the mus- cular pump, also enhances venous return. As the muscles surrounding the veins contract and relax, the blood is squeezed, or “milked,” through the veins toward the heart (Figure 11.11). Finally, the drop in pressure that occurs in the thorax just before we inhale causes the large veins near the heart to fill. Thus, the respi- ratory pump also helps return blood to the heart (see Figure 11.9).

The transparent walls of the capillaries are only one cell layer thick—just the tunica intima. Because of this exceptional thinness, substances are exchanged easily between the blood and the tissue cells. The tiny capillaries tend to form inter- weaving capillary beds. The flow of blood from an arteriole to a venule—that is, through a capillary bed—is called microcirculation. In most body regions, a capillary bed consists of two types of vessels: (1) a vascular shunt, a vessel that directly connects the arteriole and venule at opposite ends of the bed, and (2) true capillaries, the actual exchange vessels (Figure 11.12).

The true capillaries number 10 to 100 per cap- illary bed, depending on the organ or tissues served. They usually branch off the proximal end of the shunt and return to the distal end, but occa- sionally they spring from the terminal arteriole and empty directly into the postcapillary venule. A cuff of smooth muscle fibers, called a precapil- lary sphincter, surrounds the root of each true capillary and acts as a valve to regulate the flow of blood into the capillary. Blood flowing through a terminal arteriole may take one of two routes: through the true capillaries or through the shunt. When the precapillary sphincters are relaxed (open), blood flows through the true capillaries

changes in pressure without stretching out perma- nently (see Figure 11.19).

Veins, in contrast, carry blood back toward the heart, and the pressure in them tends to be low all the time. Thus veins have thinner walls. However, because the blood pressure in veins is usually too low to force the blood back to the heart, and because blood returning to the heart often flows against gravity (up the legs, for example), veins are modified to ensure that the amount of blood returning to the heart (venous return) equals the amount being pumped out of the heart (cardiac output) at any time. The lumens of veins tend to be much larger than those of corresponding arter- ies, and they tend to have a thinner tunica media but a thicker tunica externa. The larger veins have valves that prevent backflow of blood like those in the heart (see Figure 11.10).

Valve (open)

Contracted skeletal muscle

Valve (closed)

Vein

Direction of blood flow

Figure 11.11 operation of the muscular pump. When skeletal muscles contract and press against the flexible veins, the valves proximal to the area of contraction are forced open, and blood is squeezed toward the heart like toothpaste from a tube. The valves distal to the point of contraction are closed by the backflowing blood.

Chapter 11: The Cardiovascular System 399

heart as the ascending aorta, arches to the left as the aortic arch, and then plunges downward through the thorax, following the spine (thoracic aorta) finally to pass through the diaphragm into the abdominopelvic cavity, where it becomes the abdominal aorta (Figure 11.13, p. 401).

and takes part in exchanges with tissue cells. When the sphincters are contracted (closed), blood flows through the shunts and bypasses the tissue cells in that region.

Homeostatic Imbalance 11.7

Varicose veins are common in people who stand for long periods of time (for example, cashiers and hairdressers) and in obese (or pregnant) individu- als. The common factors are the pooling of blood in the feet and legs and inefficient venous return resulting from inactivity or pressure on the veins. In any case, the overworked valves give way, and the veins become twisted and dilated. A serious complication of varicose veins is thrombophlebi- tis (throm″bo-flĕ-bi′tis), inflammation of a vein that results when a clot forms in a vessel with poor circulation. Because all venous blood must pass through the pulmonary circulation before traveling through the body tissues again, a com- mon consequence of thrombophlebitis is clot detachment and pulmonary embolism, which is a life-threatening condition in which the embolism lodges in a vessel in the lung. ________________✚

Did You Get It? 13. Assume you are viewing a blood vessel under the

microscope. It has a large, lopsided lumen, relatively thick tunica externa, and a relatively thin tunica media. Which kind of blood vessel is this?

14. Arteries lack valves, but veins have them. How is this structural difference related to blood pressure?

15. How is the structure of capillaries related to their function in the body?

For answers, see Appendix A.

Gross Anatomy of Blood Vessels ➔ Learning Objective

□□ Identify the body’s major arteries and veins, and name the body region supplied by each.

Major Arteries of the Systemic Circulation

The aorta is the largest artery of the body, and it is a truly splendid vessel. In adults, the aorta is about the size of a garden hose (with an internal diameter about equal to the diameter of your thumb) where it leaves the left ventricle of the heart. It decreases only slightly in diameter as it runs to its terminus. Different parts of the aorta are named for either their location or their shape. The aorta springs upward from the left ventricle of the

Condition (a). The true capillaries would be flushed with blood to serve the working muscle cells.A:

(a) Sphincters open; blood flows through true capillaries.

(b) Sphincters closed; blood flows through vascular shunt.

Precapillary sphincters

Vascular shunt

Terminal arteriole Postcapillary venule

True capillaries

Figure 11.12 Anatomy of a capillary bed. The vascular shunt bypasses the true capillaries when precapillary sphincters controlling blood entry into the true capillaries are constricted.

Assume the capillary bed depicted here is in the biceps brachii muscle of your arm. What condition would the capillary bed be in, (a) or (b), if you were doing push-ups at the gym?

400 Essentials of Human Anatomy and Physiology

three branches: the L. gastric artery, which supplies the stomach; the splenic artery, which supplies the spleen; and the common hepatic artery, which supplies the liver.

• The unpaired superior mesenteric (mes″en- ter′ik) artery supplies most of the small intes- tine and the first half of the large intestine, or colon.

• The renal (R. and L.) arteries serve the kidneys.

• The gonadal (R. and L.) arteries supply the gonads. They are called the ovarian arteries in females (serving the ovaries) and the testicular arteries in males (serving the testes).

• The lumbar arteries (not illustrated in Figure 11.13) are several pairs of arteries serving the heavy muscles of the abdomen and trunk walls.

• The inferior mesenteric artery is a small, unpaired artery supplying the second half of the large intestine.

• The common iliac (R. and L.) arteries are the final branches of the abdominal aorta. Each di- vides into an internal iliac artery, which sup- plies the pelvic organs (bladder, rectum, and so on), and an external iliac artery, which enters the thigh, where it becomes the femoral ar- tery. The femoral artery and its branch, the deep artery of the thigh, serve the thigh. At the knee, the femoral artery becomes the popli- teal artery, which then splits into the anterior tibial artery and posterior tibial artery, which supply the leg and foot. The anterior tib- ial artery terminates in the dorsalis pedis ar- tery, which via the arcuate artery supplies the dorsum of the foot. (The dorsalis pedis is often palpated in patients with circulatory problems of the legs to determine whether the distal part of the leg has adequate circulation.)

Major Veins of the Systemic Circulation

Although arteries are generally located in deep, well-protected body areas, many veins are more superficial, and some are easily seen and palpated on the body surface. Most deep veins follow the course of the major arteries, and with a few excep- tions, the naming of these veins is identical to that of their companion arteries. Major systemic arteries branch off the aorta, whereas the veins converge on the venae cavae, which enter the right atrium of the heart. Veins draining the head and arms empty

The major branches of the aorta and the organs they serve are listed next in sequence from the heart. (Figure 11.13 shows the course of the aorta and its major branches.) As you locate the arteries on the figure, use what you already know to make your learning easier. In many cases the name of the artery tells you the body region or organs served (for example, renal artery, brachial artery, and coronary artery) or the bone followed (femoral artery and ulnar artery).

Arterial Branches of the Ascending Aorta

• The only branches of the ascending aorta are the right (R.) coronary artery and left (L.) coronary artery, which serve the heart.

Arterial Branches of the Aortic Arch

• The brachiocephalic (bra″ke-o-sĕ-fal′ik) trunk (the first branch off the aortic arch) splits into the R. common carotid (kah- rŏ′tid) artery, which further branches into the R. internal and R. external carotid arteries, and the R. subclavian (sub-kla′ve-an) artery. (See same-named vessels on the left side of the body for organs served.)

• The L. common carotid artery is the second branch off the aortic arch. It divides, forming the L. internal carotid, which serves the brain, and the L. external carotid, which serves the skin and muscles of the head and neck.

• The third branch of the aortic arch, the L. subclavian artery, gives off an important branch—the vertebral artery, which serves part of the brain. In the axilla, the subclavian artery becomes the axillary artery and then continues into the arm as the brachial artery, which supplies the arm. At the elbow, the bra- chial artery splits to form the radial artery and ulnar artery, which serve the forearm.

Arterial Branches of the Thoracic Aorta

• The intercostal arteries (10 pairs) supply the muscles of the thorax wall. Other branches of the thoracic aorta supply the lungs (bronchial arteries), the esophagus (esophageal arteries), and the diaphragm (phrenic arteries). (These arteries are not illustrated in Figure 11.13.)

Arterial Branches of the Abdominal Aorta

• The celiac trunk is the first branch of the ab- dominal aorta. It is a single vessel that has

Chapter 11: The Cardiovascular System 401

Internal carotid artery

Common carotid arteries

Subclavian artery Subclavian artery

Aortic arch Ascending aorta Coronary artery

Thoracic aorta (above diaphragm)

Renal artery

Superficial palmar arch

Radial artery

Ulnar artery

Internal iliac artery

Deep palmar arch

Vertebral artery

Brachiocephalic trunk Axillary artery

Brachial artery

Abdominal aorta

Superior mesenteric artery

Gonadal artery

External iliac artery

Digital arteries

Femoral artery

Popliteal artery

Anterior tibial artery

Posterior tibial artery

Arcuate artery

Inferior mesenteric artery

Celiac trunk

External carotid artery

Arteries of the head and trunk

Arteries that supply the upper limb

Arteries that supply the lower limb

Dorsalis pedis artery

Common iliac artery

Figure 11.13 Major arteries of the systemic circulation, anterior view. All arteries are bilateral unless otherwise stated in the text, although both may not be represented in this figure.

402 Essentials of Human Anatomy and Physiology

into the superior vena cava, and those draining the lower body empty into the inferior vena cava. (These veins are described next and shown in Figure 11.14. As before, locate the veins on the fig- ure as you read their descriptions.)

Veins Draining into the Superior Vena Cava Veins draining into the superior vena cava are listed in a distal-to-proximal direction; that is, in the same direction the blood flows into the supe- rior vena cava.

• The radial vein and ulnar vein are deep veins draining the forearm. They unite to form the deep brachial vein, which drains the arm and empties into the axillary vein in the axillary region.

• The cephalic (se-fal′ik) vein provides for the superficial drainage of the lateral aspect of the arm and empties into the axillary vein.

• The basilic (bah-sil′ik) vein is a superficial vein that drains the medial aspect of the arm and empties into the brachial vein proximally. The basilic and cephalic veins are joined at the anterior aspect of the elbow by the median cubital vein. (The median cubital vein is often chosen as the site for withdrawing blood for the purpose of blood testing.)

• The subclavian vein receives venous blood from the arm through the axillary vein and from the skin and muscles of the head through the external jugular vein.

• The vertebral vein drains the posterior part of the head.

• The internal jugular vein drains the dural si- nuses of the brain.

• The brachiocephalic (R. and L.) veins are large veins that receive venous drainage from the subclavian, vertebral, and internal jugular veins on their respective sides. The brachioce- phalic veins join to form the superior vena cava, which enters the heart.

• The azygos (āz′ı̄ -gos) vein is a single vein that drains the thorax and enters the superior vena cava just before it joins the heart. (This vein is not illustrated in Figure 11.14.)

Veins Draining into the Inferior Vena Cava The inferior vena cava, which is much longer than the superior vena cava, returns blood to the heart from all body regions inferior to the diaphragm.

As before, we will trace the venous drainage in a distal-to-proximal direction.

• The anterior tibial vein and posterior tibial vein and the fibular vein drain the leg (calf and foot). (The fibular vein is not shown in Figure 11.14.) The posterior tibial vein be- comes the popliteal vein at the knee and then the femoral vein in the thigh. The femo- ral vein becomes the external iliac vein as it enters the pelvis.

• The great saphenous (sah-fe′nus) veins are the longest veins in the body. They receive the superficial drainage of the leg. They begin at the dorsal venous arch in the foot and travel up the medial aspect of the leg to empty into the femoral vein in the thigh.

• Each common iliac (R. and L.) vein is formed by the union of the external iliac vein and the internal iliac vein (which drains the pel- vis) on its own side. The common iliac veins join to form the inferior vena cava, which then ascends superiorly in the abdominal cavity.

• The R. gonadal vein drains the right ovary in females and the right testicle in males. (The L. gonadal vein empties into the left renal vein superiorly.) (The gonadal veins are not illustrated in Figure 11.14.)

• The renal (R. and L.) veins drain the kidneys.

• The hepatic portal vein is a single vein that drains the digestive tract organs and carries this blood through the liver before it enters the systemic circulation. (We discuss the hepatic portal circulation in the next section.)

• The hepatic (R. and L.) veins drain the liver.

Did You Get It? 16. In what part of the body are the femoral, popliteal,

and arcuate arteries found? 17. In what part of the body are the axillary, cephalic,

and basilic veins located?

For answers, see Appendix A.

Special Circulations

➔ Learning Objective □□ Discuss the unique features of the arterial circulation

of the brain, and hepatic portal circulation.

Arterial Supply of the Brain and the Circle of Willis Because a lack of blood for even a few minutes

Chapter 11: The Cardiovascular System 403

Renal vein

Splenic vein

Basilic vein Brachial vein Cephalic vein

Dural venous sinuses

External jugular vein Vertebral vein Internal jugular vein

Superior vena cava

Right and left brachiocephalic veins Axillary vein

Great cardiac vein

Hepatic veins

Hepatic portal vein

Superior mesenteric vein

Inferior vena cava

Ulnar vein Radial vein

Common iliac vein

External iliac vein

Internal iliac vein

Digital veins

Femoral vein

Great saphenous vein

Popliteal vein

Posterior tibial vein

Anterior tibial vein

Small saphenous vein

Dorsal venous arch Dorsal metatarsal veins

Inferior mesenteric vein

Median cubital vein

Subclavian vein

Veins of the head and trunk

Veins that drain the upper limb

Veins that drain the lower limb

Figure 11.14 Major veins of the systemic circulation, anterior view. The vessels of the pulmonary circulation are not illustrated, accounting for the incomplete appearance of the circulation from the heart. All veins are bilateral unless specifically stated in the text.

404 Essentials of Human Anatomy and Physiology

The paired vertebral arteries pass upward from the subclavian arteries at the base of the neck. Within the skull, the vertebral arteries join to form the single basilar artery. This artery serves the brain stem and cerebellum as it travels upward. At the base of the cerebrum, the basilar artery divides to form the posterior cerebral arteries, which supply the posterior part of the cerebrum.

causes the delicate brain cells to die, a continuous blood supply to the brain is crucial. The brain is supplied by two pairs of arteries, the internal carotid arteries and the vertebral arteries (Figure 11.15).

The internal carotid arteries, branches of the common carotid arteries, run through the neck and enter the skull through the temporal bone. Once inside the cranium, each divides into the anterior cerebral artery and middle cerebral artery, which supply most of the cerebrum.

(b)

Frontal lobe

Posterior

Optic chiasma

Middle cerebral artery

Internal carotid artery

Mammillary body

Temporal lobe

Occipital lobe

Cerebral arterial circle (circle of Willis)

• Posterior cerebral artery

Basilar artery

Vertebral artery

Cerebellum

• Posterior communicating artery

(a)

Pons

• Anterior cerebral artery

• Anterior communicating artery

Figure 11.15 Arterial supply of the brain. (a) Major arteries of the brain. The four arteries composing the circle of Willis are indicated by bullet points. (The cerebellum is shown only on the left side of the brain.) (b) Colorized arteriograph of the brain’s arteries.

Chapter 11: The Cardiovascular System 405

The anterior and posterior blood supplies of the brain are united by small communicating arte- rial branches. The result is a complete circle of connecting blood vessels called the cerebral arte- rial circle or the circle of Willis, which sur- rounds the base of the brain. The cerebral arterial circle protects the brain by providing more than one route for blood to reach brain tissue in case of a clot or impaired blood flow anywhere in the system.

Hepatic Portal Circulation The veins of the hepatic portal circulation drain the digestive organs, spleen, and pancreas and deliver this blood to the liver through the hepatic portal vein (Figure 11.16). When you have just eaten, the hepatic portal blood contains large amounts of nutrients. Because the liver is a key body organ involved in maintain- ing the proper glucose, fat, and protein concentra- tions in the blood, this system allows blood to “take a detour” to ensure that the liver processes these substances before they enter the systemic circula- tion. The liver also helps detoxify blood by remov- ing and processing toxins absorbed by the stomach and intestines. As blood flows slowly through the liver, some of the nutrients are removed to be

Hepatic portal system

Arterial blood

Venous blood

Hepatic vein

Inferior vena cava

Hepatic portal vein

First capillary bed

Nutrients and toxins absorbed

Liver cells (hepatocytes)

Nutrients and toxins leave

Second capillary bed (liver sinusoids)

Stomach and intestine Liver

Figure 11.16 The basic scheme of the hepatic portal system. Note the presence of two capillary beds within the portal system.

Nutrients and toxins picked up from capillaries in the stomach and intestine are transported to the liver for processing. From the liver

sinusoids, the blood continues into the hepatic veins and inferior vena cava.

➔ConCeptLinkLike the portal circulation that links the hypothala-mus of the brain and the anterior pituitary gland (Chapter 9, p. 339), the hepatic portal circulation is a unique and unusual circulation. Normally, arteries feed capillary beds, which in turn drain into veins. In the hepatic portal circulation, veins feed the liver circula- tion (Figure 11.16). ➔

The major vessels composing the hepatic por- tal circulation (Figure 11.17, p. 406) include the inferior and superior mesenteric veins, the splenic vein, and the left gastric vein. The inferior mes- enteric vein, draining the terminal part of the large intestine, drains into the splenic vein, which itself drains the spleen, pancreas, and the left side of the stomach. The splenic vein and superior mesenteric vein (which drains the small intestine and the first part of the colon) join to form the hepatic portal vein. The L. gastric vein, which drains the right side of the stomach, drains directly into the hepatic portal vein.

stored or processed in various ways for later release to the blood. The liver is drained by the hepatic veins that enter the inferior vena cava.

406 Essentials of Human Anatomy and Physiology

Arterial Pulse

The alternating expansion and recoil of an artery that occurs with each beat of the left ventricle cre- ates a pressure wave—a pulse—that travels through the entire arterial system. Normally the pulse rate (pressure surges per minute) equals the heart rate (beats per minute). The pulse averages 70 to 76 beats per minute in a healthy resting per- son. It is influenced by activity, postural changes, and emotions.

You can feel a pulse in any artery lying close to the body surface by compressing the artery against firm tissue; this provides an easy way of counting heart rate. Because it is so accessible, the point where the radial artery surfaces at the wrist (the radial pulse) is routinely used to take a pulse measurement, but there are several other clinically important arterial pulse points (Figure 11.18).

Did You Get It? 18. Which vessel—the hepatic portal vein, hepatic vein, or

hepatic artery—has the highest content of nutrients after a meal?

19. In what two important ways is the pulmonary circulation different from the systemic circulation?

For answers, see Appendix A.

Physiology of Circulation ➔ Learning Objective

□□ Define pulse, and name several pulse points.

A fairly good indication of the efficiency of a per- son’s circulatory system can be obtained by taking arterial pulse and blood pressure measurements. These measurements, along with those of respira- tory rate and body temperature, are referred to collectively as vital signs in clinical settings.

Liver Spleen

Gastric veins

Inferior vena cava (not part of hepatic portal system)

Splenic vein

Inferior mesenteric vein

Superior mesenteric vein

Large intestine

Hepatic portal vein

Small intestine

Pancreas

Stomach

Figure 11.17 The hepatic portal circulation.

Chapter 11: The Cardiovascular System 407

Because these same points are compressed to stop blood flow into distal tissues during significant blood loss or hemorrhage, they are also called pressure points. For example, if you seriously cut your hand, you can stop the bleeding some- what by compressing the brachial artery.

• Palpate each of the pulse points shown in Figure 11.18 by placing the tips of your first two or three fingers of one hand over the artery at the site indicated. Do not use your thumb, because it has its own pulse. Compress the artery firmly as you begin and then imme- diately ease up on your pressure slightly. In each case, notice the regularity and relative strength of the pulse.

Did You Get It? 20. Which artery is palpated at the wrist? At the groin?

At the side of the neck?

For the answer, see Appendix A.

Blood Pressure

➔ Learning Objectives □□ Define blood pressure, and list factors affecting

and/or determining blood pressure.

□□ Define hypertension and atherosclerosis, and describe possible health consequences of these conditions.

Any system equipped with a pump that forces fluid through a one-way network of closed tubes operates under pressure. Blood pressure is the pressure the blood exerts against the inner walls of the blood vessels, and it is the force that keeps blood circulating continuously even between heartbeats. Unless stated otherwise, the term blood pressure in this discussion is understood to mean the pressure within the large systemic arteries near the heart.

Blood Pressure Gradient When the ventricles contract, they force blood into large, thick-walled elastic arteries close to the heart that expand as the blood is pushed into them.

Common carotid artery

Brachial artery

Radial artery

Femoral artery

Popliteal artery

Posterior tibial artery

Dorsalis pedis artery

Superficial temporal artery

Facial artery

Figure 11.18 Body sites where the pulse is most easily palpated. (Most of the specific arteries indicated are discussed on p. 400.)

➔ConCeptLinkAs you remember, in the passive process of filtration, substances move from areas of high pressure to areas of low pressure through a filter (Chapter 3, p. 102). Blood flow is driven by these same differences in pres- sure, but without a filter. ➔

The high pressure in these elastic arteries forces the blood to continuously move into areas where the pressure is lower. The pressure is high- est in the large arteries closest to the heart and continues to drop throughout the systemic path- way, reaching zero at the right atrium (Figure 11.19). Recall that the blood flows into the smaller arter- ies, then arterioles, capillaries, venules, veins, and finally back to the large venae cavae entering the right atrium of the heart. It flows continuously along a pressure gradient (from high to low pres- sure) as it makes its circuit, day in and day out. The valves in the large veins, the milking activity of the skeletal muscles, and pressure changes in the thorax are so important because together they ensure blood flows back to the heart to be pumped out to the body again. The pressure dif- ferences between arteries and veins become very clear when these vessels are cut. If a vein is cut, the blood flows evenly from the wound; a lacer- ated artery produces rapid spurts of blood. A simi- lar pressure drop occurs as blood flows through the pulmonary pathway.

408 Essentials of Human Anatomy and Physiology

the higher systolic pressure written first—120/80 (read “120 over 80”) translates to a systolic pres- sure of 120 mm Hg and a diastolic pressure of 80 mm Hg. Most often, systemic arterial blood pres- sure is measured indirectly by the auscultatory (os-kul′tuh-tor-e) method. This procedure is used to measure blood pressure in the brachial artery of the arm (Figure 11.20).

Effects of Various Factors on Blood Pressure Arterial blood pressure (BP) is directly related to cardiac output (CO; the amount of blood pumped out of the left ventricle per minute) and peripheral resistance (PR). This relationship is expressed by the equation BP = CO × PR. We have already considered regulation of cardiac output, so we will concentrate on peripheral resistance here.

Peripheral resistance is the amount of fric- tion the blood encounters as it flows through the blood vessels. Many factors increase peripheral resistance, but probably the most important is the constriction, or narrowing, of blood vessels, espe- cially arterioles, as a result of either sympathetic nervous system activity or atherosclerosis. Increased blood volume or increased blood vis- cosity (thickness) also raises peripheral resistance. Any factor that increases either cardiac output or peripheral resistance causes an almost immediate rise in blood pressure. Many factors can alter blood pressure—age, weight, time of day, exer- cise, body position, emotional state, and various drugs, to name a few. The influence of a few of these factors is discussed next.

• Neural factors: the autonomic nervous system. The parasympathetic division of the autonomic nervous system has little or no ef- fect on blood pressure, but the sympathetic division is important. The major action of the sympathetic nerves on the vascular system is to cause vasoconstriction (vās″o-kon- strik′shun), or narrowing of the blood vessels, which increases the blood pressure. The sym- pathetic center in the medulla of the brain is activated to cause vasoconstriction in many different circumstances (Figure 11.21, p. 410). For example, when we stand up suddenly af- ter lying down, gravity causes blood to pool very briefly in the vessels of the legs and feet, and blood pressure drops. This activates pressoreceptors, also called baroreceptors

Continuous blood flow absolutely depends on the stretchiness of the larger arteries and their abil- ity to recoil and keep exerting pressure on the blood as it flows into the rest of the vascular sys- tem. Think of a garden hose with relatively hard walls. When the water is turned on, the water spurts out under high pressure because the hose walls don’t expand. However, when the water fau- cet is suddenly turned off, the flow of water stops just as abruptly. The reason is that the walls of the hose cannot recoil to keep pressure on the water; therefore, the pressure drops, and the flow of water stops. The importance of the elasticity of the arteries is best appreciated when it is lost, as hap- pens in arteriosclerosis. Arteriosclerosis is also called “hardening of the arteries” (see “A Closer Look” on pp. 412–413).

Did You Get It? 21. How does blood pressure change throughout the

systemic circulatory pathway?

For the answer, see Appendix A.

Measuring Blood Pressure The off-and-on flow of blood into the arteries as the heart alternately contracts and relaxes causes the blood pressure to rise and fall during each beat. Thus, two arterial blood pressures are usually measured: systolic (sis-tŏ′lik) pressure, the pressure in the arteries at the peak of ventricular contraction, and diastolic (di″us-tŏ′lik) pressure, the pressure when the ventricles are relaxing. Blood pressures are reported in millimeters of mercury (mm Hg), with

Ao rta

Ar te

rie s

Ar te

rio le

C ap

illa rie

Ve nu

le s

Ve in

Ve na

e ca

va e

B lo

od p

re ss

ur e

(m m

H g) Systolic pressure

Diastolic pressure

100

120

Figure 11.19 Blood pressure in the systemic circuit of the cardiovascular system.

Chapter 11: The Cardiovascular System 409

11 The final example concerns sympathetic nervous system activity when we exercise vigorously or are frightened and have to make a hasty escape. Under these conditions, generalized vasoconstriction occurs except in the skeletal muscles. The vessels of the skeletal muscles dilate to increase the blood flow to the working muscles. (However, note that the sympathetic nerves never cause vaso- constriction of blood vessels of the heart or brain.)

• Renal factors: the kidneys. The kidneys play a major role in regulating arterial blood pressure by altering blood volume. As blood pressure (and/or blood volume) increases above normal, the kidneys allow more water

(baro = pressure), in the large arteries of the neck and chest. They send off warning signals that result in reflexive vasoconstric- tion, quickly increasing blood pressure back to the homeostatic level.

When blood volume suddenly decreases, as in hemorrhage, blood pressure drops, and the heart begins to beat more rapidly as it tries to compensate. However, because blood loss reduces venous return, the heart also beats weakly and inefficiently. In such cases, the sympathetic nervous system causes vasocon- striction to increase the blood pressure so that (it is hoped) venous return increases and cir- culation can continue. This phenomenon also happens during severe dehydration.

Blood pressure 120 systolic 70 diastolic (to be measured)

Rubber cuff inflated with air

Brachial artery

Brachial artery closed

Pressure in cuff above 120; no sounds audible

120 mm Hg

Pressure in cuff below 120, but above 70

120 mm Hg

Sounds audible in stethoscope

Pressure in cuff below 70; no sounds audible

70 mm Hg

(a) (b) (c) (d)

70 mm Hg

The course of the brachial artery of the arm. Assume a blood pressure of 120/70 in a young, healthy person.

The blood pressure cuff is wrapped snugly around the arm just above the elbow and inflated until the cuff pressure exceeds the systolic blood pressure. At this point, blood flow into the arm is stopped, and a brachial pulse cannot be felt or heard.

The pressure in the cuff is gradually reduced while the examiner listens (auscultates) for sounds in the brachial artery with a stethoscope. The pressure read as the first soft tapping sounds are heard (the first point at which a small amount of blood is spurting through the constricted artery) is recorded as the systolic pressure.

As the pressure is reduced still further, the sounds become louder and more distinct; when the artery is no longer constricted and blood flows freely, the sounds can no longer be heard. The pressure at which the sounds disappear is recorded as the diastolic pressure.

Figure 11.20 Measuring blood pressure.

410 Essentials of Human Anatomy and Physiology

the blood, water follows. Thus, blood volume and blood pressure both rise in response to aldosterone.

• Temperature. In general, cold has a vaso- constricting effect. This is why your exposed skin feels cold to the touch on a winter day and why cold compresses are recommended to prevent swelling of a bruised area. Heat has a vasodilating effect. This explains why skin reddens during exercise as body temper- ature increases and why warm compresses are used to speed the circulation into an in- flamed area.

• Chemicals. The effects of chemical sub- stances, many of which are drugs, on blood pressure are widespread and well known in many cases. We will give just a few examples here. Epinephrine increases both heart rate and blood pressure.

to leave the body in the urine. Because the source of this water is the bloodstream, blood volume decreases, which in turn de- creases blood pressure. However, when arte- rial blood pressure falls, the kidneys retain body water, maintaining blood volume and blood pressure (see Figure 11.21). In order to increase blood volume and blood pressure, fluids must be ingested or administered intra- venously.

In addition, when arterial blood pressure is low, certain kidney cells release the enzyme renin into the blood. Renin triggers a series of chemical reactions that result in the forma- tion of angiotensin II, a potent vasoconstric- tor chemical. Angiotensin also stimulates the adrenal cortex to release aldosterone, a hor- mone that enhances sodium ion reabsorption by the kidneys. As sodium ions move into

Increases, stimulates Initial stimulus Physiological response End result

Increased arterial blood pressure

Exercise Postural changes Chemicals (renin, nicotine and others)

Increased blood viscosity

Decreased blood volume

Sympathetic nervous system centers

Kidney conserves water and salt

Increased stroke volume

Increased heart rate

Vasoconstriction

Increased peripheral resistanceIncreased cardiac output

KEY:

Figure 11.21 Summary of factors that increase arterial blood pressure.

Chapter 11: The Cardiovascular System 411

tion called orthostatic hypotension. Because an aging sympathetic nervous system reacts more slowly to postural changes, blood pools briefly in the lower limbs, reducing blood pressure and, consequently, blood delivery to the brain. Making postural changes more slowly to give the nervous system time to make the necessary adjustments usually prevents this problem. ________________✚

Chronic hypotension (not explained by physi- cal conditioning) may hint at poor nutrition and inadequate levels of blood proteins. Because blood viscosity is low, blood pressure is also lower than normal. Acute hypotension is one of the most important warnings of circulatory shock, a con- dition in which the blood vessels are inadequately filled and blood cannot circulate normally. The most common cause is blood loss.

A brief elevation in blood pressure is a normal response to fever, physical exertion, and emo- tional upset, such as anger or fear. Persistent hypertension (high blood pressure), is patho- logical and is defined as a condition of sustained elevated arterial pressure of 140/90 or higher.

Homeostatic Imbalance 11.9

Chronic hypertension is a common and danger- ous disease that warns of increased peripheral resistance. Although it progresses without symp- toms for the first 10 to 20 years, it slowly and surely strains the heart and damages the arteries. For this reason, hypertension is often called the “silent killer.” Because the heart is forced to pump against increased resistance, it must work harder, and in time, the myocardium enlarges. When finally strained beyond its capacity to respond, the heart weakens and its walls become flabby. Hypertension also ravages blood vessels, causing small tears in the endothelium that accelerate the progress of atherosclerosis (the early stage of arte- riosclerosis).

Although hypertension and atherosclerosis are often linked, it is difficult to blame hypertension on any distinct anatomical pathology. In fact, about 90 percent of hypertensive people have primary (essential) hypertension, which cannot be attrib- uted to any specific organic cause. However, factors such as diet, obesity, heredity, race, and stress appear to be involved. For instance, more women than men and more blacks than whites are hypertensive.

Nicotine increases blood pressure by caus- ing vasoconstriction. Both alcohol and hista- mine cause vasodilation and decrease blood pressure. The reason a person who has “one too many” becomes flushed is that alcohol di- lates the skin vessels.

• Diet. Although medical opinions tend to change and are at odds from time to time, it is generally thought that a diet low in salt, satu- rated fats, and cholesterol helps to prevent hy- pertension, or high blood pressure.

Did You Get It? 22. What is the effect of hemorrhage on blood pressure?

Why?

For the answer, see Appendix A.

Variations in Blood Pressure In normal adults at rest, systolic blood pressure varies between 110 and 140 mm Hg, and diastolic pressure between 70 and 80 mm Hg—but blood pressure varies considerably from one person to another and cycles over a 24-hour period, peaking in the morning. What is normal for you may not be nor- mal for your grandfather or your neighbor. Blood pressure varies with age, weight, race, mood, physical activity, and posture. Nearly all these variations can be explained in terms of the factors affecting blood pressure that we have already discussed.

Hypotension, or low blood pressure, is gen- erally considered to be a systolic blood pressure below 100 mm Hg. In many cases, it simply reflects individual differences and is no cause for concern. In fact, low blood pressure is an expected result of physical conditioning and is often associated with long life and an old age free of illness.

Homeostatic Imbalance 11.8

Elderly people may experience temporary low blood pressure and dizziness when they rise sud- denly from a reclining or sitting position—a condi-

➔ConCeptLinkRecall that epinephrine is the “fight-or-flight” hor-mone, which is produced by the adrenal medulla and helps us deal with short-term stress (Chapter 9, p. 349). ➔

412

mounds of muscle begin to protrude into the vessel wall (and ultimately the vessel lumen), the condition is called atherosclerosis (see photo).

Arteriosclerosis is the end stage of the disease. As enlarging plaques hinder diffusion of nutrients from the blood to the deeper tissues of the artery wall, smooth muscle cells in the tunica media die, and the elastic fibers deteriorate and are gradually replaced by nonelastic scar tissue. Then, cal- cium salts are deposited in the

summons the immune system to repair the damage. Most plaques grow slowly, through a series of inju- ries that heal, only to be ruptured again and again. As the plaque grows, the injured endothelial cells release chemicals that make the endothelium more permeable, allowing fats and cholesterol to take up residence just deep to the tunica intima. Monocytes attracted to the area migrate beneath the endothelium, where they become macrophages that gorge themselves on the fat in particular. These cells can become so filled with oxidized fats that they are transformed into “foam cells” that lose their ability to function. Soon they are joined by smooth muscle cells migrating from the tunica media of the blood vessel wall. These cells deposit collagen and elastin fibers in the area and also take in fat, becoming foam cells. The result is the erroneously named fatty streak stage, characterized by thickening of the tunica intima by lesions called fibrous plaques or atherosclerotic plaques. When these small, fatty

When arteries are narrowed by atherosclerosis, the clogging process begins on the inside: the walls of the vessels thicken and then pro- trude into the vessel lumen. Once this happens, a roaming blood clot or arterial spasms can close the vessel completely. All blood vessels are sus- ceptible to atherosclerosis, but for some unknown reason the aorta and the coronary arteries are most often affected.

onset and Stages of Atherosclerosis What triggers this scourge of blood vessels that indirectly causes half of the deaths in the Western world? The ini- tial event is damage to the tunica intima caused by bloodborne chemi- cals such as carbon monoxide (present in cigarette smoke or auto exhaust); by bacteria or viruses; or by physical fac- tors such as a blow or persistent hyper- tension. Once a break has occurred, blood platelets cling to the injured site and initiate clotting to prevent blood loss. The injured endothelium

Atherosclerosis? Get Out the Cardiovascular Drāno!

A CLOSER

LOOK

Atherosclerotic plaques nearly close a human artery.

Hypertension runs in families. The child of a hyper- tensive parent is twice as likely to develop high blood pressure as is a child of parents with normal blood pressure. High blood pressure is common in obese people because the total length of their blood vessels is greater than that in thinner individuals. For each pound of fat, miles of additional blood vessels are required, making the heart work harder to pump blood over longer distances. ___________________ ✚

Capillary Exchange of Gases and nutrients

➔ Learning Objective □□ Describe the exchanges that occur across capillary

walls.

Capillaries form an intricate network among the body’s cells, and no substance has to diffuse very far to enter or leave a cell. The substances to be exchanged diffuse through the interstitial fluid (tissue fluid) between cells.

Substances tend to move to and from body cells according to their concentration gradients. Thus, oxygen and nutrients leave the blood and move into the tissue cells, and carbon dioxide and other wastes exit the tissue cells and enter the blood. Basically, substances entering or leav- ing the blood may take one of four routes across the plasma membranes of the single layer of endothelial cells forming the capillary wall (Figure 11.22).

413

Sometimes after angioplasty, a metal- mesh tube called a stent is placed in the artery to keep it open.

When a blood clot is trapped by the diseased vessel walls, the answer may be a clot-dissolving agent, for example, tissue plasminogen activator (tPA), a naturally occurring substance now being produced by genetic engi- neering techniques. Intravenously injecting tPA can restore blood flow quickly and put an early end to a heart attack in progress.

There is little doubt that lifestyle factors—emotional stress, smoking, obesity, high-fat and high-cholesterol diets, and lack of exercise—contribute to both atherosclerosis and hyperten- sion. With these changeable risk fac- tors, why not just have patients change their lifestyle? This is easier said than done. Although taking anti- oxidants (vitamins C and E and beta carotene) and exercising more may “undo” some of the damage, old habits die hard, and North Americans like their burgers and butter. If athero- sclerosis could be reversed to give the heart a longer and healthier life, many more people with diseased arteries may be more willing to trade lifelong habits for a healthy old age!

statins, cholesterol-reducing drugs, show promise. Even the humble aspi- rin is gaining new respect, and many cardiologists recommend that people at high risk take one “baby aspirin” (81 mg) daily.

So what can help when the dam- age is done and the heart is at risk because of atherosclerotic coronary vessels? In the past, the only choice has been coronary artery bypass sur- gery, in which vessels removed from the legs or thoracic cavity are implanted in the heart to restore cir- culation. A more recently developed technique, balloon angioplasty, uses a catheter with a balloon packed into its tip. When the catheter reaches the blockage, the balloon is inflated, and the fatty mass is compressed against the vessel wall. However, this proce- dure is useful to clear only very local- ized obstructions. A newer catheter device uses a laser beam to vaporize the arterial clogs. Although these intravascular devices are faster, cheaper, and less risky than bypass surgery, they carry with them the same major shortcoming: they do nothing to stop the underlying dis- ease, and in time new blockages occur in 30 to 50 percent of cases.

lesions, forming complicated plaques. Collectively, these events cause the arterial wall to fray and ulcerate, conditions that encourage thrombus formation. The increased rigidity of the vessels leads to hyper- tension. Together, these events increase the risk of myocardial infarc- tions, strokes, and aneurysms.

The popular view that most heart attacks are the consequence of severe vessel narrowing and hardening is now being challenged. It appears that the body’s defense system betrays it. The inflammatory process that occurs in the still soft, unstable, cholesterol- rich plaques changes the biology of the vessel wall and makes the plaques susceptible to rupture, exploding off fragments that trigger massive clots that can cause lethal heart attacks.

Treatment and Prevention Some medical centers test heart patients for elevated levels of choles- terol and C-reactive protein, a marker of inflammation. Electron beam CT scans may be able to identify people at risk by detecting calcium deposits in their coronary arteries. Antibiotics and anti-inflammatory drugs are being tested as preventive measures, and

1. Direct diffusion through membrane. As with all cells, substances can diffuse directly through (cross) their plasma membranes if the substances are lipid-soluble (such as the respi- ratory gases oxygen and carbon dioxide).

2. Diffusion through intercellular clefts. Limited passage of fluid and small solutes is allowed by intercellular clefts (gaps between cells in the capillary wall). Most of our capillar- ies have intercellular clefts, except for brain capillaries, which are entirely secured together by tight junctions (the basis of the blood-brain barrier, described in Chapter 7).

3. Diffusion through pores. Very free passage of small solutes and fluids is allowed by

fenestrated capillaries. These unique capil- laries are found where absorption is a priority (intestinal capillaries or capillaries serving endocrine glands) or where filtration occurs (the kidney). A fenestra is an oval pore, or opening (fenestra = window), and is usu- ally covered by a delicate membrane (see Figure 11.22). Even so, a fenestra is much more permeable than other regions of the plasma membrane.

4. Transport via vesicles. Certain lipid- insoluble substances may enter or leave the blood and/or pass through the plasma mem- branes of endothelial cells within vesicles, that is, by endocytosis or exocytosis.

414 Essentials of Human Anatomy and Physiology

Only substances unable to pass by one of these routes are prevented from leaving (or entering) the capillaries. These include protein molecules (in plasma or interstitial fluid) and blood cells.

Fluid Movements at Capillary Beds

In addition to the exchanges made via passive dif- fusion through capillary endothelial cell plasma membranes, clefts, or fenestrations, and via vesi- cles, there are active forces operating at capillary beds. Because of their intercellular clefts and fenes- trations, some capillaries are leaky, and bulk fluid flow (fluid moving all at once) occurs across their plasma membranes. Hence, blood pressure tends to force fluid (and solutes) out of the capillaries, and osmotic pressure tends to draw fluid into them because blood has a higher solute concentration (due to its plasma proteins) than does interstitial fluid. Whether fluid moves out of or into a capillary depends on the difference between the two pres- sures. As a rule, blood pressure is higher than osmotic pressure at the arterial end of the capillary bed, and lower than osmotic pressure at the venous end. Consequently, fluid moves out of the capillar- ies at the beginning of the bed and is reclaimed at the opposite (venule) end (Figure 11.23). However,

Fenestra (pore)

Intercellular cleft

Lumen of capillary

Interstitial fluid

Vesicles

1 Direct diffusion through membrane

2 Diffusion through intercellular cleft

3 Diffusion through pore

4 Transport via vesicles

Figure 11.23 Bulk fluid flow across capillary walls. Fluid flow depends largely on the difference between the blood pressure and the osmotic pressure at different regions of the capillary bed.

Blood pressure is higher than osmotic pressure Osmotic pressure

remains steady in capillary bed

Blood pressure is lower than osmotic pressure

At the arterial end of a capillary, blood pressure is more than osmotic pressure, and fluid flows out of the capillary and into the interstitial fluid.

At the venule end of the capillary, blood pressure is less than osmotic pressure, and fluid flows from the interstitial fluid into the capillary.

Net fluid movement out

Arterial end of capillary

Venule end of capillary

Net fluid movement inBlood flow

Blo od

flo w

Tissue cell Interstitial fluid

Assume there is a bacterial infection in the interstitial fluid. How would this affect bulk fluid flow across the capillary walls in the area?

Q: Bacterial infection would increase fluid flow because

the osmotic pressure of the interstitial fluid would rise as inflammatory molecules and debris

accumulated in the area.

Figure 11.22 Capillary transport mechanisms. The four possible pathways or routes of transport across the wall of an endothelial cell of a capillary. (The endothelial cell is illustrated as if cut in cross section.)

Chapter 11: The Cardiovascular System 415

directly into the left atrium through the foramen ovale (fo-ra′men o-val′e), a flaplike opening in the interatrial septum. Blood that does manage to enter the right ventricle is pumped out to the pul- monary trunk, where it meets a second shunt, the ductus arteriosus (ar-ter″e-o′sus), a short vessel that connects the aorta and the pulmonary trunk. Because the collapsed lungs are a high-pressure area, blood tends to enter the systemic circulation through the ductus arteriosus. The aorta carries blood to the tissues of the fetal body and ulti- mately back to the placenta through the umbilical arteries.

At birth, or shortly after, the foramen ovale closes. Its remnant, the fossa ovalis, is visible in the right atrium (see Figure 11.3b). The ductus arteriosus collapses and is converted to the fibrous ligamentum arteriosum (lig″ah-men′tum ar- ter″e-o′sum) (see Figure 11.3a). As blood stops flowing through the umbilical vessels, they col- lapse, and the circulatory pattern converts to that of an adult.

Homeostatic Imbalance 11.10

Congenital heart defects account for about half of infant deaths resulting from all congenital defects. Environmental interferences, such as maternal infec- tion and drugs ingested during the first 3 months of pregnancy (when the embryonic heart is forming), seem to be the major causes of such problems. Congenital heart defects may include a ductus arte- riosus that does not close, septal openings, and other structural abnormalities of the heart. Such problems can usually be corrected surgically. ____ ✚

In the absence of congenital heart problems, the heart usually functions smoothly throughout a long lifetime for most people. Homeostatic mecha- nisms are so effective that we rarely are aware of when the heart is working harder. The heart will hypertrophy and its cardiac output will increase substantially if we exercise regularly and aerobi- cally (that is, vigorously enough to force it to beat at a higher-than-normal rate for extended periods of time). The heart becomes not only a more pow- erful pump but also a more efficient one: pulse rate and blood pressure decrease. An added ben- efit of aerobic exercise is that it clears fatty depos- its from the blood vessel walls, helping to slow the progress of atherosclerosis. However, let’s raise a

not quite all of the fluid forced out of the blood is reclaimed at the venule end. Returning fluid left in tissues to the blood is the chore of the lymphatic system (discussed in Chapter 12).

Did You Get It? 23. Would you expect fluid to be entering or leaving the

capillaries at the venous end of a capillary bed?

For the answer, see Appendix A.

Developmental Aspects of the Cardiovascular System ➔ Learning Objectives

□□ Briefly describe the development of the cardiovascular system.

□□ name the fetal vascular modifications, or “fetal shunts,” and describe their function before birth.

□□ Describe changes in the cardiovascular system that occur with aging, and list several factors that help maintain cardiovascular health.

The heart begins as a simple tube in the embryo. It is beating and busily pumping blood by the fourth week of pregnancy. During the next 3 weeks, the heart continues to change and mature, finally becoming a four-chambered structure capa- ble of acting as a double pump—all without miss- ing a beat!

Because the lungs and digestive system are immature and not functioning in a fetus, all nutri- ent, excretory, and gas exchanges occur through the placenta. Nutrients and oxygen move from the mother’s blood into the fetal blood, and fetal wastes move in the opposite direction. The umbil- ical cord contains three blood vessels: one large umbilical vein and two smaller umbilical arter- ies (Figure 11.24, p. 416). The umbilical vein car- ries blood rich in nutrients and oxygen to the fetus. The umbilical arteries carry blood laden with carbon dioxide and metabolic waste products from the fetus to the placenta. As blood flows superiorly toward the heart of the fetus, most of it bypasses the immature liver through the ductus venosus (duk′tus ve-no′sus) and enters the infe- rior vena cava, which carries the blood to the right atrium of the heart.

Because the nonfunctional fetal lungs are col- lapsed, two shunts (diverting vessels) make sure that they are almost entirely bypassed. Some of the blood entering the right atrium is shunted

(Text continues on page 418.)

416 Essentials of Human Anatomy and Physiology

Superior vena cava

Foramen ovale

Pulmonary trunk

Inferior vena cava

Hepatic vein

Ductus venosus

Hepatic portal vein

Umbilical vein

Fetal umbilicus

Umbilical cord

Umbilical arteries

Ductus arteriosus

Pulmonary veins

Pulmonary artery

Inferior vena cava

Aorta

Common iliac artery

External iliac artery

Internal iliac artery

Urinary bladder

Placenta

Placenta Umbilical vein

Ductus venosus

Inferior vena cava

Right atrium

Aorta Umbilical arteries

Right ventricle

Pulmonary trunk

Ductus arteriosus

Foramen ovale

Left atrium

Left ventricle

High oxygenation

Moderate oxygenation

Low oxygenation

Very low oxygenation

KEY:

Figure 11.24 Schematic of the fetal circulation.

Homeostatic Relationships between the Cardiovascular System and Other Body Systems

SYSTEMS IN SYNC

417

• The cardiovascular system delivers oxygen and nutrients; carries away wastes • The skin’s blood vessels provide an important blood reservoir and a site for heat loss from body

• The cardiovascular system delivers oxygen and nutrients; carries away wastes; blood pressure maintains kidney function • The urinary system helps regulate blood volume and pressure by altering urine volume and releasing renin

• The cardiovascular system delivers oxygen and nutrients; carries away wastes • Aerobic exercise enhances cardiovascular efficiency and helps prevent arteriosclerosis; the muscle “pump” aids venous return

• The cardiovascular system delivers oxygen and nutrients to lymphoid organs, which house immune cells; transports lymphocytes and antibodies; carries away wastes • The lymphatic system picks up leaked fluid and plasma proteins and returns them to the cardiovascular system; its immune cells protect cardiovascular organs from specific pathogens

• The cardiovascular system delivers oxygen and nutrients; carries away wastes • The digestive system provides nutrients to the blood including iron and B vitamins essential for RBC (and hemoglobin) formation

• The cardiovascular system delivers oxygen and nutrients; carries away wastes • Estrogen maintains vascular health in women

• The cardiovascular system delivers oxygen and nutrients; carries away wastes • The respiratory system carries out gas exchange: loads oxygen and unloads carbon dioxide from the blood; respiratory “pump” aids venous return

• The cardiovascular system delivers oxygen and nutrients and carries away wastes • Bones are the site of hematopoiesis; protect cardiovascular organs by enclosure; provide a calcium depot

• The cardiovascular system delivers oxygen and nutrients; removes wastes

• ANS regulates cardiac rate and force; sympathetic division maintains blood pressure and controls blood distribution according to need

• The cardiovascular system delivers oxygen and nutrients; carries away wastes; blood serves as a transport vehicle for hormones • Several hormones influence blood pressure (epinephrine, ANP, thyroxine, ADH); estrogen maintains vascular health in women

Integumentary System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System Reproductive System

Respiratory System

Skeletal System

Endocrine System

Nervous System

Cardiovascular System

418 Essentials of Human Anatomy and Physiology

hypertensive heart disease. The insidious filling of the blood vessels with fatty, calcified deposits leads most commonly to coronary artery dis- ease. Also, the roughening of the vessel walls encourages thrombus formation (see Chapter 10). At least 30 percent of the population in the United States has hypertension by the age of 50, and car- diovascular disease causes more than one-half of the deaths in people over age 65. Although the aging process itself contributes to changes in the walls of the blood vessels that can lead to strokes or myocardial infarctions, most researchers feel that diet, not aging, is the single most important contributing factor to cardiovascular diseases. There is some agreement that the risk is lowered if people eat less animal fat, cholesterol, and salt. Other recommendations include avoiding stress, eliminating cigarette smoking, and taking part in a regular, moderate exercise program.

caution flag here: The once-a-month or once-a- year tennis player or snow shoveler has not built up this type of heart endurance and strength. When such an individual pushes his or her heart too much, it may not be able to cope with the sud- den demand. This is why many “weekend ath- letes” are myocardial infarction (heart attack) victims.

As we get older, more and more signs of car- diovascular system disturbances start to appear. In some people, the venous valves weaken, and pur- ple, snakelike varicose veins appear. Not everyone has varicose veins, but we all have progressive atherosclerosis. Some say the process begins at birth, and even children’s arteries show atheroscle- rotic plaques. There’s an old saying, “You are only as old as your arteries,” referring to this degenerative process. The gradual loss in elasticity of the blood vessels leads to hypertension and

of the right and left coronary arteries and their branches, and is drained by the cardiac veins and the coronary sinus.

6. Cardiac muscle is able to initiate its own contrac- tion in a regular way, but its rate is influenced by both intrinsic and extrinsic factors. The intrinsic conduction system increases the rate of heart contraction and ensures that the heart beats as a unit. The SA node is the heart’s pacemaker. Extrinsic factors include neural and hormonal stimuli.

Summary

The Heart (pp. 383–396)

1. The heart, located in the thorax, is flanked laterally by the lungs and enclosed in a multi-layered peri- cardium.

2. The bulk of the heart wall (myocardium) is com- posed of cardiac muscle. The heart has four hollow chambers—two atria (receiving chambers) and two ventricles (discharging chambers), each lined with endocardium. (The heart is divided longitudinally by a septum.)

3. The heart functions as a double pump. The right ventricle is the pulmonary pump (right ventricle to lungs to left atrium). The left ventricle is the systemic pump (left ventricle to body tissues to right atrium).

4. Four valves prevent backflow of blood in the heart. The AV valves (mitral, or bicuspid, and tri- cuspid) prevent backflow into the atria when the ventricles are contracting. The semilunar valves (pulmonary and aortic) prevent backflow into the ventricles when the heart is relaxing. The valves open and close in response to pressure changes in the heart.

5. The myocardium is nourished by the coronary cir- culation, which branches off the aorta and consists

Complete an interactive tutorial: > Study Area > Interactive Physiology > Cardiovascular System > Electrical Activity of the Heart

7. The time and events occurring from one heartbeat to the next are the cardiac cycle.

8. As the heart beats, sounds resulting from the clos- ing of the valves (“lub-dup”) can be heard. Faulty valves reduce the efficiency of the heart as a pump and result in abnormal heart sounds (murmurs).

Complete an interactive tutorial: > Study Area > Interactive Physiology > Cardiovascular System > Cardiac Cycle

Chapter 11: The Cardiovascular System 419

veins superior to the diaphragm drain into the superior vena cava, and those inferior to the diaphragm drain into the inferior vena cava. Both venae cavae enter the right atrium of the heart. (See pp. 400–403 for the names and locations of the sys- temic veins.)

8. The arterial circulation of the brain is formed by branches of paired vertebral and internal carotid arteries. The cerebral arterial circle provides alter- nate routes for blood flow in case of a blockage in the brain’s arterial supply.

9. The hepatic portal circulation is formed by veins draining the digestive organs, which empty into the hepatic portal vein. The hepatic portal vein carries the nutrient-rich blood to the liver, where it is pro- cessed before the blood is allowed to enter the systemic circulation.

10. The pulse is the alternating expansion and recoil of a blood vessel wall (the pressure wave) that occurs as the heart beats. It may be felt easily over any superficial artery; such sites are called pressure points.

11. Blood pressure is the pressure that blood exerts on the walls of the blood vessels. It is the force that causes blood to flow down its pressure gradient in the blood vessels. It is high in the arteries, lower in the capillaries, and lowest in the right atrium. Both systolic and diastolic pressures are recorded when measuring blood pressure.

12. Arterial blood pressure is directly influenced by heart activity (increased heart rate leads to increased blood pressure) and by resistance to blood flow. The most important factors increasing the peripheral resistance are a decrease in the diameter or stretchiness of the arteries and arteri- oles and an increase in blood viscosity.

13. Many factors influence blood pressure, including the activity of the sympathetic nerves and kidneys, drugs, and diet.

10. SV rises or falls with the volume of venous return. HR is influenced by the nerves of the autonomic nervous system, drugs (and other chemicals), and ion levels in the blood.

Blood Vessels (pp. 396–415)

1. Arteries, which transport blood away from the heart, and veins, which carry blood back to the heart, are conducting vessels. Only capillaries play a role in actual exchanges with tissue cells.

2. Except for capillaries, blood vessels are composed of three tunics: The tunica intima forms a friction- reducing lining for the vessel. The tunica media is the bulky middle layer of muscle and elastic tissue. The tunica externa is the protective, outermost con- nective tissue layer. Capillary walls are formed of the tunica intima only.

3. Artery walls are thick and strong to withstand blood pressure fluctuations. They expand and recoil as the heart beats. Vein walls are thinner, their lumens are larger, and they are equipped with valves. These modifications reflect the low pressure of the blood flowing in veins.

4. Capillary beds have two types of vessels—a vascu- lar shunt and true capillaries. Blood flow into true capillaries is controlled by precapillary sphincters. Exchanges with tissue cells occur across the walls of the true capillaries. When the precapillary sphincters are closed, blood flows directly through the vascular shunt.

5. Varicose veins, a structural defect due to incompe- tent valves, is a common vascular problem, espe- cially in people who are obese or who stand for long hours. It is a predisposing factor for thrombo- phlebitis.

6. All the major arteries of the systemic circulation are branches of the aorta, which leaves the left ventri- cle. They branch into smaller arteries and then into the arterioles, which feed the capillary beds of the body tissues. (For the names and locations of the systemic arteries, see pp. 399–401.)

7. The major veins of the systemic circulation ulti- mately converge on one of the venae cavae. All

Complete an interactive tutorial: > Study Area > Interactive Physiology > Cardiovascular System > Factors that Affect Blood Pressure

14. Hypertension, which reflects an increase in periph- eral resistance, strains the heart and damages blood vessels. In most cases, the precise cause is unknown.

15. Substances move to and from the blood and tissue cells through capillary walls. Some substances are transported in vesicles, but most move by diffusion—directly through the endothelial cell

Complete an interactive tutorial: > Study Area > Interactive Physiology > Cardiovascular System > Cardiac Output

9. Cardiac output, the amount of blood pumped out by each ventricle in one minute, is the product of heart rate (HR) × stroke volume (SV). SV is the amount of blood ejected by a ventricle with each beat.

420 Essentials of Human Anatomy and Physiology

the fetus to the placenta. Shunts bypassing the lungs and liver are also present.

3. Congenital heart defects account for half of all infant deaths resulting from congenital problems.

4. Arteriosclerosis is an expected consequence of aging. Gradual loss of elasticity in the arteries leads to hypertension and hypertensive heart disease, and clogging of the vessels with fatty substances leads to coronary artery disease and stroke. Cardiovascular disease is an important cause of death in individuals over age 65.

5. A healthy diet low in fat, cholesterol, and salt; stop- ping smoking; and regular aerobic exercise may help to reverse the atherosclerotic process and pro- long life.

plasma membranes, through intercellular clefts, or through fenestrations. Fluid is forced from the bloodstream by blood pressure and drawn back into the blood by osmotic pressure.

Developmental Aspects of the Cardiovascular System (pp. 415–418)

1. The heart begins as a tubelike structure that is beat- ing and pumping blood by the fourth week of embryonic development.

2. The fetal circulation is a temporary circulation seen only in the fetus. It consists primarily of three special vessels: the single umbilical vein that carries nutrient- and oxygen-laden blood to the fetus from the placenta, and the two umbilical arteries that carry carbon dioxide and waste-laden blood from

Multiple Choice

More than one choice may apply.

1. Both venae cavae deliver oxygen-poor blood from the body to the

a. right ventricle.

b. left ventricle.

c. right atrium.

d. left atrium.

2. Given a volume of 150 ml at the end of diastole, a volume of 50 ml at the end of systole, and a heart rate of 60 bpm, the cardiac output is

a. 600 ml/min.

b. 6 liters/min.

c. 1200 ml/min.

d. 3 liters/min.

3. Which of the following depolarizes next after the AV node?

a. Atrial myocardium

b. Ventricular myocardium

c. Bundle branches

d. AV bundle

4. During atrial diastole,

a. the atrial pressure exceeds the ventricular pressure.

b. blood from the pulmonary and systemic circuits fills atria.

c. the semilunar valves are forced open.

d. the SA node depolarizes.

5. Atrial repolarization coincides in time with the

a. P wave.

b. T wave.

c. QRS wave.

d. P-Q interval.

6. Soon after the onset of ventricular systole, the

a. AV valves close.

b. semilunar valves open.

c. first heart sound is heard.

d. aortic pressure increases.

7. To count the heart rate for an apical pulse, you would place your stethoscope approximately at the level of the

a. third intercostal space.

b. fourth intercostal space.

c. fifth intercostal space.

d. sixth intercostal space.

review Questions Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 11: The Cardiovascular System 421

15. The most external part of the pericardium is the

a. parietal layer of serous pericardium.

b. fibrous pericardium.

c. visceral layer of serous pericardium.

d. epicardium.

16. Which of the following are features of capillaries?

a. They allow direct diffusion of nutrients.

b. They allow transport by vesicles.

c. They have large pores to allow transport.

d. They rely on valves to allow transport.

17. How many cusps does the semilunar valve at the base of the aorta have?

a. Two

b. Three

c. Four

d. Six

18. Which layer of the heart wall is an endothelium?

a. Endocardium

b. Myocardium

c. Epicardium

d. Pericardium

Short Answer Essay

19. Draw a diagram of the heart showing the three layers composing its wall and its four chambers. Label each. Show where the AV and semilunar valves are, and name them. Show and label all blood vessels entering and leaving the heart chambers.

20. Trace one drop of blood from the time it enters the right atrium of the heart until it enters the left atrium. What is this circuit called?

21. What is the function of the fluid that fills the pericardial sac?

22. Define systole and diastole.

23. Define stroke volume and cardiac cycle.

24. How does the heart’s ability to contract differ from that of other muscles of the body?

25. Name the elements of the intrinsic conduction system, in order, beginning with the pacemaker.

26. Which system is responsible for decreasing heart rate after a crisis has passed?

8. In comparing a parallel artery and vein, you would find that

a. the artery wall is thicker.

b. the artery diameter is greater.

c. the artery lumen is smaller.

d. the artery endothelium is thicker.

9. Which of these vessels is bilaterally symmetrical (i.e., one vessel of the pair occurs on each side of the body)?

a. Internal carotid artery

b. Brachiocephalic trunk

c. Azygos vein

d. Renal vein

10. A stroke that occludes a posterior cerebral artery will most likely affect

a. hearing.

b. vision.

c. smell.

d. higher thought processes.

11. Most of the small intestine is supplied by the

a. superior mesenteric artery.

b. inferior mesenteric artery.

c. lumbar arteries.

d. celiac trunk.

12. Which layer of the artery wall thickens most in atherosclerosis?

a. Tunica media

b. Tunica intima

c. Tunica adventitia

d. Tunica externa

13. Which of the following are associated with aging?

a. Increasing blood pressure

b. Weakening of venous valves

c. Arteriosclerosis

d. Collapse of the ductus arteriosus

14. What effects do the kidneys have on blood pressure?

a. They decrease pressure by causing vasodilation.

b. They increase pressure by increasing blood volume.

c. They decrease pressure by decreasing blood volume.

d. They increase pressure by decreasing blood volume.

422 Essentials of Human Anatomy and Physiology

36. Which artery is palpated at the inner elbow? At the dorsum of the foot?

37. Define systolic pressure and diastolic pressure.

38. Which part of the nervous system plays an important role in regulating blood pressure? By which action does it increase blood pressure?

39. In which position—sitting, lying down, or standing— is the blood pressure normally highest? Lowest? Explain why.

40. Microcirculation is the flow of blood through a capillary bed consisting of a vascular shunt and true capillaries. What is the difference between a vascular shunt and a true capillary?

41. What are varicose veins? What factors seem to promote their formation?

42. Explain why blood flow in arteries is pulsatile and blood flow in veins is not.

43. What is the relationship between cross-sectional area of a blood vessel and velocity (speed) of blood flow in that vessel?

44. Which type of blood vessel is most important in regulating vascular resistance, and how does it achieve this?

45. What is the foramen ovale, and what is its function?

27. What is congestive heart failure? What is the difference between pulmonary congestion and peripheral congestion?

28. Describe the structure of capillary walls.

29. How is blood supply to the myocardium organized?

30. Name three factors that are important in promoting venous return.

31. Arteries are often described as vessels that carry oxygen-rich blood, and veins are said to carry oxygen-poor (carbon dioxide–rich) blood. Name two sets of exceptions to this rule that were discussed in this chapter.

32. Trace a drop of blood from the left ventricle of the heart to the wrist of the right hand and back to the heart. Now trace it to the dorsum of the right foot and back to the right heart.

33. What is the function of the hepatic portal circulation? In what way is a portal circulation a “strange” circulation?

34. In a fetus, the liver and lungs are almost entirely bypassed by blood. Why is this? Name the vessel that bypasses the liver. Name two lung bypasses. Three vessels travel in the umbilical cord; which of these carries oxygen- and nutrient-rich blood?

35. Define pulse. Palpate your pulse. Which pulse point did you use?

46. John is a 30-year-old man who is overweight and smokes. He has been diagnosed with hypertension and arteriosclerosis. Define each of these conditions. How are they often related? Why is hypertension called the “silent killer”? Name three changes in your lifestyle that might help prevent cardiovascular disease in your old age.

47. Following a viral infection, George has been feeling a sharp stabbing pain in the retrosternal region. The pain gets worse when he breathes deeply and also when he lies down. A physical examination reveals that he has pericarditis. Can you explain why pericarditis causes this severe pain?

48. Hannah, a 14-year-old girl undergoing a physical examination before being admitted to summer camp, was found to have a loud heart murmur at the second intercostal space on the left side of the sternum. The murmur takes the form of a swishing sound with no high-pitched whistle. What, exactly, is producing the murmur?

49. Colin is in the hospital to have a coronary angiography so that a possible coronary stenosis can be detected. A catheter is inserted through the right radial artery. What route must the catheter travel to reach the coronary arteries?

50. Stuart, a 49-year-old man, has been diagnosed with coronary artery disease. He needs a bypass graft in

Critical Thinking and Clinical Application Questions

Chapter 11: The Cardiovascular System 423

52. Grandma tells Hailey not to swim for 30 minutes after eating. Explain why taking a vigorous swim immediately after lunch is more likely to cause indigestion than cramping of your muscles.

53. The guards at the royal palace in London stand at attention while on duty. On a very hot day, it is not unusual for one (or more) to become lightheaded and faint. Explain this phenomenon.

his circumflex artery and right coronary artery. Which parts of his heart muscle are most likely to see an improved blood flow after the procedure?

51. Mr. Grimaldi was previously diagnosed as having a posterior pituitary tumor that causes hypersecretion of ADH. He comes to the clinic regularly to have his blood pressure checked. Would you expect his blood pressure to be chronically elevated or depressed? Why?

424

12 The Lymphatic System and Body Defenses WHAT

HOW

WHY

The lymphatic system carries excess interstitial fluid

from tissues back to cardiovascular circulation and provides locations

for immune cells to monitor the body.

Porous lymphatic vessels take in fluids from the

extracellular space and carry them through lymph nodes, where

immune cells scan the fluids for foreign particles.

Fluid must be returned to circulation via lymphatics to

maintain cardiovascular function, and lymph nodes are necessary

to monitor the body for infection.

PART I: THE LYMPHATIC SYSTEM ➔ Learning Objectives

□□ Explain how the lymphatic system is functionally related to the cardiovascular system and the immune defenses.

□□ Name the two major types of structures composing the lymphatic system.

□□ Describe the source of lymph, and explain its formation and transport.

When we mentally tick off the names of the body’s organ systems, the lymphatic (lim-fat′ik) system is

probably not the first that comes to mind. Yet without this system, our cardiovascular system would stop working, and our immune system would be hopelessly impaired. The lymphatic system consists of two semi-independent parts: (1) a meandering network of lymphatic vessels and (2) various lymphoid tissues and organs scattered throughout the body. The lymphatic vessels trans- port fluids that have escaped from the blood back to the cardiovascular system. The lymphoid tissues and organs house phagocytic cells and lympho- cytes, which play essential roles in body defense and resistance to disease.

INSTRUCTORS

New Building Vocabulary Coaching Activities for this chapter are assignable in

Chapter 12: The Lymphatic System and Body Defenses 425

12 from leaking back out and forcing it along the vessel.

Lymphatic Vessels As blood circulates through the body, nutrients, wastes, and gases are exchanged between the blood and the interstitial fluid.

➔CONCEPTLINKRecall that the hydrostatic and osmotic pressures operating at capillary beds force fluid out of the blood at the arterial ends of the beds (“upstream”) and cause most of the expelled fluid to be reabsorbed at the venous ends (“downstream”) (Chapter 11, p. 414). ➔

The fluid that remains behind in the tissue spaces, as much as 3 L daily, becomes part of the interstitial fluid. This excess tissue fluid, as well as any plasma proteins that escape from the blood, must be carried back to the blood if the vascular system is to have sufficient blood volume to con- tinue to operate properly. If it does not, fluid accu- mulates in the tissues, producing edema, or swelling. Excessive edema impairs the ability of cells to make exchanges with the interstitial fluid and ultimately the blood. The function of the lym- phatic vessels is to form an elaborate drainage system that picks up this excess interstitial fluid, now called lymph (lymph = clear water), and returns it to the blood (Figure 12.1).

The lymphatic vessels, also called lymphat- ics, form a one-way system, and lymph flows only toward the heart. The microscopic lymph capillaries weave between the tissue cells and blood capillaries in the loose connective tissues of the body (Figure 12.2a, p. 426) and absorb the leaked fluid. Although similar to blood capillar- ies, lymphatic capillaries are so remarkably per- meable that they were once thought to be open at one end like a straw. Not so—instead, the edges of the endothelial cells forming their walls loosely overlap one another, forming flaplike minivalves (Figure 12.2b) that act as one-way swinging doors. The flaps, anchored by fine col- lagen fibers to surrounding structures, gape open when the fluid pressure is higher in the intersti- tial space, allowing fluid to enter the lymphatic capillary. However, when the pressure is higher inside the lymphatic vessels, the endothelial cell flaps are forced together, preventing the lymph

Venous system

Arterial system

Heart

Lymph duct Lymph trunk

Lymph node

Lymphatic system

Lymphatic collecting vessels, with valves

Lymph capillary Tissue fluid (becomes lymph)

Loose connective tissue around capillaries

Blood capillaries

Figure 12.1 Relationship of lymphatic vessels to blood vessels. Beginning at the bottom of this figure, we see that lymph, which begins as tissue fluid derived from blood capillaries, enters the lymph capillaries, travels through the lymphatic vessels and lymph nodes, and enters the bloodstream via the great veins at the root of the neck.

➔CONCEPTLINKThis is very similar to the way that valves in veins work to ensure blood returns to the heart, despite being under low pressure (Chapter 11, p. 398). ➔

426 Essentials of Human Anatomy and Physiology

same mechanisms that aid return of venous blood—the milking action of the skeletal muscles and pressure changes in the thorax during breath- ing (that is, the muscular and respiratory “pumps”). In addition, smooth muscle in the walls of the larger lymphatics contracts rhythmically, helping to “pump” the lymph along.

Did You Get It? 1. What is the function of lymphatic vessels? 2. How do lymphatic capillaries and blood capillaries

differ structurally from each other?

For answers, see Appendix A.

Lymph Nodes ➔ Learning Objective

□□ Describe the function(s) of lymph nodes, tonsils, the thymus, Peyer’s patches, and the spleen.

In addition to returning tissue fluid to circulation, the lymphatic system plays a major role in immu- nity. Cells in lymph nodes in particular help pro- tect the body by removing foreign material such as bacteria and tumor cells from the lymphatic stream and by producing lymphocytes that function in the immune response.

Proteins, and even larger particles such as cell debris, bacteria, and viruses, are normally pre- vented from entering blood capillaries, but they enter the lymphatic capillaries easily, particularly in inflamed areas. But there is a problem here— bacteria, viruses, and cancer cells that enter the lymphatics can then use them to travel throughout the body. This dilemma is partly resolved by the fact that white blood cells (WBCs) can also travel in lymph and lymph takes “detours” through the lymph nodes, where it is cleansed of debris and “examined” by cells of the immune system, as we explain in more detail shortly.

Lymph is transported from the lymph capillaries through successively larger lymphatic vessels, referred to as lymphatic collecting vessels, until it is finally returned to the venous system through two large ducts in the thoracic region. The right lym- phatic duct drains lymph from the right arm and the right side of the head and thorax. The large tho- racic duct receives lymph from the rest of the body (Figure 12.3). Both ducts empty the lymph into the subclavian vein on their own side of the body.

Like veins of the cardiovascular system, lym- phatic vessels are thin walled, and the larger ones have valves. The lymphatic system is a low-pressure, pumpless system. Lymph is transported by the

Tissue cell

Blood capillaries

Tissue fluid

Venule

Fibroblast in loose connective tissue

Flaplike minivalve

Endothelial cell

Filaments anchored to connective tissueArteriole

Lymphatic capillary

(a) (b)

Figure 12.2 Special structural features of lymphatic capillaries. (a) Structural relationship between blood capillaries and lymph capillaries. Black arrows indicate direction of fluid movement. (b) Lymph capillaries begin as blind-ended tubes. The endothelial cells forming their walls overlap one another, forming flaplike minivalves.

Chapter 12: The Lymphatic System and Body Defenses 427

respond to foreign substances in the lymphatic stream. Although we are not usually aware of the protective nature of the lymph nodes, most of us have had “swollen glands” during an active infec- tion. What are actually swollen in these cases are not glands at all but rather lymph nodes (the term swollen glands is a misnomer), which swell as a result of the trapping function of the nodes.

Lymph nodes vary in shape and size, but most are kidney-shaped, about 1 centimeter long, and “buried” in the connective tissue that surrounds them. Each node is surrounded by a fibrous cap- sule from which connective tissue strands called trabeculae (trah-bek′yu-le) extend inward to divide the node into a number of compartments (Figure 12.4, p. 428). The internal framework is a

As lymph is transported toward the heart, it is filtered through the thousands of lymph nodes that cluster along the lymphatic vessels (see Figure 12.1). Particularly large clusters are found in the inguinal, axillary, and cervical regions of the body (see Figure 12.3). Within the lymph nodes are macrophages (mak′ro-fāj-ez), which engulf and destroy bacteria, viruses, and other foreign substances in the lymph before it is returned to the blood. Collections of lymphocytes (a type of white blood cell, covered in Chapter 10) are also strategically located in the lymph nodes and

Edema in the areas that drain into the thoracic duct.A:

Drained by the right lymphatic duct Drained by the thoracic duct

Regional lymph nodes:

Cervical nodes

Entrance of right lymphatic duct into right subclavian vein

Internal jugular vein

Thoracic duct entry into left subclavian vein

Thoracic duct

Aorta

Spleen

Cisterna chyli (receives lymph drainage from digestive organs)

Lymphatics

Axillary nodes

Inguinal nodes

KEY:

Figure 12.3 Distribution of lymphatic vessels and lymph nodes. Note that until recently, scientists thought that the CNS had no lymphatics. However, we now know the CNS is drained by lymphatics near the venous dural sinuses.

What would be the consequence of blockage of the thoracic duct?Q:

428 Essentials of Human Anatomy and Physiology

surveillance role. The medullary cords are inward extensions of cortical tissue that contain both B and T cells. Phagocytic macrophages are located in the central medulla of the lymph node.

Lymph enters the convex side of a lymph node through afferent lymphatic vessels. It then flows through a number of sinuses (spaces) that meander through the lymph node and finally exits from the node at its indented region, the hilum (hi′lum), via efferent lymphatic vessels. Because there are fewer efferent vessels draining the node than afferent vessels feeding it, the flow of lymph through the node is very slow, kind of like sand flowing through an hourglass. This allows time for the lymphocytes and macrophages to perform their protective functions.

Homeostatic Imbalance 12.1

Lymph nodes help rid the body of infectious agents and cancer cells, but sometimes they are overwhelmed by the very agents they are trying to destroy. For example, when large numbers of bacteria or viruses are trapped in the nodes, the nodes become inflamed, swollen, and tender to the touch. The lymph nodes can also become secondary cancer sites, particularly in cancers that use lymphatic vessels to spread throughout the body. Swelling is temporary in lymph nodes fighting infection. In contrast, swelling persists or nodes enlarge over time if they contain cancer cells. _____________________________________ ✚

Other Lymphoid Organs Lymph nodes are just one of the many types of lymphoid organs in the body. Others include the spleen, thymus, tonsils, Peyer’s patches, and the appendix (Figure 12.5), as well as bits of lymphoid tissue scattered in epithelial and connective tissues. All these organs have a predominance of reticular connective tissue and lymphocytes. Although all lymphoid organs have roles in protecting the body, only the lymph nodes filter lymph.

The spleen is a soft organ located in the left side of the abdominal cavity, just beneath the dia- phragm, that curls around the anterolateral aspect of the stomach. Instead of filtering lymph, the spleen filters and cleanses blood of bacteria, viruses, and other debris. As with the other lymphoid

network of soft reticular connective tissue, like “cellular bleachers,” that provides a place for the continually changing population of lymphocytes to “sit” as they monitor the lymphatic stream. Lymphocytes arise from the red bone marrow but then migrate to the lymph nodes and other lym- phoid organs, where they reproduce further (see Chapter 10).

The outer part of the node, its cortex, con- tains collections of lymphocytes called follicles, many of which have dark-staining centers called germinal centers. These centers enlarge when specific B lymphocytes (the B cells) are generating daughter cells called plasma cells, which release antibodies. The rest of the cortical cells are lym- phocytes “in transit,” the so-called T cells that cir- culate continuously between the blood, lymph nodes, and lymphatic stream, performing their

Because the outlet is smaller than the inlet to the lymph node, the lymph fluid stops flowing briefly in

the lymph node, giving macrophages and lymphocytes time to monitor the lymph for

pathogens and process them.

Afferent lymphatic vessels

Germinal center in follicle

Efferent lymphatic vessels

Cortex

Follicle

Hilum

Medullary sinus

Medullary cord

Capsule

Subcapsular sinus

Trabecula

Figure 12.4 Structure of a lymph node. Longitudinal section of a lymph node and associated lymphatics. Notice that several afferent lymphatics enter the node, whereas fewer efferent lymphatics exit at its hilum.

What is the benefit of having fewer efferent than afferent lymphatic vessels?Q:

Chapter 12: The Lymphatic System and Body Defenses 429

Did You Get It? 3. In which three regions of the body are the lymph

nodes most densely located? 4. What anatomical characteristic ensures that lymph

flows through the lymph nodes slowly? 5. Which lymphoid organ gets rid of aged red

blood cells? 6. What is MALT?

For answers, see Appendix A.

PART II: BODY DEFENSES Every second of every day, an army of hostile microbes swarms on our skin and invades our inner passageways—yet we remain healthy most of the time. The body seems to have developed a single-minded approach toward such foes—if you’re not with us, you’re against us!

Two types of mechanisms defend the body against these tiny but mighty enemies: innate and adaptive defense mechanisms (Figure 12.6). Together they make up the immune system. Although certain body organs (lymphoid organs and blood vessels) are intimately involved with the immune response, the immune system is a functional system rather than an organ system in an anatomical sense. Its “structures” are a variety

organs, the spleen provides a site for lymphocyte proliferation and immune surveillance, but its most important function is to destroy worn-out red blood cells and return some of their breakdown products to the liver. For example, iron is used again for making hemoglobin, and the rest of the hemoglo- bin molecule is secreted in bile.

Other functions of the spleen include storing platelets and acting as a blood reservoir (like the liver). During hemorrhage, both the spleen and liver contract and empty their blood into circula- tion to help bring the blood volume back to its normal level. In the fetus, the spleen is an important hematopoietic (blood cell–forming) site, but as a rule the adult spleen produces only lymphocytes.

The thymus, which functions at peak level only during youth, is a lymphoid mass found in the anterior mediastinum overlying the heart.

Tonsils (in pharyngeal region)

Thymus (in thorax; most active during youth)

Spleen (curves around left side of stomach)

Peyer’s patches (in intestine)

Appendix

Figure 12.5 Lymphoid organs. Locations of the tonsils, spleen, thymus, Peyer’s patches, and appendix.➔CONCEPTLINKRemember that the thymus produces hormones, thy-mosin and others, that function in the programming of T lymphocytes so they can carry out their protective

roles in the body (Chapter 9, p. 346). ➔

The tonsils are small masses of lymphoid tis- sue deep to the mucosa surrounding the pharynx (throat). Their job is to trap and remove bacteria or other foreign pathogens entering the throat. They carry out this function so efficiently that sometimes they become congested with bacteria and become red, swollen, and sore, a condition called tonsillitis.

Peyer’s patches, which resemble tonsils, are found in the wall of the distal small intestine. Lymphoid follicles are also located in the wall of the appendix, a tubelike offshoot of the proximal large intestine. The macrophages of Peyer’s patches and the appendix are in an ideal position to capture and destroy harmful bacteria (always present in tremendous numbers in the intestine), thereby preventing them from penetrating the intestinal wall. Peyer’s patches, the appendix, and the tonsils are part of the collection of small lym- phoid tissues referred to as mucosa-associated lymphoid tissue (MALT). Collectively, MALT acts as a sentinel to protect the upper respiratory and digestive tracts from the constant attacks of foreign matter entering those cavities.

430 Essentials of Human Anatomy and Physiology

exposure to a foreign substance (antigen) before it can protect the body against the invader. Another important feature of the adaptive response is that it “remembers” which invaders it has fought. Nonetheless, what the adaptive system lacks in speed it makes up for in the precision of its coun- terattacks. Although we consider them separately, keep in mind that innate and adaptive defenses always work hand-in-hand to protect the body.

Did You Get It? 7. How do the innate and adaptive defenses differ?

For the answer, see Appendix A.

Innate Body Defenses Some innate resistance to disease is inherited. For instance, there are certain diseases that humans never get, such as some forms of tuberculosis that affect birds. Most often, however, the term innate, or nonspecific, body defense refers to the mechani- cal barriers that cover body surfaces and to the cells and chemicals that act on the initial battle- fronts to protect the body from invading patho- gens (harmful microorganisms). Table 12.1 pro- vides a detailed summary of the innate defenses.

Surface Membrane Barriers ➔ Learning Objective

□□ Describe the protective functions of skin and mucous membranes.

The body’s first line of defense against the invasion of disease-causing microorganisms includes the

of molecules and trillions of immune cells that inhabit lymphoid tissues and organs and circu- late in body fluids. The most important of the immune cells are lymphocytes, dendritic cells, and macrophages. Macrophages actually play an important role in both innate and adaptive mech- anisms. The innate defense system, also called the nonspecific defense system, responds immediately to protect the body from all foreign substances, whatever they are. We are born with our innate defenses, which include intact skin and mucous membranes, the inflammatory response, and a number of proteins produced by body cells. These innate mechanisms reduce the workload of the adaptive defense mechanisms by generally preventing the entry and spread of microorgan- isms throughout the body.

The adaptive defense system, or specific defense system, fights invaders that get past the innate defenses by mounting an attack against one or more particular foreign sub- stances.

When our immune system is operating effec- tively, it protects us from most bacteria, viruses, transplanted organs or grafts, and even our own cells that have “turned against” us (cancer cells). The immune system does this both directly, by cell attack, and indirectly, by releasing mobilizing chemicals and protective antibody molecules. The resulting highly specific resistance to disease is called immunity (immun = free).

Unlike the innate defenses, which are always prepared to defend the body, the adaptive system must first “meet,” or be sensitized by, an initial

Innate (nonspecific) defense mechanisms

The Immune System

First line of defense

• Skin • Mucous membranes • Secretions of skin and mucous membranes

Second line of defense

• Phagocytic cells • Natural killer cells • Antimicrobial proteins • The inflammatory response • Fever

Adaptive (specific) defense mechanisms

Third line of defense

• Lymphocytes • Antibodies • Macrophages and other antigen-presenting cells

Figure 12.6 An overview of the body’s defenses.

Chapter 12: The Lymphatic System and Body Defenses 431

Table 12.1 Summary of Innate (Nonspecific) Body Defenses

Category and associated elements Protective mechanism

Surface membrane barriers—first line of defense

Intact skin (epidermis) Forms mechanical barrier that prevents entry of pathogens and other harmful substances into body.

• Acid mantle Skin secretions make epidermal surface acidic, which inhibits bacterial growth; sebum also contains bacteria-killing chemicals.

• Keratin Provides resistance against acids, alkalis, and bacterial enzymes.

Intact mucous membranes Form mechanical barrier that prevents entry of pathogens.

• Mucus Traps microorganisms in respiratory and digestive tracts.

• Nasal hairs Filter and trap microorganisms and other airborne particles in nasal passages.

• Cilia Propel debris-laden mucus away from lower respiratory passages.

• Gastric juice Contains concentrated hydrochloric acid and protein-digesting enzymes that destroy pathogens in stomach.

• Acid mantle of vagina Inhibits growth of bacteria and fungi in female reproductive tract.

• Lacrimal secretion (tears); saliva Continuously lubricate and cleanse eyes (tears) and oral cavity (saliva); contain lysozyme, an enzyme that destroys microorganisms.

Cellular and chemical defenses—second line of defense

Phagocytes Engulf and destroy pathogens that breach surface membrane barriers; macrophages also contribute to immune response.

Natural killer cells Promote cell lysis by direct cell attack against virus-infected or cancerous body cells; do not depend on specific antigen recognition.

Inflammatory response Prevents spread of injurious agents to adjacent tissues, disposes of pathogens and dead tissue cells, and promotes tissue repair; releases chemical mediators that attract phagocytes (and immune cells) to the area.

Antimicrobial chemicals

• Complement Group of plasma proteins that lyses microorganisms, enhances phagocytosis by opsonization, and intensifies inflammatory response.

• Interferons Proteins released by virus-infected cells that protect uninfected tissue cells from viral takeover; mobilize immune system.

• Fluids with acid pH Normally acid pH inhibits bacterial growth; urine cleanses the lower urinary tract as it flushes from the body.

Fever Systemic response triggered by pyrogens; high body temperature inhibits multiplication of bacteria and enhances body repair processes.

432 Essentials of Human Anatomy and Physiology

Natural Killer Cells

➔ Learning Objective □□ Explain the role of natural killer cells.

Natural killer (NK) cells roam the body in blood and lymph. They are a unique group of aggressive lymphocytes that can lyse (burst) and kill cancer cells, virus-infected body cells, and some other nonspecific targets well before the adaptive arm of the immune system is enlisted in the fight. Natural killer cells can act spontane- ously against any such target by recognizing cer- tain sugars on the “intruder’s” surface as well as its lack of certain “self” cell surface molecules. NK cells are not phagocytic. They attack the tar- get cell’s membrane and release lytic chemicals called perforin and granzymes (enzymes), which degrade target cell contents. Shortly thereafter, the target cell dies as its membrane and nucleus disintegrate. NK cells also release powerful inflam- matory chemicals.

Inflammatory Response

➔ Learning Objective □□ Describe the inflammatory process.

The inflammatory response is a nonspecific response that is triggered whenever body tissues are injured (Figure 12.7). For example, it occurs in response to physical trauma, intense heat, irritating chemicals, and infection by viruses and bacteria. The four most common indicators, or cardinal signs, of acute inflammation are redness, heat (inflamm = set on fire), pain, and swelling (edema). It is easy to understand why these signs and symptoms occur, once you understand the events of the inflammatory response.

The inflammatory process begins with a chem- ical “alarm.” When cells are damaged, they release inflammatory chemicals, including histamine and kinins (ki′ninz), that (1) cause blood vessels in the area to dilate, (2) make capillaries leaky, and (3) attract phagocytes and white blood cells to the area. (This third phenomenon is called positive chemotaxis because the cells are moving toward a high concentration of signaling molecules.) Dilation of the blood vessels increases the blood flow to the area, accounting for the redness and heat observed. Increased permeability of the capil- laries allows fluid to leak from the blood into the

skin and mucous membranes. As long as the skin is unbroken, its keratinized epidermis is a strong physical barrier to most microorganisms that swarm on the skin. Intact mucous membranes pro- vide similar mechanical barriers within the body. Recall that mucous membranes line all body cavi- ties open to the exterior. Besides serving as physi- cal barriers, these membranes produce a variety of protective secretions:

• The acidic pH of skin secretions (called the acid mantle) and usually of urine (pH of 4.5 to 6) inhibits bacterial growth, and sebum con- tains chemicals that are toxic to bacteria. The vaginal secretions of adult females are also very acidic.

• Sticky mucus traps many microorganisms that enter digestive and respiratory passageways.

• The stomach mucosa secretes gastric juice con- taining hydrochloric acid and protein-digesting enzymes that both kill pathogens.

• Saliva and lacrimal secretions (tears) contain lysozyme, an enzyme that destroys bacteria.

Some mucosae also have structural modifica- tions that fend off potential invaders. Mucus- coated hairs inside the nasal cavity trap inhaled particles, and the respiratory tract mucosa is cili- ated. The cilia sweep dust- and bacteria-laden mucus (think “sticky trap”) superiorly toward the mouth, preventing it from entering the lungs, where the warm, moist environment provides ideal conditions for bacterial growth.

Although the surface barriers are quite effec- tive, some microbes may get through and they are broken from time to time by small nicks and cuts resulting, for example, from shaving or a paper cut. When this happens and microorganisms do invade deeper tissues, the internal innate mecha- nisms come into play.

Internal Defenses: Cells and Chemicals For its second line of defense, the body uses an enor- mous number of cells and chemicals. These defenses rely on the destructive powers of cells called phago- cytes and natural killer cells, on the inflammatory response, and on a variety of chemical substances that kill pathogens and help repair tissue. Fever is another nonspecific protective response.

Chapter 12: The Lymphatic System and Body Defenses 433

stage for repair. Let’s look at how it accomplishes these tasks by mobilizing phagocytes (Figure 12.8, p. 434). Once the inflammatory process has begun:

Neutrophils, responding to a gradient of dif- fusing inflammatory chemicals, enter the blood from the bone marrow and roll along the blood vessel walls, following the “scent.”1

At the point where the chemical signal is the strongest, they flatten out and squeeze through the capillary walls, a process called diapedesis.

tissue spaces, causing local edema (swelling) that also activates pain receptors in the area as pres- sure in the tissues builds. If the swollen, painful area is a joint, its function (movement) may be impaired temporarily. This forces the injured part to rest, which aids healing. Some authorities con- sider limitation of joint movement to be the fifth cardinal sign of inflammation.

The inflammatory response prevents the spread of damaging agents to nearby tissues, dis- poses of cell debris and pathogens, and sets the

Removal of damaged/dead tissue cells and

pathogens from area

HeatRedness

Increased blood flow into area

Blood vessels dilate

Neutrophils and then monocytes

(and other WBCs) enter area

Capillaries become “leaky”

Brings more nutrients and

oxygen to area

Increases metabolic

rate of tissue cells

Clotting proteins

enter area

Fibrin barrier

Possible temporary limitation of

joint movement

Healing

Release kinins, histamine, and other chemicals

Injurious agents

Cells damaged

Pain Swelling

Edema (fluid in tissue spaces)

Figure 12.7 Flowchart of inflammatory events. The four cardinal signs (most common indicators) of acute inflammation are shown in red boxes. Limitation of joint movement (box outlined in red) occurs in some cases and is considered to be the fifth cardinal sign of acute inflammation.

434 Essentials of Human Anatomy and Physiology

(think of a small fish net) also forms scaffolding for permanent repair. The local heat increases the metabolic rate of the tissue cells, speeding up their defensive actions and repair processes.

If the damaged area contains pathogens that have previously invaded the body, the third line of defense also comes into play—the adaptive response mediated by lymphocytes.

Homeostatic Imbalance 12.2

In severely infected areas, the battle takes a con- siderable toll on both sides, and creamy, yellow pus may form in the wound. Pus is a mixture of dead or dying neutrophils, broken-down tissue cells, and living and dead pathogens. If the inflammatory mechanism fails to fully clear the area of debris, the sac of pus may become walled off, forming an abscess. Surgical drainage of abscesses is often necessary before healing can occur.

Still drawn by the gradient of inflammatory chemicals (positive chemotaxis), the neutro- phils gather at the site of tissue injury, and within an hour, they are busily devouring any foreign material present.

As the counterattack continues, monocytes follow neutrophils into the inflamed area. Monocytes are fairly poor phagocytes, but within 12 hours after entering the tissues they become macrophages with insatiable appetites. The mac- rophages continue to wage the battle, replacing the short-lived neutrophils at the site of damage. Macrophages are the central actors in the final disposal of cell debris as the inflammation subsides.

Besides phagocytosis, other protective events are also occurring at the inflamed site. Clotting proteins, leaked into the area from the blood, are activated and begin to wall off the damaged area with fibrin to prevent the spread of harmful agents to neighboring tissues. The fibrin mesh

1 2Enter blood from bone marrow and roll along the vessel wall

EndotheliumCapillary wall

Diapedesis

Positive chemotaxis

Inflammatory chemicals diffusing from the inflamed site act as chemotactic agents

Basement membrane

Neutrophils

Figure 12.8 Phagocyte mobilization during inflammation.

Chapter 12: The Lymphatic System and Body Defenses 435

Pathogens that make it through the mechanical barriers are confronted by phagocytes (fag′o-sı̄tz″; phago = eat) in nearly every body organ. A phago- cyte, such as a macrophage or neutrophil, engulfs a foreign particle by the process of phagocyto- sis (Figure 12.9). (Recall what you learned in Chapter 3.) Flowing cytoplasmic extensions bind to the particle and then pull it inside, forming a phagocytic vesicle. The vesicle then fuses with a lysosome, where enzymes digest its contents.

Antimicrobial Proteins

➔ Learning Objective □□ Name several antimicrobial substances produced by

the body that act in innate body defense.

A variety of antimicrobial proteins enhance the innate defenses either by attacking microorgan- isms directly or by hindering their ability to repro- duce. The most important of these are complement and interferon.

Complement The term complement refers to a group of at least 20 plasma proteins that circulate in the blood in an inactive state, much like inactive

Did You Get It? 8. What are the four common indicators of

inflammation?

For the answer, see Appendix A.

Phagocytes

➔ Learning Objective □□ Explain the importance of phagocytes.

Abscess on foot.

__________________________________________✚

Lysosome

Phagosome (phagocytic vesicle)

Acid hydrolase enzymes(a) A macrophage (purple) uses its cytoplasmic

extensions to ingest bacillus-shaped bacteria (pink) by phagocytosis. Scanning electron micrograph.

(b) Events of phagocytosis

Phagocyte engulfs the particles, forming a phagosome.

Phagocyte adheres to pathogens.

Lysosome fuses with the phagocytic vesicle, forming a phagolysosome.

Lysosomal enzymes digest the pathogens or debris, leaving a residual body.

Exocytosis of the vesicle removes indigestible and residual material.

Figure 12.9 Phagocytosis by a macrophage.

436 Essentials of Human Anatomy and Physiology

Body temperature is regulated by the hypothala- mus, referred to as the body’s “thermostat.” Normally the thermostat is set at approximately 37°C (98.6°F), but it can be reset upward in response to pyrogens (pyro = fire), chemicals secreted by white blood cells and macrophages exposed to foreign cells or substances in the body.

Although high fevers are dangerous because excess heat “scrambles” (destroys protein struc- ture) enzymes and other body proteins, rendering them nonfunctional, mild or moderate fever seems to benefit the body. Bacteria require large amounts of iron and zinc to multiply, but during a fever the liver and spleen gather up these nutrients, making them less available. Fever also increases the meta- bolic rate of tissue cells in general, speeding up repair processes.

Did You Get It? 9. How does complement cause lysis of a pathogenic

microorganism? 10. Which type of infectious microorganism causes the

body’s level of interferons to rise?

For answers, see Appendix A.

Adaptive Body Defenses Most of us would find it wonderfully convenient if we could walk into a single clothing store and buy custom outfits for every occasion—that fit us “to a T.” We know that such a service is nearly impossi- ble to find—yet we take for granted our built-in specific defense system, which stalks and eliminates with nearly equal precision almost any type of pathogen that intrudes into the body.

The immune response to a threat involves tremendously increased internal nonspecific defenses (inflammatory responses and others) and also provides protection that is carefully targeted against specific antigens. Furthermore, the initial exposure to an antigen “primes” (sensitizes) the body to react more vigorously to later meetings with the same antigen.

Referred to as the body’s third line of defense (see Figure 12.6), the adaptive, or specific, defense mechanism is a functional system that recognizes foreign molecules called antigens and acts to inac- tivate or destroy them. Normally it protects us from a wide variety of pathogens, as well as from abnormal body cells. When it fails, malfunctions, or is disabled, some of the most devastating

clotting proteins. However, when complement becomes attached, or fixed, to foreign cells such as bacteria, fungi, or mismatched red blood cells, it is activated and becomes a major factor in the fight against foreign cells (Figure 12.10). This com- plement fixation occurs when complement pro- teins bind to certain sugars or proteins (such as antibodies) on the foreign cell’s surface. One result of complement fixation is 1

the formation of membrane attack complexes (MAC) that pro- duce holes, or pores, in the foreign cell’s surface. 1

These pores allow water to rush into the cell,

causing it to burst, or lyse. Activated complement also amplifies the inflammatory response. Some of the molecules released during the activation process are vasodilators, and some are chemotaxis chemi- cals that attract neutrophils and macrophages into the region. Still others cause the cell membranes of the foreign cells to become sticky so they are easier to phagocytize; this effect is called opsonization. Complement is a nonspecific defensive mechanism that “complements,” or enhances, the effectiveness of both innate and adaptive defenses. Complement proteins must be activated in a particular sequence called a cascade, which insures that complement is not accidentally activated.

Interferon Viruses are unique, acellular parti- cles—essentially nucleic acids surrounded by a pro- tein coat—that lack the cellular machinery required to generate ATP or make proteins. They do their “dirty work” in the body by taking over target cells (different cells for different viruses), then using cel- lular machinery and nutrients to reproduce them- selves. Although the virus-infected cells can do little to save themselves, they help defend cells that have not yet been infected by secreting small proteins called interferons (in-ter-feer′onz). The interferon molecules diffuse to nearby cells and bind to their membrane receptors. This binding stimulates the synthesis of proteins that “interfere” with the ability of viruses to multiply within these still-healthy cells, reducing the spread of the virus. Interferons do not assist with fighting bacterial or fungal infections.

Fever

➔ Learning Objective □□ Describe how fever helps protect the body.

Fever, or abnormally high body temperature, is a systemic response to invading microorganisms.

Chapter 12: The Lymphatic System and Body Defenses 437

• It is systemic—immunity is not restricted to the initial infection site.

• It has “memory”—it recognizes and mounts even stronger attacks on previously encoun- tered pathogens.

As the pieces began to fall into place, two separate but overlapping arms of the adaptive defense system were recognized. Humoral (hu′mor-al) immunity, also called antibody- mediated immunity, is provided by antibodies (immune proteins) present in the body’s “humors,” or fluids. When lymphocytes them- selves defend the body, the immunity is called cellular immunity or cell-mediated immunity because the protective factor is living cells. The cellular arm also has cellular targets—virus- infected cells, cancer cells, and cells of foreign grafts. The lymphocytes act against such targets either directly, by lysing the foreign cells, or indi- rectly, by releasing chemicals that enhance the inflammatory response or activate other immune cells. However, before we describe the humoral and cellular responses individually, we will con- sider both the remarkable cells involved in these immune responses and the antigens that trigger their activity.

diseases—such as cancer, rheumatoid arthritis, and AIDS—may result.

Although immunology, the study of immunity, had not been established yet, the ancient Greeks knew that once someone had suffered a certain infectious disease, that person was unlikely to have the same disease again. The basis of this immunity was revealed in the late 1800s, when it was shown that animals surviving a serious bacte- rial infection have “factors” in their blood that pro- tect them from future attacks by the same pathogen. (These factors are now known to be unique proteins called antibodies.) Furthermore, it was demonstrated that if antibody-containing serum from the surviving animals (immune serum) were injected into animals that had not been exposed to the pathogen, those animals would also be protected. This was exciting news. But then, in the mid-1900s, it was discovered that injection of serum containing antibodies did not always protect a recipient from diseases the donor had survived. In such cases, however, injection of the donor’s lymphocytes did provide immunity. These landmark experiments revealed three important aspects of adaptive defense:

• It is antigen specific—it recognizes and acts against particular pathogens or foreign substances.

Membrane attack complex forming

Cytoplasm

Antibodies attached to pathogen’s membrane

Pore

H2OH2O

Activated complement proteins attach to pathogen’s membrane in step-by-step sequence, forming a membrane attack complex (a MAC attack).

MAC pores in the membrane allow water to rush into the cell.

This influx of water causes cell lysis.

1 2 3

Figure 12.10 Activation of complement, resulting in lysis of a target cell.

438 Essentials of Human Anatomy and Physiology

detergents, hair dyes, cosmetics, and other com- monly used household and industrial products.

Homeostatic Imbalance 12.3

Perhaps the most dramatic and familiar example of a drug hapten’s provoking an immune response involves the binding of penicillin to blood pro- teins, which causes a penicillin reaction in some people. The combination of blood protein and penicillin is recognized as foreign. In such cases, the immune system mounts such a vicious attack that the person’s life is endangered. __________✚

Did You Get It? 11. What is the difference between an antigen and a

self-antigen?

For the answer, see Appendix A.

Cells of the Adaptive Defense System: An Overview ➔ Learning Objectives

□□ Name the two arms of the adaptive defense system, and relate each to a specific lymphocyte type (B or T cell).

□□ Compare and contrast the development of B and T cells.

□□ State the roles of B cells and T cells.

□□ Explain the importance of antigen-presenting cells in immunity.

The crucial cells of the adaptive system are lympho- cytes and antigen-presenting cells (APCs). We already discussed the unique lymphocytes called NK cells as part of the innate defenses. Lymphocytes exist in two other major “flavors.” The B lymphocytes, or B cells, produce antibodies and oversee humoral immunity, whereas the T lymphocytes, or T cells, constitute the cell-mediated arm of the adaptive defenses and do not make antibodies. T lympho- cytes can recognize and eliminate specific virus- infected or tumor cells, and B lymphocytes can target specific extracellular antigens. In contrast, APCs do not respond to specific antigens but instead play an essential role in activating the lymphocytes that do.

Lymphocytes

Like all blood cells, lymphocytes originate from hemocytoblasts in red bone marrow. The imma- ture (called naive) lymphocytes released from the marrow are essentially identical. Whether a

Antigens ➔ Learning Objective

□□ Define antigen and hapten, and name substances that act as complete antigens.

An antigen (an′t ı̆-jen; Ag) is any substance capa- ble of provoking an immune response. Foreign antigens are large, complex molecules that are not normally present in our bodies. Consequently, they are foreign intruders, or nonself. An almost limitless variety of substances can act as antigens, including virtually all foreign proteins, nucleic acids, many large carbohydrates, and some lipids. Of these, proteins provoke the strongest responses. Pollen grains and microorganisms such as bacteria, fungi, and virus particles are antigenic because their surfaces bear such foreign molecules.

Our own cells are richly studded with a variety of protein and carbohydrate molecules. As our immune system develops, it takes inventory of all these molecules so that, thereafter, they are recog- nized as self. Although these self-antigens do not trigger an immune response in us, they are strongly antigenic to other people. This helps explain why our bodies reject cells of transplanted organs or foreign grafts unless special measures (drugs and others) are taken to cripple or stifle the immune response. In the unusual situations where self-antigens do elicit an immune response, auto- immune disease results.

➔CONCEPTLINKLikewise, this explains why only people of compatible blood types can donate blood to one another. Recall our discussion of blood antigens (Chapter 10, p. 378). Depending on your individual blood type, one or more blood antigen types may be self to your body, and one or more may be nonself. For example, if you are blood type A, type A antigen is self, but type B antigen is nonself. However, in a person with type AB blood, both A and B antigens are self-antigens. ➔

As a rule, small molecules are not antigenic, but when they link up with our own proteins, the immune system may recognize the combination as foreign and mount an attack that is harmful rather than protective. In such cases, the troublesome small molecule is called a hapten (haptein = to grasp) or incomplete antigen. Besides certain drugs, chemicals that act as haptens are found in poison ivy, animal dander, and even in some

Chapter 12: The Lymphatic System and Body Defenses 439

the sharpest ability to identify foreign antigens survive. Lymphocytes capable of binding strongly with self-antigens (and of acting against body cells) are vigorously weeded out and destroyed. Thus, the development of self-tolerance for the body’s own cells is an essential part of a lympho- cyte’s “education.” This is true not only for T cells but also for B cells. B cells develop immunocom- petence in bone marrow, but less is known about the factors that regulate B cell maturation.

Once a lymphocyte is immunocompetent, it will be able to react to one and only one distinct

given lymphocyte matures into a B cell or a T cell depends on where in the body it becomes immu- nocompetent, that is, capable of responding to a specific antigen by binding to it with antigen- specific receptors that appear on the lymphocyte’s surface. T cells arise from lymphocytes that migrate to the thymus (Figure 12.11 1

). There they undergo a maturation process lasting 2 to 3 days, directed by thymic hormones (thymosin and others). Within the thymus, the immature lympho- cytes divide rapidly and their numbers increase enormously, but only those maturing T cells with

Red bone marrow

Bone marrow Thymus

Lymph nodes, spleen, and other lymphoid tissues

Immature (naive) lymphocytes

Red bone marrow: site of lymphocyte origin

Secondary lymphoid organs: sites of antigen encounter, and activation to become effector and memory B or T cells

Primary lymphoid organs: sites of development of immunocompetence as B or T cells

Lymphocytes destined to become T cells migrate (in blood) to the thymus and develop immunocompetence there. B cells develop immunocompetence in red bone marrow.

Immunocompetent but still naive lymphocytes leave the thymus and bone marrow. They “seed” the lymph nodes, spleen, and other lymphoid tissues, where they encounter their antigens and become activated.

Antigen-activated (mature) immunocompetent lymphocytes (effector cells and memory cells) circulate continuously in the bloodstream and lymph and throughout the lymphoid organs of the body.

KEY:

Figure 12.11 Lymphocyte differentiation and activation.

440 Essentials of Human Anatomy and Physiology

tem. Macrophages tend to remain fixed in the lymphoid organs (as if waiting for antigens to come to them).

When they present antigens, dendritic cells and macrophages activate T cells. Activated T cells, in turn, release chemicals (see Table 12.3, pp. 450–451) that prod macrophages to become activated macrophages, true “killers” that are insa- tiable phagocytes and secrete bactericidal (bacteria- killing) chemicals. As you will see, interactions between lymphocytes, and between lymphocytes and APCs, play roles in virtually all phases of the immune response.

In addition to T cell circulation and passive delivery of antigens to lymphoid organs by lym- phatics, there is a third antigen capture and deliv- ery system—migration of dendritic cells to secondary lymphoid organs. With their long, wispy extensions, dendritic cells are very efficient antigen catchers. Once they have engulfed anti- gens by phagocytosis, they enter nearby lymphat- ics to get to the lymphoid organ where they will present the antigens to T cells. Indeed, dendritic cells are the most effective antigen presenters known—it’s their only job. Dendritic cells are a key link between innate and adaptive immunity. They initiate adaptive immune responses particu- larly tailored to the type of pathogen that they have encountered.

To summarize, the adaptive immune system is a two-fisted defensive system, with a humoral arm and a cellular arm. It uses lymphocytes, APCs, and specific molecules to identify and destroy all sub- stances—both living and nonliving—that are in the body but are not recognized as self. The immune system’s ability to respond to such threats depends on the ability of its cells (1) to recognize foreign substances (antigens) in the body by bind- ing to them and (2) to communicate with one another so that the system as a whole mounts a response specific to those antigens.

Did You Get It? 12. What are the two types of lymphocytes involved in

adaptive immune responses, and how do their functions in body protection differ?

13. Where does the “programming” phase leading to immunocompetence occur for T cells?

14. What is the essential role of macrophages and dendritic cells in adaptive immunity?

For answers, see Appendix A.

antigen, because all the antigen receptors on its external surface are the same, meaning they are specific for the same antigen. For example, the receptors of one lymphocyte can recognize only a part of the hepatitis A virus, those of another lym- phocyte can recognize only pneumococcus bacte- ria, and so forth.

Although all the details of the maturation pro- cess are still being worked out, we know that lym- phocytes become immunocompetent before meeting the antigens they may later attack. Thus, it is our genes, not antigens, that determine what foreign substances our immune system will be able to recog- nize and resist. Only some of the possible antigens our lymphocytes are programmed to resist will ever invade our bodies. Consequently, only some mem- bers of our army of immunocompetent cells will be mobilized during our lifetime.

After they become immunocompetent, both T cells and B cells migrate to the lymph nodes and spleen (and loose connective tissues), where their encounters with antigens will occur (Figure 12.11

). Then, when the lymphocytes bind with recognized antigens, they complete their differentiation from naive cells into fully mature T cells and B cells. Mature lymphocytes, especially T cells, circulate continuously through the body (Figure 12.11

). This makes sense because circulating greatly increases a lymphocyte’s chance of coming into contact with antigens, as well as with huge numbers of macrophages and other lymphocytes along the route.

Antigen-Presenting Cells

The major role of antigen-presenting cells (APCs) in immunity is to engulf antigens and then present fragments of them, like signal flags, on their own surfaces, where they can be recognized by T cells. In other words, they present antigens to the cells that will actually deal with the antigens. The major types of cells acting as APCs are den- dritic cells, macrophages, and B lymphocytes.

All of these cell types are in sites that make it easy to encounter and process antigens. Dendritic cells are present in connective tissues and in the epidermis (where they are also called Langerhans cells). These cells are therefore at the body’s fron- tiers, best situated to act as mobile sentinels. Macrophages are widely distributed throughout the lymphoid organs and connective tissues, where they act as phagocytes in the innate defense sys-

Chapter 12: The Lymphatic System and Body Defenses 441

on” and undergo clonal selection. In this process, the lymphocyte begins to grow and then multiplies rapidly to form an army of cells exactly like itself and bearing the same antigen-specific receptors (Figure 12.12). The resulting family of identical cells descended from the same ancestor cell are called clones, and clone formation is the primary humoral response to that antigen. (As described later, a special group of T cells called helper T cells also influence B cell activation.)

Most of the B cell clone members, or descen- dants, become plasma cells. After an initial lag

Humoral (Antibody-Mediated) Immune Response ➔ Learning Objectives

□□ Define humoral immunity.

□□ State the role of plasma cells.

An immunocompetent but as yet immature B lym- phocyte is stimulated to complete its development (from naive to fully mature B cell) when antigens bind to its surface receptors. This binding event sensitizes, or activates, the lymphocyte to “switch

Primary response (initial encounter with antigen)

Free antigen

Antigen binding to a receptor on a specific B cell (B cells with non-complementary receptors remain inactive)

Proliferation to form a cloneActivated B

cells

Plasma cells

Secreted antibody molecules

Clone of cells identical to ancestral cells

Subsequent challenge by same antigen results in more rapid response

Memory B cell

Memory B cells

Plasma cells

Secreted antibody molecules

Secondary response (can be years later)

Figure 12.12 Clonal selection of a B cell. The initial meeting with the antigen stimulates the primary response, in which the B cell divides rapidly, forming many clones (clonal selection), most of which become

antibody-producing plasma cells. Cells that do not differentiate into plasma cells become memory cells, which are primed to respond to subsequent exposures to the same antigen. Should such a meeting occur, the

memory cells quickly produce more memory cells and larger numbers of effector plasma cells with the same antigen specificity. Responses generated by memory cells are called secondary responses.

442 Essentials of Human Anatomy and Physiology

Did You Get It? 15. Suzy had the flu last winter and got it again this year.

However, her doctor says this year’s flu is a new strain. Is Suzy’s body mounting a primary or a secondary immune response?

For the answer, see Appendix A.

Active and Passive Humoral Immunity

➔ Learning Objectives □□ Explain the function(s) of antibodies, and describe

clinical uses of monoclonal antibodies.

□□ Distinguish between active and passive immunity.

When your B cells encounter antigens and pro- duce antibodies against them, you are exhibiting active immunity (Figure 12.14). Active immunity is (1) naturally acquired during bacterial and viral infections, during which we may develop the signs and symptoms of the disease and suffer a little (or a lot), and (2) artificially acquired when we receive vaccines. It makes little difference whether the antigen invades the body under its own power or is introduced in the form of a vaccine. The response of the immune system is pretty much the same. Indeed, once it was recognized that secondary responses are so much more vigorous, the race was on to develop vaccines to “prime” the immune response by providing a first meeting with the anti- gen. Most vaccines contain pathogens that are dead or attenuated (living, but extremely weakened).

We receive two benefits from vaccines: (1) they spare us most of the signs and symptoms (and dis- comfort) of the disease that would otherwise occur during the primary response and (2) the weakened antigens are still able to stimulate antibody produc- tion and promote immunological memory. So-called booster shots, which may intensify the immune response at later meetings with the same antigen, are also available. Vaccines have virtually wiped out smallpox and are currently available against micro- organisms that cause pneumonia, polio, tetanus, diphtheria, whooping cough, measles, and many other diseases. In the United States, active immuni- zation programs have dramatically reduced the inci- dence of many potentially serious childhood diseases. The more children who are vaccinated, the better the herd immunity, a phenomenon in which a population of people are generally protected because most of a given population is immune to a disease or infection. Herd immunity prevents an outbreak of the disease or infection and thus helps

period, these antibody-producing “factories” swing into action, producing the same highly specific antibodies at an unbelievable rate of about 2,000 antibody molecules per second. However, this flurry of activity lasts only 4 or 5 days; then the plasma cells begin to die. The level of antibody in the blood during this primary response peaks about 10 days after the response begins and then slowly declines (Figure 12.13).

B cell clone members that do not become plasma cells become long-lived memory cells capable of responding to the same antigen if they “see” it again. Memory cells are responsible for the immunological memory mentioned earlier. These later immune responses, called secondary humoral responses, are produced much faster, are more prolonged, and are more effective than the events of the primary response because all the preparations for this attack have already been made. Within hours after recognition of the “old- enemy” antigen, a new army of plasma cells is being generated, and antibodies flood into the bloodstream. Within 2 to 3 days, the blood anti- body level peaks (at a much higher level than seen in the primary response), and the level remains high for weeks to months. We will describe how antibodies work shortly.

0 1 2 3 4 5 6

Secondary response

Primary response

Time (weeks) Antigen injected

Antigen injected

R el

at iv

e an

tib od

y co

nc en

tr at

io n

in b

lo od

p la

sm a

Figure 12.13 Primary and secondary humoral responses to an antigen. In the primary response, the level of antibodies in the blood gradually rises and then rapidly declines. The secondary response is both more rapid and more intense, and the antibody level remains high for a much longer time.

Chapter 12: The Lymphatic System and Body Defenses 443

Antibodies, also referred to as immunoglobu- lins (im″mu-no-glob′u-linz; Igs), constitute the gamma globulin part of blood proteins. Antibodies are soluble proteins secreted by activated B cells or by their plasma-cell offspring in response to an antigen, and they are capable of binding specifi- cally with that antigen.

Antibodies are formed in response to a huge number of different antigens. Despite their variety, they all have a similar basic anatomy that allows them to be grouped into five Ig classes, each slightly different in structure and function.

Basic Antibody Structure Regardless of its class, every antibody has a basic structure consisting of four polypeptide chains linked together by disul- fide (sulfur-to-sulfur) bonds (Figure 12.15, p. 444). Two of the four chains are identical; these are the heavy chains. The other two chains, the light chains, are also identical to each other but are only about half as long as the heavy chains. When the four chains are combined, the antibody molecule formed has two identical halves, each consisting of a heavy and a light chain, and the molecule as a whole is commonly described as being Y-shaped.

When scientists began investigating antibody structure, they discovered something very peculiar. Each of the four chains forming an antibody had a variable (V) region at one end and a constant (C) region at the other end. Antibodies responding

to protect individuals who have not been immu- nized.

Passive immunity is quite different from active immunity, both in the antibody source and in the degree of protection it provides (see Figure 12.14). Instead of being made by your plasma cells, the antibodies are obtained from the serum of an immune human or animal donor. As a result, your B cells are not challenged by the antigen, immuno- logical memory does not occur, and the temporary protection provided by the “donated antibodies” ends when they naturally degrade in the body.

Passive immunity is conferred naturally on a fetus when the mother’s antibodies cross the pla- centa and enter the fetal circulation, and after birth during breastfeeding. For several months after birth, the baby is protected from all the antigens to which the mother has been exposed.

Passive immunity is artificially conferred when a person receives immune serum or gamma glob- ulin (donated antibodies). Gamma globulin is commonly administered after exposure to hepati- tis. Other immune sera are used to treat poisonous snakebites (an antivenom), botulism, rabies, and tetanus (an antitoxin) because these diseases will kill a person before active immunity can be estab- lished. The donated antibodies provide immediate protection, but their effect is short-lived (2 to 3 weeks). Meanwhile, however, the body’s own defenses take over.

In addition to their use to provide passive immunity, antibodies are prepared commercially for use in research, in clinical testing for diagnostic purposes, and in treating certain cancers. Monoclonal antibodies used for such purposes are descendants of a single cell and are pure anti- body preparations that exhibit specificity for one, and only one, antigen. Besides their use in deliver- ing cancer-fighting drugs to cancerous tissue, monoclonal antibodies are being used for early cancer diagnosis and to track cancers hidden deep within the body. They are also used for diagnosing pregnancy, hepatitis, and rabies.

Antibodies

➔ Learning Objectives □□ Describe the structure of an antibody monomer.

□□ List the five antibody classes, and describe their specific roles in immunity.

□□ Describe several ways in which antibodies act against antigens.

PassiveActive

Humoral immunity

Artificially acquired

Injection of donated antibodies (gamma globulin)

Naturally acquired

Antibodies passed from mother to fetus via placenta; or to infant in her milk

Artificially acquired

Vaccine; dead or attenuated pathogens

Naturally acquired

Infection; contact with pathogen

Figure 12.14 Types of humoral immunity. There are two overall types of humoral immunity, active and passive. Darker green boxes signify naturally acquired immunity, whereas lighter green boxes show artificially acquired immunity.

444 Essentials of Human Anatomy and Physiology

(Remember the name MADGE to recall the five Ig types.) Antibodies IgD, IgG, and IgE have the same basic Y-shaped structure described previ- ously and are referred to as monomers (Table 12.2). IgA antibodies occur in both monomer and dimer (two linked monomers) forms. (Only the dimer form is shown in the table.) Because they are con- structed of five linked monomers, IgM antibodies are called pentamers (penta = five).

The antibodies of each class have slightly dif- ferent biological roles and locations in the body. For example, IgG is the most abundant antibody in blood plasma and is the only type that can cross the placental barrier. Hence, the passive immunity that a mother transfers to her fetus is “with the compliments” of her IgG antibodies. Only IgM and IgG can fix complement. The IgA dimer, secretory IgA, is found mainly in secretions that bathe body surfaces, such as mucus and tears. It plays a major role in preventing pathogens from gaining entry into the body. IgE antibodies are the “troublemaker” antibodies involved in allergies.

Did You Get It? 17. Which class of antibody is found in saliva and tears?

For the answer, see Appendix A.

to different antigens had different variable regions, but their constant regions were the same or nearly so. This made sense when it was discovered that the variable regions of the heavy and light chains combine their efforts to form an antigen-binding site (see Figure 12.15) uniquely shaped to “fit” a specific antigen. Hence, each antibody has two such antigen-binding regions.

The constant regions that form the “stem” of an antibody can be compared to the handle of a key. A key handle has a common function for all keys: it allows you to hold the key and place its tumbler-moving portion into the lock. Similarly, the constant regions of the antibody chains serve common functions in all antibodies: they deter- mine the type of antibody formed (antibody class), as well as how the antibody class will carry out its immune roles in the body, and the cell types or chemicals with which the antibody can bind.

Did You Get It? 16. What is the importance of the variable region of

antibodies?

For the answer, see Appendix A.

Antibody Classes There are five major immuno- globulin classes—IgM, IgA, IgD, IgG, and IgE.

(b)(a)

Antigen-binding sites

Light chain

Heavy chain

Disulfide bonds

V C

C C

Figure 12.15 Basic antibody structure. (a) Computer-generated image. (b) Simplified diagram. The monomer of each type of antibody is formed by four polypeptide chains

(two light chains and two heavy chains) that are joined by disulfide bonds. Each chain has a variable (V) region (antigen-binding site that differs in different antibodies) and a

constant (C) region (essentially identical in different antibodies of the same class).

Chapter 12: The Lymphatic System and Body Defenses 445

Table 12.2 Immunoglobulin Classes

Class Generalized structure Where found Biological function

IgM Attached to B cell; free in plasma

When bound to B cell membrane, serves as antigen receptor; first Ig class released by plasma cells during primary response; potent agglutinating agent; fixes complement.

IgA Some (monomer) in plasma; dimer in secretions such as saliva, tears, intestinal juice, and milk

Bathes and protects mucosal surfaces from attachment of pathogens.

IgD Almost always attached to B cell

Believed to be cell surface receptor of immunocompetent B cell; important in activation of B cell.

IgG Most abundant antibody in plasma; represents 75–85% of circulating antibodies

Main antibody of both primary and secondary responses; crosses placenta and provides passive immunity to fetus; fixes complement.

IgE Secreted by plasma cells in skin, mucosae of gastrointestinal and respiratory tracts, and tonsils

Binds to mast cells and basophils and triggers release of histamine and other chemical mediators of inflammation and some allergic responses.

Antibody Function Antibodies inactivate anti- gens in a number of ways—by complement fixa- tion, neutralization, agglutination, opsonization, and precipitation (Figure 12.16, p. 446). Of these, complement fixation and neutralization are most important to body protection.

Complement is the chief antibody ammunition used against cellular antigens, such as bacteria or mismatched red blood cells. As noted earlier, com- plement is fixed (activated) during innate defenses and when it binds to antibodies attached to cellu- lar targets (Look back at Figure 12.10, p. 437).

Neutralization occurs when antibodies bind to specific sites (usually at or close to the site where a cell would bind) on bacterial exotoxins (toxic chemicals secreted by bacteria) or on viruses that can cause cell injury. In this way, they block the harmful effects of the exotoxin or virus by pre- venting them from binding to body cells.

Because antibodies have more than one anti- gen-binding site, they can bind to more than one antigen at a time; consequently, antigen-antibody complexes can be cross-linked into large lattices. When the cross-linking involves cell-bound anti- gens, the process causes clumping of the foreign cells, a process called agglutination. This type of antigen-antibody reaction occurs when mis- matched blood is transfused (the foreign red blood cells are clumped) and is the basis of tests used for blood typing. (Look back at Figure 10.7, p. 378.) When the cross-linking process involves soluble antigenic molecules, the resulting antigen-anti- body complexes are so large that they become insoluble and settle out of solution. This cross- linking reaction is more precisely called precipi- tation. Such agglutinated bacteria and immobilized (precipitated) antigen molecules are much more easily captured and engulfed by the body’s phago- cytes than are freely moving antigens.

446 Essentials of Human Anatomy and Physiology

Cellular (Cell-Mediated) Immune Response ➔ Learning Objective

□□ Distinguish the roles of helper, regulatory, and cytotoxic T cells.

The main difference between the two arms of the adaptive response is that B cells secrete their anti- body “weapons,” whereas T cells fight their anti- gens directly. Like B cells, immunocompetent T cells are activated to form a clone by binding with a “recognized” antigen (look ahead to Figure 12.19, p. 449). However, unlike B cells, T cells are not able to bind with free antigens. Instead, the antigens must be “presented” by a macrophage (or other antigen-presenting cell), and a double recognition

Did You Get It? 18. Regarding the action of antibodies, what is

neutralization? 19. Distinguish among precipitation, agglutination,

and opsonization.

For the answers, see Appendix A.

Enhances Enhances Leads to

Phagocytosis

Chemo- taxis

Histamine release

Inflammation Cell lysis

Agglutination (cell-bound antigens)

Precipitation (soluble

antigens)

Neutralization (masks dangerous parts of bacterial

exotoxins; viruses)

Complement

Antigen-antibody complex

Inactivates by

Antigen Antibody

Fixes and activates

Figure 12.16 Mechanisms of antibody action. Note that opsonization, the process by which antibodies “mark” antigens for phagocytes, is not pictured here.

Complement fixation opsonizes, or provides “knobs” for the phagocyte to attach to, whereas

agglutination binds antigens (typically microorganisms) together into large complexes that become nonmobile and easier to destroy.

Complement and agglutination aid phagocytosis, but they assist it in different ways. What is this difference?Q:

Chapter 12: The Lymphatic System and Body Defenses 447

Cytotoxic T cells specialize in killing virus- infected, cancer, or foreign graft cells directly (Figure 12.18, p. 448). One way a cytotoxic T cell accomplishes this is by 1

binding tightly to a foreign cell and

releasing toxic chemicals called perforin and granzymes from its granules. The perforin

enters the foreign cell’s plasma mem- brane (delivering the so-called lethal hit). Shortly thereafter, pores appear in the target cell’s mem- brane, allowing

the granzymes (protein-digesting enzymes) to enter and kill the foreign cell. The cytotoxic T cell then

detaches and seeks other foreign prey to attack.

Helper T cells are the T cells that act as the “directors” or “managers” of the adaptive immune response. Once activated, they circulate through the body, recruiting other cells to fight the invad- ers. For example, helper T cells interact directly with B cells (that have already attached to anti- gens), prodding the B cells into more rapid divi- sion (clone production) and then, like the “boss” of an assembly line, signaling for antibody forma- tion to begin. The helper T cells also release a variety of cytokines (see Table 12.3) that act indi- rectly to rid the body of antigens by (1) stimulating

must occur. The APC engulfs an antigen and pro- cesses it internally. Parts of the processed antigen are then displayed on the external surface of the presenting cell in combination with one of the APC’s own proteins. Note that there are different classes of effector T cells, those that carry out direct activities. Two types that are involved in the activation process are called helper T cells and cytotoxic T cells (Figure 12.17). We will discuss more about the functions of these cells as we work through this process.

A T cell must recognize “nonself,” the antigen fragment presented by the APC, and “self,” by cou- pling with a specific glycoprotein on the APC’s surface at the same time. Antigen binding alone is not enough to sensitize T cells. They must be “spoon-fed” the antigens by APCs, and something like a “double handshake” must occur. This pro- cess of antigen presentation is now known to be essential for the activation and clonal selection of T cells. Without the “presenters,” the immune response is severely impaired. Cytokine chemi- cals released by macrophages and dendritic cells also play important roles in the immune response (Table 12.3, pp. 450–451).

Cell-mediated immunity (attack on infected cells)

Humoral immunity (secretion of antibodies by plasma cells)

“Presented” antigen

Cytokines

Dendritic cell

Cytotoxic (killer) T cell

Helper T cell

B cell

Antigen

Antigen processing

Self- protein

T cell antigen receptor

Figure 12.17 T cell activation and interactions with other cells of the immune response. Dendritic cells are important antigen- presenting cells (APCs). After they ingest an antigen, they display parts of it on their surface, where it can be

recognized by a helper T cell specific for the same antigen. During the binding process, the helper T cell binds simultaneously to the antigen and to the APC (self) receptor, which leads to helper T cell activation and cloning (not illustrated). If the APC is

a macrophage, it releases cytokines, which enhance helper T cell activation. Activated helper T cells release cytokines, which stimulate proliferation and activity of other helper T cells and help activate B cells and cytotoxic T cells.

448 Essentials of Human Anatomy and Physiology

long-lived memory cells that remain behind to provide the immunological memory for each anti- gen encountered and enable the body to respond quickly to subsequent invasions.

The major elements of the immune response are summarized in Figure 12.19.

Did You Get It? 20. T cells must take part in what is sometimes called the

“double handshake” in order to be activated. What does this mean?

21. How is the lethal hit accomplished? 22. What is the role of regulatory T cells in the adaptive

immune response? 23. In which branch of the adaptive response do helper

T cells participate?

For answers, see Appendix A.

Organ Transplants and Rejection For people suffering with end-stage heart or kid- ney disease, organ transplants are a desirable treat- ment option. However, organ transplants have had mixed success because the immune system is ever vigilant, and rejection is a real problem.

cytotoxic T cells and B cells to grow and divide; (2) attracting other types of protective white blood cells, such as neutrophils, into the area; and (3) enhancing the ability of macrophages to engulf and destroy microorganisms. (Actually, macro- phages are good phagocytes even in the absence of cytokines, but in the presence of cytokines they develop an even bigger appetite.) As the released cytokines summon more and more cells into the battle, the immune response gains momentum, and the antigens are often overwhelmed by the sheer numbers of immune elements acting against them.

Another T cell population, regulatory T cells (formerly called suppressor T cells), release chemi- cals that suppress the activity of both T and B cells. Regulatory T cells are vital for winding down and stopping the immune response after an anti- gen has been successfully inactivated or destroyed. This helps prevent uncontrolled or unnecessary immune system activity.

Most of the T cells enlisted to fight in a partic- ular immune response are dead within a few days. However, a few members of each clone are

Cytotoxic T cell binds tightly to the foreign target cell.

Perforin molecules insert into the target cell membrane and form pores similar to those produced by complement activation.

Granzymes enter the target cell via the pores and degrade cellular contents.

The cytotoxic T cell detaches and searches for another prey.

Cytotoxic T cell releases perforin and granzyme molecules from its granules by exocytosis.

Cytotoxic T cell

Target cell

Perforin

TC cell membrane

Target cell membrane

Perforin pore

Granzymes

Granule

1 2

Figure 12.18 A proposed mechanism by which cytotoxic T cells kill target cells.

Chapter 12: The Lymphatic System and Body Defenses 449

CELLULAR (CELL-MEDIATED) ADAPTIVE IMMUNE RESPONSE

HUMORAL (ANTIBODY-MEDIATED) ADAPTIVE IMMUNE RESPONSE

Free antigens directly activate

Antigens displayed by infected cells activate

Engulfed by

Becomes

Stimulates

Macrophage

Gives rise toGives rise to

Secrete

Defend against intracellular pathogens and cancer by binding to and lysing the infected cells or cancer cells.

Antigen- presenting cell

Active cytotoxic T cells

Cytotoxic T cell

Memory B cells

Memory T cells

Plasma cells

B cell

Antigen (2nd exposure)

Stimulates

Stimulates StimulatesStimulates

Helper

Memory

Antibodies

Defend against extracellular pathogens by binding to antigens and making them easier targets for phagocytes and complement.

Stimulates

Presents antigen

Stimulates

Antigen (1st exposure)

T cell

Figure 12.19 A summary of the adaptive immune responses. In this flowchart, green arrows track the primary response, and blue arrows track the secondary response.

450 Essentials of Human Anatomy and Physiology

Table 12.3 Functions of Cells and Molecules Involved in Immunity

Element Function in the immune response

Cells

B cell Lymphocyte that resides in the lymph nodes, spleen, or other lymphoid tissues, where it is induced to replicate by antigen-binding and helper T cell interactions; its progeny (clone members) form plasma cells and memory cells.

Plasma cell Antibody-producing “machine”; produces huge numbers of the same antibody (immunoglobulin); specialized B cell clone descendant.

Helper T cell A T cell that binds with a specific antigen presented by an APC; it stimulates the production of other immune cells (cytotoxic T cells and B cells) to help fight the invader; acts both directly and indirectly by releasing cytokines.

Cytotoxic T cell Activity enhanced by helper T cells; its specialty is killing virus-invaded body cells, as well as body cells that have become cancerous; involved in graft rejection.

Regulatory T cell Slows or stops the activity of B and T cells once the infection (or attack by foreign cells) has been conquered. Thought to be important in preventing autoimmune diseases.

Memory cell Descendant of an activated B cell or T cell; generated during both primary and secondary immune responses; may exist in the body for years thereafter, enabling it to respond quickly and efficiently to subsequent infections or meetings with the same antigen.

Antigen-presenting cell (APC)

Any of several cell types (macrophage, dendritic cell, B cell) that engulfs and digests antigens that it encounters and presents parts of them on its plasma membrane for recognition by T cells bearing receptors for the same antigen; this function, antigen presentation, is essential for normal cell-mediated responses. Macrophages and dendritic cells also release chemicals (cytokines) that activate many other immune cells.

Molecules

Antibody (immunoglobulin) Protein produced by a B cell or its plasma-cell offspring and released into body fluids (blood, lymph, saliva, mucus, etc.), where it attaches to antigens, causing neutralization, opsonization, precipitation, or agglutination, which “marks” the antigens for destruction by phagocytes or complement.

Cytokines Chemicals released by sensitized T cells, macrophages, and certain other cells:

• Migration inhibiting factor (MIF)—”inhibits” macrophage migration and keeps them in the local area.

• Interleukin 2—stimulates T cells and B cells to proliferate; activates NK cells.

• Helper factors—enhance antibody formation by plasma cells.

• Suppressor factors—suppress antibody formation or T cell–mediated immune responses (interleukin-10 transforming growth factor and others).

• Chemotactic factors—attract leukocytes (neutrophils, eosinophils, and basophils) into inflamed area.

• Gamma interferon—secreted by lymphocytes; helps make tissue cells resistant to viral infection; activates macrophages and NK cells; enhances maturation of cytotoxic T cells.

Tumor necrosis factor (TNF) Like perforin, causes cell killing; attracts granulocytes; activates T cells and macrophages.

Complement Group of bloodborne proteins activated after binding to antibody-covered antigens; when activated, complement causes lysis of the microorganism and enhances inflammatory response.

Chapter 12: The Lymphatic System and Body Defenses 451

protect the body against other foreign agents. Explosive infection (bacterial, fungal, viral) is the most frequent cause of death in these patients. Even under the best conditions, roughly 50 per- cent of patients have rejected their transplant within 10 years of receiving it. In rare cases, trans- plant patients have been able to “ditch the drugs,” and finding a way to induce such tolerance is the goal of many current research projects.

Did You Get It? 24. Sheila is receiving a kidney transplant. The donor

is her fraternal twin. What name is given to this type of graft?

For the answer, see Appendix A.

Disorders of Immunity ➔ Learning Objective

□□ Describe allergies, autoimmune diseases, and immunodeficiencies.

Homeostatic Imbalance 12.4

The most important disorders of the immune sys- tem are allergies, autoimmune diseases, and immu- nodeficiencies.

Allergies At first, the immune response was thought to be purely protective. However, it was not long before its dangerous potentials were discovered. Allergies, or hypersensitivities, are abnormally vigorous immune responses in which the immune system causes tissue damage as it fights off a perceived “threat” that would otherwise be harmless to the body. The term allergen (allo = altered; erg = reaction) is used to distinguish this type of antigen from those producing normal immune responses. Most of the time, people do not die of allergies; they are just miserable with them.

There are four major types of transplants, or grafts:

• Autografts are tissue grafts transplanted from one site to another in the same person.

• Isografts are tissue grafts donated by a geneti- cally identical person, the only example being an identical twin.

• Allografts are tissue grafts taken from a per- son other than an identical twin.

• Xenografts are tissue grafts harvested from a different animal species, such as a porcine (pig) heart valve transplanted into a human.

Autografts and isografts are ideal donor organs or tissues and are just about always successful given an adequate blood supply and no infection. Although pig heart valves have been transplanted with success, xenografts of whole organs are virtually never successful. The graft type most used is an allograft taken from a recently deceased person.

Before an allograft is even attempted, the ABO and other blood group antigens of both the donor and recipient must be determined and must match. Then, the cell membrane antigens of their tissue cells are typed to determine how closely they match. At least a 75 percent match is needed to attempt a graft; as you might guess, good tissue matches between unrelated people are hard to find.

After surgery, to prevent rejection, the patient receives immunosuppressive therapy, including one or more of the following: corticosteroids to suppress inflammation, antiproliferative drugs, radiation (X-ray) therapy, and immunosuppressor drugs. Many of these drugs kill rapidly dividing cells (such as activated lymphocytes), and all of them have severe side effects. However, the major problem with immunosuppressive therapy is that while the immune system is suppressed, it cannot

Element Function in the immune response

Antigen Substance capable of provoking an immune response; typically a large, complex molecule not normally present in the body.

Cytotoxins Perforin, granzymes—cell toxins released by cytotoxic T cells and NK cells.

Table 12.3 (continued)

452 Essentials of Human Anatomy and Physiology

There are several different types of allergies, including reactions where cells are lysed, such as during a transfusion reaction, and inflammation due to antigen-antibody complexes, such as those of glomerulonephritis. The most common type is immediate hypersensitivity, or acute hyper- sensitivity. After sensitization to a particular aller- gen, this type of response is triggered when that allergen is encountered again. This secondary response starts with the release of a flood of hista- mine when IgE antibodies bind to mast cells. Histamine causes small blood vessels in the area to become dilated and leaky and is largely to blame for the best-recognized symptoms of allergy: a runny nose, watery eyes, and itchy, reddened skin (hives). When the allergen is inhaled, symp- toms of asthma appear because smooth muscle in the walls of the bronchioles contracts, constricting the passages and restricting air flow. Over-the- counter (OTC) anti-allergy drugs contain antihista- mines that counteract these effects. Most of these reactions begin within seconds after contact with the allergen and last about half an hour.

Fortunately, the bodywide, or systemic, acute allergic response known as anaphylactic (an″ah- f ı̆-lak′tik) shock is fairly rare. Anaphylactic shock occurs when the allergen directly enters the blood and circulates rapidly through the body, as might happen with certain bee stings, spider bites, or an injection of a foreign substance (such as horse serum, penicillin, or other drugs that act as hap- tens) into susceptible individuals. Food allergies (peanut or wheat allergies) may also trigger ana- phylaxis, and can be fatal. The mechanism of ana- phylactic shock is essentially the same as that of local responses, but when the entire body is involved, the outcome is life-threatening. For example, it is difficult to breathe when the smooth muscles of lung passages contract, and the sudden vasodilation (and fluid loss) that occurs may cause circulatory collapse and death within minutes. Epinephrine, found in emergency EpiPen® shots, is the drug of choice to reverse these histamine- mediated effects.

Delayed hypersensitivities, mediated mainly by a special subgroup of helper T cells, cytotoxic T cells, and macrophages, take much longer to appear (1 to 3 days) than any of the acute reac- tions produced by antibodies. Instead of hista- mine, the chemicals mediating these reactions are

Antigen (allergen) invades body.

Plasma cells produce large amounts of class IgE antibodies against allergen.

Mast cell with fixed IgE antibodies

Granules containing histamine

IgE

More of same allergen invades body.

Allergen binding to IgE on mast cells triggers release of histamine (and other chemicals).

Histamine causes blood vessels to dilate and become leaky, which promotes edema; stimulates release of large amounts of mucus; and causes smooth muscles to contract.

IgE antibodies attach to mast cells in body tissues (and to circulating basophils).

Sensitization stage

Subsequent (secondary) responses

Antigen

Histamine

Mast cell granules release contents after antigen binds with IgE antibodies

Outpouring of fluid from capillaries

Release of mucus

Constriction of bronchioles

Mechanism of an immediate (acute) hypersensitivity response.

Chapter 12: The Lymphatic System and Body Defenses 453

• Glomerulonephritis, a severe impairment of kidney function due to acute inflammation (see p. 559)

Current therapies include treatments that depress certain aspects of the immune response.

How does the normal state of self-tolerance break down? It appears that one of the following may trigger it:

• New self-antigens appear. Such “hidden” antigens are found in sperm cells, the eye lens, and certain proteins in the thyroid gland. In addition, “new” self-antigens may appear as a result of gene mutations that change the struc- ture of self-proteins or as a result of alterations in self-proteins by hapten attachment or by bacterial or viral damage.

• Foreign antigens resemble self-antigens. For instance, antibodies produced during an infection caused by streptococcus bacteria (such as strep throat or scarlet fever) are known to cross-react with heart antigens, caus- ing damage to both the heart muscle and its valves, as well as to the joints and kidneys. This age-old disease is called rheumatic fever.

Immunodeficiencies The immunodeficiencies include both congeni- tal and acquired conditions in which the produc- tion or function of immune cells or complement is abnormal. The most devastating congenital condi- tion is severe combined immunodeficiency disease (SCID), in which there is a marked deficit of both B and T cells. Because T cells are abso- lutely required for normal operation of both arms of the adaptive response, afflicted children have essentially no protection against pathogens of any type. Minor infections easily shrugged off by most children are lethal to those with SCID. Bone mar- row transplants and umbilical cord blood, which provide normal lymphocyte stem cells, have helped some SCID victims. Without such treat- ment, the only hope for survival is living behind protective barriers (in a plastic “bubble”) that keep out all infectious agents.

Currently, the most important and most devas- tating of the acquired immunodeficiencies is ac - quired immune deficiency syndrome (AIDS). AIDS cripples the immune system by interfe - ring with the activity of helper T cells (see “A Closer Look” on pp. 454–455). ______________________✚

cytokines released by the activated T cells. Hence, antihistamine drugs are not helpful against the delayed types of allergies. Corticosteroid drugs are used to provide relief.

The most familiar examples of delayed hyper- sensitivity reactions are those classed as allergic contact dermatitis, which follow skin contact with poison ivy, some heavy metals (lead, mercury, and others), and certain cosmetic and deodorant chemi- cals. These agents act as haptens; after diffusing through the skin and attaching to body proteins, they are perceived as foreign by the immune system. The Mantoux and tine tests, skin tests for detecting tuberculosis, depend on delayed hypersensitivity reactions. When the tubercle antigens are injected just under the skin, a small, hard lesion forms if the person has been sensitized to the antigen.

Autoimmune Diseases Occasionally, the immune system loses its ability to distinguish friend from foe, that is, to tolerate self-antigens while still recognizing and attacking foreign antigens. When this happens, the body produces antibodies (auto-antibodies) and sensi- tized T cells that attack and damage its own tis- sues. This puzzling phenomenon is called au - toimmune disease because it is a person’s own immune system that produces the disorder.

Some 5 percent of adults in North America— two-thirds of them women—are afflicted with auto- immune disease. Most common are the following:

• Rheumatoid arthritis (RA), which systemati- cally destroys joints (see pp. 198–199)

• Myasthenia gravis, which impairs communi- cation between nerves and skeletal muscles (see p. 244)

• Multiple sclerosis (MS), which destroys the white matter (myelin sheaths) of the brain and spinal cord (see p. 257)

• Graves’ disease, in which the thyroid gland produces excessive amounts of thyroxine (see p. 345) in response to autoantibodies that mimic TSH

• Type 1 diabetes mellitus, which destroys pancreatic beta cells, resulting in deficient production of insulin (see p. 351)

• Systemic lupus erythematosus (SLE), a systemic disease that occurs mainly in young women and particularly affects the kidneys, heart, lungs, and skin

454

A CLOSER

LOOK AIDS: An Ongoing Pandemic

A pandemic is a widespread epi-demic, or ongoing outbreak of disease. The AIDS pandemic has been going on for over 30 years.

Although AIDS was first identified in the United States in 1981 among homosexual men and intravenous drug users of both sexes, it had begun afflicting the heterosexual population of Africa several years earlier. AIDS is characterized by severe weight loss, night sweats, swollen lymph nodes, and increas- ingly frequent infections, including a fungal brain infection called crypto- coccal meningitis, a rare type of pneumonia called pneumocystis pneumonia, an eye infection called cytomegalovirus retinitis, and the bizarre malignancy Kaposi’s sarcoma, a cancerlike condition of blood ves- sels evidenced by purple lesions of

the skin. Some AIDS victims develop slurred speech and severe dementia. The course of untreated AIDS is grim and thus far nearly inescapable, finally ending in complete debilitation and death from cancer or over- whelming infection.

AIDS is caused by a virus transmit- ted in blood, breast milk, semen, and vaginal secretions. Most commonly, the virus enters the body via blood transfusions or blood-contaminated needles and during intimate sexual contact in which the mucosa is torn or where open lesions caused by sex- ually transmitted infections allow the virus access to the blood. It is not transmitted by casual contact, because the virus dries and dies when exposed to air.

The virus, named HIV (human immunodeficiency virus), specifically

targets and destroys helper T cells, severely depressing the adaptive immune response. Without helper T cells, B cells and cytotoxic T cells cannot be activated or maintained, and the whole immune system is turned topsy-turvy. It is now clear that the virus multiplies steadily in the lymph nodes throughout most of the chronic asymptomatic period. Symptomatic AIDS appears when the lymph nodes can no longer con- tain the virus and the immune sys- tem collapses. This process can take 10 years or more. The virus also invades the brain, which accounts for the dementia of some AIDS patients.

As of 2014, nearly 37 million peo- ple worldwide were living with HIV. Since the pandemic began, 39 million people have died. Sub-Saharan Africa

27. What is the principal problem common to all immunodeficiency diseases?

28. What are two possible causes of autoimmune disease?

For answers, see Appendix A.

Did You Get It? 25. What is an allergy? 26. What causes the difficulty in breathing that occurs

with anaphylactic shock?

1 2 3 4

A new HIV virus emerges from an infected human cell.

455

has the highest population of infected individuals, accounting for nearly 70 percent of the world HIV cases. Because there is a 6-month “window” during which antibodies may develop after exposure to HIV, there are prob- ably 100 asymptomatic carriers of the virus for every newly diagnosed case. Moreover, the disease has a long incu- bation period (from a few months to 10 years) between exposure and the appearance of clinical symptoms. The Centers for Disease Control and Pre- vention (CDC) estimates that over 1.2 million U.S. residents are HIV infected and that one of every eight of those people does not realize he or she is infected.

The “face of AIDS” in the United States is changing. Victims have begun to include people who do not belong to the original high-risk groups. Before reliable testing of donated blood was available, some people contracted the virus from blood transfusions. Hemophiliacs have been especially vulnerable because the blood-clotting factors they need are isolated from pooled blood donations. Although manu- facturers began taking measures to kill the virus in 1984, by then an estimated 60 percent of the hemo- philiacs in the United States were already infected. The virus can also be transmitted from an infected mother to her fetus. Homosexual men still account for the bulk of

cases transmitted by sexual contact, but more and more heterosexuals are contracting this disease. Particu- larly disturbing is the near-epidemic increase in diagnosed cases among teenagers and young adults; among adolescents aged 10–19, the death rate from AIDS has tripled since 2000. As of 2013, AIDS is the eighth leading killer of all Americans ages 25 to 34 and ranks ninth for Ameri- cans 35 to 44.

Large-city hospitals are seeing more HIV/AIDS patients daily, and sta- tistics reporting the number of cases in the inner city, where intravenous drug use is the chief means of HIV transmission, are alarming. Currently, about 13 percent of the 12.2 million IV drug users are thought to have HIV. Furthermore, 75 percent of HIV/AIDS cases in newborns occur where drug abuse abounds.

Diagnostic tests to identify carriers of the AIDS virus are increasingly sophisticated. For example, a urine test provides a painless alternative to the standard HIV tests. However, no sure cure has yet been found. Over 100 drugs are now in the U.S. Food and Drug Administration pipeline, and several vaccines are undergoing clini- cal trials.

Clinically available are several anti- retroviral therapies (ART)—with drugs named like alphabet soup—that act by inhibiting the enzymes that the HIV virus needs to multiply in body

cells. The reverse transcriptase inhibi- tors, such as AZT, were early on the scene. AZT was followed by others, including ddC and 3TC. These were followed by protease inhibitors (saquinavir, ritonavir, and others) and fusion inhibitors, which block the virus’s entry into the cell in the first place. New drugs are constantly appearing. In the early days, treat- ment required taking many pills at varying times, and the schedule for taking them was difficult to follow. Now most patients take one pill daily that contains three drugs. Combina- tion therapy using drugs from each class delivers a one-two punch to the HIV virus, postpones drug resistance (a problem with single-drug therapy using AZT alone), and substantially reduces the amount of HIV in the blood. Earlier treatment and more effective combination drug therapy have turned AIDS into a controllable illness with a life expectancy well beyond 8 years. However, preventing infection is the preferred way to go. Perhaps, as urged in the media, the best defense is to practice “safer sex” by using condoms and knowing one’s sexual partner’s history, but the only foolproof method is sexual abstinence.

The lymphatic vessels, which bud from the veins of the blood vascular system, and the main clus- ters of lymph nodes are obvious by the fifth week of development. Except for the thymus and the spleen, the lymphoid organs are poorly developed before birth. However, soon after birth, they become heavily populated with lymphocytes as the immune system begins to function.

PART III: DEVELOPMENTAL ASPECTS OF THE LYMPHATIC SYSTEM AND BODY DEFENSES ➔ Learning Objectives

□□ Describe the origin of the lymphatic vessels.

□□ Describe the effects of aging on immunity.

456

SYSTEMS IN SYNC

Homeostatic Relationships between the Lymphatic System and Other Body Systems

Cardiovascular System

Urinary System

Muscular System

Digestive System

Reproductive System

Respiratory System

Nervous System

Integumentary System

Endocrine System

• Lymphatic vessels pick up leaked plasma and proteins; spleen destroys aged RBCs, stores iron, and removes debris from blood; immune cells protect cardiovascular organs from specific pathogens • Blood is the source of lymph; lymphatics develop from veins; blood provides the route for circulation of immune elements

• Lymphatic vessels pick up leaked fluid and proteins from urinary organs; immune cells protect urinary organs from specific pathogens • Urinary system eliminates wastes and maintains homeostatic water/ acid-base/electrolyte balance of the blood for immune cell functioning; urine flushes some pathogens out of the body

• Lymphatic vessels pick up leaked fluid and proteins from skeletal muscle; immune cells protect muscles from specific pathogens • The skeletal muscle “pump” aids the flow of lymph; muscles protect superficial lymph nodes

• Lymphatic vessels pick up leaked fluids and proteins from digestive organs; lymph transports some products of fat digestion to the blood; lymphoid nodules in the wall of the intestine prevent invasion of pathogens • The digestive system digests and absorbs nutrients needed by cells of lymphoid organs; gastric acidity inhibits pathogens’ entry into blood

• Lymphatic vessels pick up leaked fluid and proteins from reproductive organs; immune cells protect the organs from specific pathogens • Acidity of vaginal secretions prevents bacterial multiplication

• Lymphatic vessels pick up leaked fluid and proteins from respiratory organs; immune cells protect respiratory organs from specific pathogens; plasma cells in the respiratory mucosa secrete IgA to prevent pathogen invasion of deeper tissues • The lungs provide oxygen needed by lymphoid/immune cells and eliminate carbon dioxide; the pharynx houses tonsils; the respiratory “pump” aids lymph flow

• The lymphatic vessels pick up leaked plasma fluid, proteins, and immune cells from both central and peripheral nervous system structures; immune cells protect nervous system structures from specific pathogens • The nervous system innervates larger lymphatic vessels; lymphatics drain excess fluid from the brain

• Lymphatic vessels pick up leaked plasma fluid and proteins from the dermis; lymphocytes in lymph enhance the skin’s protective role by defending against specific pathogens • The skin’s keratinized epithelium provides a mechanical barrier to pathogens; acid pH of skin secretions inhibits growth of bacteria on skin

Skeletal System

• Lymphatic vessels pick up leaked plasma fluid and proteins from the periostea; immune cells protect bones from specific pathogens • The bones house hematopoietic tissue (red marrow), which produces the cells that populate the lymphoid organs and provide body immunity

• Lymphatic vessels pick up leaked fluids and proteins; lymph distributes hormones; immune cells protect endocrine organs from pathogens • The thymus produces hormones that promote development of lymphoid organs and “program” T lymphocytes

Lymphatic System/Immunity

Chapter 12: The Lymphatic System and Body Defenses 457

organs” (thymus and bone marrow) and then pop- ulate the other lymphoid tissues. Upon meeting “their” specific antigens, the T and B cells complete their development to fully mature immune cells.

Although the ability of our adaptive immune system to recognize foreign substances is deter- mined by our genes, the nervous system may help to control the activity of the immune response. The immune response is definitely impaired in individuals who are under severe stress. Our immune system normally serves us well through- out our lifetime, until old age. But during the later years, its efficiency begins to wane. As a result, the body becomes less able to fight infections and destroy cells that have become cancerous. Additionally, we become more susceptible to both autoimmune and immunodeficiency diseases.

Lymphatic system problems are relatively uncommon, but when they do occur they are painfully obvious. For example, when the lym- phatic vessels are blocked (as in elephantiasis, a tropical disease in which the lymphatics become clogged with parasitic worms) or when lymphatics are removed (as in radical breast surgery), severe edema may result. However, lymphatic vessels removed surgically do grow back in time.

Stem cells of the immune system originate in the spleen and liver during the first month of embryonic development. Later, the bone marrow becomes the predominant source of stem cells (hemocytoblasts), and it persists in this role into adult life. In late fetal life and shortly after birth, the young lymphocytes develop self-tolerance and immunocompetence in their “programming

Summary

PART I: THE LYMPHATIC SYSTEM (pp. 424–429) 1. The lymphatic system consists of the lymphatic ves-

sels, lymph nodes, and certain other lymphoid organs in the body.

2. Extremely porous, blind-ended lymphatic capillaries pick up excess fluid leaked from the blood capillar- ies. The fluid (lymph) flows into the larger lymphat- ics and finally into the blood vascular system through the right lymphatic duct and the left thoracic duct.

3. Lymph transport is aided by the muscular and respiratory pumps and by contraction of smooth muscle in the walls of the lymphatic vessels.

4. Lymph nodes are clustered along lymphatic vessels, and the lymphatic stream flows through them. Lymph nodes serve as multiplication sites for lym- phocytes; phagocytic cells within them remove bacteria, viruses, and the like from the lymph stream before it is returned to the blood.

5. Other lymphoid organs include the tonsils, which remove bacteria trying to enter the digestive or re- spiratory tracts; the thymus, a programming region for T lymphocytes; Peyer’s patches, which prevent bacteria in the intestine from penetrating deeper into the body; and the spleen, a red blood cell graveyard and blood reservoir.

PART II: BODY DEFENSES (pp. 429–455)

Innate Body Defenses (pp. 430–436)

1. Surface membranes (skin and mucous membranes) provide mechanical barriers to pathogens. Some produce secretions and/or have structural modifica- tions that enhance their defensive effects; the skin’s acidity, lysozyme, mucus, keratin, and ciliated cells are examples.

2. The inflammatory response prevents the spread of harmful agents, disposes of pathogens and dead tissue cells, and promotes healing. Protective leu- kocytes enter the area, fibrin walls off the area, and tissue repair occurs.

3. Natural killer cells are lymphocytes that act nonspe- cifically to lyse virus-infected and malignant cells.

4. Phagocytes (macrophages and neutrophils) engulf and destroy pathogens that penetrate epithelial bar- riers. This process is enhanced when the patho- gen’s surface is altered by attachment of antibodies and/or complement.

5. When complement (a group of plasma proteins) becomes fixed on the membrane of a foreign cell, lysis of the target cell occurs. Complement also enhances phagocytosis and the inflammatory and immune responses.

6. Interferons are a group of proteins synthesized by virus-infected cells. They prevent viruses from mul- tiplying in other body cells.

458 Essentials of Human Anatomy and Physiology

b. Other clone members become memory B cells, capable of mounting a rapid attack against the same antigen in subsequent meetings (second- ary humoral responses). These cells provide immunological “memory.”

c. Active humoral immunity is acquired during an infection or via vaccination and provides immu- nological memory. Passive immunity is conferred when a donor’s antibodies are injected into the bloodstream or when the mother’s antibodies cross the placenta. It does not provide immuno- logical memory.

d. Basic antibody structure

(1) Antibodies are proteins produced by sensi- tized B cells or plasma cells in response to an antigen, and they are capable of binding specifically with that antigen.

(2) An antibody is composed of four polypep- tide chains (two heavy and two light) that form a Y-shaped molecule.

(3) Each polypeptide chain has a variable and a constant region. Variable regions form anti- gen-binding sites, one on each arm of the Y. Constant regions determine antibody func- tion and class.

(4) Five classes of antibodies exist: IgM, IgA, IgD, IgG, and IgE. They differ structurally and functionally.

(5) Antibody functions include agglutination, complement fixation, neutralization, opsoniza - tion, and precipitation.

(6) Monoclonal antibodies are pure preparations of a single antibody type useful in diagnos- ing various infectious disorders and cancer and in treating certain cancers.

5. Cellular (cell-mediated) immune response

a. T cells are sensitized by binding simultaneously to an antigen and a self-protein displayed on the surface of a macrophage or another type of antigen-presenting cell. Clonal selection occurs, and clone members differentiate into effector T cells or memory T cells.

b. There are several different classes of T cells. Cytotoxic (killer) T cells directly attack and lyse infected and cancerous cells. Helper T cells inter- act directly with B cells bound to antigens. They also release cytokines, chemicals that enhance the killing activity of macrophages, attract other leukocytes, or act as helper factors that stimulate activity of B cells and cytotoxic T cells. Regulatory T cells “turn off” the normal immune response by releasing suppressor chemicals.

7. Fever enhances the fight against infectious micro- organisms by increasing metabolism (which speeds up repair processes) and by causing the liver and spleen to store iron and zinc (which bacteria need for multiplication).

Adaptive Body Defenses (pp. 436–455)

1. The immune system recognizes something as for- eign and acts to inactivate or remove it. The adaptive immune response is antigen specific, is systemic, and has memory. The two arms are humoral immu- nity (mediated by antibodies) and cellular immunity (mediated by living cells, T lymphocytes).

2. Antigens

a. Antigens are large, complex molecules (or parts of them) recognized as foreign by the body. Foreign proteins are the strongest antigens.

b. Complete antigens provoke an immune response and bind with products of that response (anti- bodies or sensitized lymphocytes).

c. Incomplete antigens, or haptens, are small mol- ecules that are unable to cause an immune response by themselves but do so when they bind to body proteins and the complex is recog- nized as foreign.

3. Cells of the adaptive defense system: an overview

a. Two main cell populations, lymphocytes and antigen-presenting cells, are involved in adaptive defense.

b. Lymphocytes arise from hemocytoblasts of bone marrow. T cells develop immunocompetence in the thymus and oversee cell-mediated immunity. B cells develop immunocompetence in bone marrow and provide humoral immunity. Immunocompetent lymphocytes seed lymphoid organs, where antigen challenge occurs, and circulate through blood, lymph, and lymphoid organs.

c. Immunocompetence is signaled by the appear- ance of antigen-specific receptors on the sur- faces of lymphocytes.

d. Macrophages arise from monocytes produced in bone marrow. They and other APCs phagocytize pathogens and present parts of the antigens on their surfaces, for recognition by T cells.

4. Humoral (antibody-mediated) immune response

a. Clonal selection of B cells occurs when antigens bind to their receptors, causing them to prolifer- ate. Most clone members become plasma cells, which secrete antibodies. This is called the pri- mary humoral response.

Chapter 12: The Lymphatic System and Body Defenses 459

c. Immunodeficiencies result from abnormalities in any immune element. Most serious is severe combined immunodeficiency disease (a congen- ital disease) and AIDS, an acquired immunodefi- ciency disease caused by a virus that attacks and cripples helper T cells.

PART III: DEVELOPMENTAL ASPECTS OF THE LYMPHATIC SYSTEM AND BODY DEFENSES (pp. 455–457) 1. Lymphatic vessels form by budding off veins. The

thymus and the spleen are the first lymphoid organs to appear in the embryo. Other lymphoid organs remain relatively undeveloped until after birth.

2. The immune response develops around the time of birth.

3. The ability of immunocompetent cells to recognize foreign antigens is genetically determined. Stress appears to interfere with normal immune response.

4. Efficiency of the immune response wanes in old age, and infections, cancer, immunodeficiencies, and autoimmune diseases become more prevalent.

6. Organ transplants include autografts, isografts, allografts, and xenografts. The most common graft is an allograft. Blood group and tissue matching are done to ensure the best match possible, and organ transplant is followed by immunosuppressive therapy.

7. Disorders of immunity

a. Autoimmune disease occurs when the body’s self-tolerance breaks down and antibodies and/ or T cells attack the body’s own tissues. Most forms of autoimmune disease result from the appearance of formerly hidden self-antigens or changes in the structure of self-antigens, and antibodies formed against foreign antigens that resemble self-antigens.

b. In hypersensitivity (allergy), the immune system overreacts to an otherwise harmless antigen, and tissue destruction occurs. Immediate (acute) hypersensitivity, as seen in hay fever, hives, and anaphylaxis, is due to IgE antibodies. Delayed hypersensitivity (for example, contact dermatitis) reflects activity of T cells, macrophages, and cytokines.

Multiple Choice

More than one choice may apply.

1. The body’s largest lymphatic vessel is the

a. lymph capillary.

b. lymphatic collecting vessel.

c. right lymphatic duct.

d. thoracic duct.

2. Which parts of the lymph node show increased activity when antibody production is high?

a. Germinal centers

b. Outer follicle

c. Medullary cords

d. Medullary sinuses

3. Which of the following drain into the right lymphatic duct?

a. Inguinal nodes

b. Cisterna chyli

c. Cervical nodes

d. Axillary nodes

4. Which of the following are part of MALT?

a. Tonsils

b. Thymus

c. Peyer’s patches

d. Any lymphoid tissue along the digestive tract

5. Developmentally, embryonic lymphatic vessels are most closely associated with the

a. veins.

b. arteries.

c. nerves.

d. thymus gland.

6. Which of the following are among the most common indicators of inflammation?

a. Phagocytosis

b. Edema

c. Leukocytosis

d. Pain

7. Chemical mediators of inflammation include

a. interferon.

b. complement.

c. histamine.

d. antibodies.

Review Questions Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

460 Essentials of Human Anatomy and Physiology

in other ways. Cite the common body locations and the importance of mucus, lysozyme, keratin, acid pH, and cilia.

18. What is complement? Besides bacterial lysis, what are some of the roles of complement?

19. Why is the term “swollen glands” a misnomer in the case of an infection?

20. Define immune response.

21. What happens to pathogens during phagocytosis?

22. Differentiate clearly between humoral and cellular immunity.

23. Although the immune system has two arms, it has been said, “No T cells, no immunity.” How is this so?

24. Define immunocompetence. What indicates that a B cell or T cell has developed immunocompetence? Where does the “programming phase” occur in the case of B cells?

25. Binding of antigens to receptors of immunocompe- tent lymphocytes leads to clonal selection. Describe the process of clonal selection. What nonlympho- cyte cell is a central actor in this process, and what is its function?

26. What is the connection between T cells and memory cells?

27. Describe the specific roles of helper, cytotoxic, and regulatory T cells in cell-mediated immunity. Which is thought to be disabled in AIDS?

28. Compare and contrast a primary and a secondary immune response. Which is more rapid, and why?

29. Describe the structure of an antibody, and explain the importance of its variable and constant regions.

30. Name the five classes of immunoglobulins. Which is most likely to be found attached to a B cell membrane? Which is most abundant in plasma? Which is important in allergic responses? Which is the first Ig to be released during the primary response? Which can cross the placental barrier?

31. How can mild fever be beneficial to the body?

32. Distinguish between immediate types of allergy and delayed allergic reactions in terms of cause and consequences.

33. What events can result in the loss of self-tolerance and autoimmune disease?

8. The second line of defense of the innate defense mechanisms includes

a. the mucous membranes.

b. the macrophages.

c. fever.

d. the natural killer cells.

9. Which of these antibody classes is usually arranged as a pentamer?

a. IgG

b. IgM

c. IgA

d. IgD

10. A damaged cell releases

a. perforin.

b. kinins.

c. histamine.

d. granzymes.

11. Which of the following antibody capabilities causes a transfusion reaction with A or B erythrocyte antigens?

a. Neutralization

b. Precipitation

c. Complement fixation

d. Agglutination

12. Which of the following is/are examples of auto- immune disease?

a. Type 1 diabetes

b. Multiple sclerosis

c. Graves’ disease

d. Rheumatoid arthritis

13. Helper T cells directly stimulate

a. B cells.

b. cytotoxic T cells.

c. memory B cells.

d. plasma cells.

Short Answer Essay

14. What is the most important function of the lymph nodes?

15. Compare and contrast blood, interstitial fluid, and lymph.

16. Where would you find Peyer’s patches and the appendix? What are their functions?

17. Besides acting as mechanical barriers, the skin and mucosae of the body contribute to body protection

Chapter 12: The Lymphatic System and Body Defenses 461

36. As an infant receives her first dose of oral polio vaccine, the nurse explains to her parents that the vaccine is a preparation of weakened virus. What type of immunity will the infant develop?

37. Some people with a deficit of IgA exhibit recurrent paranasal sinus and respiratory tract infections. Explain these symptoms.

38. Mr. James, an 80-year-old man, is grumbling about having to receive a flu shot every year. Flu viruses have a high mutation rate (undergo rapid genetic changes), which results in the appearance of new proteins on the flu virus’s “coat.” How does this help explain the need to get a flu shot each year?

39. Mrs. Morrow, a 59-year-old woman, has undergone a left radical mastectomy (removal of the left breast and left axillary lymph nodes and vessels). Her left arm is severely swollen and painful, and she is unable to raise it more than shoulder height. (a) Ex- plain her signs and symptoms. (b) Can she expect relief from these symptoms in time? How so?

40. Lymphocytes continuously circulate through the body using blood and lymph as their transport vehicles. What is the importance of this recirculation behavior?

41. Capillary permeability increase and plasma proteins leak into the interstitial fluid as part of the inflammatory process. Why is this desirable?

Critical Thinking and Clinical Application Questions

34. Would lack of memory B cells for a particular antigen impact the primary or the secondary humoral response?

35. Contrast lysozyme, perforin and granzymes, and a membrane attack complex. Include which cell(s) or proteins secrete or produce each.

462

The trillions of cells in the body require an abundant and endless supply of oxygen to carry out their vital functions. We cannot “do without” oxygen for even a little while, as we can without food or water. Furthermore, as cells use oxygen, they give off carbon dioxide, a waste product the body must get rid of.

The cardiovascular and respiratory systems share responsibility for supplying the body with oxygen and disposing of carbon dioxide. The respiratory system organs oversee the gas exchanges that occur between the blood and the external environment. Using blood as the trans- porting fluid, the cardiovascular system organs

transport respiratory gases between the lungs and the cells in the rest of the body. If either system fails, cells begin to die from oxygen starvation and accumulation of carbon dioxide.

Functional Anatomy of the Respiratory System ➔ Learning Objectives

□□ Name the organs forming the respiratory passageway from the nasal cavity to the alveoli of the lungs (or identify them on a diagram or model), and describe the function of each.

□□ Describe several protective mechanisms of the respiratory system.

The Respiratory System

HOW

WHAT

WHY

The respiratory system provides oxygen

to the body, disposes of carbon dioxide, and helps reguate

blood pH.

Gas exchange occurs in the air sacs of

the lungs, called alveoli, and at capillary beds

around the body.

Without oxygen, cells will eventually die.

Too much carbon dioxide in the blood will cause the blood pH to become acidic, which will

interfere with cellular function.

INSTRucToRS

New Building Vocabulary coaching Activities for this chapter are assignable in

Chapter 13: The Respiratory System 463

system. During breathing, air enters the nose by passing through the nostrils, or nares. The inte- rior of the nose consists of the nasal cavity, divided by a midline nasal septum. The olfactory receptors for the sense of smell are located in the mucosa in the slitlike superior part of the nasal cavity, just beneath the ethmoid bone.

The organs of the respiratory system include the nose, pharynx, larynx, trachea, bronchi and their smaller branches, and the lungs, which contain the alveoli (al-ve′o-li), or terminal air sacs (Figure 13.1). Because gas exchange with the blood happens only in the alveoli, the other respiratory system structures are really just conducting passageways that carry air through the lungs. The passageways from the nose to the larynx are called the upper respiratory tract, and those from the trachea to the alveoli are the lower respiratory tract. These con- ducting passageways also purify, humidify, and warm incoming air. Thus, the air finally reaching the lungs has fewer irritants (such as dust or bacte- ria) than the air outside, and it is warm and damp.

The Nose The nose, whether “button” or “hooked” in shape, is the only externally visible part of the respiratory

Nasal cavity

Trachea

Left main (primary) bronchus

Right main (primary) bronchus

Right lung

Left lung

Nostril

Oral cavity

Pharynx

Larynx

Diaphragm

Figure 13.1 The major respiratory organs shown in relation to surrounding structures.

➔ConCeptLinkRecall that any area open to the outside of the body, including respiratory passages, is lined with mucous membrane (mucosa), which is a “wet,” or moist, mem- brane (Chapter 4, p. 136).

The rest of the mucosa lining the nasal cavity, called the respiratory mucosa, rests on a rich net- work of thin-walled veins that warms the air as it flows past. In addition, the sticky mucus produced by this mucosa’s glands moistens the air and traps

464 Essentials of Human Anatomy and Physiology

and postnasal drip. Because the nasal mucosa is continuous throughout the respiratory tract and extends tentacle-like into the nasolacrimal (tear) ducts and paranasal sinuses, nasal cavity infections often spread to those regions as well. Sinusitis, or sinus inflammation, is difficult to treat and can cause marked changes in voice quality. When the passageways connecting the sinuses to the nasal cavity are blocked with mucus or infectious matter, the air in the sinus cavities is absorbed. The result is a partial vacuum and a sinus headache localized over the inflamed area. _______________________ ✚

The Pharynx The pharynx (far′inks) is a muscular passageway about 13 cm (5 inches) long that vaguely resem- bles a short length of red garden hose. Commonly called the throat, the pharynx serves as a common passageway for food and air (see Figure 13.2). It is continuous with the nasal cavity anteriorly via the posterior nasal aperture.

The pharynx has three regions (Figure 13.2a). Air enters the superior portion, the nasopharynx (na″zo-far′inks), from the nasal cavity and then descends through the oropharynx (o″ro-far′inks) and laryngopharynx (lah-ring″go-far′inks) to enter the larynx below. Food enters the mouth, then trav- els along with air through the oropharynx and laryn- gopharynx. But instead of entering the larynx, food is directed into the esophagus (ĕ-sof′ah-gus) posteri- orly by a flap called the epiglottis (ep″ı̆-glot′is).

The pharyngotympanic tubes, which drain the middle ears, open into the nasopharynx. The muco- sae of these two regions are continuous, so ear infections such as otitis media (o-ti′tis me′de-ah) may follow a sore throat or other types of pharyn- geal infections.

Clusters of lymphatic tissue called tonsils are also found in the pharynx. The single pharyngeal (far-rin′je-al) tonsil, often called the adenoid, is located high in the nasopharynx. The two pala- tine tonsils are in the oropharynx at the end of the soft palate, as are the two lingual tonsils, which lie at the base of the tongue. The tonsils also play a role in protecting the body from infec- tion (see Chapter 12).

Homeostatic Imbalance 13.3

If the pharyngeal tonsil becomes inflamed and swollen (as during a bacterial infection), it obstructs

incoming bacteria and other foreign debris, and lysozyme enzymes in the mucus destroy bacteria chemically. The ciliated cells of the nasal mucosa create a gentle current that moves the sheet of contaminated mucus posteriorly toward the throat (pharynx), where it is swallowed and digested by stomach juices. We are usually unaware of this important ciliary action, but when the external temperature is extremely cold, these cilia become sluggish, allowing mucus to accumulate in the nasal cavity and to dribble outward through the nostrils. This helps explain why you might have a “runny” nose on a crisp, wintry day.

The lateral walls of the nasal cavity are uneven, owing to three mucosa-covered projections, or lobes, called conchae (kong′ke). The conchae greatly increase the surface area of the mucosa exposed to the air (as shown in Figure 13.2). The conchae also increase the air turbulence in the nasal cavity. As the air swirls through the twists and turns, inhaled particles are deflected onto the mucus-coated surfaces, where they are trapped and prevented from reaching the lungs.

The nasal cavity is separated from the oral cav- ity below by a partition, the palate (pal′et). Anteriorly, where the palate is supported by bone, is the hard palate; the unsupported posterior part is the soft palate.

Homeostatic Imbalance 13.1

The genetic defect cleft palate (failure of the bones forming the palate to fuse medially) results in breath- ing difficulty as well as problems with oral cavity functions, such as nursing and speaking. ________✚

The nasal cavity is surrounded by a ring of para- nasal sinuses located in the frontal, sphenoid, eth- moid, and maxillary bones. (See Figure 5.13, p. 177.) The sinuses lighten the skull and act as resonance chambers for speech. They also produce mucus, which drains into the nasal cavities. The suctioning effect created by nose blowing helps to drain the sinuses. The nasolacrimal ducts, which drain tears from the eyes, also empty into the nasal cavities.

Homeostatic Imbalance 13.2

Cold viruses and various allergens can cause rhinitis (ri-ni′tis), inflammation of the nasal mucosa. The excessive mucus produced results in nasal congestion

Chapter 13: The Respiratory System 465

13 they were routinely removed. Now, with the widespread use of antibiotics, this is no longer necessary. _________________________________ ✚

The Larynx The larynx (lar′inks), or voice box, routes air and food into the proper channels and plays a role in

the nasopharynx and forces the person to breathe through the mouth. In mouth breathing, air is not properly moistened, warmed, or filtered before reaching the lungs. Many children routinely get this condition, called tonsillitis. Years ago the belief was that the tonsils, though protective, were more trouble than they were worth, and

Sphenoidal sinus Frontal sinus

Pharynx • Nasopharynx

• Oropharynx • Laryngopharynx

• Nasal meatuses (superior, middle, and inferior)

Nasopharynx

• Uvula

• Palatine tonsil

Posterior nasal aperture

• Opening of pharyngotympanic tube

• Pharyngeal tonsil

Oropharynx

Laryngopharynx

• Vocal fold

Esophagus

Trachea

(b) Detailed anatomy of the upper respiratory tract

(a) Regions of the pharynx

• Nasal conchae (superior, middle, and inferior)

• Nasal vestibule • Nostril

Nasal cavity

Hard palate

Soft palate

Tongue• Lingual tonsil

• Epiglottis

Hyoid bone

Larynx

• Thyroid cartilage

• Cricoid cartilage

Cribriform plate of ethmoid bone

Figure 13.2 Basic anatomy of the upper respiratory tract, sagittal section.

466 Essentials of Human Anatomy and Physiology

Homeostatic Imbalance 13.4

Because the trachea is the only way air can enter the lungs, tracheal obstruction is life-threatening. Many people have suffocated after choking on a piece of food that suddenly closed off the trachea (or the glottis of the larynx). The Heimlich maneuver, a procedure in which the air in a per- son’s own lungs is used to “pop out,” or expel, an obstructing piece of food, has saved many people from becoming victims of choking. The Heimlich maneuver is simple to learn and easy to do. However, it is best learned by demonstration because cracked ribs are a distinct possibility when it is done incorrectly. In some cases of obstructed breathing, an emergency tracheostomy (tra′ke-ost′o-me; surgical opening of the trachea) is done to provide an alternative route for air to reach the lungs. Individuals with tracheostomy tubes in place form huge amounts of mucus the first few days because of irritation to the trachea. Thus, they must be suctioned frequently during this time to pre- vent the mucus from pooling in their lungs. ______✚

The trachea is lined with a ciliated mucosa (Figure 13.3b). The cilia beat continuously in a superior direction. They are surrounded by goblet cells that produce mucus. The cilia propel this mucus, loaded with dust particles and other debris, away from the lungs to the throat, where it can be swallowed or spat out.

Homeostatic Imbalance 13.5

Smoking inhibits and ultimately destroys the cilia. Without these cilia, coughing is the only means of preventing mucus from accumulating in the lungs. Smokers with respiratory congestion should avoid medications that inhibit the cough reflex. ______✚

Did You Get It? 1. Why is nose breathing preferable to mouth

breathing? 2. What is the specific protective function of cilia in the

trachea?

For answers, see Appendix A.

The Main Bronchi The right and left main (primary) bronchi (brong′ki) are formed by the division of the tra- chea. Each main bronchus runs obliquely before it

speech. Located inferior to the pharynx (Figure 13.2b), it is formed by eight rigid hyaline cartilages and a spoon-shaped flap of elastic cartilage, the epiglottis. The largest of the hyaline cartilages is the shield-shaped thyroid cartilage, which pro- trudes anteriorly and is commonly called the Adam’s apple.

Sometimes referred to as the “guardian of the airway,” the epiglottis protects the superior open- ing of the larynx. During regular breathing, the epiglottis allows the passage of air into the lower respiratory passages. When we swallow food or fluids, the situation changes dramatically; the lar- ynx is pulled upward, and the epiglottis tips, form- ing a lid over the larynx’s opening. This routes food into the esophagus, which leads to the stom- ach, posteriorly. If anything other than air enters the larynx, a cough reflex is triggered to prevent the substance from continuing into the lungs. Because this protective reflex does not work when we are unconscious, never try to give fluids to an unconscious person when attempting to revive him or her.

• Palpate your larynx by placing your hand midway on the anterior surface of your neck. Swallow. Can you feel the larynx rising as you swallow?

Part of the mucous membrane of the larynx forms a pair of folds, called the vocal folds, or true vocal cords, which vibrate with expelled air. This ability of the vocal folds to vibrate allows us to speak. The vocal folds and the slitlike passage- way between them are called the glottis.

The Trachea Air entering the trachea (tra′ke-ah), or windpipe, from the larynx travels down its length (10–12 cm, or about 4 inches) to the level of the fifth thoracic vertebra, which is approximately midchest (see Figure 13.1).

The trachea is fairly rigid because its walls are reinforced with C-shaped rings of hyaline carti- lage (Figure 13.3a). These rings serve a double pur- pose. The open parts of the rings abut the esophagus and allow it to expand anteriorly when we swallow a large piece of food. The solid portions support the trachea walls and keep it patent, or open, in spite of the pressure changes that occur during breathing. The trachealis muscle lies next to the esophagus and completes the wall of the trachea posteriorly.

Chapter 13: The Respiratory System 467

Superiorly toward the mouth, to prevent unwanted substances from entering the lungs.A:

(a)

Hyaline cartilage

Submucosa

Mucosa

Seromucous gland in submucosa

(b)

Posterior

Lumen of trachea

Esophagus

Trachealis muscle

Adventitia

Figure 13.3 Anatomy of the trachea and esophagus. (a) A cross-sectional view of the structural relationship between the trachea and esophagus. (b) Cilia in the trachea. The cilia are the yellow, grasslike projections surrounded by the mucus-secreting goblet cells, which exhibit short microvilli (orange). (Scanning electron micrograph, 1,800×.)

In what direction is the power stroke of these cilia directed—superiorly toward the mouth or inferiorly toward the lungs?Q:

plunges into the medial depression (hilum) of the lung on its own side (see Figure 13.1). The right main bronchus is wider, shorter, and straighter than the left. Consequently, it is the more com- mon site for an inhaled foreign object to become lodged. By the time incoming air reaches the bronchi, it is warm, cleansed of most impurities, and humid. The smaller subdivisions of the main bronchi within the lungs are direct routes to the air sacs.

The Lungs ➔ Learning Objectives

□□ Describe the structure and function of the lungs and the pleural coverings.

□□ Describe the structure of the respiratory membrane.

The lungs are fairly large organs. They occupy the entire thoracic cavity except for the most central area, the mediastinum (me″de-as-ti′num), which

houses the heart, the great blood vessels, bronchi, the esophagus, and other organs (Figure 13.4, p. 468). The narrow superior portion of each lung, the apex, is just deep to the clavicle. The broad lung area resting on the diaphragm is the base. Each lung is divided into lobes by fissures; the left lung has two lobes, and the right lung has three.

The surface of each lung is covered with its own visceral serosa, called the pulmonary pleura or visceral pleura (ploor′ah), and the walls of the thoracic cavity are lined by the parietal pleura. The pleural membranes produce pleural fluid, slippery serous fluid, which allows the lungs to glide easily over the thorax wall during breathing and causes the two pleural layers to cling together. The pleurae can slide easily from side to side across one another, but they strongly resist being pulled apart. Consequently, the lungs are held tightly to the thorax wall, and the pleural space is more of a potential space than an actual one. As we describe shortly, this tight adherence of the pleural membranes is absolutely essential for nor- mal breathing. (Figure 13.4a shows the position of the pleurae on the lungs and the thorax wall.)

View Histology

468 Essentials of Human Anatomy and Physiology

Trachea

Apex of lung

Thymus

Right superior lobe

Horizontal fissure Right middle lobe

Oblique fissure Right inferior lobe

Heart (in pericardial cavity of mediastinum) Diaphragm

Base of lung

Left superior lobe

Oblique fissure

Left inferior lobe

Lung Pleural cavity Parietal pleura

Rib

Intercostal muscle

Visceral pleura

(a) Anterior view. The lungs flank mediastinal structures laterally.

Esophagus (in posterior mediastinum)

Right lung

Parietal pleura

Visceral pleura

Pleural cavity

Pericardial membranes

Sternum

Posterior Vertebra

Root of lung at hilum

Left lung

Thoracic wall

Pulmonary trunk

Heart (in mediastinum)

Anterior mediastinum

(b) Transverse section through the thorax, viewed from above

• Left main bronchus • Left pulmonary artery • Left pulmonary vein

Figure 13.4 Anatomical relationships of organs in the thoracic cavity. Part (a) includes a close-up illustration of the pleurae. In (b), the size of the pleural (and pericardial) cavity is exaggerated for clarity.

Chapter 13: The Respiratory System 469

est of the conducting passageways, the bronchioles (brong′ke-ōlz). Because of this branching and rebranching of the respiratory passageways within the lungs, the network formed is often referred to as the bronchial, or respiratory, tree. All but the smallest branches have reinforcing cartilage in their walls.

Respiratory Zone Structures and the Respiratory Membrane

The terminal bronchioles lead into respiratory zone structures, even smaller conduits that eventu- ally terminate in alveoli (al-ve′o-li; alveol = small cavity), or air sacs. The respiratory zone, which includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli, is the only site of gas exchange (Figure 13.5). All other respiratory

Homeostatic Imbalance 13.6

Pleurisy (ploo′r ı̆-se), inflammation of the pleurae, can be caused by insufficient secretion of pleural fluid. The pleural surfaces become dry and rough, resulting in friction and stabbing pain with each breath. Conversely, the pleurae may produce excessive amounts of fluid, which exerts pressure on the lungs. This type of pleurisy hinders breath- ing movements, but it is much less painful than the dry rubbing type. ________________________✚

The Bronchial Tree

After entering the lungs, the main bronchi subdivide into smaller and smaller branches (secondary and ter- tiary bronchi, and so on), finally ending in the small-

Respiratory bronchioles

Terminal bronchiole

Alveolar duct Alveoli

Alveolar duct

Alveolar sac

(a) Diagrammatic view of respiratory bronchioles, alveolar ducts, and alveoli

Alveolar pores

Alveolar duct

Alveolus

(b) Light micrograph of human lung tissue, showing the final divisions of the respiratory tree (120×)

Figure 13.5 Respiratory zone structures.

View Histology

470 Essentials of Human Anatomy and Physiology

membrane (air-blood barrier) (Figure 13.6). The respiratory membrane has gas (air) flowing past on one side and blood flowing past on the other. Gas exchange occurs by simple diffusion through the respiratory membrane—oxygen passes from the alveolar air into the capillary blood, and carbon dioxide leaves the blood to enter the alveoli (Figure 13.6). The total gas exchange surface provided by the alveolar walls of a healthy man is estimated to be 50 to 70 square meters, or approximately 40 times greater than the surface area of his skin!

The final line of defense for the respiratory system is in the alveoli. Remarkably efficient alve- olar macrophages, sometimes called “dust cells,” wander in and out of the alveoli picking up bacte- ria, carbon particles, and other debris. Also scat- tered amid the epithelial cells that form most of the alveolar walls are cuboidal surfactant-secreting cells, which look very different from the squamous epithelial cells. These cells produce a lipid (fat) molecule called surfactant, which coats the gas- exposed alveolar surfaces and is very important in lung function (as described on p. 483).

passages are conducting zone structures that serve as conduits to and from the respiratory zone. There are millions of the clustered alveoli, which resemble bunches of grapes, and they make up the bulk of the lungs. Consequently, the lungs are mostly air spaces. The balance of the lung tissue, its stroma, is mainly elastic connective tissue that allows the lungs to stretch and recoil (spring back) as we breathe. Thus, in spite of their relatively large size, the lungs weigh only about 2½ pounds, and they are soft and spongy.

The walls of the alveoli are composed largely of a single, thin layer of simple squamous epithelial cells. The thinness of their walls is hard to imagine, but a sheet of tissue paper is much thicker. Alveolar pores connect neighboring air sacs and provide alternative routes for air to reach alveoli whose feeder bronchioles have been clogged by mucus or otherwise blocked. The external surfaces of the alveoli are covered with a “cobweb” of pulmonary capillaries. Together, the alveolar and capillary walls, their fused basement membranes, and occa- sional elastic fibers construct the respiratory

Capillary endothelium

Fused basement membranes

Alveolar epithelium

Red blood cell

Capillary

Alveolus

CO2

Surfactant- secreting cell

Squamous epithelial cell of alveolar wall

Endothelial cell nucleus

Macrophage

Alveoli (gas- filled air spaces)

Red blood cell in capillary

Alveolar pores

Respiratory membrane

Capillary

Nucleus of squamous epithelial cell

Figure 13.6 Functional anatomy of the respiratory membrane (air-blood barrier). As shown in the left illustration, the respiratory membrane is composed of squamous epithelial cells of the alveoli, the

capillary endothelium, and the scant basement membranes between. Surfactant-secreting cells in the alveoli are also shown. Small pores connect neighboring alveoli. As shown in the right illustration,

oxygen diffuses from the alveolar air across this membrane into the pulmonary capillary blood; carbon dioxide diffuses from the pulmonary blood into the alveolus.

Chapter 13: The Respiratory System 471

oxygen to produce ATP and carbon dioxide, is the cornerstone of all energy-producing chemical reactions and occurs in all cells (see Chapter 14).

Mechanics of Breathing Breathing, or pulmonary ventilation, is a mechani- cal process that depends on volume changes occurring in the thoracic cavity. Keep this rule in mind about the mechanics of breathing: Volume changes lead to pressure changes, which lead to the flow of gases to equalize the pressure.

A gas, like a liquid, always conforms to the shape of its container. However, unlike a liquid, a gas fills its container. Therefore, in a large volume, the gas molecules will be far apart, and the pres- sure (created by the gas molecules hitting each other and the walls of the container) will be low. Assuming the amount of gas remains constant, if the volume is reduced, the gas molecules will be closer together, and the pressure will rise.

Did You Get It? 3. Name the order of the following parts of the human

respiratory system from the site where air enters the nostrils to the site where air reaches the end passages of the lungs—bronchi, larynx, nasal cavity, alveoli, trachea, pharynx, bronchioles.

4. Which main bronchus is the most likely site for an inhaled object to become lodged? Why?

5. The lungs are mostly passageways and elastic tissue. What is the role of the passageways? Of the elastic tissue?

6. Name the four structures that make up the respiratory zone.

For answers, see Appendix A.

Respiratory Physiology ➔ Learning Objectives

□□ Define cellular respiration, external respiration, internal respiration, pulmonary ventilation, expiration, and inspiration.

□□ Explain how the respiratory muscles cause volume changes that lead to air flow into and out of the lungs (breathing).

The major function of the respiratory system is to supply the body with oxygen and to dispose of carbon dioxide. To do this, at least four distinct events, collectively called respiration, must occur:

1. Pulmonary ventilation. Air must move into and out of the lungs so that the gases in the alveoli of the lungs are continuously refreshed. This process of pulmonary ventila- tion is commonly called breathing.

2. External respiration. Gas exchange (oxy- gen loading and carbon dioxide unloading) between the pulmonary blood and alveoli must take place. Remember that in external respiration, gas exchanges are being made between the blood and the body exterior.

3. Respiratory gas transport. Oxygen and car- bon dioxide must be transported to and from the lungs and tissue cells of the body via the bloodstream.

4. Internal respiration. At systemic capillaries, gas exchange occurs between the blood and cells inside the body.

Although only the first two processes are the special responsibility of the respiratory system, all four processes are necessary for gas exchange to occur. Note that cellular respiration, the use of

➔ConCeptLinkRecall that pressure changes also drive other processes in the body, such as filtration (passive transport; Chapter 3, pp. 102–103) and blood flow (Chapter 11). In these processes, substances move from high to low pressure and achieve a specific function, such as mem- brane transport or circulation.

Let’s see how volume changes relate to the two phases of breathing—inspiration, when air is flowing into the lungs, and expiration, when air is leaving the lungs.

Inspiration

When the inspiratory muscles, the diaphragm and external intercostals, contract, the size of the thoracic cavity increases. As the dome-shaped diaphragm contracts inferiorly, the superior- inferior dimension (height) of the thoracic cavity increases (Figure 13.7a, p. 472). Contraction of the external intercostals lifts the rib cage and thrusts the sternum forward, which increases the antero- posterior and lateral dimensions of the thorax. The lungs adhere tightly to the thorax walls (because of the surface tension of the fluid between the pleural membranes), so they are stretched to the new, larger size of the thorax. As intrapulmonary volume (the volume within the lungs) increases, the gases within the lungs spread out to fill the larger space. The resulting decrease in gas

472 Essentials of Human Anatomy and Physiology

Full inspiration (External

intercostals contract)

Expiration (External

intercostals relax)

Ribs are depressed as external intercostals relax

Diaphragm moves superiorly as it relaxes

(b) Expiration: Air (gases) flows out of the lungs

Ribs are elevated as external intercostals contract

Diaphragm moves inferiorly during contraction

External intercostal muscles

(a) Inspiration: Air (gases) flows into the lungs

Changes in lateral dimensionsChanges in anterior-posterior and superior-inferior dimensions

Figure 13.7 Rib cage and diaphragm positions during breathing. Black arrows represent the direction of diaphragm movement. (a) At the end of a normal inspiration: Chest is expanded laterally, rib cage is

elevated, and diaphragm is depressed. Lungs are stretched to the larger thoracic volume, causing the intrapulmonary pressure to fall and air to flow into the lungs. (b) At the end of a normal expiration: The chest is depressed, and the lateral

dimension is reduced; the rib cage is descended; and the diaphragm is elevated and dome-shaped. Lungs recoil to a smaller volume, intrapulmonary pressure rises, and air flows out of the lungs.

Chapter 13: The Respiratory System 473

pressure in the lungs produces a partial vacuum (pressure less than atmospheric pressure outside the body), which causes air to flow into the lungs. Air continues to move into the lungs until the intrapulmonary pressure equals atmospheric pres- sure (Figure 13.8a). This series of events is called inspiration (inhalation).

Expiration

Expiration (exhalation) in healthy people is largely a passive process that depends more on the natu- ral elasticity of the lungs than on muscle contrac- tion. As the inspiratory muscles relax and resume their initial resting length, the rib cage descends, the diaphragm relaxes superiorly, and the lungs recoil (Figure 13.7b). Thus, both the thoracic and intrapulmonary volumes decrease. As the intrapul- monary volume decreases, the gases inside the lungs are forced more closely together, and the intrapulmonary pressure rises to a point higher

V ol

um e

(L )

Volume of breath

0.5

–0.5

Inspiration Expiration

Intrapulmonary pressure

P re

ss ur

e re

la tiv

e to

a tm

os ph

er ic

p re

ss ur

e +2

–1

–2

(a)

(b)

Figure 13.8 changes in (a) intrapulmonary pressure and (b) air flow during inspiration and expiration.

A colored chest X-ray film showing a pneumothorax, or collapsed lung. The patient’s right lung (on left side of X-ray film, bright blue) has collapsed because of a buildup of air (purple) between the lung and chest wall.

than atmospheric pressure (see Figure 13.8a). This causes the gases to passively flow out to equalize the pressure with the outside.

However, if the respiratory passageways are narrowed by spasms of the bronchioles (as in asthma) or clogged with mucus or fluid (as in chronic bronchitis or pneumonia), expiration becomes an active process. In such cases of forced expiration, the internal intercostal muscles are activated to help depress the rib cage, and the abdominal muscles contract and help to force air from the lungs by squeezing the abdominal organs upward against the diaphragm.

Normally the pressure within the pleural space, the intrapleural pressure, is always negative. This is the major factor preventing lung collapse. If for any reason the intrapleural pressure becomes equal to the atmospheric pressure, the lungs immediately recoil and collapse.

Homeostatic Imbalance 13.7

During atelectasis (a″teh-lek′tuh-sis), or lung col- lapse, the lung is useless for ventilation. This phe- nomenon occurs when air enters the pleural space through a chest wound, but it may also result from a rupture of the visceral pleura, which allows air to enter the pleural space from the respiratory tract. The presence of air in the intrapleural space, which disrupts the fluid bond between the pleurae, is referred to as pneumothorax (nu″mo-tho′raks).

474 Essentials of Human Anatomy and Physiology

exhaled beyond tidal expiration, the expiratory reserve volume (ERV), is approximately 1,200 ml.

Even after the most strenuous expiration, about 1,200 ml of air still remains in the lungs and cannot voluntarily be expelled. This is the resid- ual volume. Residual volume air is important because it allows gas exchange to go on continu- ously even between breaths and helps to keep the alveoli open (inflated).

The total amount of exchangeable air (around 4,800 ml in healthy young men and 3,100 ml in healthy young women) is the vital capacity (VC). The vital capacity is the sum of the tidal volume plus the inspiratory and expiratory reserve vol- umes. Respiratory volumes of a male are summa- rized in Figure 13.9.

Note that some of the air that enters the respira- tory tract remains in the conducting zone passage- ways and never reaches the alveoli. This is called the dead space volume and during a normal tidal breath is about 150 ml. The functional volume—air that actually reaches the respiratory zone and con- tributes to gas exchange—is about 350 ml.

Respiratory capacities are measured with a spi- rometer (spi-rom′ĕ-ter). As a person breathes, the volumes of air exhaled can be read on an indica- tor, which shows the changes in air volume inside the apparatus. Spirometer testing is useful for evaluating losses in respiratory functioning and in following the course of some respiratory diseases. In pneumonia, for example, inspiration is obstructed, and the IRV and VC decrease. In emphysema, where expiration is hampered, the

Pneumothorax is reversed by drawing air out of the intrapleural space with a chest tube, which allows the lung to reinflate and resume its normal function. ____________________________________✚

Did You Get It? 7. What is the most basic function of respiration? 8. What causes air to flow out of the lungs during

expiration?

For answers, see Appendix A.

Respiratory Volumes and capacities ➔ Learning Objectives

□□ Define the following respiratory volumes: tidal volume, vital capacity, expiratory reserve volume, inspiratory reserve volume, and residual air.

□□ Name several nonrespiratory air movements, and explain how they modify or differ from normal respiratory air movements.

Many factors affect respiratory capacity—for exam- ple, a person’s size, sex, age, and physical condi- tion. Normal quiet breathing moves approximately 500 ml of air (about a pint) into and out of the lungs with each breath (Figure 13.8b). This respiratory volume is referred to as the tidal volume (TV).

As a rule, a person is capable of inhaling much more air than is taken in during a tidal breath. The amount of air that can be taken in forcibly above the tidal volume is the inspiratory reserve vol- ume (IRV), which is around 3,100 ml.

Similarly, after a normal expiration, more air can be exhaled. The amount of air that can be forcibly

Inspiratory reserve volume

3,100 ml

Tidal volume 500 ml

M ill

ili te

rs (

m l)

1,000

2,000

3,000

4,000

5,000

6,000

Expiratory reserve volume

1,200 ml

Residual volume 1,200 ml

Vital capacity 4,800 ml Total lung

capacity 6,000 ml

Figure 13.9 Graph of the various respiratory volumes in a healthy young adult male.

Chapter 13: The Respiratory System 475

Did You Get It? 9. Which is the largest respiratory volume—ERV, IRV, TV,

or VC? Which is the smallest? 10. Dead space volume accounts for about 150 ml of

tidal volume. How much of a tidal breath actually reaches the alveoli?

11. Jimmy broke a left rib when he fell from his bike. The rib punctured the chest wall. What happened to his left lung? Why?

For answers, see Appendix A.

External Respiration, Gas Transport, and Internal Respiration ➔ Learning Objectives

□□ Describe the process of gas exchanges in the lungs and tissues.

□□ Describe how oxygen and carbon dioxide are transported in the blood.

As explained earlier, external respiration is the actual exchange of gases between the alveoli and the blood (pulmonary gas exchange), and internal respiration is the gas exchange process that occurs between the blood and the tissue cells (systemic capillary gas exchange). Keep in mind that all gas exchanges obey the laws of diffusion; that is, movement occurs toward the area of lower con- centration of the diffusing substance. The relative amounts of O2 and CO2 in the alveolar tissues, and in the arterial and venous blood, are illustrated in Figure 13.10 (p. 476).

ERV is much lower than normal, and the residual volume is higher.

Nonrespiratory Air Movements Many situations other than breathing move air into or out of the lungs and may modify the normal respiratory rhythm. Coughs and sneezes clear the air passages of debris or collected mucus. Laughing and crying reflect our emotions. For the most part, these nonrespiratory air movements are a result of reflex activity, but some may be produced volun- tarily. (Examples of nonrespiratory movements are given in Table 13.1.)

Respiratory Sounds As air flows into and out of the respiratory tree, it produces two recognizable sounds that can be picked up with a stethoscope. Bronchial sounds are produced by air rushing through the large respiratory passageways (trachea and bronchi). Vesicular (vĕ-sik′u-lar) breathing sounds occur as air fills the alveoli. The vesicular sounds are soft murmurs that resemble a muffled breeze.

Homeostatic Imbalance 13.8

Diseased respiratory tissue, mucus, or pus can pro- duce abnormal sounds such as crackle (a bubbling sound), wheezing (a whistling sound), and rales. Rales are abnormal bronchial sounds produced by the presence of mucus or exudate in the lung pas- sages or by thickening of the bronchial walls. ____✚

Table 13.1 Nonrespiratory Air (Gas) Movements

Movement Mechanism and result

Cough Taking a deep breath, closing glottis, and forcing air superiorly from lungs against glottis. Then, glottis opens suddenly, and a blast of air rushes upward. Coughs act to clear the lower respiratory passageways.

Sneeze Similar to a cough, except that expelled air is directed through nasal cavities instead of through oral cavity. The uvula (u′vu-lah), a dangling tag of tissue hanging from the soft palate, becomes depressed and closes oral cavity off from pharynx, routing air through nasal cavities. Sneezes clear upper respiratory passages.

Crying Inspiration followed by release of air in a number of short expirations. Primarily an emotionally induced mechanism.

Laughing Essentially same as crying in terms of the air movements produced. Also an emotionally induced response.

Hiccups Sudden inspirations resulting from spasms of diaphragm; initiated by irritation of diaphragm or phrenic nerves, which serve diaphragm. The sound occurs when inspired air hits vocal folds of closed glottis.

Yawn Very deep inspiration, taken with jaws wide open; ventilates all alveoli (some alveoli may remain collapsed during normal quiet breathing).

476 Essentials of Human Anatomy and Physiology

body cells continually remove oxygen from blood, there is always more oxygen in the alveoli than in the blood. Thus, oxygen tends to diffuse from the air of the alveoli through the respiratory mem- brane into the more oxygen-poor blood of the pulmonary capillaries.

In contrast, as tissue cells remove oxygen from the blood in the systemic circulation, they release carbon dioxide into the blood. Because the con- centration of carbon dioxide is much higher in the pulmonary capillaries than it is in the alveolar air, it will diffuse from the blood into the alveoli and be flushed out of the lungs during expiration. Relatively speaking, blood draining from the lungs into the pulmonary veins is rich in oxygen and poor in carbon dioxide.

Gas Transport in the Blood

Oxygen is transported in the blood in two ways. Most attaches to hemoglobin molecules inside the red blood cells (RBCs) to form oxyhemoglobin (ok″se-he″mo-glo′bin)—HbO2 (Figure 13.11a). A very small amount of oxygen is carried dissolved in the plasma.

Carbon dioxide is twenty times more soluble in plasma compared to oxygen. As a result, most carbon dioxide is transported in plasma as bicar- bonate ion (HCO3

−), which plays a very impor- tant role in buffering blood pH. Carbon dioxide is enzymatically converted to bicarbonate ion within red blood cells; then the newly formed bicarbon- ate ions diffuse into the plasma.

Inspired air:

External respiration

Blood leaving lungs and entering tissue capillaries:

Pulmonary veins

O2 CO2 O2 CO2

O2CO2

O2 CO2

CO2 O2

Pulmonary arteries Alveolarcapillaries

Tissue capillaries

Tissue cells:

Alveoli of lungs:

Heart

Blood leaving tissues and entering lungs:

Systemic veins

Systemic arteries

Internal respiration

Figure 13.10 Gas exchanges in external and internal respiration. Note that these exchanges occur in the body according to the laws of diffusion.

External Respiration

During external respiration, dark red blood flow- ing through the pulmonary circuit is transformed into the scarlet river that is returned to the heart for distribution to the systemic circuit. Although this color change is due to oxygen pickup by hemoglobin in the lungs, carbon dioxide is being unloaded from the blood equally fast. Because

➔ConCeptLinkRemember that blood pH should remain between 7.35 and 7.45 (Chapter 10, p. 364). Buffers, such as bicar-bonate ion, minimize changes in pH in order to main- tain homeostasis.

A smaller amount of the transported CO2 (between 20 and 30 percent) is carried inside the RBCs bound to hemoglobin. Carbon dioxide binds to hemoglobin at a different site from oxygen, so it does not interfere with oxygen transport.

Before carbon dioxide can diffuse out of the blood into the alveoli, it must first be released from its bicarbonate ion form. For this to occur, bicarbon- ate ions (HCO3

–) must enter the red blood cells, where they combine with hydrogen ions (H+) to form carbonic acid (H2CO3). Carbonic acid quickly splits to form water and carbon dioxide, and carbon dioxide then diffuses from the blood into the alveoli.

Chapter 13: The Respiratory System 477

monoxide is a very successful competitor—so much so that it crowds out or displaces oxygen.

Carbon monoxide poisoning is the leading cause of death from fire. It is particularly danger- ous because it kills its victims softly and quietly. It does not produce the characteristic signs of hypoxia—cyanosis and respiratory distress. Instead, the victim becomes confused and has a throbbing headache. In rare cases, the skin becomes cherry red (the color of the hemoglobin- CO complex), which is often interpreted as a healthy “blush.” People with CO poisoning are given 100 percent oxygen until the carbon monox- ide has been cleared from the body. __________✚

Internal Respiration

Internal respiration, the exchange of gases between the blood and the tissue cells, is the

Homeostatic Imbalance 13.9

Impaired oxygen transport: Whatever the cause, inadequate oxygen delivery to body tissues is called hypoxia (hi-pok′se-ah). This condition is easy to recognize in light-skinned people because their skin and mucosae take on a bluish cast (become cyanotic; si″ah-not′ik). In dark-skinned individuals, this color change can be observed only in the mucosae and nailbeds. Hypoxia may be the result of anemia, pulmonary disease, or impaired or blocked blood circulation.

Carbon monoxide poisoning represents a unique type of hypoxia. Carbon monoxide (CO) is an odorless, colorless gas that competes vigor- ously with oxygen for the same binding sites on hemoglobin. Moreover, because hemoglobin binds to CO more readily than to oxygen, carbon

Alveoli (air sacs)

Hb + O2 HbO2

Red blood cell

Loading of O2 Unloading

of CO2

CO2O2

HCO3 –+ H+ H2CO3 CO2+ H2O

Bicar- bonate

ion

Carbonic acid

Water

Plasma

Pulmonary capillary

(Oxyhemoglobin is formed)

Tissue cells

Plasma

Red blood cell

Systemic capillary

HbO2 Hb + O2

CO2 + H2O H2CO3 H ++ HCO3

–

Water Carbonic acid

Bicar- bonate

ion

O2 CO2

Loading of CO2

Unloading of O2

(a) External respiration in the lungs (pulmonary gas exchange)

Oxygen is loaded into the blood, and carbon dioxide is unloaded.

(b) Internal respiration in the body tissues (systemic capillary gas exchange)

Oxygen is unloaded and carbon dioxide is loaded into the blood.

Figure 13.11 The loading and unloading of oxygen (o2) and carbon dioxide (co2) in the body. Note that although the conversion of CO2 to bicarbonate ion

and the reverse reaction are shown occurring in the plasma in (b), most such conversions occur within the red blood cells. Additionally, though not illustrated, some CO2 is carried

within red blood cells bound to hemoglobin, and very small amounts of O2 and CO2 are carried dissolved in blood plasma.

478 Essentials of Human Anatomy and Physiology

(Figure 13.12). Much remains to be discovered about neural regulation of breathing. A brief sum- mary of what we know follows.

• The medulla contains two respiratory centers. The first, the ventral respiratory group (VRG), contains both inspiratory and expiratory neu- rons that alternately send impulses to control the rhythm of breathing. The inspiratory neu- rons stimulate the diaphragm and external in- tercostal muscles via the phrenic and intercos- tal nerves, respectively, during quiet breathing. Impulses from the expiratory neurons stop the stimulation of the diaphragm and external in- tercostal muscles, allowing passive exhalation to occur. Impulses from the VRG maintain a normal quiet breathing rate of 12 to 15 respira- tions/minute, a rate called eupnea (ūp-ne′ah).

• The other medullary center, the dorsal respira- tory group (DRG), integrates sensory informa- tion from chemoreceptors and peripheral stretch receptors. The DRG communicates this information to the VRG to help modify breath- ing rhythms.

• The pons respiratory centers, which also com- municate with the VRG, help to smooth the transitions (modify timing) between inhalation and exhalation during activities such as sing- ing, sleeping or exercising.

The bronchioles and alveoli have stretch receptors that respond to extreme overinflation (which might damage the lungs) by initiating pro- tective reflexes. In the case of overinflation, the vagus nerves send impulses from the stretch receptors to the medulla; soon thereafter, inspira- tion ends and expiration occurs. This is one exam- ple of DRG integration during respiratory control.

During exercise, we breathe more vigorously and deeply because the brain centers send more impulses to the respiratory muscles. This respiratory pattern is called hyperpnea (hy-perp′ne-ah). After strenuous exercise, expiration becomes active, and the abdominal muscles and any other muscles capa- ble of depressing the ribs are used to aid expiration.

Homeostatic Imbalance 13.10

If the medullary centers are completely suppressed (as with an overdose of sleeping pills, morphine, or alcohol), respiration stops completely, and death occurs. ________________________________✚

opposite of what occurs in the lungs. In this pro- cess, oxygen leaves and carbon dioxide enters the blood (see Figure 13.11b). In the blood, carbon dioxide combines with water to form carbonic acid (H2CO3), which quickly releases bicarbonate ions. As previously mentioned, most of the con- version of carbon dioxide to bicarbonate ions occurs inside the RBCs, where a special enzyme (carbonic anhydrase) speeds up this reaction. Then the bicarbonate ions diffuse out into plasma, where they are transported. At the same time, oxy- gen is released from hemoglobin, and the oxygen diffuses quickly out of the blood to enter the cells. As a result of these exchanges, venous blood in the systemic circulation is much poorer in oxygen and richer in carbon dioxide than blood leaving the lungs.

Did You Get It? 12. Which type of cellular transport moves respiratory

gases between the blood and the body’s cells? 13. What is the major form in which CO2 is transported in

the blood? 14. What is cyanosis?

For answers, see Appendix A.

control of Respiration ➔ Learning Objectives

□□ Name the brain areas involved in control of respiration.

□□ Name several physical factors that influence respiratory rate.

□□ Explain the relative importance of oxygen and carbon dioxide in modifying breathing rate and depth.

□□ Explain why it is not possible to stop breathing voluntarily.

□□ Define apnea, hyperventilation, and hypoventilation.

Although our tidelike breathing seems so beauti- fully simple, its control is fairly complex. We will cover only the most basic aspects of the respira- tory controls.

Neural Regulation: Setting the Basic Rhythm

The activity of the respiratory muscles, the dia- phragm and external intercostals, is regulated by nerve impulses transmitted from the brain by the phrenic nerves and intercostal nerves. Neural centers that control respiratory rhythm and depth are located mainly in the medulla and pons

Chapter 13: The Respiratory System 479

will simply ignore messages from the cortex (our wishes) when the oxygen supply in the blood is getting low or blood pH is falling. All you need do to prove this is to try to talk normally or to hold your breath after running at breakneck speed for a few minutes. It simply cannot be done. Many tod- dlers try to manipulate their parents by holding their breath “to death.” Even though this threat causes many parents to become anxious, they need not worry because the involuntary controls take over and normal respiration begins again.

Emotional Factors Emotional factors also modify the rate and depth of breathing. Have you ever watched a horror movie with bated (held) breath or been so scared by what you saw that you were nearly panting? Have you ever touched something cold and clammy and gasped? All of these result

Nonneural Factors Influencing Respiratory Rate and Depth

Physical Factors Although the medulla’s respira- tory centers set the basic rhythm of breathing, physical factors such as talking, coughing, and exercising can modify both the rate and depth of breathing. We have already examined some of these factors in our discussion of nonrespiratory air movements. Increased body temperature also causes an increase in the rate of breathing.

Volition (Conscious Control) We all have con- sciously controlled our breathing pattern at one time or another. During singing and swallowing, breath control is extremely important, and many of us have held our breath for short periods to swim underwater. However, voluntary control of breathing is limited, and the respiratory centers

Figure 13.12 Neural control of respiration. This figure shows sensory inputs to the breathing control centers, which send signals through effector nerves to the respiratory muscles.

• Pons centers • Medulla centers

• Intercostal nerves

Breathing control centers:

Breathing control centers stimulated by:

• Phrenic nerves

O2 sensor in aortic body of aortic arch

Diaphragm

Intercostal muscles

CO2 and H + increase in

tissue. Nerve impulse from O2 sensor indicating O2 decrease

Efferent nerve impulses from medulla trigger contraction of inspiratory muscles:

Afferent impulses to medulla

exceeds the body’s need to remove carbon dioxide. In other words, during hyperventilation, we exhale more CO2 than we should, resulting in elevated blood pH (there is less carbonic acid).

By contrast, when blood starts to become slightly alkaline, or basic, breathing slows and becomes shallow. Slower breathing allows carbon dioxide to accumulate in the blood and brings the blood pH back into the normal range.

Indeed, control of breathing during rest is aimed primarily at regulating the hydrogen ion concentration in the brain. Hypoventilation (extremely slow or shallow breathing) or hyper- ventilation can dramatically change the amount of carbonic acid in the blood. Carbonic acid increases dramatically during hypoventilation and decreases substantially during hyperventilation. In both situ- ations, the buffering ability of the blood is likely to be overwhelmed; the result is acidosis or alkalosis.

Homeostatic Imbalance 13.12

Hyperventilation, often brought on by anxiety attacks, frequently leads to brief periods of apnea (ap′ne-ah), cessation of breathing, until the carbon dioxide builds up in the blood again. If breathing stops for an extended time, cyanosis may occur as a result of insufficient oxygen in the blood. In addi- tion, the hyperventilating person may get dizzy and faint because the resulting alkalosis causes cerebral blood vessels to constrict. Such attacks can be pre- vented by having the hyperventilating person breathe into a paper bag. Because exhaled air con- tains more carbon dioxide than atmospheric air, it upsets the normal diffusion gradient that causes carbon dioxide to be unloaded from the blood and leave the body. As a result, the carbon dioxide (and thus carbonic acid) level begins to rise in the blood, ending alkalosis. ____________________________ ✚

Did You Get It? 15. Which brain area is most important for setting the

basic respiratory rate and rhythm? 16. What chemical factor in blood normally provides the

most powerful stimulus to breathe?

For answers, see Appendix A.

Respiratory Disorders Learning Objective

□□ Describe symptoms and probable causes of coPD and lung cancer.

from reflexes initiated by emotional stimuli acting through centers in the hypothalamus.

Chemical Factors Although many factors can modify respiratory rate and depth, the most impor- tant factors are chemical—the levels of carbon dioxide and oxygen in the blood (see Figure 13.12). An increased level of carbon dioxide and a decreased blood pH are the most important stim- uli leading to an increase in the rate and depth of breathing. An increase in the carbon dioxide level can cause a decreased blood pH because high CO2 results in more carbonic acid, which lowers blood pH. However, a low blood pH could also result from metabolic activities independent of breathing. Changes in the carbon dioxide concen- tration or H+ ion concentration (which affects pH) in brain tissue seem to act directly on the medulla centers by influencing the pH of local tissues in the brain stem (see Figure 13.12).

Conversely, changes in oxygen concentration in the blood are detected by peripheral chemore- ceptor regions in the aorta (aortic body in the aor- tic arch) and in the fork of the common carotid artery (the carotid body). These, in turn, send impulses to the medulla when the blood oxygen level is dropping. When oxygen levels are low, these same chemoreceptors are also able to detect high carbon dioxide levels. Although every cell in the body must have oxygen to live, it is the body’s need to rid itself of carbon dioxide that is the most important stimulus for breathing. A decrease in the oxygen level becomes an important stimulus only when the level is dangerously low.

Homeostatic Imbalance 13.11

In people who retain carbon dioxide, such as peo- ple with emphysema, chronic bronchitis, or other chronic lung diseases, the brain no longer recog- nizes an increased level of carbon dioxide as impor- tant. In such cases, a dropping oxygen level becomes the respiratory stimulus. This interesting fact explains why such patients are always given a low level of oxygen; it helps to maintain the prevailing stimulus for breathing (a low oxygen level). _____________✚

The respiratory system in healthy individuals has homeostatic mechanisms. As carbon dioxide or other sources of acids begin to accumulate in the blood and tissues, and pH starts to drop, you begin to breathe more deeply and more rapidly. Hyperventilation is an increase in the rate and depth of breathing that

Chapter 13: The Respiratory System 481

Lung Cancer Lung cancer is the leading cause of cancer death for both men and women in North America, caus- ing more deaths than breast, prostate, and colorec- tal cancer combined. This is tragic, because lung cancer is largely preventable—nearly 90 percent of lung cancers result from smoking. Lung cancer is aggressive and metastasizes rapidly and widely, so most cases are not diagnosed until they are well advanced. The cure rate for lung cancer is notori- ously low; most victims die within 1 year of diag- nosis. The overall 5-year survival of people with lung cancer is about 17 percent.

Ordinarily, nasal hairs, sticky mucus, and the action of cilia do a fine job of protecting the lungs from irritants, but smoking overwhelms these cleans- ing devices, and they eventually stop functioning. Continuous irritation prompts the production of more mucus, but smoking slows the movements of cilia that clear this mucus and depresses lung mac- rophages. One result is a pooling of mucus in the lower respiratory tract and an increased frequency of pulmonary infections, including pneumonia and COPD. However, it is the irritating effects of the “cocktail” of toxic chemicals in tobacco smoke that ultimately lead to lung cancer.

The three most common types of lung cancer are (1) adenocarcinoma (40 percent of cases), which originates as solitary nodules in peripheral

Homeostatic Imbalance 13.13

The respiratory system is particularly vulnerable to infections because it is open to airborne patho- gens. We have already considered some of these inflammatory conditions, such as rhinitis and ton- sillitis. Now we will turn our attention to the most disabling respiratory disorders, the group of dis- eases collectively referred to as chronic obstructive pulmonary disease (COPD) and lung cancer. These disorders are “living proof” of cigarette smoking’s devastating effects on the body. Long known to promote cardiovascular disease, cigarettes are per- haps even more effective at destroying the lungs.

Chronic Obstructive Pulmonary Disease (COPD) The chronic obstructive pulmonary diseases, exemplified by chronic bronchitis and emphysema (em″f ı̆-se′mah), are a major cause of death and disability in the United States. These diseases have certain features in common: (1) Patients almost always have a history of smoking; (2) dyspnea (disp′ne-ah), difficult or labored breathing, often referred to as “air hunger,” occurs and becomes progressively worse; (3) coughing and frequent pulmonary infections are common; and (4) most COPD victims are hypoxic, retain carbon dioxide and have respiratory acidosis, and ultimately develop respiratory failure (see the figure).

In chronic bronchitis, the mucosa of the lower respiratory passages becomes severely inflamed and produces excessive mucus. The pooled mucus impairs ventilation and gas exchange and dramatically increases the risk of lung infec- tions, including pneumonias. Chronic bronchitis patients are sometimes called “blue bloaters” because hypoxia and carbon dioxide retention occur early in the disease and cyanosis is common.

In emphysema, the walls of some alveoli are destroyed, causing the remaining alveoli to be enlarged. In addition, chronic inflammation pro- motes fibrosis of the lungs. As the lungs become less elastic, the airways collapse during expiration and obstruct outflow of air. As a result, these patients use an incredible amount of energy to exhale, and they are always exhausted. Because air is retained in the lungs, oxygen exchange is surprisingly efficient, and cyanosis does not usu- ally appear until late in the disease. Consequently, emphysema sufferers are sometimes referred to as “pink puffers.” However, overinflation of the lungs leads to a permanently expanded barrel chest.

• Tobacco smoke • Air pollution

Respiratory failure

Chronic bronchitis • Excessive mucus produced • Chronic productive cough

Emphysema • Destruction of alveolar walls • Loss of lung elasticity

Breakdown of elastin in connective tissue of lungs

Continual bronchial irritation and inflammation

• Airway obstruction or air trapping • Dyspnea • Frequent infections

The pathogenesis of COPD.

482

Too Clean for Our Own Good? A

CLOSER LOOK

Ah, spring is here! You can tell by the sounds of wheezing and sneezing, not to mention the glut of advertisements for allergy medica- tions. It seems that just about every- one is allergic to something. A century ago, allergies were rare—a disease of rich city folk who had the best food, water, and medical care that money could buy. In cities of North America and Europe, allergies are now com- monplace, and asthma is a serious problem for many children.

Asthma is chronic inflammation of small lung passages. In allergic asthma, allergens trigger constriction of these passages, making it difficult to breathe. Over the past 20 years, the cases of asthma have increased by about 75 percent in the United States and Canada, yet asthma and allergy are still rare in developing countries.

What is behind this? The cause can’t be genetic, because the increase in allergy and asthma has been much too rapid. Clearly, there is something about living in an industrialized soci- ety that increases the risk of allergy. The observation that the increase in allergy is correlated with the decrease in childhood diseases (smallpox, mea- sles, whooping cough, and others) has led to the hygiene hypothesis. The twentieth century saw remarkable advances in public health. Improve- ments in public sanitation and clean water supplies together with vaccina- tion and the use of antibiotics have kept us living longer, healthier lives and have drastically reduced infant mortality in the industrialized world. The cost? Allergy.

Back in the “good old days” before improvements in sanitation, newborns

fell prey to a wide variety of childhood diseases. Those that survived child- hood diseases ended up with an immune system that had received a well-rounded education. Today, chil- dren are immunized against childhood infections and are often given antibi- otics that wipe out beneficial symbi- otic bacteria of the gut. Kept inside houses sterilized by antiseptic soaps, children don’t play in (and eat) dirt as much as they used to. According to the hygiene hypothesis, the result is that children’s immune systems aren’t exposed to the large numbers of pathogens and toxins they once were. Voilà! Allergy.

This simple story doesn’t tell all, though. First, it isn’t quite true that people in developing countries don’t have allergies—although they make IgE, they don’t respond when exposed to allergens. It turns out that many people in developing countries are infected by parasites, particularly intestinal worms. When cured of the worm infections, they get allergies. The worms seem to actively suppress the immune system.

A second problem with the hy - giene hypothesis is that allergy isn’t the only immune disorder on the rise. Autoimmune diseases in children and young adults, such as type 1 diabetes and multiple sclerosis, have also increased in industrial societies.

Normally, as regulatory T cells (a group of lymphocytes that help con- trol the immune response) become established during infancy, the host becomes tolerant to the antigens these cells have been exposed to (meaning they will not be attacked), preventing allergy and autoimmune

disorders. Regulatory T cell develop- ment may also depend on the pres- ence of the very bacteria depleted by antibiotics and excessive cleanliness. Thus, allergy and autoimmune disor- ders may be the result of faulty regu- latory T cell development.

Perhaps, in our haste to rid our- selves of household dust, dirt, bacte- ria, and parasites, we have eliminated the signals our immune systems need to fully mature. Scientists are now seeking ways to imitate these lost signals.

Instead of relying on injections of synthetic “dirt,” perhaps we need to rethink our obsession with cleanliness. Clearly, no one wants to return to the “good old days” when 25 percent of infants died, but it may be that allow- ing babies to get a little dirtier early in life should be looked at as a golden opportunity to prevent allergy and asthma later on!

Chapter 13: The Respiratory System 483

lung areas and develops from bronchial glands and alveolar cells; (2) squamous cell carcinoma (25–30 percent), which arises in the epithelium of the larger bronchi and tends to form masses that hollow out and bleed; and (3) small cell carcinoma (about 20 percent), which contains lymphocyte- like cells that originate in the main bronchi and grow aggressively in small grapelike clusters within the mediastinum, a site from which metas- tasis is especially rapid.

The most effective treatment for lung cancer is complete removal of the diseased lung lobes in an attempt to halt metastasis. However, removal is an option only if metastasis has not already occurred. In most cases, radiation therapy and chemother- apy are the only options, but most lung cancers are resistant to these treatments.

Fortunately, there are several new therapies on the horizon. These include (1) antibodies that tar- get specific molecules required by the tumor or that deliver toxic agents directly to the tumor, (2) cancer vaccines to stimulate the immune system, and (3) various forms of gene therapy to replace the defective genes that make tumor cells divide continuously. As clinical trials progress, we will learn about the effectiveness of these approaches. However, prevention is worth a pound of cure. Quitting smoking is a valuable goal. ___________✚

Did You Get It? 17. Alvin, a smoker, sees his doctor because he has a

persistent cough and is short of breath after very little exertion. He has a barrel chest and explains that it is difficult for him to exhale. What diagnosis will the doctor make?

For the answer, see Appendix A.

Developmental Aspects of the Respiratory System Learning Objective

□□ Describe normal changes that occur in respiratory system functioning from infancy to old age.

In the fetus, the lungs are filled with fluid, and all respiratory exchanges are made by the placenta. At birth, the fluid-filled pathway is drained, and the respiratory passageways fill with air. The alve- oli inflate and begin to function in gas exchange, but the lungs are not fully inflated for 2 weeks. The success of this change—that is, from nonfunc-

tional to functional respiration—depends on the presence of surfactant (sur-fak′tant), a fatty mol- ecule made by the cuboidal alveolar cells (see Figure 13.6). Surfactant lowers the surface tension of the film of water lining each alveolar sac so that the alveoli do not collapse between each breath. Surfactant is not usually present in large enough amounts to accomplish this function until late in pregnancy (between 28 and 30 weeks).

Homeostatic Imbalance 13.14

Infants who are born prematurely (before week 28) or in whom surfactant production is inade- quate (as in many infants born to diabetic moth- ers) have infant respiratory distress syndrome (IRDS). These infants have dyspnea within a few hours after birth and use tremendous amounts of energy just to reinflate their alveoli, which collapse after each breath. Although IRDS still accounts for over 20,000 newborn deaths a year, many babies with IRDS survive now because of the use of equipment that supplies a positive pressure con- tinuously and keeps the alveoli open until the maturing lungs produce enough surfactant.

Important birth defects of the respiratory system include cleft palate and cystic fibrosis. Cystic fibro- sis (CF), the most common lethal genetic disease in the United States, strikes in 1 out of every 2,400 births, and every day two children die of it. CF causes oversecretion of a thick mucus that clogs the respiratory passages and puts the child at risk for fatal respiratory infections. It affects other secretory processes as well. Most important, it impairs food digestion by clogging ducts that deliver pancreatic enzymes and bile to the small intestine. Also, sweat glands produce an extremely salty perspiration. At the heart of CF is a faulty gene that codes for the CFTR protein. CFTR works as a chloride ion (Cl−) channel to control the flow of Cl− into and out of cells. In people with the mutated gene, CFTR gets “stuck” in the endoplasmic reticulum and is unable to reach the plasma membrane to perform its normal role. Consequently, less Cl− is secreted from the cells and less water follows, resulting in the thick mucus typical of CF. The goal of CF research is to restore normal salt and water movement. Conventional ther- apy for CF is mucus-dissolving drugs, “clapping” the chest to loosen the thick mucus, and antibiotics to prevent infection. A surprisingly simple approach is to inhale hypertonic saline droplets. ____________✚

484

SYSTEMS IN SYNC

Homeostatic Relationships between the Respiratory System and Other Body Systems

• Respiratory system provides oxygen; disposes of carbon dioxide; carbon dioxide present in blood as HCO3

− and H2CO3 contributes to blood buffering

• Blood transports respiratory gases

• Respiratory system provides oxygen; disposes of carbon dioxide

• Kidneys dispose of metabolic wastes (other than carbon dioxide) of respiratory system organs

• Respiratory system provides oxygen needed for muscle activity; disposes of carbon dioxide

• The diaphragm and intercostal muscles produce volume changes necessary for breathing; regular exercise increases respiratory efficiency

• Respiratory system provides oxygen; disposes of carbon dioxide; tonsils in pharynx house immune cells

• Lymphatic system helps to maintain blood volume required for respiratory gas transport; immune system protects respiratory organs from pathogens and cancer

• Respiratory system provides oxygen; disposes of carbon dioxide

• Digestive system provides nutrients needed by respiratory system

• Respiratory system provides oxygen; disposes of carbon dioxide

• Respiratory system provides oxygen needed for normal neural activity; disposes of carbon dioxide

• Medullary and pons centers regulate respiratory rate/depth; stretch receptors in lungs and chemoreceptors in large arteries provide feedback

• Respiratory system provides oxygen; disposes of carbon dioxide

• Bones enclose and protect lungs and bronchi

• Respiratory system provides oxygen; disposes of carbon dioxide

• Skin protects respiratory system organs by forming surface barriers

• Respiratory system provides oxygen; disposes of carbon dioxide

• Epinephrine dilates the bronchioles; testosterone promotes laryngeal enlargement in males at puberty

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System

Reproductive System

Skeletal System

Integumentary System

Endocrine System Nervous System

Respiratory System

Chapter 13: The Respiratory System 485

cockroach droppings, dog dander, and fungi) with dyspnea, coughing, and wheezing. “A Closer Look” (see p. 482) discusses how hygiene may be related to recent increases in cases of asthma.

For many years, tuberculosis and pneumonia were the worst killers in the United States. Antibiotics have largely decreased their lethal threat, but they are still dangerous diseases. Newly diag- nosed, and frequently drug-resistant, tuberculosis cases in AIDS patients are increasing by leaps and bounds the world over, but at present the most damaging and disabling respiratory diseases are still those described previously, COPD and lung cancer.

As we age, the chest wall becomes more rigid and the lungs begin to lose their elasticity, resulting in a slowly decreasing ability to ventilate the lungs. Vital capacity decreases by about one-third by the age of 70. In addition, the blood oxygen level decreases, and sensitivity to the stimulating effects of carbon dioxide decreases, particularly in a reclining or supine position. As a result, many old people tend to become hypoxic during sleep and exhibit sleep apnea.

Additionally, many of the respiratory system’s protective mechanisms also become less efficient with age. Ciliary activity of the mucosa decreases, and the phagocytes in the lungs become sluggish. The net result is that the elderly population is more at risk for respiratory tract infections, partic- ularly pneumonia and influenza.

Did You Get It? 18. What happens to the alveoli if surfactant is not

produced in a newborn baby? What name is given to this condition?

For the answer, see Appendix A.

The respiratory rate is highest in newborn infants, about 40 to 80 respirations per minute. It continues to drop through life: In the infant it is around 30 per minute, at 5 years it is around 25 per minute, and in adults it is 12 to 18 per min- ute. However, the rate often increases again in old age. The lungs mature throughout childhood, and more alveoli are formed until young adulthood. But in people who begin smoking during the early teens, the lungs never completely mature, and those additional alveoli are lost forever.

Homeostatic Imbalance 13.15

In sudden infant death syndrome (SIDS), also called crib death, apparently healthy infants stop breathing and die in their sleep. Believed to be a problem with neural control of respiration, most cases occur in infants placed on their tummy to sleep. Since 1992, the recommendation to place infants on their back to sleep has resulted in a 40 percent decline in SIDS cases in the United States. _________________________ ✚

Except for sneezes or coughs (responses to irritants) and the occasional common cold that blocks the upper respiratory passageways with mucus, the respiratory system works so efficiently and smoothly that we are mostly not even aware of it. Most problems that occur are a result of external factors—for example, obstruction of the trachea by a piece of food or aspiration of food particles or vomitus (which leads to aspiration pneumonia). Some unfortunate individuals are plagued by asthma, caused by chronically inflamed, hypersensitive bronchial passages that respond to many irritants (such as dust mite and

2. The pharynx (throat) is a mucosa-lined, muscular tube with three regions—nasopharynx, orophar- ynx, and laryngopharynx. The nasopharynx func- tions in respiration only; the others serve both respiratory and digestive functions. The pharynx contains tonsils, which act as part of the body’s defense system.

3. The larynx (voice box) is a cartilaginous structure; most prominent is the thyroid cartilage (Adam’s

Summary

Functional Anatomy of the Respiratory System (pp. 462–471)

1. The nasal cavity (the chamber within the nose) is divided medially by a nasal septum and separated from the oral cavity by the palate. The nasal cavity is lined with mucosa, which warms, filters, and moistens incoming air. The mucosa also contains receptors for sense of smell. Paranasal sinuses and nasolacrimal ducts drain into the nasal cavity.

486 Essentials of Human Anatomy and Physiology

2. Respiratory volumes and capacities: Air volumes exchanged during breathing are TV, IRV, ERV, and VC (see p. 474 for values). Residual volume, which is nonexchangeable respiratory volume, allows gas exchange to go on continually.

3. Nonrespiratory air movements: These are voluntary or reflex activities that move air into or out of the lungs. They include coughing, sneezing, laughing, crying, hiccuping, and yawning.

4. Respiratory sounds: Bronchial sounds are sounds of air passing through large respiratory passageways. Vesicular breathing sounds occur as air fills alveoli.

5. External respiration, gas transport, and internal respi- ration: Gases move according to the laws of diffusion. Oxygen moves from alveolar air into pulmonary blood. Most oxygen is transported bound to hemoglo- bin inside RBCs. Carbon dioxide moves from pulmo- nary blood into alveolar air. Most carbon dioxide is transported as bicarbonate ion in plasma. At body tis- sues, oxygen moves from blood to the tissues, whereas carbon dioxide moves from the tissues to blood.

6. Control of respiration

a. Nervous control: Neural centers for control of respiratory rhythm are in the medulla and pons. The medulla is the respiratory rate “pacemaker.” Reflex arcs initiated by stretch receptors in the lungs also play a role in respiration by notifying neural centers of excessive overinflation.

b. Physical factors: Increased body temperature, exercise, speech, singing, and nonrespiratory air movements modify both rate and depth of breathing.

c. Volition: To a degree, breathing may be con- sciously controlled if it does not interfere with homeostasis.

d. Emotional factors: Some emotional stimuli can modify breathing. Examples are fear, anger, and excitement.

e. Chemical factors: Changes in carbon dioxide lev- els are the most important stimuli affecting respi- ratory rhythm and depth. Carbon dioxide acts directly on the medulla via its effect on reduc- ing the pH of blood and brainstem tissue. A ris- ing level of carbon dioxide or a drop in pH independent of CO2 in the medulla results in faster, deeper breathing; falling levels lead to shallow, slow breathing. Hyperventilation may result in apnea and dizziness due to alkalosis.

apple). The larynx connects the pharynx with the trachea inferiorly. The laryngeal opening (glottis) is hooded by the epiglottis, which prevents entry of food or drink into respiratory passages when swal- lowing. The larynx contains the vocal folds (true vocal cords), which produce sounds used in speech.

4. The trachea (windpipe) extends from the larynx to the main bronchi. The trachea is a smooth-muscle tube lined with a ciliated mucosa and reinforced with C-shaped cartilaginous rings, which keep the trachea patent (open).

5. Right and left main (primary) bronchi result from division of the trachea. Each plunges into the hilum of the lung on its side.

6. The lungs are paired organs flanking the mediasti- num in the thoracic cavity. Each lung is covered with pulmonary (visceral) pleura; the thorax wall is lined with parietal pleura. Pleural fluid decreases friction during breathing. The lungs are primarily elastic tissue and passageways of the bronchial tree. The smallest passageways end in clusters of air sacs called alveoli.

7. The conducting zone includes all respiratory pas- sages from the nasal cavity to the terminal bronchi- oles; they conduct air to and from the alveoli of the lungs. Respiratory bronchioles, alveolar ducts and sacs, and alveoli—which have thin walls through which all gas exchange occurs with pulmonary capillary blood—are respiratory zone structures.

Respiratory Physiology (pp. 471–480)

1. Mechanics of breathing: Gas travels from high- pressure to low-pressure areas. Pressure outside the body is atmospheric pressure; pressure inside the lungs is intrapulmonary pressure; pressure between the pleurae is intrapleural pressure (which is always negative). Movement of air into and out of the lungs is called pulmonary ventilation, or breathing. When inspiratory muscles contract, intra- pulmonary volume increases, its pressure decreases, and air rushes in (inspiration). When inspiratory muscles relax, the lungs recoil and air rushes out (expiration). Expansion of the lungs is helped by cohesion between pleurae and by the presence of surfactant in alveoli.

Complete an interactive tutorial: > Study Area > Interactive Physiology > Respiratory System > Pulmonary Ventilation

Chapter 13: The Respiratory System 487

Developmental Aspects of the Respiratory System (pp. 483–485)

1. Premature infants have problems keeping their lungs inflated because of lack of surfactant in their alveoli. (Surfactant is formed late in pregnancy.)

2. The most important birth defects of the respiratory system are cleft palate and cystic fibrosis.

3. The lungs continue to mature until young adult- hood. Smoking in the teen years can lead to irrepa- rable damage.

4. During youth and middle age, most respiratory sys- tem problems are a result of external factors, such as infections and substances that physically block respiratory passageways.

5. In old age, the thorax becomes more rigid and lungs become less elastic, leading to decreased vital capacity. Protective mechanisms of the respi- ratory system become less effective in elderly per- sons, predisposing them to more respiratory tract infections.

Oxygen, monitored by peripheral chemorecep- tors, is less important as a respiratory stimulus in healthy people. It is the stimulus for people whose systems have become accustomed to a high level of carbon dioxide as a result of disease.

Respiratory Disorders (pp. 480–483)

1. The major respiratory disorders are COPD (emphy- sema and chronic bronchitis) and lung cancer. Cigarette smoking is a significant cause of both.

2. Emphysema is characterized by permanent destruc- tion and enlargement of alveoli. The lungs lose their elasticity, and expiration becomes an active process.

3. Chronic bronchitis is characterized by excessive pro- duction and pooling of mucus in lower respiratory passageways, which severely impairs ventilation and gas exchange. Patients may become cyanotic as a result of chronic hypoxia.

4. Lung cancer is extremely aggressive and metasta- sizes rapidly. The three most common lung cancers are adenocarcinoma, squamous cell carcinoma, and small cell carcinoma.

Review Questions

Multiple choice

More than one choice may apply.

1. When you exhale, air flows through respiratory structures in which sequence?

a. Alveolus, bronchiole, bronchus, larynx, trachea, pharynx, nasal cavity

b. Alveolus, trachea, bronchus, bronchiole, larynx, pharynx, nasal cavity

c. Alveolus, bronchus, bronchiole, trachea, larynx, pharynx, nasal cavity

d. Alveolus, bronchiole, bronchus, trachea, larynx, pharynx, nasal cavity

2. The structures that open into the nasal cavity include the

a. sphenoidal sinus.

b. esophagus.

c. pharyngotympanic tubes.

d. nasolacrimal ducts.

3. Which of the following is (are) located in the larynx?

a. Glottis

b. Epiglottis

c. Hyoid bone

d. Thyroid cartilage

4. Lung collapse is prevented by

a. high surface tension of alveolar fluid.

b. adhesion of the pleural membranes.

c. high pressure in the pleural cavities.

d. high elasticity of lung tissue.

5. Disorders classified as COPDs include

a. pneumonia.

b. emphysema.

c. bronchitis.

d. sleep apnea.

6. Which of the following changes will accompany the loss of lung elasticity associated with aging?

a. Increase in tidal volume

b. Increase in inspiratory reserve volume

c. Increase in residual volume

d. Increase in vital capacity

7. Which of the following is (are) not present in the alveoli?

a. Surfactant-secreting cells

b. Dust cells

c. Red blood cells

d. Squamous epithelial cells

e. Capillary endothelial cells

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

488 Essentials of Human Anatomy and Physiology

17. The contraction of the diaphragm and the external intercostal muscles begins inspiration. What happens, in terms of volume and pressure changes in the lungs, when these muscles contract?

18. How can you recognize whether an isolated lung is a right or a left lung?

19. What determines in which direction carbon dioxide and oxygen will diffuse in the lungs? In the tissues?

20. Name the two major brain areas involved in the nervous control of breathing.

21. Why are elderly people more at risk for respiratory tract infections?

22. Name two chemical factors that modify respiratory rate and depth. Which is usually more important?

23. Define hyperventilation. If you hyperventilate, do you retain or expel more carbon dioxide? What effect does hyperventilation have on blood pH? On breathing rate?

24. Compare and contrast the signs and symptoms of emphysema and chronic bronchitis.

25. Contrast the stimulus for and the result of hyperventilation with the stimulus for and result of hyperpnea.

Short Answer Essay

8. Clearly explain the difference between external and internal respiration.

9. Trace the route of air from the nares to an alveolus.

10. Why is it important that the trachea be reinforced with cartilaginous rings? What is the advantage of the fact that the rings are incomplete posteriorly?

11. Name the lymphatic tissue present in the oropharynx.

12. What distinguishes the mucosa in the slitlike superior part of the nasal cavity from that in the rest of the nasal cavity?

13. In terms of general health, what is the importance of the fact that the pharyngotympanic tubes and the sinuses drain into the nasal cavities and nasopharynx?

14. What is it about the structure of the alveoli that makes them an ideal site for gas exchange?

15. What do TV, ERV, and VC mean?

16. Name several nonrespiratory air movements, and explain how each differs from normal breathing.

26. Anne has a bad cold, and she is also suffering from a continuing headache. An X-ray image of her head shows that her nasal sinuses have a cloudy white aspect, indicating the presence of mucus or infectious matter. Does this explain her headache? Justify your answer.

27. Why doesn’t Laney have to worry when her 3-year- old son Ethan threatens to “hold his breath till he dies”?

28. Mr. Alvarez bumped a bee’s nest while making repairs on his roof. Not surprisingly, he was promptly stung several times. Because he knew he was allergic to bee stings, he rushed to the hospital. While waiting, he went into a state of shock and had extreme difficulty breathing. Examination showed his larynx to be edematous, and a trache- ostomy was performed. Why is edema of the larynx

likely to obstruct the airway? What is a tracheos- tomy, and what purpose does it serve?

29. As a result of a stroke, Mrs. Minnick’s swallowing is uncoordinated. What detrimental effect might this have on her ability to breathe?

30. An elderly lady living alone is found dead on her bathroom floor. There are no signs of violence, but the rescue team notices a gas-powered heating sys- tem and the lack of proper ventilation in the room. What is a likely cause of the death of this lady?

31. Nineteen-year-old Tyler stumbled into the drug- store gasping for breath. Blood was oozing from a small hole in his chest wall. When the paramedics arrived they said that Tyler had been shot and suf- fered a pneumothorax and atelectasis. What do both of these terms mean, and how do you explain his respiratory distress? How will it be treated?

critical Thinking and clinical Application Questions

489

PART I: AnATomy And PhysIology of The dIgesTIve sysTem

Anatomy of the digestive system ➔ Learning Objectives

□□ name the organs of the alimentary canal and accessory digestive organs, and identify each on an appropriate diagram or model.

Children are fascinated by the workings of the digestive system: They relish crunching a potato chip, delight in making “mustaches”

The digestive system and Body Metabolism14

WHAT

HOW

WHY

The digestive system breaks down the food you

eat into nutrients needed for metabolic processes, such as

making ATP, and rids the body of materials that cannot be

used, such as fiber.

Chewing breaks down food into small pieces easy

for enzymes to access; enzymes then chemically digest food into nutrients

that are actively transported into blood and delivered to cells

around the body.

The digestive system is essential for providing the

body with the energy and building blocks it requires to

maintain life.

with milk, and giggle when their stomach “growls.” As adults, we know that a healthy digestive system is essential for good health because it converts food into the raw materials that build and fuel our body’s cells. Specifically, the digestive system ingests food (takes it in), digests it (breaks it down) into nutrient molecules, absorbs the nutrients into the blood- stream, and then defecates (excretes) to rid the body of the indigestible wastes.

InsTRucToRs

New Building Vocabulary Coaching Activities for this chapter are assignable in

490 Essentials of Human Anatomy and Physiology

digestive functions (ingests, digests, absorbs, and defecates) as it propels the foodstuffs along its length. The accessory organs (teeth, tongue, and several large digestive glands) assist digestion in various ways, as described shortly.

organs of the Alimentary canal The alimentary canal, also called the gastroin- testinal (GI) tract or gut, is a continuous, coiled,

□□ Identify the overall function of the digestive system as digestion and absorption of foodstuffs, and describe the general activities of each digestive system organ.

We can separate the organs of the digestive system into two main groups: those forming the alimen- tary (al″ĕ-men′tar-e; aliment = nourish) canal and the accessory digestive organs (Figure 14.1). The alimentary canal performs the whole menu of

Mouth (oral cavity)

Tongue

Esophagus

Liver

Gallbladder

Anus

Duodenum

Jejunum

Ileum

Small intestine

Parotid gland

Sublingual gland Submandibular gland

Salivary glands

Pharynx

Stomach Pancreas

(Spleen)

Transverse colon

Descending colon

Ascending colon

Cecum

Sigmoid colon

Rectum

Appendix

Anal canal

Large intestine

Figure 14.1 The human digestive system: Alimentary canal and accessory organs. The organs of the alimentary canal have boldfacedlabels, whereas accessory organs do not. The liver and gallbladder are reflected superiorly and to the right side of the body.

Chapter 14: The Digestive System and Body Metabolism 491

Mouth

Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane–lined cavity (Figure 14.2). The lips, or labia, protect its ante- rior opening; the cheeks form its lateral walls; the hard palate forms its anterior roof; and the soft palate forms its posterior roof. The uvula (u′vu- lah) is a fleshy fingerlike projection of the soft pal- ate, which dangles from the posterior edge of the soft palate. The space between the lips and cheeks externally and the teeth and gums internally is the vestibule. The area contained by the teeth is the oral cavity proper. The muscular tongue occu- pies the floor of the mouth. The tongue has sev- eral bony attachments—two of these are to the hyoid bone and the styloid processes of the skull. The lingual frenulum (ling′gwal fren′u-lum), a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements (see Figure 14.2a).

Homeostatic Imbalance 14.1

Children born with an extremely short lingual frenulum are often referred to as “tongue-tied” because movement of the tongue is restricted, leading to distorted speech. This congenital

hollow muscular tube that winds through the ven- tral body cavity from mouth to anus. Its organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The large intestine leads to the terminal opening, or anus. In a cadaver, the alimentary canal is approximately 9 m (about 30 feet) long, but in a living person, it is considerably shorter because of its muscle tone. Food material within this tube is technically out- side the body, because it has contact only with cells lining the tract and the tube is open to the external environment at both ends. This relation- ship becomes clearer if you think of your ali- mentary canal as an elongated doughnut. Then it’s easy to see that if you stick your finger through a doughnut hole, your finger is not really in the doughnut. Each organ of the ali- mentary canal is described next (find the organs in Figure 14.1).

Nasopharynx

Oropharynx

Epiglottis

Laryngopharynx

Esophagus

Palatine tonsil

Lingual tonsil

Uvula

Hard palate

Tongue

Gingivae (gums)

(b)

Upper lip

Soft palate

Hard palate

Oral cavity

Lips (labia)

Lingual frenulum

Vestibule

Tongue

Hyoid bone

Trachea

(a)

Figure 14.2 Anatomy of the mouth (oral cavity). (a) Sagittal view of the oral cavity and pharynx. (b) Anterior view of the oral cavity.

What is the protective value of having several sets of tonsils at the oral entrance to the pharynx?Q:

The mouth is a favored site of body entry by bacteria, and the presence of the tonsils (lymphocyte- and

macrophage-filled organs) is very effective in preventing many pathogens from getting further

into the digestive tract.

492 Essentials of Human Anatomy and Physiology

The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same four tissue layers, or tunics (Figure 14.3):

1. The mucosa is the innermost layer, a moist mucous membrane that lines the hollow cav- ity, or lumen, of the organ. It consists primar- ily of surface epithelium plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer. Beyond the esophagus, which has a friction-resisting strati- fied squamous epithelium, the epithelium is mostly simple columnar.

2. The submucosa is found just beneath the mucosa. It is soft connective tissue contain- ing blood vessels, nerve endings, mucosa- associated lymphoid tissue (MALT), and lymphatic vessels.

3. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.

4. The serosa is the outermost layer of the wall. As half of a serous membrane pair, the vis- ceral peritoneum (per″ ı̆-to-ne′um) consists of a single layer of flat, serous fluid– producing cells. The visceral peritoneum is continu- ous with the slippery parietal peritoneum, which lines the abdominopelvic cavity by way of a membrane extension, the mesentery (mes′en-ter″e). (These relationships are illus- trated in Figure 14.5 on p. 495.)

Homeostatic Imbalance 14.2

When the peritoneum is infected, a condition called peritonitis (per″ ı̆-to-ni′tis), the peritoneal membranes tend to stick together around the infection site. This helps to seal off and localize many intraperitoneal infections (at least initially), providing time for macrophages in the lymphatic tissue to mount an attack. ____________________✚

The alimentary canal wall contains two impor- tant intrinsic nerve plexuses—the submucosal nerve plexus and the myenteric (mi-en′ter-ik; “intestinal muscle”) nerve plexus. These net- works of nerve fibers are actually part of the auto- nomic nervous system. They help regulate the mobility and secretory activity of GI tract organs.

condition can be corrected surgically by cutting the frenulum. _______________________________✚

At the posterior end of the oral cavity are paired masses of lymphatic tissue, the palatine tonsils. The lingual tonsil covers the base of the tongue just beyond. The tonsils, along with other lymphatic tissues, are part of the body’s defense system (look back at Figure 13.2, p. 465). When the tonsils become inflamed and enlarge, they par- tially block the entrance into the throat (pharynx), making swallowing difficult and painful, as described in Homeostatic Imbalance 13.3, p. 464.

As food enters the mouth, it is mixed with saliva and masticated (chewed). The cheeks and closed lips hold the food between the teeth during chewing. The nimble tongue continuously mixes food with saliva and initiates swallowing. Thus, the breakdown of food begins before it has even left the mouth.

➔ConCeptLinkRecall that papillae containing taste buds, or taste receptors, are found on the tongue surface (see Chapter 8, p. 326). So, besides its food-manipulating function, the tongue allows us to enjoy and appreciate the food we eat.

Pharynx

From the mouth, food passes posteriorly into the oropharynx and laryngopharynx, both of which are common passageways for food, fluids, and air. The pharynx is subdivided into the nasopharynx, part of the respiratory passageway; the orophar- ynx, posterior to the oral cavity; and the laryngo- pharynx, which is continuous with the esophagus inferiorly (see Chapter 13).

The walls of the pharynx contain two skeletal muscle layers. The cells of the outer layer run lon- gitudinally; those of the inner layer (the constrictor muscles) run around the wall in a circular fashion. Alternating contractions of these two muscle layers propel food through the pharynx inferiorly into the esophagus. Later we describe this propelling mechanism, called peristalsis (per″ ı̆-stal′sis).

Esophagus

The esophagus (ĕ-sof′ah-gus), or gullet, runs from the pharynx through the diaphragm to the stomach. About 25 cm (10 inches) long, it is essen- tially a passageway that conducts food (by peri- stalsis) to the stomach.

Chapter 14: The Digestive System and Body Metabolism 493

The stomach varies from 15 to 25 cm (6 to 10 inches) in length, but its diameter and volume depend on how much food it contains. When it is full, it can hold about 4 liters (1 gallon) of food. When it is empty, it collapses inward on itself, and its mucosa is thrown into large folds called rugae (roo′ge; ruga = wrinkle, fold).

The lesser omentum (o-men′tum), a double layer of peritoneum, extends from the liver to the lesser curvature of the stomach. The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall (Figure 14.5a, p. 495). The

stomach

The C-shaped stomach (Figure 14.4a and b, p. 494) is on the left side of the abdominal cavity, nearly hidden by the liver and diaphragm. Different regions of the stomach have been named. The cardial region, or cardia (named for its posi- tion near the heart), surrounds the cardioesopha- geal (kar″de-o-ĕ-sof″ah-je′al) sphincter, through which food enters the stomach from the esopha- gus. The fundus is the expanded part of the stom- ach lateral to the cardial region. The body is the midportion of the stomach; in the body, the con- vex lateral surface is the greater curvature, and its concave medial surface is the lesser curva- ture. As it narrows inferiorly, the body becomes the pyloric antrum and then the funnel-shaped pylorus (pi-lor′us), the terminal part of the stom- ach. The pylorus is continuous with the small intestine through the pyloric sphincter, or pyloric valve.

Intrinsic nerve plexuses • Myenteric nerve plexus • Submucosal nerve plexus

Submucosal glands

Visceral peritoneum

Mucosa • Surface epithelium • Lamina propria • Muscle layer

Submucosa

Muscularis externa • Longitudinal muscle layer • Circular muscle layer

Serosa (visceral peritoneum)

Lumen

Lymphoid tissueDuct of gland outside alimentary canal

Gland in mucosa

Nerve Artery VeinMesentery

Figure 14.3 Basic structure of the alimentary canal wall.

➔ConCeptLinkRecall that valves control the flow of fluids, including blood flow in veins and through the heart (see Chapter 11, pp. 387–389). The valves of the digestive system (formed by sphincter muscles) control the flow of food and digestive juices through the GI tract.

Cardia

Esophagus

Pylorus

Pyloric sphincter (valve)

Pyloric antrum

Pyloric sphincter

Rugae of mucosa

Body

Serosa

Fundus

Lesser curvature

Greater curvature

Muscularis externa • Longitudinal layer • Circular layer • Oblique layer

Duodenum

(a) (b)

(d)

Mucous neck cells

Parietal cells

Surface epithelium

Pyloric sphincter

Gastric glands

Chief cells

(c)

G as

tr ic

p it

G as

tr ic

g la

Gastric pits

Pepsinogen Pepsin HCl

Enteroendocrine cell

What modification of the stomach muscularis externa allows it to mechanically digest food?Q:

Its third layer of smooth muscle, the oblique layer, which allows it to knead or pummel the food.A:

Figure 14.4 Anatomy of the stomach. Gross internal anatomy (frontal section). (a) Diagram. (b) Photo. (c) Enlarged view of gastric pits and glands (longitudinal section). (d) Pepsinogen produced by the chief cells is activated (converted to pepsin) by HCl secreted by the parietal cells.

Chapter 14: The Digestive System and Body Metabolism 495

The chief cells produce inactive protein-digesting enzymes, mostly pepsinogens. The parietal cells produce corrosive hydrochloric acid (HCl), which makes the stomach contents acidic and activates the enzymes, as in the conversion of pepsinogen to pepsin (shown in Figure 14.4d). The mucous neck cells produce a thin acidic mucus with an unknown function that is quite different from that secreted by the mucous cells of the mucosa. Still other cells, the enteroendocrine cells (entero = gut), produce local hormones, such as gastrin, that are important in regulating the digestive activities of the stomach (see Table 14.1 on p. 497).

Most digestive activity occurs in the pyloric region of the stomach. After food has been pro- cessed in the stomach, it is thick like heavy cream and is called chyme (kı̄m). The chyme enters the small intestine through the pyloric sphincter.

Did You Get It? 1. What is the sequential order (mouth to anus) of the

digestive organs making up the alimentary canal? 2. In which organ of the alimentary canal does protein

digestion begin? 3. The stomach epithelium secretes several substances,

including alkaline mucus and intrinsic factor. What is the function of each of these two secretions?

For answers, see Appendix A.

greater omentum is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs. It also has large collections of lymphoid follicles containing macrophages and defensive cells of the immune system.

The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown. Besides the usual longitudinal and circular muscle layers, its wall contains a third, obliquely arranged layer in the muscularis externa (see Figure 14.4a). This arrangement allows the stomach not only to move food along the tract, but also to churn, mix, and pummel the food, physically breaking it down into smaller fragments. In addition, chemical break- down of proteins begins in the stomach.

The mucosa of the stomach is a simple colum- nar epithelium composed entirely of mucous cells. They produce a protective layer of bicarbonate- rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands (Figure 14.4c) that secrete the components of gas- tric juice. For example, some stomach cells pro- duce intrinsic factor, a substance needed for absorption of vitamin B12 from the small intestine.

Falciform ligament Liver

Lesser omentum

Gallbladder

Stomach Duodenum

Visceral peritoneum Transverse colonGreater omentum

Small intestine

Cecum

Urinary bladder

Diaphragm

Pancreas

Anus

Rectum

Peritoneal cavity

Mesenteries

Spleen

Large intestine

Parietal peritoneum

Uterus

(a) (b)

Figure 14.5 Peritoneal attachments of the abdominal organs. (a) Anterior view; the greater omentum is shown in its normal position, covering the abdominal viscera. (b) Sagittal view of the abdominopelvic cavity of a female.

Explore Cadaver

496 Essentials of Human Anatomy and Physiology

widths long”), the jejunum (jĕ-joo′num; “empty”), and the ileum (il′e-um; “twisted intestine”), which contribute 5 percent, nearly 40 percent, and almost 60 percent of the length of the small intestine, respectively (see Figure 14.1). The ileum joins the large intestine at the ileocecal (il″e-o-se′kal) valve (look forward to Figure 14.8, p. 499).

Chemical digestion of foods begins in earnest in the small intestine. The small intestine is able to process only a small amount of food at one time. The pyloric sphincter (literally, “gatekeeper”) con- trols the movement of chyme into the small intes- tine from the stomach and prevents the small intestine from being overwhelmed. In the C-shaped duodenum, some enzymes are produced by the intestinal cells. More important are enzymes that are produced by the pancreas and then delivered to the duodenum through the pancreatic ducts, where they complete the chemical breakdown of foods in the small intestine. Bile (formed by the liver) also enters the duodenum through the bile duct in the same area (Figure 14.6). The main pan- creatic and bile ducts join at the duodenum to

small Intestine

➔ Learning Objective □□ Explain how villi aid digestive processes in the

small intestine.

The small intestine is the body’s major digestive organ. Within its twisted passageways, usable nutrients are finally prepared for their journey into the cells of the body. The small intestine is a mus- cular tube extending from the pyloric sphincter to the large intestine (see Figure 14.1, p. 490). It is the longest section of the alimentary tube, with an average length of 2 to 4 m (7 to 13 feet) in a living person. Except for the initial part of the small intestine, which mostly lies in a retroperitoneal position (posterior to the parietal peritoneum), the small intestine hangs in sausagelike coils in the abdominal cavity, suspended from the posterior abdominal wall by the fan-shaped mesentery (Figure 14.5b, p. 495). The large intestine encircles and frames it in the abdominal cavity.

The small intestine has three subdivisions: the duodenum (doo″ uh-de′num; “twelve finger

Jejunum

Cystic duct

Duodenum Hepatopancreatic ampulla and sphincter

Gallbladder

Right and left hepatic ducts from liver

Bile duct and sphincter

Main pancreatic duct and sphincter

Pancreas

Common hepatic duct

Duodenal papilla

Accessory pancreatic duct

Figure 14.6 The duodenum of the small intestine and related organs.

Chapter 14: The Digestive System and Body Metabolism 497

appearance, sometimes referred to as the brush border. The plasma membranes bear enzymes (brush border enzymes) that complete the diges- tion of proteins and carbohydrates in the small intestine. Circular folds, also called plicae cir- culares (pli′se ser-ku-la′res), are deep folds of both mucosa and submucosa layers. Unlike the rugae of the stomach, the circular folds do not disappear when food fills the small intestine. Instead, they form an internal “corkscrew slide” to increase surface area and force chyme to travel slowly through the small intestine so nutrients can be absorbed efficiently. All these structural modi- fications, which increase the surface area, decrease in number toward the end of the small intestine. In contrast, local collections of lym- phatic tissue (called Peyer’s patches) found in the submucosa increase in number toward the end of the small intestine. This reflects the fact that the remaining (undigested) food residue in

form the flasklike hepatopancreatic ampulla (hĕ-pă″to-pan-kre-ă′tik am-pu′lah), literally, the “liver-pancreatic enlargement.” From there, the bile and pancreatic juice travel through the duodenal papilla and enter the duodenum together.

Nearly all nutrient absorption occurs in the small intestine. The small intestine is well suited for its function. Its wall has three structures that increase the absorptive surface tremendously— villi, microvilli, and circular folds (Figure 14.7, p. 498). Villi are fingerlike projections of the mucosa that give it a velvety appearance and feel, much like the soft nap of a towel. Within each vil- lus is a rich capillary bed and a modified lym- phatic capillary called a lacteal. The nutrients are absorbed through the mucosal cells into both the capillaries and the lacteal (indicated in Figure 14.13 on p. 505). Microvilli (mi″kro-vil′i) are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy

Table 14.1 Hormones and Hormonelike Products That Act in Digestion

hormone Source Stimulus for secretion Action

Gastrin Stomach Food in stomach, particularly partially digested proteins; ACh released by nerve fibers

• Stimulates release of gastric juice

• Stimulates stomach emptying

Intestinal gastrin Duodenum Food in stomach • Stimulates gastric secretion and emptying

Histamine Stomach Food in stomach • Activates parietal cells to secrete hydrochloric acid

Somatostatin Stomach and duodenum

Food in stomach stimulated by sympathetic nerve fibers

• Inhibits secretion of gastric juice and pancreatic juice

• Inhibits emptying of stomach and gallbladder

Secretin Duodenum Acidic chyme and partially digested foods in duodenum

• Increases output of pancreatic juice rich in bicarbonate ions

• Increases bile output by liver

• Inhibits gastric mobility and gastric gland secretion

Cholecystokinin (CCK)

Duodenum Fatty chyme and partially digested proteins in duodenum

• Increases output of enzyme-rich pancreatic juice

• Stimulates gallbladder to expel stored bile

• Relaxes sphincter of duodenal papilla to allow bile and pancreatic juice to enter the duodenum

Gastric inhibitory peptide (GIP)

Duodenum Food in duodenum • Inhibits secretion of gastric juice

• Stimulates insulin release

498 Essentials of Human Anatomy and Physiology

Blood vessels serving the small intestine

Muscle layers

Villi

(a) Small intestine

(b) Villi

Lumen

Circular folds (plicae circulares)

Absorptive cells

Lacteal

Intestinal crypt

Lymphoid tissue

Muscularis mucosae

Submucosa

Venule

Lymphatic vessel

Microvilli (brush border)

Blood capillaries

Villus

They tremendously increase the surface area available for absorption of digested foodstuffs.A:

Figure 14.7 Structural modifications of the small intestine. (a) Several circular folds (plicae circulares), seen on the inner surface of the small intestine. (b) Enlargement of a villus extension of the circular fold. (c) Enlargement of an absorptive cell to show microvilli (brush border).

What is the functional value of the microvilli in the absorptive cells of the small intestine?Q:

the intestine contains huge numbers of bacteria, which must be prevented from entering the bloodstream if at all possible.

Did You Get It? 4. Which muscular sphincter regulates the flow of

chyme into the small intestine? 5. What are villi, and why are they important?

For answers, see Appendix A.

large Intestine

The large intestine is much larger in diameter than the small intestine (thus its name) but shorter in length. About 1.5 m (5 feet) long, it extends from the ileocecal valve to the anus (Figure 14.8, p. 499). Its major functions are to dry out the indi- gestible food residue by absorbing water and to eliminate these residues from the body as feces (fe′sēz). It frames the small intestine on three sides and has these subdivisions: cecum (se′kum), appendix, colon, rectum, and anal canal.

The saclike cecum is the first part of the large intestine. Hanging from the cecum is the wormlike

Chapter 14: The Digestive System and Body Metabolism 499

two valves: the external anal sphincter, com- posed of skeletal muscle, is voluntary, and the internal anal sphincter, formed by smooth mus- cle, is involuntary. These sphincters, which act rather like purse strings to open and close the anus, are ordinarily closed except during defeca- tion, when feces are eliminated from the body.

Because most nutrients have been absorbed before the large intestine is reached, no villi are present in the large intestine, but there are tremen- dous numbers of goblet cells in its mucosa that produce alkaline (bicarbonate-rich) mucus. The mucus lubricates the passage of feces to the end of the digestive tract.

In the large intestine, the longitudinal layer of the muscularis externa is reduced to three bands

appendix, a potential trouble spot. Because it is usually twisted, it is an ideal location for bacteria to accumulate and multiply. Inflammation of the appendix, appendicitis, is the usual result.

The colon is divided into several distinct regions. The ascending colon travels up the right side of the abdominal cavity and makes a turn, the right colic (or hepatic) flexure, to travel across the abdominal cavity as the transverse colon. It then turns again at the left colic (or splenic) flexure and continues down the left side as the descending colon to enter the pelvis, where it becomes the S-shaped sigmoid (sig′moyd) colon. The sigmoid colon, rectum, and anal canal lie in the pelvis.

The anal canal ends at the anus (a′nus), which opens to the exterior. The anal canal has

Right colic (hepatic) flexure

Transverse colon

Haustrum

Ascending colon

IIeum (cut)

IIeocecal valve

Appendix

Cecum

Left colic (splenic) flexure

Transverse mesocolon

Descending colon

Teniae coli

Sigmoid colon

Cut edge of mesentery

External anal sphincter

Rectum

Anal canal

Figure 14.8 The large intestine. A section of the cecum is removed to show the ileocecal valve.

500 Essentials of Human Anatomy and Physiology

exert pressure and cause a good deal of pain, so they usually must be removed surgically. Wisdom teeth are the most commonly impacted. _______✚

We classify the teeth according to shape and function as incisors, canines, premolars, and molars (see Figure 14.9). The chisel-shaped inci- sors are adapted for cutting; the fanglike canines (eyeteeth) are for tearing or piercing. The premo- lars (bicuspids) and molars have broad crowns with rounded cusps (tips) and are best suited for crushing and grinding.

A tooth consists of two major regions, the crown and the root (Figure 14.10). The enamel- covered crown is the exposed part of the tooth

of muscle called teniae coli (ten′ne-e ko′li; “rib- bons of the colon”). Because these muscle bands usually display some degree of tone (are partially contracted), they cause the wall to pucker into small pocketlike sacs called haustra (haws′trah).

Did You Get It? 6. What are the two main functions of the large

intestine?

For the answer, see Appendix A.

Accessory digestive organs ➔ Learning Objectives

□□ List the accessory digestive organs, and describe the general function of each.

□□ Name the deciduous and permanent teeth, and describe the basic anatomy of a tooth.

□□ Describe the composition and functions of saliva.

□□ Name the main digestive products of the pancreas and of the liver.

Teeth

The role teeth play in processing food needs little introduction. We masticate, or chew, by opening and closing our jaws and moving them from side to side while continuously using our tongue and cheek muscles to keep the food between our teeth. In the process, the teeth tear and grind the food, breaking it down into smaller fragments.

Ordinarily, by the age of 21, two sets of teeth have been formed (Figure 14.9, p. 500). The first set are deciduous (de-sid′u-us) teeth, also called baby teeth or milk teeth. The deciduous teeth begin to erupt around 6 months; the first teeth to appear are the lower central incisors. A baby has a full set (20 teeth) by the age of 2 years.

As the second set of teeth, the deeper perma- nent teeth, enlarge and develop, the roots of the milk teeth are reabsorbed, and between the ages of 6 and 12 years they loosen and fall out. All of the permanent teeth but the third molars have erupted by the end of adolescence. The third molars, also called wisdom teeth, emerge between the ages of 17 and 25. Although there are 32 permanent teeth in a full set, the wisdom teeth often fail to erupt; sometimes they are completely absent.

Homeostatic Imbalance 14.3

When teeth remain embedded in the jawbone, they are said to be impacted. Impacted teeth can

Incisors Central (6–8 mo)

Incisors Central (7 yr)

Canine (eyetooth) (16–20 mo)

Canine (eyetooth) (11 yr)

Premolars (bicuspids) First premolar (11 yr)

Molars First molar (10–15 mo)

Molars First molar (6–7 yr)

Lateral (8–10 mo)

Lateral (8 yr)

Second molar (about 2 yr)

Second molar (12–13 yr)

Third molar (wisdom tooth) (17–25 yr)

Permanent teeth

Deciduous (milk) teeth

Second premolar (12–13 yr)

Figure 14.9 Deciduous and permanent teeth. Approximate time of tooth eruption is shown in parentheses. The same number and arrangement of teeth exist in both upper and lower jaws, so only the lower jaw is shown in each case. The shapes of individual teeth are shown on the right.

Chapter 14: The Digestive System and Body Metabolism 501

the bony jaw. Dentin, a bonelike material, under- lies the enamel and forms the bulk of the tooth. It surrounds a central pulp cavity, which contains a number of structures (connective tissue, blood vessels, and nerve fibers) collectively called pulp. Pulp supplies nutrients to the tooth tissues and provides for tooth sensations. Where the pulp cav- ity extends into the root, it becomes the root canal, which provides a route for blood vessels, nerves, and other pulp structures to enter the pulp cavity of the tooth.

Did You Get It? 7. What is the general function of each of the four types

of teeth?

For the answer, see Appendix A.

salivary glands

Three pairs of salivary glands empty their secre- tions into the mouth. The large parotid (pah- rot′id) glands lie anterior to the ears. Mumps, a common childhood disease, is an inflammation of the parotid glands. If you look at the location of the parotid glands (see Figure 14.1), you can read- ily understand why people with mumps complain that it hurts to open their mouth or chew.

The submandibular glands and the small sublingual (sub-ling′gwal) glands empty their secretions into the floor of the mouth through tiny ducts. The product of the salivary glands, saliva, is a mixture of mucus and serous fluids. The mucus moistens and helps to bind food together into a mass called a bolus (bo′lus), which makes chewing and swallowing easier. The clear serous portion contains an enzyme, salivary amylase (am′ ı̆-lās), in a bicarbonate-rich (alkaline) juice that begins the process of starch digestion in the mouth. Saliva also contains substances such as lysozyme and antibod- ies (IgA) that inhibit bacteria; therefore, it has a pro- tective function as well. Last but not least, saliva dissolves food chemicals so they can be tasted.

Pancreas

The pancreas is a soft, pink, triangular gland that extends across the abdomen from the spleen to the duodenum (look back at Figure 14.6). Most of the pancreas lies posterior to the parietal peritoneum; hence its location is referred to as ret- roperitoneal.

Only the pancreas produces enzymes (described later) that break down all categories of digestible

above the gingiva (jin′-j ı̆vah), or gum. The crown is covered with enamel, a ceramic-like substance as thick as a dime, that directly bears the force of chewing. It is the hardest substance in the body and is fairly brittle because it is heavily mineral- ized with calcium salts.

The portion of the tooth embedded in the jaw- bone is the root; the root and crown are con- nected by a region called the neck. The outer surface of the root is covered by a substance called cement, which attaches the tooth to the periodontal (per″e-o-don′tal) membrane (liga- ment). This ligament holds the tooth in place in

Dentin.A:

Crown

Neck

Root

Enamel

Dentin

Pulp cavity (contains blood vessels and nerves)

Gum (gingiva)

Cement

Root canal

Periodontal membrane (ligament)

Bone

What substance forms the bulk of the tooth?Q:

Figure 14.10 Longitudinal section of a canine tooth.

502 Essentials of Human Anatomy and Physiology

may occur when the gallbladder contracts (an event called a gallbladder attack).

Blockage of the common hepatic or bile ducts (for example, by wedged gallstones) prevents bile from entering the small intestine, and it begins to accumulate and eventually backs up into the liver. This exerts pressure on the liver cells, and bile salts and bile pigments begin to enter the blood- stream. As the bile pigments circulate through the body, the tissues become yellow, or jaundiced. Blockage of the ducts is just one cause of jaun- dice. More often it results from actual liver prob- lems such as hepatitis, an inflammation of the liver, or cirrhosis (sir-ro′sis), a chronic inflamma- tory condition in which the liver is severely dam- aged and becomes hard and fibrous. Hepatitis is most often due to viral infection resulting from drinking contaminated water or transmitted in blood via transfusion or contaminated needles. Cirrhosis is almost guaranteed when someone drinks alcoholic beverages in excess for many years, and it is a common consequence of severe hepatitis. __________________________________ ✚

Did You Get It? 8. Mary has a dry mouth—very little saliva is being

secreted. Digestion of which type of food will be affected (decreased) by this situation?

9. What is the digestive role of bile? What organ produces bile?

10. Which organ is the only one that produces enzymes capable of digesting all groups of food?

For answers, see Appendix A.)

Functions of the Digestive system ➔ Learning Objectives

□□ List and describe the six main activities of the digestive system.

□□ Describe how foodstuffs in the digestive tract are mixed and moved along the tract.

□□ Describe the function of local hormones in digestion.

□□ List the major enzymes or enzyme groups involved in digestion, and name the foodstuffs on which they act.

□□ Describe the mechanisms of swallowing, vomiting, and defecation.

□□ Name the end products of protein, fat, and carbohydrate digestion.

foods. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid that neutralizes the acidic chyme coming in from the stomach. The pancreas also has an endocrine function; it pro- duces the hormones insulin and glucagon (as explained in Chapter 9).

Liver and Gallbladder

The liver is the largest gland in the body. Located under the diaphragm, more to the right side of the body, it overlies and almost completely covers the stomach (look back at Figure 14.5a). The liver has four lobes and is suspended from the diaphragm and abdominal wall by a deli- cate mesentery cord, the falciform (fal′s ı̆-form) ligament.

The liver has many critical metabolic and regulatory roles; however, its digestive function is to produce bile. Bile leaves the liver through the common hepatic duct and enters the duo- denum through the bile duct (look back at Figure 14.6).

Bile is a yellow-to-green, watery solution con- taining bile salts, bile pigments (chiefly bilirubin, a breakdown product of hemoglobin), cholesterol, phospholipids, and a variety of electrolytes. Of these components, only the bile salts (derived from cholesterol) and phospholipids help the digestive process. Bile does not contain enzymes, but its bile salts emulsify fats by physically break- ing large fat globules into smaller ones, thus pro- viding more surface area for the fat-digesting enzymes to work on.

The gallbladder is a small, thin-walled green sac that snuggles in a shallow fossa in the inferior surface of the liver (look back at Figure 14.6). When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored. While in the gallbladder, bile is con- centrated by the removal of water. Later, when fatty food enters the duodenum, a hormonal stimulus prompts the gallbladder to contract and spurt out stored bile, making it available to the duodenum.

Homeostatic Imbalance 14.4

If bile is stored in the gallbladder for too long or too much water is removed, the cholesterol it con- tains may crystallize, forming gallstones. Because gallstones tend to be quite sharp, agonizing pain

Chapter 14: The Digestive System and Body Metabolism 503

Overview of Gastrointestinal Processes and controls The major functions of the digestive tract are usu- ally summarized in two words—digestion and absorption. However, many of its specific activities (such as smooth muscle activity) and certain regu- latory events are not really covered by either term. To describe digestive system processes a little more accurately, we need to consider a few more functional terms. The essential activities of the GI tract include the following six processes (summa- rized in Figure 14.11).

1. Ingestion. Food must be placed into the mouth before it can be acted on. This is an active, voluntary process called ingestion.

2. Propulsion. To be processed by more than one digestive organ, foods must be propelled from one organ to the next. Swallowing is one example of food move- ment that depends largely on the propul- sive process called peristalsis. Peristalsis is involuntary and involves alternating waves of contraction and relaxation of the longi- tudinal muscles in the organ wall (Figure 14.12a). The net effect is to squeeze the food along the tract.

3. Food breakdown: Mechanical breakdown. Mechanical breakdown physically fragments food into smaller particles, increasing surface area and preparing food for further degrada- tion by enzymes. Chewing and mixing of food in the mouth by the teeth and tongue, and churning of food in the stomach are exam- ples of processes contributing to mechanical food breakdown. In addition, segmentation (Figure 14.12b) in the small intestine moves food back and forth across the internal wall of the organ, mixing it with the digestive juices. Although segmentation may also help to pro- pel foodstuffs through the small intestine, it is more an example of mechanical digestion than of propulsion.

4. Food breakdown: Digestion. The sequence of steps in which large food molecules are chemically broken down to their building blocks by enzymes (protein molecules that act as catalysts) is called digestion.

Food Ingestion

Propulsion

Esophagus

Stomach

Pharynx

•

Mechanical breakdown

Digestion

• Chewing (mouth) • Churning (stomach) • Segmentation (small intestine)

Small intestine

Large intestine

Defecation Anus

Feces

Mainly H2O

Blood vessel

Lymph vessel

Absorption

Swallowing (oropharynx)

• Peristalsis (esophagus, stomach, small intestine, large intestine)

Figure 14.11 Schematic summary of gastro intestinal tract activities. Gastrointestinal tract activities include ingestion, mechanical breakdown, chemical (enzymatic) breakdown or digestion, propul- sion, absorption, and defecation. Sites of chemical digestion are also sites that produce enzymes or that receive enzymes or other secretions made by accessory organs. The entire GI tract mucosa secretes mucus, which protects and lubricates.

➔ConCeptLinkRecall that these enzymatic reactions are called hydro-lysis reactions because a water molecule is added to each bond to be broken (see Chapter 2, p. 69). Water is also necessary as a dissolving medium and a soften- ing agent for food digestion.

504 Essentials of Human Anatomy and Physiology

when we talk about blood sugar level, we are referring to glucose. Fructose is the most abundant sugar in fruits, and galactose is found in milk.

The only carbohydrates that our digestive sys- tem digests, or breaks down to simple sugars, are sucrose (table sugar), lactose (milk sugar), maltose (malt sugar), and starch. Sucrose, maltose, and lac- tose are referred to as disaccharides, or double sugars, because each consists of two simple sugars linked together. Starch is a polysaccharide (liter- ally, “many sugars”) formed by linking hundreds of glucose units. Although we eat foods containing other polysaccharides, such as cellulose, we do not have enzymes capable of breaking them down. The indigestible polysaccharides do not provide us with any nutrients, but they help move the foodstuffs along the gastrointestinal tract by pro- viding bulk, or fiber, in our diet.

Proteins are digested to their building blocks, which are amino (ah-me′no) acids. The interme- diate product of protein digestion is polypeptides.

When lipids (fats) are digested, they yield two different types of building blocks—fatty acids and an alcohol called glycerol (glis′er-ol). The diges- tion, or chemical breakdown, of carbohydrates, proteins, and fats is summarized in Figure 14.13, and we describe it in more detail shortly.

5. Absorption. Absorption is the transport of digestive end products from the lumen of the GI tract to the blood or lymph. For absorp- tion to occur, the digested foods must first enter the mucosal cells by active or passive transport processes. The small intestine is the major absorptive site.

6. Defecation. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces.

Some of these processes are the job of a sin- gle organ. For example, only the mouth ingests, and only the large intestine defecates. But most digestive system activities occur bit by bit as food is moved along the tract. Thus, in one sense, the digestive tract can be viewed as a “disassembly line” in which food is carried from one stage of its processing to the next and its nutrients are made available to the cells in the body along the way.

Throughout this book we have stressed the body’s drive to maintain a constant internal envi- ronment, particularly in terms of homeostasis of the blood, which comes into contact with all body

Because each of the major food groups has very different building blocks, let’s take a little time to review these organic molecules (which we first introduced in Chapter 2). The building blocks, or units, of carbohydrate foods are monosaccha- rides (mon″o-sak′ah-rı̄dz), or simple sugars. We need to remember only three of these that are common in our diet—glucose, fructose, and galac- tose. Glucose is by far the most important, and

(a)

(b)

Figure 14.12 Peristaltic and segmental move ments of the digestive tract. (a) In peristalsis, adjacent or neighboring segments of the intestine (or other alimentary canal organs) alternately contract and relax, thereby moving food distally along the tract. (b) In segmentation, single segments of the intestine alternately contract and relax. Because active segments are separated by inactive ones, the food is moved forward and then backward. Thus the food is mixed rather than simply propelled along the tract.

Starch and disaccharides

Oligosaccharides* and disaccharides

Lactose Maltose Sucrose

Glucose Fructose

Salivary amylase Mouth

Pancreatic amylase

Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase)

Small intestine

Stomach

Foodstuff Enzyme(s) and source Site of action

Protein

Large polypeptides

Pepsin (stomach glands) in the presence of HCl

Unemulsified fats

Emulsified by the detergent action of bile salts from the liver

Pancreatic lipase

Small polypeptides

Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase)

Amino acids (some dipeptides and tripeptides)

Monoglycerides and fatty acids

*Oligosaccharides consist of a few linked monosaccharides.

Glycerol and fatty acids

Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase)

Galactose

The monosaccharides glucose, galactose, and fructose enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein.

Amino acids enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein.

Fatty acids and monoglycerides enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. (Glycerol and short-chain fatty acids are absorbed into the capillary blood in the villi and transported to the liver via the hepatic portal vein.)

Digestion of carbohydrates

Digestion of proteins

Digestion of fats

Absorption of proteins

Absorption of carbohydrates

Absorption of fats

Figure 14.13 Flowchart of digestion and absorption of foodstuffs.

505

506 Essentials of Human Anatomy and Physiology

mouth, much larger amounts of saliva pour out. However, the simple pressure of anything put in the mouth and chewed, such as rubber bands or sugarless gum, will also stimulate the release of saliva. Some emotional stimuli can also cause sali- vation. For example, the mere thought of a hot fudge sundae will make many a mouth water. All these reflexes, though initiated by different stimuli, are brought about by parasympathetic fibers in cranial nerves VII and IX.

Essentially no food absorption occurs in the mouth. (However, some drugs, such as nitroglyc- erine, are absorbed easily through the oral mucosa.) The pharynx and esophagus have no digestive function; they simply provide passage- ways to carry food to the next processing site, the stomach.

Food Propulsion—Swallowing and Peristalsis

For food to be sent on its way from the mouth, it must first be swallowed. Deglutition (de″gloo- tish′un), or swallowing, is a complex process that involves the coordinated activity of several struc- tures (tongue, soft palate, pharynx, and esopha- gus). It has two major phases, the buccal and the pharyngeal-esophageal. The first phase, the volun- tary buccal phase, occurs in the mouth. Once the food has been chewed and well mixed with saliva, the bolus (food mass) is forced into the pharynx by the tongue. As food enters the pharynx, it passes out of our control and into the realm of reflex activity.

The second phase, the involuntary pharyn- geal-esophageal phase, transports food through the pharynx and esophagus. The parasympathetic division of the autonomic nervous system (pri- marily the vagus nerve) controls this phase and promotes the mobility of the digestive organs from this point on. All routes that the food might take, except the desired route further into the digestive tract, are blocked off. The tongue blocks off the mouth, and the soft palate closes off the nasal passages. The larynx rises so that its open- ing (into the respiratory passageways) is covered by the flaplike epiglottis. Food is moved through the pharynx and then into the esophagus inferi- orly by wavelike peristaltic contractions of their muscular walls—first the longitudinal muscles contract, and then the circular muscles contract. (The events of the swallowing process are illus- trated in Figure 14.14.)

cells. The digestive system, however, creates an optimal environment for itself to function in the lumen (cavity) of the alimentary canal, an area that is actually outside the body. Conditions in that lumen are controlled so that digestive processes occur efficiently.

Digestive activity is mostly controlled by reflexes via the parasympathetic division of the autonomic nervous system. (This division is the “rest-and-digest” arm. See Chapter 7.) The sensors (mechanoreceptors, chemoreceptors) involved in these reflexes are located in the walls of the ali- mentary canal organs and respond to a number of stimuli, the most important being stretch of the organ by food in its lumen, pH of the contents, and presence of certain breakdown products of digestion. When these receptors are activated, they trigger reflexes that activate or inhibit (1) the glands that secrete digestive juices into the lumen or hormones into the blood, and (2) the smooth muscles of the muscularis that mix and propel the food along the tract.

Now that we have summarized some points that apply to the function of the digestive organs as a group, we are ready to look at their special capabilities.

Did You Get It? 11. What is the proper order of the following stages of

food processing—defecation, absorption, digestion, ingestion?

12. How do mechanical breakdown and digestion differ from each other?

For answers, see Appendix A.

Activities Occurring in the Mouth, Pharynx, and Esophagus Food Ingestion and Breakdown

Once food has been placed in the mouth, both mechanical and digestive (chemical) processing begins. First the food is physically broken down into smaller particles by chewing. Then, as the food is mixed with saliva, salivary amylase begins the digestion of starch, chemically breaking it down into maltose (see Figure 14.13). The next time you eat a piece of bread, chew it for a few minutes before swallowing it. You will notice that it begins to taste sweet as the sugars are released.

Saliva is normally secreted continuously to keep the mouth moist, but when food enters the

Chapter 14: The Digestive System and Body Metabolism 507

Activities of the stomach Food Breakdown

Secretion of gastric juice is regulated by both neural and hormonal factors. The sight, smell, and taste of food stimulate parasympathetic nervous system reflexes, which increase the secretion of gastric juice by the gastric (stomach) glands. In addition, the presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin. Gastrin prods the gastric glands to produce still more of the protein- digesting enzymes (such as pepsinogen), mucus, and hydrochloric acid. Under normal conditions, 2 to 3 liters of gastric juice are produced every day.

Hydrochloric acid makes the stomach contents very acidic. This can be dangerous because both

If we try to talk or laugh while swallowing, we confuse our bodies with mixed messages, and as a result food may enter the respiratory passages. This triggers still another protective reflex—cough- ing—during which air rushes upward from the lungs in an attempt to expel the food.

Once food reaches the distal end of the esoph- agus, it presses against the cardioesophageal sphincter, causing it to open, and the food enters the stomach. The movement of food through the pharynx and esophagus is so automatic that a per- son can swallow and food will reach the stomach even if he is standing on his head. Gravity plays no part in the transport of food once it has left the mouth, which explains why astronauts in the zero gravity of outer space can still swallow and get nourishment.

Tongue

Trachea

Pharynx Epiglottis up

Glottis (lumen) of larynx Larynx up

Bolus of food

Upper esophageal sphincter contracted

Upper esophageal sphincter relaxed

Epiglottis down

Esophagus

Uvula

Bolus

Upper esophageal sphincter contracted

Relaxed muscles

Cardioesophageal sphincter open

Cardioesophageal sphincter relaxed

Upper esophageal sphincter

1 2 3

Figure 14.14 Swallowing. 1

With the upper esophageal sphincter contracted, the tongue pushes the food bolus posteriorly and against the soft palate. 1

The soft palate rises to close off the nasal passages as the bolus enters the pharynx. The larynx rises so that the epiglottis covers its opening as peristalsis carries the food through the pharynx and into the esophagus. The upper esophageal sphincter relaxes to allow food entry.

The upper esophageal sphincter contracts again as the larynx and epiglottis return to their former positions and peristalsis moves the food bolus inferiorly to the stomach.

The cardioesophageal sphincter opens, and food enters the stomach.

508 Essentials of Human Anatomy and Physiology

on milk protein and converts it to a substance that looks like sour milk. Many parents mistakenly think that the curdy substance infants spit up after feeding is milk that has soured in their stomach. Rennin, pro- duced in large amounts in infants, is the same enzyme used to make milk curdle into cheese. It is not believed to be produced in adults.

Other than the beginning of protein digestion, little chemical digestion occurs in the stomach. Except for aspirin and alcohol (which seem some- how to have a “special pass”), virtually no absorp- tion occurs through the stomach walls.

As food enters and fills the stomach, its wall begins to stretch (at the same time the gastric juices are being secreted, as just described). Then the three muscle layers of the stomach wall become active. They compress and pummel the food, breaking it apart physically, all the while continuously mixing the food with the enzyme- containing gastric juice so that the thick fluid chyme is formed. The process looks something like the preparation of a cake mix, in which the floury mixture is repeatedly folded on itself and mixed with the liquid until it reaches a uniform texture.

Food Propulsion

Once the food has been well mixed, a rippling peristalsis begins in the upper half of the stom- ach (Figure 14.15, 1 ). The contractions increase in force as the food approaches the pyloric

hydrochloric acid and the protein-digesting enzymes have the ability to digest the stomach itself, causing ulcers (see “A Closer Look” on p. 512). However, as long as enough mucus is made, the stomach is protected.

Homeostatic Imbalance 14.5

Occasionally, the cardioesophageal sphincter fails to close tightly and gastric juice backs up into the esophagus, which has little mucus protection. This results in a characteristic pain known as heart- burn, which, if uncorrected, leads to inflammation of the esophagus (esophagitis [ĕ-sof′ah-ji′tis]) and perhaps even to ulceration of the esophagus. A common cause is a hiatal hernia, a structural abnormality in which the superior part of the stomach protrudes slightly above the diaphragm. Because the diaphragm no longer reinforces the relatively weak cardioesophageal sphincter, gastric juice flows into the unprotected esophagus. Conservative treatment involves restricting food intake after the evening meal, taking antacids, and sleeping with the head elevated. ______________✚

The extremely acidic environment that hydro- chloric acid provides is necessary, because it activates pepsinogen to pepsin, the active protein-digesting enzyme. Rennin, the second protein-digesting enzyme produced by the stomach, works primarily

Propulsion: Peristaltic waves move from the fundus toward the pylorus.

Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum.

Retropulsion: The peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorus backward into the stomach.

Pyloric valve closed

Pyloric valve slightly opened

Pyloric valve closed

1 2 3

Figure 14.15 Peristaltic waves in the stomach.

Chapter 14: The Digestive System and Body Metabolism 509

Activities of the small Intestine Chyme Breakdown and Absorption

Chyme reaching the small intestine is only par- tially digested. Carbohydrate and protein diges- tion has begun, but virtually no fats or nucleic acids have been digested up to this point. Here the process of chemical digestion is accelerated as the food now takes a rather wild 3- to 6-hour journey through the looping coils and twists of the small intestine. By the time the food reaches the end of the small intestine, digestion will be complete, and nearly all food absorption will have occurred.

As mentioned earlier, the microvilli of small intestine cells bear several important enzymes, the so-called brush border enzymes, that break down double sugars into simple sugars and complete pro- tein digestion (see Figure 14.13). Intestinal juice itself is relatively enzyme poor, and protective mucus is probably the most important intestinal gland secretion. However, foods entering the small intestine are literally deluged with enzyme-rich pancreatic juice delivered via a duct from the pan- creas, as well as bile from the liver.

Pancreatic juice contains enzymes that (1) along with brush border enzymes, complete the digestion of starch (pancreatic amylase); (2) carry out about half of protein digestion (via the action of trypsin, chymotrypsin, carboxypeptidase, and others); (3) are totally responsible for fat diges- tion, because the pancreas is essentially the only source of lipases; and (4) digest nucleic acids (nucleases).

In addition to enzymes, pancreatic juice con- tains a rich supply of bicarbonate ions, which makes it very basic (about pH 8). When pancreatic juice reaches the small intestine, it neutralizes the acidic chyme coming in from the stomach and provides the proper environment for activation and activity of intestinal and pancreatic digestive enzymes.

Homeostatic Imbalance 14.7

Pancreatitis (pan″kre-ah-ti′tis) is a rare but extremely serious inflammation of the pancreas that results from activation of pancreatic enzymes in the pancreatic duct. Because pancreatic enzymes break down all categories of biological molecules, they cause digestion of the pancreatic tissue and

valve, grinding the food into chyme. The pylorus of the stomach, which holds about 30 ml of chyme, acts like a meter that allows only liquids and very small particles to pass through the pyloric sphincter. Because the pyloric sphincter barely opens, each contraction of the stomach muscle squirts 3 ml or less of chyme into the small intestine (Figure 14.15,

). The contrac- tion also closes the valve, so the rest of the chyme (about 27 ml) is propelled backward into the stomach for more mixing, a process called retropulsion (Figure 14.15,

). When the duodenum is filled with chyme and

its wall is stretched, a nervous reflex, the entero- gastric (en″ter-o-gas′trik) reflex, occurs. This reflex “puts the brakes on” gastric activity. It slows the emptying of the stomach by inhibiting the vagus nerve and tightening the pyloric sphincter, thus allowing time for intestinal processing to catch up. Generally, it takes about 4 hours for the stomach to empty completely after the person eats a well- balanced meal and 6 hours or more if the meal has a high fat content.

Homeostatic Imbalance 14.6

Local irritation of the stomach, such as occurs with bacterial food poisoning, may activate the emetic (ē-met′ik) center in the brain (medulla). The emetic center, in turn, causes vomiting, or eme- sis. Vomiting is essentially a reverse peristalsis occurring in the stomach (and perhaps the small intestine), accompanied by contraction of the abdominal muscles and the diaphragm, which increases the pressure on the abdominal organs. The emetic center may also be activated through other pathways; disturbance of the equilibrium apparatus of the inner ear during a boat ride on rough water is one example. _________________✚

Did You Get It? 13. What happens during the buccal stage of

swallowing? 14. How does segmentation differ from peristalsis in

terms of moving substances along the GI tract? 15. What does it mean when we say that food “went

down the wrong tube”? 16. Why is it necessary for the stomach contents to be so

acidic?

For answers, see Appendix A.

510 Essentials of Human Anatomy and Physiology

small intestine together. As mentioned before, bile is not an enzyme. Instead, it acts like a detergent to emulsify, or mechanically separate, large fat globules into thousands of tiny ones, providing a much greater surface area for the pancreatic lipases to work on. Bile is also necessary for absorption of fats—and the fat-soluble vitamins (K, D, E, and A) that are absorbed along with them—from the intestinal tract.

Homeostatic Imbalance 14.8

If either bile or pancreatic juice is absent, essen- tially no fat digestion or absorption goes on, and fatty, bulky stools are the result. In such cases, blood-clotting problems also occur because the liver needs fat-soluble vitamin K to make pro- thrombin, an important clotting factor. _________✚

Absorption of water and of the end products of digestion occurs all along the length of the

duct. This painful condition can lead to nutritional deficiencies because pancreatic enzymes are essen- tial for digestion in the small intestine. _________ ✚

The release of pancreatic juice into the duode- num is stimulated by both the vagus nerve and local hormones. When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones (Table 14.1, p. 511). Two of these hormones, secretin (se-kre′tin) and chole- cystokinin (ko″le-sis″to-kin′in) (CCK), influence the release of pancreatic juice and bile.

The hormones enter the blood and circulate to their target organs, the pancreas, liver, and gall- bladder. Both hormones work together to stimu- late the pancreas to release its enzyme- and bicarbonate-rich product (Figure 14.16, p. 510). In addition, secretin causes the liver to increase its output of bile, and cholecystokinin causes the gall- bladder to contract and release stored bile into the bile duct so that bile and pancreatic juice enter the

Chyme entering duodenum causes duodenal enteroendocrine cells to release cholecystokinin (CCK) and secretin.

CCK (red dots) and secretin (blue dots) enter the bloodstream.

3 Upon reaching the pancreas, CCK induces secretion of enzyme-rich pancreatic juice; secretin causes secretion of bicarbonate- rich pancreatic juice.

4 Secretin causes the liver to secrete more bile; CCK stimulates the gallbladder to release stored bile and the hepatopancreatic sphincter to relax (allows bile from both sources to enter the duodenum).

5 Stimulation by vagal nerve fibers causes release of pancreatic juice and weak contractions of the gallbladder.

Figure 14.16 Regulation of pancreatic juice and bile secretion and release. Hormonal controls exerted by secretin and cholecystokinin 1

–

, are the main regulatory factors. Neural control

is mediated by the vagus nerves.

Chapter 14: The Digestive System and Body Metabolism 511

to the absorption of these vitamins, some ions, and most of the remaining water. Feces, the more or less solid product delivered to the rectum, con- tains undigested food residues, mucus, millions of bacteria, and just enough water to allow its smooth passage.

Propulsion of Food Residue and Defecation

When presented with food residue, the colon begins contractions, but they are sluggish or short-lived. The movements most seen in the colon are haus- tral contractions, slow segmenting movements lasting about 1 minute that occur every 30 minutes or so. As a haustrum fills with food residue, the dis- tension stimulates its muscle to contract, which pro- pels the luminal contents into the next haustrum. These movements also mix the residue, which aids in water absorption.

Mass movements are long, slow-moving but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum. Typically, they occur during or just after eating, when food begins to fill the stomach and small intestine. Bulk, or fiber, in the diet increases the strength of colon contractions and softens the stool, allowing the colon to perform its function more effectively.

Homeostatic Imbalance 14.9

When the diet lacks bulk, the colon narrows and its circular muscles contract more powerfully, increasing the pressure on its walls. This encour- ages formation of diverticula (di″ver-tik′u-lah), in which the mucosa protrudes through the colon walls, a condition called diverticulosis. Diverticulitis, a condition in which the diver- ticula become inflamed, can be life-threatening if ruptures occur. ___________________________ ✚

The rectum is generally empty, but when feces are forced into it by mass movements and its wall is stretched, the defecation reflex is initiated. The defecation reflex is a spinal (sacral region) reflex that causes the walls of the sigmoid colon and the rectum to contract and the anal sphincters to relax. As the feces are forced through the anal canal, messages reach the brain giving us time to decide whether the external voluntary sphincter should

small intestine. Most substances are absorbed through the intestinal cell plasma membranes by the process of active transport. Then they enter the capillary beds in the villi to be trans- ported in the blood to the liver via the hepatic portal vein. The exception seems to be lipids, or fats, which are absorbed passively by the process of diffusion. Lipid breakdown products enter both the capillary beds and the lacteals in the villi and are carried to the liver by both blood and lym- phatic fluids.

At the end of the ileum, all that remains is some water, indigestible food materials (plant fibers such as cellulose), and large amounts of bacteria. This debris enters the large intestine through the ileocecal valve. (The complete pro- cess of food digestion and absorption is summa- rized in Figure 14.13 on p. 505.)

Chyme Propulsion

As mentioned previously, peristalsis is the major means of propelling chyme through the digestive tract. It involves waves of contraction that move along the length of the intestine, followed by waves of relaxation. The net effect is that the food is moved through the small intestine in much the same way that toothpaste is squeezed from a tube. Rhythmic segmental movements produce local constrictions of the intestine (see Figure 14.12b) that mix the chyme with the diges- tive juices and help to propel food through the intestine.

Activities of the large Intestine Nutrient Breakdown and Absorption

What is finally delivered to the large intestine con- tains few nutrients, but that residue still has 12 to 24 hours more to spend there. The colon itself produces no digestive enzymes. However, the “resident” bacteria that live in its lumen metabolize some of the remaining nutrients, releasing gases (methane and hydrogen sulfide) that contribute to flatulence and the odor of feces. About 500 ml of gas (flatus) is produced each day, much more when certain carbohydrate-rich foods (such as beans) are eaten.

Bacteria residing in the large intestine also make some vitamins (vitamin K and some B vita- mins). Absorption by the large intestine is limited

512

A CLOSER

LOOK Peptic Ulcers: “Something Is Eating at Me”

For years the stereotypical ulcer patient was the overworked business executive. Although a stressful lifestyle does seem to aggravate existing ulcers, recent studies indicate that most such ulcers are caused by a strain of acid-resistant bacteria (Heli- cobacter pylori) that burrow through the mucus coat of the stomach, leav- ing bare areas. This corkscrew-shaped bacterium is found in the stomachs of 50 percent of healthy people and 70 to 90 percent of people with ulcers (see photo b).

Peptic ulcers can produce serious complications. In about 20 percent of cases, eroded blood vessels bleed into the GI tract. In such cases, anemia may result from severe blood loss. In 5 to 10 percent of patients, scarring within the stomach obstructs the pyloric opening, blocking digestion. About 5 percent of peptic ulcers per- forate, leaking stomach or duodenal contents into the peritoneal cavity. A perforated ulcer is a life-threatening condition.

Archie, a 53-year-old factory worker, began to experience a burning pain in his upper abdomen an hour or two after each meal. At first, he blamed his home cooking, but his abdominal pain became markedly worse during a hectic week when he worked 15 hours of overtime. Archie consulted a physi- cian and was told that he had a peptic ulcer.

Peptic ulcers affect one of every eight Americans. A peptic ulcer is typically a round, sharply defined crater 1 to 4 cm in diameter in the mucosa of any part of the GI tract exposed to secretions of the stom- ach (see photo a). A few peptic ulcers occur in the lower esophagus, following the reflux of stomach con- tents, but most (98 percent) occur in the pyloric part of the stomach (gas- tric ulcers) or the first part of the duodenum (duodenal ulcers). Peptic ulcers may appear at any age, but they develop most frequently between ages 50 and 70. They tend to recur—healing, then flaring up periodically—for the rest of a per- son’s life if not treated.

Duodenal ulcers are about three times more common than gastric ulcers. Gastric and duodenal ulcers may produce a gnawing or burning epigastric pain that seems to bore through to your back. Other symp- toms include loss of appetite, bloat- ing, nausea, and vomiting. Not all people with ulcers experience these symptoms, however, and some exhibit no symptoms at all.

In spite of these potential complica- tions, most peptic ulcers heal readily and respond well to treatment. The first steps are to avoid smoking, alcohol (especially wine), ibuprofen, and aspi- rin, all of which aggravate ulcers. Ant- acid drugs are suggested to neutralize the stomach acids. In ulcers colonized by H. pylori, a simple 2-week regimen of antibiotics permanently cures nearly all such patients. For active ulcers, a his- tamine blocker (a medication that sup- presses acid production in the stomach) can be combined with the antibiotic therapy to speed recovery.

The relatively few peptic ulcers not caused by H. pylori generally result from long-term use of NSAIDs, such as ibuprofen. Treatment with histamine-blocker drugs (such as Zantac) is the therapy of choice.

Recent animal trials using a newly developed vaccine against H. pylori have been successful. Preventive vaccination, coupled with antibiotic cures, may eradicate peptic ulcers within the next 25 years.

Bacteria

Mucosa layer of stomach

(a) A peptic ulcer lesion (b) H. pylori bacteria

Chapter 14: The Digestive System and Body Metabolism 513

replace worn-out parts. However, most foods are used as metabolic fuels. That is, they are oxidized and transformed into adenosine triphosphate (ATP), the chemical energy form body cells need to drive their many activities. The energy value of foods is measured in units called kilocalories (kcal), or Calories (with a capital C), the units conscientiously counted by dieters.

We have just considered how foods are digested and absorbed. But what happens to these foods once they enter the blood? Why do we need bread, meat, and fresh vegetables? Why does everything we eat seem to turn to fat? We will try to answer these questions in this section.

Nutrition A nutrient is a substance in food that the body uses to promote normal growth, maintenance, and repair. The nutrients divide neatly into six catego- ries. The major nutrients—carbohydrates, lipids, and proteins—make up the bulk of what we eat. Minor nutrients—vitamins and minerals— although equally crucial for health, are required in very small amounts. Water, which accounts for about 60 percent of the volume of the food we eat, is also considered to be a major nutrient. However, because we described its importance as a solvent and in many other aspects of body func- tioning in the chapter on basic chemistry (see Chapter 2), we will consider only the other five groups of nutrients here.

Most foods offer a combination of nutrients. For example, a bowl of cream of mushroom soup contains all of the major nutrients plus some vita- mins and minerals. A diet consisting of foods selected from each of the five food groups (Table 14.2, p. 514), that is, grains, fruits, vegeta- bles, meats and meat alternatives, and milk prod- ucts, normally guarantees adequate amounts of all of the needed nutrients.

dietary Recommendations Several types of detailed food guide pyramids have been developed in the United States to provide dietary recommendations. The version issued in 1992 by Walter Willett, called the Healthy Eating Pyramid (Figure 14.17a, p. 515), looks at six major food groups, subdividing some of them further. It uses the traditional (horizontal) orientation of food groups; emphasizes eating whole-grain foods and

remain open or be constricted to stop passage of feces. If it is not convenient, defecation (bowel movement) can be delayed temporarily. Within a few seconds, the reflex contractions end, and the rectal walls relax. With the next mass movement, the defecation reflex is initiated again.

Homeostatic Imbalance 14.10

Watery stools, or diarrhea (di″ah-re′ah), result from any condition that rushes food residue through the large intestine before that organ has had sufficient time to absorb the water (as in irrita- tion of the colon by bacteria). Because fluids and ions are lost from the body, prolonged diarrhea may result in dehydration and electrolyte imbal- ance, which can be fatal if severe.

If food residue remains in the large intestine for extended periods, too much water is absorbed, and the stool becomes hard and diffi- cult to pass. This condition, called constipation, may result from lack of fiber in the diet, poor bowel habits (“failing to heed the call”), narcotic pain medications such as hydrocodone, and lax- ative abuse. Stool softeners can help increase water content of material in the colon to ease passage during defecation. _________________ ✚

Did You Get It? 17. What are the building blocks (and digestion products)

of proteins? 18. What is the role of CCK in digestion? 19. What are brush border enzymes?

For answers, see Appendix A.

PART II: nuTRITIon AND METABOLISM ➔ Learning Objectives

□□ define nutrient and kilocalorie.

□□ List the six nutrient categories. Note important dietary sources and their main cellular uses.

Although it seems at times that people can be divided into two camps—those who live to eat and those who eat to live—we all recognize the vital importance of food for life. The common say- ing “You are what you eat” is true in that part of the food we eat is converted to our living flesh. In other words, a certain fraction of nutrients is used to build cellular molecules and structures and to

514 Essentials of Human Anatomy and Physiology

diet by age, sex, and activity level by going to the MyPlate website (www.choosemyplate.gov).

Nutrition advice is constantly in flux and often mired in the self-interest of food companies. Nonetheless, basic dietary principles have not changed in years and are not in dispute: Eat less overall; eat plenty of fruits, vegetables, and whole grains; avoid junk food; and exercise regularly.

Dietary Sources of the Major Nutrients Carbohydrates

Except for milk sugar (lactose) and small amounts of glycogen in meats, all the carbohydrates—sugars

lots of fruits and vegetables; and recommends sub- stituting plant oils for animal fats and restricting red meat, sweets, and starchy foods.

MyPlate is a more recent food guide from 2011 that uses a different symbol—a round dinner plate. Issued by the United States Department of Agriculture (USDA), it shows food categories in healthy proportions in sections of a place setting rather than as segments of a pyramid (Figure 14.17b). Occupying half the plate are fruits and vegetables (more vegetable than fruit). The other half shows grains and proteins (more grain than protein). A glass represents dairy, the fifth food group. Links provide details on healthy choices in each food group. You can personalize your MyPlate

Table 14.2 Five Basic Food Groups and Some of Their Major Nutrients

Major nutrients supplied in significant amounts

Group Example foods By all in group By only some in group

Fruits Apples, bananas, dates, oranges, tomatoes

Carbohydrate

Water

Vitamins: A, C, folic acid

Minerals: iron, potassium

Fiber

Vegetables Broccoli, cabbage, green beans, lettuce, potatoes

Carbohydrate

Water

Vitamins: A, C, E, K, and B vitamins except B12 Minerals: calcium, magnesium, iodine, manganese, phosphorus

Fiber

Grain products (preferably whole grain; otherwise, enriched or fortified)

Breads, rolls, bagels; cereals, dry and cooked; pasta; rice, other grains; tortillas, pancakes, waffles; crackers; popcorn

Carbohydrate Protein

Vitamins: thiamine (B1), niacin

Water

Fiber

Minerals: iron, magnesium, selenium

Milk products Milk, yogurt; cheese; ice cream, ice milk, frozen yogurt

Protein

Fat

Vitamins: riboflavin, B12 Minerals: calcium, phosphorus

Water

Carbohydrate

Vitamins: A, D

meats and meat alternatives

Meat, fish, poultry; eggs; seeds; nuts, nut butters; soybeans, tofu; other legumes (peas and beans)

Protein

Vitamins: niacin, B6 Minerals: iron, zinc

Carbohydrate

Fat

Vitamins: B12, thiamine (B1)

Water

Fiber

http://www.choosemyplate.gov

Chapter 14: The Digestive System and Body Metabolism 515

Major sources of cholesterol are egg yolk, meats, and milk products.

Proteins

Animal products contain the highest-quality pro- teins, molecules that are amino acid polymers. Eggs, milk, fish, and most meat proteins are com- plete proteins that meet all of the body’s amino acid requirements for tissue maintenance and growth.

Legumes (beans and peas), nuts, and cereals are also protein rich, but their proteins are nutri- tionally incomplete because they are low in one or more of the essential amino acids. The essential amino acids are the eight amino acids (listed in Figure 14.18, p. 516) that our bodies cannot make. Hence we must obtain these amino acids from our diet. As you can see, strict vegetarians must care- fully plan their diets to obtain all the essential amino acids and prevent protein malnutrition. Cereal grains and legumes when eaten together provide all the needed amino acids, and some variety of this combination is found in the diets of all cultures (for example, rice and beans in the Mexican tradition).

vitamins

Vitamins are organic nutrients of various forms that the body requires in small amounts. Although

and starches—we ingest are derived from plants. Sugars come mainly from fruits, sugar cane, and milk. The polysaccharide starch is found in grains, legumes, and root vegetables. The polysaccharide cellulose, which is plentiful in most vegetables, is not digested by humans, but it provides fiber that increases the bulk of the stool and aids defecation.

Lipids

Although we also ingest cholesterol and phospho- lipids, most dietary lipids are triglycerides (neu- tral fats). We eat saturated fats in animal products such as meat and dairy foods and in a few plant products, such as coconut. Unsaturated fats are present in seeds, nuts, and most vegetable oils.

(b) USDA’s MyPlate

Red meat, butter: use sparingly

Vegetables in abundance

Whole-grain foods at most meals Daily excercise and weight control

(a) Healthy Eating Pyramid

Dairy or calcium supplement: 1–2 servings

White rice, white bread, potatoes, pasta, sweets:

use sparingly

Fish, poultry, eggs: 0–2 servings

Nuts, legumes: 1–3 servings

Fruits: 2–3 servings

Plant oils at most

meals

Figure 14.17 Two visual food guides.

Corn and other grains

Beans and other legumes

Tryptophan

Methionine

Valine

Threonine

Phenylalanine

Leucine

Isoleucine

Lysine

Figure 14.18 The eight essential amino acids. Vegetarian diets must be carefully constructed to provide all essential amino acids. A meal of corn and beans fills the bill: Corn provides the essential amino acids not in beans, and vice versa.

516 Essentials of Human Anatomy and Physiology

(ah-nab′o-lizm), the building of larger molecules or structures from smaller ones. During catabo- lism, bond energy of foods is released and cap- tured to make adenosine triphosphate (ATP), the energy-rich molecule used to energize all cel- lular activities, including catabolic reactions.

Carbohydrate, Fat, and Protein Metabolism in Body Cells Not all foodstuffs are treated in the same way by body cells. For example, carbohydrates, particularly glucose, are usually broken down to make ATP. Fats are used to build cell membranes, make myelin sheaths around neurons, and insulate the body with a fatty cushion. They are also used as the body’s main fuel for making ATP when there are inadequate carbohydrates in the diet. Proteins are the major structural materials used for building cell structures.

Carbohydrate Metabolism

Just as an oil furnace uses oil (its fuel) to pro- duce heat, the cells of the body use carbohy- drates as their preferred fuel to produce cellular energy (ATP). Glucose, also known as blood sugar, is the major breakdown product of car- bohydrate digestion. Glucose is also the major fuel used for making ATP in most body cells. The liver is an exception; it routinely uses fats as well, thus saving glucose for other body cells. Essentially, glucose is broken apart piece by piece, and some of the chemical energy released when its bonds are broken is captured and used to bind phosphate to ADP molecules to make ATP.

The carbon atoms released from glucose leave the cells as carbon dioxide, and the hydrogen atoms removed (which contain energy- rich electrons) are eventually combined with oxy- gen to form water. (The overall reaction is summed up simply in Figure 14.19.) These oxygen-using events are referred to collectively as cellular res- piration. The three main metabolic pathways involved in cellular respiration are glycolysis, the citric acid cycle, and the electron transport chain (shown schematically in Figure 14.20).

Oxidation via the removal of hydrogen atoms (which are temporarily passed to vitamin-containing coenzymes) is a major role of glycolysis and the citric acid cycle. Glycolysis, which takes place in the cytosol, also energizes each glucose molecule

vitamins are found in all major food groups, no one food contains all the required vitamins. Thus, a balanced diet is the best way to ensure a full vitamin complement, particularly because certain vitamins (A, C, and E) appear to have anticancer effects. Diets rich in broccoli, cabbage, and brussels sprouts (all good sources of vitamins A and C) appear to reduce cancer risk. However, contro- versy abounds concerning the ability of vitamins to work wonders.

Most vitamins function as coenzymes (or parts of coenzymes); that is, they act with an enzyme to accomplish a particular type of catalysis.

minerals

The body also requires adequate supplies of seven minerals (which are inorganic substances)— calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium—as well as trace amounts of about a dozen others.

Fats and sugars have practically no minerals, and cereals and grains are poor sources. The most mineral-rich foods are vegetables, legumes, milk, and some meats.

The section on metabolism discusses the main uses of the major nutrients in the body. (Appendix D details some important roles of vitamins and miner- als in the body.)

Did You Get It? 20. What is the major source of carbohydrates in

our diet? 21. Why is it important to include cellulose in a healthy

diet even though we cannot digest it? 22. Are oils saturated or unsaturated lipids? 23. What is the most significant role that vitamins play

in the body?

For answers, see Appendix A.

Metabolism ➔ Learning Objectives

□□ define metabolism, anabolism, and catabolism.

□□ Recognize the uses of carbohydrates, fats, and proteins in cell metabolism.

Metabolism (mĕ-tab′o-lizm; metabol = change) is a broad term referring to all chemical reactions that are necessary to maintain life. It involves catabolism (kah-tab′o-lizm), the breakdown of substances to simpler substances, and anabolism

Chapter 14: The Digestive System and Body Metabolism 517

from phosphorylated substances to ADP, a process called substrate-level phosphorylation. Free oxygen is not involved.

The electron transport chain is where the action is for ATP production. The hydrogen atoms removed during the first two metabolic phases are loaded with energy. These hydrogens are delivered

so that it can be split into two pyruvic acid mole- cules and yield a small amount of ATP in the pro- cess (Figure 14.20). The citric acid cycle occurs in the mitochondria and produces virtually all the carbon dioxide that results during cell respiration. Like glycolysis, it yields a small amount of ATP by transferring high-energy phosphate groups directly

H2OO2 CO2C6H12O6 6 6 6+ + +

Glucose Water EnergyOxygen gas

Carbon dioxide

ATP

Figure 14.19 Summary equation for cellular respiration.

Via oxidative phosphorylation

Via substrate-level phosphorylation

CO2

H2O

Mitochondrion Mitochondrial cristae

Cytosol of cell

Citric acid cycle

Glucose

Glycolysis

Pyruvic acid

Electron transport chain and oxidative phosphorylation

Chemical energy (high-energy electrons)

Chemical energy

28 ATP

2 ATP

During glycolysis, each glucose molecule is broken down into two molecules of pyruvic acid as hydrogen atoms containing high-energy electrons are removed.

The pyruvic acid enters the mitochondrion, where citric acid cycle enzymes remove more hydrogen atoms and decompose it to CO2. During glycolysis and the citric acid cycle, small amounts of ATP are formed.

Energy-rich electrons picked up by coenzymes are transferred to the electron transport chain, built into the cristae membrane. The electron transport chain carries out oxidative phosphorylation, which accounts for most of the ATP generated by cellular respiration, and finally unites the removed hydrogen with oxygen to form water.

1 2 3

Figure 14.20 The formation of ATP in the cytosol and the mitochondria during cellular respiration. The maximum energy yield per glucose molecule is 32 ATP.

518 Essentials of Human Anatomy and Physiology

foods such as candy and other sugary sweets causes a rapid deposit of fat in the body’s adipose tissues. When the blood glucose level is too low (hypoglycemia), the liver breaks down stored glycogen and releases glucose to the blood for cellular use. (These various fates of carbohydrates are shown in Figure 14.22a.)

Did You Get It? 24. What name is given to the process in which glucose is

combined with oxygen to yield CO2, H2O, and ATP? 25. What are the major end products of the citric acid

cycle? 26. What are the major products of the electron transport

chain?

For answers, see Appendix A.

Fat Metabolism

As we will describe shortly, the liver handles most lipid, or fat, metabolism that goes on in the body. The liver cells use some fats to make ATP for their own use; use some to synthesize lipoproteins, thromboplastin (a clotting protein), and choles- terol; and then release the rest to the blood in the form of relatively small fat-breakdown products. Body cells remove the fat products and cholesterol from the blood and build them into their mem- branes or steroid hormones as needed. Fats are also used to form myelin sheaths (see Chapter 7)

by the coenzymes to the protein carriers of the electron transport chain, which form part of the mitochondrial cristae membranes (Figure 14.21a). There the hydrogen atoms are split into hydrogen ions (H+) and electrons (e–). The electrons “fall down an energy hill,” going from each carrier to a carrier of lower energy. They give off their “load” of energy in a series of steps in small enough amounts to enable the cell to attach phosphate to ADP and make ATP. Ultimately, free oxygen is reduced (the electrons and hydrogen ions are united with molecular oxygen), forming water and a large amount of ATP. This more complicated process of ATP formation is called oxidative phosphorylation. The beauty of this system is that, unlike the explo- sive reaction (Figure 14.21b) that usually happens when fuels are burned (O2 is combined with hydrogen), relatively small amounts of energy are lost as heat (and light).

Because glucose is the major fuel for making ATP, homeostasis of the blood glucose level is criti- cally important. If there is an excessively high level of glucose in the blood (hyperglycemia [hi″per- gli-se′me-ah]), some of the excess is stored in body cells (particularly liver and muscle cells) as glycogen. If the blood glucose level is still too high, excesses are converted to fat. There is no question that eating large amounts of empty-calorie

Energy released and now

available for making ATP

Protein carriers

of the electron transport chain

2e–

e–

NADH NAD+ H++

Electron flow

(a) (b)

Energy released as heat and light

Figure 14.21 energy release in the electron transport chain versus onestep reduction of oxygen. (a) In cellular respiration, cascading electrons release energy in

small steps and finally reduce O2 and form water. The energy released is in quantities easily used to form ATP. (NADH is a niacin-containing coenzyme that delivers electrons to

the electron transport chain.) (b) When O2 is reduced (combined with hydrogen) in one step, the result is an explosion.

Chapter 14: The Digestive System and Body Metabolism 519

Cellular uses

To capillary

Excess stored as glycogen or fat

Glycogen and fat broken down for ATP formation

Broken down to glucose and released to blood

Metabolized by liver to acetic acid, etc.

Cellular uses

Insulation and fat cushions to protect body organs

Fats are the primary fuels in many cells

Fats build myelin sheaths and cell membranes

ATP formation if inadequate glucose and fats or if essential amino acids are lacking

(a) Carbohydrates: polysaccharides, disaccharides; composed of simple sugars (monosaccharides)

Polysaccharides

GI digestion to simple sugars

Monosaccharides

(b) Fats: composed of 1 glycerol molecule and 3 fatty acids; triglycerides

Lipid (fat) Fatty acids GI digestion

to fatty acids and glycerol

Glycerol

Protein

GI digestion to amino acids

Normally infrequent

Cellular uses

Amino acids

Functional proteins (enzymes, antibodies, hemoglobin, etc.)

Structural proteins (connective tissue fibers, muscle proteins, etc.)

(d) ATP formation (fueling the metabolic furnace): all categories of food can be oxidized to provide energy molecules (ATP)

Monosaccharides

Fatty acids

Amino acids (amine first removed and combined with CO2 by the liver to form urea)

Carbon dioxide

Water Cellular

metabolic “furnace”:

Citric acid cycle and

electron transport chain

ATP

Figure 14.22 Metabolism by body cells. (a) Carbohydrate metabolism. (b) Fat metabolism. (c) Protein metabolism. (d) ATP formation.

520 Essentials of Human Anatomy and Physiology

acid supply. They use ATP to actively transport amino acids into their interior even though in many cases they may contain more of those amino acids than are in the blood flowing past them. Even though this may appear to be “cellu- lar greed,” there is an important reason for this active uptake of amino acids. Cells cannot build their proteins unless all the needed amino acids, which number around 20, are present. As men- tioned earlier, because cells cannot make the essential amino acids, they are available to the cells only through the diet. This helps explain the avid accumulation of amino acids, which ensures that all amino acids needed will be available for pres- ent and (at least some) future protein-building needs of the cells (Figure 14.22c).

Amino acids are used to make ATP only when proteins are overabundant and/or when carbohydrates and fats are not available. When it is necessary to oxidize amino acids for energy (Figure 14.22d), their amine groups are removed as ammonia, and the rest of the molecule enters the citric acid cycle pathway in the mitochondria. The ammonia that is released during this process is toxic to body cells, especially nerve cells. We will discuss how the liver comes to the rescue in this situation shortly.

Did You Get It? 27. Other than for ATP production, list two uses of fats

in the body.

For the answer, see Appendix A.

The central Role of the liver in Metabolism ➔ Learning Objective

□□ Describe the metabolic roles of the liver.

The liver is one of the most versatile and com- plex organs in the body. Without it, we would die within 24 hours. Its role in digestion (manu- facturing bile) is important to the digestive pro- cess to be sure, but it is only one of the many functions of liver cells. The liver cells detoxify drugs and alcohol, degrade hormones, and make many substances vital to the body as a whole (cholesterol, blood proteins such as albumin and clotting proteins, and lipoproteins). They play a central role in metabolism as they process nearly every class of nutrient. Because of the liver’s key roles, nature has provided us with a surplus of

and fatty cushions around body organs. In addi- tion, stored fats are the body’s most concentrated source of energy. (Catabolism of 1 gram of fat yields twice as much energy as the breakdown of 1 gram of carbohydrate or protein.)

For fat products to be used for ATP synthesis, they must first be broken down to acetic acid (Figure 14.22b). Within the mitochondria, the ace- tic acid (like the pyruvic acid product of carbohy- drates) is then completely oxidized, and carbon dioxide, water, and ATP are formed.

When there is not enough glucose to fuel the needs of the cells for energy, larger amounts of fats are used to produce ATP. Under such conditions, fat oxidation is fast but incomplete, and some of the intermediate products, such as acetoacetic acid (two acetic acids linked together) and acetone, begin to accumulate in the blood. These cause the blood to become acidic (a condition called acidosis or ketoaci- dosis), and the breath takes on a fruity odor as acetone diffuses from the lungs. Ketoacidosis is a common consequence of “no-carbohydrate” diets, uncontrolled diabetes mellitus, and starva- tion in which the body is forced to rely almost totally on fats to fuel its energy needs. Although fats are an important energy source, cholesterol is never used as a cellular fuel. Its importance lies in the functional molecules and in the struc- tures it helps to form.

Excess fats are stored in fat deposits such as the hips, abdomen, breasts, and subcutaneous tis- sues. Although the fat in subcutaneous tissue is important as insulation for the deeper body organs, excessive amounts restrict movement and place greater demands on the circulatory system. (The metabolism and uses of fats are shown in Figure 14.22b.)

Protein Metabolism

Proteins make up the bulk of cellular structures. Ingested proteins are broken down to amino acids. Once the liver has finished processing the blood draining the digestive tract and has taken its “fill” of amino acids, the remaining amino acids circulate to the body cells. The cells remove amino acids from the blood and use them to build proteins, both for their own use (enzymes, membranes, mitotic spindle proteins, muscle pro- teins) and for export (mucus, hormones, and others). Cells take few chances with their amino

Chapter 14: The Digestive System and Body Metabolism 521

the liver. The liver also makes cholesterol and secretes cholesterol’s breakdown products in bile.

All blood proteins made by the liver are built from the amino acids its cells pick up from the blood. The completed proteins are then released back into the blood to travel throughout the circu- lation. Albumin, the most abundant protein in blood, holds fluid in the bloodstream. When insuf- ficient albumin is present in blood, fluid leaves the bloodstream and accumulates in the tissue spaces, causing edema. (In Chapter 10, we discussed the role of the protective clotting proteins made by the liver.) Liver cells also synthesize nonessential amino acids and, as mentioned earlier, detoxify ammonia. In this process, ammonia is combined with carbon dioxide to form urea (u-re′ah). Urea, which is not harmful to the body cells, is then flushed from the body in urine.

Nutrients not needed by the liver cells, as well as the products of liver metabolism, are released into the blood and drain from the liver in the hepatic vein. These materials then enter the sys- temic circulation, where they become available to other body cells.

Cholesterol Metabolism and Transport

Although it’s a very important lipid in the diet, as we mentioned, cholesterol is not used as an energy fuel. Instead, it serves as the structural basis of steroid hormones and vitamin D and is a major building block of plasma membranes. Because we hear so much about “cutting down our cholesterol intake” in the media, it may be sur- prising to learn that only about 15 percent of blood cholesterol comes from the diet. The other 85 percent or so is made by the liver. Cholesterol is lost from the body when it is broken down and secreted in bile salts, which eventually leave the body in feces.

Fatty acids, fats, and cholesterol are insoluble in water, so they cannot circulate freely in the bloodstream. Instead they are transported bound to the small lipid-protein complexes called lipo- proteins. Although the entire story is complex, the important thing to know is that the low- density lipoproteins, or LDLs, transport cho- lesterol and other lipids to body cells, where they are used in various ways. If large amounts of LDLs are circulating, the chance that fatty sub- stances will be deposited on the arterial walls, initiating atherosclerosis, is high. Because of this

liver tissue. We have much more than we need, and even if part of it is damaged or removed, it is one of the few body organs that can regener- ate rapidly and easily.

A unique circulation, the hepatic portal circula- tion, brings nutrient-rich blood draining from the digestive viscera directly to the liver (see Chapter 11). The liver is the body’s major metabolic organ, and this detour that nutrients take through the liver ensures that the liver’s needs will be met first. As blood circulates slowly through the liver, liver cells remove amino acids, fatty acids, and glucose from the blood. These nutrients are stored for later use or processed in various ways. At the same time, the liv- er’s phagocytic cells remove and destroy bacteria that have managed to get through the walls of the diges- tive tract and into the blood.

General Metabolic Functions

The liver is vitally important in helping to maintain the blood glucose level within the normal range (around 100 mg glucose/100 ml of blood). After a carbohydrate-rich meal, thousands of glucose mol- ecules are removed from the blood and combined to form the large polysaccharide molecules called glycogen (gli′ko-jen), which are then stored in the liver. This process is glycogenesis (gli″ko- jen′ĕ-sis), literally, “glycogen formation” (genesis = beginning).

Later, as body cells continue to remove glu- cose from the blood to meet their needs, the blood glucose level begins to drop. At this time, liver cells break down the stored glycogen by a process called glycogenolysis (gli″ko-jen-ol′ ı̆-sis), which means “glycogen splitting.” The liver cells then release glucose bit by bit to the blood to maintain blood glucose homeostasis.

If necessary, the liver can also make glucose from noncarbohydrate substances, such as fats and proteins. This process is gluconeogenesis (glu″ko- ne″o-jen′ĕ-sis), which means “formation of new sugar” (Figure 14.23, p. 522). Hormones, such as thyroxine, insulin, and glucagon are vitally impor- tant in controlling the blood sugar level and in the handling of glucose in all body cells (see Chapter 9).

Some of the fats and fatty acids picked up by the liver cells are oxidized for energy (to make ATP) for use by the liver cells themselves. The rest are broken down to simpler substances such as acetic acid and acetoacetic acid and then are released into the blood or stored as fat reserves in

522 Essentials of Human Anatomy and Physiology

walls. In general, aerobic exercise, a diet low in saturated fats and cholesterol, and abstaining from smoking and drinking coffee all appear to favor a desirable HDL/LDL ratio.

Did You Get It? 28. What is gluconeogenesis? 29. What happens to the ammonia released when amino

acids are “burned” for energy? 30. If you had your choice, would you prefer to have high

blood levels of HDLs or LDLs? Explain your answer.

For answers, see Appendix A.

possibility, the LDLs are unkindly tagged as “bad lipoproteins.” By contrast, the lipoproteins that transport cholesterol from the tissue cells (or arteries) to the liver for disposal in bile are high- density lipoproteins, or HDLs. A high HDL level is considered “good” because the choles- terol is destined to be broken down and elimi- nated from the body. Obviously both LDLs and HDLs are “good and necessary”; it is just their relative ratio in the blood that determines whether or not potentially lethal cholesterol deposits are likely to be laid down on the artery

Glycogenesis: Glucose converted to glycogen and stored

Glycogenolysis: Stored glycogen converted to glucose

Gluconeogenesis: Amino acids and fats converted to glucose

Stimulus: Rising blood glucose level

Stimulus: Falling blood glucose level

HOMEOSTATIC BLOOD SUGAR

IMBALANCE

Figure 14.23 Metabolic events occurring in the liver as the blood glucose level rises and falls. When the blood glucose level is rising, the liver removes glucose

from the blood and stores it as glycogen (glycogenesis). When the blood glucose level falls, the liver breaks down stored glycogen (glycogenolysis) and makes new

glucose from amino acids and fats (gluconeogenesis). The glucose is then released to the blood to restore homeostasis of blood sugar.

Chapter 14: The Digestive System and Body Metabolism 523

most importantly on neural signals from the digestive tract, bloodborne signals related to body energy stores, and hormones. To a smaller degree, body temperature and psychological fac- tors also seem to play a role. All these factors appear to operate through feedback signals to the feeding centers of the brain. Brain receptors include thermoreceptors, chemoreceptors (for glucose, insulin, and others), and receptors that respond to leptin and other polypeptides. Sensors in peripheral locations have also been suggested, with the liver and gut itself (alimentary canal) the prime candidates.

Metabolic Rate and Body Heat Production

➔ Learning Objective □□ List several factors that influence metabolic rate,

and indicate each one’s effect.

When nutrients are broken down to produce cel- lular energy (ATP), they yield different amounts of energy. As mentioned earlier, the energy value of food is measured in a unit called the kilocalo- rie (kcal). In general, carbohydrates and proteins each yield 4 kcal/gram, and fats yield 9 kcal/ gram when they are broken down for energy production. Most meals, and even many individ- ual foods, are mixtures of carbohydrates, fats, and proteins. To determine the caloric value of a meal, we must know how many grams of each type of foodstuff it contains. For most of us, this is a difficult chore indeed, but approximations can easily be made with the help of a simple app such as MyFitnessPal.

Basal Metabolic Rate The amount of energy used by the body is also measured in kilocalories. The basal metabolic rate (BMR) is the amount of heat produced by the body per unit of time when it is under basal conditions—that is, at rest. It reflects the energy supply a person’s body needs just to perform essential life activities such as breathing, maintaining the heartbeat, and kidney function. An average 70-kg (154-pound) adult has a BMR of about 60 to 72 kcal/hour.

Many factors influence BMR, including surface area and gender. For example, small, thin males tend to have a higher BMR than large, obese females (Table 14.3). Age is also important; chil- dren and adolescents require large amounts of energy for growth and have relatively high BMRs.

Body Energy Balance ➔ Learning Objective

□□ Explain the importance of energy balance in the body, and indicate consequences of energy imbalance.

When any fuel is burned, it consumes oxygen and liberates heat. The “burning” of food fuels by body cells is no exception. Energy cannot be created or destroyed—it can only be converted from one form to another (see Chapter 2). If we apply this principle to cellular metabolism, it means that a dynamic balance exists between the body’s energy intake and its energy output:

Energy intake = total energy output (heat + work + energy storage)

Energy intake is the energy liberated during food oxidation—that is, during the reactions of glycolysis, the citric acid cycle, and the electron transport chain. Energy output includes the energy we immediately lose as heat (about 60 percent of the total), plus energy used to do work (driven by ATP), plus energy that is stored in the form of fat or glycogen. Energy storage is impor- tant only during periods of growth and during net fat deposit.

Regulation of Food Intake

When energy intake and energy output are bal- anced, body weight remains stable. When they are not, we either gain or lose weight. Because body weight in most people is surprisingly stable, mech- anisms that control food intake or heat production or both must exist. Unhappily for many people, the body’s weight-controlling systems appear to be designed more to protect us against weight loss than weight gain. (Some weight-control methods used by people who are obese are discussed in “A Closer Look” on pp. 529–530.)

But how is food intake controlled? That is a difficult question, and one that is still not fully answered. For example, what type of receptor could sense the body’s total calorie content and alert one to start eating or to put down that fork? Despite heroic research efforts, no such single receptor type has been found.

We have known for some time that the hypo- thalamus releases several peptides that influence feeding behavior. Current theories of how feed- ing behavior and hunger are regulated focus

524 Essentials of Human Anatomy and Physiology

total amount of kilocalories the body must con- sume to fuel all ongoing activities. Muscular work is the major body activity that increases the TMR. Even slight increases in skeletal muscle activity cause remarkable leaps in metabolic rate. When a well-trained athlete exercises vigorously for sev- eral minutes, the TMR may increase to 15 to 20 times normal, and it remains elevated for several hours afterward.

When the total number of kilocalories con- sumed is equal to the TMR, homeostasis is main- tained, and our weight remains constant. However, if we eat more than we need to sustain our activi- ties, excess kilocalories appear in the form of fat deposits. Conversely, if we are extremely active and do not properly feed the “metabolic furnace,” we begin to break down fat reserves and even tis- sue proteins to satisfy our TMR. This principle is used in every good weight-loss diet. (The total kilocalories needed are calculated on the basis of body size and age. Then, 20 percent or more of the requirements are cut from the daily diet.) If the dieting person exercises regularly, weight drops off even more quickly because the TMR increases above the person’s former rate.

Did You Get It? 31. Which of the following would you expect to yield a

relatively high BMR: old age, large surface area relative to body volume, female sex, deficient thyroxine production?

For the answer, see Appendix A.

In old age, the BMR decreases dramatically as the muscles begin to atrophy.

The amount of thyroxine produced by the thyroid gland is probably the most important fac- tor in determining a person’s BMR; hence, thyrox- ine has been dubbed the “metabolic hormone.” The more thyroxine produced, the higher the oxy- gen consumption and ATP use, and the higher the metabolic rate. In the past, most BMR tests were done to determine whether sufficient thyroxine was being made. Today, thyroid activity is more easily assessed by blood tests.

Homeostatic Imbalance 14.11

Hyperthyroidism causes a host of effects due to the excessive metabolic rate it produces. The body catabolizes stored fats and tissue proteins, and despite increased hunger and food intake, the per- son often loses weight. Bones weaken, and body muscles, including the heart, atrophy. In contrast, hypothyroidism results in slowed metabolism, obesity, and diminished thought processes. ____✚

Total Metabolic Rate When we are active, the body must oxidize more glucose to provide energy for the additional activities. Digesting food and even modest physical activity increase the body’s caloric requirements dramatically. These additional fuel requirements are above and beyond the energy required to maintain the body in the basal state. Total metabolic rate (TMR) refers to the

Table 14.3 Factors Determining the Basal Metabolic Rate (BMR)

factor variation Effect on BMR

Surface area Large surface area in relation to body volume, as in thin, small individuals

Small surface area in relation to body volume, as in large, heavy individuals

Increased

Decreased

Sex Male

Female

Increased

Decreased

Thyroxine production Increased

Decreased

Increased

Decreased

Age Young, rapid growth

Aging, elderly

Increased

Decreased

Strong emotions (anger or fear) and infections

Increased

Chapter 14: The Digestive System and Body Metabolism 525

When the core body temperature (the temper- ature of the deep organs) drops to the point beyond which simple constriction of skin capillar- ies can handle the situation, shivering begins. Shivering, involuntary shuddering contractions of the voluntary muscles, is very effective in increas- ing the body temperature because skeletal muscle activity produces large amounts of heat.

Homeostatic Imbalance 14.13

Extremely low body temperature resulting from prolonged exposure to cold is hypothermia. In hypothermia, the individual’s vital signs (respira- tory rate, blood pressure, heart rate) decrease. The person becomes drowsy and oddly comfortable, even though previously he or she felt extremely cold. Uncorrected, the situation progresses to coma and finally death as metabolic processes grind to a stop. _____________________________✚

Heat Loss Mechanisms Just as the body must be protected from becoming too cold, it must also be protected from excessively high temperatures. Most heat loss occurs through the skin via radia- tion or evaporation. When body temperature increases above the normal range, the blood ves- sels serving the skin dilate, and capillary beds in the skin become flushed with warm blood. As a result, heat radiates from the skin surface. However, if the external environment is as hot as or hotter than the body, heat cannot be lost by radiation. In such cases, the only means of getting rid of excess heat is by the evaporation of perspi- ration off the skin surface. This is an efficient means of liberating body heat as long as the air is dry. If it is humid, evaporation occurs at a much slower rate. Our heat-liberating mechanisms don’t work well, and we feel miserable and irritable.

Homeostatic Imbalance 14.14

When normal heat loss processes become ineffec- tive, the resulting hyperthermia, or elevated body temperature, depresses the activity of the hypothal- amus. As a result, a vicious positive feedback cycle occurs: The soaring body temperature increases the metabolic rate, which in turn increases heat pro- duction. The skin becomes hot and dry; and, as the temperature continues to spiral upward, permanent

Body Temperature Regulation

➔ Learning Objective □□ Describe how body temperature is regulated.

Although we have been emphasizing that foods are “burned” to produce ATP, remember that ATP is not the only product of cell catabolism. Most of the energy released during food oxida- tion escapes as heat. Less than 40 percent of available food energy is actually captured to form ATP. The heat released warms the tissues and, more important, the blood, which circu- lates to all body tissues, keeping them at homeo- static temperatures. This allows metabolism to occur efficiently.

Body temperature reflects the balance between heat production and heat loss. The body’s thermostat is in the hypothalamus of the brain. Through autonomic nervous system path- ways, the hypothalamus continuously regulates body temperature around a set point of 37°C, fluctuating within the range of 35.6° to 37.8°C (96° to 100°F). The hypothalamus does this by initiating heat-promoting mechanisms or heat loss mechanisms (Figure 14.24, p. 526).

Heat-Promoting Mechanisms When the envi- ronmental temperature is low, the body must produce more heat to maintain normal body tem- perature. And if, for whatever reason, the tem- perature of circulating blood falls, body heat must be conserved and more heat generated to restore normal body (blood) temperature. Short- term means of accomplishing this are vasocon- striction of blood vessels of the skin and shivering.

When the skin vasculature constricts, the blood temporarily bypasses the skin and is rerouted to the deeper, more vital body organs. When this happens, the temperature of the exposed skin drops to that of the external environment.

Homeostatic Imbalance 14.12

Restriction of blood delivery to the skin is no prob- lem for brief periods of time. But if it is extended, the skin cells, chilled by internal ice crystals and deprived of oxygen and nutrients, begin to die. This condition, called frostbite, can cause the loss of extremities such as fingers and toes. ________ ✚

526 Essentials of Human Anatomy and Physiology

IMBALANCE

Activates heat loss center in hypothalamus

Blood warmer than hypothalamic set point

Sweat glands are activated: Secrete perspiration, which is vaporized by body heat, helping to cool the body

Stimulus: Increased body temperature (e.g., when exercising or the climate is hot)

HOMEOSTASIS = NORMAL BODY TEMPERATURE (35.6˚C–37.8˚C)

Stimulus: Decreased body temperature (e.g., due to cold environmental temperatures)

Blood cooler than hypothalamic set point

Skin blood vessels constrict: Blood is diverted from skin capillaries and withdrawn to deeper tissues; minimizes

overall heat loss from skin surface

Skin blood vessels dilate: Capillaries become flushed with warm blood; heat radiates from skin surface

Activates heat- promoting center in hypothalamus

Skeletal muscles are activated when more heat must be generated; shivering begins

Body temperature decreases: Blood temperature declines, and hypothalamus heat-loss center “shuts off”

Body temperature increases: Blood temperature rises, and hypothalamus heat-promoting center “shuts off”

Figure 14.24 Mechanisms of body temperature regulation.

Chapter 14: The Digestive System and Body Metabolism 527

PART III: develoPmenTAl AsPecTs of The dIgesTIve SYSTEM AND METABOLISM ➔ Learning Objectives

□□ Name important congenital disorders of the digestive system and significant inborn errors of metabolism.

□□ Describe the effect of aging on the digestive system.

The very young embryo is flat and pancake- shaped. However, it soon folds to form a cylindri- cal body, and its internal cavity becomes the cavity of the alimentary canal. By the fifth week of devel- opment, the alimentary canal is a continuous tube- like structure extending from the mouth to the anus. Shortly after, the digestive glands (salivary glands, liver, and pancreas) bud out from the mucosa of the alimentary tube. These glands retain their connections (ducts) and can easily empty their secretions into the alimentary canal to pro- mote its digestive functions.

Homeostatic Imbalance 14.15

The digestive system is susceptible to many con- genital defects that interfere with feeding. The most common is the cleft palate/cleft lip defect.

brain damage becomes a distinct possibility. This condition, called heat stroke, can be fatal unless rapid corrective measures are taken immediately (immersion in cool water and administration of fluids).

Heat exhaustion is the term used to describe the heat-associated collapse of an individual dur- ing or following vigorous physical activity. Heat exhaustion results from excessive loss of body flu- ids (dehydration) and is evidenced by low blood pressure, a rapid heartbeat, and cool, clammy skin. In contrast to heat stroke, heat loss mecha- nisms still operate in heat exhaustion. _________✚

Fever is controlled hyperthermia. Most often, it results from infection somewhere in the body, but it may be caused by other conditions (cancer, aller- gic reactions, and CNS injuries). Macrophages, white blood cells, and injured tissue cells release chemical substances called pyrogens (pi′ro-jenz) that act directly on the hypothalamus, causing its thermostat to be set to a higher temperature (pyro = fire). After thermostat resetting, heat-promoting mechanisms are initiated. Vasoconstriction causes the skin to become cool, and shivering begins to generate heat. This situation, called “the chills,” is a sure sign that body temperature is rising. Body temperature is allowed to rise until it reaches the new setting. Then, it is maintained at the “fever set- ting” until natural body defense processes or anti- biotics reverse the disease process. At that point, the thermostat is reset again to the lower normal level, causing heat loss mechanisms to swing into action—the individual begins to sweat, and the skin becomes flushed and warm. Physicians have long recognized that these signs signal a turn for the better, saying that the patient’s fever has “bro- ken” once body temperature begins falling.

Fever, by increasing the metabolic rate, helps speed the various healing processes and also appears to inhibit bacterial growth (see Chapter 12). The danger of fever is that if the body thermostat is set too high, body proteins may be denatured (unfolded), and permanent brain damage may occur.

Did You Get It? 32. What are two means of either maintaining or

increasing body temperature? 33. How does vasodilation of skin blood vessels affect

body temperature on a hot day?

For answers, see Appendix A.

A baby born with a cleft lip and palate.

Of the two, cleft palate is more serious because the child is unable to suck properly. Another rela- tively common congenital defect is tracheoesoph- ageal fistula (tra″ke-o-ĕ-sof′ah-je-al fis′tu-lah), an abnormal connection between the esophagus and the trachea. In addition, the esophagus often (but not always) ends in a blind sac and does not

528 Essentials of Human Anatomy and Physiology

mation of the gastrointestinal tract, or gastroen- teritis [gas″tro-en-ter-i′tis]). Inflammation of the appendix, appendicitis, is particularly common in teenagers for some unknown reason.

Between middle age and early old age, the meta- bolic rate decreases by 5 to 8 percent in every 10-year period. This is the time of life when the weight seems to creep up, and obesity often becomes a fact of life. To maintain desired weight, we must be aware of this gradual change and be prepared to reduce caloric intake. Two distinctly middle-age digestive problems are ulcers (see “A Closer Look” on p. 512) and gallbladder problems (inflammation of the gallbladder or gallstones). For some unknown reason, irritable bowel syndrome (IBS), characterized by alternating bouts of diarrhea and constipation, is becoming much more prevalent in adults, but its cause has been difficult to pin down.

During old age, activity of the GI tract declines. Fewer digestive juices are produced, and peristalsis slows. Taste and smell become less acute, and peri- odontal disease often develops. Many elderly indi- viduals live alone or on a reduced income. These factors, along with increasing physical disability, tend to make eating less appealing, and nutrition is inadequate in many of our elderly citizens. Diverticulosis, the bulging of the colon wall into the lumen, and cancer of the gastrointestinal tract are fairly common problems. Cancer of the stomach and colon rarely show early signs and often pro- gress to an inoperable stage because they spread to distant parts of the body before the individual seeks medical attention. However, when detected early, both diseases are treatable. A diet high in plant fiber and low in fat may help to decrease the inci- dence of colon cancer. Additionally, because most colorectal cancers derive from initially benign mucosal tumors called polyps, and because the incidence of polyp formation increases with age, colonoscopy screenings should begin at the age of 50 and be repeated every 10 years. However, very sensitive fecal occult blood tests, which look for blood in the stool, should be completed annually.

Did You Get It? 34. How does cystic fibrosis affect digestion? 35. What dietary changes must be made to prevent brain

damage in children with PKU? 36. What occurs when your total caloric intake exceeds

your TMR?

For answers, see Appendix A.

connect to the stomach. The baby chokes, drools, and becomes cyanotic during feedings because food is entering the respiratory passageways. All three defects can be corrected surgically.

There are many types of genetic problems that interfere with metabolism, but perhaps the two most common are cystic fibrosis and phenylketonuria (fen″il-ke′to-nu″re-ah). Cystic fibrosis (CF) primar- ily affects the lungs, but it also significantly impairs the activity of the pancreas. In CF, huge amounts of mucus are produced, which block the passages of involved organs. Blockage of the pancreatic duct prevents pancreatic fluid from reaching the small intestine. As a result, fats and fat-soluble vitamins are not digested or absorbed, and bulky, fat-laden stools result. This condition is usually handled by administering pancreatic enzymes with meals.

Phenylketonuria (PKU) involves an inability of tissue cells to use phenylalanine (fen″il-al′ah- nin), an amino acid present in all protein foods. In such cases, brain damage and intellectual dysfunc- tion occur unless a special diet low in phenylala- nine is prescribed. ___________________________✚

The developing infant receives all its nutrients through the placenta, and at least at this period of life, obtaining and processing nutrients is no prob- lem (assuming the mother is adequately nour- ished). Obtaining nutrition is the most important activity of the newborn baby, and several reflexes present at this time help in this activity. For exam- ple, the rooting reflex helps the infant find the nipple (mother’s or bottle), and the sucking reflex helps him or her to hold on to the nipple and swallow. The stomach of a newborn infant is very small, so feeding must be frequent (every 3 to 4 hours). Peristalsis is rather inefficient at this time, and vomiting is not at all unusual.

Teething begins around age 6 months and con- tinues until about the age of 2 years. During this interval, the infant progresses to more and more solid foods and usually is eating an adult diet by toddlerhood. Appetite decreases in the elementary school–aged child and then increases again during the rapid growth of adolescence. (Parents of ado- lescents can expect higher grocery bills!)

All through childhood and into adulthood, the digestive system usually operates with relatively few problems unless there are abnormal interfer- ences, such as contaminated food or extremely spicy or irritating foods (which may cause inflam-

529

➤

A CLOSER

LOOK

ow fat is too fat? What distin- guishes an obese person from one who is merely overweight?

The bathroom scale is a poor guide, because body weight alone cannot reveal body composition. Dense bones and well-developed muscles may make a person technically overweight.

The most common view of obesity is that it is a condition of excessive fat storage. A body fat content that is 18 to 22 percent of body weight (males and females, respectively) is consid- ered normal for adults. Anything over that is defined as obesity.

Body fat content can be estimated using the body mass index (BMI) scale, an index of a person’s weight relative to height. To estimate BMI, multiply weight in pounds (lb) by 703 and then divide by your height in inches squared:

technique that measures bone min- eral content and uses that informa- tion to estimate the amounts of lean tissue and fat.

• Air displacement plethysmography technology, known widely as the Bod Pod, is a way of measuring body composition using air displace- ment to determine body volume and thus body density (density is the mass, or weight, divided by volume).

• Underwater weighing is similar to the Bod Pod but is performed with a water-filled rather than air-filled chamber.

Of course, the most accurate method for determining body compo- sition is dissection, which can only be performed after death. Although the three techniques described here are more accurate than BMI, they are also more expensive and less readily avail- able to the general population.

Obesity: Magical Solution Wanted

However it’s defined, obesity is a perplexing and poorly understood dis- ease. Despite its adverse effects on health (a higher incidence of arterio- sclerosis, hypertension, coronary artery disease, and diabetes mellitus), it is the most common health problem in the United States. According to the Centers for Disease Control and Pre- vention (CDC), nearly 35% of adults and about 17% of all youth ages 2–19 are obese in the United States. Overeating, consuming fried foods and processed foods, and sedentary activities, such as video gaming or watching television, have replaced healthy eating and exercise.

With all its health and social prob- lems, it’s a fair bet that few people choose to be obese. So what causes obesity? Let’s look at three theories.

• Overeating during childhood. Some researchers believe that the

BMI = (height in inches)

2 wt(lb)

* 703

A BMI of 25 or more is defined as overweight, and a BMI of 30 or above is defined as obese. Although BMI cal- culations are simple and inexpensive, it is important to note that they are not diagnostic. In other words, BMI does not measure body composition directly, so a high BMI does not necessarily rep- resent a health crisis. For example, a healthy athlete with a high percentage of lean muscle mass may have a BMI in the overweight or even obese range. In order to verify high body fat content, other testing would need to be done. Let’s look at three such tests.

• DEXA, or dual-energy X-ray absorption, is a special X-ray

A CLOSER LOOK Obesity: Magical Solution Wanted (continued) “clean your plate” syndrome early in life sets the stage for adult obe- sity by increasing the number of fat cells formed during childhood. Then, during adulthood, the more cells there are, the more fat can be stored.

• Greater fuel efficiency in obese people. Obese people may be more fuel efficient and more effec- tive “fat storers.” Although it is often assumed that obese people eat more than other people, this is not necessarily true—many actually eat less than people of normal weight.

Fat packs more wallop per cal- orie (is more fattening) than pro- teins or carbohydrates because very little energy is used to pro- cess fat. For example, when someone ingests 100 excess car- bohydrate calories, 23 calories are used in metabolic processing, and 77 are stored. However, if the excess calories come from fat, only 3 calories are “burned,” and 97 are stored. These facts apply to everyone, but for obese individu- als, the picture is even bleaker. Fat cells of overweight people “sprout” more receptors that favor fat accumulation, and their lipoprotein lipase enzyme, which unloads fat from the blood, is exceptionally efficient.

• Genetic predisposition. Morbid obesity is the fate of people inherit- ing two obesity genes. However, a true genetic predisposition for “fat- ness” accounts for only about 5 percent of obesity in the United States. These people, given excess

calories, will always deposit them as fat.

False and Risky Cures Many people have “tried every- thing” to lose weight. Poor choices for fighting obesity include diuretics; amphetamines; over-the-counter medications containing ephedra; and drugs that interfere with fat, and thus fat-soluble vitamin, absorp- tion. All of these have serious health risks such as electrolyte imbalance, heart valve malfunction, cardiovas- cular problems, liver damage, stroke, seizures, and, in some cases, death. Because weight-loss supplements are not monitored by the U.S. Food and Drug Administration (FDA), the true extent of the damage they cause is not known.

Diet products and books sell well, but which ones really work? At the present time, many dieters find themselves choosing between two “dueling” schools of thought: those promoting low-carbohydrate (high- protein and high-fat) diets such as the Atkins and South Beach diets, and those espousing the traditional low-fat diet (high in complex carbo- hydrates). Clinical studies show that people on low-carbohydrate diets lose weight more quickly at first but tend to plateau at 6 months. Among long-term dieters, people on low-fat diets lost just as much weight as did those on the low-carbohydrate diets.

Surgery In desperation, some people turn to surgery. Options for bariatric surgery

(to reduce stomach size) mainly focus on reducing food intake, reducing the amount of food absorbed, or both. Procedures such as gastric bypass or sleeve gastrectomy can make the stomach smaller through placement of a gastric band or surgical removal of part of the stomach. These solu- tions help patients feel full after a small meal. To reduce food absorp- tion, one approach is to route food away from a portion of the small intestine, as in biliopancreatic diver- sion (BPD). During BPD surgery, the lower half of the small intestine is connected directly to the stomach in such a way that bile and pancre- atic juices do not pass through this “new” segment of the small intestine. Because pancreatic enzymes are absent, fewer nutrients are available for reabsorption.

Liposuction directly removes fat. In this procedure, suction is used to remove adipose tissue in order to reshape the body. Like BPD, it carries all the risks of surgery, and unless the patient changes his or her eating hab- its, the remaining fat deposits else- where in the body will overfill.

eat less, exercise more Unfortunately, there is no magical solution for obesity. The best way for most of us to lose weight is to take in fewer fat calories and increase physi- cal activity. Resistance exercise also helps because it increases muscle mass, and muscle consumes more energy at rest compared to fat. The only way to keep the weight off is to make healthy eating and exercise life- long habits.

530

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Reproductive System

Respiratory System

Nervous System

Integumentary System

Endocrine System

• Digestive system provides nutrients to heart and blood vessels; absorbs iron needed for hemoglobin synthesis; absorbs water necessary for normal blood volume

• Cardiovascular system transports nutrients absorbed by alimentary canal to all tissues of the body; distributes hormones of the digestive tract

• Digestive system provides nutrients for energy fuel, growth, and repair

• Kidneys transform vitamin D to its active form, which is needed for calcium absorption; excrete some bilirubin produced by the liver

• Digestive system provides nutrients for energy fuel, growth, and repair; liver removes lactic acid, resulting from muscle activity, from the blood

• Skeletal muscle activity increases contractions of GI tract

• Digestive system provides nutrients for normal functioning; HCl of stomach provides nonspecific protection against bacteria

• Lacteals drain fatty lymph from digestive tract organs and convey it to blood; Peyer's patches and lymphoid tissue in mesentery house macrophages and immune cells that protect digestive tract organs against infection

• Digestive system provides nutrients for energy fuel, growth, and repair and extra nutrition needed to support fetal growth

• Digestive system provides nutrients for energy metabolism, growth, and repair

• Respiratory system provides oxygen and carries away carbon dioxide produced by digestive system organs

• Digestive system provides nutrients for normal neural functioning

• Neural controls of digestive function; in general, parasympathetic fibers accelerate and sympathetic fibers inhibit digestive activity; reflex and voluntary controls of defecation

• Digestive system provides nutrients for energy fuel, growth, and repair; supplies fats that provide insulation in subcutaneous tissues

• The skin synthesizes vitamin D needed for calcium absorption from the intestine; protects by enclosure

Skeletal System

• Digestive system provides nutrients for energy fuel, growth, and repair; absorbs calcium needed for bone salts

• Skeletal system protects some digestive organs by bone; cavities store some nutrients (e.g., calcium, fats)

• Liver removes hormones from blood, ending their activity; digestive system provides nutrients needed for energy fuel, growth, and repair; pancreas has hormone-producing cells

• Local hormones help regulate digestive function

Digestive System

Homeostatic Relationships between the Digestive System and Other Body Systems

SYSTEMS IN SYNC

531

532 Essentials of Human Anatomy and Physiology

teeth (32 of them) begin to replace deciduous teeth around age 7. A typical tooth consists of crown covered with enamel and root covered with cement. Most of the tooth is bonelike dentin. The pulp cav- ity contains blood vessels and nerves.

4. Salivary glands (three pairs—parotid, submandibu- lar, and sublingual) secrete saliva into the oral cav- ity. Saliva contains mucus and serous fluids. The serous component contains salivary amylase.

5. Several accessory organs deposit substances into the alimentary tube via ducts.

a. The pancreas is a soft gland lying in the mesen- tery between the stomach and small intestine. Pancreatic juice contains enzymes (which digest all categories of foods) in an alkaline fluid.

b. The liver is a four-lobed organ overlying the stomach. Its digestive function is to produce bile, which it ducts into the small intestine.

c. The gallbladder is a muscular sac that stores and concentrates bile. When fat digestion is not occurring, the continuously made bile backs up the cystic duct and enters the gallbladder.

Functions of the Digestive System (pp. 502–513)

1. Foods must be broken down to their building blocks to be absorbed. Building blocks of carbohydrates are simple sugars, or monosaccharides. Building blocks of proteins are amino acids. Building blocks of fats, or lipids, are fatty acids and glycerol, and building blocks of nucleic acids are nucleotides.

2. Both mechanical (chewing) and chemical food breakdown begin in the mouth. Saliva contains mucus, which helps bind food together into a bolus, and salivary amylase, which begins the chemical breakdown of starch. Saliva is secreted in response to food in the mouth, mechanical pres- sure, and psychic stimuli. Essentially no food absorption occurs in the mouth.

3. Swallowing has two phases: The buccal phase is voluntary; the tongue pushes the bolus into the pharynx. The involuntary pharyngeal-esophageal phase involves the closing off of nasal and respira- tory passages and the conduction of food to the stomach by peristalsis.

4. When food enters the stomach, gastric secretion is stimulated by the vagus nerve and by gastrin (a local hormone). Hydrochloric acid activates the protein- digesting enzyme pepsin, and digestion of proteins

Summary

PART I: AnATomy And PhysIology OF THE DIGESTIVE SYSTEM (pp. 489–513)

Anatomy of the digestive system (pp. 489–502)

1. The digestive system consists of the alimentary canal (a hollow tube extending from mouth to anus) and several accessory digestive organs. The wall of the alimentary canal has four main tissue layers—mucosa, submucosa, muscularis externa, serosa. The serosa (visceral peritoneum) is continuous with the parietal peritoneum, which lines the abdominal cavity wall.

2. Organs of the alimentary canal:

a. The mouth, or oral cavity, contains teeth and tongue and is bound by lips, cheeks, and palate. Tonsils guard its posterior margin.

b. The pharynx is a muscular tube that provides a passageway for food and air.

c. The esophagus is a muscular tube that com- pletes the passageway from the pharynx to the stomach.

d. The stomach is a C-shaped organ located on the left side of the abdomen beneath the diaphragm. Food enters it through the cardioesophageal sphincter and leaves it to enter the small intestine through the pyloric sphincter. The stomach has a third oblique layer of muscle in its wall that allows it to perform mixing or churning movements. Gastric glands produce hydrochloric acid, pepsin, rennin, mucus, gastrin, and intrinsic factor. Mucus protects the stomach itself from being digested.

e. The tubelike small intestine is suspended from the posterior body wall by the mesentery. Its subdivisions are the duodenum, jejunum, and ileum. Food digestion and absorption are com- pleted here. Pancreatic juice and bile enter the duodenum through a sphincter at the distal end of the bile duct. Microvilli, villi, and circular folds increase the surface area of the small intestine for enhanced absorption.

f. The large intestine frames the small intestine. Subdivisions are the cecum; appendix; ascend- ing, transverse, and descending colon; sigmoid colon; rectum; and anal canal. The large intes- tine delivers undigested food residue (feces) to the body exterior.

3. Two sets of teeth are formed. The first set consists of 20 deciduous teeth that begin to appear at 6 months and are lost by 12 years of age. Permanent

Chapter 14: The Digestive System and Body Metabolism 533

begins. Food is also mechanically broken down by the churning activity of stomach muscles. Movement of chyme into the small intestine is controlled by the enterogastric reflex.

5. Digestion of fats, proteins, and carbohydrates is completed in the small intestine by intestinal enzymes and, more important, pancreatic enzymes. Alkaline pancreatic juice neutralizes acidic chyme and provides the proper environment for the oper- ation of its enzymes. Both pancreatic juice (the only source of lipases) and bile (formed by the liver) are necessary for normal fat breakdown and absorption. Bile acts as a fat emulsifier. Secretin and cholecystokinin, hormones produced by the small intestine, stimulate release of bile and pan- creatic juice. Segmental movements mix foods; peristaltic movements move foodstuffs along the small intestine.

6. Most nutrient absorption occurs by active transport into the capillary blood of the villi. Fats are absorbed by diffusion into both capillary blood and lacteals in the villi.

7. The large intestine receives bacteria-laden indigest- ible food residue. Activities of the large intestine are absorption of water, of salts, and of vitamins made by resident bacteria. When feces are deliv- ered to the rectum by mass peristalsis, the defeca- tion reflex is initiated.

PART II: NuTRITION AND METABOLISM (pp. 513–527)

Nutrition (pp. 513–516)

1. Most foods are used as fuels to form ATP; proteins and cholesterol are exceptions.

2. A nutrient is a substance in food used to promote growth, maintenance, and repair of the body.

3. The major nutrients are carbohydrates, lipids, pro- teins, and water. Vitamins and minerals are required in very small amounts.

4. Dietary carbohydrates (sugars and starch) are included in fruits and vegetables (plant products).

5. Dietary lipids are found in meats, dairy products, and vegetable oils.

6. Eggs, milk, meats, poultry, and fish are rich sources of protein.

7. Vitamins, found mainly in fruits, vegetables, and milk, function mainly as coenzymes in the body.

8. Minerals, most plentiful in vegetables and legumes, are mainly important for enzyme activity. Iron is important to make hemoglobin, and calcium is im- portant for building bone, blood clotting, nerve im- pulses, muscle contraction, and secretory activities.

Metabolism (pp. 516–527)

1. Metabolism includes all chemical breakdown (cata- bolic) and building (anabolic) reactions needed to maintain life.

2. Carbohydrates, the most important of which is glu- cose, are the body’s major energy fuel. As glucose is oxidized, carbon dioxide, water, and ATP are formed. The sequential pathways of glucose catab- olism are glycolysis (which occurs in the cytosol) and the citric acid cycle and electron transport chain (which function in the mitochondria). During hyperglycemia, glucose is stored as glycogen or converted to fat. In hypoglycemia, glycogenolysis, gluconeogenesis, and fat breakdown occur to restore the normal blood glucose level.

3. Fats insulate the body, protect organs, build some cell structures (membranes and myelin sheaths), and provide reserve energy. When carbohydrates are in limited supply, more fats are oxidized to pro- duce ATP. Excessive fat breakdown causes blood to become acidic. Excess dietary fat is stored in sub- cutaneous tissue and other fat deposits.

4. Proteins form the bulk of cell structure and most functional molecules. They are carefully conserved by body cells. Amino acids are actively taken up from blood by tissue cells; those that cannot be made by body cells are called essential amino acids. Amino acids are oxidized to form ATP when other fuel sources are not available. Ammonia is released as amino acids are catabolized.

5. The liver is the body’s key metabolic organ. Its cells remove nutrients from hepatic portal blood. It performs glycogenesis, glycogenolysis, and gluconeogenesis to maintain homeostasis of the blood glucose level. Its cells make blood proteins and other substances and release them to blood. Liver cells also detoxify ammonia by combining it with carbon dioxide to form urea. Liver cells burn fats to provide some of their energy (ATP); excesses are stored or released to blood in simpler forms that can be used by other tissue cells. Phagocytic cells remove bacteria from hepatic portal blood. Most cholesterol is made by the liver; cholesterol breakdown products are secreted in bile. Fats and cholesterol are transported in the blood by

534 Essentials of Human Anatomy and Physiology

ering) as necessary to maintain body tempera- ture within the normal range. Fever (hyperthermia) represents body temperature regulated at a higher- than-normal level.

PART III: develoPmenTAl AsPecTs of The dIgesTIve sysTem And METABOLISM (pp. 527–531) 1. The alimentary canal forms as a hollow tube.

Accessory glands form as outpocketings from this tube.

2. Common congenital defects include cleft palate, cleft lip, and tracheoesophageal fistula, all of which interfere with normal nutrition. Common inborn errors of metabolism are cystic fibrosis (CF) and phenylketonuria (PKU).

3. Various inflammatory conditions plague the diges- tive system throughout life. Appendicitis is com- mon in adolescents; gastroenteritis and food poisoning can occur at any time (if irritating fac- tors are present); and ulcers and gallbladder problems increase in middle age. Obesity and diabetes mellitus are bothersome during later middle age.

4. Efficiency of all digestive system processes de- creases in the elderly. Gastrointestinal cancers, such as stomach and colon cancer, appear with increas- ing frequency in an aging population.

lipoproteins. LDLs transport cholesterol to body cells; HDLs carry cholesterol to the liver for degra- dation. Cholesterol is used to make functional mol- ecules and for some structural purposes; it is not used for ATP synthesis.

6. A dynamic balance exists between energy intake and total energy output (heat + work + energy stor- age). Interference with this balance results in obe- sity or in malnutrition leading to body wasting.

7. When the three major types of foods are oxidized for energy, they yield different amounts of energy. Carbohydrates and proteins yield 4 kcal/gram; fats yield 9 kcal/gram. Basal metabolic rate (BMR) is the total amount of energy used by the body in a basal (resting) state. Age, sex, body surface area, and amount of thyroxine produced influ- ence BMR.

8. Total metabolic rate (TMR) is the number of calo- ries used by the body to accomplish all ongoing daily activities. It increases dramatically as muscle activity increases. When TMR equals total caloric intake, weight remains constant.

9. As foods are catabolized to form ATP, more than 60 percent of energy released escapes as heat, warming the body. The hypothalamus initiates heat loss processes (radiation of heat from skin and evaporation of sweat) or heat-promoting processes (vasoconstriction of skin blood vessels and shiv-

Review Questions

Multiple Choice

More than one choice may apply.

1. Which of the following digestive organs are accessory organs?

a. Esophagus

b. Stomach

c. Pancreas

d. Liver

2. Ducts that are part of the digestive system include the

a. bile duct.

b. pancreatic duct.

c. thoracic duct.

d. ejaculatory duct.

e. cystic duct.

3. The GI tube tissue layer responsible for the actions of segmentation and peristalsis is the

a. serosa.

b. mucosa.

c. muscularis externa.

d. submucosa.

4. Match the digestive organ listed in column B with the function listed in column A.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Column A Column B

____ 1. produces bile ____ 2. absorbs water ____ 3. churning occurs here ____ 4. muscular tube con-

necting the laryn- gopharynx with the stomach

____ 5. produces both endo- crine and exocrine secretions

____ 6. secretes a substance that initiates carbohy- drate digestion

____ 7. stores bile ____ 8. segmentation occurs

here

a. salivary glands

b. esophagus

c. stomach

d. small intestine

e. liver

f. gallbladder

g. pancreas

h. large intestine

Chapter 14: The Digestive System and Body Metabolism 535

13. Complete this statement. In glycolysis, _________ is oxidized, and _________ is reduced.

a. vitamin-containing coenzyme; glucose

b. ATP; ADP

c. glucose; oxygen

d. glucose; vitamin-containing coenzyme

Short Answer Essay

14. Make a simple line drawing of the organs of the alimentary tube, and label each organ.

15. Add three labels to your drawing—salivary glands, liver, and pancreas—and use arrows to show where each of these organs empties its secretion into the alimentary tube.

16. Name the layers of the alimentary tube wall from the lumen outward.

17. What is the mesentery? The peritoneum?

18. Name the subdivisions of the small intestine beginning near the stomach. Do the same for the subdivisions of the large intestine, beginning closest to the small intestine.

19. Name the two major regions of a tooth. Briefly describe them. What connects these two regions?

20. Describe how salivary secretions assist the digestive process.

21. Assume you have been chewing a piece of bread for five or six minutes. How would you expect its taste to change during this time? Why?

22. Name the three subdivisions of the pharynx, and briefly describe their locations.

23. What hormones and hormonelike products are secreted by the stomach?

24. Bile emulsifies fat. Define emulsify.

25. Where are CCK and secretin produced? What is the stimulus for their release?

26. Describe the two phases of swallowing.

27. How do segmental and peristaltic movements differ?

28. A cream cheese and jelly sandwich contains proteins, carbohydrates, and fats. Describe what happens to the sandwich when you eat it relative to events occurring in ingestion, digestion, ab- sorption, and defecation.

29. Which structures greatly increase the absorptive surface of the small intestine?

30. How is the passage of feces facilitated?

31. Define defecation reflex, constipation, and diarrhea.

5. Which of the following is (are) true concerning the stomach?

a. It is located within the peritoneum.

b. The pylorus is continuous with the large intestine.

c. It covers the liver.

d. The breakdown of fats starts in it.

6. Which of these organs lie in the left hypochondriac region of the abdomen?

a. Spleen

b. Stomach

c. Cecum

d. Duodenum

7. Release of secretin leads to

a. contraction of smooth muscle in the duodenal papilla.

b. increased secretory activity of liver cells.

c. contraction of the gallbladder wall.

d. release of bicarbonate-rich fluid by the pancreas.

8. The pH of chyme entering the duodenum is adjusted by

a. bile.

b. intestinal juice.

c. enzyme secretions from the pancreas.

d. bicarbonate-rich secretions from the pancreas.

9. A 3-year-old girl is rewarded with a hug because she is now completely toilet trained. Which muscle is one that she has learned to control?

a. Levator ani

b. Internal anal sphincter

c. Internal and external obliques

d. External anal sphincter

10. Hormones that act to decrease the blood glucose level include

a. insulin.

b. glucagon.

c. epinephrine

d. growth hormone.

11. Which events occur shortly after eating?

a. Use of amino acids as a major source of energy

b. Lipogenesis (and fat deposit)

c. Breakdown of fat reserves

d. Increased uptake of glucose by skeletal muscles and other body tissues

12. What is the approximate time of eruption of the third molars, also called wisdom teeth?

a. 6–7 years

b. 12–13 years

c. 17–25 years

d. They do not erupt sometimes.

536 Essentials of Human Anatomy and Physiology

Critical Thinking and Clinical Application Questions

45. Ryan, a 6-year-old child who is allergic to milk, has extremely bowed legs. What condition do you sus- pect, and what is the connection to not drinking milk?

46. Kathy’s baby is not thriving because she has prob- lems sucking. Outwardly, no abnormalities can be observed, but a close examination of the baby’s mouth reveals a possible cause of her condition. Explain what the cause is, and how this condition can be corrected.

47. An anorexic girl shows a high level of acetone in her blood. What is this condition called, and what has caused it?

48. Every year dozens of elderly people are found dead in their unheated apartments—victims of hypothermia. What is hypothermia, and how does it kill?

49. Charlie has been vomiting ever since he ate last night’s fast food for breakfast. What could be a pos- sible cause of his emesis?

50. Kayla has been on antibiotics for an extended period. A visit to her physician reveals that she has a vitamin K deficiency. What is the cause-and-effect relationship here?

41. After chopping wood for about two hours on a hot but breezy afternoon, John stumbled into the house and then fainted. His T-shirt was wringing wet with perspiration, and his pulse was faint and rapid. Was he suffering from heat stroke or heat exhaustion? Explain your reasoning, and note what you should do to help John recover.

42. When Serge went to his doctor with the reports of his annual health checkup, the doctor was not happy with the HDL/LDL ratio in Serge’s blood. The LDL level being far too high, he was concerned that Serge might have atherosclerosis. How can an undesirable HDL/LDL ratio lead to atherosclerosis?

43. A young woman is put through an extensive bat- tery of tests to determine the cause of her stomach pains. She is diagnosed with gastric ulcers. An anti- histamine drug is prescribed, and she is sent home. What is the mechanism of her medication? What life-threatening problems can result from a poorly managed ulcer? Why did the clinic doctor warn the woman not to take aspirin?

44. Continuing from the previous question, the wom- an’s ulcer got worse. She started complaining of back pain. The physician discovered that the back pain occurred because the pancreas was now dam- aged. Use logic to deduce how a perforating gastric ulcer could come to damage the pancreas.

Critical Thinking and Clinical Application Questions

20 percent protein, 30 percent carbohydrate, and 10 percent fat, how many kilocalories did you consume?

37. In terms of surface area, which kinds of individuals will have a high basal metabolic rate (BMR)?

38. Name two ways in which heat is lost from the body.

39. What is fever? What does it indicate?

40. Why must the feeding of a newborn baby be frequent? Which reflexes help a newborn to obtain nutrition?

32. Define metabolism, anabolism, and catabolism.

33. Define gluconeogenesis, glycogenolysis, and glyco- genesis.

34. What are the major food groups?

35. What is the harmful result when excessive amounts of fats are burned to produce ATP? Name two conditions that might lead to this result.

36. How many calories are produced when 1 gram of carbohydrate is oxidized? 1 gram of protein? 1 gram of fat? If you just ate 100 grams of food that was

537

Disposing of wastes and excess ions is only part of the kidneys’ work. As they perform these excretory functions, the kidneys also regulate blood volume and maintain the proper balance between water and salts, and between acids and bases.

The kidneys have other regulatory functions, too:

• By producing the enzyme renin, they help regulate blood pressure.

• The hormone erythropoietin, released by the kidneys, stimulates red blood cell production in bone marrow (see Chapter 10).

• Kidney cells convert vitamin D to its active form.

The kidneys, which maintain the purity and constancy of our internal fluids, are perfect examples of homeostatic organs. Like sanita- tion workers who keep a city’s water supply drink- able and dispose of its waste, the kidneys are usually unappreciated until there is a malfunction and “internal garbage” piles up. Every day, the kid- neys filter gallons of fluid from the bloodstream. They then process this filtrate, allowing wastes and excess ions to leave the body in urine while return- ing needed substances to the blood in just the right proportions. Although the lungs and the skin also play roles in excretion, the kidneys bear the major responsibility for eliminating nitrogenous (nitrogen- containing) wastes, toxins, and drugs from the body.

The Urinary System15 WHAT

HOW

WHY

The urinary system filters nitrogenous wastes from the blood and helps

regulate water, electrolyte, and acid-base balances.

The kidneys filter blood in nephrons and

secrete or reabsorb electrolytes under the direction of

hormones.

The kidneys are directly involved in maintaining proper

blood pressure which is essential for delivering oxygen and nutrients to tissues and for proper filtration of

blood by the kidneys themselves.

InSTrUcTorS

New Building Vocabulary coaching Activities for this chapter are assignable in

538 Essentials of Human Anatomy and Physiology

□□ Identify the following regions of a kidney (longitudinal section): hilum, cortex, medulla, medullary pyramids, calyces, pelvis, and renal columns.

Although many people believe the kidneys are located in the lower back, this is not the case. These small, dark red organs with a kidney-bean shape lie against the dorsal body wall in a retroperitoneal position (behind the parietal peritoneum) in the superior lumbar region (Figure 15.1b). The kidneys extend from the T12 to the L3 vertebra; thus they receive some protection from the lower part of the rib cage. Because it is crowded by the liver, the right kidney is slightly lower than the left.

The kidneys alone perform the functions just described and manufacture urine in the process. The other organs of the urinary system—the paired ureters and the single urinary bladder and urethra (Figure 15.1a)—provide temporary storage for urine or serve as transportation channels to carry it from the kidneys to the outside of the body.

Kidneys Location and Structure ➔ Learning Objectives

□□ Describe the location of the kidneys in the body.

12th rib

(b)

(a)

Inferior vena cava

Adrenal gland

Hepatic veins (cut)

Renal artery

Renal hilum

Renal vein

Iliac crest Kidney

Ureter

Urinary bladder

Urethra

Aorta

Rectum (cut)

Uterus (part of female reproductive system)

Figure 15.1 organs of the urinary system. (a) Anterior view of urinary organs of a female. (b) Posterior in situ view of a male showing the relationship of the kidneys to the 12th rib pair.

Chapter 15: The Urinary System 539

calyx), form cup-shaped “drains” that enclose the tips of the pyramids. The calyces collect urine, which continuously drains from the tips of the pyra- mids into the renal pelvis. Urine then flows from the pelvis into the ureter, which transports it to the blad- der for temporary storage until it leaves the body.

Blood Supply

The kidneys continuously cleanse the blood and adjust its composition, so it is not surprising that they have a rich blood supply (see Figure 15.2b and c). Approximately one-quarter of the total blood supply of the body passes through the kidneys each minute. The artery supplying each kidney is the renal artery. As the renal artery approaches the hilum, it divides into segmental arteries, each of which gives off several branches called interlobar arteries, which travel through the renal columns to reach the cortex. At the cortex-medulla junction, interlobar arteries give off the arcuate (ar′ku-at) arteries, which arch over the medullary pyramids. Small cortical radiate arteries then branch off the arcuate arteries to sup- ply the renal cortex. Venous blood draining from the kidney flows through veins that trace the pathway of the arterial supply but in a reverse direction— cortical radiate veins to arcuate veins to interlo bar veins to the renal vein, which emerges from the kidney hilum and empties into the inferior vena cava. (There are no segmental veins.)

Did You Get It? 1. The kidneys are retroperitoneal. What does that mean? 2. Mary lost lots of weight, and suddenly she was having

problems with her urine flow. What would you guess happened, and what caused the problem?

3. From the most superficial aspect of a kidney to its ureter, name its three major regions.

For answers, see Appendix A.

nephrons ➔ Learning Objective

□□ recognize that the nephron is the structural and functional unit of the kidney, and describe its anatomy.

Nephrons (nef′ronz) are the structural and func- tional units of the kidneys and are responsible for forming urine. Each kidney contains over a million of these tiny filters. In addition, there are thou- sands of collecting ducts, each of which collects fluid from several nephrons and conveys it to the renal pelvis. Figure 15.3a and b, p. 541, show the

Kidney Structure

An adult kidney is about 12 cm (5 inches) long, 6 cm (2.5 inches) wide, and 3 cm (1 inch) thick, or about the size of a large bar of soap. It is convex laterally and has a medial indentation called the renal hilum. Several structures, including the ureters, the renal blood vessels, and nerves, enter or exit the kidney at the hilum (Figure 15.2, p. 540). Sitting atop each kid- ney is an adrenal gland, which is part of the endo- crine system and is a separate organ.

The kidney has three protective layers. Deep to superficial, they are as follows:

• A transparent fibrous capsule encloses each kidney and gives it a glistening appearance.

• A fatty mass, the perirenal fat capsule, surrounds each kidney and cushions it against blows.

• The renal fascia, the most superficial layer made of dense fibrous connective tissue, an- chors the kidney and adrenal gland to sur- rounding structures.

Homeostatic Imbalance 15.1

The fat surrounding the kidneys is important in holding them in their normal body position. If the amount of fatty tissue dwindles (as with rapid weight loss), the kidneys may drop to a lower posi- tion, a condition called ptosis (to′sis; “a fall”). Ptosis creates problems if the ureters, which drain urine from the kidneys, become kinked. When this hap- pens, urine that can no longer pass through the ure- ters backs up and exerts pressure on the kidney tis- sue. This condition, called hydronephrosis (hi″dro- nĕ-fro′sis), can severely damage the kidney. ____✚

When a kidney is cut lengthwise, three distinct regions become apparent (see Figure 15.2). The outer region, which is light in color, is the renal cortex (cortex = bark). Deep to the cortex is a darker reddish brown area, the renal medulla. The medulla has triangular regions with a striped appearance, the renal pyramids, or medullary (med′u-lar″e), pyramids. The broad base of each pyramid faces toward the cortex; its tip, the apex, points toward the inner region of the kidney. The pyramids are separated by extensions of cortexlike tissue, called renal columns.

Lateral to the hilum is a flat, funnel-shaped tube, the renal pelvis. The pelvis is continuous with the ureter leaving the hilum (see Figure 15.2b). Extensions of the pelvis, calyces (kal′ ı̆-sēz; singular

(Text continues on page 542.)

540 Essentials of Human Anatomy and Physiology

Cortical radiate vein

Renal pyramid

Minor calyx

Major calyx

Renal column

Fibrous capsule

Renal cortex

Cortical radiate artery Arcuate vein

Arcuate artery

Interlobar vein

Interlobar artery

Segmental arteries

Renal vein

Renal artery

Renal pelvis

Major calyx Ureter

(b)(a)

Aorta Renal artery

Segmental artery

Inferior vena cava

Renal vein

Interlobar vein

Arcuate vein

Cortical radiate vein

Efferent arteriole

Peritubular capillaries

Glomerulus (capillaries)

Interlobar artery

Arcuate artery

Cortical radiate artery

Afferent arteriole

(c)

Figure 15.2 Internal anatomy of the kidney. (a) Photograph of a coronally sectioned kidney. (b) Diagram of coronal kidney section illustrating major blood vessels. (c) Summary of the pathway of renal blood vessels.

Chapter 15: The Urinary System 541

Glomerular capillary covered by podocytes

Afferent arteriole

PCT Glomerular capsular space

Efferent arteriole

(c)

Foot processes

Filtration slits

Podocyte cell body

(a)

(b)

Renal cortex

Renal medulla

Cortical nephron Fibrous capsule

Collecting duct

Proximal convoluted tubule Glomerulus Distal convoluted tubule

Juxtamedullary nephron

Nephron loop

Distal convoluted tubule (DCT)

Peritubular capillaries

Proximal convoluted tubule (PCT)

Glomerular capillaries

Glomerular (Bowman’s) capsule

Efferent arteriole

Afferent arteriole

Cells of the juxtaglomerular apparatus

Cortical radiate artery

Arcuate artery

Cortical radiate vein

Arcuate vein

Nephron loop

Collecting duct

(d)

Renal cortex

Renal medulla

Renal pelvis

Ureter

Figure 15.3 Structure of the nephron. (a) Wedge- shaped section of kidney tissue indicating the position of nephrons in the kidney. (b) Detailed anatomy of a nephron and its associated blood supply. Part of the distal convoluted tubule and afferent arteriole have been sectioned to reveal the location of the juxtaglomerular apparatus. (c) Diagrammatic view of the relationship of the visceral layer of the glomerular capsule to the glomerular capillaries. (d) Scanning electron micrograph of podocytes clinging to the glomerular capillaries.

542 Essentials of Human Anatomy and Physiology

Each and every nephron is associated with two capillary beds—the glomerulus (mentioned earlier) and the peritubular (per″ ı̆-tu′bu-lar) capillary bed (see Figure 15.3b). The glomerulus is fed by the afferent arteriole, which arises from a cortical radiate artery. It is the “feeder vessel,” and the effer ent arteriole receives the blood that has passed through the glomerulus. The glomerulus, specialized for filtration, differs from any other capillary bed in the entire body because it is both fed and drained by arterioles. Also, the afferent arteriole has a larger diameter than the efferent, resulting in a much higher blood pressure in the glomerular capillaries compared to other capillary beds. This high pres- sure forces fluid and small solutes out of the blood into the glomerular capsule. Most of this filtrate (99 percent) is eventually reclaimed by the renal tubule cells and returned to the blood in the peritubular capillary beds.

The second capillary bed, the peritubular capillaries, arises from the efferent arteriole that drains the glomerulus. Unlike the high-pressure glomerulus, these capillaries are low-pressure, porous vessels adapted for absorption instead of filtration. They cling closely to the whole length of the renal tubule, where they are in an ideal posi- tion to receive solutes and water from the tubule cells as these substances are reabsorbed from the filtrate percolating through the tubule. The peritu- bular capillaries ultimately drain into interlobar veins leaving the cortex.

Urine Formation and characteristics ➔ Learning Objectives

□□ Describe the process of urine formation, identifying the areas of the nephron that are responsible for filtration, reabsorption, and secretion.

□□ Describe the function of the kidneys in excretion of nitrogen-containing wastes.

□□ Define anuria and oliguria.

Urine formation is a result of three processes— glomerular filtration, tubular reabsorption, and tubular secretion (Figure 15.4).

Glomerular Filtration

As just described, the glomerulus acts as a filter. Glomerular filtration is a nonselective, passive process in which fluid passes from the blood into the glomerular capsule part of the renal tubule.

anatomy and relative positioning of nephrons and collecting ducts in each kidney.

Each nephron consists of two main structures: a renal corpuscle and a renal tubule. Each renal corpuscle consists of a glomerulus (glo-mer′u- lus), which is a knot of capillaries (glom = little ball), and a cup-shaped hollow structure that completely surrounds the glomerulus like a well- worn baseball glove encloses a ball. This portion of the renal corpuscle is called the glomerular capsule or Bowman’s capsule. The inner (vis- ceral) layer of the capsule is made up of highly modified octopus-like cells called podocytes (pod′o-sı̄ tz; Figure 15.3c and d). Podocytes have long branching extensions called foot processes that intertwine with one another and cling to the glomerulus. Openings called filtration slits between these processes allow the podocytes to form a porous, or “holey,” membrane around the glomerulus ideal for filtration.

The renal tubule, which makes up the rest of the nephron, is about 3 cm (approximately 1.25 inches) long. As it extends from the glomer- ular capsule, it coils and twists before forming a hairpin loop and then again becomes coiled and twisted before entering a collecting duct. These different regions of the tubule have specific names (see Figure 15.3a and b); in order from the glomerular capsule, they are the proximal convoluted tubule (PCT); the nephron loop, or loop of Henle (hen′le); and the distal con voluted tubule (DCT). The surfaces of the tu - bule cells exposed to the filtrate in the proximal convoluted tubules are covered with dense microvilli, which increases their surface area tremendously. Microvilli also occur on the tubule cells in other parts of the tubule, but in much reduced numbers.

Most nephrons are called cortical nephrons because they are located almost entirely within the cortex. In a few cases, the nephrons are called juxtamedullary nephrons because they are situ- ated close to the cortex-medulla junction, and their nephron loops dip deep into the medulla (see Figure 15.3a). The collecting ducts, each of which receives urine from many nephrons, run downward through the medullary pyramids, giv- ing the pyramids a striped appearance. They deliver the final urine product into the calyces and renal pelvis.

Chapter 15: The Urinary System 543

Once in the capsule, the fluid is called fil trate; it is essentially blood plasma without blood proteins. Both proteins and blood cells are nor- mally too large to pass through the filtration mem- brane, and when either of these appears in the urine, there is usually a problem with the glomer- ular filters. As long as the systemic blood pressure is normal, filtrate will be formed. If arterial blood pressure drops too low, glomerular pressure becomes inadequate to force substances out of the blood into the tubules, and filtrate formation stops.

Homeostatic Imbalance 15.2

An abnormally low urinary output is called oligu ria (ol″i-gu′re-ah) if it is between 100 and 400 ml/ day, and anuria (ah-nu′re-ah) if it is less than 100 ml/day. Low urinary output usually indicates that glomerular blood pressure is too low to cause fil- tration, but anuria may also result from transfusion reactions and acute inflammation or from crushing injuries to the kidneys. _______________________✚

Tubular reabsorption

Besides wastes and excess ions that must be removed from the blood, the filtrate contains many useful substances (including water, glucose, amino acids, and ions), which must be reclaimed from the filtrate and returned to the blood. Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule (Figure 15.5, p. 544). The tubule cells are “transporters,” taking up needed substances from the filtrate and then passing them out their posterior aspect into the extracellular space, from which they are absorbed into peritubular capillary blood. Some reabsorp- tion is done passively (for example, water passes by osmosis), but reabsorption of most substances depends on active transport processes, which use membrane carriers, require ATP, and are very selective. There is an abundance of carriers for substances that need to be retained, and few or no carriers for substances of no use to the body.

➔ConCeptLinkRecall that filtration, as a passive process, requires a pressure gradient (Chapter 3, p. 104). The capillaries of the glomerulus are under higher pressure compared to the glomerular capsule; as a result, fluids move down the pressure gradient, from the blood into the glomerular capsule.

Cortical radiate artery

Afferent arteriole Glomerular capillaries

Efferent arteriole

Glomerular capsule

Rest of renal tubule containing filtrate

Peritubular capillary

To cortical radiate vein

Urine

Glomerular filtration: Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule.

Tubular reabsorption: Water, glucose, amino acids, and needed ions are transported out of the filtrate into the tubule cells and then enter the capillary blood.

Tubular secretion: H1, K1, creatinine, and drugs are removed from the peritubular blood and secreted by the tubule cells into the filtrate.

Three major renal processes:

Figure 15.4 The kidney depicted schematically as a single large, uncoiled nephron. A kidney actually has millions of nephrons acting in parallel. Three processes in the kidneys adjust the composition of plasma.

The amount of renal filtrate formed is a function of filtration (blood) pressure and blood osmotic pressure

(exerted largely by blood proteins). In this situation, more filtrate than normal will be formed because the

blood pressure is opposed by lower-than-normal osmotic pressure of the blood, caused by low blood

protein levels.

How would liver disease, in which the liver is unable to make many of the blood proteins, affect process 1? (Reviewing Chapter 10 might help.)

544 Essentials of Human Anatomy and Physiology

nitrogenous Wastes

Nitrogenous waste products are poorly reab- sorbed, if at all. Tubule cells have few membrane carriers to reabsorb these substances because we do not need them. They tend to remain in the fil- trate and are found in high concentrations in urine excreted from the body. Various ions are reabsorbed or allowed to go out in the urine, according to what is needed at a particular time to maintain the proper pH and electrolyte (solute)

Needed substances (for example, glucose and amino acids) are usually entirely removed from the filtrate. Most reabsorption occurs in the proxi- mal convoluted tubules, but the distal convoluted tubule and the collecting duct are also active.

Tubular Secretion

Tubular secretion is essentially tubular reabsorp- tion in reverse. Some substances, such as hydrogen and potassium ions (H+ and K+) and creatinine, also move from the blood of the peritubular capil- laries through the tubule cells or from the tubule cells themselves into the filtrate to be eliminated in urine. This process seems to be important for get- ting rid of substances not already in the filtrate, such as certain drugs or excess potassium ions, or as an additional means for controlling blood pH (see Figure 15.5).

Proximal tubule Distal tubule

NaCl H2O Glucose and amino acids

Some drugs and poisons

H+

NaCl HCO3

–

K+ and some drugs

Collecting duct

Cortex

Medulla

Glomerular capsule

Blood

NaCl

H2O

NaCl H2O Urea

H2O

Urine (to renal pelvis)

Filtrate

H2O Salts (NaCl, etc.) HCO3

– (bicarbonate) H+ Urea Glucose; amino acids Some drugs

Reabsorption

Active transport

Secretion

Nephron loop

Passive transport

(active transport)

K+

Figure 15.5 Sites of filtration, reabsorption, and secretion in a nephron.

➔ConCeptLinkRecall that pH is a measure of hydrogen ion (H+) con-centration (see Chapter 2, p. 67). When the body experiences a high level of hydrogen ions, which can lower pH, the kidneys help by eliminating excess hydro- gen ions from the body via the urine.

Chapter 15: The Urinary System 545

might be a result of eating certain foods (beets, for example) or the presence of bile or blood in the urine.

When formed, urine is sterile, and its odor is slightly aromatic. If it is allowed to stand, it takes on an ammonia odor caused by the action of bac- teria on the urine solutes. Some drugs, vegetables (such as asparagus), and various diseases (such as diabetes mellitus) alter the usual odor of urine.

Urine pH is usually slightly acid (around 6), but changes in body metabolism and certain foods may cause it to be much more acidic or basic. For example, a diet with large amounts of protein (eggs and cheese) and whole-wheat products causes urine to become quite acidic. Conversely, a vegetarian diet makes urine quite alkaline as the kidneys excrete excess bases. Bacterial infection of the urinary tract also may cause the urine to be alkaline.

Because urine is water plus solutes, urine is denser (weighs more) than distilled water. Specific gravity compares how much heavier urine is than distilled water. Whereas the specific gravity of pure water is 1.0, the specific gravity of urine usu- ally ranges from 1.001 to 1.035 (dilute to concen- trated urine, respectively). Urine is generally dilute (has a low specific gravity) when a person drinks excessive fluids, uses diuretics (drugs that increase urine output), or has chronic renal failure, a condi- tion in which the kidney loses its ability to con- centrate urine (see “A Closer Look” on p. 549). Conditions that produce concentrated urine with a high specific gravity include inadequate fluid intake, fever, and kidney inflammation called pyelonephritis (pi″ĕ-lo-nĕ-fri′tis).

Solutes normally found in urine include sodium ions (Na+) and potassium ions (K+), urea, uric acid, creatinine, ammonia, bicarbonate ions, and various other ions, depending on blood com- position. With certain diseases, urine composition can change dramatically, and the presence of abnormal substances in urine is often helpful in diagnosing the problem. This is why a routine urinalysis should always be part of a physical examination.

Substances not normally found in urine are glucose, blood proteins, red blood cells, hemoglo- bin, white blood cells (pus), and bile. Table 15.1, p. 546 lists conditions that may involve abnormal urinary constituents and volumes.

composition of the blood. Common nitrogenous wastes include the following:

• Urea (u-re′ah), formed by the liver as an end product of protein breakdown when amino acids are used to produce energy

• Uric acid, released when nucleic acids are metabolized

• Creatinine (kre-at′ ı̆-nin), associated with creatine (kre′ah-tēn) metabolism in muscle tissue

Did You Get It? 4. What is the structural and functional unit of the

kidney? 5. What are two functions of the renal tubule and one

function of the peritubular capillaries? 6. How does a decrease in blood pressure affect

glomerular pressure?

For answers, see Appendix A.

characteristics of Urine

➔ Learning Objectives □□ Describe the composition of normal urine.

□□ List abnormal urinary components.

In 24 hours, the marvelously complex kidneys fil- ter some 150 to 180 liters of blood plasma through their glomeruli into the tubules, which process the filtrate by taking substances out of it (reabsorp- tion) and adding substances to it (secretion). In the same 24 hours, only about 1.0 to 1.8 liters of urine are produced. Therefore, urine and filtrate are quite different. Filtrate contains everything that blood plasma does (except proteins), but by the time it reaches the collecting ducts, the filtrate has lost most of its water and just about all of its nutri- ents and necessary ions. What remains, urine, contains nitrogenous wastes and unneeded or excess substances. Assuming we are healthy, our kidneys can keep our blood composition fairly constant despite wide variations in diet and cell activity.

Freshly voided urine is generally clear and pale to deep yellow. The normal yellow color is due to urochrome (u′ro-krōm), a pigment that results from the body’s destruction of hemoglobin. The more solutes are in the urine, the deeper yel- low its color. Dilute urine is a pale, straw color. At times, urine may be a color other than yellow. This

546 Essentials of Human Anatomy and Physiology

could help drain urine to the bladder, it would not transport urine while a person is lying down. Urine reaches the bladder even when standing on your head because the ureters do play an active role in urine transport. Smooth muscle layers in their walls contract to propel urine by peristalsis. Once urine has entered the bladder, it is prevented from flowing back into the ureters by small valve- like folds of bladder mucosa that cover the ureter openings.

Homeostatic Imbalance 15.3

When urine becomes extremely concentrated, sol- utes such as uric acid salts form crystals that pre- cipitate in the renal pelvis. These crystals are called renal calculi (kal′ku-li; calculus = little stone) or kidney stones (see photo).

Excruciating pain that radiates to the flank (lateral, posterior lower back) occurs when the ureter walls close in on the sharp calculi as they are being eased through the ureter by peristalsis or when the calculi become wedged in a ureter. Frequent bacterial infections of the urinary tract, urinary retention, and alkaline urine all favor calculi formation. Surgery is one treatment option, but a noninvasive procedure (litho- tripsy) that uses ultrasound waves to shatter the calculi is also available. The pulverized rem- nants of the calculi are then painlessly elimi- nated in urine.

Did You Get It? 7. Is the specific gravity of urine higher or lower than

that of water? Why?

For the answer, see Appendix A.

Ureters, Urinary Bladder, and Urethra ➔ Learning Objectives

□□ Describe the general structure and function of the ureters, urinary bladder, and urethra.

□□ compare the course and length of the male urethra to that of the female.

□□ Define micturition.

□□ Describe the difference in control of the external and internal urethral sphincters.

Ureters The ureters (yer′ĕ-terz) are two slender tubes each 25 to 30 cm (10 to 12 inches) long and 6 mm (¼ inch) in diameter. Each ureter runs behind the peritoneum from the renal hilum to the posterior aspect of the bladder, which it enters at a slight angle (Figure 15.6). The superior end of each ureter is continuous with the renal pelvis, and its mucosal lining is continuous with the mucosa lining the renal pelvis and the blad- der inferiorly.

The ureters carry urine from the kidneys to the bladder. Although it might appear that gravity

Table 15.1 Abnormal Urinary constituents

Substance name of condition Possible causes

Glucose Glycosuria (gli″ko-su′re-ah) Nonpathological: Excessive intake of sugary foods

Pathological: Diabetes mellitus

Proteins Proteinuria (pro″te- ı̆-nu′re-ah) (also called albuminuria)

Nonpathological: Physical exertion, pregnancy

Pathological: Glomerulonephritis, hypertension

Pus (WBCs and bacteria) Pyuria (pi-u′re-ah) Urinary tract infection

RBCs Hematuria (he″mah-tu′re-ah) Bleeding in the urinary tract (due to trauma, kidney stones, infection)

Hemoglobin Hemoglobinuria (he″mo-glo-b ı̆-nu′re-ah)

Various: Transfusion reaction, hemolytic anemia

Bile pigment Bilirubinuria (bil″ ı̆-roo-b ı̆-nu′re-ah) Liver disease (hepatitis)

Chapter 15: The Urinary System 547

15Trigone

External urethral sphincter

Urogenital diaphragm

Urethra

Ureteral orifice

Ureter

Urinary bladder

Internal urethral orifice

Internal urethral sphincter

A urogram of a 35-year-old man after an injection of 80 ml of iodized contrast medium. The contrast medium (in yellow) travels from the kidneys toward the bladder. The presence of a kidney stone is highlighted in red.

Urinary Bladder The urinary bladder is a smooth, collapsible, muscular sac that stores urine temporarily. It is located retroperitoneally in the pelvis just poste- rior to the pubic symphysis. A scan of the interior of the bladder reveals three openings—the two ureter openings (ureteral orifices), and the single opening of the urethra (u-re′thrah) (the internal urethral orifice), which drains the bladder (see Figure 15.6). The smooth triangular region of the bladder base outlined by these three openings is called the trigone (tri′gon). The trigone is impor- tant clinically because infections tend to persist in this region. In males, the prostate (part of the male reproductive system) surrounds the neck of the bladder where it empties into the urethra.

The bladder wall contains three layers of smooth muscle, collectively called the detrusor muscle, and its mucosa is a special type of epithe- lium, transitional epithelium (see Chapter 3, p. 119). Both of these structural features make the bladder uniquely suited for its function of urine storage. When the bladder is empty, it is collapsed, 5 to 7.5 cm (2 to 3 inches) long at most, and its walls are thick and thrown into folds. As urine accumu- lates, the bladder expands and rises superiorly in the abdominal cavity (Figure 15.7, p. 549). Its mus- cular wall stretches, and the transitional epithelial

____________________________________________✚

Figure 15.6 Basic structure of the female urinary bladder and urethra.

548 Essentials of Human Anatomy and Physiology

both urine and sperm (in semen) from the body, but never at the same time. Thus, in males, the ure- thra is part of both the urinary and reproductive systems.

In women, the urethra is about 3 to 4 cm (1½ inches) long, and its external orifice, or opening, lies anterior to the vaginal opening (look ahead to Figure 16.8a, p. 575). Its only function is to con- duct urine from the bladder to the body exterior.

Homeostatic Imbalance 15.4

The female urinary orifice is close to the anal opening, so improper toileting habits (wiping from back to front rather than from front to back) can carry fecal bacteria into the urethra. And because the mucosa of the urethra is continuous with that of the rest of the urinary tract organs, inflam- mation of the urethra, or urethritis (u″re-thri′tis), can easily ascend the tract to cause bladder inflam- mation (cystitis) or even kidney inflammation (pyelonephritis, or pyelitis). Symptoms of uri nary tract infection (UTI) include dysuria (pain- ful urination), urinary urgency and frequency, fever, and sometimes cloudy or blood-tinged urine. When the kidneys are involved, back pain and a severe headache are common. __________✚

Micturition Micturition (mik″tu-rish′un), or voiding, is the act of emptying the bladder. As noted, two sphinc- ters, or valves—the internal urethral sphincter (more superiorly located) and the external urethral sphincter (more inferiorly located)—control the flow of urine from the bladder (see Figure 15.6). Ordinarily, the bladder continues to collect urine until about 200 ml have accumulated. At this point, stretching of the bladder wall activates stretch receptors. Impulses transmitted to the sacral region of the spinal cord and then back to the bladder via the pelvic splanchnic nerves cause the bladder to go into reflex contractions. As the contractions become stronger, stored urine is forced past the internal urethral sphincter (the smooth muscle, involuntary sphincter) into the upper part of the urethra. The person will then feel the urge to void. Because the lower external sphincter is skeletal muscle and is controlled voluntarily, we can choose to keep it closed and postpone bladder emptying temporarily. However, if it is convenient, the exter- nal sphincter can be relaxed so that urine is flushed

layer thins, increasing its volume and allowing the bladder to store more urine without substantially increasing its internal pressure. A moderately full bladder is about 12.5 cm (5 inches) long and holds about 500 ml (1 pint) of urine, but it is capable of holding more than twice that amount. When the bladder is really distended, or stretched by urine, it becomes firm and pear-shaped and may be felt just above the pubic symphysis. Urine is formed con- tinuously by the kidneys, but it is stored in the bladder until its release is convenient.

Urethra The urethra is a thin-walled tube that carries urine by peristalsis from the bladder to the outside of the body. At the bladder-urethra junction, a thickening of the smooth muscle forms the inter nal urethral sphincter (see Figure 15.6), an involuntary sphincter (think “internal is involun- tary”) that keeps the urethra closed when urine is not being passed. A second sphincter, the exter nal urethral sphincter, is formed by skeletal muscle as the urethra passes through the pelvic floor. This sphincter is voluntarily controlled.

The length and relative function of the urethra differ in the two sexes. In men, the urethra is approximately 20 cm (8 inches) long and has three named regions (look ahead to Figure 16.2a, p. 566): the prostatic, membranous, and spongy (or penile) urethrae. The urethra opens at the tip of the penis after traveling down its length. The ure- thra of the male has a double function. It carries

Umbilicus

Superior wall of distended bladder

Superior wall of empty bladder

Pubic symphysis

Figure 15.7 Position and shape of a distended and an empty urinary bladder in an adult male.

549

Renal Failure and the Artificial Kidney

One out of every 10 Americans has some degree of kidney dysfunc-

lead) or organic solvents (dry- cleaning fluids, paint thinner)

• Prolonged pressure on skeletal muscles (causes release of myoglo- bin, a muscle pigment that can clog renal tubules)

In renal failure, filtrate formation decreases significantly or stops com- pletely. Because toxic wastes accumu- late quickly in the blood, dialysis (dialys = separate) is necessary to cleanse the blood while the kidneys are shut down. In hemodialysis, which uses an “artificial kidney” apparatus (see photo), the patient’s blood is passed through tubing that is perme- able only to selected substances, and the tubing is immersed in a bathing solution that differs slightly from nor- mal “cleansed” plasma.

As blood circulates through the tubing, nitrogenous wastes and potassium ions (K+) present in the blood (but not in the bath) diffuse out of the blood through the tubing into the surrounding solution. Sub- stances to be added to the blood (mainly buffers for hydrogen ions and glucose for malnourished patients) move from the bathing

solution into the blood. In this way, needed substances are retained in the blood or added to it, while wastes and ion excesses are re - moved. Hemodialysis is routinely done three times weekly, and each session takes 4 to 8 hours, depend- ing on patient size and how well the kidneys are functioning. Hemodialy- sis patients sometimes encounter serious problems such as thrombo- sis, infection, and ischemia at the shunt site. Hemorrhage is an added risk because the blood must be treated with an anticoagulant, such as heparin, to prevent clotting dur- ing hemodialysis.

When renal damage is nonrevers- ible, as in chronic renal failure, a kid- ney transplant is the only answer. Unless the new kidney comes from an identical twin, recipients must take immunosuppressive drugs for the rest of their lives to prevent rejection. If this sounds bleak, consider that 40 years ago people who reached end- stage renal failure survived for only a few days. Today, they can live for years or even decades.

A CLOSER

LOOK

A renal patient undergoing hemodialysis.

tion, and nearly 4 percent require dial- ysis or a kidney transplant just to stay alive. The incidence of kidney disease is growing fastest among adults age 60 and above.

Warning signs of kidney trouble may include high blood pressure, fre- quent urination, painful urination, puffy eyes, or swollen hands or feet. But all too often, there are few noticeable symptoms until a signifi- cant percentage of kidney function has already been lost. Urinalysis can detect proteinuria, a sensitive and early marker of kidney damage, and a simple blood test can determine the creatinine level, which can be used to estimate the rate of blood filtration.

In adults, chronic renal disease often develops together with other chronic health conditions. The leading causes are diabetes mellitus, which accounts for approximately 44 percent of new cases each year, and hyperten- sion, accounting for about 28 percent of cases. Hypertension is both a cause and a symptom: High blood pressure impairs kidney function by damaging renal blood vessels and reducing circulation to the organs, even as hypertension-battered kidneys push blood pressure upward. Atherosclero- sis compounds the problem by further impairing circulation.

Other possible causes of renal failure include the following:

• Repeated kidney infections • Physical trauma to the kidneys

(crush injury and others)

• Chemical poisoning of the tubule cells by heavy metals (mercury or

550 Essentials of Human Anatomy and Physiology

roles to play, which help keep the blood composi- tion relatively constant:

• Excreting nitrogen-containing wastes

• Maintaining water balance of the blood

• Maintaining electrolyte balance of the blood

• Ensuring proper blood pH

Excretion of nitrogenous wastes has already been considered; the other roles of the kidneys are discussed next.

Maintaining Water Balance of Blood ➔ Learning Objectives

□□ Name and localize the three main fluid compart- ments of the body.

□□ Explain the role of antidiuretic hormone (ADH) in the regulation of water balance by the kidney.

□□ Explain the role of aldosterone in sodium ion and potassium ion balance of the blood.

□□ Define diuresis and polyuria.

Body Fluids and Fluid compartments

If you are a healthy young adult, water probably accounts for half or more of your body weight—50 percent in women and about 60 percent in men. These differences reflect the fact that women have relatively less muscle and a larger amount of body fat (of all body tissues, fat contains the least water). Babies, with little fat and low bone mass, are about 75 percent water. This accounts for their “dewy” skin, like that of a freshly picked peach. After infancy, total body water content declines through life, finally accounting for only about 45 percent of body mass in old age. Water is the universal body solvent within which all solutes (including the very important electrolytes) are dissolved. (We described the importance of water to the functioning of the body and its cells in Chapter 2.)

Water occupies three main locations within the body, referred to as fluid compartments (Figure 15.8). About two-thirds of body fluid, the so-called intracellular fluid (ICF), is contained within the living cells. The remainder, called extracellular fluid (ECF), includes all body fluids located out- side the cells. ECF includes blood plasma, intersti- tial fluid (IF), lymph, and transcellular fluid, which includes cerebrospinal and serous fluids, the humors of the eye, and others.

As plasma circulates throughout the body deliv- ering substances, it serves as the “highway” that links the external and internal environments

from the body. When a person chooses not to void, the reflex contractions of the bladder stop within a minute or so, and urine collection continues. After 200 to 300 ml more have been collected, the mictu- rition reflex occurs again. Eventually, micturition occurs whether the person wills it or not.

Homeostatic Imbalance 15.5

Incontinence (in-kon′tı̆-nens) occurs when a person is unable to voluntarily control the external sphincter. Incontinence is normal in children 2 years old or younger because they have not yet gained control over their voluntary sphincter. It may also occur in older children who sleep so soundly that they are not awakened by the stimulus. However, after the toddler years, incontinence is usually a result of emotional problems, pressure (as in pregnancy), or nervous sys- tem problems (stroke or spinal cord injury).

Urinary retention is essentially the opposite of incontinence. In this condition, the bladder is unable to expel its contained urine. There are vari- ous causes for urinary retention. It often occurs after surgery in which general anesthesia has been given, because it takes a little time for the smooth muscles to regain their activity. Another cause of urinary retention, occurring primarily in older men, is enlargement, or hyperplasia, of the pros- tate, which surrounds the neck of the bladder. As the prostate gland enlarges, it narrows the urethra, making it very difficult to void. When urinary retention is prolonged, a slender flexible drainage tube called a catheter (kath′ ı̆-ter) must be inserted through the urethra to drain the urine and prevent bladder trauma from excessive stretching. ______✚

Did You Get It? 8. A kidney stone blocking a ureter would interfere with

the flow of urine to which organ? 9. Why is the presence of transitional epithelium in the

urinary bladder important? 10. Structurally and functionally, how does the male

urethra differ from the female urethra? 11. What is a synonym for micturition?

For answers, see Appendix A.

Fluid, Electrolyte, and Acid-Base Balance The composition of the blood depends on three major factors: diet, cellular metabolism, and urine output. In general, the kidneys have four major

Chapter 15: The Urinary System 551

(Figure 15.9). Exchanges occur almost continuously in the lungs, gastrointestinal tract, and kidneys. Although these exchanges alter plasma composition and volume, adjustments in the other two fluid com- partments follow quickly so that balance is restored.

The Link between Water and Electrolytes

Water certainly accounts for nearly the entire vol- ume of body fluids, which are all similar, regardless of type. But there is more to fluid balance than just water. Recall from Chapter 2 that electrolytes are charged particles (ions) that conduct an electrical current in an aqueous solution. The types and amounts of solutes in body fluids, especially elec- trolytes such as sodium, potassium, and calcium ions, are also vitally important to overall body homeostasis. Water and electrolyte balance, though we consider them separately here, are tightly linked as the kidneys continuously process the blood.

regulation of Water Intake and output

If the body is to remain properly hydrated, we cannot lose more water than we take in. Most

P lasm

a V

olum e =

3 L, 20% of E

C F

Total body water Volume = 40 L 60% body weight

Extracellular fluid (ECF) Volume = 15 L 20% body weight

Intracellular fluid (ICF) Volume = 25 L 40% body weight

Interstitial fluid (IF) Volume = 12 L 80% of ECF

Figure 15.8 The major fluid compartments of the body. Approximate values are noted for a 70-kg (154-pound) man. For simplicity, this figure shows only interstitial fluid and plasma in the ECF.

Lungs

Interstitial fluid

Intracellular fluid in tissue cells

Blood plasma

O2 CO2 H2O, Ions

H2O, Ions

Nitrogenous wastes

Nutrients

O2 CO2 H2O Ions Nitrogenous wastes

Nutrients

KidneysGastrointestinal tract

Figure 15.9 The continuous mixing of body fluids. Nutrients and wastes are exchanged between the intracellular fluid and the plasma through the interstitial fluid. Blood plasma transports nutrients and wastes between the cells and the external environment of the body.

552 Essentials of Human Anatomy and Physiology

How would the values shown here be affected by (1) drinking a six-pack of beer and (2) fasting (ingesting only water)?

water intake results from fluids and foods we ingest in our diet (Figure 15.10). However, a small amount (about 10 percent) is produced during cel- lular metabolism (explained in Chapter 14).

The thirst mechanism is the driving force for water intake. An increase in plasma solute content of only 2 to 3 percent excites highly sensitive cells in the hypothalamus called osmoreceptors (oz″mo-re-sep′torz), which in turn activate the hypothalamic thirst center (Figure 15.11). The mouth also becomes dry because the salivary glands obtain the water they require from the blood. When less fluid leaves the bloodstream, less saliva is produced, reinforcing the thirst response. The same response results from a decline in blood volume (or pressure), although this is a less potent stimulus.

Feces 4%

Sweat 8% Insensible losses via skin and lungs 28%

Urine 60%

25 00

m l

Average output per day

Average intake per day

Beverages 60%

Foods 30%

Metabolism 10%

1500 ml

700 ml

200 ml

100 ml

1500 ml

750 ml

250 ml

Figure 15.10 Water intake and output. Major sources of body water and routes of water loss from the body are shown. When intake and output are in balance, the body is adequately hydrated.

In case 1, the fluid ingested would greatly increase, as would the amount of urinary output. In case 2, there

would be no fluid intake from ingested foods and less fluid resulting from metabolism. Fluid output

would be severely curtailed—no output in feces and less in urine to conserve body water. Insensible losses

would not change. Loss of water in perspiration would depend on body and ambient temperatures.

Dry mouth

Saliva Osmoreceptors in hypothalamus

Hypothalamic thirst center

Sensation of thirst; person takes a drink

Water absorbed from GI tract

Plasma solutes

Water moistens mouth, throat; stretches stomach, intestine

Plasma solutes

Increases, stimulates Reduces, inhibits

Initial stimulus

Physiological response

Result

KEY:

Figure 15.11 The thirst mechanism for regulating water intake. The major stimulus is increased osmolarity (concentration of solutes) of blood plasma. (A decrease in plasma volume and blood pressure, not shown, is a less important stimulus that acts primarily to control blood pressure.)

Water leaves the body by several routes. Some water vaporizes out of the lungs (insensible water loss), some is lost in perspiration, and some leaves the body in the stool (see Figure 15.10). The job

Chapter 15: The Urinary System 553

Homeostatic Imbalance 15.6

When ADH is not released (perhaps because of injury or destruction of the hypothalamus or pos- terior pituitary gland), huge amounts of very dilute urine (up to 25 liters/day) flush from the body day after day. This condition, diabetes insipidus (in-sip′ ı̆-dus), can lead to severe dehydration and electrolyte imbalances. Affected individuals are always thirsty and have to drink fluids almost con- tinuously to maintain normal fluid balance. ____✚

Maintaining Electrolyte Balance Very small changes in the electrolyte concentrations in the various fluid compartments cause water to move from one compartment to another. This movement not only alters blood volume and blood pressure, but also can severely impair the activity of irritable cells, such as nerve and muscle cells. For example, when the level of potassium ions in the ECF is low, muscle cells may not be able to repolar- ize properly, which can lead to muscle cramps.

Besides ADH, aldosterone (al″dos′ter-on) is a second hormone that helps to regulate blood com- position and blood volume by acting on the kidney. Aldosterone is the major factor regulating sodium ion content of the ECF and in the process helps regulate the concentration of other ions (potassium, chloride [Cl–], and magnesium [Mg2+]) as well.

Sodium ions are the electrolytes most responsi- ble for osmotic water flow. When too few sodium ions are in the blood, water will leave the blood and enter tissues, causing edema. If serious, this sit- uation can cause the circulatory system to shut down. Regardless of whether aldosterone is present or not, about 80 percent of the sodium ions in the filtrate are reabsorbed in the proximal convoluted tubules of the kidneys. When the aldosterone con- centration is high, most of the remaining sodium ions are actively reabsorbed in the distal convoluted tubules and the collecting ducts. Generally speak- ing, for each sodium ion reabsorbed, a chloride ion follows, and a potassium ion is secreted into the filtrate. Thus, as the sodium ion content of the blood increases, the potassium ion concentration decreases, bringing these two ions back to their normal balance in the blood. Still another effect of aldosterone is to increase water reabsorption by the tubule cells; as sodium ions are reclaimed, water follows it passively back into the blood. Here is a little rule to keep in mind: Water follows salt.

of the kidneys is like that of a juggler. If large amounts of water are lost in other ways, the kid- neys compensate by producing less urine to con- serve body water. However, when water intake is excessive, the kidneys excrete generous amounts of water in urine to reduce fluid volume.

In addition to regulating fluid volume, the kid- neys ensure that proper concentrations of the vari- ous electrolytes are present in both intracellular and extracellular fluids. Most electrolytes enter the body in foods and “hard” (mineral-rich) water. Although very small amounts are lost in perspiration and in feces, the major factor regulating the electrolyte composition of body fluids is the kidneys.

Reabsorption of water and electrolytes by the kidneys is regulated primarily by hormones. When blood volume drops for any reason (for example, as a result of hemorrhage or excessive water loss through sweating or diarrhea), arterial blood pres- sure drops, which in turn decreases the amount of filtrate formed by the kidneys. In addition, the hypothalamic osmoreceptors react to the change in blood composition (that is, less water and more solutes) by becoming more active. The result is that nerve impulses are sent to the posterior pitu- itary (Figure 15.12, p. 554), which then releases antidiuretic hormone (ADH). (The term antidi- uretic is derived from diuresis [di″u-re′sis], which means “flow of urine from the kidney,” and anti, which means “against.”) As you might guess, this hormone prevents excessive water loss in the urine.

➔ConCeptLinkRemember the concept of interrelationships among organ systems (see Figure 1.3, p. 34), and notice that the interdependent events regulating sodium ion and water balance (see Figure 15.12, p. 554) involve four body systems: urinary, nervous, endocrine, and cardiovascular.

ADH travels in the blood to its main target, the kidney’s collecting ducts, where it causes the duct cells to reabsorb more water. As more water is returned to the bloodstream, blood volume and blood pressure increase to normal levels, and only a small amount of concentrated urine is formed. ADH is released more or less continuously unless the solute concentration of the blood drops too low. When this happens, the osmoreceptors become “quiet,” and excess water is allowed to leave the body in the urine.

554 Essentials of Human Anatomy and Physiology

Falling systemic blood pressure/volume

Na+ reabsorption (and H2O absorption)

JG cells of kidneys

Renin

Posterior pituitary

Systemic arterioles

Angiotensin II formed in blood

Systemic arterioles

Vasoconstriction Aldosterone

Blood volume

Rising blood pressure

Kidney tubules

Adrenal cortex

Vasoconstriction

Peripheral resistance

(+)

(+) (+)

(+)

Peripheral resistance

H2O reabsorption

Inhibits baroreceptors in blood vessels

Sympathetic nervous system

ADH (antidiuretic hormone)

Collecting ducts of kidneys

Hypothalamic osmoreceptors

Reduced filtrate volume or solute content in renal tubules

Causes

Results in

Targets

Results in

Releases

Release

Leads to

(+) (+)

(+)

Causes Secretes

(+) = stimulates

Renin-angiotensin system

Neural regulation (sympathetic nervous system effects) Effects of ADH release

KEY:

Figure 15.12 Flowchart of mechanisms regulating sodium ion and water balance to help maintain blood pressure homeostasis.

Chapter 15: The Urinary System 555

15. Where are osmoreceptors located, and to what stimulus do they respond?

For answers, see Appendix A.

Maintaining Acid-Base Balance of Blood Learning Objective

□□ compare and contrast the relative speed of buffers, the respiratory system, and the kidneys in maintaining the acid-base balance of the blood.

For the cells of the body to function properly, blood pH must be maintained between 7.35 and 7.45, a very narrow range. Whenever the pH of arterial blood rises above 7.45, a person is said to have alkalosis (al″kah-lo′sis). A drop in arterial pH to below 7.35 results in acidosis (as″ ı̆-do′sis). Because a pH of 7.0 is neutral, any pH between 7.0 and 7.35 is not acidic, chemically speaking; however, it rep- resents a lower-than-optimal pH for the functioning of most body cells. Therefore, any arterial pH in this range is called physiological acidosis.

Although small amounts of acidic substances enter the body in ingested foods, most hydrogen ions originate as by-products of cellular metabolism, which continuously adds substances to the blood that tend to disturb its acidbase balance. Many dif- ferent acids are produced (for example, phosphoric acid, lactic acid, and many types of fatty acids). In addition, carbon dioxide, which is released during energy production, forms carbonic acid. Ammonia and other basic substances are also released to the blood as cells go about their usual “business.” Although the chemical buffers in the blood can tem- porarily “tie up” excess acids and bases, and the lungs have the chief responsibility for eliminating carbon dioxide from the body, the kidneys assume most of the load for maintaining acid-base balance of the blood. Before describing how the kidneys func- tion in acid-base balance, let’s look at how blood buffers and the respiratory system help regulate pH.

Blood Buffers

Chemical buffers are systems of one or two mole- cules that act to prevent dramatic changes in the hydrogen ion concentration when acids or bases are added. They do this by binding to hydrogen ions whenever the pH drops and by releasing hydrogen ions when the pH rises. The chemical buffers act within a fraction of a second, so they are the first line of defense in resisting pH changes.

Recall that aldosterone is produced by the adre- nal cortex. Although a rising potassium ion level or a falling sodium ion level in the ECF directly stimu- lates the adrenal cells to release aldosterone, the most important trigger for aldosterone release is the reninangiotensin mechanism (see Figure 15.12) mediated by the juxtaglomerular (JG) apparatus of the renal tubules. The juxtaglomerular apparatus (look back at Figure 15.3b) consists of a complex of modified smooth muscle cells (JG cells) in the afferent arteriole plus some modified epithelial cells forming part of the distal convoluted tubule. The naming of this cell cluster reflects its location close to (juxta) the glomerulus. When the cells of the JG apparatus are stimulated by low blood pressure in the afferent arteriole or changes in solute content of the filtrate, they respond by releasing the enzyme renin (re′nin) into the blood. (Notice the different spelling and pronunciation of this enzyme from rennin (rĕn′in), an enzyme secreted by stomach glands.) Renin initiates the series of reactions that produce angiotensin II. Angiotensin II in turn acts directly on the blood vessels to cause vasoconstric- tion (leading to an increase in peripheral resistance) and on the adrenal cortical cells to promote aldo- sterone release. As a result, blood volume and blood pressure increase (see Figure 15.12). The renin-angiotensin mechanism is extremely impor- tant for regulating blood pressure.

When pressure drops, baroreceptors in large blood vessels are also excited. These baroreceptors alert sympathetic nervous system centers of the brain to cause vasoconstriction (via release of epinephrine and norepinephrine) (see Figure 15.12). However, this neural mechanism’s major focus is blood pres- sure regulation, not water or electrolyte balance.

Homeostatic Imbalance 15.7

People with Addison’s disease (hypoaldosteron- ism) have polyuria (excrete large volumes of urine) and so lose tremendous amounts of salt and water to urine. As long as adequate amounts of salt and fluids are ingested, people with this con- dition can avoid problems, but they are constantly teetering on the brink of dehydration. _________✚

Did You Get It? 12. What are the four main functions of the kidneys? 13. What is the chief mechanism prompting water intake? 14. What is the role of aldosterone in water balance?

556 Essentials of Human Anatomy and Physiology

The three major chemical buffer systems of the body are the bicarbonate, phosphate, and protein buffer systems, each of which helps to maintain the pH in one or more of the fluid compartments. They all work together, and anything that causes a shift in hydrogen ion concentration in one com- partment also causes changes in the others. Thus, any abnormal pH level is resisted by the entire buffering system. Because all three systems oper- ate in a similar way, we will examine just one, the bicarbonate buffer system, which is important in preventing changes in blood pH.

The bicarbonate buffer system is a mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3). Carbonic acid is a weak acid, so it does not dissociate much in neutral or acidic solutions. Thus, when a strong acid such as hydrochloric acid is added, most of the carbonic acid remains intact. However, the bicarbonate ions (HCO3

–) of the salt act as bases to tie up the hydrogen ions released by the stronger acid, form- ing more carbonic acid:

To better understand how a chemical buffer system works, let’s review strong and weak acids and bases (covered in Chapter 2). Acids are proton (H+) donors, and the acidity of a solution reflects only the free hydrogen ions, not those still bound to anions. Strong acids such as hydrochloric acid (HCl) dissociate completely and liberate all their hydrogen ions (H+) in water (Figure 15.13a). Consequently they can cause large changes in pH. By contrast, weak acids such as carbonic acid (H2CO3) release only some of their H

+ (partial dis- sociation) and so have a lesser effect on pH (Figure 15.13b). However, weak acids are very effective at preventing pH changes because they are forced to dissociate and release more H+ when the pH rises over the desirable pH range. This fea- ture allows them to play a very important role in the chemical buffer systems.

Also recall that bases are proton, or hydrogen ion, acceptors. Strong bases such as hydroxides dissociate easily in water and quickly tie up H+, but weak bases such as bicarbonate ion (HCO3

–) and ammonia (NH3) are slower to accept H

+. However, as pH drops, the weak bases become “stronger” and begin to tie up more H+. Thus, like weak acids, they are valuable members of the chemical buffer systems.

Figure 15.13 Dissociation of strong and weak acids in water. Undissociated molecules are shown in colored ovals.

(a) A strong acid such as HCI dissociates completely into its ions.

(b) A weak acid such as H2CO3 does not dissociate completely.

H2CO3

HCO3 –

H2CO3

H2CO3H1

HCO3 –

H1 CI–

CI–

CI–CI– CI–

H1 H1

H1H 1

H1 H1

H2CO3

HCI

HCl strong acid

+ NaHCO3 weak base

4 H2CO3 weak acid

+ NaCl salt

Because the strong acid is (effectively) changed to a weak one, it lowers the pH of the solution only very slightly.

Similarly, if a strong base such as sodium hydroxide (NaOH) is added to a solution contain- ing the bicarbonate buffer system, NaHCO3 will not dissociate further under such alkaline condi- tions. However, carbonic acid (H2CO3) will be forced to dissociate further by the presence of the strong base—releasing more H+ to bind with the OH– released by NaOH.

NaOH strong acid

+ H2CO3 weak acid

4 NaHCO3 weak base

+ H2O water

The net result is replacement of a strong base by a weak one, so that the pH of the solution rises very little.

respiratory Mechanisms

The respiratory system eliminates carbon dioxide from the blood while it “loads” oxygen into the blood. When carbon dioxide (CO2) enters the blood from the tissue cells, most of it enters the red blood cells, where it is converted to bicarbonate ion (HCO3

–) for transport in the plasma, as shown by the equation

Chapter 15: The Urinary System 557

by excreting bicarbonate ions and reabsorbing or generating new bicarbonate ions. Look back again at the equation showing how the carbonic acid– bicarbonate buffer system operates. Notice that losing a bicarbonate ion (HCO3

–) from the body has the same effect as gaining a hydrogen ion (H+) because it pushes the equation to the right (that is, it leaves a free H+). By the same token, reabsorb- ing or generating new HCO3

– is the same as losing H+ because it tends to combine with an H+ and pushes the equation to the left.

Renal mechanisms undertake these adjust- ments: As blood pH rises, bicarbonate ions are excreted and hydrogen ions are retained by the tubule cells. Conversely, when blood pH falls, bicarbonate is reabsorbed and generated, and hydrogen ions are secreted. Urine pH varies from 4.5 to 8.0, which reflects the ability of the renal tubules to excrete basic or acid ions to maintain blood pH homeostasis.

Did You Get It? 16. Why is blood pH between 7.0 and 7.35 considered

acidic even though basic chemistry defines any pH above 7.0 as basic?

17. To minimize the pH shift that occurs when a strong acid is added to water, would it be better to add a strong base or a weak base? Why?

18. Brian’s ultralight plane has crashed in the desert. It is hot, and he neglected to bring a canteen of water. Assume he is there all day. How would you expect this situation to affect the solute concentration of his blood?

19. What are the two main ways the kidneys maintain the acid-base balance of the blood?

For answers, see Appendix A.

Developmental Aspects of the Urinary System ➔ Learning Objectives

□□ Describe three common congenital problems of the urinary system.

□□ Describe the effect of aging on urinary system functioning.

When you trace the embryonic development of the kidneys, it almost seems as though they can’t “make up their mind” about whether to come or go. The first tubule system forms and then begins to degenerate as a second, lower, set appears. The second set, in turn, degenerates as a third set

The double arrows reveal that this reaction goes both ways. However, an increase in carbon dioxide pushes the reaction to the right, producing more carbonic acid (refer back to Figure 13.11, p. 477). Likewise, an increase in hydrogen ions pushes the reaction to the left, producing more carbonic acid.

In healthy people, carbon dioxide is expelled from the lungs at the same rate as it is formed in the tissues. Thus the H+ released when CO2 is loaded into the blood is not allowed to accumu- late, because it is tied up in H2O when CO2 is unloaded in the lungs. So, under normal condi- tions, the H+ produced by CO2 transport has essentially no effect on blood pH.

However, when CO2 accumulates in the blood (for example, during restricted breathing) or more H+ is released to the blood by metabolic pro- cesses, the chemoreceptors in the respiratory con- trol centers of the brain (or in peripheral blood vessels) are activated. As a result, breathing rate and depth increase, and the excess H+ is “blown off” as more CO2 is removed from the blood.

In contrast, when blood pH begins to rise (alkalosis), the respiratory center is depressed. Consequently, the respiratory rate and depth fall, allowing CO2 (and hence, H

+) to accumulate in the blood. Again blood pH is restored to the nor- mal range. Generally, these respiratory system cor- rections of blood pH (via regulation of CO2 content of the blood) are accomplished within a minute or so.

renal Mechanisms

Chemical buffers can tie up excess acids or bases temporarily, but they cannot eliminate them from the body. And although the lungs can dispose of carbonic acid by eliminating carbon dioxide, only the kidneys can rid the body of other acids gener- ated during metabolism. Additionally, only the kid- neys have the power to regulate blood levels of alkaline substances. Thus, although the kidneys act slowly and require hours or days to change blood pH, they are the most potent of the pH- regulating mechanisms.

The most important means by which the kid- neys maintain acid-base balance of the blood are

CO2 carbon dioxide

+ H2O water

carbonic anhydrase

∆ H2CO3 carbonic

acid

∆ H+ hydrogen

ion

+ HCO3 -

bicarbonate ion

558

FOCUS ON

CAREERS Licensed Practical Nurse (LPN)

Knowledge of anatomy and physiology helps licensed practical nurses care for their patients.

and alert team members to any changes. “Hospice patients take many medications, including narcotics,” she notes. The narcotics also slow down the digestive process and can make patients constipated. “I monitor patients for side effects and report any symptoms immediately to the RN on my team so we can adjust their medications.”

In cancer patients, Davila can often gauge the extent of the disease and recognize when it spreads. “In class we learned that many types of cancer spread to the bone. Sometimes patients will complain of sudden pain in their ribs, which could indicate their cancer has progressed. With lung cancer patients, Davila monitors for shortness of breath, a bluish tinge in skin and fin- gernails, and mental confusion—signs of reduced oxygen, indicating that the cancer is impairing lung function. She promptly alerts team members so appropriate treatment can begin.

LPN training applicants must hold a high school diploma or the equivalent and pass the school’s entrance examina- tion. A one-year program that includes anatomy and physiology, as well as mathematics and other general educa- tional requirements, is typical.

Davila’s anatomy and physiology

training enable her to monitor patients

closely.

After graduating from training, an LPN must also pass a state licensing examination and earn continuing edu- cation credits to maintain licensure. Accreditation procedures vary from state to state. For more information about licensed practical nursing and testing requirements, contact:

National League for Nursing The Watergate 2600 Virginia Ave, NW Eighth Floor Washington, DC 20037 800-669-1656 www.nln.org

For additional information on this career and others, click the Focus on Careers link at Mastering A&P.

Originally, the term practical nurse was used to describe health work- ers who went into private homes to provide bedside nursing. Some licensed practical nurses (LPNs) still provide care in patients’ homes, but others play important roles in hospitals, doctors’ offices, and health care clinics.

Linda Davila, an LPN employed by Stein Hospice in Sandusky, Ohio, spe- cializes in caring for the terminally ill. “Most of our patients, perhaps 60 percent, have cancer,” she says. “Oth- ers have life-threatening kidney, lung, or cardiac conditions. Most hospice patients have been diagnosed as hav- ing 6 months or less to live, and they have chosen to forgo aggressive medi- cal treatments. My job is to help them stay as comfortable and pain free as possible for the remaining days of their lives and to help maintain quality of life for them and their families.”

Hospice care provides a multidis- ciplinary approach. Davila’s team includes social workers, spiritual advis- ers, registered nurses (RNs), and other LPNs. Explains Davila, “I provide per- sonal care for our patients by helping them with bathing and grooming and monitoring their medications. I also work closely with their families and teach them how to care for the patient: how to empty catheters, bathe someone in bed, and change the sheets for someone who’s bedridden.”

Davila’s clinical skills and anatomy/ physiology training enable her to monitor a patient’s condition closely

http://www.nln.org

Chapter 15: The Urinary System 559

time control is achieved. Complete nighttime con- trol usually does not occur before the child is 4 years old.

During childhood and through late middle age, most urinary system problems are infectious or inflammatory conditions. Many types of bacte- ria may invade the urinary tract to cause urethritis, cystitis, or pyelonephritis. Escherichia coli (esh″er- ı̆ke-ah ko′li) bacteria are normal residents of the digestive tract and generally cause no problems there, but they act as pathogens (disease-causing agents) in the sterile environment of the urinary tract and account for 80 percent of urinary tract infections. Bacteria and viruses responsible for sexually transmitted infections (STIs), which are primarily reproductive tract infections, may also invade and cause inflammation in the urinary tract, which leads to the clogging of some of its ducts.

Homeostatic Imbalance 15.9

Childhood streptococcal (strep″to-kok′al) infec- tions, such as strep throat and scarlet fever, may cause inflammatory damage to the kidneys if the original infections are not treated promptly and properly. A common sequel to untreated child- hood strep infections is glomerulonephritis (glo- mer″u-lo-nĕ-fri′tis), in which the glomerular filters become clogged with antigen-antibody complexes resulting from the strep infection. _____________✚

As we age, kidney size and function progres- sively decline. By age 70, the rate of filtrate forma- tion is only about half that of the middle-aged adult. This is believed to result from impaired renal circulation due to atherosclerosis, which affects the entire circulatory system of the aging person. In addition to a decrease in the number of functional nephrons, the tubule cells become less efficient in their ability to concentrate urine.

Another consequence of aging is bladder shrinkage and loss of bladder tone, causing many elderly individuals to experience urgency (a feel- ing that it is necessary to void) and frequency (frequent voiding of small amounts of urine). Nocturia (nok-tu′re-ah), the need to get up dur- ing the night to urinate, plagues almost two-thirds of this population. In many, incontinence is the final outcome of the aging process. Urinary reten- tion is another common problem; most often it is a

makes its appearance. This third set develops into the functional kidneys, which are excreting urine by the third month of fetal life. However, the fetal kidneys do not work nearly as hard as they will after birth, because exchanges with the mother’s blood through the placenta allow her system to clear many of the undesirable substances from the fetal blood.

Homeostatic Imbalance 15.8

There are many congenital abnormalities of the urinary system. Two of the most common are polycystic kidney and hypospadias.

Adult polycystic (pol″e-sis′tik) kidney dis ease is a degenerative condition that appears to run in families. One or both kidneys enlarge, sometimes to the size of a football, and have many blisterlike sacs (cysts) containing urine. These cysts interfere with renal function by obstructing but initially not stopping urine drainage. Currently, little can be done for this condition except to pre- vent further kidney damage by avoiding infection. Renal failure is the eventual outcome, but kidney transplants improve chances for survival.

In the rarer, infantile form of the disease, the kidney has blind pouches into which the filtrate flows, totally blocking drainage. The disease pro- gresses rapidly, resulting in death by 2 years of age.

Hypospadias (hi″po-spa′de-as) is a condition found in male babies only. It occurs when the ure- thral orifice is located on the ventral surface of the penis. Corrective surgery is generally done when the child is around 12 months old. ____________✚

Because the bladder is very small and the kid- neys are unable to concentrate urine for the first 2 months, a newborn baby voids from 5 to 40 times per day, depending on fluid intake. By 2 months of age, the infant is voiding approximately 400 ml/ day, and the amount steadily increases until ado- lescence, when adult urine output (about 1500 ml/ day) is achieved.

Control of the voluntary urethral sphincter goes hand in hand with nervous system develop- ment. By 15 months, most toddlers are aware when they have voided. By 18 months they can hold urine in their bladder for about 2 hours, which is the first sign that potty training can begin. Daytime control usually occurs well before night-

560

SYSTEMS IN SYNC

Homeostatic Relationships between the Urinary System and Other Body Systems

Urinary System

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood; renal regulation of Na+ and water balance is essential for blood pressure homeostasis. Na+, K+, and Ca2+ regulation help maintain normal heart function

• Systemic arterial blood pressure is the driving force for glomerular filtration; heart secretes atrial natriuretic peptide (ANP); blood vessels transport nutrients, oxygen, etc. to urinary organs

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood

• By returning leaked plasma fluid to cardiovascular system, lymphatic vessels help maintain normal systemic blood pressure needed for kidney function; immune cells protect urinary organs from infection, cancer, and other foreign substances

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood; also, metabolize vitamin D to the active form needed for calcium absorption

• Digestive organs provide nutrients needed for kidney health; liver synthesizes most urea, a nitrogenous waste that must be excreted by the kidneys

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood

• Respiratory system provides oxygen required by kidney; disposes of carbon dioxide; cells in the lungs convert angiotensin I to angiotensin II

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood; renal regulation of Na+, K+, and Ca2+ content in ECF is essential for normal neural function

• Neural controls are involved in micturition; sympathetic nervous system activity triggers the renin-angiotensin mechanism

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood

• Bones of rib cage provide some protection to kidneys

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood

• Skin provides external protective barrier; serves as site for vitamin D synthesis and water loss (via perspiration)

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood; produce the hormone erythropoietin; renal regulation of Na+ and water balance is essential for blood pressure homeostasis

• ADH, aldosterone, ANP, and other hormones help regulate renal reabsorption of water and electrolytes

Cardiovascular System

• Kidneys dispose of nitrogenous wastes; maintain fluid, electrolyte, and acid-base balance of blood; renal regulation of Na+, K+, and Ca2+ content in ECF is crucial for muscle activity

• Muscles of pelvic diaphragm and external urethral sphincter function in voluntary control of micturition; muscle metabolism generates creatinine, a nitrogenous waste product that must be excreted by the kidneys

Muscular System

Lymphatic System/Immunity

Digestive System

Reproductive System

Respiratory System

Skeletal System

Integumentary System

Endocrine System

Nervous System

Chapter 15: The Urinary System 561

Did You Get It? 20. What is hypospadias? 21. What is urgency as related to the urinary system?

What is frequency? 22. What is the difference between polycystic kidney

disease and glomerulonephritis?

For answers, see Appendix A.

result of hypertrophy of the prostate gland in males. Some of the problems of incontinence and retention can be avoided by a regular regimen of activity that keeps the body as a whole in opti- mum condition and promotes alertness to elimina- tion signals.

ous ions (always including sodium and potassium). Substances normally absent from urine include glucose, blood proteins, blood, pus (WBCs), and bile.

Ureters, Urinary Bladder, and Urethra (pp. 546–550)

1. The ureters are slender tubes that conduct urine by peristalsis from kidney to bladder.

2. The bladder is a muscular sac posterior to the pubic symphysis. It has two inlets (ureters) and one outlet (urethra). In males, the prostate gland sur- rounds its outlet. The function of the bladder is to temporarily store urine.

3. The urethra carries urine from the bladder to the body exterior. In females, it is 3 to 4 cm long and conducts only urine. In males, it is 20 cm long and conducts both urine and sperm. The internal sphinc- ter of smooth muscle is at the bladder-urethra junc- tion. The external sphincter of skeletal muscle is located more inferiorly.

4. Micturition is emptying the bladder. The micturition reflex causes the involuntary internal sphincter to open when stretch receptors in the bladder wall are stimulated. The external sphincter is voluntarily controlled, so micturition can ordinarily be tempo- rarily delayed. Incontinence is the inability to con- trol micturition.

Fluid, Electrolyte, and Acid-Base Balance (pp. 550–557)

1. Blood composition depends on diet, cellular me- tabolism, and urinary output. To maintain blood composition, the kidneys must do the following:

a. Allow nitrogen-containing wastes (urea, ammo- nia, creatinine, uric acid) to leave the body in the urine.

b. Maintain water and electrolyte balance by absorb- ing more or less water and reclaiming ions in

Summary

Kidneys (pp. 538–546)

1. The paired kidneys are retroperitoneal in the supe- rior lumbar region. Each kidney has a medial indentation (hilum), where the renal artery, renal vein, and ureter attach. Each kidney is enclosed in a tough fibrous capsule. A fatty cushion holds the kidneys against the body trunk wall.

2. A longitudinal section of a kidney reveals an outer cortex, deeper medulla, and medial pelvis. Ex - tensions of the pelvis (calyces) surround the tips of medullary pyramids and collect urine draining from them.

3. The renal artery, which enters the kidney, breaks up into segmental and then interlobar arteries that travel outward through the medulla. Interlobar arteries split into arcuate arteries, which branch to produce cortical radiate arteries, which serve the cortex.

4. Nephrons are structural and functional units of the kidneys. Each consists of a glomerulus and a renal tubule. Subdivisions of the renal tubule (from the glomerulus onward) are glomerular capsule, proxi- mal convoluted tubule, nephron loop, and distal convoluted tubule. A second (peritubular) capillary bed is also associated with each nephron.

5. Nephron functions include filtration, reabsorption, and secretion. Filtrate formation is the role of the high-pressure glomerulus. Filtrate is essentially plasma without blood proteins. In reabsorption, done by tubule cells, needed substances are removed from filtrate (amino acids, glucose, water, some ions) and returned to blood. The tubule cells also secrete additional substances into filtrate. Secretion is important to rid the body of drugs and excess ions and to maintain acid-base balance of blood.

6. Urine is clear, yellow, and usually slightly acidic, but its pH value varies widely. Substances normally found in urine are nitrogenous wastes, water, vari-

562 Essentials of Human Anatomy and Physiology

response to hormonal signals. ADH increases water reabsorption and conserves body water. Aldosterone increases reabsorption of sodium and water and decreases potassium reabsorption.

c. Maintain acid-base balance by actively secreting bicarbonate ions (and retaining hydrogen ions [H+] ) or by absorbing bicarbonate ions (and secreting H+). Chemical buffers tie up excess H+ or bases temporarily; respiratory centers modify blood pH by retaining CO2 (decreases pH) or by eliminating more CO2 from the blood (increases blood pH). Only the kidney can remove meta- bolic acids and excess bases from the body.

Developmental Aspects of the Urinary System (pp. 557–561)

1. The kidneys begin to develop in the first few weeks of embryonic life and are excreting urine by the third month.

2. Common congenital abnormalities include polycys- tic kidney and hypospadias.

3. Common urinary system problems in children and young to middle-aged adults are infections caused by fecal microorganisms, microorganisms causing sexually transmitted infections, and Streptococcus.

4. Renal failure is an uncommon, but extremely seri- ous, problem in which kidneys are unable to con- centrate urine. Dialysis must be done to maintain chemical homeostasis of blood.

5. With age, filtration rate decreases, and tubule cells become less efficient at concentrating urine, leading to urgency, frequency, and incontinence. In men, urinary retention is another common problem.

review Questions

Multiple choice

More than one choice may apply.

1. Microscopic examination of a section of the kid- ney shows a thick-walled vessel with glomeruli scattered in the tissue on one side of the vessel but not on the other side. What vessel is this?

a. Segmental vein

b. Cortical radiate artery

c. Interlobar vein

d. Arcuate artery

2. What is the glomerulus?

a. The same as the renal tubule

b. The same as Bowman’s capsule

c. The same as the nephron

d. Capillaries

3. Which of the following substances should not be present in normal urine?

a. Red blood cells

b. Uric acid

c. Creatinine

d. Glucose

4. Effects of aldosterone include

a. increase in sodium ion excretion.

b. increase in water retention.

c. increase in potassium ion concentration in the urine.

d. higher blood pressure.

5. Which of the following is dependent on tubular secretion?

a. Clearing penicillin from the blood

b. Removal of nitrogenous wastes that have been reabsorbed

c. Removal of excess potassium ions from the blood

d. Control of blood pH

6. What is the length of the urethra in women?

a. 25–30 cm

b. 18–20 cm

c. 3–4 cm

d. 10–15 cm

7. The smallest fluid compartment is the

a. intracellular fluid.

b. extracellular fluid.

c. plasma.

d. interstitial fluid.

8. In the carbonic acid–bicarbonate buffer system, strong acids are buffered by

a. carbonic acid.

b. water.

c. bicarbonate ion.

d. the salt of the strong acid.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 15: The Urinary System 563

Short Answer Essay

9. Name the organs of the urinary system, and describe the general function of each organ.

10. Describe the location of the kidneys in the body.

11. Make a diagram of a nephron. Identify and label the glomerulus, proximal convoluted tubule, distal convoluted tubule, and nephron loop.

12. About 150 L of plasma filters into the renal tubules each day, but only 1–2 L of urine are formed. What happens to the rest of the plasma fluid?

13. What is the function of the collecting ducts? What do they contain?

14. What is the function of the glomerulus?

15. Where do the peritubular capillaries arise from, and how do they differ from the glomerular capillaries?

16. Which part of the glomerulus is under the highest pressure? What is the significance of this?

17. How does aldosterone modify the chemical com- position of urine?

18. What is the juxtaglomerular apparatus, and what is its role in the regulation of blood pressure?

19. Through which parts of the nephron is NaCl trans- ported out? Is this active transport or passive transport?

critical Thinking and clinical Application Questions

31. Jessica has been experiencing a raging and insatia- ble thirst for some months. The unfortunate conse- quence of her increased water intake is a constant need to urinate. She is finding it difficult to get enough sleep and this is now interfering with her work. Explain her symptoms.

32. Mitchell’s parents bring him to the clinic because his urine is tinged with blood. Two days before, he was diagnosed with strep throat. His face and hands are swollen. What is the probable cause of Mitchell’s current kidney problem?

33. Lucy is worried about the variation in the color of her urine. Sometimes her urine is pale yellow and sometimes it is very deep yellow. Is there really a cause for concern? Explain.

34. What happens to the rate of RBC production in a patient on dialysis with total renal failure? What could be given to the patient to counteract such a

problem? (Hint: You might want to check Table 9.2 on p. 356 for help with this one.)

35. Alexander is 72 and is in good health. However, he is recently having difficulties emptying his bladder. What is the likely cause?

36. Mr. Jessup, a 55-year-old man, is operated on for a cerebral tumor. About one month later, he com- plains that he is excessively thirsty and that he has been voiding almost continuously. A urine sample is collected, and its specific gravity is 1.001. What is your diagnosis of Mr. Jessup’s problem, and how might it be related to his previous surgery?

37. Raymond is hypertensive and was recently diag- nosed with impaired kidney function based on uri- nalysis and a blood test for creatinine. What sorts of test results would you expect, and how is hyper- tension related to kidney function?

20. In which condition presence of proteins can be observed in urine? Is this condition pathological or nonpathological? Explain.

21. Describe the role of the respiratory system in controlling acid-base balance.

22. Why is urinalysis a routine part of any good physical examination?

23. How do the internal and external urethral sphincters differ structurally and functionally?

24. Define micturition, and describe the micturition reflex.

25. What sometimes happens when urine becomes too concentrated or remains too long in the bladder?

26. Define urinary retention.

27. Contrast the following homeostatic imbalances: oliguria, anuria, polyuria, and nocturia.

28. Why is the trigone of the urinary bladder important clinically?

29. What type of problem most commonly affects the urinary system organs?

30. Describe the changes that occur in kidney and bladder function in old age.

564

Although male and female reproductive sys tems are quite different, their joint purpose is to produce offspring. The reproductive role of the man is to manufacture male gametes called sperm and deliver them to the woman’s reproductive tract. The woman, in turn, produces female gam- etes, called ova (singular ovum), or eggs. If the conditions are suitable, a sperm and egg fuse to produce a fertilized egg, called a zygote. Once fer- tilization has occurred, the female uterus provides a protective environment in which the embryo, later called the fetus, develops until birth.

The sex hormones play vital roles both in the development and function of the reproductive organs, and in sexual behavior and drives. These hormones

➔ Learning Objective □□ Discuss the common purpose of the reproductive

system organs.

Most organ systems of the body function almost continuously to maintain the well-being of the individual. The reproductive system, however, appears to “slumber” until puberty, when reproductive organs become func- tional. The gonads (go′nadz; “seeds”), or pri mary sex organs, are the testes in men and the ovaries in women. The gonads produce sex cells, or gametes (gam′ēts), and secrete sex hormones. The remaining reproductive system structures are accessory reproductive organs.

The Reproductive System16 WHAT

HOW

WHY

The reproductive organs of men and

women produce gametes and also contribute to the development of secondary

sex characteristics. Male testes produce

sperm, whereas female ovaries produce eggs. When an egg and a sperm join, an

embryo is formed.

The reproductive systems of men and

women produce offspring to ensure the survival of the

human species.

InSTRucToRS

New Building Vocabulary coaching Activities for this chapter are assignable in

Chapter 16: The Reproductive System 565

is testosterone. Thus, the sperm-producing and hormone-producing functions of the testes are car- ried out by completely different cell populations.

Duct System The accessory organs forming the male duct sys- tem, which transports sperm from the body, are the epididymis, ductus deferens, and urethra (Fig ure 16.2, p. 566).

Epididymis

The cup-shaped epididymis is a highly convoluted tube about 6 m (20 feet) long that hugs the poste- rior side of the testis (see Figure 16.1). The epididy- mis is the first part of the male duct system and provides a temporary storage site for the immature sperm. While the sperm make their way along the tortuous course of the epididymis (a trip that takes about 20 days), they mature, developing the ability to swim. When a man is sexually stimulated and ejaculates (ejac = to shoot forth), the walls of the epididymis contract to expel the sperm into the next part of the duct system, the ductus deferens.

also influence the growth and development of many other organs and tissues of the body.

Anatomy of the Male Reproductive System ➔ Learning Objectives

□□ When provided with a model or diagram, identify the organs of the male reproductive system, and discuss the general function of each.

□□ name the endocrine and exocrine products of the testes.

□□ Discuss the composition of semen, and name the glands that produce it.

□□ Trace the pathway followed by a sperm from the testis to the body exterior.

□□ Define circumcision, erection, and ejaculation.

As already noted, the primary reproductive organs of the male are the paired testes (tes′tēz), which have both an exocrine (sperm-producing) function and an endocrine (testosterone-producing) function. The accessory reproductive structures are ducts or glands that aid in the delivery of sperm to the body exterior or to the female reproductive tract.

Testes Each golf ball–sized testis is approximately 4 cm (1.5 inches) long and 2.5 cm (1 inch) wide and is connected to the trunk via the spermatic cord, a connective tissue sheath that encloses blood ves- sels, nerves, and the ductus deferens. A fibrous connective tissue capsule, the tunica albuginea (tu′nı̆-kah al″bu-jin′e-ah; “white coat”) surrounds each testis. Extensions of this capsule (septa) plunge into the testis and divide it into a large number of wedge-shaped lobules (Figure 16.1). Each lobule contains one to four tightly coiled seminiferous (sem′ ı̆-nif′er-us) tubules, the ac - tual “sperm-forming factories.” Seminiferous tu - bules of each lobule empty sperm into another set of tubules, the rete (re′te) testis, located to one side of the testis. Sperm travel through the rete testis to enter the first part of the duct system, the epididymis (ep″ ı̆-did′ ı̆-mis), which hugs the external surface of the testis.

Lying in the soft connective tissue surrounding the seminiferous tubules are the interstitial (in″ter-stish′al) cells, functionally distinct cells that produce androgens—the most important of which

Spermatic cord

Lobule Septum

Tunica albuginea

Seminiferous tubule

Blood vessels and nerves

Epididymis

Ductus (vas) deferens

Rete testis

Figure 16.1 Sagittal section of the testis and associated epididymis.

566 Essentials of Human Anatomy and Physiology

propel live sperm from their storage sites—the epididymis and distal part of the ductus deferens— into the urethra. At the moment of ejaculation, the thick layers of smooth muscle in its walls create peristaltic waves that rapidly squeeze the sperm forward.

Part of the ductus deferens lies in the scrotum, a skin sac that hangs outside the body cavity and holds the testes (see Figure 16.2). Some men vol- untarily opt to take full responsibility for birth con- trol by having a vasectomy (vah-sek′to-me; “cutting the vas”). In this relatively minor operation, the surgeon makes a small incision into the scrotum and then cuts through and ties off the ductus def- erens. Sperm are still produced, but they can no longer reach the body exterior, and eventually they deteriorate and are phagocytized. A man is sterile after this procedure, but because testosterone is

Ductus Deferens

The ductus deferens (duk′tus def′er-enz; “carry- ing away”), or vas deferens, runs upward from the epididymis via the spermatic cord through the inguinal canal into the pelvic cavity and arches over the superior aspect of the urinary bladder. The ductus deferens then loops medially over the ureter and descends along the posterior blad- der wall. The end of the ductus deferens expands as the ampulla and then empties into the ejac ulatory (e-jak′u-lah-to″re) duct, which passes through the prostate to merge with the urethra. The main function of the ductus deferens is to

Seminal vesicle

Ampulla of ductus deferens

Ejaculatory duct

Rectum

Prostate

Bulbo-urethral gland

Epididymis

Ureter

Urinary bladder

Prostatic urethra

Pubis

Membranous urethra

Urogenital diaphragm

Erectile tissue of the penis

Glans penis

Prepuce External urethral orifice

Spongy urethra

Shaft of the penis

Testis Scrotum

Ductus (vas) deferens

(a)

How might hypertrophy of the prostate lead to urinary system problems?Q:

Because the prostate encircles the superior part of the urethra, prostate hypertrophy constricts the

urethra in that region, impairing urination.

Figure 16.2 Male reproductive organs. (a) Sagittal view.

Chapter 16: The Reproductive System 567

ducts, the bladder sphincter (internal urethral sphincter) constricts. This event not only prevents urine from passing into the urethra, but also pre- vents sperm from entering the urinary bladder.

Did You Get It? 1. What are the two major functions of the testes? 2. What is the role of the seminiferous tubules? 3. Name the organs of the male duct system, in order,

from the scrotum to the body exterior.

For answers, see Appendix A.

Accessory Glands and Semen The accessory glands include the paired seminal vesicles, the single prostate, and the paired bulbo- urethral (bul-bo-u-re′thral) glands (see Figure 16.2). These glands produce the bulk of semen (se′men), the sperm-containing fluid that is propelled out of the male’s reproductive tract during ejaculation.

Seminal Vesicles

The seminal (sem′ ı̆-nul) vesicles are located at the base of the bladder. These large hollow glands, each 6 to 7 cm (about the shape and size of the little fin- ger), produce about 60 percent of seminal fluid, the fluid portion of semen. Their thick, yellowish secre- tion is rich in sugar (fructose), vitamin C, prostaglan- dins, and other substances that nourish and activate the sperm passing through the tract. The duct of each seminal vesicle joins that of the ductus defer- ens on the same side to form the ejaculatory duct (see Figure 16.2). Thus, sperm and seminal fluid enter the urethra together during ejaculation.

Prostate

The prostate is a single doughnut-shaped gland about the size of a peach pit (see Figure 16.2). It encircles the upper (prostatic) part of the urethra just inferior to the urinary bladder. Prostate fluid is milky and plays a role in activating sperm. During ejaculation, the fluid enters the urethra through several small ducts. Because the prostate is located immediately anterior to the rectum, its size and texture can be palpated (felt) by digital (finger) examination through the anterior rectal wall.

Homeostatic Imbalance 16.1

The prostate has a reputation as a common cause of health problems. Hypertrophy of the prostate, an increase in its size independent of the body’s

still produced, he retains his sex drive and second- ary sex characteristics.

urethra

The urethra, which extends from the base of the urinary bladder to the tip of the penis, is the termi- nal part of the male duct system. It has three named regions: (1) the prostatic urethra, sur- rounded by the prostate gland; (2) the membra nous urethra, spanning the distance from the prostatic urethra to the penis; and (3) the spongy (penile) urethra, running within the length of the penis and opening to the body exterior via the external urethral orifice (see Figure 16.2). The male urethra carries both urine and sperm to the body exterior; thus, it serves two masters, the uri- nary and reproductive systems (recall Chapter 15). However, urine and sperm never leave the body at the same time. When ejaculation occurs and sperm enter the prostatic urethra from the ejaculatory

Urinary bladder

(b)

Bulbourethral gland

Erectile tissues

Spongy urethra

Glans penis

Prepuce

External urethral orifice

Ductus deferens

Ampulla of ductus deferens

Prostatic urethra

Prostate

Membranous urethra

Root of penis

Shaft (body) of penis

Ejaculatory duct

Seminal vesicle

Epididymis

Testis

Ureter

Figure 16.2 (continued) (b) Frontal view; posterior aspect of penis.

568 Essentials of Human Anatomy and Physiology

system during ejaculation is relatively small, only 2 to 5 ml (about a teaspoonful), but there are between 50 and 150 million sperm in each milliliter.

Homeostatic Imbalance 16.2

Male infertility, the inability to conceive a child, may be due to obstructions of the duct system, hormonal imbalances, environmental estrogens, pesticides, excessive alcohol, or many other fac- tors. One of the first series of tests done when a couple has been unable to conceive is semen analysis. Factors analyzed include sperm count, motility and morphology (shape and maturity), and semen volume, pH, and fructose content. A sperm count lower than 20 million per milliliter makes impregnation improbable. _____________✚

External Genitalia The male external genitalia (jen″i-tāl′e-ah) include the scrotum and the penis (see Figure 16.2). The scrotum (skro′tum; “pouch”) is a divided sac of skin with sparse hairs that hangs outside the abdominal cavity, between the legs at the root of the penis. Under normal conditions, the scrotum hangs loosely from its attachments, providing the testes with a temperature that is below body tem- perature. This is a rather exposed location for a man’s testes, which contain his entire genetic heri- tage, but apparently viable sperm cannot be pro- duced at normal body temperature. The scrotum, which provides a temperature about 3°C (5.4°F) lower, is necessary for the production of healthy sperm. When the external temperature is very cold, the scrotum becomes heavily wrinkled as it pulls the testes closer to the warmth of the body wall. Thus, changes in scrotal surface area can maintain a temperature that favors viable sperm production.

The penis (pe′nis; “tail”) functions to deliver sperm into the female reproductive tract. The skin- covered penis consists of a shaft, which ends in an enlarged tip, the glans penis. The skin cover- ing the penis is loose, and it folds downward to form a sleeve of skin, the prepuce (pre′pūs), or foreskin, around the proximal end of the glans penis. Frequently in the United States, the foreskin is surgically removed shortly after birth, in a pro- cedure called circumcision.

Internally, the spongy urethra (see Figure 16.2) is surrounded by three elongated areas of erectile tissue, a spongy tissue that fills with blood during

growth, affects nearly every older man and stran- gles the urethra. This troublesome condition makes urination difficult and enhances the risk of bladder infections (cystitis) and kidney damage. Traditional treatment has been surgery, but some newer options are becoming more popular. These include the following:

• Using drugs or microwaves to shrink the prostate

• Inserting a small inflatable balloon to compress prostate tissue away from the prostatic urethra

• Inserting a tiny needle that emits bursts of radiofrequency radiation, which incinerate excess prostate tissue

Prostatitis, inflammation of the prostate, is the single most common reason for a man to con- sult a urologist, and prostate cancer is the third most prevalent cancer in men. Most cases of pros- tate cancer are slow-growing, but it can also be a swift and deadly killer. _______________________✚

Bulbourethral Glands

The bulbourethral glands are tiny, pea-sized glands inferior to the prostate gland. They produce a thick, clear mucus that drains into the penile urethra. This secretion is the first to pass down the urethra when a man becomes sexually excited. It cleanses the urethra of trace acidic urine prior to ejaculation, and it serves as a lubricant during sexual intercourse.

Semen

Semen is a milky white, somewhat sticky mixture of sperm and accessory gland secretions. The liquid portion acts as a transport medium for nutrients and chemicals that protect the sperm and aid their movement. Mature sperm cells are streamlined cel- lular “tadpoles” containing little cytoplasm or stored nutrients. The fructose in the seminal vesicle secre- tion provides essentially all of their energy fuel. Sperm are very sluggish under acidic conditions (below pH 6). The relative alkalinity of semen as a whole (pH 7.2–7.6) helps neutralize the acidic envi- ronment (pH 3.5–4) of the female’s vagina, protect- ing the delicate sperm. Semen also contains antibi- otic chemicals that destroy certain bacteria, the hormone relaxin, enzymes that enhance sperm motility, and substances that inhibit an immune response in the female reproductive tract.

Semen also dilutes sperm; without such dilu- tion, sperm motility is severely impaired. The amount of semen propelled out of the male duct

Chapter 16: The Reproductive System 569

Late spermatids

Early spermatids

Secondary spermatocytes

Primary spermatocyte

Daughter cell type B (moves toward tubule lumen)

Daughter cell type A (remains at basement membrane as a stem cell)

Spermatogonium (stem cell)

M ei

os is

S pe

rm io

ge ne

si s

S pe

rm at

og en

es is

Mitosis

Seminiferous tubule Basement membrane

Growth

Lumen of seminiferous tubule

Sperm

Enters prophase of meiosis I

Meiosis I completed

Meiosis II

2n 2n

n n

n n n

n n n n

sexual excitement. This causes the penis to enlarge and become rigid. This event, called erection, helps the penis serve as the male organ of copula- tion (sexual intercourse) to deliver the semen into the female reproductive tract.

Did You Get It? 4. What is the function of the erectile tissue of the penis? 5. What is an important function of each of these two

components of semen—sperm and seminal fluid? 6. Adolph, a 68-year-old gentleman, has trouble

urinating and is given a rectal exam. What is his most probable condition, and what is the purpose of the rectal exam?

For answers, see Appendix A.)

Male Reproductive Functions ➔ Learning Objectives

□□ Define spermatogenesis and meiosis.

□□ Describe the structure of a sperm, and relate its structure to its function.

□□ Describe the effect of FSH and LH on testis functioning.

The chief role of the male in the reproductive pro- cess is to produce sperm and the hormone testos- terone.

Spermatogenesis Sperm production, or spermatogenesis (sper″mah- to-jen′ĕ-sis), begins during puberty and continues throughout life. A man makes millions of sperm daily. Only one sperm fertilizes an egg, so it seems that nature has made sure that the human species will not be endangered for lack of sperm.

Sperm are formed in the seminiferous tubules of the testis, as noted earlier. The process is begun by primitive stem cells called spermatogonia (sper″mah-to-go′ne-ah), found in the outer edge, or periphery, of each tubule (Figure 16.3). Spermatogonia go through rapid mitotic divisions to build up the stem cell line. From birth until puberty, all such divisions simply produce more stem cells. During puberty, however, follicle-stimulating hor- mone (FSH) is secreted in increasing amounts by the anterior pituitary gland.

➔ConCeptLinkRecall that FSH is a tropic hormone that, in males, tar-gets the testes and stimulates sperm production (Chapter 9, p. 343). Figure 16.3 Spermatogenesis. Flowchart showing the relative position of the spermatogonia in the wall of the seminiferous tubule. Although the stem cells and the primary spermatocytes have the same number of chromosomes (46, designated as 2n) as other body cells, the products of meiosis (spermatids and sperm) have only half as many (23, designated as n).

570 Essentials of Human Anatomy and Physiology

As meiosis occurs, the dividing cells (primary and then secondary spermatocytes) are pushed toward the lumen of the tubule. Thus, we can follow the progress of meiosis from the tubule periphery to the lumen. The spermatids, produced by meiosis, are not functional sperm. They are nonmotile and have too much excess baggage to function well in reproduction. They must undergo further changes, in which their excess cytoplasm is stripped away and a tail is formed (see Figure 16.3). In this last stage of sperm development, called spermiogene sis (sper″me-o-gen′ĕ-sis), all the excess cytoplasm is sloughed off, and what remains is compacted into the three regions of the mature sperm—the head, midpiece, and tail (Figure 16.5). The mature sperm is a greatly streamlined cell equipped with a high rate of metabolism and a means of propelling itself, enabling it to move long distances in a short time to get to the egg. It is a prime example of the relation- ship between structure and function. (Remember, we talked about this in Chapter 1.)

Sperm “pack” light. The sperm head is the nucleus and contains compacted DNA, the genetic material. Anterior to the nucleus is the helmetlike acrosome (ak′ro-sōm), which is produced by the Golgi appara- tus and is similar to a large lysosome. When a sperm comes into close contact with an egg (or more pre- cisely, an oocyte), the acrosomal membrane breaks down and releases enzymes that help the sperm pen- etrate the capsule of follicle cells that surround the egg. Filaments, which form the long tail, arise from centrioles in the midpiece. Mitochondria wrapped tightly around these filaments provide the ATP needed for the whiplike movements of the tail that propel the sperm along the female reproductive tract.

The entire process from the formation of a pri- mary spermatocyte to release of immature sperm in the tubule lumen takes 64 to 72 days. But sperm in the lumen are still unable to “swim” and so are still incapable of fertilizing an egg. They are moved by peristalsis through the tubules of the testes into the epididymis. There they undergo further matu- ration, which results in increased motility and fer- tilizing power.

Homeostatic Imbalance 16.3

Environmental threats can alter the normal process of sperm formation. For example, some common antibiotics, such as penicillin and tetracycline, may suppress sperm formation. Radiation, lead, certain

From puberty on, each division of a spermato- gonium produces one stem cell, called a type A daughter cell, and another cell, called a type B daughter cell. The type A cell remains at the tubule periphery to maintain the stem cell popula- tion. The type B cell gets pushed toward the tubule lumen, where it becomes a primary sper matocyte destined to undergo meiosis (mi-o′sis) and form four sperm.

Meiosis is a special type of nuclear division that occurs for the most part only in the gonads (testes and ovaries). It differs from mitosis (described in Chapter 3) in two major ways. Meiosis consists of two successive divisions of the nucleus (called meiosis I and II), instead of only one division. It results in four (instead of two) daughter cells, or gametes.

In spermatogenesis, the gametes are called spermatids (sper′mah-tidz). Spermatids have only half as much genetic material as other body cells. In humans, this is 23 chromosomes (or the haploid number [n] of chromosomes) rather than the usual 46 (diploid, or 2n). Then, when the sperm and the egg (which also has 23 chromosomes) unite, form- ing the fertilized egg, or zygote, the normal 2n number of 46 chromosomes is reestablished and is maintained in subsequent body cells by the pro- cess of mitosis (Figure 16.4).

FertilizationMeiosis

Gametes (n = 23)

Zygote (2n = 46)

Multicellular adults (2n = 46)

Egg

Sperm

Mitosis and development

Figure 16.4 The human life cycle.

Chapter 16: The Reproductive System 571

the seminiferous tubules are being prodded by FSH to produce sperm, the interstitial cells are being activated by luteinizing hormone (LH), which is also released by the anterior pituitary gland (Figure 16.6, p. 572). From this time on, tes- tosterone is produced continuously (more or less) for the rest of a man’s life. The rising blood level of testosterone in the young man stimulates the adolescent growth spurt, prompts his reproductive organs to develop to their adult size, underlies the sex drive, and causes the male secondary sex characteristics to appear. Secondary sex charac teristics are features induced in nonreproductive organs by sex hormones. Male secondary sex characteristics include the following:

• Deepening of the voice as the larynx enlarges

• Increased hair growth all over the body, partic- ularly in the axillary and pubic regions and on the face

• Enlargement of skeletal muscles to produce the heavier muscle mass typical of the male physique

pesticides, marijuana, tobacco, and excessive alco- hol can cause production of abnormal sperm (two- headed, multiple-tailed, and so on).

An abnormal, multi-tailed human sperm from a 66-year-old man (colorized). _____________________________________________✚

Testosterone Production As noted earlier, the interstitial cells produce tes tosterone (tes-tos′tĕ-rōn), the most important hor- monal product of the testes. During puberty, as

They provide the ATP energy the tail needs to make the sperm motile.A:

Neck

Head

Proximal centriole

Nucleus

Acrosome

Plasma membrane

Mitochondria

Tail

(a)

(b)

Axial filament of tail

Midpiece

Provides energy for mobility

Provides genetic instructions and a means of penetrating the follicle cell capsule and oocyte membrane

Provides for mobility

Figure 16.5 Structure of sperm. (a) Scanning electron micrograph of mature sperm (1,525*). (b) Diagrammatic view of a sperm.

What is the importance of the mitochondria in the sperm midpiece?Q:

572 Essentials of Human Anatomy and Physiology

Hypothalamus

Testosterone

FSH and testosterone

stimulate sperm

production

Supporting cell

Spermatogonia

Seminiferous tubule in testis

Primary and secondary sex characteristics

Spermatogenesis

LHFSH

Stimulates

Inhibits

Anterior pituitary

GnRH

The hypothalamus releases gonadotropin-releasing hormone (GnRH).

5 5

The rising level of testosterone exerts negative feedback control on the hypothalamus and pituitary.

GnRH stimulates the anterior pituitary to release gonadotropins —FSH and LH.

4 LH stimulates the interstitial cells to release testosterone, which serves as the final trigger for spermatogenesis. Testosterone then enhances spermatogenesis.

3 4

FSH stimulates spermatogonia to produce sperm.

KEY:

Figure 16.6 Hormonal control of testosterone release and sperm production.

What is the effect of negative feedback of testosterone on anterior pituitary and hypothalamic cells?Q:

It “turns off” the hypothalamic releasing factor that prompts FSH and LH secretion; hence, FSH and LH

secretion also fall.

Chapter 16: The Reproductive System 573

• Increased heaviness of the skeleton due to bone growth in both size and density

Because testosterone is responsible for the appearance of these typical masculine characteris- tics, it is often referred to as the “masculinizing” hormone.

Homeostatic Imbalance 16.4

If testosterone is not produced in a young man, his secondary sex characteristics never appear, and his other reproductive organs remain child- like. This condition is called sexual infantilism. Castration of the adult male (or the inability of his interstitial cells to produce testosterone) results in a decrease in the size and function of his reproductive organs, as well as a decrease in his sex drive. Sterility also occurs because tes- tosterone is necessary for the final stages of sperm production. ___________________________✚

Did You Get It? 7. Which pituitary hormone stimulates spermatogenesis? 8. How does the final product of meiosis differ from the

final product of mitosis? 9. How are nonmotile spermatids converted to func-

tional sperm? 10. Which pituitary hormone prompts testosterone

production?

For answers, see Appendix A.

Anatomy of the Female Reproductive System ➔ Learning Objectives

□□ Identify the organs of the female reproductive system, and discuss the general function of each.

□□ Describe the functions of the vesicular follicle and corpus luteum of the ovary.

□□ Define endometrium, myometrium, and ovulation.

□□ Indicate the location of the following regions of the female uterus: cervix, fundus, body.

The reproductive role of the female is much more complex than that of the male. Not only must she produce the female gametes (ova), but her body must also nurture and protect a developing fetus during 9 months of pregnancy. Ovaries (o′vah- rēz) are the primary female reproductive organs.

Like the testes, ovaries produce both an exocrine product (eggs, or ova) and endocrine products (estrogens and progesterone). The other organs of the female reproductive system serve as accessory structures to transport, nurture, or otherwise serve the needs of the reproductive cells and/or the developing fetus.

ovaries The paired ovaries are pretty much the shape of almonds but are nearly twice as large. An internal view of an ovary reveals many tiny saclike struc- tures called ovarian follicles (Figure 16.7, p. 574). Each follicle consists of an immature egg, called an oocyte (o′o-sı̄t; oo = egg), surrounded by one or more layers of very different cells called folli cle cells. As a developing egg within a follicle begins to ripen or mature, the follicle enlarges and develops a fluid-filled central region called an antrum. At this stage, the follicle, called a vesicu lar follicle or Graafian (graf′e-an) follicle, is mature, and the developing egg is ready to be ejected from the ovary, an event called ovulation. After ovulation, the ruptured follicle is transformed into a very different-looking structure called a cor pus luteum (kor′pus lu′te-um; “yellow body”), which eventually degenerates. Ovulation generally occurs every 28 days, but it can occur more or less frequently in some women.

The ovaries flank the uterus laterally (Figure 16.8, p. 575). These organs are secured to the lateral walls of the pelvis by the suspensory ligaments and are anchored to the uterus medially by the ovarian ligaments. In between, they are enclosed and held in place by a fold of peritoneum, the broad ligament.

Duct System The uterine tubes, uterus, and vagina form the duct system of the female reproductive tract (see Figure 16.8).

uterine Tubes

The uterine (u′ter-in) tubes, or fallopian (fal- lo′pe-an) tubes, form the initial part of the duct system. They receive the ovulated oocyte and pro- vide a site where fertilization can occur. Each of the uterine tubes is about 10 cm (4 inches) long and extends medially from an ovary to empty into the superior region of the uterus. Like the ovaries,

574 Essentials of Human Anatomy and Physiology

the uterine tubes are enclosed and supported by the broad ligament.

Unlike in the male duct system, which is con- tinuous with the tubule system of the testes, there is little or no actual contact between the uterine tubes and the ovaries. The distal end of each uterine tube expands as the funnel-shaped infundibulum, which has fingerlike projections called fimbriae (fim′bre-e) that partially sur- round the ovary (see Figure 16.8b). As an oocyte is expelled from an ovary during ovulation, the waving fimbriae create fluid currents that carry the oocyte into the uterine tube, where it begins its journey toward the uterus. (Many potential eggs, however, are lost in the peritoneal cavity.) The oocyte is carried toward the uterus by a combination of peristalsis and the rhythmic beat- ing of cilia.

Because the journey to the uterus takes 3 to 4 days and the oocyte is viable for at most 24 hours after ovulation, the usual site of fertiliza- tion is the uterine tube. To reach the oocyte, the sperm must swim upward through the vagina and uterus to reach the uterine tubes. Because they must swim against the downward current created by the cilia, it is rather like swimming against the tide!

Homeostatic Imbalance 16.5

The fact that the uterine tubes are not continuous distally with the ovaries places women at risk for infections spreading from the reproductive tract into the peritoneal cavity. Gonorrhea (gon″o-re′ah) and other sexually transmitted bacteria some- times infect the peritoneal cavity in this way, caus- ing a severe inflammation called pelvic inflam matory disease (PID). Unless treated promptly, PID can cause scarring and closure of the narrow uterine tubes, which is one of the major causes of female infertility. __________________________✚

uterus

The uterus (u′ter-us; “womb”), located in the pel- vis between the urinary bladder and rectum, is a hollow organ that functions to receive, retain, and nourish a fertilized egg. In a woman who has never been pregnant, it is about the size and shape of a pear. During pregnancy, the uterus increases tremendously in size and during the latter part of pregnancy can be felt well above the umbilicus. The uterus is suspended in the pelvis by the broad ligament and anchored anteriorly and posteriorly by the round ligaments and uterosacral liga ments, respectively (see Figure 16.8).

Degenerating corpus luteum

Primary follicle Growing follicles

Mature vesicular (Graafian) follicle

Germinal epithelium

Ruptured follicle

Developing corpus luteum

Ovulation

Corpus luteum

Secondary oocyte

Antrum

Corona radiata

Blood vessels

Figure 16.7 Sagittal section of a human ovary showing the developmental stages of an ovarian follicle.

Chapter 16: The Reproductive System 575

Infundibulum Uterine tube Ovary

Uterus (fundus)

Round ligament

Urinary bladder Pubic symphysis Mons pubis

Urethra Clitoris

Hymen

Labium minus Labium majus

Suspensory ligament of ovary (part of broad ligament)

Uterosacral ligament

Rectum

Cervix

Vagina

Anus

Greater vestibular gland

Fimbriae

Vagina

Cervical canal

Wall of uterusPerimetrium

Myometrium Endometrium

Round ligament of uterus

Uterine tube

Infundibulum Fimbriae

Lumen (cavity) of uterus

Suspensory ligament of ovary

Uterine (fallopian) tube

Ovarian blood vessels

Broad ligament

Ovary

Ovarian ligament

Body of uterus

Ureter

Uterine blood vessels Uterosacral ligament Cervix

(b)

(a)

Fundus of uterus

Figure 16.8 The human female reproductive organs. (a) Sagittal section. (The plurals of labium minus and majus are labia minora and majora, respectively.) (b) Posterior view. The posterior organ walls have been removed on the right side to reveal the shape of the lumen of the uterine tube, uterus, and vagina.

576 Essentials of Human Anatomy and Physiology

The major portion of the uterus is referred to as the body. Its rounded region superior to the entrance of the uterine tubes is the fundus, and its narrow outlet, which protrudes inferiorly into the vagina, is the cervix.

The thick wall of the uterus is composed of three layers. The inner layer, or mucosa, is the endometrium (en-do-me′tre-um). If fertilization occurs, the fertilized egg (actually the young embryo by the time it reaches the uterus) burrows into the endometrium (in a process called implantation) and resides there for the rest of its development. When a woman is not pregnant, the endometrial lin- ing sloughs off periodically, usually about every 28 days, in response to changes in the levels of ovarian hormones in the blood. This process, called men- struation or menses, is often referred to as a woman getting her “period” (discussed on p. 580).

Homeostatic Imbalance 16.6

Cervical cancer is relatively common among women between the ages of 30 and 50. Risk factors include sexually transmitted diseases such as human papillomavirus (HPV), frequent cervical inflamma- tion, multiple pregnancies, and many sexual partners. Although HPV infection is a major cause of cervical cancer, other factors may also be involved. A yearly Pap smear is the single most important diagnostic test for detecting this slow-growing cancer. When results are inconclusive, a test for HPV can be done from the same Pap sample or from a blood sample.

Gardasil, a three-dose vaccine that provides protection from the most common types of HPV- induced cervical cancer, is a recent addition to the official childhood immunization schedule. It is rec- ommended for all 11- and 12-year-old girls but can also be given to boys and girls ages 13–26. In unexposed girls, the vaccine specifically blocks two cancer-causing kinds of HPV as well as two additional types that are not associated with cervi- cal cancer. Whether or not this vaccine will become a requirement for school is currently decided on a state-to-state basis. ______________✚

The bulky middle layer of the uterus wall is the myometrium (mi-o-me′tre-um), which is composed of interlacing bundles of smooth mus- cle (see Figure 16.8b). The myometrium plays an active role during childbirth, when it contracts rhythmically to force the baby out of the mother’s body. The outermost serous layer of the uterus

wall is the perimetrium (per- ı̆-me′tre-um), or the visceral peritoneum.

Vagina

The vagina (vah-ji′nah) is a thin-walled tube 8 to 10 cm (3 to 4 inches) long. It lies between the bladder and rectum and extends from the cervix to the body exterior (see Figure 16.8). Often called the birth canal, the vagina provides a passageway for the delivery of an infant and for the menstrual flow to leave the body. Because it receives the penis (and semen) during sexual intercourse, it is the female organ of copulation.

The distal end of the vagina is partially closed by a thin fold of the mucosa called the hymen (hi′men). The hymen is very vascular and tends to bleed when it is ruptured during the first sexual intercourse. However, its durability varies. In some women, it is torn during a sports activity, tampon insertion, or pelvic examination. Occasionally, it is so tough that it must be ruptured surgically if intercourse is to occur.

External Genitalia and Female Perineum The female reproductive structures that are located external to the vagina are the external genitalia (Figure 16.9). The external genitalia, also called the vulva, include the mons pubis, labia, clitoris, urethral and vaginal orifices, and greater vestibular glands.

Mons pubis

Labia majora

Prepuce of clitoris

Vestibule

Labia minora

Urethral orifice Vaginal orifice

Opening of duct of greater vestibular gland

Perineum

Anus

Clitoris

Figure 16.9 External genitalia of the human female.

Chapter 16: The Reproductive System 577

The mons pubis (“mountain on the pubis”) is a fatty, rounded area overlying the pubic symphy- sis. After puberty, this area is covered with pubic hair. Running posteriorly from the mons pubis are two elongated hair-covered skin folds, the labia majora (la′be-ah ma-jo′ra), which enclose two delicate, hair-free folds, the labia minora. The labia majora enclose a region called the vestibule, which contains the external openings of the ure- thra,* followed posteriorly by that of the vagina.

Just anterior to the vestibule is the clitoris (kli′to-ris; “hill”), a small, protruding structure that corresponds to the male penis. Like the penis, it is hooded by a prepuce and is composed of sensi- tive erectile tissue that becomes swollen with blood during sexual excitement. The clitoris differs from the penis in that it lacks a reproductive duct.

A pair of mucus-producing glands, the greater vestibular glands, flank the vagina, one on each side (see Figure 16.8a). Their secretion lubricates the distal end of the vagina during intercourse. The diamond-shaped region between the anterior end of the labial folds, the anus posteriorly, and the ischial tuberosities laterally is the perineum (per″ ı̆-ne′um).

Did You Get It? 11. What is the exocrine product of the ovary? 12. Which organ of the female duct system serves as an

“incubator” for fetal development? What is the most common site of fertilization?

13. What name is given to an ovarian follicle that is ready to ovulate?

For answers, see Appendix A.

Female Reproductive Functions and cycles Although sperm production in men continues throughout life, the production of eggs in women is quite different. The total supply of eggs that a female can release is already determined by the time she is born. In addition, a female’s reproduc- tive ability (that is, her ability to release eggs) begins during puberty, heralded by the beginning of her menstrual cycle, and usually ends in her fif- ties. The period in which a woman’s reproductive

capability ends is called menopause (see pp. 596 and 598).

oogenesis and the ovarian cycle ➔ Learning Objectives

□□ Define oogenesis.

□□ Describe the influence of FSH and LH on ovarian function.

Meiosis, the special kind of cell division that occurs in the testes to produce sperm, also occurs in the ovaries. But in this case, ova or female gametes are produced, and the process is called oogenesis (o″o-jen′ĕ-sis; “the beginning of an egg”). This pro- cess is shown in Figure 16.10, p. 578.

In the developing female fetus, oogonia (o″o- go′ne-ah), the female stem cells, multiply rapidly to increase their number, producing daughter cells called primary oocytes. These primary oocytes then push into the ovary connective tissue, where they become surrounded by a single layer of cells to ultimately form the primary follicles. By birth, the oogonia no longer exist, and a female’s life- time supply of primary oocytes (approximately 1 million of them) is already in place in the ovarian follicles, awaiting the chance to undergo meiosis to produce functional eggs. Because the primary oocytes remain in this state of suspended anima- tion all through childhood, their wait is a long one—usually from 10 to 14 years.

At puberty, the anterior pituitary gland begins to release FSH (follicle-stimulating hormone), which stimulates a small number of primary folli- cles to grow and mature each month, and ovula- tion begins to occur each month. These cyclic changes that occur monthly in the ovary constitute the ovarian cycle. At puberty, perhaps 300,000 oocytes remain; and, beginning at this time, a small number of oocytes are activated each month. As the reproductive life of a female is about 40 years (from the age of 11 to approximately 51), and there is typically only one ovulation event per month, fewer than 500 ova out of the potential 300,000 are released during her lifetime. Again, nature has provided us with a generous oversup- ply of sex cells.

As a follicle prodded by FSH grows larger, it accumulates fluid in the central chamber called the antrum (look back at Figure 16.7), and the primary oocyte it contains replicates its chromosomes and begins meiosis. The first meiotic division produces

*Unlike the male urethra, which carries both urine and semen, the female urethra has no reproductive function—it is strictly a passageway for urine.

578 Essentials of Human Anatomy and Physiology

two cells that are very dissimilar in size (see Figure 16.10). The larger cell is a secondary oocyte and the other, very tiny cell is a polar body. By the time a follicle has ripened to the mature (vesicular follicle) stage, it contains a secondary oocyte and protrudes from the external surface of the ovary. Follicle development to this stage takes about 14 days, and ovulation (of a secondary oocyte) occurs at just about that time in response to the burstlike release of a second anterior pituitary hormone,

luteinizing hormone (LH) (Figure 16.11). The ovu- lated secondary oocyte is still surrounded by its follicle-cell capsule, now called the corona radiata (“radiating crown”) (as shown in Figure 16.7).

Some women experience a twinge of abdomi- nal pain in the lower abdomen when ovulation occurs. This phenomenon, called mittelschmerz (mit′el-shmerts; German for “middle pain”), is caused by the intense stretching of the ovarian wall during ovulation.

Meiotic Events Follicle Development in Ovary

Before birth

Childhood

Primary oocyte

Primary oocyte (still arrested in prophase I)

Mature vesicular (Graafian) follicle

Primary follicle

Oocyte

Ovulated secondary oocyte

Growing follicle

Primary oocyte (arrested in prophase I; present at birth)

Oogonium (stem cell)

Monthly from puberty to menopause

Meiosis I (completed by one primary oocyte each month)

First polar body

Mitosis

Growth

Meiosis II of polar body (may or may not occur)

Polar bodies (all polar bodies degenerate)

OvumSecond polar body

Meiosis II completed (only if sperm penetration occurs)

Sperm

Ovulation

Secondary oocyte (arrested in metaphase II)

Follicle cells

(ovary inactive)

nnn n

Figure 16.10 Events of oogenesis. Left, flowchart of meiotic events. Right, correlation with follicular development and ovulation in the ovary.

Chapter 16: The Reproductive System 579

Generally speaking, one of the developing fol- licles outstrips the others each month to become the dominant follicle. Just how this follicle is selected or selects itself is not understood, but the follicle that is at the proper stage of maturity when the LH level surges is the one that ovulates its oocyte. The mature follicles that are not ovulated soon become overripe and deteriorate. In addition to playing a major role in triggering ovulation, LH also causes the ruptured follicle to change into a very different hormone-producing structure, the corpus luteum. (Both the maturing follicles and the corpus luteum produce hormones, as we will describe later.)

If the ovulated secondary oocyte is fertilized by a sperm in one of the uterine tubes, the oocyte quickly completes the second meiotic division that produces the ovum and another polar body. Once the ovum is formed, its 23 chromosomes are com- bined with those of the sperm to form the zygote. However, if a sperm does not penetrate the sec- ondary oocyte, it simply deteriorates without ever completing meiosis to form a functional egg. Although meiosis in males results in four func- tional sperm, meiosis in females yields only one functional ovum and three tiny polar bodies. The polar bodies, produced to reduce the number of chromosomes in the developing oocyte, have essentially no cytoplasm, so they die quickly.

Another major difference between male and female gametes concerns the size and structure

of these cells. Sperm are tiny and equipped with tails for locomotion. They have little nutrient- containing cytoplasm; thus, the nutrients in semi- nal fluid are vital to their survival. In contrast, the egg is a large, nonmotile cell, well stocked with nutrient reserves that nourish the developing embryo until it can take up residence in the uterus.

Did You Get It? 14. Besides the one functional gamete (ovum), what

other cell types are produced during oogenesis, and what happens to them?

15. Which anterior pituitary hormone promotes follicle development in the ovary?

16. Which anterior pituitary hormone causes ovulation?

For answers, see Appendix A.

Hormone Production by the ovaries As the ovaries become active at puberty and start to produce oocytes, they also begin to produce ovarian hormones. The follicle cells of the grow- ing and mature follicles produce estrogens,* which cause the appearance of secondary sex characteristics in the young woman. Such changes include the following:

• Enlargement of the accessory organs of the female reproductive system (uterine tubes, uterus, vagina, external genitals)

• Development of the breasts

• Appearance of axillary and pubic hair

• Increased deposits of fat beneath the skin in general, and particularly in the hips and breasts

• Widening and lightening of the pelvis

• Onset of menses, or the menstrual cycle

Estrogen also has metabolic effects. For exam- ple, it helps maintain low total blood cholesterol levels (and a high HDL level) and facilitates cal- cium ion uptake, which sustains bone density.

A second ovarian hormone, progesterone, is produced by the glandular corpus luteum (look back at Figure 16.7). As mentioned earlier, after ovulation occurs, the ruptured follicle is converted to the corpus luteum, which looks and acts com- pletely different from the growing and mature fol-

Oocyte

Figure 16.11 ovulation. A secondary oocyte is released from a follicle at the surface of the ovary. The orange mass below the ejected oocyte is part of the ovary. The “halo” of follicle cells around the secondary oocyte is the corona radiata.

*Although the ovaries produce several different estrogens, the most important are estradiol, estrone, and estriol. Of these, estradiol is the most abundant and is most responsible for estrogenic effects.

580 Essentials of Human Anatomy and Physiology

licle. Once formed, the corpus luteum produces progesterone (and some estrogen) as long as LH is still present in the blood. Generally speaking, the corpus luteum stops producing hormones by 10 to 14 days after ovulation. Except for working with estrogens to establish the menstrual cycle, proges- terone does not contribute to the appearance of the secondary sex characteristics. Its other major effects are exerted during pregnancy, when it helps to maintain the pregnancy and prepare the breasts for milk production. However, the source of progesterone during pregnancy is the placenta, not the ovaries.

uterine (Menstrual) cycle ➔ Learning Objective

□□ Describe the phases and controls of the menstrual cycle.

Although the young embryo implants and devel- ops in the uterus, this organ is receptive to implan- tation only for a very short period each month. Not surprisingly, this brief interval coincides ex - actly with the time when a fertilized egg would begin to implant, approximately 7 days after ovu- lation. The uterine cycle, or menstrual cycle, is a series of cyclic changes that the endometrium (mucosa of the uterus) goes through each month as it responds to changing blood levels of ovarian hormones.

The cyclic production of estrogens and pro- gesterone by the ovaries is, in turn, regulated by the anterior pituitary gonadotropic hormones, FSH and LH. It is important to understand how these “hormonal pieces” fit together. Generally speak- ing, both female cycles (the ovarian and the uter- ine cycles) are about 28 days long. Ovulation typically occurs midway in the cycles, on or about day 14. Figure 16.12 (p. 581) illustrates the events of the ovarian cycle and the uterine cycle at the same time.

The three phases of the menstrual cycle are as follows:

• Days 1–5: Menstrual phase. During this in- terval, the superficial functional layer of the thick endometrial lining of the uterus is sloughing off (detaching) from the uterine wall. The detached tissues and blood pass through the vagina as menstrual flow (the “period”) for 3 to 5 days. The average blood

loss during this period is 50 to 150 ml (or about ¼ to ½ cup). By day 5, growing ovarian folli- cles are beginning to produce more estrogens.

• Days 6–14: Proliferative phase. Stimulated by a rising level of estrogens produced by the growing follicles of the ovaries, the basal layer of the endometrium regenerates the functional layer, glands form in it, and the endometrial blood supply increases. The endometrium once again becomes velvety, thick, and well vascularized. (Ovulation occurs in the ovary at the end of this stage, in response to the sud- den surge of LH in the blood.)

• Days 15–28: Secretory phase. A rising level of progesterone production by the corpus lu- teum acts on the estrogen-primed endome- trium and increases its blood supply even more. Progesterone also causes the endome- trial glands to grow and begin secreting nutri- ents into the uterine cavity. These nutrients will sustain a developing embryo (if one is present) until it has implanted. If fertilization does oc- cur, the embryo produces a hormone very sim- ilar to LH that causes the corpus luteum to continue producing its hormones.

If fertilization does not occur, the corpus luteum begins to degenerate toward the end of this period as the LH blood level declines. Lack of ovarian hormones in the blood causes the blood vessels supplying the functional layer of the endo- metrium to go into spasms and kink. When deprived of oxygen and nutrients, those endome- trial cells begin to die, which sets the stage for menses to begin again on day 28.

Although this explanation assumes a classic 28-day cycle, the length of the menstrual cycle is quite variable. It can be as short as 21 days or as long as 40 days. Only one interval is fairly con- stant in all females—the time from ovulation to the beginning of menses is almost always 14 or 15 days.

Mammary Glands ➔ Learning Objective

□□ Describe the structure and function of the mammary glands.

The mammary glands are present in both sexes, but they normally function only in women.

Chapter 16: The Reproductive System 581

16Menstrual phase

Menstrual flow

Endometrial glands

Blood vessels

F un

ct io

na l l

ay er

B as

al la

ye r

Proliferative phase

Secretory phase

1 5 10 15 20 25 Days28

FSH

P la

sm a

ho rm

on e

le ve

Progesterone

Estrogens

P la

sm a

ho rm

on e

le ve

Primary follicle

Secondary follicle

Vesicular follicle

Ovulation Corpus luteum

Degenerating corpus luteum

Follicular phase

Ovulation (Day 14)

Luteal phase

(a) Fluctuation of gonadotropin levels: Fluctuating levels of pituitary gonadotropins (FSH and LH) in the blood regulate the events of the ovarian cycle.

(b) Ovarian cycle: Structural changes in the ovarian follicles during the ovarian cycle are correlated with (d) changes in the endometrium of the uterus during the uterine cycle.

(c) Fluctuation of ovarian hormone levels: Fluctuating levels of ovarian hormones (estrogens and progesterone) cause the endometrial changes of the uterine cycle.

The high estrogen levels are also responsible for the LH/FSH surge in (a).

(d) The three phases of the uterine cycle: • Menstrual: Shedding of the functional

layer of the endometrium. • Proliferative: Rebuilding of the functional

layer of the endometrium. • Secretory: Begins immediately after

ovulation. Enrichment of the blood supply and glandular secretion of nutrients prepare the endometrium to receive an embryo.

The menstrual and proliferative phases occur before ovulation and together correspond to the follicular phase of the ovarian cycle. The secretory phase corresponds in time to the luteal phase of the ovarian cycle.

Figure 16.12 Hormonal interactions of the female cycles. Shown is the correlation of relative levels of anterior pituitary gonadotropins with hormonal and follicular changes of the ovary and with the menstrual cycle.

582 Essentials of Human Anatomy and Physiology

Because the biological role of the mammary glands is to produce milk to nourish a newborn baby, they are actually important only when repro- duction has already been accomplished. Stimula- tion by female sex hormones, especially estrogens, causes the female mammary glands to increase in size at puberty.

The mammary glands are modified sweat glands that are part of the skin. Each mammary gland is contained within a rounded skin-covered breast anterior to the pectoral muscles. Slightly inferior to the center of each breast is a pigmented area, the areola (ah-re′o-lah), which surrounds a central protruding nipple (Figure 16.13).

Internally, each mammary gland consists of 15 to 25 lobes that radiate around the nipple. The lobes are padded and separated from one another

by connective tissue and fat. Within each lobe are smaller chambers called lobules, which contain clusters of alveolar glands that produce milk when a woman is lactating (producing milk). Milk produced by the alveolar glands exits each lobule by passing into lactiferous (lak-tif′er-us) ducts, which open to the outside at the nipple. Just deep to the areola, each duct has a dilated region called a lactiferous sinus, where milk accumulates during nursing.

Homeostatic Imbalance 16.7

Breast cancer is the second most common cause of death in American women; one woman in eight will develop this condition. Some 10 per- cent of breast cancers stem from hereditary defects, and half of these can be traced to dan- gerous mutations in a pair of genes (BRCA1 and BRCA2). Fifty to 80 percent of women who carry Adipose tissue.A:

Skin (cut)

Pectoralis major muscle

Connective tissue suspensory ligament

Adipose tissue

Lobe

Areola

Nipple

Opening of lactiferous duct

Lactiferous sinus

Lactiferous duct

Hypodermis (superficial fascia)

Intercostal muscles

Lobule containing alveoli

(a) (b)

First rib

Figure 16.13 Female mammary glands. (a) Anterior view. (b) Sagittal section.

Women with smaller breasts are perfectly able to nurse their newborn infants; hence, it is not glandular tissue that accounts for the bulk of the breast tissue. So, what does?Q:

Chapter 16: The Reproductive System 583

the altered gene develop breast cancer. With the possible exception of family history, most risk factors reflect lifelong exposure to estrogens (early menses, late menopause, estrogen replace- ment therapy, and others).

Breast cancer is often signaled by a change in skin texture, puckering, or leakage from the nip- ple. Early detection by breast self-examination and mammography is unquestionably the best way to increase a woman’s chances of surviving breast cancer. Because most breast lumps are dis- covered by women themselves in routine monthly breast exams, this simple examination should be a priority in every woman’s life. Currently the American Cancer Society recommends scheduling mammography—X-ray examination that detects breast cancers too small to feel (less than 1 cm)— yearly for women between 45 and 54 years old and every 2 years thereafter if the results are nor- mal (Figure 16.14). ___________________________✚

Did You Get It? 17. Which ovarian hormone promotes the formation of

female secondary sex characteristics? 18. What happens during the proliferative stage of the

uterine cycle? 19. What are three important functions of progesterone

in women? 20. What problems do mutations of the BRCA genes

cause?

For answers, see Appendix A.

Pregnancy and Embryonic Development ➔ Learning Objectives

□□ Define fertilization and zygote.

□□ Describe implantation.

□□ Distinguish between an embryo and a fetus.

□□ List the major functions of the placenta.

Because the birth of a baby is such a familiar event, we tend to lose sight of the wonder of this accomplishment. In every instance it begins with a single cell, the fertilized egg, and ends with an extremely complex human being consisting of tril- lions of cells. The development of an embryo is very complex, and the details of this process can fill a book. Our intention here is simply to outline the important events of pregnancy and embryonic development.

Let’s begin by defining some terms. The term pregnancy refers to events that occur from the time of fertilization (conception) until birth. The pregnant woman’s developing offspring is called the conceptus (kon-sep′tus; “that which is con- ceived”). Development occurs during the gesta tion period (gestare = to carry), which extends by convention from the last menstrual period (a date the woman is likely to remember) until birth, approximately 280 days.

From fertilization through week 8, the embry- onic period, the conceptus is called an embryo, and from week 9 through birth, the fetal period, the conceptus is called a fetus (“the young in the

(b) Film of normal breast(a) Mammogram procedure (c) Film of breast with tumor

Malignancy

Figure 16.14 Mammograms.

584 Essentials of Human Anatomy and Physiology

womb”). Figure 16.15 shows the changing size and shape of the conceptus as it progresses from fertil- ization to the early fetal stage. At birth, the fetus becomes an infant.

Accomplishing Fertilization For fertilization to occur, a sperm must reach the ovulated secondary oocyte. The oocyte is viable for 12 to 24 hours after ovulation, and sperm gen- erally retain their fertilizing power within the female reproductive tract for 24 to 48 hours after ejaculation. Consequently, for fertilization to occur, sexual intercourse must occur no more than 2 days before ovulation and no later than 24 hours after. At this point, the oocyte is approximately one-third of the way down the uterine tube. Remember that sperm are motile cells that can propel themselves by lashing movements of their tails. If sperm are deposited in a female’s vagina at the approximate time of ovulation, they are attracted to the oocyte by chemicals that act as “homing devices,” allowing them to locate the oocyte.

It takes 1 to 2 hours for sperm to complete the journey up the female duct system into the uterine tubes, even though they are only about 12 cm (5 inches) away. However, millions of sperm leak out of the vagina, and of those remaining, millions more are destroyed by the vagina’s acidic environ- ment. Only a few hundred to a few thousand sperm make it to the egg’s vicinity.

When the swarming sperm reach the oocyte (Figure 16.16), their cell surface hyaluronidase enzymes break down the “cement” that holds the follicle cells of the corona radiata together around the oocyte. Once a path has been cleared through the corona, thousands of sperm undergo the acro somal reaction, in which the acrosome mem- branes break down, releasing enzymes that digest holes in the surrounding oocyte membrane. Then, when the membrane is adequately weakened and

Fertilization 1-week conceptus

3-week embryo (3 mm)

Embryo

12-week fetus (90 mm)

8-week embryo (22 mm)

5-week embryo (10 mm)

Figure 16.15 Diagrams showing the approximate size of a human conceptus from fertilization to the early fetal stage. Measurements indicate the length from crown to rump.

➔ConCeptLinkRecall the concept of chemotaxis, in which cells move toward or away from a stimulus (positive and negative chemotaxis, respectively) (Chapter 12, p. 432).

Chapter 16: The Reproductive System 585

a single sperm makes contact with one of the oocyte’s membrane receptors for sperm, the head (nucleus) of the sperm fuses with the oocyte membrane, and the snakelike sperm contents enter the oocyte cytoplasm. This is one case that does not bear out the adage “The early bird catches the worm,” because sperm that are in the best position to be the fertilizing sperm are the ones that come along after hundreds of sperm have undergone acrosomal reactions to expose the oocyte membrane. Once a single sperm has pene- trated the oocyte, the oocyte nucleus completes the second meiotic division, forming the ovum and a polar body.

Once the sperm has entered, the ovum sheds its remaining membrane surface receptors for sperm, preventing other sperm from gaining entry. In humans, of the millions of sperm ejaculated, only one can penetrate an oocyte. Fertilization occurs at the moment the genetic material of a sperm combines with that of an ovum to form a fertilized egg, or zygote (zi′gōt), with a complete set of 46 chromosomes. The zygote represents the first cell of the new individual.

Events of Embryonic and Fetal Development As the zygote journeys down the uterine tube (propelled by peristalsis and cilia), it begins to undergo rapid mitotic cell divisions—forming first two cells, then four, and so on. This early stage of embryonic development is called cleavage (Figure 16.17, p. 586). Because there is not much

time for cell growth between divisions, the daugh- ter cells become smaller and smaller. Cleavage provides a large number of cells to serve as build- ing blocks for constructing the embryo. Consider for a moment how difficult it would be to con- struct a building from one huge block of granite. If you now consider how much easier your task would be if you could use hundreds of brick-sized granite blocks, you will quickly grasp the impor- tance of cleavage.

By the time the developing embryo reaches the uterus (about 3 days after ovulation, or on day 17 of the woman’s cycle), it is a morula, a tiny ball of 16 cells that looks like a microscopic raspberry. The uterine endometrium is still not fully prepared to receive the embryo at this point, so the embryo floats free in the uterine cavity, temporarily using the uterine secretions for nutrition. While still unattached, the embryo continues to develop until it has about 100 cells, and then it hollows out to form a ball-like structure called a blastocyst (blas′to-sist). At the same time, it secretes an LH- like hormone called human chorionic gonad otropin (hCG), which prods the corpus luteum of the ovary to continue producing its hormones. (If this were not the case, the functional layer of the endometrium would be sloughing off shortly in menses.) Many home pregnancy tests detect hCG in urine.

The blastocyst has two important functional areas: the trophoblast, which forms the large fluid-filled sphere, and the inner cell mass, a small cluster of cells on one side (see Figure 16.17e). By day 7 after ovulation, the blastocyst has attached to the endometrium and has eroded away the lining in a small area, embedding itself in the thick velvety mucosa. All of this is occurring even while development is continuing and the three primary germ layers are being formed from the inner cell mass (Figure 16.18, p. 587). The pri- mary germ layers are the ectoderm (which will become the nervous system and the epidermis of the skin), the endoderm (which forms mucosae and associated glands), and the mesoderm (which will become virtually everything else). Implantation has usually been completed and the uterine mucosa has grown over the burrowed-in embryo by day 14 after ovulation—the day the woman would ordinarily be expecting to start her period (menses).

Figure 16.16 Sperm and oocyte during fertilization.

586 Essentials of Human Anatomy and Physiology

After it is securely implanted, the trophoblast part of the blastocyst develops elaborate projec- tions, called chorionic villi, which combine with the tissues of the mother’s uterus to form the pla centa (plah-sen′tah) (Figure 16.19). Once the pla- centa has formed, the platelike embryonic body,

now surrounded by a fluid-filled sac called the amnion (am′ne-on), is attached to the placenta by a blood vessel–containing stalk of tissue, the umbilical cord (see Figure 16.19). (We discussed the special features of the umbilical blood vessels and fetal circulation in Chapter 11.)

Generally by the third week, the placenta is functioning to deliver nutrients and oxygen to and remove wastes from the embryonic blood. All exchanges are made through the placental barrier. By the end of the second month of pregnancy, the placenta has also become an endocrine organ and

(a) Zygote (fertilized egg)

(b) 4-cell stage 2 days

(d) Early blastocyst (morula hollows out and fills with fluid) 4 days

Blastocyst cavity

Zona pellucida

Inner cell mass

Blastocyst cavity

Trophoblast

(e) Implanting blastocyst (consists of a sphere of trophoblast cells and an eccentric cell cluster called the inner cell mass) 7 days

Cavity of uterus

Uterus

Endometrium Ovulation

Ovary

Fertilization (sperm meets and enters egg) Uterine tube

Oocyte (egg)

Sperm

Figure 16.17 From fertilization and cleavage to implantation. Cleavage is a rapid series of mitotic divisions that begins with the zygote and ends with the blastocyst. The zygote begins to divide about

24 hours after fertilization and continues to divide rapidly (undergo cleavage) as it travels down the uterine tube. The embryo reaches the uterus 3 to 4 days after ovulation and floats freely for another 2 to 3

days, nourished by secretions of the endometrial glands. At the late blastocyst stage, the embryo implants into the endometrium; this begins at about day 7 after ovulation.

Because as the zygote and then its descendants divide, little or no time is provided for growth between

subsequent division cycles. As a result, the cells get smaller and smaller, and the size of the cell mass stays

approximately the same size as the initial zygote.

Why is the multicellular blastocyst only slightly larger than the single-celled zygote?Q:

Chapter 16: The Reproductive System 587

is producing estrogens, progesterone, and other hormones that help to maintain the pregnancy. At this time, the corpus luteum of the ovary becomes inactive and degenerates.

By the eighth week of embryonic develop- ment, all the groundwork has been completed. All the organ systems have been laid down, at least in rudimentary form, and the embryo looks distinctly human. Beginning in the ninth week of develop- ment, we refer to the embryo as a fetus. From this point on, the major activities are growth and organ specialization, accompanied by changes in body proportions. During the fetal period, the develop- ing fetus grows from a crown-to-rump length of about 3 cm (slightly more than 1 inch) and a weight of approximately 1 g (0.03 ounce) to about 36 cm (14 inches) and 2.7 to 4.1 kg (6 to 10 pounds) or more. (Total body length at birth is about 55 cm, or 22 inches.) As you might expect with such tremendous growth, the changes in fetal appearance are quite dramatic (Figure 16.20, p. 589). The most significant of these changes are summarized in Table 16.1 on pp. 588–589. By approximately 270 days after fertilization, the fetus is said to be “full-term” and is ready to be born.

Did You Get It? 21. How does cleavage differ from cell divisions occurring

after birth? 22. What are three roles of the placenta?

For answers, see Appendix A.

Uterine cavity

Chorion

Amnion

Umbilical cord

Chorionic villi

Ectoderm

EmbryoFormingmesoderm

Endoderm

Figure 16.18 Embryo of approximately 18 days. At this stage of development, embryonic membranes are present.

Yolk sac

Placenta

Cut edge of chorion

Amniotic sac Umbilical cord Umbilical vein

Chorionic villi

Figure 16.19 The 7week embryo. A 7-week embryo, encased in its amniotic sac, and the chorionic villi (to the right), which cooperate with maternal uterine tissues to form the placenta.

588 Essentials of Human Anatomy and Physiology

Table 16.1 Development of the Human Fetus

Time changes/accomplishments

8 weeks (end of embryonic period)

Head nearly as large as body; all major brain regions present

Liver disproportionately large and begins to form blood cells

Limbs present; though initially webbed, fingers and toes are free by the end of this interval

Bone formation has begun

Heart has been pumping blood since the fourth week

All body systems present in at least rudimentary form

Approximate crown-to-rump length: 22 mm (0.9 inch); weight: 2 grams (0.07 ounce)

9–12 weeks (third month)

Head still dominant, but body elongating; brain continues to enlarge

Facial features present in crude form

Walls of hollow visceral organs gaining smooth muscle

Blood cell formation begins in bone marrow

Bone formation accelerating

Sex readily detected from the genitalia

Approximate crown-to-rump length at end of interval: 90 mm (9 cm)

13–16 weeks (fourth month)

General sensory organs are present; eyes and ears assume characteristic position and shape; blinking of eyes and sucking motions of lips occur

Face looks human, and body is beginning to outgrow head

Kidneys attain typical structure

Most bones are distinct, and joint cavities are apparent

Approximate crown-to-rump length at end of interval: 140 mm (14 cm)

17–20 weeks (fifth month)

Vernix caseosa (fatty secretions of sebaceous glands) covers body; silklike hair (lanugo) covers skin

Fetal position (body flexed anteriorly) assumed because of space restrictions

Limbs achieve near-final proportions

Quickening occurs (mother feels spontaneous muscular activity of fetus)

Approximate crown-to-rump length at end of interval: 190 mm

21–30 weeks (sixth and seventh months)

Substantial increase in weight (may survive if born prematurely at 27–28 weeks, but hypothalamus still too immature to regulate body temperature, and surfactant production by the lungs is still inadequate)

Myelination of spinal cord begins; eyes are open

Skin is wrinkled and red; fingernails and toenails are present

Body is lean and well proportioned

Approximate crown-to-rump length at end of interval: 280 mm

8 weeks

12 weeks

16 weeks

Chapter 16: The Reproductive System 589

Effects of Pregnancy on the Mother ➔ Learning Objectives

□□ Indicate several ways that pregnancy alters or modifies the functioning of the mother’s body.

□□ List several agents that can interfere with normal fetal development.

The period from conception to birth can be a dif- ficult time for the mother. Pregnancy causes not only obvious anatomical changes in the mother, but also striking changes in her physiology.

Anatomical changes

The ability of the uterus to enlarge during preg- nancy is nothing less than remarkable. Starting as a fist-sized organ, the uterus grows to occupy most of the pelvic cavity by 16 weeks. As pregnancy continues, the uterus pushes higher and higher into the abdominal cavity (Figure 16.21, p. 590). As birth nears, the uterus reaches the level of the xiphoid process and occupies the bulk of the abdominal cavity. The crowded abdominal organs press superiorly against the diaphragm, which

(a) (b)

Figure 16.20 Examples of fetal development. (a) Fetus in month 3, about 6 cm (2.5 inches) long. (b) Fetus late in month 5, about 19 cm (8 inches) long.

Time changes/accomplishments

30–40 weeks (term) (eighth and ninth months)

Bone marrow becomes sole site of blood cell formation

Testes enter scrotum in seventh month (in males)

Fat laid down in subcutaneous tissue of skin

Approximate crown-to-rump length at end of interval: 360 mm (14 inches); weight: 3.2 kg (7 pounds)

At birth

Table 16.1 (continued)

590 Essentials of Human Anatomy and Physiology

intrudes on the thoracic cavity. As a result, the ribs flare, causing the thorax to widen.

The increasing bulkiness of the abdomen changes the woman’s center of gravity, and many women develop an accentuated lumbar curvature (lordosis), often accompanied by backaches, dur- ing the last few months of pregnancy. Placental production of the hormone relaxin causes pelvic ligaments and the pubic symphysis to relax, widen, and become more flexible. This increases the width of the birth canal to ease birth passage, but it may also result in a waddling gait during pregnancy.

Good maternal nutrition is necessary throughout pregnancy if the developing fetus is to have all the building materials (proteins, cal- cium, iron, and the like) to form its tissues and organs. The old expression “A pregnant woman is eating for two” has encouraged many women to eat twice the amount of food actually needed during pregnancy, which, of course, leads to ex - cessive weight gain. Actually, a pregnant woman needs only about 300 additional calories daily to sustain proper fetal growth. The emphasis should be on high-quality nutrition, not just more calories.

Homeostatic Imbalance 16.8

Many potentially harmful substances can cross through the placental barrier into the fetal blood; therefore, a pregnant woman should be very aware of what she is taking into her body. Substances that may cause life-threatening birth defects (and even fetal death) include alcohol, nicotine, and many types of drugs (anticoagulants, antihypertensives, sedatives, and some antibiotics). Maternal infec- tions, particularly German measles (rubella), may also cause severe fetal damage. Natural termination of a pregnancy before a fetus can survive on its own is called miscarriage. A medical termination of a pregnancy is called abortion. ____________✚

Physiological changes

Gastrointestinal System Many women suffer nausea, commonly called morning sickness, dur- ing the first few months of pregnancy, until their system adjusts to the elevated levels of progester- one and estrogens. Heartburn is common because the esophagus is displaced and the stomach is crowded by the growing uterus, which favors reflux of stomach acid into the esophagus. Another problem is constipation due to decline in motility of the digestive tract during pregnancy.

(a) Before conception (Uterus is the size of a fist and resides in the pelvis.)

(b) 4 months (Fundus of the uterus is halfway between the pubic symphysis and the umbilicus.)

(d) 9 months (Fundus reaches the xiphoid process.)

Figure 16.21 Relative size of the uterus before conception and during pregnancy.

Chapter 16: The Reproductive System 591

Urinary System The kidneys have the additional burden of disposing of fetal metabolic wastes, and they produce more urine during pregnancy. Because the uterus compresses the bladder, urina- tion becomes more frequent, more urgent, and sometimes uncontrollable. The last condition is called stress incontinence.

Respiratory System The nasal mucosa responds to estrogens by becoming swollen and congested; thus, nasal stuffiness and occasional nosebleeds may occur. Vital capacity and respiratory rate increase during pregnancy, but residual volume declines, and many women exhibit dyspnea (diffi- cult breathing) during the later stages of pregnancy.

Cardiovascular System Perhaps the most dra- matic physiological changes occur in the cardiovas- cular system. Total body water rises, and blood volume increases by 25 to 40 percent to accommo- date the additional needs of the fetus. The rise in blood volume also acts as a safeguard against blood loss during birth. Blood pressure and pulse typically rise and increase cardiac output by 20 to 40 percent; this helps propel the greater blood vol- ume around the body. Because the uterus presses on the pelvic blood vessels, venous return from the lower limbs may be impaired somewhat, resulting in varicose veins or swollen ankles and feet.

childbirth ➔ Learning Objective

□□ Describe how labor is initiated, and briefly discuss the three stages of labor.

Childbirth, also called parturition (par″tur-ish′un; “bringing forth young”), is the culmination of preg- nancy. It usually occurs within 15 days of the cal- culated due date (which is 280 days from the last menstrual period). The series of events that expel the infant from the uterus is referred to as labor.

Initiation of Labor

Several events work together to trigger labor. During the last few weeks of pregnancy, estrogens reach their highest levels in the mother’s blood. This has two important consequences: it causes the myometrium to form abundant oxytocin recep- tors (so that it becomes more sensitive to the hor- mone oxytocin), and it interferes with progester- one’s quieting influence on the uterine muscle. As

a result, weak, irregular uterine contractions begin to occur. These contractions, called Braxton Hicks contractions, have caused many women to go to the hospital, only to be told that they were in false labor and sent home.

As birth nears, two more chemical signals cooperate to convert these false labor pains into the real thing. Certain cells of the fetus begin to produce oxytocin, which in turn stimulates the placenta to release prostaglandins. Both hormones stimulate more frequent and powerful contractions of the uterus. At this point, the increasing emo- tional and physical stresses activate the mother’s hypothalamus, which signals for oxytocin release by the posterior pituitary. The combined effects of rising levels of oxytocin and prostaglandins initiate the rhythmic, expulsive contractions of true labor. Once the hypothalamus is involved, a positive feedback mechanism is propelled into action: stronger contractions cause the release of more oxytocin, which causes even more vigorous con- tractions, forcing the baby ever deeper into the mother’s pelvis (Figure 16.22, p. 592).

➔ConCeptLinkRemember the concept of the feedback loop (see Chapter 1, p. 45). A stimulus triggers a receptor, the information is sent to the brain for processing, and a sig- nal is sent to an effector with instructions for a response. Most of the feedback in the body is negative feedback, in which the response decreases the initial stimulus. Labor, however, involves positive feedback: The response (stronger contractions) actually increases the initial stimu- lus (oxytocin release) until the child is born.

Because both oxytocin and prostaglandins are needed to initiate labor, anything that interferes with production of either of these hormones will hinder the onset of labor. For example, antiprosta- glandin drugs such as aspirin and ibuprofen can inhibit labor at the early stages, and such drugs are used occasionally to prevent premature births.

Stages of Labor

The process of labor is commonly divided into three stages (Figure 16.23, p. 592).

Stage 1: Dilation Stage The dilation stage is the time from the appearance of true contractions until the cervix is fully dilated by the baby’s head (about 10 cm in diameter). As labor starts, regular

592 Essentials of Human Anatomy and Physiology

but weak uterine contractions begin in the upper part of the uterus and move inferiorly toward the vagina. Gradually, the contractions become more vigorous and more rapid, and, as the infant’s head is forced against the cervix with each contraction, the cervix begins to soften, becomes thinner (effaces), and dilates. Eventually, the amnion rup- tures, releasing the amniotic fluid, an event com- monly referred to as the woman’s “water breaking.” The dilation stage is the longest part of labor and usually lasts for 6 to 12 hours or more.

Stage 2: Expulsion Stage The expulsion stage is the period from full dilation to delivery of the infant. In this stage, the infant passes through the cervix

Positive feedback mechanism continues to cycle until interrupted by birth of baby

5 Posterior pituitary releases oxytocin to blood; oxytocin targets mother’s uterine muscle

4 Hypothalamus sends efferent impulses to posterior pituitary, where oxytocin is stored

6 Uterus responds by contracting more vigorously

1 Baby moves deeper into mother’s birth canal

2 Pressoreceptors in cervix of uterus excited

3 Afferent impulses to hypothalamus

Figure 16.22 oxytocin promotes labor contractions during birth by a positive feedback mechanism.

Umbilical cord

Uterus

Cervix Vagina

Placenta

Sacrum

Uterus

Placenta (detaching)

Umbilical cord

(a) Dilation (of cervix)

(b) Expulsion (delivery of infant)

Figure 16.23 The three stages of labor.

Chapter 16: The Reproductive System 593

and vagina to the outside of the body. During this stage, a mother experiencing natural childbirth (that is, undergoing labor without local anesthesia) has an increasing urge to push, or bear down, with the abdominal muscles. Although this phase can take as long as 2 hours, it is typically 50 minutes in a first birth and around 20 minutes in subsequent births.

When the infant is in the usual head-first (ver- tex) position, the skull (its largest diameter) acts as a wedge to dilate the cervix. The head-first presen- tation also allows the baby to be suctioned free of mucus and to breathe even before it has com- pletely exited from the birth canal. Once the head has been delivered, the rest of the baby’s body is delivered much more easily. After birth, the umbil- ical cord is clamped and cut. In breech (buttocks- first) presentations and other nonvertex presen- tations, these advantages are lost and delivery is often much more difficult, sometimes requiring the use of forceps or a vacuum extractor.

Homeostatic Imbalance 16.9

During an extremely prolonged or difficult stage 2, a condition called dystocia (dis-to′se-ah) may occur. In dystocia, oxygen delivery to the infant is inadequate, leading to fetal brain damage (result- ing in cerebral palsy or epilepsy) and decreased viability of the infant. To prevent these outcomes, a cesarean (se-zayr′e-an) section, also called a Csection, may be performed. A C-section is deliv- ery of the infant through a surgical incision made through the abdominal and uterine walls. ______✚

Stage 3: Placental Stage The placental stage, or the delivery of the placenta, is usually accomplished within 15 minutes after birth of the infant. The strong uterine contractions that continue after birth com- press uterine blood vessels, limit bleeding, and cause the placenta to detach from the uterine wall. The placenta and its attached fetal membranes, collec- tively called the afterbirth, are then easily removed by a slight tug on the umbilical cord. All placental fragments must be removed to prevent continued uterine bleeding after birth (postpartum bleeding).

Did You Get It? 23. Explain how pregnancy affects a woman’s respiratory

and digestive processes. 24. What are the three stages of labor?

For answers, see Appendix A.

Developmental Aspects of the Reproductive System ➔ Learning Objectives

□□ Describe the importance of the presence/absence of testosterone during embryonic development of the reproductive system organs.

□□ Define menarche and menopause.

□□ List common reproductive system problems seen in adult and aging men and women.

Although the genetic sex of an individual is deter- mined at the time of fertilization (males have X and Y sex chromosomes and females have two X sex chromosomes), the gonads do not begin to form until about the eighth week of embryonic develop- ment. Prior to this time, the embryonic reproductive structures of males and females are identical and are said to be in the indifferent stage. After the gonads have formed, development of the accessory structures and external genitalia begins. Whether male or female structures will form depends entirely on whether testosterone is present or absent. The usual case is that, once formed, the embryonic tes- tes produce testosterone, and the development of the male duct system and external genitalia follows. When testosterone is not produced, as is the case in female embryos that form ovaries, the female ducts and external genitalia result.

Homeostatic Imbalance 16.10

Any interference with the normal pattern of sex hormone production in the embryo results in abnormalities. For example, if the embryonic testes fail to produce testosterone, a genetic male devel- ops the female accessory structures and external genitalia. If a genetic female is exposed to testoster- one (as might happen if the mother has an andro- gen-producing tumor of her adrenal gland), the embryo has ovaries but develops male accessory ducts and glands, as well as a penis and an empty scrotum. Individuals with external genitalia that do not “match” their gonads are called pseudoher maphrodites (su″do-her-maf′ro-dı̄tz) to distinguish them from true hermaphrodites, rare individuals who possess both ovarian and testicular tissues. In recent years, many pseudohermaphrodites have sought sex change operations to match their outer selves (external genitalia) with their inner selves (gonads).

(Text continues on page 596.)

594

In a society such as ours, where many women opt for professional

ous cardiovascular side effects that occurred (rarely) with earlier forms of the pill.

Presently, the pill is one of the most widely used drugs in the world; well over 50 million women use these drugs to prevent pregnancy. The incidence of failure is less than 1 percent.

A different combination hormone pill, the morning-after pill (MAP) or emergency contraceptive pill (EC), is the therapy of choice for rape vic- tims and is now available over the counter. Taken within 3 days of unprotected intercourse, the con- centrated estrogen-progesterone combination pills “mess up” the

Contraception: Preventing Pregnancy

A CLOSER

LOOK

Intrauterine device (IUD); progestin-only minipill, implant, or injection

Morning- after pill (MAP)

Implantation of blastocyst in properly prepared endometrium

Birth

Union of sperm and egg

Meeting of sperm and oocyte in uterine tube

Sperm move through the

female’s repro- ductive tract

Transport down the

uterine tube

Spermicides, diaphragm, cervical cap, vaginal pouch, or progestin-only minipill, implant, or injection

Tubal ligation

Capture of the oocyte by the uterine tube

Sperm deposited in the female

vagina

Coitus interruptus

Condom

Vasectomy

Abstinence

Technique Production of viable sperm

Male

Event

Transport down the male duct

system

Event Production of viable oocytes

Female

Technique

Ovulation

Combination "pill," birth control patch, monthly shot, vaginal ring, or implant

Flowchart of points at which the events that must occur to produce a baby can be interrupted. Techniques or products that interfere with this process are indicated by red arrows at the site of interference; they act to prevent the next step.

careers or must work for economic reasons, contraception (contra = against; cept = taking), or birth con- trol, is often seen as a necessity.

The key to birth control is depend- ability. As shown by the red arrows in the accompanying flowchart, the birth control techniques and products cur- rently available have many sites of action for blocking the reproductive process. Let’s examine the relative advantages of a few of these methods more closely.

The most-used contraceptive product in the United States is the birth control pill, or simply, “the pill,” a preparation taken daily that contains tiny amounts of estrogens and progestins (progesterone-like hormones), except that for the last 7 days of the 28-day cycle the tablets are hormone-free. The pill tricks the hypothalamic-pituitary control sys- tem and “lulls it to sleep” because the relatively constant blood levels of ovarian hormones make it appear that the woman is pregnant (both estrogen and progesterone are pro- duced throughout pregnancy). Ovar- ian follicles do not mature, ovulation ceases, and menstrual flow is much reduced.

The pill has adverse cardiovascu- lar effects, such as strokes, heart attacks, and blood clots in a small number of users, and there is still debate about whether it increases the incidence of ovarian, uterine, and particularly breast cancer. How- ever, it appears that the new, very- low-dose preparations may actually help protect against ovarian and endometrial cancer and may also have reduced the incidence of seri-

595

normal hormonal signals so much that fertilization is prevented alto- gether or a fertilized egg is pre- vented from implanting.

Other hormonal approaches are progestin-only products that cause cervical mucus to become thick, blocking sperm passage and making the endometrium inhospitable to implantation. These include the minipill (a tablet); implants of a tiny silicon rod that releases the hormone over several years; and Depo- Provera, an injectable synthetic pro- gesterone that lasts for 3 months. The failure rate of the implant or injection is even lower than that of the pill.

For several years, the second most used contraceptive method was the intrauterine device (IUD), a plastic or metal device inserted into the uterus that prevented implantation of the fertilized egg (see flowchart). Although the failure rate of the IUD was nearly as low as that of the pill, IUDs were taken off the market in the United States because of problems with occasional contraceptive failure, uterine perforation, or pelvic inflam- matory disease (PID). New IUD prod- ucts that deliver sustained doses of synthetic progesterone to the endo- metrium are currently being recom- mended for women who have given birth and for women with a lower risk of developing PID in monogamous relationships.

Sterilization techniques, such as tubal ligation and vasectomy (cutting

or cauterizing the uterine tubes or ductus deferens, respectively), are nearly foolproof and are the choice of approximately 33 percent of cou- ples of childbearing age in the United States. Both procedures can be done in the physician’s office. However, these changes are usually permanent, so they are not for indi- viduals who plan to have children at a later time.

Coitus interruptus, or withdrawal of the penis just before ejaculation, is simply against nature, and control of ejaculation is never assured. Rhythm, or fertility awareness, methods depend on avoiding intercourse during the period of ovulation or fertility. With a failure rate of 10 to 20 percent, rhythm techniques require

accurate recordkeeping for several cycles before they can be used with confidence.

Barrier methods, such as dia- phragms, cervical caps, condoms (see photo), spermicidal foams, gels, and sponges, are quite effective, especially when used by both partners. But many people avoid them because they can reduce the spontaneity of sexual encounters.

A large number of experimental birth control drugs are now awaiting clinical trials, and other methods are sure to be developed in the near future. In the final analysis, however, the only 100 percent effective means of birth control is the age-old one— abstinence.

Condom

Diaphragm

Contraceptive sponge

Contraceptive jelly

Spermicidal inserts

Birth control pills

Some contraceptive devices.

596 Essentials of Human Anatomy and Physiology

Additionally, abnormal separation of chromo- somes during meiosis can lead to congenital defects of this system. For example, males who have an extra female sex chromosome (XXY) have the normal male accessory structures, but their tes- tes atrophy, causing them to be sterile. Other abnormalities occur when a child has only one sex chromosome. An XO female appears normal but lacks ovaries; YO males die during development. Other, much less serious, conditions affect males primarily; these include phimosis (fi-mo′sis), which essentially is a narrowing of the foreskin of the penis, and misplaced urethral openings.

The male testes, formed in the abdominal cav- ity at approximately the same location as the female ovaries, descend to enter the scrotum about one month before birth. Failure of the testes to make their normal descent leads to a condition called cryptorchidism (krip-tor′kı̆-dizm). Be - cause this condition results in sterility of a male (and also puts him at risk for cancer of the testes), surgery is usually performed during childhood to rectify this problem. _________________________✚

Because the reproductive system organs do not function until puberty, there are few problems with this system during childhood. Puberty is the period of life, generally between the ages of 10 and 15 years, when the reproductive organs grow to their adult size and become functional under the influence of rising levels of gonadal hormones (testosterone in men and estrogens in women). After this time, reproductive capability continues until old age in men and menopause in women. Earlier we described the secondary sex character- istics and major events of puberty, so we will not repeat these details here. It is important to remem- ber, however, that puberty represents the earliest period of reproductive system activity.

The events of puberty occur in the same sequence in all individuals, but the age at which they occur varies widely. In boys, the event that signals puberty’s onset is enlargement of the testes and scrotum, around the age of 13 years, followed by the appearance of pubic, axillary, and facial hair. Growth of the penis goes on over the next 2 years, and sexual maturation is indicated by the presence of mature sperm in the semen. In the meantime, the young man has unexpected erec- tions and occasional nocturnal emissions (“wet dreams”) as his hormones surge and hormonal controls struggle to achieve a normal balance.

The first sign of puberty in girls is budding breasts, often apparent by the age of 11 years. The first menstrual period, called menarche (mĕ- nar′ke), usually occurs about 2 years later. Dependable ovulation and fertility are deferred until the hormonal controls mature, an event that takes nearly 2 more years.

Homeostatic Imbalance 16.11

In adults, the most common reproductive system problems are infections. Vaginal infections are more common in young and elderly women and in women whose immune resistance is low. Common infections include those caused by Escherichia coli (spread from the digestive tract); sexually transmitted microorganisms (such as gonorrhea, syphilis, and genital warts); and yeasts (a type of fungus). Untreated vaginal infections may spread throughout the female reproductive tract, causing pelvic inflammatory disease and sterility. Problems involving painful or abnormal menses may result from infection or hormone imbalance.

The most common inflammatory conditions in men are urethritis, prostatitis, and epididymitis (ep″ ı̆-did-ı̆-mi′tis), all of which may follow sexual contacts in which sexually transmitted microorgan- isms are transmitted. Orchitis (or″ki′tis), inflamma- tion of the testes, is rather uncommon but is serious because it can cause sterility. Orchitis most commonly follows sexually transmitted infection (STI) or mumps in an adult man.

As noted earlier, neoplasms represent a major threat to reproductive system organs. Tumors of the breast and cervix are the most common repro- ductive cancers in adult women, and prostate can- cer (a common sequel to prostatic hypertrophy) is a widespread problem in adult men. __________✚

Most women reach peak reproductive abili- ties in their late twenties. After that, a natural decrease in ovarian function occurs. As produc- tion of estrogens declines, ovulation becomes irregular, and menstrual periods become scanty and shorter in length. Eventually, ovulation and menses cease entirely, ending childbearing abil- ity. This event, called menopause, normally occurs between the ages of 46 and 54 years. A woman is said to have reached menopause when she has gone a whole year without men- struation.

Homeostatic Relationships between the Reproductive System and Other Body Systems

SYSTEMS IN SYNC

• Estrogens lower blood cholesterol levels and promote cardiovascular health in premenopausal women; pregnancy increases workload of the cardiovascular system

• Cardiovascular system transports needed substances to organs of reproductive system; local vasodilation is involved in erection; blood transports sex hormones

• Hypertrophy of the prostate inhibits urination; compression of bladder during pregnancy leads to urinary frequency and urgency

• Kidneys dispose of nitrogenous wastes and maintain acid-base balance of blood of mother and fetus; semen exits the body through the urethra of the male

• Androgens promote increased muscle mass

• Abdominal muscles are active during childbirth; muscles of the pelvic floor support reproductive organs and aid erection of penis/clitoris

• Developing embryo/fetus escapes immune surveillance (not rejected)

• Lymphatic vessels drain leaked tissue fluids; transport sex hormones; immune cells protect reproductive organs from disease; IgA is present in breast milk

• Digestive organs crowded by developing fetus; heartburn, constipation common during pregnancy

• Digestive system provides nutrients needed for health

• Late stages of pregnancy impair descent of the diaphragm, causing difficult breathing • Respiratory system provides oxygen; disposes of carbon dioxide; vital capacity and respiratory rate increase during pregnancy

• Sex hormones masculinize or feminize the brain and influence sex drive

• Hypothalamus regulates timing of puberty; neural reflexes regulate sexual response

• Androgens masculinize the skeleton and increase bone density; estrogen feminizes skeleton and maintains bone mass in women

• The bony pelvis encloses some reproductive organs; if narrow, the bony pelvis may hinder vaginal delivery of an infant

• Male sex hormones (androgens) activate oil glands, which lubricate skin and hair; gonadal hormones stimulate characteristic fat distribution and appearance of pubic and axillary hair; estrogen increases skin hydration; enhances facial skin pigmentation during pregnancy

• Skin protects all body organs by enclosing them externally; mammary gland secretions (milk) nourish the infant

• Gonadal hormones exert feedback effects on hypothalamic-pituitary axis; placental hormones help to maintain pregnancy

• Gonadotropins help regulate function of gonads

Cardiovascular System

Urinary System

Muscular System

Lymphatic System/Immunity

Digestive System

Respiratory System

Skeletal System

Integumentary System

Endocrine System

Nervous System

Reproductive System

597

598 Essentials of Human Anatomy and Physiology

Although production of estrogens continues for a while after menopause, the ovaries eventu- ally stop functioning as endocrine organs. When deprived of the stimulatory effects of estrogens, the reproductive organs and breasts begin to atro- phy. The vagina becomes dry; intercourse may become painful (particularly if infrequent), and vaginal infections become increasingly common. Other consequences of the deficit of estrogens include irritability and other mood changes (depression in some); intense vasodilation of the skin’s blood vessels, which causes uncomfortable sweat-drenching “hot flashes”; gradual thinning of the skin and loss of bone mass; and slowly rising blood cholesterol levels, which place postmeno- pausal women at risk for cardiovascular disorders. At one time, physicians prescribed low-dose estrogen–progesterone preparations to help women through this often difficult period and to prevent the skeletal and cardiovascular complica- tions. These seemed like great bonuses, and until July 2002, some 14 million American women were taking some form of estrogen-containing hormone replacement therapy (HRT). Then, on July 9, the Women’s Health Initiative (WHI) abruptly ended a clinical trial of 16,000 postmenopausal women, reporting that in those taking a popular estrogen– progesterone hormone combination there was an

increase of 51 percent in heart disease, 24 percent in invasive breast cancer, 31 percent in stroke, and a doubling of the risk of dementia compared to those taking placebos. The backlash of this infor- mation is still spreading through physicians’ offices and research labs, and it has dampened enthusi- asm for HRT in both the medical community and postmenopausal women. A new and encouraging study in 2006 reported a significant drop in breast cancers, with the drop due nearly entirely to fewer women using HRT.

There is no equivalent of menopause in males. Although aging men exhibit a steady decline in testosterone secretion and a longer latent period after orgasm, a condition called andropause, their reproductive capability continues until death. Healthy men well into their eighties and beyond are able to father offspring.

Did You Get It? 25. Which sex chromosome combination yields a boy—

XX or XY? What hormone must be produced by an XY fetus during development to stimulate the formation of the male duct system?

26. What is cryptorchidism, and what results if it is not rectified?

27. What are the major health threats to the adult woman’s reproductive system?

For answers, see Appendix A.

b. Penis—consists of three columns of erectile tissue surrounding the urethra. Erectile tissue provides a way for the penis to become rigid so it may serve as a penetrating device during sexual intercourse.

Male Reproductive Functions (pp. 569–573)

1. Spermatogenesis (sperm production) begins at puberty in seminiferous tubules in response to FSH. Spermatogenesis involves meiosis, a special nuclear division that halves the chromosomal num- ber in resulting spermatids. An additional process that strips excess cytoplasm from the spermatid, called spermiogenesis, is necessary for production of functional, motile sperm.

2. Testosterone production begins at puberty in re- sponse to LH. Testosterone is produced by intersti- tial cells of the testes. Testosterone causes the ap- pearance of male secondary sex characteristics and is necessary for sperm maturation.

Summary

Anatomy of the Male Reproductive System (pp. 565–569)

1. The paired testes, the male gonads, reside in the scrotum outside the abdominopelvic cavity. Testes produce both sperm (exocrine function) and testos- terone (endocrine function).

2. The male duct system includes the epididymis, ductus deferens, and urethra. Sperm mature in the epididy- mis. When ejaculation occurs, sperm are propelled through duct passageways to the body exterior.

3. Male accessory glands include the seminal vesicles, prostate, and bulbourethral glands. Collectively, these glands produce a fluid that activates and nourishes sperm.

4. External genitalia:

a. Scrotum—a skin sac that hangs outside the abdominopelvic cavity and provides the proper temperature for producing viable sperm.

Chapter 16: The Reproductive System 599

Anatomy of the Female Reproductive System (pp. 573–577)

1. The ovaries, the female gonads, are located against the lateral walls of the pelvis. They produce oocytes (exocrine function) and estrogen and progesterone (endocrine function).

2. The duct system:

a. Uterine (fallopian) tubes extend from the vicinity of an ovary to the uterus. Ends form fringed fim- briae that “wave” to direct ovulated oocytes into uterine tubes. These tubes then conduct the oocyte (or embryo) to the uterus by peristalsis and ciliary action.

b. The uterus is a pear-shaped muscular organ in which the embryo implants and develops. Its mucosa (endometrium) sloughs off each month in menses unless an embryo has become embed- ded in it. The myometrium contracts rhythmi- cally during the birth of a baby.

c. The vagina is a passageway between the uterus and the body exterior that allows a baby or the menstrual flow to leave the body. It also receives the penis and semen during sexual intercourse.

3. Female external genitalia include labia majora and minora (skin folds), clitoris, and urethral and vagi- nal openings.

Female Reproductive Functions and cycles (pp. 577–580)

1. Oogenesis (production of female sex cells) occurs in ovarian follicles, which are activated at puberty by FSH and LH to mature and eject oocytes (ovula- tion) on a cyclic basis. The egg (ovum) is formed only if sperm penetrates the secondary oocyte. In females, meiosis produces only one functional ovum (plus three nonfunctional polar bodies), as compared with the four functional sperm per meio- sis event produced by males.

2. Hormone production: Estrogens are produced by ovarian follicles in response to FSH. Progesterone, produced in response to LH, is the main hor- monal product of the corpus luteum. Estrogens stimulate development of female secondary sex characteristics.

3. The menstrual cycle involves changes in the endo- metrium in response to fluctuating blood levels of ovarian hormones. There are three phases:

a. Menstrual phase. Endometrium sloughs off and bleeding occurs. Ovarian hormones are at their lowest levels.

b. Proliferative phase. Endometrium is repaired, thickens, and becomes well vascularized in response to increasing levels of estrogens.

c. Secretory phase. Endometrial glands begin to secrete nutrients, and lining becomes more vas- cular in response to increasing level of proges- terone.

4. If fertilization does not occur, the phases are re- peated about every 28 days.

Mammary Glands (pp. 580–583)

1. Mammary glands are milk-producing glands found in the breasts. After the birth of a baby, they pro- duce milk in response to hormonal stimulation.

Pregnancy and Embryonic Development (pp. 583–593)

1. An oocyte can be fertilized up to 24 hours after release; sperm are viable within the female repro- ductive tract for up to 48 hours. Hundreds of sperm must release their acrosomal enzymes to break down the oocyte’s plasma membrane.

2. Following sperm penetration, the secondary oocyte completes meiosis II. Then ovum and sperm nuclei fuse (fertilization), forming a zygote.

3. If fertilization occurs, embryonic development begins immediately. Cleavage, a rapid series of mitotic divisions without intervening growth, begins with the zygote and ends with a blastocyst.

4. By day 14 after ovulation, the young embryo (blas- tocyst) has implanted in the endometrium, and the placenta is being formed. Human chorionic gonad- otropin (hCG) released by the blastocyst maintains hormone production of the corpus luteum, pre- venting menses, until the placenta assumes its endocrine role.

5. The placenta serves respiratory, nutritive, and excretory needs of the embryo and produces hor- mones of pregnancy.

6. All major organ systems have been formed by 8 weeks, and at 9 weeks the embryo is called a fetus. Growth and tissue/organ specialization are the major events of the fetal period.

7. A pregnant woman has increased respiratory, circu- latory, and urinary demands placed on her system by the developing fetus. Good nutrition is neces- sary to produce a healthy baby.

8. Childbirth (parturition) includes a series of events called labor. It is initiated by several factors but

600 Essentials of Human Anatomy and Physiology

most importantly by rising levels of oxytocin and prostaglandins, which promote vigorous uterine contractions. The three stages of labor are dilation, expulsion, and placental stages.

Developmental Aspects of the Reproductive System (pp. 593–598)

1. Reproductive system structures of males and females are identical during early development. Gonads begin to develop in the eighth week. The presence or absence of testosterone determines whether male or female accessory reproductive organs are formed.

2. Important congenital defects result from abnor- mal separation of sex chromosomes during sex cell formation.

3. The reproductive system is inactive during child- hood. Reproductive organs mature and become functional for childbearing at puberty.

4. Common reproductive problems during young adulthood are infections of the reproductive tract. Neoplasms of breast and cervix are major threats to women. Prostate cancer is the most common repro- ductive system cancer in men.

5. During menopause, female reproductive capabili- ties end, and reproductive organs begin to atro- phy. Hot flashes and mood changes may occur. Reproductive capacity does not appear to decline significantly in aging men.

Review Questions

Multiple choice

More than one choice may apply.

1. Which of the following can be found in semen?

a. Sperm

b. Relaxin

c. Cholesterol

d. Glucose

2. In terms of development, which of these pairs is mismatched ?

a. Vagina—penis

b. Testis—ovary

c. Labia majora—scrotum

d. Uterine tube—ductus deferens

3. The myometrium is the muscular layer of the uterus, and the endometrium is the _________ layer.

a. serosa

b. adventitia

c. submucosa

d. mucosa

4. Which of the following is (are) true concerning human spermatids?

a. They are the female gametes.

b. They contain 46 chromosomes.

c. They contain 23 chromosomes.

d. They are motile.

5. The ductus deferens can be found in the

a. scrotum.

b. prostate.

c. inguinal canal.

d. penis.

6. Which of the following attach to the ovary?

a. Fimbriae

b. Ovarian ligament

c. Suspensory ligaments

d. Broad ligament

7. Human ova and sperm are similar in that

a. about the same number of each is produced per month.

b. they have the same degree of motility.

c. they are about the same size.

d. they have the same number of chromosomes.

8. Select the false statement about the cervix of the uterus.

a. It is the superiormost part of the uterus.

b. It projects into the vagina.

c. Its cervical glands secrete mucus.

d. It contains the cervical canal.

Access additional practice questions using your smartphone, tablet, or computer: > Study Area > Practice Tests & Quizzes

Chapter 16: The Reproductive System 601

9. Each month, typically only one

a. primary follicle is stimulated.

b. follicle secretes estrogens.

c. vesicular follicle undergoes ovulation.

d. ovary is stimulated.

10. After ovulation, the ruptured follicle

a. degenerates.

b. becomes a corpus luteum.

c. sloughs off as waste material.

d. mends and produces another oocyte.

11. The outer layer of the blastocyst, which attaches to the uterine wall, is the

a. yolk sac.

b. inner cell mass.

c. amnion.

d. trophoblast.

12. The crown-to-rump length of an 8-week human embryo is

a. 3 mm.

b. 10 mm.

c. 22 mm.

d. 90 mm.

13. During human embryonic development, organogenesis occurs

a. during the first trimester.

b. during the second trimester.

c. during the third trimester.

d. just before birth.

Short Answer Essay

14. What are the primary sex organs, or gonads, of males? What are their two major functions?

15. What is the function of seminal fluid? Name the three types of glands that help produce it.

16. The penis contains erectile tissue that becomes engorged with blood during sexual excitement. What term is used to describe this event?

17. Define ejaculation.

18. Why are the male gonads not found in the abdominal cavity? Where are they found?

19. How does enlargement of the prostate interfere with a man’s reproductive function?

20. When does spermatogenesis begin? What causes it to begin?

21. Testosterone causes the male secondary sex characteristics to appear at puberty. Name three examples of male secondary sex characteristics.

22. What is circumcision? Does it affect secondary sex characteristics?

23. Name the female gonad, and describe its two major functions.

24. Why is the term urogenital system more applicable to males than females?

25. Name the structures of the female duct system, and describe the important functions of each.

26. How is the uterus anchored in the pelvis?

27. What is a follicle? What is ovulation?

28. What is mittelschmerz, and when is it most likely to occur?

29. What is the most important diagnostic method to detect cervical cancer?

30. List and describe the events of the menstrual cycle. Why is the menstrual cycle so important?

31. Define menopause. What does this mean to a woman?

32. Define fertilization. Where does fertilization usually occur? Describe the process of implantation.

33. How is body functioning of a pregnant woman altered by her pregnancy?

34. What events trigger labor?

35. Delivery of the infant occurs during which stage of labor?

36. What is the indifferent stage of embryonic development?

37. What are the major events of puberty?

38. How important are hereditary defects in the development of breast cancer? What are some of the methods of early detection of this cancer?

602 Essentials of Human Anatomy and Physiology

critical Thinking and clinical Application Questions

39. A pregnant woman in substantial pain called her doctor and explained (between sobs) that she was about to have her baby “right away.” The doctor calmed her and asked how she had come to that conclusion. She said that her water had broken and that her husband could see the baby’s head. (a) Was she right to believe that birth was immi- nent? If so, what stage of labor was she in? (b) Do you think she had time to make it to the hospital 60 miles away? Why or why not?

40. Crystal had both her left ovary and her right uterine tube removed surgically at age 17 because of a cyst and a tumor in these organs. Now, at age 32, she remains healthy and is expecting her second child. How could Crystal conceive a child with just one ovary and one uterine tube, separated on opposite sides of the pelvis?

41. A sperm enters a polar body, and their nuclei fuse. Why would it be unlikely for the resulting cell to develop into a healthy embryo?

Chapter 1 Did You Get It? 1. Anatomy and physiology are related. A given function can occur only if the corresponding structure allows it. 2. False. They are all topics of physiology. 3. The stomach exhibits the organ level of structural organization. Glucose is at the chemical level. 4. These organs are part of the respiratory system. 5. The urinary system rids the body of waste and helps regulate blood pressure. 6. Survival also depends on the ability to maintain one’s boundaries, to move, to respond to stimuli, and to reproduce. 7. Oxygen is required for the chemical reactions that release energy from food and form ATP. Metabolism is the life function in which oxygen directly participates. 8. The anatomical position is a standard position that serves as a reference point in descriptions of the body and its structures. In this position, the body is standing erect, feet are parallel, arms hang at the sides, and palms face forward. It’s important to understand it because most descriptions of the body using anatomical terminology refer to body regions as if the body is in the anatomical position regardless of its actual position. 9. The axillary region is the armpit. The acromial region is the point of the shoulder. 10. Left posterior antebrachial region. 11. To separate the thoracic and abdominal cavities, you would make a transverse section (cross section). 12. Of these organs, the small intestine and uterus are in the abdominopelvic cavity. 13. Joe might have appendicitis. 14. No. We mean that they vary within a narrow and regulated range. 15. Thirst is part of a negative feedback system. Thirst prods us to drink fluids (the response), which in turn causes the thirst sensation to decrease and end. Were it a positive feedback mechanism, we would become even more thirsty (the stimulus for drinking would increase).

Review Questions 1. d; 2. a, c, d, e; 3. superior, superficial, proximal, lateral, medial, posterior; 4. 1-e, 2-c, 3-i, 4-f, 5-h, 6-a, 7-b, 8-d, 9-g; 5. c; 6. c; 7. b, c, d; 8. a, c, d, e

Chapter 2 Did You Get It? 1. Matter is the substance of living and nonliving objects. Energy is the mover of the substance—the ability to do work. 2. Electrical and chemical energy are both involved in transmitting messages in the body. 3. Potential energy and kinetic energy, respectively. 4. Every time energy changes from one form to another, some heat is given off to the environment (is lost) and is unusable. 5. Carbon, oxygen, hydrogen, and nitrogen. 6. An atom is the smallest particle of an element that still retains the element’s properties. 7. The atom’s atomic number (#p) is 4; its atomic mass number (#n + p) is 9. 8. Radioisotope. 9. A molecule is two or more atoms chemically bound together. 10. A molecule of an element is a chemical combination of two or more atoms of the same kind. In a molecule of a compound, the atoms differ. 11. In ionic bonds, electrons are completely transferred from one atom to another. In covalent bonds, the interacting atoms share one or more electron pairs. 12. Hydrogen bonds. 13. It is a decomposition reaction. 14. A reversible reaction is shown by double reaction arrows. 15. The high heat capacity of water prevents rapid changes in body temperature. 16. Acids are proton donors. 17. A pH of 11 is basic. A pH of 5 is 1,000,000-fold more acidic than pH 11. 18. All chemical reactions in the body take place in a watery environment. 19. They conduct an electrical current when dissolved in water. 20. The structural units of carbohydrates are monosaccharides. Those of lipids are glycerol and fatty acids. 21. Phospholipids and cholesterol

Answers to Did You Get It? Questions and Multiple Choice Review Questions

are in cellular membranes; phospholipids are the most abundant. 22. Dehydration synthesis. 23. A chain of amino acids. 24. Fibrous proteins. 25. The enzyme is able to specifically bind with its substrate(s). 26. DNA contains the bases A, T, G, and C, and the sugar deoxyribose. RNA contains the bases A, G, C, and U, and its sugar is ribose. 27. ATP is the immediately useful form of energy for all body cells.

Review Questions 1. b, c, e; 2. b, c, d, f; 3. a, b, c, d, e; 4. c; 5. b, c; 6. b; 7. a, e; 8. c; 9. a; 10. a, c

Chapter 3 Did You Get It? 1. A cell is the (living) unit of life. 2. What its cells can do. 3. Plasma membrane = external barrier that regulates what enters and leaves the cell. Cytoplasm = the area where most cell activities occur. Nucleus = control center of the cell. 4. The generalized cell is a concept that describes organelles and functions common to all cells. 5. Nucleoli are the sites of synthesis of ribosomes, which are important in protein synthesis. 6. The phospholipids have both polar (heads) and nonpolar (tails) regions. Polar aligns with polar (water and other polar molecules inside and outside the cell). Nonpolar aligns with nonpolar in the membrane interior. 7. They act as receptors, determine blood type, and play a role in cell-to-cell interactions. 8. Communication and binding together, respectively. 9. The cytosol is the liquid portion of the cytoplasm. Cytoplasm includes cytosol, organelles, and inclusions. 10. Lysosomes break down ingested bacteria, worn-out organelles, and dead cells. Peroxisomes detoxify a number of harmful toxic substances and disarm free radicals. 11. Mitochondria are the major site of ATP synthesis. The Golgi apparatus is the packaging site.

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12. Microtubules and microfilaments are involved in cell mobility. 13. The basis of centrioles is microtubules; that of microvilli is a core of actin filaments. 14. Microvilli increase the cell surface area for absorption. 15. Fibroblasts and erythrocytes. 16. Neurons gather information and control body functions. 17. Kinetic energy is the energy source for diffusion. 18. The concentration gradient determines the direction that water and solutes move by diffusion. Movement is from high to low concentration of a given substance. 19. A channel protein is an opening formed by membrane proteins for diffusion of certain small solutes. A carrier protein undergoes shape changes that allow diffusion of a specific substance through the membrane. 20. “Down the concentration gradient” means to move from where there is a lot of something (high concentration) to where there is not a lot (low concentration). 21. When the pump protein is phosphorylated, it changes shape. When K+ binds, phosphate is released. 22. Phagocytosis moves large particles into the cell. 23. Receptor- mediated endocytosis. 24. DNA is double-stranded. When it is replicated, each strand serves as a template to build a complementary strand. Thus, if the template strand is ACT, the complementary strand formed at that site is TGA. 25. Cytokinesis is the division of the cytoplasm. If cytokinesis does not occur, the result is a binucleate cell. 26. mRNA carries the coded information for building proteins from the DNA to the ribosome where protein synthesis occurs. tRNA delivers amino acids to the ribosome and “checks” the location by recognizing the mRNA codon with its anticodon. 27. Transcription and translation. Proteins are synthesized during translation. Transcription is the production of mRNA using DNA as a template. 28. Cell shape and cell arrangement (layers) are the two criteria used to classify epithelium. 29. Exocrine glands have ducts that carry their secretion (typically a protein-containing secretion other than hormones) to a free body surface. Endocrine glands produce only hormones and are ductless glands. 30. Can repair itself; cells have specialized cell junctions. 31. The two hallmarks of epithelial tissue include having one free surface and being avascular. 32. The

two unique features of connective tissues include variations in blood supply and the production of a nonliving, extracellular matrix that surrounds their living cells. 33. Collagen fibers provide strength. 34. Skeletal muscle. 35. It allows neurons to control structures some distance away. 36. Nervous tissue is irritable, meaning it is able to respond to stimuli. 37. Dense fibrous connective tissue is the basis for fibrosis, or scar formation. 38. Epithelium and some connective tissues remain mitotic. 39. Neoplasm means “new growth.” It is an abnormal growth that occurs when normal control of cell division is lost; this was also described as “mitosis gone wild.” 40. Endocrine activity tends to decline with age.

Review Questions 1. a; 2. a, d; 3. b; 4. e; 5. c; 6. a, c; 7. b; 8. b; 9. a; 10. a; 11. b, e; 12. c

Chapter 4 Did You Get It? 1. Serous membranes line ventral body cavities closed to the exterior. Mucous membranes line body cavities open to the exterior (respiratory, digestive, urinary, and reproductive organ cavities). 2. Parietal pleura, visceral pleura, (lung), visceral pleura, parietal pleura, parietal pericardium, visceral pericardium, (heart). 3. Lining a fibrous capsule surrounding a joint. 4. The skin is the epithelial membrane that covers the body surface. Cutaneous membrane is a synonym for skin, as is integument, which means “covering.” The integumentary system is the skin and its derivatives (nails, hair, glands). 5. Any three of the functions listed in Table 4.1. 6. Keratinocytes are the most abundant cell type in the epidermis. 7. The stratum basale. 8. The stratum corneum. 9. Cells begin to die as they leave the stratum granulosum. 10. The dermal papillae in the papillary layer of the dermis are responsible for fingerprints. 11. Just the epidermis. 12. Melanin, carotene, and hemoglobin all contribute to skin color. 13. The sebaceous glands, which produce oily secretions. 14. Both are dilute salt solutions containing vitamins and wastes. Apocrine secretion also contains proteins and fatty acids. 15. No, the nail won’t regrow, because the growth region (nail

matrix) is torn off. 16. Loss of body fluids containing needed proteins and electrolytes, resulting in circulatory shock, and overwhelming infection. 17. First-degree burns are red and swollen; only epidermis damaged. Second-degree burns damage the epidermis and the superficial part of the dermis; blisters appear, but epithelial regeneration can occur. Third-degree burns destroy the entire skin thickness and can extend into the subcutaneous tissue; the burn is gray and painless; must graft. Fourth-degree burns destroy all layers of the skin; appear charred; extend into deep tissues such as muscle or bone. 18. ABCDE rule. 19. UV radiation exposure (sun or tanning bed). 20. Because stratum corneum cells are dead and thus no longer are dividing. 21. Loss of subcutaneous fat. 22. The sebaceous glands of the baby.

Review Questions 1. c; 2. b, e; 3. a, b, c; 4. a, c, e; 5. a, c, d; 6. c, d; 7. c; 8. a, c; 9. 1-e, 2-d, 3-f, 4-b, 5-g, 6-c

Chapter 5 Did You Get It? 1. Muscles use bones as levers to bring about body movements. 2. Red bone marrow provides a site for hematopoiesis, and yellow bone marrow is a storage site for fat. 3. Most long bones are found in the limbs. 4. Shaft = diaphysis; bone ends = epiphyses. 5. Compact bones appear solid and very dense with few holes. Spongy bone areas look like the cross-beams of a house with lots of space between the bone spicules. 6. They carry nutrients to the bone cells. 7. Membranes or cartilage. 8. The hormonal stimulus (PTH) maintains blood calcium homeostasis. 9. Bones will become thinner and weaker because osteoclasts are bone-destroying cells. 10. A fracture is a break in a bone. Compression and comminuted fractures are common in the elderly. 11. Skull, vertebral column, and bony thorax. 12. Eating or talking. These are the temporomandibular joints. 13. Maxillae. 14. Ethmoid bone. 15. Frontal joins with the parietals at the coronal suture. Parietals join one another at the sagittal suture. 16. Cervical, thoracic, lumbar, sacral, coccygeal. 17. All typical cervical vertebrae are small, have holes in their transverse processes, and have a

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split spinous process. Lumbar vertebrae are large, blocklike vertebrae with a blunt spinous process that projects directly back. They have a large body and no foramina in their transverse processes. 18. A true rib is attached directly to the sternum by its own costal cartilage. A false rib attaches indirectly (by the costal cartilage of a superior rib) or not at all. 19. Vertebrae. 20. Flat bones. 21. The thoracic and sacral curvatures are present. 22. At birth the newborn’s spine is an arc (C-shaped), whereas an adult’s spine has two additional curvatures and is S-shaped. 23. The axial skeleton forms the body axis and protects the brain and viscera. The appendicular skeleton allows for mobility and manipulation of the external environment. 24. The clavicle attaches medially to the sternum. 25. The humerus forms the skeleton of the arm. 26. Carpals are the short bones found in the wrist. 27. Radius and ulna. 28. Ilium, ischium, and pubis form the hip bone. Each pelvic girdle is formed by two coxal (hip) bones and the sacrum. 29. The female pelvis is broader, lighter, has a less acute pubic angle, a wider inlet and outlet, and shorter ischial spines. 30. Tibia and fibula form the skeleton of the leg. 31. The talus allows toe pointing. 32. The femur has those markings. 33. Joints connect bones together while allowing flexibility of the body. 34. The material between the articulating bone ends, which is connective tissue fibers in fibrous joints and cartilage in cartilaginous joints. 35. Lining a synovial joint capsule to provide a source of lubricating fluid in the joint. 36. The shoulder and hip joints are ball-and- socket joints. The carpometacarpal joint of the thumb is a saddle joint. 37. Compression fracture; osteoporosis. 38. Lower limbs and facial skeleton grow most rapidly during childhood.

Review Questions 1. a, b, d; 2. d; 3. a, d; 4. d; 5. a, b, c, d, e; 6. a, c; 7. d; 8. b, d, e; 9. c; 10. b; 11. 1-a, b, 2-a, 3-a, 4-a, 5-b, 6-c, 7-c, 8-a, 9-c; 12. 1-a, d, f; 2-b, e; 3-c

Chapter 6 Did You Get It? 1. Skeletal muscle fibers are long multinucleate cells with obvious striations. Cardiac cells are branching, typically

uninucleate cells with less obvious striations but obvious junctions. Smooth muscle fibers are spindle-shaped uninucleate cells, nonstriated. 2. Skeletal muscle. 3. Striped or having bands = striated. 4. Skeletal muscle movements can be very forceful and rapid, whereas smooth muscle movements tend to be slow and often rhythmic. 5. The alignment of the bands on the myofilaments is responsible for the banding pattern in skeletal muscle cells. 6. The axon ending of a motor neuron and the sarcolemma of the skeletal muscle fiber. 7. Sodium ions enter the cells during action potential generation. 8. Calcium stimulates the release of neurotransmitter (ACh) at the axon terminal of the motor neuron and causes the regulatory proteins blocking the myosin-binding sites on actin to move so cross bridges can form and allow contraction. 9. Calcium ions trigger the sliding of the myofilaments. 10. Pulling a bucket out of a well. 11. Phosphorylation of ADP by CP, stored ATP, and ATP generated by glucose oxidation. 12. Stored ATP. 13. Isometric contraction. 14. Oxygen deficit occurs when a person is not able to take in oxygen fast enough to keep the muscles supplied with all the oxygen they need when working vigorously. 15. Resistance exercise leads to increased muscle size. 16. Abduction. 17. Flexion and extension. 18. They anchor or aid the activity of the prime mover. 19. Tibialis anterior—a muscle overlying the tibia anteriorly. Erector spinae—muscles that straighten the spine. Rectus abdominus— muscle that runs straight up the abdomen. Extensor carpi radialis longus—long muscle on the posterior of the forearm that extends the wrist and attaches at the distal radius. 20. Circular. 21. Frontalis raises the eyebrows. 22. Masseter and temporalis are synergists in jaw closure. 23. Erector spinae. 24. It’s like plywood. The various abdominal muscles run in different directions across the abdomen, making the abdominal wall very strong. 25. Latissimus dorsi. 26. Triceps brachii. 27. Quadriceps on anterior thigh. 28. The medial gluteal site and the deltoid muscle of the shoulder. 29. Soleus and gastrocnemius. They plantar flex the foot. 30. Nerve fibers must be myelinated. 31. Exercise defers or reduces the natural loss in muscle mass and strength that occurs in old age.

Review Questions 1. c, e; 2. a; 3. c; 4. a, d; 5. a, b, c, d; 6. a, b; 7. a; 8. b, d

Chapter 7 Did You Get It? 1. CNS = brain and spinal cord. PNS = nerves that extend to and from the CNS. 2. Astrocytes are the most numerous neuroglia. Oligodendrocytes produce myelin. 3. Neuroglia can divide. Most neurons cannot. A criterion of cancer cells is their uncontrolled division. 4. A tract is a bundle of nerve fibers in the CNS. A nerve is a bundle of nerve fibers in the PNS. 5. A ganglion is a cluster of nerve cell bodies in PNS; a nucleus is a cluster of nerve cell bodies in the CNS. 6. Dendrites conduct impulses toward the nerve cell body; the axon terminal releases neurotransmitters. 7. The fiber that conducts at 40 m/sec. 8. A graded potential is a local current that dies out with distance. An action potential is a current that is continuously regenerated along the length of the axon and does not die out. 9. The synapse is the location where an impulse passes from one neuron to another or to a target cell. The synaptic cleft is the gap at the synapse that neurotransmitter must cross to transmit the signal. Chemically via the release of a neurotransmitter and binding of the neurotransmitter to the postsynaptic membrane. 10. Dendrites. 11. A reflex is a rapid, predictable, and involuntary response to a stimulus. 12. Cerebral cortex, white matter, and basal nuclei. 13. Mostly myelinated nerve fibers. 14. The brain stem controls vital activities. 15. The cerebellum provides precise timing for skeletal muscle act- ivity and helps control our balance and equilibrium. 16. Diencephalon. 17. Ventricles. 18. Blood-brain barrier. 19. Arachnoid mater. 20. Contusion. 21. Nerve cell bodies of interneurons and motor neurons. 22. Sensory pathways are ascending pathways. 23. Because the leash of nerve fibers there looks like a horse’s tail, the literal translation of cauda equina. 24. Around each nerve fiber. 25. Vagus nerves. 26. Nerve plexus = complex network of nerves. 27. Sciatic nerve of the sacral plexus. 28. Visceral organs (smooth and cardiac muscles and glands) are served by the ANS. Skeletal muscles are served

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by the somatic nervous system. 29. The ANS has a two-motor neuron pathway from the CNS to the organ to be served. The somatic nervous system has just one motor neuron in the motor pathway. 30. Sympathetic division. 31. They are unable to regulate their body temperature until the hypothalamus matures. 32. It is hypotension caused by a rapid change in position, such as getting up quickly from a reclining position. The sympathetic nervous system, which regulates blood pressure, is less efficient in old age.

Review Questions 1. d; 2. b, c; 3. b; 4. d; 5. c; 6. 1-e, 2-d, 3-f, 4-b, 5-e, 6-h, 7-a; 7. b, c; 8. c; 9. b; 10. a; 11. b; 12. b, c, d

Chapter 8 Did You Get It? 1. The eyelids protect the eyes. 2. The tarsal glands, ciliary glands, conjunctiva, and lacrimal glands all contribute to lubricating the eye, though their secretions differ. 3. Lysozyme is a component of tears that helps to destroy bacteria and protect the eye from infection. 4. They direct the eyeball toward what you wish to view. 5. The blind spot contains no photoreceptors; it is the site where the optic nerve leaves the eyeball. 6. Both contain pigment, which prevents light scattering in the eye. 7. Rods have a rodlike outer segment containing the photopigment, whereas cones have a shorter cone- shaped outer segment. Rods respond to low light conditions and produce black- and-white vision; cones need bright light and provide color vision. 8. Refractory media include the cornea, aqueous humor, the lens, and vitreous humor. 9. Accommodation. 10. The optic nerves leave the eyeballs, and the medial half of the fibers of each optic nerve cross over to the opposite side, joining there with the fibers from the outside half of the opposite eye to form the optic tracts. 11. It causes pupillary constriction in very bright light. Intense light stimulation can injure the photoreceptors. 12. Astigmatism results from unequal curvatures on the lens surface, not from an eyeball that is too long or too short to focus the image on the retina. The unequal curvatures of astigmatism result in points of light that focus on the retina as lines, not points, leading to blurry images. 13. External

and middle ears serve hearing only. 14. The ossicles (hammer, anvil, and stirrup). 15. Balance or equilibrium. 16. Dynamic receptors located in the semicircular canals (crista ampullaris) have embedded in the gel-like cupula; static receptors located in the vestibule (maculae) have otoliths that move when the head moves, causing hairs embedded in the otolithic membrane to bend. 17. Otoliths are tiny stones made of calcium salts that are located in the maculae of the vestibule. They respond to static equilibrium cues relative to the position of the head in space. 18. Tympanic membrane to bones of ossicles to fluids of the cochlear scalae. 19. Cochlear nerve (division of cranial nerve VIII). 20. Close to the oval window. 21. Sensorineural deafness results from damage to neural structures involved in hearing (cochlear nerve, auditory region of the brain), whereas conductive deafness results from anything that prevents sound vibrations from reaching the cochlea (earwax, fusion of the ossicles, fluid in the middle ear). 22. Chemoreceptors, because they respond to chemicals in solution. 23. On the tongue. 24. Odor receptors are located at the superior aspect of the nasal cavity. Sniffing brings the air upward. 25. Presbyopia; caused by reduced elasticity of the lens as a result of aging. 26. Vision. 27. Deafness of old age.

Review Questions 1. d; 2. b, c; 3. a, c; 4. d; 5. b; 6. a, c; 7. a, b, c, d; 8. a, c; 9. b; 10. a, b, c

Chapter 9 Did You Get It? 1. The endocrine system delivers its commands slowly via hormones carried by the blood. The nervous system uses rapid electrical messages that are much faster, allowing you to lift your foot off the glass more quickly. 2. A hormone is a chemical messenger used by the endocrine system. The target organ of a hormone is the specific cells or tissues that the hormone affects. 3. Because cAMP is not a first messenger, which is a molecule that attaches to a receptor on the cell’s plasma membrane and triggers the reactions leading to synthesis of the second messenger in the cell. 4. Endocrine organs are stimulated by hormones, by chemicals other than hormones, and by the nervous system.

5. Endocrine glands are ductless, and they release their hormone products directly into the intercellular fluid. Exocrine glands release their nonhormonal products to an epithelial surface via a duct. 6. The posterior pituitary is nervous tissue that acts as a storage and releasing area for hormones sent to it by the hypothalamus. 7. Diabetes insipidus, in which copious amounts of urine are voided as a result ofhyposecretion of ADH. 8. Tropic hormones stimulate certain endocrine organs to secrete their hormones. 9. Functional thyroid hormone has iodine as part of its structure. 10. Melatonin, produced by the pineal gland. 11. The parathyroid glands reside on the posterior thyroid gland. 12. PTH, produced by the parathyroid glands, increases the blood calcium ion level. 13. Calcitonin, produced by the parafollicular cells of the thyroid gland, reduces the blood calcium ion level. 14. Thymosin programs the T lymphocytes, which essentially direct our immune responses. 15. Aldosterone stimulates the kidneys to reabsorb more sodium ions. 16. Glucocorticoids such as cortisol. 17. Her blood glucose level would be elevated because of her stress. When we are stressed, both glucocorticoids (adrenal cortical hormones) and epinephrine and norepinephrine (adrenal medullary hormones) are produced in increased amounts. Both sets of hormones promote a rise in the blood glucose level. 18. Insulin stimulates cellular uptake of glucose. 19. Estrogen. 20. Stomach and small intestine. 21. Placenta. 22. Ovaries. 23. Growth hormone decline results in muscle atrophy; estrogen decline leads to osteoporosis.

Review Questions 1. d; 2. b, c; 3. a, c; 4. c; 5. a, b, d; 6. a, b, c, d; 7. c, d; 8. b; 9. b

Chapter 10 Did You Get It? 1. The liver is the major source of plasma proteins. 2. Erythrocytes, leukocytes, and platelets. 3. The color of blood varies with the amount of oxygen it is carrying. From most oxygen to least, the blood goes from scarlet to dull red or purple. 4. Hemoglobin transports oxygen and a small amount of carbon dioxide in the blood. 5. Lymphocytes

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are the main actors in body immunity. 6. Infection in the body causes an increase in WBC count, thus 15,000/mm3. 7. Anemia. 8. The hemocytoblast gives rise to all formed elements. 9. Lack of a nucleus; therefore, they cannot carry out transcription and translation to produce proteins (enzymes and others). 10. The stem cell (megakaryocyte) undergoes mitosis many times, forming a large multinucleate cell, which then fragments into platelets. 11. Inactivity, leading to blood pooling, and anything that roughens or damages the lining of a blood vessel (laceration, atherosclerosis, or physical trauma). 12. The self-antigens (agglutinogens) the RBCs bear. 13. A transfusion reaction in which the RBCs are lysed and hemoglobin enters the bloodstream, potentially leading to kidney shutdown. 14. O-positive. 15. An antigen is a substance foreign to the body that activates and is attacked by the immune system. An antibody is a protein released by immune cells that binds with a specific antigen and inactivates it in some way. 16. Fetal HbF has a greater ability to bind oxygen and binds it more strongly than adult HbA. 17. Leukemia, pernicious anemia, and clotting disorders are particularly common in the elderly.

Review Questions 1. a, b, d; 2. a, b, c, d; 3. d; 4. c; 5. a, c; 6. c, d; 7. b, d; 8. c; 9. b, c, d; 10. a, d; 11. d; 12. a

Chapter 11 Did You Get It? 1. The heart is in the mediastinum between the lungs. 2. The left ventricle has the thickest walls. This reflects its function, which is to pump blood through the whole body. 3. Pulmonary circulation strictly serves gas exchange. Oxygen is loaded and carbon dioxide is unloaded from the blood in the lungs. Systemic circulation provides oxygen- laden blood to all body organs. 4. Heart valves keep blood flowing in one direction through the heart. 5. The coronary arteries supply the myocardium (cardiac muscle) with oxygen. If that circulation fails, the heart fails. 6. The intrinsic conduction system of the heart coordinates the action of the heart chambers and causes the heart to beat faster than it would otherwise. 7. Left

ventricle 8. The atria are relaxing, and the ventricles are contracting. 9. The operation of the heart valves. 10. CO = amount of blood pumped out by each side of the heart in 1 minute. 11. Fever increases the heart rate because the rate of metabolism of the cardiac muscle increases. 12. Venous return. 13. It is a vein. 14. Blood pressure in veins is much lower than that in arteries because veins are farther along in the circulatory pathway. Hence, veins need extra measures to force blood back to the heart. 15. Capillary walls consist only of the innermost intima layer, which is very thin. Capillaries are the exchange vessels between the blood and tissue cells; thus, thin walls are desirable. 16. Lower limb. 17. Upper limb. 18. Hepatic portal vein. 19. The pulmonary circulation is much shorter and requires a less powerful pump than the systemic circulation does. Pulmonary arteries carry oxygen-depleted/carbon dioxide– rich blood, whereas the pulmonary veins carry oxygen-rich/carbon dioxide– depleted blood. The opposite is true of the arteries and veins of the systemic circulation. 20. Radial artery at the wrist; femoral at the groin; common carotid at the side of the neck. 21. It decreases from heart to venae cavae. 22. Hemorrhage reduces blood pressure initially because blood volume drops. 23. Fluid enters the capillary bed at its venous end.

Review Questions 1. c; 2. b; 3. d; 4. b; 5. c; 6. a, c; 7. c; 8. a, c; 9. a, d; 10. b; 11. a; 12. b; 13. a, b, c; 14. b, c; 15. b; 16. a, b, c; 17. b; 18. a

Chapter 12 Did You Get It? 1. Lymphatic vessels pick up fluid and proteins leaked from the blood into the interstitial space. 2. Lymphatic capillaries are blind-ended and not fed by arteries as blood capillaries are. They also have flaplike minivalves that make them more permeable than blood capillaries. 3. Particularly large collections of lymph nodes occur in the axillary, inguinal, and cervical regions. 4. The number of afferent lymphatic vessels entering the node is greater than the number of efferent vessels leaving the node at the hilum. Therefore, lymph flow stagnates somewhat. 5. The spleen destroys worn-

out RBCs. 6. The lymphoid tissue associated with the mucosa—tonsils in the throat, and Peyer’s patches in the intestinal walls—which acts to prevent pathogens from entering the body through the mucosa. 7. Innate defenses are nonspecific and always ready to protect the body. They include intact membranes (mucosa, skin), inflammatory response, and several protective cell types and chemicals. Adaptive defenses must be programmed and specifically target particular pathogens or antigens. 8. Redness, heat, swelling (edema), and pain. 9. It attaches to foreign cells, and when it is activated, membrane attack complexes (MACs) are inserted in the foreign cell’s membrane and produce lesions that allow water entry and cause lysis. 10. Viruses elicit interferon formation. 11. An antigen is a foreign substance in the body. A self-antigen is a body protein, typically displayed in the plasma membrane, that is foreign to anyone but oneself. 12. B lymphocytes and T lymphocytes. B lymphocytes mount a humoral response by producing antibodies. T lymphocytes mount the cellular response by activating B cells and cytotoxic T cells and stimulating the inflammatory response. 13. In the thymus. 14. They are antigen presenters to the T cells. 15. Primary response— this year’s new strain will be seen by the body as a different virus. 16. This is the region that binds to the antigen. 17. IgA, a dimer. 18. Neutralization occurs when antibodies attach to viruses or bacterial toxins, thereby blocking the virus or toxin from injuring the body. 19. Agglutination cross-links cell-bound antigen; precipitation cross-links soluble antigen, which then falls out of solution like clotting proteins that have been activated; opsonization is the “marking” of antigen for phagocytosis, which also makes it a bit sticky so it is easier for phagocytes to “catch.” 20. T cells have to bind to both an antigenic particle and to a self-protein on the antigen- presenting cell. 21. The cytotoxic T cell inserts perforin (toxic chemicals) into the foreign antigen’s plasma membrane, which then develops holes. Then granzymes, enzymes from the T cell’s cytoplasmic granules, enter and kill the foreign cell. 22. To slow or stop the immune response once the “enemy” has been conquered. 23. Helper T cells “direct” both humoral and cell-mediated

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adaptive responses; they interact with both B cells and T cells to stimulate cell division; release cytokines to recruit other phagocytes, such as neutrophils; and enhance macrophage activity. 24. Allograft. 25. An overzealous immune response against an otherwise harmless substance that causes injury to the body. 26. In anaphylactic shock, released histamine causes constriction of the bronchioles, sudden vasodilation, and fluid loss. 27. Abnormal production or functioning of immune cells or complement. 28. Self-proteins that were not previously exposed to the immune system appear in the circulation, or foreign antigens that resemble self- antigens arouse antibodies that attack the self-antigens.

Review Questions 1. d; 2. a; 3. c, d; 4. a, c; 5. a; 6. b, d; 7. c; 8. c, d; 9. b; 10. b, c; 11. d; 12. a, b, c, d; 13. a, b

Chapter 13 Did You Get It? 1. Because the respiratory mucosa rests on thin-walled veins that warm the incoming air, mucus produced by mucous glands and goblet cells moistens the air and traps dust and bacteria. The oral mucosa performs none of these functions. 2. Ciliated cells of the mucosa move the sheet of contaminated mucus away from the lungs and toward the throat for swallowing. 3. Nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli. 4. The right bronchus, because it’s wider and straighter. 5. The passageways conduct air. The elastic tissue allows lungs to recoil passively when exhaling, saving energy. 6. Respiratory bronchiole, alveolar duct, alveolar sac, and individual alveoli. 7. To exchange gases between the external environment and the blood—oxygen in, carbon dioxide out. 8. Increased air pressure in the lungs as they recoil. 9. VC is largest; TV is smallest. 10. About 350 ml reaches the alveoli. 11. His left lung collapsed because the pressure in the intrapleural space (normally negative) became equal to atmospheric pressure. 12. Diffusion. 13. As bicarbonate ion. 14. Cyanosis is a bluish cast to the skin (in light-skinned people), mucous membranes, and nailbeds due to inadequate oxygenation of the blood. 15. The medulla oblongata. 16. Increased

CO2, which results in a drop in blood pH. 17. He has COPD, specifically, emphysema. 18. The lung alveoli will collapse after every breath; IRDS.

Review Questions 1. d; 2. a, d; 3. a, b, d; 4. b; 5. b, c; 6. c; 7. c, e

Chapter 14 Did You Get It? 1. Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus. 2. Protein digestion begins in the stomach. 3. Alkaline mucus protects the stomach wall from being digested. Intrinsic factor is needed for intestinal absorption of vitamin B12. 4. The pyloric sphincter. 5. Villi are extensions of the mucosa of the small intestine. They increase the surface area of the small intestine tremendously for nutrient absorption. 6. Water absorption and absorption of some vitamins made by resident bacteria. 7. The incisors cut, the canines tear or pierce, the premolars and molars crush and grind. 8. Starch. Saliva contains salivary amylase. 9. Bile, produced and secreted by the liver, acts as a detergent to mechanically break up large fat masses into smaller ones for enzymat ic d iges t ion. 10. The pancreas. 11. Ingestion, digestion, absorption, and defecation. 12. Mechanical breakdown breaks food down physically by squeezing and pummeling it. Digestion uses enzymes to break the chemical bonds of the food molecules and release the nutrients. 13. This is the voluntary stage in which the chewed food is pushed into the pharynx by the tongue. 14. The main goal of segmentation is to mix the food thoroughly, though some propulsion occurs. Peristalsis is a propulsive activity. 15. Food went down the air passageway (trachea) instead of the digestive passage (esophagus). 16. Pepsin, the main enzyme of the stomach, needs acidic conditions to work. 17. Amino acids. 18. CCK causes the pancreas to secrete enzyme-rich fluid and stimulates the gallbladder to contract and the duodenal papilla to open. 19. Enzymes located on the microvilli of intestinal absorptive cells. 20. Plants—fruits, vegetables, and grains. 21. It provides fiber, which is important for moving feces along the colon and defecation. 22. Oils are

unsaturated lipids. 23. Most are coenzymes that work with enzymes to bring about metabolic reactions. 24. Cellular (aerobic) respiration. 25. Carbon dioxide and reduced coenzymes. 26. ATP and water. 27. Fats are used in synthesis of myelin sheaths and of cellular membranes, and as body insulation. 28. Making glucose from noncarbohydrate molecules, such as fats and proteins. 29. The liver combines ammonia with carbon dioxide to form urea. Urea is then eliminated from the body in urine. 30. HDLs, because they are headed back to the liver for breakdown and elimination from the body. 31. Large surface area relative to body volume. 32. Vasoconstriction of blood vessels serving the skin, and shivering. 33. It decreases body temperature. 34. Cystic fibrosis causes excessive production of mucus, which clogs the pancreatic ducts so that pancreatic enzymes cannot reach the small intestine; as a result, fat digestion is virtually stopped, resulting in fatty stools and an inability to absorb fat- soluble vitamins. 35. A diet low in phenylalanine. 36. You gain weight (fat).

Review Questions 1. c, d; 2. a, b, e; 3. c; 4. 1-e, 2-h, 3-c, 4-b, 5-g, 6-a, 7-f, 8-d; 5. a; 6. a, b; 7. b, d; 8. d; 9. d; 10. a; 11. b, d; 12. c, d; 13. d

Chapter 15 Did You Get It? 1. Retroperitoneal = behind, or in back of, the peritoneum. 2. Fat helps keep the kidneys anchored against the body trunk wall. When she lost weight, the amount of fat decreased, and the kidneys fell to a lower position (ptosis), causing the ureter(s) to kink and inhibit urine flow. 3. Cortex, medulla, pelvis. 4. The nephron. 5. The renal tubule carries out tubular reabsorption and secretion. The peritubular capillaries receive the fluid, nutrients, and needed ions to be returned to the general circulation. 6. It decreases glomerular pressure. 7. The specific gravity of urine is higher because it contains more solutes than water. 8. The bladder. 9. It allows the cavity of the bladder to increase in volume to store more urine when necessary. 10. It is longer (8″ vs. 1.5″), and it transports both urine and semen. The female urethra transports urine only. 11. Voiding or urination. 12. Excreting nitrogenous

Appendix A: Answers to Did You Get It? Questions and Multiple Choice Review Questions 609

wastes, maintaining acid-base balance, water balance, and electrolyte balance of the blood. 13. Thirst. 14. Aldosterone increases sodium ion reabsorption by the kidney tubules, and water follows (if able). 15. Osmoreceptors are in the brain (hypothalamus), and they respond to changing (increasing) solute content (osmolarity) of the blood. 16. The homeostatic pH of blood in the body varies between 7.35 and 7.45. Any blood pH below 7.35 is considered too acidic for normal body function. 17. A weak base, because a weak base will neutralize acidity by tying up only enough hydrogen ions to maintain a physiological pH, and there is no chance of overshoot and making the pH strongly basic. 18. It will increase, because water will be lost in sweat (and urine). 19. Kidneys maintain acid-base balance by excreting bicarbonate ions (HCO3

−) and reabsorbing or generating new HCO3

−. 20. A condition in a newborn baby boy, in which the urethral opening is on the ventral surface of the penis. 21. Urgency is the urgent feeling that you have to urinate. Frequency is the condition in which you urinate small amounts of urine frequently. 22. Polycystic kidney disease is a degenerative condition in which cysts (blisterlike sacs) of urine interfere with normal kidney function and lead to kidney failure; glomerulonephritis is clogging of the glomerular filter with antigen-antibody complexes.

Review Questions 1. d; 2. d; 3. a, d; 4. b, c, d; 5. a, b, c, d; 6. c; 7. c; 8. c

Chapter 16 Did You Get It? 1. The testes produce male gametes (sperm) and secrete sex hormones, mainly testosterone. 2. The seminiferous tubules produce sperm. 3. Epididymis, ductus deferens, ejaculatory duct, and urethra (prostatic, membranous, and spongy regions). 4. To make the penis firm by allowing it to fill with blood, so that it can serve as a penetrating device during sexual activity. 5. Sperm are male gametes that fertilize the female’s “eggs.” Seminal fluid serves as a transport medium for sperm and contains various substances that nourish and protect the sperm or aid their movement. 6. His prostate is probably hypertrophied. The prostate is immediately anterior to the rectum and can be examined digitally through the anterior rectal wall. 7. FSH stimulates spermatogenesis. 8. The final product of mitosis is two diploid cells that are genetically identical to each other and to the mother cell. The final product of meiosis is four cells, each with half the normal number of chromosomes (haploid). Genetic variation is introduced by meiosis. 9. Excess cytoplasm is discarded, and what remains is compacted into the head, midpiece, and tail regions. Final maturation processes in the epididymis result in increased motility. 10. Luteinizing hormone stimulates testosterone production. 11. Female gametes (eggs, or ova). 12. The uterus serves as an incubator. The uterine (fallopian) tubes are the most common site of fertilization. 13. Vesicular, or Graafian,

follicle. 14. Polar bodies are also produced. They deteriorate and die because they lack sustaining cytoplasm. 15. FSH promotes follicle development. 16. LH stimulates ovulation. 17. Estrogen. 18. The functional layer of the endometrium is rebuilt. 19. Progesterone (1) causes the secretory phase of the menstrual cycle, (2) helps prepare the breasts for lactation, and (3) helps quiet the uterine muscle to maintain pregnancy. 20. A large percentage (80%) of women who carry the altered gene(s) develop breast cancer. 21. Cleavage involves successive divisions not separated by growth of the cells, so the cells get smaller with each division. 22. The placenta produces hormones of pregnancy, delivers nutrients and oxygen to the fetus, and removes wastes from the fetal blood. 23. During pregnancy, many women have heartburn because the uterus crowds the stomach. Constipation is another problem because mobility of the digestive tract decreases. Nasal stuffiness and difficulty breathing are common. 24. Dilation, expulsion, and placental s tages . 25. XY; tes tos te rone. 26. Cryptorchidism is failure of the testes to descend into the scrotum; if uncorrected, the result is infertility. 27. Vaginal infections (E. coli, STIs, and fungal infections), which may lead to PID; abnormal or painful menses.

Review Questions 1. a, b; 2. a; 3. d; 4. c; 5. a, c; 6. b, c, d; 7. d; 8. a; 9. c; 10. b; 11. d; 12. c; 13. a

610 Essentials of Human Anatomy and Physiology

Word Roots, Prefixes, and Suffixes

Appendix B

Word element Meaning

Prefixes a-, an- absence, lack ab- away from acro- extreme, extremities ad- toward, to adeno- gland ambi- around, on both

sides amyl- starch ante- before, preceding ant-, anti- opposed to,

preventing, inhibiting

bi- two bili- bile bio- life brachi- arm brady- slow cholecysto- gallbladder circum- around co, con- together contra- against, opposite cost- ribs demi- half derm- skin dis- from dors- back dys- difficult, faulty,

painful electro- electric en-, em- in, inside equi- equal eryth- red exo- outside, outer

layer extra- outside, beyond ferr- iron fibro- fiber fore- before, in front of glyco- sugar hemi- half

Word element Meaning

hemo- blood hist- tissue homo- same hydro- water hygro- moisture hyper- excess hypo- below, deficient ileo- ileum in- in, within, into inter- between intra- within, inside juxta- near, close to lapar- abdomen laryngo- larynx later- side leuko- white macro- large mal- bad mast- breast medi- middle mega- large men- month mono- single multi- many myelo- marrow, spinal

cord myo- muscle neo- new nitro- nitrogen non- not ob- before, against ortho- straight, direct osteo- bone pan- all, universal para- beside, near path- disease ped- child, foot per- by, through peri- around pharyngo- pharynx

Word element Meaning

phren- diaphragm, mind pod- foot poly- multiple post- after, behind pre- before, ahead procto- rectum, anus pseudo- false psycho- mind pyel- pelvis of the kidney pyo- pus pyro- fire quadri- four radio- radiation re- back, again ren- kidney retro- backward, behind rhin- nose sacro- sacrum salpingo- fallopian tube sarco- flesh sclero- hard semi- half sex- six skeleto- skeleton steno- narrow sub- beneath, under super-, supra-

above, upon

syn- together tachy- rapid thyro- a shield trache- trachea trans- across, through tri- three ultra- beyond un- not, back reversal uni- one uretero- ureter urethro- urethra uro- urine, urinary organs

610

Appendix B: Word Roots, Prefixes, and Suffixes 611

Word Roots, Prefixes, and Suffixes (continued)

Suffixes -able able to -algia pain, in a certain

spot -cele tumor, swelling -centesis surgical puncture

to remove fluid -cide kill, destroy -cule little -ectasia dilating,

stretching -ectomy cutting out, surgical

removal -esis action -form shape genesis, formation, genetic origin -gram data that are

systematically recorded, a record

-graph an instrument used for recording data or writing

-ism condition -itis inflammation -ize to treat -lith stone, calculus -lithiasis presence of stones -lysis loosening or

breaking down -megaly enlargement -meter instrument that

measures -oid like, resemble -oma tumor -orrhaphy surgical repair -osis disease, condition of -ostomy to form an opening

or outlet

-otomy to incise -pathy disease -pexy fixation -phage ingesting -phobia fear -plasty plastic surgery -plegia paralysis -rhexis rupture -rrhagia abnormal or excessive

discharge -rrhea flow or discharge -scope lighted instrument for

visual examination -scopy to examine visually -stomy establishment of an

artificial opening -tomy incision into -uria urine -zyme enzyme, ferment

Word element Meaning

612 Essentials of Human Anatomy and Physiology

Appendix C

Periodic Table of the Elements*

Fe 26

55.845

Representative (main group)

elements

IIA

IIIB

IVB VB VIB VIIB VIIIB

IB IIB

IIIA IVA VA VIA VIIA

VIIIA 1

H 1.0079

Li 6.941

Na 22.990

K 39.098

Rb 85.468

Cs 132.905

Fr 223

Ra 226

Y 88.906

Sc 44.956

Ti 47.867

Zr 91.224

Hf 178.49

104

Rf 105

Ta 180.948

Nb 92.906

V 50.942

Cr 51.996

Mo 95.96

W 183.84

106

Sg 107

Re 186.207

Tc 98

Mn 54.938

Ru 101.07

Os 190.23

108

Co 58.933

Rh 102.906

Ir 192.22

Ni 58.69

Pd 106.42

Pt 195.08

Cu 63.546

Ag 107.868

Au 196.967

Zn 65.38

Cd 112.411

Hg 200.59

Ga 69.723

Al 26.982

B 10.811

In 114.82

Tl 204.383

La 138.906

Ac 227 267 268 271 272 270 276 281 280 285

Ge 72.64

Si 28.086

C 12.011

Sn 118.71

Pb 207.2

As 74.922

P 30.974

14.007

Sb 121.76

Bi 208.980

Se 78.96

S 32.065

15.999

Te 127.60

Po 209

284 289 288 293 294

Br 79.904

35.453

18.998

I 126.905

At 210

83.8

39.948

20.180

4.003

131.29 86

N O

Rn 222

109

Ba 137.327

Sr 87.62

Ca 40.078

Mg 24.305

Be 9.012

110

Ds 111

Rg 112

Cn 114 116

Representative (main group)

elements

Transition metals

Periodic Table of the Elements

113 115

Uuq Uuh 292

117

Uus 118

UuoUut Uup

Lanthanides

Actinides

Ce 140.116

Th 232.038

Pr 140.908

Pa 231.036

Nd 144.24

U 238.029

145 93

237.048

Sm 150.36

244

Eu 151.964

243

Gd 157.25

247

Tb 158.925

247

Dy 162.5

251

Ho 164.93

252

Er 167.26

100

257

Tm 168.934

101

258

Yb 173.054

102

259

Lu 174.967

103

Np Pu Am Cm Bk Cf Es Fm Md No Lr 262

Rare earth elements

*Atomic weights of the elements per IUPAC Commission on Isotopic Abundances and Atomic Weights, 2007.

The periodic table arranges elements according to atomic num- ber and atomic weight into horizontal rows called periods and 18 vertical columns called groups or families. The elements in the groups are classified as being in either A or B classes.

Elements of each group of the A series have similar chemical and physical properties. This reflects the fact that members of a particular group have the same number of valence shell electrons, which is indicated by the number of the group. For example, group IA elements have one valence shell electron, group IIA elements have two, and group VA elements have five. In contrast, as you progress across a period from left to right, the properties of the elements change in discrete steps, varying gradually from the very metallic properties of groups IA and IIA elements to the nonmetallic properties seen in group VIIA (chlorine and

others), and finally to the inert elements (noble gases) in group VIIIA. This change reflects the continual increase in the number of valence shell electrons seen in elements (from left to right) within a period.

Class B elements are referred to as transition elements. All transition elements are metals, and in most cases they have one or two valence shell electrons. (In these elements, some electrons occupy more distant electron shells before the deeper shells are filled.)

In this periodic table, the colors are used to convey information about the phase (solid, liquid, or gas) in which a pure element exists under standard conditions (25 degrees centigrade and 1 atmosphere of pressure). If the element's symbol is solid black, then the element exists as a solid. If its symbol is red, then it exists as a gas. If its symbol is dark blue, then it is a liquid. If the element’s symbol is green, the element does not exist in nature and must be created by some type of nuclear reaction.

612

Appendix D: Key Information about Vitamins and Many Essential Minerals 613

Appendix D

Key Information about Vitamins and Many Essential Minerals

613

*Abbreviations: RDA, Recommended Dietary Allowance; UL, upper limit; Al, adequate intake.

Vitamin/ mineral

Primary functions

Recommended intake*

Reliable food sources

Toxicity/deficiency symptoms

Fat-Soluble A (retinol, retinal, retinoic acid)

Required for ability of eyes to adjust to changes in light Protects color vision Assists cell differentiation Required for sperm production in men and fertilization in women Contributes to healthy bone Contributes to healthy immune system

RDA: Men: 900 μg/day Women: 700 μg/day UL: 3,000 μg/day

Preformed retinol: Beef and chicken liver, egg yolks, milk Carotenoid precursors: Spinach, carrots, mango, apricots, cantaloupe, pumpkin, yams

Toxicity: Fatigue; bone and joint pain; spontaneous abortion and birth defects of fetuses in pregnant women; nausea and diarrhea; liver damage; blurred vision; hair loss; skin disorders Deficiency: Night blindness, xerophthalmia; impaired growth, immunity, and reproductive function

D (cholecalciferol) Regulates blood calcium levels Maintains bone health Assists cell differentiation

AI (assumes that person does not get adequate sun exposure): Adult aged 19 to 50: 5 μg/day Adult aged 50 to 70: 10 μg/day Adult aged >70: 15 μg/day UL: 50 μg/day

Canned salmon and mackerel, milk, fortified cereals

Toxicity: Hypercalcemia Deficiency: Rickets in children; osteomalacia and/or osteoporosis in adults

E (tocopherol) As a powerful antioxidant, protects cell membranes, polyunsaturated fatty acids, and vitamin A from oxidation Protects white blood cells Enhances immune function Improves absorption of vitamin A

RDA: Men: 15 mg/day Women: 15 mg/day UL: 1,000 mg/day

Sunflower seeds, almonds, vegetable oils, fortified cereals

Toxicity: Rare Deficiency: Hemolytic anemia; impairment of nerve, muscle, and immune function

K (phylloquinone, menaquinone, menadione)

Serves as a coenzyme during production of specific proteins that assist in blood coagulation and bone metabolism

AI: Men: 120 μg/day Women: 90 μg/day

Kale, spinach, turnip greens, brussels sprouts

Toxicity: None known Deficiency: Impaired blood clotting; possible effect on bone health

Water-Soluble Thiamine (vitamin B1)

Required as enzyme cofactor for carbohydrate and amino acid metabolism

RDA: Men: 1.2 mg/day Women: 1.1 mg/day

Pork, fortified cereals, enriched rice and pasta, peas, tuna, legumes

Toxicity: None known Deficiency: Beriberi; fatigue, apathy, decreased memory, confusion, irritability, muscle weakness

Riboflavin (vitamin B2)

Required as enzyme cofactor for carbohydrate and fat metabolism

RDA: Men: 1.3 mg/day Women: 1.1 mg/day

Beef liver, shrimp, milk and dairy foods, fortified cereals, enriched breads and grains

Toxicity: None known Deficiency: Ariboflavinosis; swollen mouth and throat; seborrheic dermatitis; anemia

Niacin, nicotinamide, nicotinic acid

Required for carbohydrate and fat metabolism Plays role in DNA replication and repair and cell differentiation

RDA: Men: 16 mg/day Women: 14 mg/day UL: 35 mg/day

Beef liver, most cuts of meat/fish/poultry, fortified cereals, enriched breads and grains, canned tomato products

Toxicity: Flushing, liver damage, glucose intolerance, blurred vision differentiation Deficiency: Pellagra; vomiting, constipation, or diarrhea; apathy

•

614 Appendix D: Key Information about Vitamins and Many Essential Minerals

Vitamin/ mineral

Primary functions

Recommended intake*

Reliable food sources

Toxicity/deficiency symptoms

Water-Soluble Pyridoxine, pyridoxal, pyridoxamine (vitamin B6)

Required as enzyme cofactor for carbohydrate and amino acid metabolism Assists synthesis of blood cells

RDA: Men and women aged 19 to 50: 1.3 mg/day Men aged >50: 1.7 mg/day Women aged >50: 1.5 mg/day UL: 100 mg/day

Chickpeas (garbanzo beans), most cuts of meat/fish/poultry, fortified cereals, white potatoes

Toxicity: Nerve damage, skin lesions Deficiency: Anemia; seborrheic dermatitis; depression, confusion, and convulsions

Folate (folic acid) Required as enzyme cofactor for amino acid metabolism Required for DNA synthesis Involved in metabolism of homocysteine

RDA: Men: 400 μg/day Women: 400 μg/day UL: 1,000 μg/day

Fortified cereals, enriched breads and grains, spinach, legumes (lentils, chickpeas, pinto beans), greens (spinach, romaine lettuce), liver

Toxicity: Masks symptoms of vitamin B12 deficiency, specifically signs of nerve damage Deficiency: Macrocytic anemia; neural tube defects in a developing fetus; elevated homocysteine levels

Cobalamin (vitamin B12)

Assists with formation of blood Required for healthy nervous system function Involved as enzyme cofactor in metabolism of homocysteine

RDA: Men: 2.4 μg/day Women: 2.4 μg/day

Shellfish, all cuts of meat/fish/poultry, milk and dairy foods, fortified cereals

Toxicity: None known Deficiency: Pernicious anemia; tingling and numbness of extremities; nerve damage; memory loss, disorientation, and dementia

Pantothenic acid Assists with fat metabolism AI: Men: 5 mg/day Women: 5 mg/day

Meat/fish/poultry, shiitake mushrooms, fortified cereals, egg yolk

Toxicity: None known Deficiency: Rare

Biotin Involved as enzyme cofactor in carbohydrate, fat, and protein metabolism

RDA: Men: 30 μg/day Women: 30 μg/day

Nuts, egg yolk Toxicity: None known Deficiency: Rare

Ascorbic acid (vitamin C)

Antioxidant in extracellular fluid and lungs Regenerates oxidized vitamin E Assists with collagen synthesis Enhances immune function Assists in synthesis of hormones, neurotransmitters, and DNA Enhances iron absorption

RDA: Men: 90 mg/day Women: 75 mg/day Smokers: 35 mg more per day than RDA UL: 2,000 mg

Sweet peppers, citrus fruits and juices, broccoli, strawberries, kiwi

Toxicity: Nausea and diarrhea, nosebleeds, increased oxidative damage, increased information of kidney stones in people with kidney disease Deficiency: Scurvy; bone pain and fractures, depression, and anemia

Major Minerals Sodium Fluid balance

Acid-base balance Transmission of nerve impulses Muscle contraction

AI: Adults: 1.5 g/day (1,500 mg/day)

Table salt, pickles, most canned soups, snack foods, cured luncheon meats, canned tomato products

Toxicity: Water retention, high blood pressure, loss of calcium in urine Deficiency: Muscle cramps, dizziness, fatigue, nausea, vomiting, mental confusion

Potassium Fluid balance Transmission of nerve impulses Muscle contraction

AI: Adults: 4.7 g/day (4,700 mg/day)

Most fresh fruits and vegetables: potatoes, bananas, tomato juice, orange juice, melons

Toxicity: Muscle weakness, vomiting, irregular heartbeat Deficiency: Muscle weakness, paralysis, mental confusion, irregular heartbeat

Key Information about Vitamins and Many Essential Minerals (continued)

*Abbreviations: RDA, Recommended Dietary Allowance; UL, upper limit; Al, adequate intake.

Appendix D: Key Information about Vitamins and Many Essential Minerals 615

Key Information about Vitamins and Many Essential Minerals (continued)

*Abbreviations: RDA, Recommended Dietary Allowance; UL, upper limit; Al, adequate intake.

Vitamin/ mineral

Primary functions

Recommended intake*

Reliable food sources

Toxicity/deficiency symptoms

Phosphorus Fluid balance Bone formation Component of ATP, which provides energy for our bodies

RDA: Adults: 700 mg/day

Milk/cheese/yogurt, soy milk and tofu, legumes (lentils, black beans), nuts (almonds, peanuts and peanut butter), poultry

Toxicity: Muscle spasms, convulsions, low blood calcium Deficiency: Muscle weakness, muscle damage, bone pain, dizziness

Chloride Fluid balance Transmission of nerve impulses Component of stomach heartbeat acid (HCL) Antibacterial

AI: Adults: 2.3 g/day (2,300 mg/day)

Table salt Toxicity: None known Deficiency: Dangerous blood acid- base imbalances, irregular heartbeat

Calcium Primary component of bone Acid-base balance Transmission of nerve impulses Muscle contraction

AI: Adults aged 19 to 50: 1,000 mg/day Adults aged >50: 1,200 mg/day UL: 2,500 mg/day

Milk/yogurt/cheese (best-absorbed form of calcium), sardines, collard greens and spinach, calcium- fortified juices

Toxicity: Mineral imbalances, shock, kidney failure, fatigue, mental confusion Deficiency: Osteoporosis, convulsions, heart failure

Magnesium Component of bone Muscle contraction Assists more than 300 enzyme systems

RDA: Men aged 19 to 30: 400 mg/day Men aged >30: 420 mg/day Women aged 19 to 30: 310 mg/day Women aged >30: 320 mg/day UL: 350 mg/day

Greens (spinach, kale, collard greens), whole grains, seeds, nuts, legumes (navy and black beans)

Toxicity: None known Deficiency: Low blood calcium, muscle spasms or seizures, nausea, weakness, increased risk of chronic diseases such as heart disease, hypertension, osteoporosis, and type 2 diabetes

Sulfur Component of certain B vitamins and amino acids Acid-base balance Detoxification in liver

No DRI Protein-rich foods Toxicity: None known Deficiency: None known

Trace Minerals Selenium Required for carbohydrate

and fat metabolism RDA: Adults: 55 μg/day UL: 400 μg/day

Nuts, shellfish, meat/ fish/poultry, whole grains

Toxicity: Brittle hair and nails, skin rashes, nausea and vomiting, weakness, liver disease Deficiency: Specific forms of heart disease and arthritis, impaired immune function, muscle pain and wasting, depression, hostility

Fluoride Development and maintenance of healthy teeth and bones

RDA: Men: 4 mg/day Women: 3 mg/day UL: 2.2 mg/day for children aged 4 to 8; 10 mg/day for children aged >8

Fish, seafood, legumes, whole grains, drinking water (variable)

Toxicity: Fluorosis of teeth and bones Deficiency: Dental caries, low bone density

Iodine Synthesis of thyroid hormones Temperature regulation Reproduction and growth

RDA: Adults: 150 μg/day UL: 1,100 μg/day

Iodized salt, saltwater seafood

Toxicity: Goiter Deficiency: Goiter, hypothyroidism, cretinism in infant of mother who is iodine deficient

•

616 Appendix D: Key Information about Vitamins and Many Essential Minerals

Vitamin/ mineral

Primary functions

Recommended intake*

Reliable food sources

Toxicity/deficiency symptoms

Trace Minerals Chromium Glucose transport

Metabolism of DNA and RNA Immune function and growth

AI: Men aged 19 to 50: 35 μg/day Men aged >50: 30 μg/day Women aged 19 to 50: 25 μg/day Women aged >50: 20 μg/day

Whole grains, brewers yeast

Toxicity: None known Deficiency: Elevated blood glucose and blood lipids, damage to brain and nervous system

Manganese Assists many enzyme systems Synthesis of protein found in bone and cartilage

AI: Men: 2.3 mg/day Women: 1.8 mg/day UL: 11 mg/day adults

Whole grains, nuts, leafy vegetables, tea

Toxicity: Impairment of neuromuscular system Deficiency: Impaired growth and reproductive function, reduced bone density Impaired glucose and lipid metabolism, skin rash

Iron Component of hemoglobin in blood cells Component of myoglobin in muscle cells Assists many enzyme systems

RDA: Adult men: 8 mg/day Women aged 19 to 50: 18 mg/day Women aged >50: 8 mg/day

Meat/fish/poultry (best-absorbed form of iron), fortified cereals, legumes, spinach

Toxicity: Nausea, vomiting and diarrhea; dizziness, confusion; rapid heartbeat, organ damage, death Deficiency: Iron-deficiency microcytic (small red blood cells), hypochromic anemia

Zinc Assists more than 100 enzyme systems Immune system function Growth and sexual maturation Gene regulation

RDA: Men: 11 mg/day Women: 8 mg/day UL: 40 mg/day

Meat/fish/poultry (best-absorbed form of zinc), fortified cereals, legumes

Toxicity: Nausea, vomiting, and diarrhea; headaches, depressed immune function, reduced absorption of copper Deficiency: Growth retardation, delayed sexual maturation, eye and skin lesions, hair loss, increased incidence of illness and infection

Copper Assists many enzyme systems Iron transport

RDA: Adults: 900 μg/day UL: 10 μg/day

Shellfish, organ meats, nuts, legumes

Toxicity: Nausea, vomiting, and diarrhea; liver damage Deficiency: Anemia, reduced levels of white blood cells, osteoporosis in infants and growing children

*Abbreviations: RDA, Recommended Dietary Allowance; UL, upper limit; Al, adequate intake.

Key Information about Vitamins and Many Essential Minerals (continued)

with other molecules of complementary shape and charge (such as substrate). Active transport net movement of a sub- stance across a membrane against a concen- tration or electrical gradient; requires release and use of cellular energy. Adaptive defense system branch of the immune system that targets specific antigen; involves B and T lymphocytes and antigen- presenting cells; also called specific defense system. Adenosine triphosphate (ATP) ah-den″o- sēn tri-fos′fāt the compound that is the impor- tant intracellular energy source; cellular energy. Adipose tissue ad′ ı̆-pōs fat. Adrenal glands ah-dre′nal glanz hormone- producing glands located superior to the kid- neys; each consists of a medulla and a cortex. Aerobic respiration respiration in which oxygen is consumed and glucose is broken down entirely; water, carbon dioxide, and large amounts of ATP are the final products. Afferent af′er-ent carrying to or toward a center. Agglutinins ah-gloo′t̆ı-ninz antibodies in blood plasma that cause clumping of corpus- cles or bacteria. Agglutinogens ag″loo-tin′o-jenz (1) anti- gens that stimulate the formation of a specific agglutinin; (2) antigens found on red blood cells that are responsible for determining the ABO blood group classification. Albumin al-bu′min a protein found in vir- tually all animals; the most abundant plasma protein. Alkalosis al″kah-lo′sis a condition in which the blood has a lower hydrogen ion concentra- tion than normal and an increased pH. Allergy al′er-je overzealous immune re- sponse to an otherwise harmless antigen, re- sulting in tissue damage; also called hypersen- sitivity. Alveolus al-ve′o-lus (1) a general term refer- ring to a small cavity or depression; (2) an air sac in the lungs. Amino acid ah-me′no an organic com- pound containing nitrogen, carbon, hydrogen, and oxygen; the building block of protein. Amphiarthrosis am″fe-ar-thro′sis a slightly movable joint. Anabolism ah-nab′o-lizm the energy- requiring building phase of metabolism in which simpler substances are combined to form more complex substances. Anaerobic glycolysis an-a′er-ōb-ik a pro- cess in which glucose is broken down in the absence of oxygen, resulting in the formation of 2 ATP molecules and lactic acid. Anatomical position reference point for re- gional and directional terminology; standing, feet parallel, palms facing forward.

Pronunciations in the text and this glossary use the following rules:

1. Accent marks follow stressed syllables. The primary stress is shown by′, and the sec- ondary stress by ″.

2. Unless otherwise noted, assume that vow- els at the ends of syllables are long and vowels followed by consonants are short. Exceptions to this rule are indicated by a bar (-) over the vowel, which indicates a long vowel, or a breve sign (˘) over the vowel, indicating that the vowel is short.

For example, the phonetic spelling of “throm- bophlebitis” is throm″bo-flĕ-bi′tis. The next-to- last syllable (bi′) receives the greatest stress, and the first syllable (throm″ ) gets the second- ary stress. The vowel in the second syllable comes at the end of the syllable and is long. The vowel that comes at the end of the third syllable is short because it has a breve sign.

Abdominal ab-dom′ ı̆nal pertaining to the anterior body trunk inferior to the ribs. Accommodation (1) adaptation in response to differences or changing needs; (2) adjustment of the eye for seeing objects at close range. Acetylcholine (ACh) a″-se-til-ko′lēn a chem- ical transmitter substance released by certain nerve endings. Acid a substance that liberates hydrogen ions when in an aqueous solution; proton do- nor; compare with base. Acid-base balance the maintenance of proper pH in the body; involves buffers, the respiratory system, and the kidneys. Acidosis as″ ı̆-do′sis a condition in which the blood has an excess hydrogen ion con- centration and a decreased pH; also called ketoacidosis. Acquired immune deficiency syndrome (AIDS) immunodeficiency caused by hu- man immunodeficiency virus (HIV) attacking T cells; symptoms include severe weight loss, night sweats, swollen lymph nodes, and op- portunistic infections. Acromial ah-kro′me-ul pertaining to the point of the shoulder. Acrosome ak′ro-sōm an enzyme-containing structure covering the nucleus of the sperm. Actin ak′tin one of the principal contractile proteins found in muscle; makes up the thin filaments. Action potential an electrical event occurring when a stimulus of sufficient intensity is applied to a neuron or muscle cell, allowing sodium ions to move into the cell and reverse the polarity. Active immunity immunity produced by an encounter with an antigen; provides immuno- logical memory. Active site region on the surface of a globular protein (such as an enzyme) where it interacts

Glossary

Anatomy the science of the structure of living organisms. Anemia ah-ne′me-ah reduced oxygen- carrying capacity of the blood caused by a de- creased number of erythrocytes or decreased percentage of hemoglobin in the blood. Antebrachial an″te-bra′ke-ul pertaining to the forearm. Antecubital anterior surface of the elbow. Antibody an″t̆ı-bod′e a specialized sub- stance produced by the body that can provide immunity against a specific antigen. Antigen (Ag) an′t̆ı-jen any substance—in- cluding toxins, foreign proteins, or bacteria— that, when introduced to the body, is recognized as foreign and activates the immune system. Antigen-presenting cells (APCs) cells that display portions of foreign antigens to T cells; include dendritic cells, macrophages, and B lym- phocytes. Antimicrobial proteins defensive proteins that assist the innate defenses by attacking mi- crobes directly or by preventing their repro- duction; see complement and interferon. Aorta a-or′tah the major systemic artery; arises from the left ventricle of the heart. Apocrine gland ap′o-krin the less numer- ous type of sweat gland. Its secretions contain water, salts, fatty acids, and proteins. Aponeurosis ap″o-nu-ro′sis fibrous or mem- branous sheet connecting a muscle and the part it moves. Appendicular skeleton ap″en-dik′u-lar bones of the limbs and limb girdles that are attached to the axial skeleton. Aqueous humor a′kwe-us hu′mer the wa- tery fluid in the anterior chambers of the eye. Arachnoid granulation special projections of the arachnoid mater that protrude through the dura mater; site of cerebrospinal fluid ab- sorption into the dural venous sinuses. Arachnoid mater middle layer of the me- ninges; has weblike extensions attach to the inner layer, the pia mater. Arrector pili ah-rek′tor pi′li tiny, smooth muscles attached to hair follicles; when acti- vated, they cause the hair to stand upright. Arteriole ar-ter′e-ōl minute artery. Arteriosclerosis ar-tēr′e-o-skler-o′sis any of a number of proliferative and degenerative chang- es in the arteries leading to their decreased elas- ticity and hardening; end stage of atherosclerosis. Artery a vessel that carries blood away from the heart. Arthritis ar-thri′tis inflammation of the joints. Articular cartilage hyaline cartilage cover- ing the epiphysis of a bone at a joint. Asthma az′mah disease or allergic response characterized by bronchial spasms and difficult breathing.

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617

618 Glossary

Burn tissue damage resulting in cell death caused by electricity, chemicals, too much heat or UV radiation. Burns vary in degree of sever- ity. See rule of nines. Bursa ber′sah a small sac filled with fluid and located at friction points, especially joints. Calcaneal kal-ka′ne-ul pertaining to the heel of the foot. Calyx ka′liks a cuplike extension of the pel- vis of the kidney. Canaliculus kan″ah-lik′u-lus extremely small tubular passage or channel. Cancer a malignant, invasive cellular neo- plasm that has the capability of spreading throughout the body or body parts. Carbohydrate kar″bo-hi′drāt organic com- pound composed of carbon, hydrogen, and oxygen; includes starches, sugars, cellulose. Cardiac cycle sequence of events encom- passing one complete contraction and relax- ation of the atria and ventricles of the heart. Cardiac muscle specialized muscle of the heart with striations and intercalated discs; in- voluntary muscle. Cardiac output (CO) the blood volume (in liters) ejected per minute by each ventricle. Cardiac veins veins that drain the myocar- dium and empty into the coronary sinus. Cardiovascular system organ system that distributes blood to all parts of the body. Carpal kar′pal (1) one of the eight bones of the wrist; (2) pertaining to the wrist. Cartilage kar′t ı̆-lij white, semiopaque con- nective tissue. Cartilaginous joint kar″ti-laj′ ı̆-nus bones united by cartilage; no joint cavity is present. Catabolism kah-tab′o-lizm the process in which living cells break down substances into simpler substances; destructive metabolism. Catalyst kat′ah-list substance that increases the rate of a chemical reaction without itself becoming chemically changed or part of the product; see enzyme. Cataract kat′ah-rakt partial or complete loss of transparency of the crystalline lens of the eye. Catecholamines kat″ĕ-kol′ah-mēnz epineph- rine and norepinephrine. Cauda equina kaw′da e-kwi′nah the collec- tion of spinal nerves at the inferior end of the vertebral canal. Cell the basic biological unit of living organ- isms, enclosed by a limiting membrane; cells in more complex organisms contain a nucleus and a variety of organelles. Cell body the part of a neuron containing the nucleus; the metabolic center of a neuron. Cell division the phase of a cell’s life cycle when it reproduces itself. Cellular immunity sel′u-lar ı̆-mu′n̆ı-te im- munity conferred by lymphocytes called T cells; also called cell-mediated immunity. Central nervous system (CNS) the brain and the spinal cord. Centriole sen′tre-ōl a minute body found near the nucleus of the cell composed of mi- crotubules; active in cell division.

Basal nuclei nu′kle-i gray matter areas deep within the white matter of the cerebral hemispheres; also called basal ganglia. Base (1) a substance that accepts hydrogen ions; proton acceptor; compare with acid; (2) the broad, posterosuperior aspect of the heart where large vessels enter or leave the heart; (3) the broad inferior area of each lung resting on the diaphragm. Basement membrane a thin layer of extra- cellular material to which epithelial cells are attached in mucosa surfaces. Basilar membrane the membrane in the co- chlear duct that vibrates to transmit sound waves and also houses the receptor cells for hearing. Basophils ba′so-filz white blood cells whose granules stain deep blue with basic dye; have a relatively pale nucleus and granu- lar-appearing cytoplasm. Benign be-nı̄n′ not malignant. Bicarbonate buffer system system com- posed of bicarbonate ion (HCO3

–) and its salt, sodium bicarbonate (NaHCO3), that resists changes in blood pH. Bile a greenish yellow or brownish fluid pro- duced in and secreted by the liver, stored in the gallbladder, and released into the small intestine. Blind spot gap in vision caused by a lack of photoreceptors at the optic disc. Blood liquid connective tissue composed of plasma and cells responsible for transport- ing substances such as nutrients and oxygen around the body. Blood pressure the pressure exerted by blood against the inner walls of blood vessels. Blood sugar the amount of glucose in the blood. Bone markings surface features of bones where muscles, tendons, and ligaments attach, and where blood vessels and nerves pass. Bone remodeling the process of repairing or maintaining bone by adding or removing bone matrix in response to damage or me- chanical stress. Bony callus “splint” of spongy bone that replaces fibrocartilage callus during bone remodeling. Bony thorax bōn′e tho′raks bones of the thorax, including ribs, sternum, and thoracic vertebrae; also called thoracic cage. Brachial bra′ke-al pertaining to the arm. Bradycardia brad″e-kar′de-ah slow heart rate, usually defined as a rate under 60 beats per minute. Brain stem the portion of the brain consist- ing of the medulla, pons, and midbrain. Broad ligament large ligament formed by a fold of peritoneum that encloses the ovaries and holds them in place, and suspends the uterus from the pelvis. Bronchioles brong′ke-ōlz the smallest con- ducting passages in the lungs. Buccal buk′al pertaining to the cheek. Buffer a substance or substances that help to stabilize the pH of a solution. Bulbourethral glands tiny glands inferior to the prostate that produce clear mucus that neutralizes urine prior to ejaculation.

Astigmatism ah-stig′mah-tizm a visual defect resulting from irregularity in the lens or cornea of the eye causing the image to be out of focus. Astrocyte as′tro-sı̄t type of CNS supporting cell; assists in exchanges between blood capil- laries and neurons. Atherosclerosis a″ther-o″skler-o′sis changes in the walls of large arteries consisting of lipid deposits on the artery walls; the early stage of arteriosclerosis and increased rigidity. Atlas the first cervical vertebra; articulates with the occipital bone of the skull and the second cervical vertebra (axis). Atom at′um the smallest part of an element; indivisible by ordinary chemical means. Atomic mass number the sum of the num- ber of protons and neutrons in the nucleus of an atom; also known as mass number. Atomic number the number of protons in an atom. Atomic symbol a one-or two-letter symbol indicating a particular element. Atrioventricular (AV) node a″tre-o-ven- trik′u-lar a specialized mass of conducting cells located at the atrioventricular junction in the heart. Atrioventricular (AV) valves two valves set between the atria and ventricles to prevent backflow; includes the mitral (bicuspid) on the left and the tricuspid on the right. Atrium a′tre-um a chamber of the heart re- ceiving blood from the veins; superior heart chamber. Atrophy at′ro-fe a reduction in size or wast- ing away of an organ or cell resulting from disease or lack of use. Auricle the external part of the ear sur- rounding the auditory canal; also called pinna. Autonomic nervous system the division of the nervous system that functions involuntarily; innervates cardiac muscle, smooth muscle, and glands; also called involuntary nervous system. Axial skeleton ak′se-al the skull, vertebral column, ribs, and sternum. Axilla ak-sih′lah armpit; pertaining to axil- lary region. Axis (1) the second cervical vertebra; has a vertical projection called the dens around which the atlas rotates; (2) the imaginary line about which a joint or structure revolves. Axon ak′son neuron process that carries im- pulses away from the nerve cell body; efferent process; the conducting portion of a nerve cell. Axon terminal one of multiple ends of the axon that branch from the motor neuron axon; interacts with the sarcolemma of different muscle cells to form neuromuscular junctions. B cells lymphocytes that oversee humoral immunity; their descendants differentiate into antibody-producing plasma cells; also called B lymphocytes. Ball-and-socket joint multiaxial synovial joint in which the rounded head of one bone fits into a socket (depression) on the other. Basal metabolic rate (BMR) met″ah-bol′ik the rate at which energy is expended (heat produced) by the body per unit time under controlled (basal) conditions: 12 hours after a meal, at rest.

Glossary 619

Conducting zone structures all respiratory passages that carry air to the terminal bronchi- oles and beyond. Conduction deafness hearing loss due to interference in transmitting sound waves to the inner ear. Condylar joint kon′dı̆-ler biaxial synovial joint in which the convex surface of one bone fits into the concave surface of another. Cones one of the two types of photorecep- tor cells in the retina of the eye. Provides for color vision. Conjunctiva kon″junk-ti′vah the thin, pro- tective mucous membrane lining the eyelids and covering the anterior surface of the eyeball. Connective tissue a primary tissue; form and function vary extensively. Functions in- clude support, storage, and protection. Control center component of homeostatic control receiving sensory input and sending motor output. Contusion brain injury in which the damage cannot be reversed. Convergence kon-ver′jens turning toward a common point from different directions. Cornea kor′ne-ah the transparent anterior portion of the eyeball. Coronal suture interlocking joint formed at the intersection of the two parietal bones with the frontal bone; also called the frontal suture. Coronary arteries arteries that branch off the aorta and supply the myocardium. Coronary sinus enlarged vessel that re- ceives blood from the cardiac veins and emp- ties into the right atrium. Corpora quadrigemina cor′pora kwod″r ı̆- jem′in-uh four nuclei in the midbrain contain- ing visual and hearing reflex centers. Corpus callosum kor′pus kah-lo′sum large fiber tract of the cerebral white matter that connects the cerebral hemispheres. Corpus luteum kor′pus lu′te-um structure formed by the ruptured follicle cells after ovu- lation; secretes estrogen and progesterone. Cortex kor′teks the outer surface layer of an organ. Corticosteroids hormones secreted by the adrenal cortex, including mineralocorticoids, glucocorticoids, and sex hormones. Covalent bond ko-va′lent a bond involving the sharing of electrons between atoms. Coxal pertaining to the hip. Cranial nerves the 12 pairs of nerves that arise from the brain. Cranium part of the skull that encloses and protects the brain. Creatine phosphate (CP) kre′ah-tēn fos′fāt a compound that transfers a phosphate group to ADP to regenerate ATP in muscle fibers. Crista ampullaris receptors in the ampullae of the semicircular canals that detect dynamic equilibrium. Cross bridge the link formed when a myo- sin head binds to the corresponding binding site on actin during contraction. Crural pertaining to the anterior leg; shin. Cryptorchidism krip-tor′k̆ı-dizm a devel- opmental defect in which one or both testes fail to descend into the scrotum.

Cilia sil′e-ah tiny, hairlike projections on cell surfaces that move in a wavelike manner. Ciliary body sil′e-er-e smooth muscle of the vascular tunic of the eye that is connected to the lens by the ciliary zonule. Ciliary zonule suspensory ligament that at- taches the lens to the ciliary body in the an- terior eye. Citric acid cycle the aerobic pathway oc- curring within the mitochondria, in which energy is liberated during metabolism of car- bohydrates, fats, and amino acids and CO2 is produced. Clavicle klav′ ı̆-kl collar bone; part of the pectoral girdle that attaches medially to the manubrium and laterally to the scapula. Cleavage klēv′ij an early embryonic phase consisting of rapid cell divisions without inter- vening growth periods. Cleavage furrow indentation formed by a contractile ring of microfilaments; continues to contract until cytokinesis is complete and two daughter cells are formed. See cytokinesis. Clitoris kli′to-ris a small, erectile structure in the female, homologous to the penis in the male. Clonal selection klo′nul the process during which a B cell or T cell becomes sensitized through binding contact with an antigen. Clone descendants of a single cell. Coccyx kok′siks three to five fused bones forming the inferior section of the spine; tail- bone. Cochlea kōk′le-ah a cavity of the inner ear resembling a snail shell; houses the hearing receptor. Cochlear duct the location of the spiral organ of Corti in the cochlea; contains endo- lymph. Codon ko′don the three-base sequence on a messenger RNA molecule that provides the genetic information used in protein synthesis; codes for a given amino acid. Coenzymes nonprotein molecules, such as vitamins, that are necessary for the function of some enzymes. Colon the part of the large intestine between the cecum and the rectum. Common hepatic duct the duct that drains bile from the liver. Compact bone dense bone composed of osteons that makes up the outer layer of flat, short and irregular bones; major component of long bones. Complement a group of plasma proteins that normally circulate in inactive forms; when activated by complement fixation, causes ly- sis of foreign cells and enhances phagocytosis and inflammation. Compound substance composed of two or more different elements, the atoms of which are chemically united. Concentration gradient a difference in amount of a substance between two areas. Conchae kong′ke projections from the walls of the nasal cavity that increase air turbulence and the surface area of mucous membrane ex- posed to the air to aid in warming and moist- ening. Concussion brain injury with reversible damage.

Centromere sen′tro-mear button-like body holding sister chromatids together; also, the site of attachment to the mitotic spindle. Cephalic seh-fă′lik pertaining to the head. Cerebellum ser″e-bel′um part of the hind- brain; involved in producing smoothly coordi- nated skeletal muscle activity. Cerebral cortex sĕ-re′bral outer gray matter of the cerebrum. Cerebral white matter area of the cere- brum deep to the cerebral cortex containing fiber tracts carrying impulses to, from, or with- in the cortex. Cerebrospinal fluid (CSF) the fluid pro- duced by choroid plexuses; fills the ventricles and surrounds the central nervous system. Cerebrovascular accident (CVA) a con- dition in which brain tissue is deprived of a blood supply, as in blockage of a cerebral blood vessel; also called a stroke. Cerebrum sĕ-rē′brum the largest part of the brain; consists of right and left cerebral hemi- spheres. Cerumen sĕ-roo′men earwax. Cervical ser′v̆ı-kal term referring to the neck or the necklike portion of an organ or structure. Cervical cancer uncontrolled cellular growth in the cervix that is almost always as- sociated with one or more strains of human papilloma virus (HPV). Cervical vertebrae the seven vertebrae of the neck region. Cervix ser′viks the inferior necklike portion of the uterus leading to the vagina. Chemical bond an energy relationship holding atoms together; involves the interac- tion of electrons. Chemical reaction process in which mol- ecules are formed, changed, or broken down. Chemoreceptors ke″mo-re-sep′terz recep- tors sensitive to various chemicals in solution. Chief cells cells of the gastric glands that se- crete inactive pepsinogen. Cholesterol ko-les′ter-ol a steroid found in animal fats as well as in most body tissues; made by the liver. Chordae tendineae tendinous chords that anchor the flaps of the closed AV valves to pre- vent inversion and backflow of blood. Chorionic villi embryonic tissue projections that will combine with tissue of the uterine lin- ing to form the placenta. Choroid ko′roid the pigmented nutritive layer of the eye; part of the vascular layer. Choroid plexus structure in each of the four brain ventricles that produces cerebrospi- nal fluid (CSF). Chromatid kro′mah-tid one copy of the DNA after replication but before cell division; still paired with the other copy, together called sister chromatids, by a centromere. Chromatin kro′mah-tin the structures in the nucleus that carry the hereditary factors (genes). Chromosome kro′ma-sōm barlike body of tightly coiled chromatin; visible during cell division. Chyme kı̄m the semifluid stomach contents consisting of partially digested food and gas- tric secretions.

620 Glossary

Efferent arteriole arteriole leaving the glomerulus and connecting it to the peritubu- lar capillaries. Ejaculation e-jak″u-la′shun the sudden ejec- tion of semen from the penis. Electrical energy energy form resulting from the movement of charged particles. Electrocardiogram (ECG) e-lek″tro-kar′de- o-gram″ a graphic record of the electrical activ- ity of the heart. Electroencephalogram (EEG) (e-lek″tro-en- sef′ah-lo-gram) graphic record of the electrical activity of the brain. Electrolyte e-lek′tro-lı̄t a substance that breaks down into ions when in solution and is capa- ble of conducting an electric current. Electron (e−) negatively charged subatomic particle; orbits the atomic nucleus. Electron transport chain metabolic path- way within the mitochondria in which energy harvested from high-energy hydrogen atoms is used to make ATP. Final delivery of H to molecular oxygen produces water. Element el′ĕ-ment any of the building blocks of matter; oxygen, hydrogen, carbon, for example. Embolus a free-floating clot in an unbroken vessel. Embryo em′bre-o an organism in its early stages of development; in humans, the first 2 months after conception. Emmetropia the eye that focuses images correctly on the retina is said to have this “har- monious vision.” Endocardium en″do-kar′de-um the endo- thelial membrane lining the interior of the heart. Endocrine glands en′do-krin ductless glands that empty their hormonal products directly into the blood. Endocytosis en″do-si-to′sis means by which fairly large extracellular molecules or particle enter cells. See phagocytosis, pinocytosis, receptor-mediated endocytosis. Endolymph en′do-limf thick fluid inside the membranous labyrinth similar to perilymph. Endometrium en-do-me′tre-um the mu- cous membrane lining of the uterus. Endomysium en″do-mı̆s′e-um the thin con- nective tissue surrounding each muscle cell. Endoneurium a delicate connective tissue sheath surrounding each fiber in a nerve. Endoplasmic reticulum (ER) en″do-plas′mik rĕ-tik′u-lum a membranous network of tubular or saclike channels in the cytoplasm of a cell. Energy the ability to do work. Enteroendocrine cells cells of the gastric glands that secrete locally acting hormones, such as gastrin. Enzyme en′zı̄m a substance formed by liv- ing cells that acts as a catalyst in chemical reac- tions in the body; see catalyst. Eosinophils e″o-sin′o-filz granular white blood cells whose granules readily take up a stain called eosin. Ependymal cell ĕ-pen′dı̆-mal type of CNS supporting cell; lines the central cavities of the brain and spinal cord and circulates cerebro- spinal fluid with their beating cilia.

Diaphragm di′ah-fram (1) any partition or wall separating one area from another; (2) a muscle that separates the thoracic cavity from the abdominopelvic cavity. Diaphysis di-af′ ı̆-sis elongated shaft of a long bone. Diarthrosis di″ar-thro′sis a freely movable joint. Diastole di-as′to-le a period (between con- tractions) of relaxation of the heart during which it fills with blood. Diastolic pressure di″as-tŏ′lik the arterial blood pressure during ventricular relaxation (diastole). Diencephalon di″en-sef′ah-lon that part of the forebrain between the cerebral hemi- spheres and the midbrain including the thala- mus, the third ventricle, and the hypothala- mus; also called interbrain. Diffusion dı̆-fu′zhun the spreading of par- ticles in a gas or solution with a movement toward uniform distribution of particles; also called simple diffusion. Digestive system system that processes food into absorbable units and eliminates indi- gestible wastes. Digital pertaining to the digits; fingers, toes. Distal convoluted tubule (DCT) segment of kidney tubule between the ascending neph- ron loop and the collecting duct; site of some reabsorption and some secretion. Diverticulum di″ver-tik′u-lum a pouch or sac in the walls of a hollow organ or structure. Dorsal ramus branch of a spinal nerve that serves the skin and muscles of the posterior trunk. Dorsal root the root at which sensory neu- rons enter the spinal cord. Dorsal root ganglion the location of cell bodies of sensory neurons which enter the spinal cord via the dorsal root. Ductus arteriosus a shunt in the fetal heart that connects the aorta and pulmonary trunk in order to bypass the immature lungs. Ductus deferens duk′tus def ′er-enz segment of the male duct system that links the epididy- mis to the ejaculatory duct. Ductus venosus a shunt in the fetal heart that bypasses the immature liver. Duodenum doo″uh-de′num the first part of the small intestine. Dura mater du′rah ma′ter the outermost and toughest of the three membranes (menin- ges) covering the brain and spinal cord. Dynamic equilibrium sense that reports on angular or rotatory movements of the head in space. Dyspnea disp′ne-ah labored, difficult breathing. Eccrine gland ek′rin the more numerous type of sweat gland found all over the body, but par- ticularly abundant on the palms, soles of feet, and the forehead. Its secretions contain water, salts and metabolic wastes such as urea and uric acid. Edema ĕ-de′mah an abnormal accumula- tion of fluid in body parts or tissues; causes swelling. Effector ef-fek′tor an organ, gland, or muscle capable of being activated by nerve endings. Efferent ef′er-ent carrying away or away from.

Cupula ku′pu-lah a domelike structure; in the cristae of the inner ear, a gel-like cap cov- ering the “hairs” of receptor cells. Cushing’s syndrome koosh′ingz sin′drōm a disease produced by excess secretion of glu- cocorticoids (such as cortisol); characterized by adipose tissue accumulation, weight gain, and osteoporosis. Cutaneous membrane the skin; composed of epidermal and dermal layers. Cyanosis si″ah-no′sis a bluish coloration of the mucous membranes and skin caused by deficient oxygenation of the blood. Cystic duct duct that carries bile from the gallbladder. Cystitis sis-ti′tis an inflammation of the uri- nary bladder. Cytokines si′to-kı̄nz chemical messengers involved in immunity that enhance the im- mune and inflammatory responses. Cytokinesis si″to-k̆ı-ne′sis division of cyto- plasm that occurs after the cell nucleus has divided. Cytoplasm si′to-plazm″ the substance of a cell other than that of the nucleus. Cytoskeleton literally, cell skeleton; inter- nal framework of proteins that determine cell shape. See microtubules, intermediate fila- ments, and microfilaments. Cytosol si′to-zol the liquid component of cytoplasm containing water and solutes. Cytotoxic T cell effector T cell that directly kills foreign cells; also called a killer T cell. Dead space volume volume of air that never reaches the alveoli and does not participate in gas exchange. Decomposition reaction a destructive chemical reaction in which complex substances are broken down into simpler ones. Defecation reflex def″ih-ka′shun the elimi- nation of the contents of the bowels (feces). Deglutition de″gloo-tish′un the act of swal- lowing. Dehydration synthesis process by which a larger molecule is synthesized from smaller ones by removal of a water molecule at each site of bond formation. Dendrites den′drı̄tz the branching exten- sions of neurons that carry electrical signals to the cell body; the receptive portion of a nerve cell. Deoxyribonucleic acid (DNA) de-ok″s̆ı- ri″bo-nu-kle′ik nucleic acid found in all living cells: carries the organism’s hereditary infor- mation. Depolarization de-po″lar-i-za′shun the loss of a state of polarity; the loss of a negative charge inside the plasma membrane. Dermis der′mis the deep layer of the skin; composed of dense, irregular connective tissue. Diabetes mellitus di″ah-be′t ēz mel′ ı̆-tus a disease caused by deficient insulin release or inadequate responsiveness to insulin, leading to inability of the body cells to use carbohy- drates at a normal rate. Diapedesis di″ah-pĕ-de′sis the passage of blood cells through intact vessel walls into the tissues.

Glossary 621

Frontal (coronal) section (plane) a longi- tudinal plane that divides the body or an organ into anterior and posterior portions. Fused tetanus tet′ah-nus a completely smooth, sustained muscle contraction resulting from rapid stimulation with no time for relax- ation between stimulation events; also called complete tetanus. Gallstones particles of hardened cholesterol or calcium salts that are occasionally formed in the gallbladder and bile ducts. Gamete gam′ ēt male or female sex cell (sperm/egg). Ganglion gang′le-on a group of nerve cell bodies located in the peripheral nervous system. Gap junction a passageway between two neighboring cells; formed by transmembrane proteins called connexons. Gastric glands glands within the gastric pits of the stomach that secrete the components of gastric juice and include chief, parietal, mu- cous neck, and enteroendocrine cells. Gastric juice fluid formed by secretions of the gastric glands, characterized by extreme acidity and pepsin, a protein-digesting enzyme. Gastrin gas′trin a hormone that stimulates gastric secretion, especially release of hydro- chloric acid. Gastrointestinal (GI) tract continuous hol - low muscular tube including all structures from mouth to anus. Gene jēn biological units of heredity located in chromatin; transmits hereditary information. Gland an organ specialized to secrete or excrete substances for further use in the body or for elimination. Glaucoma glaw-ko′mah an abnormal in- crease of the pressure within the eye. Globular protein a protein whose functional structure is basically spherical. Also referred to as functional protein; includes hemoglobin, enzymes, and some hormones. Glomerular capsule glo-mer′u-ler double- walled cuplike end of a renal tubule; encloses a glomerulus; also called Bowman’s capsule. Glomerular filtration the process of forc- ing fluids and small particles out of the blood and into the glomerular capsule and renal tubule. Glomerulus glo-mer′u-lus a knot of coiled capillaries in the kidney; forms filtrate. Glottis glot′is the opening between the vo- cal cords in the larynx. Gluconeogenesis glu″ko-ne″o-jen′ĕ-sis the formation of new glucose molecules from noncarbohydrate substances, such as fats and proteins. Glucose gloo′kōs the principal sugar in the blood; a monosaccharide. Gluteal gloo′te-al pertaining to the buttock. Glycerol glis′er-ol a sugar alcohol; one of the building blocks of fats. Glycogen gli′ko-jen the main carbohydrate stored in animal cells; a polysaccharide. Glycogenesis gli″ko-jen′ĕ-sis formation of glycogen from glucose. Glycogenolysis gli″ko-jen-ol′ ı̆-sis breakdown of glycogen to glucose.

requiring a membrane protein to act as a car- rier or channel. Fallopian tube fal-lo′pe-an see uterine tube. Fascicle fas′ ı̆-kul a bundle of nerve or mus- cle fibers bound together by connective tissue. Feces fe′s ēz material discharged from the bowel composed of food residue, secretions, and bacteria. Femoral pertaining to the thigh. Fenestrated capillaries unique capillaries with large pores that allow extensive exchange. Fertilization fer′t̆ı-l̆ı-za′shun fusion of the nuclear material of an egg and a sperm. Fetus fe′tus the unborn young; in humans, the period from the third month of develop- ment until birth. Fibrillation f ı̆-br ı̆-la′shun irregular, unco- ordinated contraction of muscle cells, particu- larly of the heart musculature. Fibrin fi′brin the fibrous insoluble protein formed during the clotting of blood. Fibrocartilage callus repair tissue formed dur- ing bone remodeling; “splint” composed of carti- lage matrix, bony matrix, and collagen fibers. Fibrous joint bones joined by fibrous tis- sue; no joint cavity is present. Fibrous pericardium loose-fitting sac sur- rounding the heart that anchors the heart to surrounding structures. Fibrous protein a strandlike protein that appears most often in body structures; very important in binding structures together and for providing strength in certain body tissues; also called structural protein. Fibular pertaining to the area of the fibula, the lateral bone of the leg. Filtration fil-tra′shun the passive process of forcing water and solutes through a membrane using a fluid pressure gradient. Fimbriae fim′bre-e fingerlike projections that line the distal end of each uterine tube that create fluid currents to wave the oocyte inside after ovulation. Fissure fis′zher (1) a groove or cleft; (2) the deepest depressions or inward folds on the brain. Flaccid flă′sid soft; flabby; relaxed. Flagella flah-jel′ah long, whiplike exten- sions of the cell membrane of some bacteria and sperm; serve to propel the cell. Follicle fol′l̆ı-kul (1) structure in an ovary consisting of a developing egg surrounded by follicle cells; (2) colloid-containing structure in the thyroid gland; (3) collection of lympho- cytes in a lymph node. Follicle cells cells that surround and help nourish a developing oocyte. Fontanels fon″tah-nelz′ the fibrous mem- branes in the skull where bone has not yet formed; babies’ “soft spots.” Foramen fo-ra′men a hole or opening in a bone or between body cavities. Formed elements cellular portion of blood. Fovea centralis fo′ve-ah sen-tră′lis pit con- taining only cones lateral to the blind spot of the eye. Frequency (1) the number of sound waves per unit time, measured in Hertz; (2) the need to urinate often.

Epicardium visceral layer of a serous mem- brane that tightly hugs the external surface of the heart and is actually part of the heart wall; also called visceral pericardium. Epidermis ep″ ı̆-der′mis the outer layers of the skin; an epithelium. Epididymis ep″ ı̆-did′ ı̆-mis that portion of the male duct system in which sperm mature. Emp- ties into the ductus deferens (vas deferens). Epiglottis ep″ ı̆-glot′is the elastic cartilage at the back of the throat; covers the glottis during swallowing. Epimysium ep″ ı̆-mis′e-um the sheath of fibrous connective tissue surrounding a muscle. Epineurium a tough, fibrous sheath that binds together the fascicles in a nerve. Epiphyseal line calcified line in the epiphy- sis of a long bone that contained hyaline car- tilage during the period of long bone growth, which ends when calcification is complete. Epiphyseal plate plate of hyaline cartilage in the epiphysis of a long bone that allows for growth in length. Epiphysis ĕ-pif′ ı̆-sis the end of a long bone. Epithalamus roof of the third ventricle; composed of the pineal gland and the choroid plexus of the third ventricle. Epithelial membranes membranes com- posed of epithelial tissue with an underlying layer of connective tissue; covering and lining membranes. See mucous membrane and se- rous membrane. Epithelium ep″ ı̆-the′le-um one of the pri- mary tissues; covers the surface of the body and lines the body cavities, ducts, and vessels. Erection enlargement and increased rigidity of the penis resulting when erectile tissue fills with blood. Erythrocytes ĕ-rith′ro-sı̄ts red blood cells. Erythropoietin hormone that stimulates erythrocyte production; secreted by kidneys. Esophagus muscular tube that carries food from the pharynx to the stomach. Estrogens es′tro-jenz hormones that stimu- late female secondary sex characteristics; fe- male sex hormones. Eupnea ūp-ne′ah easy, normal breathing. Exchange reaction a chemical reaction in which bonds are both made and broken; at- oms become combined with different atoms. Excretion ek-skre′shun the elimination of waste products from the body. Exocrine glands ek′so-krin glands that have ducts through which their secretions are car- ried to a body surface (skin or mucosa). Exocytosis ek″so-si-to′sis method for the movement of substances from the cell interior to the extracellular space as a secretory vesicle fuses with the plasma membrane. Expiration eks″p ı̆-ra′shun the act of expel- ling air from the lungs; exhalation. Extracellular fluid (ECF) fluid within the body but outside the cells; see interstitial fluid. Extracellular matrix nonliving material in connective tissue consisting of ground sub- stance and fibers that separate the living cells. Facilitated diffusion passive transport pro- cess driven by a concentration gradient and

622 Glossary

thyroid hormone, causing an increased meta- bolic rate. Hypertonic hi″per-ton′ik excessive, above normal, tone or tension. Hypertrophy hi-per′tro-fe an increase in the size of a tissue or organ independent of the body’s general growth. Hyperventilation pattern of breathing in which excess carbon dioxide is exhaled in an effort to raise blood pH to its normal range. Hypodermis adipose tissue beneath the skin. See subcutaneous. Hypoglycemia low blood sugar. Hypospadias hi″pŏ-spa′de-as condition oc- curring in male babies only where the urethral opening is located on the ventral surface of the penis. Hypothalamus hi″po-thal′ah-mus the region of the diencephalon forming the floor of the third ventricle of the brain. Hypothyroidism decreased thyroid activity, resulting in a slow metabolic rate. Hypotonic hi″po-ton′ik below normal tone or tension. Hypoxia hi-pok′se-ah a condition in which inadequate oxygen is available to tissues. Ileum il′e-um the terminal part of the small intestine; between the jejunum and the cecum of the large intestine. Ilium il′e-um large flaring bone of the hip; the most superior bone of the pelvis. Immune response antigen-specific defenses mounted by activated lymphocytes (T cells and B cells). Immune system the body system respon- sible for protecting the body from pathogens and foreign cells; involves both humoral and cell-mediated defenses. Immunodeficiency im″mu-no-de-fish′en-se disease resulting from the deficient production or function of immune cells or certain mol- ecules (complement, antibodies, and so on) required for normal immunity. Immunoglobulin (Ig) im″mu-no-glob′u-lin a protein molecule, released by plasma cells, that mediates humoral immunity; an antibody. Inclusions in-klu′zhun chemical substances stored in the cytoplasm of a cell. Incontinence in-kon′t̆ı-nens the inability to control the external urethral sphincter. Infarct in′farkt a region of dead, deterio- rating tissue resulting from a lack of blood supply. Inferior vena cava large vein that carries blood from veins inferior to the diaphragm back to the right atrium. Inflammatory response in-flam′ah-tor″e a physiological response of the body to tissue injury; includes dilation of blood vessels and increased blood vessel permeability. Inguinal in′gwı̆-nal pertaining to the groin region. Innate defense system natural defenses that protect against all foreign material, not specifically for any one antigen; also called nonspecific defense system. Inorganic compound a compound that lacks carbon; for example, water.

Hiatal hernia protrusion of the superior part of the stomach above the diaphragm, resulting in entry of gastric juice into the esophagus. High-density lipoproteins proteins that carry cholesterol from the tissues to the liver for destruction. Hilum hi′lum a depressed area where ves- sels enter and leave an organ. Histamine his′tah-mēn a substance that causes vasodilation and increased vascular permeability. Homeostasis ho″me-o-sta′sis a state of dy- namic body equilibrium or stable internal en- vironment of the body. Homeostatic imbalance a disturbance or change in homeostasis that effects function. Hormones hor′mōnz chemical messengers secreted by endocrine glands; responsible for specific regulatory effects on certain cells, tis- sues, or organs. Human chorionic gonadotropin (hCG) an LH-like hormone produced by the developing embryo that tells the corpus luteum to con- tinue producing hormones to prevent menses. Humerus long bone of the upper arm; ar- ticulates with the scapula to form the shoulder joint and the radius and ulna to form the el- bow joint. Humoral immunity hu′mor-al immunity provided by antibodies released by sensitized B cells and their plasma cell progeny. Also called antibody-mediated immunity. Hyaline cartilage cartilage connective tis- sue containing collagen fibers; has glassy ap- pearance; found on ends of long bones and forms C-shaped rings in trachea. Hydrocephalus a condition in which cere- brospinal fluid accumulates and puts pressure on the brain of an adult; in an infant, the skull may expand because the skull bones are not yet fused. Hydrogen bond weak bond in which a hydrogen atom forms a bridge between two electron-hungry atoms. An important intramo- lecular bond. Hydrolysis hi-drol′ ı̆-sis the process in which water is used to split a substance into smaller particles. Hydrophilic hi″dro-fil′ik refers to mole- cules, or portions of molecules, that interact with water and charged particles. Hydrophobic hi″dro-fo′bik refers to mol- ecules, or portions of molecules, that interact only with nonpolar molecules. Hyoid bone hi′oid neck bone suspended by ligaments; does not articulate with any other bone. Hyperglycemia hi″per-gli-se′me-ah high blood sugar. Hyperopia hi″per-o′pe-ah farsightedness; dis- tant objects are focused. Hyperpnea hy-perp′ne-ah pattern of breath- ing faster and deeper during exercise. Hypersensitivities allergies; overreaction of the immune system to a perceived threat, re- sulting in tissue damage. Hyperthyroidism overactivity of the thy- roid, resulting in higher-than-normal levels of

Glycolysis gli-kol′ ı̆-sis breakdown of glu- cose to pyruvic acid; an anaerobic process. Goblet cells individual cells (simple glands) that produce mucus. Goiter goy′ter a benign enlargement of the thyroid gland. Golgi apparatus gol′je ap″uh-rat′is mem- brane-bound organelle responsible for mod- ifying, packaging, and shipping proteins produced by rough ER to the appropriate destination. Gonads go′nadz organs producing gametes; ovaries or testes. Gout gouty arthritis; caused by uric acid buildup in joints. See arthritis. Graafian follicle graf′e-an fol′l̆ı-kul see ve- sicular follicle. Graded potential a local change in mem- brane potential that varies directly with the strength of the stimulus; declines with dis- tance. Graded response a response that varies di- rectly with the strength of the stimulus. Gray matter the gray area of the central nervous system; contains unmyelinated nerve fibers and nerve cell bodies. Gum the tissue that surrounds the teeth like a tight collar; also called gingiva (jin′j ı̆-vah). Gyrus ji′rus an outward fold of the surface of the cerebral cortex. Hair flexible epithelial structure composed of keratinized cells; produced by hair follicle. Hair follicle Structure with inner root sheath of epithelial tissue that produces the hair; the outer root sheath is dermal connec- tive tissue that provides the blood supply to epithelial cells. Hard palate roof of mouth formed by the fusion of the two maxillae bones. Haustra pocketlike sacs caused by tone of the teniae coli (longitudinal muscle bands) of the colon. Haversian system hă-ver′zhen see osteon. Heartburn pain in the esophagus due to re- flux of gastric juice. Helper T cell the type of T lymphocyte that orchestrates cellular immunity by direct contact with other immune cells and by releasing chem- icals called cytokines; also helps to mediate the humoral response by interacting with B cells. Hematocrit he-mat′o-krit the percentage of erythrocytes to total blood volume. Hematoma blood-filled swelling; bruise. Hematopoiesis hem″ah-to-po-e′sis formation of blood cells. Hemocytoblasts he″mo-si′to-blastz stem cells that give rise to all the formed elements of the blood. Hemoglobin (Hb) he″mo-glo′bin the oxygen- transporting pigment of erythrocytes. Hemolysis he-mol′ ı̆-sis the rupture of eryth- rocytes. Hemostasis the stoppage of bleeding. Hepatic portal vein single vein that drains the digestive tract organs and carries nutrient- rich blood to the liver before it enters general circulation.

Glossary 623

Lordosis lor-do′sis the abnormal curvature of the lumbar spine in an anterior direction. Low-density lipoproteins proteins that carry cholesterol to the tissues. Lumbar lum′bar the portion of the back be- tween the thorax and the pelvis; the loin. Lumen lu′men the space inside a tube, blood vessel, or hollow organ. Lungs organs of the respiratory system re- sponsible for air exchange with the outside and gas exchange with the blood. Luteinizing hormone (LH) gonadotropic hormone released by the anterior pituitary gland. Lymph limf the watery fluid in the lymph vessels collected from the tissue spaces. Lymphatic system lim-fat′ik the lymphatic vessels and the lymphoid tissues and organs, including lymph nodes. Lymphocytes lim′fo-sı̄tz agranular white blood cells formed in the bone marrow that mature in the lymphoid tissue. Lymphoid organs organs in the lymphatic system, including lymph nodes, spleen, and tonsils; see lymphatic system. Lysosomes li′so-sōmz organelles that origi- nate from the Golgi apparatus and contain strong digestive enzymes. Lysozyme li′so-zı̄m an enzyme found in sweat, saliva, and tears that is capable of de- stroying certain kinds of bacteria. Macrophage mak′ro-fāj″ cell particularly abun- dant in lymphatic and connective tissues; impor- tant in the immune response as an antigen presenter to T cells and B cells. Maculae mak′u-le static equilibrium recep- tors found in the vestibule. Main (primary) bronchi divisions of the trachea that lead into the right and left lungs. Major nutrients nutrients the body requires in significant amounts; include carbohydrates, proteins, and fats. Malignant mah-lig′nant life-threatening; per- tains to neoplasms that spread and lead to death, such as cancer. Malleus mă′le-us lateral bone of the middle ear; also called the hammer. Mammary glands mam′mer-e milk-produc- ing glands of the breasts. Manubrium mah-nu′bre-um the most su- perior of the three bones of the sternum. Mass movements long, powerful, slow- moving contractions of the large intestine that move materials toward the rectum. Mastication mas″t̆ı-ka′shun the act of chewing. Matter anything that occupies space and has mass. Maxillary bones mak-sil′uh-re paired facial bones that fuse to form the upper jaw and ar- ticulate with all other facial bones except the mandible; also called maxillae. Mechanical energy energy form directly in- volved in putting matter into motion. Mechanoreceptors mek″ah-no-re-sep′terz receptors sensitive to mechanical pressures such as touch, sound, or contractions.

Isotonic contraction i″so-ton′ik refers to “same tone”; the muscle shortens as it con- tracts, and movement occurs. Isotope i′s ı̆-tōp different atomic form of the same element. Isotopes vary only in the num- ber of neutrons they contain. Jaundice jawn′dis an accumulation of bile pigments in the blood producing a yellow color of the skin. Jejunum jĕ-joo′num the part of the small in- testine between the duodenum and the ileum. Joint the junction of two or more bones; an articulation. Juxtamedullary nephrons nephrons that sit close to the cortex-medulla junction, with neph- ron loops that dip deep into the renal medulla. Keratin ker′ah-tin a tough, insoluble pro- tein found in tissues such as hair, nails, and epidermis of the skin. Ketoacidosis low blood pH due to the me- tabolism of fats to produce ATP in the absence of sufficient glucose. Kilocalories (kcal) unit used to measure the energy value of food. Kinetic energy energy of motion. Kinins ki′ninz group of polypeptides that dilate arterioles, increase vascular permeabil- ity, and induce pain. Kyphosis ki-fo′sis hump formed by the ab- normal curvature of the thoracic spine in a posterior direction. Labia la′be-ah lips. Lacrimal lak′r̆ı-mal pertaining to tears. Lactation lak-ta′shun the production and secretion of milk. Lacteal lak′te-al special lymphatic capillar- ies of the small intestine that take up lipids. Lactic acid lak′tik the product of anaerobic metabolism, especially in muscle. Lacuna lah-ku′nah a little depression or space; in bone or cartilage, lacunae are occu- pied by cells. Lambdoid suture interlocking joint between the occipital bone and the parietal bones. Lamella concentric rings of bone matrix with lacuna between them. Lamina lam′ ı̆-nah (1) a thin layer or flat plate; (2) the portion of a vertebra between the transverse process and the spinous process. Larynx lar′inks the cartilaginous organ lo- cated between the trachea and the pharynx; voice box. Leukemia lu-ke′me-ah a cancerous condi- tion in which there is an excessive production of immature leukocytes. Leukocyte lu′ko-sı̄t white blood cell. Ligament lig′ah-ment a cord of fibrous tis- sue that connects bones. Lingual tonsils pair of tonsils at the base of the tongue. Lipid lip′id organic compound formed of carbon, hydrogen, and oxygen; examples are neutral fats and cholesterol. Liver gland of the digestive tract that pro- duces bile and is the major metabolic gland of the body. Long bone a bone that is longer than it is wide. See humerus and femur.

Insertion in-ser′shun the movable attach- ment of a muscle as opposed to its origin. Inspiration in″sp ı̆-ra′shun the drawing of air into the lungs; inhalation. Inspiratory reserve volume (IRV) amount of air that can be forcibly inhaled in addition to tidal volume. Insulin pancreatic hormone that decreases blood glucose. Integration the process in which the nervous system processes and interprets sensory input and makes decisions about what should be done. Integumentary system in-teg″u-men′tuh-re the skin and its accessory organs (jin′j ı̆-vah). Intercalated discs in-ter′kah-la′ted special- ized connections for communication between cardiac muscle cells containing gap junctions and desmosomes. Intermediate filaments one of three types of rods in the cytoskeleton of a cell; resists pulling forces on the cell. Interneuron completes the pathway be- tween afferent and efferent neurons; also called an association neuron. Interosseous membrane flexible fibrous connective tissue attachment connects the ra- dius and ulna, and the tibia and fibula along their lengths. Interphase the period in a cell’s life cycle when it performs its usual metabolic activities and prepares for cell division by copying the DNA (genes). Interstitial cells in″ter-stish′al cells in the testes between the seminiferous tubules that produce testosterone. Interstitial fluid (tissue fluid) in″ter- stish′al the fluid between the cells; also called extracellular fluid. Intervertebral discs in″ter-ver′tĕ-bral the discs of fibrocartilage between the vertebrae. Intrapleural pressure pressure in the space between the pleurae; always negative com- pared to intrapulmonary pressure. Intrinsic conduction system control system built into the heart that sets the rate of contrac- tion by stimulating the myocardium in an atrial to ventricular direction; also called nodal system. Intrinsic factor in-trin′sik a substance pro- duced by the stomach that is required for vita- min B12 absorption. Ion i′on an atom with a positive or negative electric charge. Ionic bond bond formed by the complete transfer of electron(s) from one atom to another (or others). The resulting charged atoms, or ions, are oppositely charged and attract each other. Iris i′ris the pigmented, involuntary muscle that acts as the diaphragm of the eye; part of the vascular tunic of the eye. Irritability ir″ ı̆-tah-bil′ ı̆-te ability to respond to a stimulus. Ischemia is-ke′me-ah a local decrease in blood supply. Ischium is′ke-um inferior bone of the hip; “sit bone.” Isometric contraction i″so-met′rik refers to “same length”; the muscle generates tension but does not shorten, and no movement occurs.

624 Glossary

Muscularis externa smooth muscle layer of the gastrointestinal tract composed of an inner circular layer and an outer longitudinal layer. Myasthenia gravis mi″as-the′ne-ah gră′vis an autoimmune disease affecting muscles; characterized by weakness and fatigue due to a shortage of acetylcholine receptors. Myelin mi′ĕ-lin a white, fatty lipid substance. Myocardial infarction mi″o-kar′de-al in- fark′shun a condition characterized by dead tissue areas in the myocardium caused by in- terruption of blood supply to the area. Myocardium mi″o-kar′de-um the cardiac muscle layer of the heart wall. Myofibrils mi″o-fi′brilz contractile organ- elles found in the cytoplasm of muscle cells. Myofilament mi″o-fil′ah-ment filaments com- posing the myofibrils. Of two types: actin and myosin. Myometrium mi″o-me′tre-um the thick uter- ine musculature. Myopia mi-o′pe-ah nearsightedness; near ob- jects are in focus. Myosin mi′o-sin one of the principal con- tractile proteins found in muscle; makes up the thick filaments. Nasal pertaining to the nose. Nasal septum tissue at midline that sepa- rates the nasal cavity into right and left sides. Natural killer (NK) cells unique lympho- cytes that are part of the nonspecific defenses; kill cells that do not exhibit self markers. Neck (1) portion of the body connecting the head to the thorax that contains the trachea and esophagus; (2) portion of a tooth connect- ing the crown and root. Negative feedback feedback that causes the stimulus to decline or end. Nephron nef′ron structural and functional unit of the kidney. Nephron loop portion of the renal tubule between the proximal and distal convoluted tubules; also called loop of Henle. Nerve bundle of neuronal processes (axons) outside the central nervous system. Nerve impulse a self-propagating wave of depolarization; also called an action potential. Nervous system fast-acting control system that employs nerve impulses to trigger muscle contraction or gland secretion. Nervous tissue one of four main tissue types; specialized for irritability (respond to stimuli) and conductivity. Neuroglia nu-rōg′le-ah the nonneuronal tis- sue of the central nervous system that performs supportive and other functions; also called glial cells or glia. Neuromuscular junction nu″ro-mus′ku- lar the region where a motor neuron comes into close contact with a skeletal muscle cell. Neurons nu′ronz cells of the nervous sys- tem specialized to transmit messages through- out the body. Neurotransmitter chemical released by neu- rons that may, upon binding to receptors of neu- rons or effector cells, stimulate or inhibit them. Neutral fats dietary fats; also called triglyc- erides.

determine the cell shape and the distribution of cellular organelles. Microvilli mi″kro-vil′i the tiny projections on the free surfaces of some epithelial cells; increase surface area for absorption. Micturition mik″tu-rish′un urination, or void- ing; emptying the bladder. Midbrain part of the brain stem containing reflex centers for vision and hearing, and fiber tracts (cerebral peduncles) that convey both ascending and descending impulses. Minor nutrients vitamins and minerals; nu- trients the body needs in small amounts. Miscarriage termination of a pregnancy be- fore the embryo or fetus is viable outside the uterus. Mitochondria mi″to-kon′dre-ah the rod-like cytoplasmic organelles responsible for ATP generation. Mitosis mi-to′sis the division of the cell nucleus; often followed by division of the cytoplasm of a cell. Mixed nerves nerves containing the process- es of motor and sensory neurons; their impulses travel to and from the central nervous system. Molecule mol′ĕ-kyool particle consisting of two or more atoms held together by chemical bonds. Monoclonal antibodies mon″o-klōn′ul pure preparations of identical antibodies that ex- hibit specificity for a single antigen. Monocyte mon′o-sı̄t large single-nucleus white blood cell; agranular leukocyte. Monosaccharide mon″o-sak′ ı̆-rı̄d literally, one sugar; the building block of carbohydrates; examples include glucose and fructose. Motor division the part of the PNS that sends motor signals from the CNS to muscles or glands; also called efferent division. Motor unit a motor neuron and all the mus- cle cells it supplies. Mucosa innermost layer of the gastrointesti- nal tract; see mucous membrane. Mucous membrane (mucosa) membrane that forms the linings of body cavities open to the exterior (digestive, respiratory, urinary, and reproductive tracts). Multiple sclerosis (MS) autoimmune disor- der of the CNS where the myelin sheaths are converted into hardened scleroses in the brain and spinal cord; results in loss of control. Multipolar neuron structural class of neu- ron with more than two processes extending from the cell body. Muscle fatigue inability of a muscle to con- tract even while being stimulated. Muscle tissue one of four main tissue types; specialized to contract (shorten) in order to produce a force that will cause movement. Muscle tone sustained partial contraction of a muscle in response to stretch receptor in- puts; keeps the muscle healthy and ready to react. Muscle twitch a single rapid contraction of a muscle followed by relaxation. Muscular system organ system consisting of skeletal muscles and their connective tissue attachments.

Median (midsagittal) section specific sag- ittal plane that lies exactly in the midline. Mediastinum me″de-as-ti′num the region of the thoracic cavity between the lungs that houses the heart. Medulla mĕ-dul′ah the central portion of certain organs. Medulla oblongata most inferior part of brainstem involved in visceral control. Medullary cavity central cavity in the di- aphysis of a long bone containing marrow. Megakaryocytes large, multinucleate cells that fragment to produce platelets. Meiosis mi-o′sis the two successive cell di- visions in gamete formation producing nuclei with half the full number of chromosomes (haploid). Melanin mel′ah-nin the dark pigment syn- thesized by melanocytes; responsible for skin color. Membranous urethra the part of the male urethra that connects the prostatic urethra to the penis. Memory cell member of T cell and B cell clones that provides for immunological memory. Menarche mĕ-nar′ke establishment of men- strual function; the first menstrual period. Meninges mĕ-nin′jēz the membranes that cover the brain and spinal cord. Menopause men′o-pawz the physiological end of menstrual cycles. Menstrual cycle the periodic, cyclic dis- charge of blood, secretions, tissue, and mucus from the mature female uterus in the absence of pregnancy; a series of cyclic changes of the endometrium in response to changing ovar- ian hormone levels; also called menstruation, uterine cycle. Mental pertaining to the chin. Merkel cell cells associated with nerve end- ings that serve as touch receptors. Mesentery mes′en-ter″e the double-layered membrane of the peritoneum that supports most organs in the abdominal cavity. Messenger RNA (mRNA) long nucleotide strands that reflect the exact nucleotide se- quence of the DNA gene; carry this informa- tion to the ribosome for protein synthesis. Metabolism mĕ-tab′o-lizm the sum total of the chemical reactions that occur in the body. Metacarpal met″ah-kar′pal one of the five bones of the palm of the hand. Metastasis mĕ-tas′tah-sis the spread of can- cer from one body part or organ into another not directly connected to it. Metatarsal met″ah-tar′sal one of the five bones between the tarsus and the phalanges of the foot. Microfilaments mi″kro-fil′ah-ments thin strands of the contractile protein actin; one of three types of rods in the cytoskeleton of a cell. Microglia mi-krog′le-ah type of CNS sup- porting cell; phagocytes that ingest debris such as bacteria or dead cells. Microtubules mi″kro-too′bulz one of three types of rods in the cytoskeleton of a cell; hollow tubes made of spherical protein that

Glossary 625

that are lined with mucous membrane; help lighten the skull. Parasympathetic division par″ah-sim′pah- thet′ik a division of the autonomic nervous sys- tem; also referred to as the craniosacral division. Parathyroid glands par″ah-thi′roid small endocrine glands located on the posterior as- pect of the thyroid gland. Parathyroid hormone (PTH) hormone re- leased by the parathyroid glands that regulates blood calcium level. Parietal cells cells of the gastric glands that produce hydrochloric acid (HCl). Parietal pericardium outermost layer of the serous pericardium that lines the interior surface of the fibrous pericardium. Parietal peritoneum lining of the abdomi- nopelvic cavity that is continuous with the serosa, or visceral peritoneum of the gastro- intestinal tract. Parietal pleura ploor′ah serous membrane layer covering the cavity in which the lung resides. Parotid glands large salivary glands ante- rior to the ears. Passive immunity short-lived immunity re- sulting from the introduction of “donated an- tibodies” obtained from an immune animal or human donor; immunological memory is not established. Pathogen disease-causing microorganism (some bacteria, fungi, viruses, etc.). Pectoral pek′to-ral pertaining to the chest. Pectoral girdle composite of two bones, scapula and clavicle, that attach the upper limb to the axial skeleton; also called the shoulder girdle. Pedicle bony extension connecting the cen- trum and transverse process of a vertebra. Pelvic pertaining to the anterior area overly- ing the pelvis. Pelvic girdle incomplete bony basin formed by the two coxal bones and the sacrum. Penis pe′nis the male organ of copulation and urination. Pepsin an enzyme capable of digesting pro- teins in an acid pH. Perforating canal canal running at right angles to the central canal in an osteon. Also called Volkmann’s canal. Perforating fibers connective tissue fibers that secure the periosteum to the bone; also called Sharpey’s fibers. Pericardium per″ ı̆-kar′de-um the membra- nous sac enveloping the heart. Perilymph per′ ı̆-limf plasmalike fluid filling the bony labyrinth. Perimysium per″ ı̆-mis′e-um the connective tissue enveloping bundles of muscle fibers. Perineum per″i-ne′um that region of the body extending from the anus to the scrotum in males and from the anus to the vulva in females. Perineurium coarse connective tissue wrap- ping that binds groups of fibers in a nerve, forming fascicles, or fiber bundles. Periosteum per-e-os′te-um double-layered connective tissue membrane that covers and nourishes the bone.

Ossicles os′sı̆-kulz the three bones of the middle ear: hammer (malleus), anvil (incus), and stirrup (stapes); also called the auditory ossicles. Ossification the process of bone formation. Osteoarthritis (OA) most common form of ar- thritis; caused by wear and tear at joints that leads to articular cartilage breakdown. See arthritis. Osteoblasts os′te-o-blasts″ bone-forming cells. Osteoclasts os′te-o-klasts″ large cells that re- sorb or break down bone matrix. Osteocyte os′te-o-sı̄t″ a mature bone cell. Osteon os′te-on the structural and functional unit of compact bone; Haversian system. Osteoporosis os″te-o-po-ro′sis an increased softening of the bone resulting from a gradual decrease in rate of bone formation; a common condition in older people. Otolith o′to-lith one of the small calcified masses in the maculae of the vestibular ap- paratus in the inner ear. Otolithic membrane gelatinous membrane in a macula with receptor “hairs” embedded in it and studded with otoliths. Otosclerosis o″to-sklĕ-ro′sis fusion of the ossicles (bones) in the middle ear; results in conduction deafness. Oval window the superior, membrane-covered opening in the middle ear wall; sound is con- veyed to the oval window by the stirrup (stapes). Ovarian cycle o-va′re-an the monthly cycle of follicle development, ovulation, and corpus luteum formation in an ovary. Ovarian follicles saclike structures consist- ing of follicle cells and a developing oocyte. Ovarian ligaments bilateral ligaments that anchor the ovaries to the uterus. Ovary o′var-e the female sex organ in which ova (eggs) are produced. Ovulation ov″u-la′shun the release of an ovum (or oocyte) from the ovary. Ovum o′vum the female gamete (germ cell); an egg. Oxygen deficit ok′s ı̆-jen the volume of oxy- gen required after exercise to oxidize the lactic acid formed during exercise. Oxyhemoglobin ok″se-he′mo-glo″bin he- moglobin combined with oxygen. P wave wave on an ECG representing depo- larization of the atria. Pacemaker see sinoatrial node. Palate pal′et roof of the mouth. Palatine tonsils pair of tonsils flanking the oropharynx. Pancreas pan′kre-as gland posterior to the stomach, between the spleen and the duode- num; produces both endocrine and exocrine secretions. Pancreatic juice pan″kre-at′ik a secretion of the pancreas containing enzymes for diges- tion of all food categories. Papilla pah-pil′ah small nipplelike projection. Papillary layer superficial layer of dermis bordering epidermis; contains dermal papil- lae, the cause of fingerprints. See dermal papillae. Paranasal sinuses air-filled spaces within the maxillae on either side of the nasal cavity

Neutralization nu″tral-i-za′shun (1) a chem- ical reaction that occurs between an acid and a base; (2) blockage of the harmful effects of bacterial exotoxins or viruses by the binding of antibodies to their functional sites. Neutron (n0) nu′tron uncharged subatomic particle; found in the atomic nucleus. Neutrophils nu′tro-filz the most abundant of the white blood cells. Nose the only external organ of the respira- tory system. Nuclear envelope the double membrane barrier of the nucleus; separates the nucleo- plasm from the cytoplasm. Nucleic acid nu-kle′ik class of organic mol- ecules that includes DNA and RNA. Nucleoli nu-kle′o-li small spherical bodies in the cell nucleus; function in ribosome synthesis. Nucleotide nu′kle-o-tı̄d building block of nucleic acids. Nucleus nu′kle-us (1) a dense central body in most cells containing the genetic material of the cell; (2) cluster of neuronal cell bodies in the brain or spinal cord. Nutrient chemical substance used by the body for normal growth, maintenance, and repair. Occipital ok-sip′ ı̆-tal pertaining to area at the back of the head. Olecranal ol-eh-kra′nel pertaining to the posterior surface of the elbow. Oligodendrocyte ol′ ı̆-go-den′dro-sı̄t type of CNS supporting cell that has many cellular processes, each of which makes up a myelin sheath. Oocyte o′o-sı̄t an immature egg. Oogenesis o″o-jen′ĕ-sis the process of for- mation of the ova. Optic chiasma op′tik ki-as′mah the partial crossover of fibers of the optic nerves. Optic disc site where the optic nerve meets the retina; causes the blind spot. Oral cavity the mouth; contains teeth and the tongue. Orbital eye area. Organ a part of the body formed of two or more tissues that performs a specialized function. Organ system a group of organs that work together to perform a vital body function; e.g., nervous system. Organelles or″gah-nelz′ specialized struc- tures in a cell that perform specific metabolic functions. Organic compound a compound contain- ing carbon; examples include proteins, carbo- hydrates, and fats. Organism an individual living thing. Origin the muscle attachment that is not movable or is less movable than the insertion. Orthostatic hypotension temporary hypo- tension due to abrupt movement from a sitting or reclining position to an upright position. Osmoreceptor oz″mo-re-sep′tor a structure sensitive to osmotic pressure or concentration of a solution. Osmosis oz-mo′sis the diffusion of water (a solvent) through a membrane from a dilute so- lution into a more concentrated one.

626 Glossary

of uterine muscle, and helping to prepare the breasts for lactation. Proprioceptor pro″pre-o-sep′tor a receptor located in a muscle or tendon; concerned with locomotion, posture, and muscle tone. Prostate accessory organ of the male repro- ductive tract that produces a milky fluid that contributes to semen by activating sperm. Prostatic urethra the segment of the ure- thra that passes through the prostate. Protein pro′tēn a complex nitrogenous sub- stance; the main building material of cells. Proton (p+) pro′ton subatomic particle that bears a positive charge; located in the atomic nucleus. Proximal convoluted tubule (PCT) seg- ment of the renal tubule between the glomeru- lar capsule and the descending nephron loop; site of most reabsorption and some secretion. Puberty pu′ber-te the period at which re- productive organs become functional. Pubic pertaining to the genital region. Pubic symphysis cartilaginous joint formed at the anterior of the two hip bones where the pubis bones articulate. Pubis pu′bis most anterior bone of the hip; the pubis from each hip bone meets to articu- late at the pubic symphysis joint anteriorly. See pubic symphysis. Pulmonary circulation system of blood vessels that carry blood to and from the lungs for gas exchange. Pulmonary edema ĕ-de′mah a leakage of fluid into the air sacs and tissue of the lungs. Pulmonary pleura ploor′ah serous mem- brane layer covering the surface of each lung; also called visceral pleura. Pulmonary trunk large artery connecting the right ventricle with the pulmonary arteries. Pulp the part of the tooth that provides nu- trients and sensation; composed of blood ves- sels, connective tissue, and nerves. Pulse the rhythmic expansion and recoil of arteries resulting from heart contraction; can be felt from the outside of the body. Pupil an opening in the center of the iris through which light enters the eye. Purkinje fibers pur-kin′je the modified car- diac muscle fibers of the conduction system of the heart that carry impulses to the myocardium. Pus the fluid product of inflammation, com- posed of white blood cells, the debris of dead cells, and a thin fluid. Pyelonephritis pi″ĕ-lo-nĕ-fri′tis an inflam- mation of the kidney pelvis and surrounding kidney tissues. Pyloric valve valve formed by the pyloric sphincter between the stomach and small intestine. Pyramidal tracts major motor pathways concerned with voluntary movement; connect primary motor area in the frontal lobes of each cerebral hemisphere with the spinal cord; also called corticospinal tracts. Pyrogen pi′ro-jen an agent or chemical sub- stance that induces fever. QRS complex wave on an ECG represent- ing depolarization of the ventricles.

Plantar plan′tar pertaining to the sole of the foot. Plasma plaz′mah the fluid portion of the blood. Plasma cell member of a B cell clone; spe- cialized to produce and release antibodies. Plasma membrane membrane that enclos- es cell contents; outer limiting membrane. Platelet plāt′let one of the irregular cell frag- ments of blood; involved in clotting. Pleura ploo′rah the serous membrane cov- ering the lung and lining the thoracic cavity. Plexus network of nerves formed by joining ventral rami of spinal nerves; contains both sensory and motor fibers. Plicae circulares pli′kuh″ ser ku-lar′ēz cork- screw-like folds within the small intestine that increase the surface area and slow the progres- sion of food through the small intestine; also called circular folds. Pneumothorax nu″mo-tho′raks the presence of air or gas in a pleural cavity. Polar body a minute cell produced during meiosis in the ovary. Polycythemia pol″e-si-the′me-ah presence of an abnormally large number of erythrocytes in the blood. Polymer pol′ ı̆ ″mer a long, chainlike molecule consisting of many similar or repeated units. Polysaccharide pol″e-sak′ ı̆-rı̄d literally, many sugars; a polymer of linked monosaccharides; examples include starch and glycogen. Pons (1) any bridgelike structure or part; (2) the brain area connecting the medulla with the midbrain, providing linkage between upper and lower levels of the central nervous system. Popliteal related to the posterior knee. Positive feedback feedback that tends to cause a variable to change in the same direc- tion as the initial change; enhances the stimu- lus. Potential energy stored energy; energy at rest; compare to kinetic energy. Precipitation formation of insoluble com- plexes that settle out of solution. Pressure gradient difference in hydrostatic (fluid) pressure that drives filtration. Primary curvatures spinal curvatures in the thoracic and sacral areas that are present at birth. Primary (essential) hypertension high blood pressure that cannot be directly linked to any specific cause. Primary humoral response the initial re- sponse of the humoral arm of the immune system to an antigen; involves clonal selection and establishes immunological memory. Primary motor area area of the cerebral cortex responsible for voluntary movement; located in the precentral gyrus. Primary somatic sensory area area of the cerebral cortex responsible for interpreting im- pulses from cutaneous sense organs and pro- prioceptors; located in the postcentral gyrus. Progesterone ovarian hormone produced by the corpus luteum and later the placenta; responsible for maintaining the uterine lin- ing during pregnancy, inhibiting contraction

Peripheral nervous system (PNS) pĕ- rif′er-al a system of nerves that connects the outlying parts of the body with the central ner- vous system. Peripheral resistance the resistance to blood flow offered by the systemic blood ves- sels; a measure of the amount of friction en- countered by blood. Peristalsis per″ ı̆-stal′sis the waves of con- traction seen in tubelike organs; propels sub- stances along the tract. Peritoneum per″ ı̆-to-ne′um the serous mem- brane lining the interior of the abdominal cavity and covering the surfaces of the abdom- inal organs. Peritubular capillaries low-pressure capil- laries that arise from the efferent arteriole and surround the renal tubule of each nephron to aid in reabsorption. Peroxisomes per-ok′sih-sōmz membranous sacs in the cytoplasm containing powerful oxidase enzymes that use molecular oxygen to detoxify harmful or toxic substances, such as free radicals. Petechiae pĕ-te′ke-e rashlike spots on the skin that indicate widespread bleeding. pH the symbol for hydrogen ion concentra- tion; a measure of the relative acidity or alka- linity of a solution. Phagocyte fag′o-sı̄t cell capable of engulf- ing and digesting particles or cells harmful to the body using the process of phagocytosis. Phagocytosis fag″o-si-to′sis the ingestion of solid particles by cells. Phalanges fah-lan′jēz the bones of the fin- ger or toe. Pharyngeal-esophageal phase the second, involuntary phase of swallowing. Pharyngeal tonsil far-rin′je-al single tonsil, also called adenoid, located in the nasopharynx. Pharyngotympanic tube tube that con- nects the middle ear and the pharynx; allows pressure to be equalized on both sides of the eardrum; also called the auditory tube and the eustachian tube. Pharynx far′inks the muscular tube extend- ing from the posterior of the nasal cavities to the esophagus. Phospholipid fos″fo-lip′id a modified tri- glyceride containing phosphorus. Photoreceptors fo″to-re-sep′torz specialized receptor cells that respond to light energy. Physiology fiz″e-ol′o-je the science of the functioning of living organisms. Pia mater innermost meningeal layer; clings to the surface of the brain and spinal cord. Pinocytosis pin″uh-si-to′sis the engulfing of extracellular fluid by cells. Pivot joint uniaxial synovial joint in which the rounded end of one bone fits into a sleeve or ring of bone or ligaments. Placenta plah-sen′tah the temporary organ that provides nutrients and oxygen to the de- veloping fetus, carries away wastes, and pro- duces the hormones of pregnancy. Plane joint nonaxial synovial joint in which the two bones have flat articular surfaces; a plane joint allows only short gliding movements.

Glossary 627

Scapula skap′u-luh triangular bone of the pectoral girdle that articulates with the clavicle and humerus. Schwann cell type of neuroglia in the PNS; many Schwann cells form the myelin sheath on a single neuron. Sclera skle′rah the firm white fibrous outer layer of the eyeball; protects and maintains eyeball shape. Scoliosis sko″le-o′sis the abnormal curvature of the spine in a lateral direction. Scrotum skro′tum a divided sac that sus- pends the testes outside the body at the root of the penis. Sebaceous glands seh-ba′shus glands that empty their sebum secretion into hair follicles. Sebum se′bum the oily secretion of seba- ceous glands. Second messenger intracellular molecule generated by binding of a chemical to a mem- brane receptor; mediates intracellular responses. Secondary curvatures spinal curvatures in the cervical and lumbar regions that develop after birth. Secondary humoral response second and subsequent responses of the humoral arm of the immune system to a previously met anti- gen; more rapid and more vigorous than the primary response. Secondary oocyte the developing oocyte after the first meiotic division; the cell that gets ovulated. Secondary sex characteristics anatomical features that develop under influence of sex hormones that are not directly involved in the reproductive process. Include male or female pattern of muscle development, bone growth, body hair, etc. Secretion se-kre′shun (1) the passage of material formed by a cell to its exterior; (2) cell product that is transported to the cell exterior. Segmentation the mixing of foodstuffs by the alternating contraction and relaxation of the circular muscle layer in the muscularis ex- terna of the gastrointestinal tract. Selective permeability characteristic exhi- bited by a barrier, such as a membrane, that allows some substances through and excludes others. Semen se′men fluid mixture produced by male reproductive structures; contains sperm, nutrients, and mucus. Semicircular canals circular canals of the inner ear situated in three different planes; lo- cation of dynamic equilibrium receptors. Semilunar valves sem″ ı̆-lu′nar valves that prevent blood return to the ventricles after contraction; see pulmonary valve and aortic valve. Seminal vesicles sem′ ı̆-nul glands located at the base of the bladder that contribute to semen by providing a food source for sperm. Seminiferous tubules sem′ ı̆-nif′er-us highly convoluted tubes within the testes that form sperm. Sensory division the part of the PNS that sends sensory input to the CNS from sensory receptors around the body; also called afferent division.

Respiratory zone part of the lung that leads into the alveoli and is involved in gas exchange. Responsiveness the ability to sense chang- es (stimuli) in the environment and then to re- act to them; see also irritability. Rete testis re′te structures that carry sperm from the seminiferous tubules to the epididymis. Reticular layer deep layer of the dermis; contains hair follicles, sweat glands, sebaceous glands and is highly vascular. Retina ret′ ı̆-nah light-sensitive layer (tunic) of the eye; contains rods and cones. Rheumatoid arthritis (RA) chronic inflam- matory form of arthritis caused by an autoim- mune disorder in which the immune system mis- takenly attacks the joints bilaterally. See arthritis. Ribonucleic acid (RNA) ri″bo-nu-kle′ik the nucleic acid that contains ribose; acts in pro- tein synthesis. Ribosomes ri′bo-sōmz cytoplasmic organ- elles at which proteins are synthesized. Rickets disease in which the bones of chil- dren fail to calcify properly, leading to bowing of the leg bones. Rods one of the two types of photosensitive cells in the retina. Round ligaments ligaments that anchor the uterus to the pelvis anteriorly. Round window inferior, membrane-covered opening in the middle ear wall between the vestibule and the cochlea. Rule of nines method of computing the extent of burns by dividing the body into 11 areas, each accounting for 9 percent of the total body area plus an area around the geni- tals representing 1 percent of the body. Sacral sa′krul referring to the lower portion of the back, just superior to the buttocks at the base of the spine. Sacrum sa′krum bone made up of five fused bones; forms the posterior of the pelvis. Saddle joint biaxial synovial joint in which both articulating bones have both convex and concave surfaces. Sagittal section (plane) saj′ ı̆-tal a longitu- dinal (vertical) plane that divides the body or any of its parts into right and left portions. Sagittal suture interlocking joint formed by the intersection of the two parietal bones at the midline of the skull. Salivary amylase enzyme in saliva that be- gins the digestion of starch. Salivary glands three pairs of glands that produce a mixture of mucus and serous fluid containing the enzyme salivary amylase, lyso- zyme, and IgA antibodies. Salt ionic compound that dissociates into charged particles (other than hydrogen or hy- droxyl ions) when dissolved in water. Sarcomere sar′ko-mēr the smallest contrac- tile unit of muscle; extends from one Z disc to the next. Sarcoplasmic reticulum specialized smooth endoplasmic reticulum found in muscle cells that stores and releases calcium during muscle contraction. Satellite cell type of neuroglia in the PNS; protects and cushions peripheral nerve cell bodies.

Radiant energy energy of the electromag- netic spectrum, which includes heat, light, ultraviolet waves, infrared waves, and other forms. Radioactivity the process of spontaneous decay seen in some of the heavier isotopes, during which particles or energy is emitted from the atomic nucleus; results in the atom becoming more stable. Radioisotope ra″de-o-i′s̆ı-tōp isotope that exhibits radioactive behavior. Radius lateral forearm bone; articulates with the capitulum of the humerus at the elbow and the carpals distally. Real image image formed on the retina that is reversed left to right and inverted (upside down) as a result of the refraction bending of light by the lens. Receptor re-sep′tor (1) a peripheral nerve ending specialized for response to particular types of stimuli; (2) molecule that binds spe- cifically with other molecules, e.g., hormones and neurotransmitters. Receptor-mediated endocytosis the type of endocytosis in which engulfed particles at- tach to receptors on the cell surface before en- docytosis occurs. Rectum portion of the large intestine be- tween the sigmoid colon and the anal canal. Red marrow bone marrow that is the site of blood cell production; see hematopoiesis. Reduction restoring broken bone ends (or a dislocated bone) to its original position. Reflex automatic reaction to a stimulus. Refract bend; usually refers to light. Regulatory T cell type of T lymphocyte that slows or stops activity of B and T cells once the antigenic threat is ended. Renin re′nin an enzyme released by the kidneys that is involved with raising blood pressure. Renin-angiotensin mechanism system trig- gered by low blood pressure or changes to filtrate solute concentrations that results in angiotensin II production and rising blood pressure. Repolarization restoration of the mem- brane potential to the initial resting (polarized) state. Reproductive system organ system that functions to produce offspring. Residual volume air remaining in the lungs after exhalation that cannot be voluntarily exhaled. Resistance exercise refers to building mus- cle mass and strength; also called isometric exercise. Respiration the process of supplying the body with oxygen and removing carbon dioxide; includes both internal and external respiration. Respiratory membrane (air-blood bar- rier) membrane composed of the alveolar wall, the capillary wall, and their basement membranes; gases must cross this membrane for proper exchange to occur. Respiratory system organ system that car- ries out gas exchange; includes the nose, phar- ynx, larynx, trachea, bronchi, and lungs.

628 Glossary

Synovial fluid s ı̆-no′ve-al a fluid secreted by the synovial membrane; lubricates joint sur- faces and nourishes articular cartilages. Synovial joint freely movable joint exhibit- ing a joint cavity enclosed by a fibrous capsule lined with synovial membrane. Synovial membrane membrane that lines the fibrous capsule of a synovial joint. Synthesis reaction chemical reaction in which larger molecules are formed from sim- pler ones. Systemic circulation system of blood ves- sels that carries nutrient- and oxygen-rich blood to all body organs. Systole sis′to-le the contraction phase of heart activity. Systolic pressure sis-tŏ′lik the pressure gen- erated by the left ventricle during systole. T cells lymphocytes that mediate cellular im- munity; include helper, cytotoxic, regulatory, and memory cells. Also called T lymphocytes. T wave wave on an ECG representing repo- larization of the ventricles. Tachycardia tak″e-kar′de-ah an abnormal, excessively rapid heart rate; over 100 beats per minute. Talus tă′lus ankle bone; articulates with the tibia and calcaneus; allows plantar flexion. Tarsal tahr′sal (1) one of the seven bones that form the ankle and heel; (2) relating to the ankle. Tectorial membrane tek-to′re-al gel-like membrane in the cochlear duct in which the “hairs” of the receptor cells are embedded. Tendon ten′dun cord of dense fibrous tis- sue attaching a muscle to a bone. Tendon sheath elongated bursa that wraps a tendon subject to friction. Testis tes′tis the male primary sex organ that produces sperm. Testosterone tes-tos′tĕ-rōn male sex hor- mone produced by the testes; during puberty promotes virilization and is necessary for nor- mal sperm production. Thalamus tha′luh-mus a mass of gray mat- ter in the diencephalon of the brain. Thoracic tho-ras′ik refers to the area be- tween the neck and abdomen supported by the ribs, costal cartilages and sternum; chest. Thoracic vertebrae the 12 vertebrae that are in the middle part of the vertebral column and articulate with the ribs. Thrombin throm′bin an enzyme that induces clotting by converting fibrinogen to fibrin. Thrombus throm′bus a fixed clot that devel- ops and persists in an unbroken blood vessel. Thymus thi′mus an endocrine gland active in the immune response. Thyroid gland thi′roid one of the largest of the body’s endocrine glands; straddles the anterior trachea. Tidal volume (TV) amount of air inhaled or exhaled with a normal breath. Tight junction area where plasma mem- branes of neighboring cells are tightly bound together, forming an impermeable barrier. Tissue a group of similar cells specialized to perform a specific function; primary tissue

Spinal nerves 31 pairs of nerves originating from the spinal cord; formed when the dorsal and ventral roots merge. Spinous process bony projection from the posterior of the vertebral arch. Spiral organ of Corti the location of the hearing receptors in the cochlea. Splanchnic nerves splank′nik pregangli- onic sympathetic neurons that do not synapse at the sympathetic trunk ganglion, but instead synapse at a collateral ganglion anterior to the spinal cord, then innervate the visceral organs. Spongy bone internal layer of bone in flat, short, and irregular bones, and in the epiphy- ses of long bones. Spongy (penile) urethra the portion of the urethra that travels down the penis and opens to the outside of the body. Squamous suture interlocking joint formed by the intersection of temporal bones with the parietal bones. Stapes sta′pēz medial bone of the middle ear; also called the stirrup. Static equilibrium stat′ik e″kwı̆-lib′re-um balance concerned with changes in the posi- tion of the head. Sterility the inability to produce offspring. Sternum breastbone; flat bone composed of the manubrium, body, and xiphoid process. Steroids stĕ′roidz a specific group of chemi- cal substances including certain hormones and cholesterol. Stroke volume (SV) a volume of blood ejected by a ventricle during systole. Submucosa layer of the gastrointestinal tract just deep to the mucosa that contains blood vessels, nerves, lymphoid tissue (MALT), and lymphatic vessels. Sudoriferous glands su″do-rif′er-us the glands that produce a saline solution called sweat Sulcus sul′kus a furrow on the brain, less deep than a fissure. Superior vena cava large vein that carries blood from veins superior to the diaphragm back to the right atrium. Sural soo′ral related to the posterior leg; calf. Surfactant sur-fak′tant a chemical sub- stance coating the pulmonary alveoli walls that reduces surface tension, thus preventing collapse of the alveoli after expiration. Suspensory ligaments ligaments that secure the ovaries to the lateral walls of the pelvis. Swallowing complex process of moving food from the mouth into the esophagus; in- volves the buccal and pharyngeal-esophageal phases; also called deglutition. Sympathetic division a division of the au- tonomic nervous system; opposes parasympa- thetic functions; called the fight-or-flight divi- sion; also called thoracolumbar division. Synapse sin′aps the region of communica- tion between neurons, or a neuromuscular junction between a neuron and a muscle cell. Synaptic cleft s ı̆-nap′tik the fluid-filled space at a synapse between neurons. Synarthrosis sin″ar-thro′sis an immovable joint.

Serosa outermost layer of the gastrointesti- nal tract; also called the visceral peritoneum. Serous fluid ser′us a clear, watery fluid se- creted by the cells of a serous membrane. Serous membrane membrane that lines a cavity without an opening to the outside of the body (except for joint cavities, which have a synovial membrane); serosa. Serous pericardium double-layered serous membrane deep to the fibrous pericardium; the layers secrete serous fluid to reduce fric- tion as the heart beats. Serum the fluid portion of plasma minus clotting proteins. Sinoatrial (SA) node si″no-a′tre-al the mass of specialized myocardial cells in the wall of the right atrium; pacemaker of the heart. Sinus si′nus (1) a mucous membrane–lined, air-filled cavity in certain cranial bones; (2) a dilated channel for passage of blood or lymph. Skeletal muscle muscle composed of cy- lindrical multinucleate cells with obvious striations; the muscle(s) attached to the body’s skeleton; also called voluntary muscle. Skeletal system system of protection and sup- port composed primarily of bone and cartilage. Skull component of the axial skeleton con- taining cranium (bony enclosure for the brain) and facial bones. Small intestine segment of the gastroin- testinal tract between the stomach and large intestine; site where most absorption occurs. Smooth (visceral) muscle muscle consist- ing of spindle-shaped, unstriped (nonstriated) muscle cells; involuntary muscle. Soft palate portion of the roof of the mouth that is not supported by bone. Solute sol′yoot the dissolved substance in a solution. Solute pump protein carrier involved in ac- tive transport. Solution a homogenous mixture of two or more components. Solvent the substance present in the largest amount in a solution. Somatic nervous system so-mat′ik a divi- sion of the peripheral nervous system; also called the voluntary nervous system. Sperm mature male sex cell. Spermatic cord a group of blood vessels, nerves, and ducts that are wrapped in a con- nective tissue sheath that travels through the inguinal canal. Spermatids sper′mah-tidz gametes after the second meiotic division that still need to un- dergo spermiogenesis. Spermatogenesis sper″mah-to-jen′ĕ-sis the process of sperm production in the male; in- volves meiosis. Spermatogonia sper″mah-to-go′ne-ah male stem cells that will produce sperm. Spermiogenesis sper″me-o-gen′ĕ-sis the last stage of sperm development in which all ex- cess cytoplasm is sloughed off and the sperm takes its mature shape. Spinal cord part of the CNS that provides a two-way conduction system to and from the brain; also a major reflex center.

Glossary 629

Ventral root the root at which motor neu- rons of the somatic nervous system exit the spinal cord. Ventricles ven′tr ı̆-kulz (1) discharging cham- bers of the heart; (2) cavities within the brain. Venule vēn′ūl a small vein. Vertebral arch arch formed by connecting all bony projections posteriorly from the cen- trum; includes pedicles and laminae. Vertebral column the spine, formed of a number of individual bones called verte- brae and two composite bones (sacrum and coccyx). Vesicle ves′ ı̆-kul a small fluid-filled sac formed by membrane. Vesicular breathing sounds soft respira- tory sounds produced when air fills the alveoli. Vesicular follicle a mature ovarian follicle; also called Graafian follicle. Vesicular transport an active transport pro- cess in which items too large to pass through the plasma membrane are transported in bulk via a vesicle. See endocytosis and exocytosis. Vestibular apparatus name for equilibrium receptors of the inner ear; includes both static and dynamic equilibrium receptors. Vestibule ves′t̆ı-būl (1) area of the inner ear between the cochlea and semicircular canals; (2) the space in the oral cavity inside the lips and cheeks and outside the teeth and gums. Villi vil′i fingerlike projections of the small intestinal mucosa that tremendously increase its surface area for absorption. Visceral pericardium innermost layer of the serous pericardium that lines the surface of the heart, also called epicardium. Visceral peritoneum outermost layer of the gastrointestinal tract; also called the serosa. Visceral pleura ploor′ah see pulmonary pleura. Visual acuity ah-ku′ ı̆-te the ability of the eye to distinguish detail. Vital capacity (VC) the volume of air that can be expelled from the lungs by forcible expiration after the deepest inspiration; total exchangeable air. Vitamins organic compounds required by the body in very small amounts for physiologi- cal maintenance and growth. Vitreous humor a gel-like substance that helps prevent the eyeball from collapsing in- ward by reinforcing it internally; also called vitreous body. Vulva vul′va female external genitalia. White matter white substance of the central nervous system; the myelinated nerve fibers. Xiphoid process zif′oyd the most inferior of the three bones making up the sternum; be- gins as hyaline cartilage and gradually ossifies throughout adulthood. Yellow marrow bone marrow that stores adipose (fat) tissue. Zygote zi′gōt the fertilized ovum; produced by union of two gametes.

Ulna medial bone of the forearm; articulates with the trochlea of the humerus and the carpals. Umbilical pertaining to the navel area. Umbilical cord um-bı̆l′ ̆ı-kul a structure bear- ing arteries and veins connecting the placenta and the fetus. Unfused tetanus tet′ah-nus a muscle con- traction in which the muscle does not com- pletely relax between stimulation events; causes an increase in force because individual twitches are added together, or summed; also called incomplete tetanus. Unipolar neuron structural class of neuron with one process extending from the cell body. Urea u-re′ah the main nitrogen-containing waste excreted in the urine. Ureters yer′ĕ-terz tubes that carry urine from kidney to bladder. Urethra u-re′thrah the canal through which urine passes from the bladder to the outside of the body. Urinary system system primarily respon- sible for water, electrolyte, and acid-base bal- ance and the removal of nitrogen-containing wastes from the blood. Urine filtrate containing waste and excess ions excreted by the kidneys. Uterine cycle see menstrual cycle. Uterine tube the oviduct: the tube through which the ovum is transported to the uterus; also called fallopian tube. Uterosacral ligaments bilataeral ligaments that anchor the uterus posteriorly. Uterus u′ter-us hollow pelvic organ of the female reproductive system that functions to receive, retain, and nourish a fertilized egg. Uvula u′vu-lah tissue tag hanging from soft palate. Vaccine weakened or killed antigen injected into a person for the purpose of stimulating a primary immune response; conveys artifi- cial active immunity; results in immunological memory. Vagina vah-ji′nah the female copulatory or- gan that runs from the cervix to the outside of the body. Valence shell va′lens the outermost energy level of an atom that contains electrons; the electrons in the valence shell determine the bonding behavior of the atom. Valves structures that close to prevent back- flow of blood in the heart or in large veins. Vascular layer middle layer (tunic) of the eyeball wall that contains the choroid, ciliary body, and iris. Vascular shunt a vessel that directly con- nects the arteriole and venule of a capillary bed. Vasoconstriction vās″o-kon-strik′shun nar- rowing of blood vessels. Vein vān a vessel carrying blood away from the tissues toward the heart. Ventral ramus branch of a spinal nerve that serves the anterior and lateral trunk; ventral rami of spinal nerves T1–T12 form the intercos- tal nerves; all other ventral rami form the four nerve plexuses.

types are epithelial, connective, muscle, and nervous tissues. Trachea tra′ke-ah The windpipe; the respi- ratory tube extending from larynx to bronchi. Tracheoesophageal fistula tra″ke-o-ĕ-sof″ ah- je′ al fis′tu-lah congenital defect in which the esophagus and trachea are connected phys- ically; the esophagus often ends in a blind sac that does not lead to the stomach. Tract a collection of nerve fibers in the CNS having the same origin, termination, and function. Transcription one of the two major steps in protein synthesis; the transfer of information from DNA base sequence to the complemen- tary messenger RNA base sequence. Transient ischemic attack (TIA) the tem- porary restriction of blood flow to an area of the brain. Translation the second major step in pro- tein synthesis; information carried by mes- senger RNA is decoded and used to assemble amino acids into a protein. Transverse process lateral bony projection that originates at the vertebral arch; occur in pairs, one on each side. Transverse section (plane) plane that di- vides the body or its parts into superior and inferior parts; cross section. Triglycerides tri-glis′er-ı̄dz compounds com- posed of fatty acids and glycerol; fats and oils; also called neutral fats. Trigone tri′gon smooth, triangular region of the bladder formed by the openings of the two ureters and the urethra. Trochanter tro-kan′ter a large, somewhat blunt process. Tropic hormone tro′pik a hormone that reg- ulates the function of another endocrine organ. True ribs rib pairs 1–7; are attached directly to the sternum via costal cartilage. True vocal cords folds in the mucous mem- brane of the larynx that vibrate to produce sound; also called vocal folds. Tubercle tu′ber-kul a nodule or small rounded process. Tuberosity tu″bĕ-ros′ ı̆-te a broad process, larger than a tubercle. Tubular reabsorption the process of re- claiming nutrients, ions, and water that the body needs from the filtrate in the renal tubule. Tubular secretion the process of actively moving substances from peritubular blood into the renal tubule for elimination from the body. Tunica externa tu′nı̆-kah eks′tern-ah out- ermost layer of blood vessel walls made of fibrous connective tissue. Tunica intima tu′nı̆-kah in′ti-mah the in- nermost lining of blood vessels made of en- dothelium on a basement membrane; the only layer forming a capillary wall. Tunica media tu′nı̆-kah me′de-ah the middle layer of blood vessel walls containing smooth muscle and elastic fibers. Tympanic membrane tim-pan′ik the ear- drum.

630

Chapter 15 15.1b: Richard Tauber/Pearson Educa- tion, Inc.; 15.2a: Karen Karbbenhoft/Pearson Educa- tion, Inc.; 15.3b: From A Stereoscopic Atlas of Human Anatomy by David Bassett/Robert A. Chase; Homeo- static Imbalance 15.3: G. Warrick/Science Source; A Closer Look: AJPhoto/Science Source; 15.11: Susanna Blavarg/Getty Images; Focus on Careers: Design Pics/ Don Hammond/Getty Images. Chapter 16 16.5a: Juergen Berger/Science Source; Homeostatic Imbalance 16.3: Scimat/Science Source; 16.11: C. Edleman/La Vilette/Science Source; 16.14a: Thomas/Science Source; 16.14b, c: Southern Illinois University/Science Source; 16.16: David M. Phillips/ Science Source; 16.19: From A Stereoscopic Atlas of Human Anatomy by David Bassett/Robert A. Chase; 16.20a: Lennart Nilsson/Scanpix Sweden AB; 16.20b: Neil Bromhall/Science Source; A Closer Look: Susanna Price/Dorling Kindersley Ltd.

Art Credits All illustrations are by Imagineering STA Media Services unless noted below.

Chapter 3 3.2: Carla Simmons/Kristin Mount; 3.4: Tomo Narashima; 3.15: Adapted from Campbell, Neil A.; Mitchell, Lawrence G.; Reece, Jane B., Biology: Concepts and Connections, 3rd Ed., ©2000, pp. 132– 133. Reprinted and Electronically reproduced by per- mission of Pearson Education, Inc., Upper Saddle River, New Jersey. Chapter 4 4.4: Electronic Publishing Services, Inc. Chapter 5 5.3: Carla Simmons/Imagineering STA Media Services; 5.7: Carla Simmons/Imagineering; 5.9–5.12: Kristin Mount. Chapter 6 6.1: Raychel Ciemma; 6.2b: Carla Sim- mons/Kristin Mount; 6.14abcd: Electronic Publishing Services, Inc./Imagineering STA Media Services. Chapter 7 7.20: Laurie O’Keefe/Kristin Mount; 7.22: Charles W. Hoffman/Kristin Mount; 7.27: Elec- tronic Publishing Services, Inc. Chapter 11 11.1: Wendy Hiller Gee/Kristin Mount; 11.2, 11.3b, 11.4: Barbara Cousins; 11.5, 11.6: Barbara Cousins/Imagineering STA Media Services; 11.7: Bar- bara Cousins/Kristin Mount; 11.15, 11.16, 11.24: Kris- tin Mount. Chapter 14 14.17b: http://www.choosemyplate.gov. Chapter 15 15.6: Linda McVay/Imagineering STA Media Services. Chapter 16 16.3: Precision Graphics/Imagineering STA Media Services; 16.5: Precision Graphics/Imagi- neering STA Media Services; 16.8: Martha Blake/Kris- tin Mount.

Text Credits Chapter 14 Table 14.2: Thompson, Janice; Manore, Melinda; Vaughan, Linda, The Science of Nutrition, 3rd Ed., ©2014, pp. C1–C12, 57. Reprinted and Elec- tronically reproduced by permission of Pearson Edu- cation, Inc., Upper Saddle River, New Jersey. Appendix D Thompson, Janice; Manore, Melinda; Vaughan, Linda, The Science of Nutrition, 3rd Ed., ©2014, pp. C1–C12, 57. Reprinted and Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.

Perry/AFP/Getty Images; Homeostatic Imbalance 5.7: CNRI/Science Photo Library/Science Source; 5.31: Scott Camazine/Science Source; 5.34a: Professor Pietro M. Motta/Science Source; 5.34b: P. Motta/ Department of Anatomy, University “La Sapienza,” Rome/Science Source. Chapter 6 Table 6.1.1: Pearson Education, Inc.; Table 6.1.2: Elaine N. Marieb; Table 6.1.3: William Karkow/Pearson Education, Inc.; 6.4b: Eric V. Grave/ Science Source; 6.11a: Simon Balson/Alamy Stock Photo; 6.11b: JGI/Blend Images/AGE Fotostock.; 6.13: Phocasso/J W White/Pearson Education, Inc.; A Closer Look, 1: Corbis/SuperStock; 2: Rodney Turner KRT/Newscom; Homeostatic Imbalance 6.5: Dr. M.A. Ansary/Science Source. Chapter 7 7.4: Phanie/Science Source; 7.13b: A. Glauberman/Science Source; 7.17b: From A Stereo- scopic Atlas of Human Anatomy by David Bassett/ Robert A. Chase; Homeostatic Imbalance 7.6: Ulrich Doering/Alamy Stock Photo; A Closer Look: AdMe- dia/Splash News/Newscom; Homeostatic Imbalance 7.11: Jeffrey Greenberg/Science Source; A Closer Look, a: Hank Morgan/Science Source. Chapter 8 8.1: Richard Tauber/Pearson Education, Inc.; 8.4: From A Stereoscopic Atlas of Human Anat- omy by David Bassett/Robert A. Chase; Homeostatic Imbalance 8.5: Arztsamui/Shutterstock; 8.7: Thalerng- sak Mongkolsin/Shutterstock; Focus on Careers: Cre- atas/Photolibrary, Inc.; Homeostatic Imbalance 8.11: Biophoto Associates/Science Source. Chapter 9 Homeostatic Imbalance 9.2: Caters News/ZUMA Press/Newscom; A Closer Look: Ander- sen Ross/Blend Images/Getty Images; 9.6b: Michael Wiley/Pearson Education, Inc.; Homeostatic Imbal- ance 9.4.1: Alison Wright/Science Source; Homeo- static Imbalance 9.4.2: Ralph C. Eagle, Jr./Science Source; Homeostatic Imbalance 9.6: Biophoto Associ- ates/Science Source; 9.11b: Pearson Education, Inc. Chapter 10 10.2: Victor Eroschenko/Pearson Edu- cation, Inc.; Homeostatic Imbalance 10.1: Ingram Publishing/Photolibrary, Inc.; 10.6: Lennart Nilsson/ Scanpix Sweden AB; Homeostatic Imbalance 10.3: Moredun Scientific/Science Source; Focus on Careers: BSIP SA/Alamy Stock Photo; 10.7: Jack Scanlon/Holy- oke Community College. Chapter 11 Homeostatic Imbalance 11.2: David Campione/Science Source; 11.9: Mark Shaiken/ Moment/Getty Images; 11.10a: Ed Reschke/Photoli- brary/Getty Images; 11.15b: CNRI/Science Source; A Closer Look: SPL/Science Source. Chapter 12 Homeostatic Imbalance 12.2: Chris Bjornberg/Science Source; 12.9a: SPL/Science Source; 12.15a: Arthur J. Olson, Molecular Graphics Labora- tory, Scripps Research Institute; A Closer Look: Eye of Science/Science Source. Chapter 13 13.3b: CNRI/Science Source; 13.4a: Richard Tauber/Pearson Education, Inc.; 13.5b: Lisa Lee/Pearson Education, Inc.; Homeostatic Imbalance 13.7: Du Cane Medical Imaging Ltd./Science Source; A Closer Look: Stacey Green/Photographer’s Choice/ Getty Images. Chapter 14 14.4b: From A Stereoscopic Atlas of Human Anatomy by David Bassett/Robert A. Chase; 14.5a: Robert A. Chase; A Closer Look, a: CNRI/Sci- ence Source; A Closer Look, b: Eye of Science/Science Source; 14.21b: Andrew Doran/Shutterstock; Homeo- static Imbalance 14.15: Center for Craniolfacial Anom- alies; A Closer Look: Keith Srakocic/AP Images.

Photographic Credits VWT smart phone, tablet, and computer: Shutterstock. Author photo: Elaine N. Marieb, Suzanne Keller, Essentials of Human Anatomy & Physiology, 12 Ed., © 2018, Pearson Education, Inc., New York, NY.

Chapter 1 1.1: CLS Digital Arts/Shutterstock; A Closer Look, a: Southern Illinois University/Science Source; A Closer Look, b: Laurent/Hop. Americain/Science Source; A Closer Look, c: Medical Body Scans/Science Source; A Closer Look, d: Science Source; A Closer Look, e: Scott Camazine/Science Source; 1.5a2: CNRI/ Science Source; 1.5c2: James Cavallini/Science Source; 1.5a-c1: Pearson Education, Inc.; 1.5b2: Scott Cama- zine/Science Source; 1.8a: Pearson Education, Inc. Chapter 2 2.4.1: Chip Clark/Fundamental Photo- graphs, NYC; 2.4.2: Leslie Garland Picture Library/ Alamy Stock Photo; 2.4.3: Levent Konuk/Shutter- stock; 2.9b: BMJ/Shutterstock; 2.16a: Image Source/ Getty Images; 2.16b: Ivaschenko Roman/Shutter- stock; 2.21c: Will & Deni McIntyre/Science Source; Focus on Careers: Don Hammond/Photolibrary, Inc. Chapter 3 3.7a: Mary Osborn; 3.7b: Dr. Frank Solo- mon; 3.7c: Mark S. Ladinsky and J. Richard McIntosh; A Closer Look, a-c: David M Philips/Science Source; 3.12b: Birgit H. Satir, Dept. of Anatomy and Structural Biology, Albert Einstein College of Medicine.; 3.18a: Biophoto Associates/Science Source; 3.18b, c, e, f: Allen Bell/Pearson Education, Inc.; 3.18d: William Karkow/Pearson Education, Inc.; 3.19a, c, g: Allen Bell/Pearson Education, Inc.; 3.19b, h: Ed Reschke/ Photolibrary/Getty Images; 3.19d: William Karkow/ Pearson Education, Inc.; 3.19e: Ed Reschke; 3.19f: Biophoto Associates/Science Source; 3.20a: Eric Graves/Science Source; 3.20b: Marian Rice; 3.20c: Steve Gschmeissner/Science Photo Library/Getty Images; 3.21: Biophoto Associates/Science Source; Homeostatic Imbalance 3.3: Biophoto Associates/ Science Source. Chapter 4 4.5: Ed Reschke/Photolibrary/Getty Images; A Closer Look: Lauren Shear/Science Source; Homeostatic Imbalance 4.2a: Mediscan/Alamy Stock Photo; Homeostatic Imbalance 4.2b: National Insti- tute of Health; Homeostatic Imbalance 4.4: Ian Boddy/Science Source; 4.7a: Pearson Education, Inc.; 4.7b: CNRI/Science Source; 4.7c: CNRI/Photo Researchers, Inc./Science Source; 4.9a, b: Dr. P. Marazzi/Science Source; 4.9c: Medicshots/Alamy Stock Photo; 4.10b.1: Gianni Muratore/Alamy Stock Photo; 4.10b.3: John Radcliffe Hospital/Science Source; 4.10b.2: Scott Camazine/Science Source; 4.11a: Dr. P. Marazzi/Science Source; 4.11b: Girand/ Science Source; 4.11c: Custom Medical Stock Photo/ Alamy Stock Photo; Focus on Careers: Elena Dorf- man/Pearson Education, Inc. Chapter 5 5.1: Seelevel.com; 5.4c: William Krakow/ Pearson Education, Inc.; Focus on Careers: Frederic Ciroues/PhotoAlto/Alamy Stock Photo; Homeostatic Imbalance 5.1: Jeff Rotman/Science Source; 5.16: Reik/AGE Fotostock; Homeostatic Imbalance 5.5a: Princess Margaret Rose Orthopaedic Hospital/Science Source; Homeostatic Imbalance 5.5b: Nordic Photos/ SuperStock; Homeostatic Imbalance 5.5c: Steve Gorton/Dorling Kindersley Ltd.; 5.20b: Mark Nielsen and Shawn Miller; Closer Look, a: Lawrence Liver- more National Laboratory/Science Source; Closer Look, b: Elaine N. Marieb; Closer Look, c: Frank

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NOTE: Page numbers in boldface indicate a definition. A t following a page number indicates tabular material, an f indicates a figure, and a b indicates boxed material.

A. See Adenine A (dark) bands, 212, 212f ABCD/ABCDE rule, for melanoma

diagnosis, 153 Abdominal (term), 40, 40f Abdominal aorta, 399, 401f

branches of, 400 Abdominal cavity, 43, 43f Abdominal muscles, 232–233, 233f,

240f, 242t Abdominopelvic cavity, 43, 43f

quadrants/regions of, 43, 44f Abdominopelvic trauma, 43 Abducens nerve (cranial nerve VI),

284f, 286t Abduction (body movement), 223,

225f, 227f ABO blood groups, 377, 377t, 378,

378f transplantation and, 451

Abortion, 590 Abscess, 434, 435f Absorption (food), 503f, 504, 505f

in large intestine, 511 in small intestine, 497, 510–511

Absorption function, of epithelial tissue, 114

Absorptive cells, small intestine, 498f Abstinence, 594b, 595b Accessory digestive organs, 490,

490f, 500–502. See also specific structure

Accessory eye structures, 305–306, 305f, 306f

Accessory nerve (cranial nerve XI), 284f, 287t

Accessory pancreatic duct, 496f Accessory reproductive organs, 564

in male, 567–568 Accommodation, 313 Accommodation pupillary reflex, 316 Acetabulum, 188, 189f Acetic acid, 66, 520, 521 Acetoacetic acid, 520, 521 Acetone, 520 Acetylcholine (ACh), 292

in muscle contraction, 143, 213, 214, 215f

shortage of in Alzheimer’s disease, 278b

Acetylcholinesterase (AChE), in muscle contraction, 215f, 216

ACh. See Acetylcholine AChE. See Acetylcholinesterase

(AChE), in muscle contraction Achilles (calcaneal) tendon, 239,

239f, 241f Acid(s), 66, 556, 556f

pH of, 67f strong, 66, 556, 556f weak, 66, 556, 556f

Acid-base balance, 67–68, 67f, 555–557, 556f. See also Acidosis; Alkalosis; pH

buffer systems in maintenance of, 68, 476, 480, 555–556, 556f

renal mechanisms in maintenance of, 544, 544f, 557

respiratory mechanisms in maintenance of, 476, 556–557

Acid group, amino acid, 74, 74f

Acidic urine, diet and, 545 Acid mantle, in body defense, 139,

139t, 431t, 432 Acidosis, 351, 364, 480, 520, 555

in diabetes, 351, 520 physiological, 555

Acne, 145 Acoustic (auditory) canal/meatus

external, 172, 174f, 317, 317f internal, 174, 175f

Acoustic nerve. See Vestibuloco- chlear nerve

Acquired immune deficiency syndrome (AIDS), 375, 453, 454–455b

growth hormone for wasting and, 342b

Acromegaly, 341 Acromial (term), 40, 40f Acromioclavicular joint, 184, 185f Acromion, scapular, 184, 185f Acrosomal reaction, 584 Acrosome, 570, 571f ACTH (adrenocorticotropic

hormone), 340f, 343, 354t in stress response, 350f tumors secreting, 348

Actin, 96, 97f, 213 in microvilli, 100 in sliding filament theory,

216–217, 217f, 218f Actin (thin) filaments, 212f, 213

in sliding filament theory, 216–217, 217f, 218f

Action potential in muscle cell contraction,

214–216, 215f, 216f, 218f in neuron, 260–262, 261f.

See also Nerve impulses transmission at synapses and,

262–263, 263f Active immunity, 442–443, 443f Active processes, 104–108, 106f,

107f, 108f in absorption, 511 active transport, 106, 106f vesicular transport, 107–108,

107f, 108f Active sites, 77, 78f Acute (immediate) hypersensitivity,

452, 452f AD. See Alzheimer’s disease Adam’s apple (thyroid cartilage),

465f, 466 Adaptation

auditory, 322–323 olfactory, 325

Adaptive (specific) defense system, 430, 430f, 436–454, 449f, 450–451t, 457. See also Immunity/immune response

antigens and, 438, 449f, 451t cells of, 438–440, 439f, 449f, 450t cellular (cell-mediated) immune

response, 437, 446–448, 447f, 448f, 449f

humoral (antibody-mediated) immune response, 437, 441–446, 441–444f, 445t, 446f, 447f, 449f

organ transplants/rejection and, 448–451

Addison’s disease, 348, 348f, 555 Adduction (body movement), 223,

225f, 227f Adductor magnus muscle, 236f Adductor muscles of thigh, 236f,

237f, 238, 240f, 241f, 242t

Adenine (A), 78, 79f, 80, 109, 109f Adenine monophosphate, cyclic

(cAMP), in hormone mechanism of action, 336, 336f

Adenocarcinoma, of lung, 481–483 Adenoids (pharyngeal tonsils), 429,

429f, 464, 465f Adenosine triphosphate. See ATP ADH. See Antidiuretic hormone Adhesion proteins, in extracellular

matrix, 120 Adhesions, in pericarditis, 384 Adipose (fat) tissue/cells, 101, 101f,

122f, 124, 140f autonomic nervous system

affecting, 294t bone storage and, 161, 163

ADP, 81 accumulation/formation of, 63f,

64, 80f, 81 phosphorylation in ATP produc-

tion and, 219, 220f, 517, 517f Adrenal cortex, 347–349, 347f, 355t

ACTH affecting, 340f, 343 hormones of, 73t, 347–349, 347f,

348f in stress response, 347, 350f tumors of, 348

Adrenal glands, 30, 31f, 338f, 346–349, 347f, 348f, 355t, 538f, 539. See also Adrenal cortex; Adrenal medulla

autonomic innervation/effects and, 291f, 293

in stress response, 347, 349, 350f Adrenaline. See Epinephrine Adrenal medulla, 347, 347f, 349,

355t autonomic nervous system

affecting, 294t, 349 homeostatic imbalance, 349 hormones of, 349 in stress response, 349, 350f

Adrenergic fibers, 292 Adrenocorticotropic hormone

(ACTH), 340f, 343, 354t in stress response, 350f tumors secreting, 348

Adult polycystic kidney disease, 559

Adventitia, 467f Aerobic exercise. See also Exercise

heart affected by, 222 muscles affected by, 222, 222f

Aerobic respiration, ATP production and, 219, 220f

Afferent arteriole, 541f, 542, 543f Afferent (sensory) division of

peripheral nervous system, 252f, 253

Afferent lymphatic vessels, 428, 428f

Afferent nerves/neurons/tracts. See Sensory (afferent) nerves; Sen- sory (afferent) neurons; Sensory (afferent) tracts

Afferent pathway, in homeostasis, 45, 46f

Afterbirth, 593 Ag. See Antigen(s) Agglutination, 375, 378, 378f, 445,

446f Agglutinins, 375, 377t, 378, 378f Agglutinogens, 375, 377, 377t, 378,

378f Aging, 129. See also specific system

bladder and, 559–561

cardiovascular system and, 418 cell and tissue changes and, 129 chemical senses and, 329 digestive system and, 528 endocrine system and, 129, 357 growth hormone administration

and, 342b hair changes/loss and, 156 hearing and, 328–329 heart/heart rate and, 394, 418 kidneys and, 559 lacrimal glands and, 328 metabolic rate and, 523–524,

524t, 528 muscular system and, 129, 245 nervous system and, 296–297 nutrition and, 528 odor sensation and, 329 reproductive changes and,

596–598 respiratory system and, 485 skeletal system and, 129, 201–203,

203f skin changes and, 129, 156 taste sensation and, 329 urinary system and, 559–561 vision and, 328

Agranulocytes, 369, 370t Ags. See Antigen(s) AIDS, 375, 453, 454–455b

growth hormone for wasting and, 342b

AIDS dementia, 454b Air displacement plethysmography,

529 Air-blood barrier (respiratory

membrane), 470, 470f Air movements, nonrespiratory,

475, 475t Air sacs. See Alveoli Alae

ilial, 188, 189f sacral, 182, 182f

Alarm stage of stress response, 349 Albumin, 364, 365f, 521 Albuminuria, 546t Alcohol

antidiuretic hormone and, 340 blood pressure and, 411 central nervous system and, 297 cirrhosis and, 502

Aldosterone, 73t, 347, 348f, 553–555

homeostatic imbalances and, 348 water and electrolyte balance/

blood pressure and, 347, 348, 348f, 410, 553–555, 554f

Alimentary canal (gastrointestinal tract), 490–500, 490f. See also specific structure and Digestive system

walls of, 492, 493f Alkaline urine, diet and, 545 Alkalosis, 364, 480, 555 Allergen, 451 Allergic contact dermatitis, 150, 453 Allergies, 149, 451–453, 452f, 482b

hygiene hypothesis and, 482b to penicillin, 438 respiratory, 482b skin disorders, 149, 150, 453

Allografts, 451 “All-or-none” response

in muscle cell contraction, 217 in neuron, 262

Alopecia, 156 Alpha (α) cells, pancreatic, 351, 351f

Subject Index

631

632 Subject Index

Alpha (α)-helix, 75–76, 75f Alpha particles, 56 Alpha waves, EEG, 297f ALS (amyotrophic lateral sclerosis),

213–214 Alveolar ducts, 469, 469f Alveolar glands, in breast, 582,

582f Alveolar macrophages, 470, 470f Alveolar pores, 469f, 470, 470f Alveolar processes, 174f, 176f, 177,

178 Alveolar sacs, 469, 469f Alveoli

dental, 178 respiratory, 463, 469, 469f, 470f

Alzheimer’s disease, 278b PET scans in diagnosis of, 36b,

37f Amine group, amino acid, 74, 74f Amino acids, 74–76, 74f, 111–112,

515, 515f hormones derived from (amino

acid-based molecules), 335, 336f protein synthesis/digestion and,

63, 111–112, 113f, 504, 505f, 520 sequence of in protein (primary

structure), 75, 75f Aminopeptidase, 505f Amitotic tissues, 129 Ammonia, 520, 556

pH of, 67f Amnion, 586, 587f Amniotic sac, 587f Amoeboid motion, white blood

cell, 368 Amphetamines, for weight loss,

530b Amphiarthroses, 193, 196t Ampulla

of ductus deferens, 566, 566f, 567f

hepatopancreatic, 496f, 497 of semicircular canal, 319, 320f

Amylase pancreatic, 505f, 509 salivary, 501, 505f, 506

Amyotrophic lateral sclerosis. See ALS (amyotrophic lateral sclerosis)

Anabolic steroids, 231b Anabolism, 516. See also

Metabolism synthesis reactions and, 63, 63f

Anaerobic glycolysis, in cellular respiration/ATP formation, 219–220, 220f

Anal canal, 490f, 499, 499f Anal sphincters, 499, 499f Anaphase, 110f, 111 Anaphylactic shock, 452 Anatomical direction/directional

terms, 38, 39t Anatomical neck, of humerus, 186,

186f Anatomical position, 38, 39t, 40f Anatomy, 27–28

complementarity principle and, 89 levels of structural organization

and, 28–33, 29f relationship to physiology and, 28 terminology used in, 38–45, 39t,

40f, 42f, 43f, 44f Anchoring junctions (desmosomes),

92, 92f, 141f Androgens, 348, 353, 355t

adrenal, 348 apocrine glands and, 147 sebaceous glands and, 145

Andropause, 598 Androstenedione, 231b Anemia, 367, 368t, 379

Anencephaly, 295 Angina pectoris, 389 Angiogram/angiography

cerebral, 296b digital subtraction (DSA), 37b

Angioplasty, balloon, for atheroscle- rosis, 413b

Angiotensin II, 347, 348f in fluid and electrolyte balance/

blood pressure, 410, 554f, 555 Anions, 58–59 Ankle joint, muscles of, 238–239,

239f Ankylosis, in rheumatoid arthritis,

199, 199f Anosmias, 326 ANP. See Atrial natriuretic peptide ANS. See Autonomic nervous

system Antagonists (muscle), 226 Antebrachial (term), 40, 40f Antecubital (term), 40, 40f Anterior (term), 39t. See also under

Ventral Anterior arm muscles, 233f, 235,

240f, 242t Anterior association area, 267f, 268 Anterior body landmarks, 38–41, 40f Anterior border, tibial, 190f, 191 Anterior cerebral artery, 404, 404f Anterior communicating artery, 404f Anterior femoral cutaneous nerve,

288t, 289f Anterior fontanels, 200, 200f Anterior (ventral) horns, 275f, 279,

281f Anterior inferior iliac spine, 189f Anterior interventricular artery, 385f,

389 Anterior pituitary gland, 339, 339f,

340f, 354t hormones of, 340f, 341–343, 341f,

342b, 354t hypothalamus relationship and,

337, 337f, 340f Anterior (aqueous) segment of eye,

308f, 312 Anterior superior iliac spine, 188,

189f Anterior tibial artery, 400, 401f Anterior tibial vein, 402, 403f Anterior trunk muscles, 232, 233f,

240f, 242t Anti-A antibodies, 377, 377t, 378,

378f Anti-B antibodies, 377, 377t, 378,

378f Antibodies, 76, 77t, 375, 428,

437, 443–446, 444f, 445t, 446f, 449f, 450t. See also Humoral (antibody-mediated) immunity

active and passive immunity and, 442–443, 443f

autoimmune disease and, 453 blood groups/transfusion reactions

and, 375, 377, 377t, 378, 378f classes of, 444, 445t function of, 445, 446f monoclonal, 443 structure of, 443–444, 444f

Antibody-mediated (humoral) immunity, 437, 441–446, 441–444f, 445t, 446f, 447f, 449f. See also Humoral (antibody- mediated) immunity

Anticancer drugs, 131b Anticodon, 112, 113f Antidiuretic hormone (vasopressin),

339, 339f, 340, 354t, 553 homeostatic imbalance, 340–341 water balance/blood pressure

and, 340–341, 354t, 553, 554f

Antigen(s), 375, 436, 437, 438, 449f, 451t. See also Immunity/ immune response

antibody inactivation of, 445, 446f autoimmune disease and, 453 incomplete (hapten), 438 presentation of, 440, 446–447,

447f, 449f red blood cell, blood groups and,

375, 377, 377t transplantation and, 451

Antigen-antibody complexes, 445, 446f

Antigen-binding site, 444, 444f Antigen-presenting cells, 438, 440,

446–447, 447f, 449f, 450t Antihistamines, 452 Anti-inflammatory effects, of

glucocorticoids, 348 Antimicrobial proteins/chemicals,

431t, 435–436, 437f Antitoxin, 443 Antivenom, 443 Antrum, ovarian follicle, 573, 574f,

577 Anucleate cells, erythrocytes as, 366 Anuria, 543 Anus, 33, 490f, 491, 495f, 499 Anvil (incus), 317f, 318 Aorta, 385–386f, 386, 387f, 399–400

blood pressure in, 408f Aortic arch, 385f, 399, 401f

branches of, 400 in respiratory control, 479f, 480

Aortic body, in respiratory control, 479f, 480

Aortic semilunar valve, 386f, 389 APCs. See Antigen-presenting cells Apex

of heart, 383, 383f, 385f of lung, 467, 468f of renal (medullary) pyramid, 539

Aphasias, 277 Apical surface, epithelial, 115, 115f Aplastic anemia, 368t Apnea, 480

sleep, 485 Apocrine sweat glands, 147 Aponeuroses, 209, 233f Appendages, skin, 140f, 145–149,

146f, 148f, 149f Appendicitis, 499, 528 Appendicular skeleton, 160, 173f,

184–192 lower limb, 173f, 190–192, 190f,

191f pelvic girdle, 173f, 188, 189f shoulder (pectoral) girdle, 173f,

184, 185f upper limb, 173f, 184–187, 186f,

187f Appendix, 44f, 429, 429f, 490f,

499, 499f Appositional bone growth, 168,

168f Aquaporins, 103, 104f Aqueous humor, 308f, 312 Aqueous (anterior) segment of eye,

308f, 312 Arachnoid granulations, 273, 274f,

275f, 276 Arachnoid mater

brain, 273, 274f, 275f spinal, 280f, 281f

Arachnoid villi Arch(es)

of foot, 191–192, 191f pubic, 189f vertebral, 180, 180f, 181f

Arcuate arteries of foot, 400, 401f of kidney, 539, 540f, 541f

Arcuate veins, of kidney, 539, 540f, 541f

Areola, 582, 582f Areolar, 142 Areolar tissue, 122f, 123–124, 142,

142f Arm. See also Upper limbs

bones of, 184–187, 186f muscles of, 233f, 234f, 235, 240f,

241f, 242t, 243t Arrector pili muscles, 140f, 148, 148f

autonomic nervous system affect- ing, 294t

Arterial blood pressure. See Blood pressure

Arterial pulse, 406–407, 407f pregnancy and, 591

Arteries, 396, 397f. See also specific named artery

blood pressure in, 398, 407–408, 408f. See also Blood pressure

of brain and circle of Willis, 402–405, 404f

“hardening of” (arteriosclerosis), 296, 408, 412b

structure of, 397–398, 397f of systemic circulation, 399–400,

401f Arterioles, 396, 397f

blood pressure in, 408f glomerular, 541f, 542, 543f terminal, 398, 399f

Arteriosclerosis, 296, 408, 412b. See also Atherosclerosis

Arthritis, 198–199 gouty (gout), 199 joint replacement and, 192–193b rheumatoid, 198–199, 199f, 453

Articular capsule, 195, 195f Articular (hyaline) cartilage, 120,

121f, 138f, 163, 164f, 167–168 in bone formation/growth,

167–168, 167f, 168f laryngeal, 465f, 466 in synovial joint, 195, 195f tracheal, 121f, 466, 467f

Articular processes, vertebral, 180, 180f, 181f

Articulations. See Joint(s) Artificial joints, 192–193b Artificial kidney, 549b Artificially acquired immunity

active, 442, 443f passive, 443, 443f

Ascending aorta, 385f, 399, 401f branches of, 400

Ascending colon, 490f, 499, 499f Ascending (sensory) pathways, 280,

282f -ase (suffix), 78 Aspartic acid, 74f Association area

anterior, 267f, 268 posterior, 267f, 268

Association fiber tracts, 268, 270f Association neurons (interneurons),

258f, 259–260 in ascending and descending

pathways, 282f in reflex arc, 263, 264f, 265

Asthma, 452, 473, 482b, 485 Astigmatism, 316b Astrocytes, 253–254, 254f Ataxia, 273 Atelectasis, 473 Atherosclerosis, 412–413b, 418. See

also Arteriosclerosis heart failure and, 395 hypertension and, 412–413b 415

Atherosclerotic (fibrous) plaques, 412b, 413b

Athlete’s foot (tinea pedis), 149

Subject Index 633

Atlas, 179f, 180, 181f Atmospheric pressure, 35–38 Atom(s), 28, 29f, 52

orbital model of, 54, 54f planetary model of, 54, 54f structure of, 52–54, 54f, 54t, 55f

Atomic mass number, 55, 56t Atomic nucleus, 54, 54f, 54t, 56 Atomic number, 55 Atomic structure, 52–54, 54f, 54t, 55f Atomic symbols, 52 Atomic weight, 55–57, 56t ATP (adenosine triphosphate), 35,

51, 53t, 70, 80f, 81, 81f, 513 in active transport, 104, 106 energy release from, 51, 63f, 64,

81, 81f hydrolysis of, 80f, 81, 81f mitochondrial supply of, 93 for muscle contraction, 81, 81f,

211, 217, 218f, 219–220, 220f production of. See ATP (adenos-

ine triphosphate) synthesis structure of, 80f, 81

ATP (adenosine triphosphate) synthesis, 219–220, 220f, 516–520, 517f, 518f, 519f

carbohydrate/glucose metabolism and, 63f, 64, 70, 81, 219–220, 220f, 516–518, 517f, 518f, 519f

fat metabolism in, 518, 519f, 520, 521

protein metabolism in, 519f, 520 Atria, of heart, 384–386, 385f, 387f Atrial diastole (ventricular filling),

391, 392f Atrial natriuretic peptide, 347, 348f,

356t Atrial systole, 391, 392f Atrioventricular (AV) bundle (bun-

dle of His), 390f, 391 Atrioventricular groove, 385f, 389 Atrioventricular (AV) node, 390f,

391 Atrioventricular (AV) valves, 386f,

387, 388f Atrophy, 129, 221, 244 Attachments, muscle, 222–223, 223f Attenuated pathogens, for vaccines,

442 Auditory area, 267f, 322 Auditory (acoustic) canal/meatus

external, 172, 174f, 317, 317f internal, 174, 175f

Auditory ossicles, 317f, 318, 322, 324f

Auditory (pharyngotympanic) tube, 317f, 318, 464, 465f

Aura (hallucination), olfactory, 326 Auricle (pinna), 317, 317f Auscultatory method, for blood

pressure measurement, 408, 409f

Auto-antibodies, 453 Autografts, 451 Autoimmune diseases, 453

hygiene hypothesis and, 482b rheumatoid arthritis as, 198–199

Autologous chondrocyte implanta- tion, 193b

Automatic external defibrillators, 391

Autonomic nervous system, 252f, 253, 290–295, 290f, 291f, 294t. See also specific division

blood pressure and, 296, 408–409, 410f, 411

digestion and, 293–294, 294t, 506, 507

function of, 292–294, 294t heart and, 291f, 292, 294, 294t,

390, 394, 395f

homeostatic imbalances and, 293 parasympathetic division of, 252f,

253, 291, 291f, 292, 293–294 somatic nervous system compared

with, 290–291, 290f sympathetic division of, 252f, 253,

291, 291f, 292–293, 293f Autonomic reflexes, 263 Avascularity

of connective tissue, 119 of epidermis, 140 of epithelial tissue, 115

AV node. See Atrioventricular (AV) node

AV valves. See Atrioventricular (AV) valves

Axial skeleton, 160, 172–184, 173f. See also specific bone

fetal skull, 200, 200f skull, 172–178, 173f, 174f, 175f,

176f thoracic cage (bony thorax), 173f,

182–184, 183f vertebral column (spine), 173f,

178–156, 179–182f Axillary (term), 40, 40f Axillary artery, 400, 401f Axillary lymph nodes, 427f Axillary nerve, 288t, 289f Axillary vein, 402, 403f Axis, 179f, 180–182, 181f Axon(s), 213, 255, 256f, 258f

in motor unit, 213, 214f, 215f neuron classification and, 260, 260f

Axon hillock, 255, 256f Axon terminals, 213, 255, 256f

in motor unit, 213, 214f, 215f signal transmission at synapse

and, 262, 263f AZT, for HIV infection/AIDS, 455b Azygos vein, 402

Baby teeth, 500, 500f Back muscles, 233–234, 234f, 241f,

243t Bacteria

in digestion, 511 in urine, 546t

Balance. See Equilibrium Baldness, 156 Ball-and-socket joint, 197f, 198 Balloon angioplasty, for atheroscle-

rosis, 413b Baroreceptors (pressoreceptors),

blood pressure and, 408, 554f, 555

Barrier methods, for contraception, 595b

Basal cell carcinoma, of skin, 152–153, 152f

Basal cells, taste bud, 326f, 327 Basal metabolic rate, 523–524, 524t.

See also Metabolic rate age-related changes in, 523–524,

524t, 528 Basal nuclei (basal ganglia), 269t,

270, 270f in Parkinson’s disease/Hunting-

ton’s disease, 270, 278b Base

of heart, 383 of lung, 467, 468f of renal (medullary) pyramid,

539 Base(s) (chemical), 66–67, 556

pH of, 67f strong, 67, 556 weak, 67, 556

Base(s) (nucleotide), 78–80, 79f, 109, 109f

in protein synthesis, 111–112, 113f Basement membrane, 115, 117–118f

Base pair, 80 Basilar artery, 404, 404f Basilar membrane, 322, 322f, 324f Basilic vein, 402, 403f Basophils, 365f, 369, 370t

formation of, 371f B cells. See B lymphocytes Bedsores (decubitus ulcers), 144,

144f Benign neoplasm, 129, 130b Beta-amyloid, in Alzheimer’s

disease, 36b, 37f Beta-amyloid peptide, in

Alzheimer’s disease, 278b Beta (β) cells, pancreatic, 350, 351f Beta particles, 56 Beta (β)-pleated sheet, 75f, 76 Beta waves, EEG, 297f Biaxial joints, 194f, 197f, 198 Bicarbonate, in pancreatic juice,

509, 510f Bicarbonate buffer system, 476, 556 Bicarbonate ion, 67, 476, 509

in acid-base balance, 476, 556, 557

carbon dioxide transport/internal respiration and, 476, 477f, 478

Biceps brachii muscle, 233f, 235, 240f, 242t

Biceps femoris muscle, 236f, 238, 241f, 243t

Bicuspid (mitral) valve, 386f, 387 Bicuspids (premolars), 500, 500f Bile/bile salts, 73t, 496–497, 502,

505f, 510f homeostatic imbalances and, 510 in urine, 546t

Bile duct, 496–497, 496f blockage of, 502

Biliopancreatic diversion, for obesity, 530b

Bilirubin, 371 Bilirubinuria, 546t Binocular vision, 314 Binucleate cells, 111 Biochemistry, 64–81

inorganic compounds in, 64, 65–68

organic compounds in, 64–39, 68–81

Biopsy, in cancer diagnosis, 131b Bipennate muscles, 228–229, 229f Bipolar cells, retinal, 309, 310f Bipolar neurons, 260, 260f Birth (parturition), 591–593, 592f Birth canal. See Vagina Birth control, 594–595b Birth control pill, 594b Bitter receptors, 327 Black-and-blue marks, 145 Blackhead, 145 Bladder, 32f, 33, 44f, 538, 538f,

547–548, 547f, 548f autonomic innervation/effects and,

291f, 294t developmental aspects of,

559–561 emptying (micturition/voiding),

548–550 inflammation of (cystitis), 548,

559, 568 pregnancy and, 591 transitional epithelium in, 118f,

119, 547–548 Bladder trigone, 547, 547f Blanching, 145 Blastocyst, 585, 586f Bleach, pH of, 67f “Bleaching of the pigment,” 311b Bleeding disorders, 374–375

liver and, 374, 375 Blepharospasm, 143b

Blindness, night, 309 Blind spot (optic disc), 308f, 309,

310f Blister(s), 139, 151, 151f Blood, 34f, 123f, 124, 363–381

acid-base balance and, 68, 476, 480, 555–556

components of, 364, 365f composition and functions of,

363–373 developmental aspects of,

378–379 formed elements of, 364, 365f,

366–371, 366f, 367f, 370t. See also Blood cells

hematopoiesis (blood cell forma- tion) and, 371–373, 371f, 372f

hemostasis/hemostasis disorders and, 373–375, 373f, 374f

homeostatic imbalances and, 367–368, 368t, 369, 374–375, 374f, 379

pH of, 67f, 68, 364. See also pH physical characteristics/volume

of, 364 respiratory control of pH of, 476,

479, 479f, 480, 556–557 Blood-brain barrier, 276 Blood buffers, acid-base balance

and, 476, 480, 555–556, 556f Blood cells, 123f, 124, 364, 365f.

See also specific type formation of, 161, 163, 371–373,

371f, 372f, 378. See also Hematopoiesis

Blood clotting (coagulation), 373–374, 373f

undesirable, 374, 374f “Blood doping” by athletes, 368 Blood groups, 375–378, 377t, 378f

transplantation and, 451 typing and, 378, 378f, 438

Blood plasma, 124, 364, 365f Blood pressure, 407–412, 408f,

409f, 410f factors affecting, 408–411, 410f fluid movements at capillary beds

and, 414–415, 414f homeostatic imbalances and,

411–412 measuring, 408, 409f mechanisms regulating, 553–555,

554f normal, 408, 411 pregnancy and, 591 sympathetic nervous system

regulation and, 296, 408–409, 410f, 411, 554f

variations in, 411–412 Blood pressure gradient, 407–408,

408f Blood sugar. See Glucose Blood supply. See specific organ

or system and Blood vessels; Circulation

Blood transfusion blood groups and, 375–378, 377t,

378f HIV transmission and, 455b

Blood typing, 378, 378f, 438 Blood vessels, 28, 29f, 30, 31f,

396–415. See also specific type and Circulation

atherosclerosis affecting, 412–413b 412, 418

autonomic nervous system affect- ing, 292, 294t, 408–409, 410f

developmental aspects and, 418 gross anatomy of, 399–406, 401f,

403f lymphatic vessels in relationship

to, 425f, 426f

634 Subject Index

microscopic anatomy of, 396–399, 397f

physiology of circulation and, 406–415

structural differences among, 397–399, 397f, 398f, 399f

tunics of, 396–397, 397f Blood volume, 375, 393–394, 554f

blood pressure and, 409–410, 410f cardiac output and, 393, 395f pregnancy and, 591

“Blue bloaters,” 481 Blue cones, 310, 310f B lymphocytes (B cells), 438, 447f,

449f, 450t activation of, 441–442, 441f, 442f antigen presentation by, 440 clonal selection of, 441, 441f in lymph nodes, 428

BMI. See Body mass index BMR. See Basal metabolic rate Bod Pod, 529 Body of bone

of hyoid, 178, 178f of ischium, 189f of mandible, 178 of pubis, 189f of sternum, 183, 183f of vertebra, 180, 180f, 181f

Body cavities, 41–45, 43f, 44f Body defenses, 429–454, 430f.

See also Immunity/immune response; Inflammatory response/inflammation

adaptive (specific), 430, 436–454, 449f, 450–451t. See also Adap- tive (specific) defense system

developmental aspects and, 455–457

homeostatic imbalances and, 428, 434, 438, 451–453, 452f, 454–455b

innate (nonspecific), 430– 436, 431t. See also Innate (nonspecific) defense system

Body fat, in obesity, 529b Body fluids, 550–551, 551f Body heat regulation, 35, 46,

525–527, 526f skin in, 138–139, 139t, 144,

525–527, 526f Body landmarks, anterior and

posterior, 38–41, 40f Body mass index, 529b Body membranes, 135–138, 137f,

138f. See also Membrane(s) developmental aspects of, 153–156

Body movements, 222–226, 223f, 223t, 224–225f

Body of nail, 149, 149f Body planes/sections, 41, 42f Body regions, terminology for,

38–41, 40f Body of stomach, 493, 494f Body temperature, 35, 526f Body temperature regulation, 35,

46, 525–527, 526f skin in, 138–139, 139t, 144,

525–527, 526f Body tissues. See Tissue(s) Body of uterus, 575f, 576 Body water, 550, 551f, 552f Body weight, 523

obesity and, 529b Boils, 149 Bolus (food), 501, 506, 507f Bonds. See Chemical bonds Bone(s), 29–30, 31f, 120, 121f,

160–172 aging affecting, 129, 201–203, 203f classification of, 161–163, 161f, 162f

compact, 161, 161f, 164f, 165f developmental aspects of,

199–203, 200f, 201f, 203f flat, 161f, 162, 162f formation/growth of, 167–168,

167f, 168f fracture of, 170–172, 171f, 171t,

203 functions of, 161 growth hormone affecting, 168,

340f, 354t homeostatic imbalances and, 170 irregular, 162, 162f long, 162, 162f parathyroid hormone affecting,

168, 345–346 remodeling of, 168–170, 168f short, 162, 162f spongy, 161, 161f, 164f, 165f structure of, 163–167, 164f, 165f,

166t surface markings of, 163, 166t thinning of (osteoporosis),

201–203 Bone cells, 120, 163–164, 165f, 168 Bone collar, 167, 167f Bone densitometry, 36b, 37f Bone markings, 163, 166t Bone marrow

blood cell formation in, 161, 163, 371, 378

cancer of (leukemia), 369 lymphocyte differentiation/

activation in, 439, 439f, 457 red, 163, 378 yellow, 163, 164f

Bone marrow biopsy, 372 Bone matrix, 163–164, 165f 167,

168 Bone remodeling, 168–170, 168f

in fracture repair, 171f, 172 Bone spurs, 198 Bony callus, in fracture repair, 171f,

172 Bony (osseous) labyrinth, 318 Bony pelvis, 188, 189f Bony thorax (thoracic cage), 173f,

182–184, 183f Booster shots, 442 Botox. See Botulinum toxin type A Botulinum toxin type A (Botox),

143b Boundary maintenance, 33

plasma membrane in, 33 skin in, 33, 138, 139t

Bowman’s (glomerular) capsule, 541f, 542, 543f, 544f

BP. See Blood pressure BPD. See Biliopancreatic diversion,

for obesity Brachial (term), 40, 40f Brachial artery, 400, 401f

measuring blood pressure at, 408, 409f

pulse at, 407f Brachialis muscle, 233f, 235, 240f,

241f, 242t Brachial plexus, 285f, 288t, 289f Brachial vein, 402, 403f Brachiocephalic trunk, 385f, 400,

401f Brachiocephalic veins, 402, 403f Brachioradialis muscle, 233f, 235,

240f, 241f Bradycardia, 391 Brain, 30, 31f, 252f, 253, 265–273,

266f, 267f, 269t aging affecting, 296, 297 arterial supply of, 402–405, 404f blood-brain barrier and, 276 cerebrospinal fluid and, 273–276,

275f

developmental aspects of, 266f, 296–297

diagnosis of disorders and, 296–297b

functional anatomy of, 265–273, 266f, 267f, 269t

homeostatic imbalances/dysfunc- tion of, 276, 277–278, 278–279b

meninges of, 273, 274f protection of, 273–277, 274f, 275f,

276f traumatic injury of, 277

Brain stem, 266f, 267f, 269t, 271–272, 271f, 282f

development of, 266f Brain tumors, gliomas, 255 Brain waves, 296b, 297f Braxton Hicks contractions, 591 BRCA1t/BRCA2t genes, 582–583 Breast(s) (mammary glands), 32f,

580–583, 582f, 583f developmental aspects of, 596 prolactin affecting, 340f, 341, 354t

Breast cancer, 582–583, 583f, 598f Breastfeeding, 582

oxytocin and, 339, 354t Breathing, 471–474, 472f, 473f. See

also Respiration basic rhythm of, 478, 479f

Breathing sounds, vesicular, 475 Breech presentation, 593 Broad ligament, 573, 575f Broca’s area, 266, 267f Bronchi, 30, 32f, 463f, 466–467, 468f Bronchial arteries, 400 Bronchial sounds, 475 Bronchial (respiratory) tree, 469,

469f Bronchioles, 469, 469f Bronchitis, chronic, 473, 481, 481f Bruises, 145 Brush border, 497, 498f. See also

Microvilli Brush border enzymes, 497, 505f,

509 Buccal (term), 40, 40f Buccal phase of swallowing, 506 Buccinator muscle, 230, 230f, 242t Buffers, acid-base balance and, 68,

476, 480, 555–556, 556f Buffy coat, 364, 365f Bulbo urethral glands, 566f, 567,

567f, 568 Bundle branches, 390f, 391 Bundle of His (atrioventricular

bundle), 390f, 391 Burn(s), 150–152, 151f Bursae, 138, 195f, 196 Bursitis, 198 Buttocks, gluteus maximus forming,

236, 236f

C. See Carbon; Cytosine Ca. See Calcium Café coronary, 466 Calcaneal (term), 40f, 41 Calcaneal (Achilles) tendon, 239,

239f, 241f Calcaneus, 191, 191f Calcitonin, 343, 345, 354t

in calcium regulation, 345, 346f, 354t

Calcium, 53t, 56t atomic structure of, 56t bone remodeling and, 168, 170 calcitonin regulation and, 345,

346f, 354t heart rate and, 394 homeostatic imbalances and,

345–346 in hormone mechanism of action,

336

in muscle contraction, 214, 215f, 217, 218f

parathyroid (PTH) regulation and, 337f, 338, 345, 346f, 354t

rickets and, 170 signal transmission at synapse

and, 262 storage of in bone, 161

Callus, in fracture repair bony, 171f, 172 fibrocartilage, 171–172, 171f

Calories/kilocalories, 513, 523, 524 Calyces, renal, 539, 540f cAMP. See Cyclic AMP (cAMP/cyclic

adenine monophosphate), in hormone mechanism of action

Canaliculi in bone, 164, 165f lacrimal, 306, 306f

Canal of Schlemm (scleral venous sinus), 308f, 312

Cancer, 130–131b. See also specific type or organ affected

lymph node involvement and, 428 Cancer chemotherapy, 131b Canines (eyeteeth), 500, 500f Canthi (commissures), lateral and

medial, 305, 305f Capillaries (blood), 387f, 396, 397f,

399f blood pressure in, 408f fluid movements and, 414–415,

414f gas/nutrient exchange and, 387f,

412–414, 414f, 470, 470f, 475, 476f

glomerular, 541f, 542, 543f, 544f in inflammation, 433, 433f peritubular, 541f, 542, 543f structure of, 397f, 398–399

Capillaries (lymph), 425, 425f, 426f Capillary beds, 387f, 396, 397f,

398, 399f fluid movements at, 414–415, 414f in hepatic portal system, 405f

Capillary exchange, 387f, 412–414, 414f, 470, 470f, 475, 476f

Capitate, 187f Capitulum, 186f, 187 Capsule, lymph node, 427, 428f Carbohydrate(s), 35, 68, 69–70,

69f, 514–515, 514t. See also Glucose

absorption/digestion of, 504, 505f, 509

dietary sources of, 514–515, 514t metabolism of, 63, 63f, 70,

516–518, 517f, 518f, 519f Carbon, 53t, 56t, 59f

as chemically reactive element, 59–60, 59f, 61f

in organic compounds, 64–65 Carbon dioxide, 34f

in acid-base balance, 556–557 exchange/transport of, 34f, 476,

476f, 477f, 478. See also Gas exchange; Gas transport

formation of, 60, 62f in respiratory control, 479f, 480

Carbonic acid, 66 in acid-base balance, 480, 556,

556f, 557 carbon dioxide transport/internal

respiration and, 476, 477f, 478 Carbonic anhydrase, in internal

respiration, 478 Carbon monoxide poisoning, 477 Carboxypeptidase, 505f, 509 Carbuncles, 149 Carcinogens, 130b Cardia (cardial region of stomach),

493, 494f

Blood vessels (continued)

Subject Index 635

Cardiac circulation, 385f, 389 Cardiac conduction system, intrinsic,

390–391, 390f Cardiac cycle, 391–393, 392f Cardiac failure, 395–396 Cardiac hypertrophy, aerobic

exercise and, 222 Cardiac murmurs, 392 Cardiac muscle bundles, 210–211,

210f Cardiac muscle/cardiac muscle tis-

sue, 124–126, 125f, 208t, 210–211, 210f. See also Heart

contraction of, 124, 126, 208t Cardiac muscle cells, 101, 208t,

210–211, 210f Cardiac output, 393–396, 395f, 398

blood pressure and, 408, 410f muscular/respiratory pumps and,

394, 395f, 398, 398f pregnancy and, 591

Cardiac physiology, 28 Cardiac valves, 386f, 387–390, 388f Cardiac veins, 385f, 389, 403f Cardial region of stomach (cardia),

493, 494f Cardinal signs of inflammation,

432, 433f Cardioesophageal sphincter, 493,

507, 507f Cardiovascular system, 29f, 30,

31f, 382–423. See also Blood vessels; Circulation; Heart

blood vessels, 396–415 developmental aspects of,

415–418 digestive system and, 417b, 531b endocrine system and, 358b, 417b heart, 383–396 homeostatic imbalances and, 384,

389, 391, 392, 395–396, 399, 411–412, 415

homeostatic relationships of, 34f, 417b

integumentary system and, 155b, 417b

lymphatic system/immunity and, 417b, 456b

muscular system and, 246b, 417b nervous system and, 298b, 417b pregnancy and, 591 reproductive system and, 417b,

597b respiratory system and, 417b, 462,

484b skeletal system and, 202b, 417b urinary system and, 417b, 560b

Career focus licensed practical nurse, 558b medical transcriptionist, 154b pharmacy technician, 82b phlebotomy technician, 376b physical therapy assistant, 321b radiologic technologist, 169b

Carotene, skin color and, 144 Carotid arteries, 174, 400, 401f,

404, 404f pulse at, 407f

Carotid body, in respiratory control, 480

Carotid canal, 174, 175f Carpal (term), 40, 40f Carpal bones/carpus (wrist), 173f,

187, 187f Carrier-mediated facilitated

diffusion, 103, 104f Carriers (protein), 91, 103 Cartilage, 29, 31f, 120, 121f, 160.

See also specific type articular (hyaline), 120, 121f,

163, 164f, 167–168, 167f, 168f in bone growth, 167–168, 167f

connective tissue, 120, 121f elastic, 120 laryngeal, 465f, 466 thyroid, 465f, 466 tracheal, 121f, 466, 467f

Cartilaginous joints, 193, 194f, 195, 196t

Caruncle, lacrimal, 305, 305f Cascade, of complement proteins,

436 Castration, 573 CAT (computerized axial tomogra-

phy). See Computed tomography Catabolism, 516. See also

Metabolism decomposition reactions and,

63, 63f Catalase, 96 Catalyst(s), 64t, 77, 77t. See also

Enzyme(s)/enzyme activity Cataracts, 312, 312f Catecholamines, 337f, 349

stress response/sympathetic stimulation and, 349, 350f

Catheter, urinary, 550 Cations, 59 Cauda equina, 279, 280f Caudal (term), 39t. See also under

Inferior Cavities (body), 41–45, 43f, 44f Cavities (bone marking), 163 CCK. See Cholecystokinin Cecum, 490f, 495f, 498, 499f Celiac trunk, 400, 401f Cell(s), 28, 29f, 88–114, 98–100t 101f

anatomy of, 89–102, 90f, 94f, 98– 100t. See also specific structure

cytoplasm of, 89, 90f, 93–97, 94f, 95f, 97f, 98–99t

developmental aspects of, 128–129 diversity/specialization of, 100–102,

101f, 128–129 division of, 35, 108–111, 109,

109f, 110f extensions of, 97–100 generalized, 89, 90f, 94f membrane of, 89, 90–92, 90f, 91f,

92f, 94f, 98t. See also Plasma membrane

membrane transport and, 91, 102– 108, 103f, 104f, 106f, 107f, 108f

nucleus of, 89–90, 90f, 94f, 99–100t

overview of, 88–89 physiology of, 98–100t 102–114 protein synthesis and, 111–114, 113f structural organization at level of,

28, 29f Cell adhesion proteins, 120 Cell body, neuron, 126f, 255, 256f,

258f nuclei, 257

Cell division, 35, 108–111, 109, 109f, 110f. See also Cytokinesis; Mitosis

in cancer, 129b development and, 129 DNA replication preceding, 109,

109f Cell (membrane) junctions, 92, 92f Cell life cycle, 108–109 Cell-mediated (cellular) immunity,

437, 446–448, 447f, 448f, 449f Cell membrane, 89, 90–92, 90f, 91f,

92f, 94f, 98t. See also Plasma membrane

phospholipids in, 72 Cell theory, 89 Cellular basis of life, 88–89 Cellular extensions, 97–100 Cellular (cell-mediated) immunity,

437, 446–448, 447f, 448f, 449f

Cellular inclusions, 93, 98t, 99t Cellular metabolism, autonomic

nervous system affecting, 294t Cellular reproduction, 35 Cellular respiration, 471, 516–518,

517f, 518f ATP for muscle activity and, 219,

220f Cellulose, 504 Cement, 501, 501f Central artery, retinal, 308f, 310f Central (Haversian) canals, 121f,

163, 165f Central canal of spinal cord, 279,

281f cerebrospinal fluid in, 275f, 279

Central nervous system, 30, 252f, 253, 258f, 265–281, 290f. See also Brain; Spinal cord

blood-brain barrier and, 276 cerebrospinal fluid and, 273–276,

275f developmental aspects of, 266f diagnosis of problems and,

296–297b homeostatic imbalances/

dysfunction of, 276, 277–278, 278–279b, 280–281, 296–297b

meninges of, 273, 274f myelinated fibers in, 254f, 257 protection of, 273–277, 274f, 275f,

276f supporting cells in, 253–255, 254f

Central sulcus, 266, 267f Central vein, retinal, 308f, 310f Centrioles, 94f, 97, 99t

in cell division, 110–111, 110f Centromere, 110, 110f, 111 Centrosome, 97 Centrum (body) of vertebra, 180,

180f, 181f. See also under Superior

Cephalic (term), 39t, 40f, 41 Cephalic vein, 402, 403f Cerebellum, 266f, 267f, 269t, 271f,

272–273 development of, 266f

Cerebral angiography, 296b Cerebral aqueduct, 271, 271f, 274,

275f Cerebral arterial circle (circle of

Willis), 404f, 405 Cerebral arteries, 404, 404f Cerebral cortex, 266–268, 267f,

268f, 269t, 282f Cerebral edema, 277 Cerebral embolus, 374 Cerebral hemispheres (cerebrum),

265–270, 266f, 267f, 268f, 269t, 270f, 271f, 282f

development of, 266f Cerebral palsy, 295, 295f Cerebral peduncles, 271, 271f Cerebrospinal fluid, 273–276, 275f Cerebrovascular accidents (strokes),

277, 374 diagnosis of, 296–297b

Cerebrum (cerebral hemispheres), 265–270, 266f, 267f, 268f, 269t, 270f, 271f, 282f

Cerumen (earwax), 317 Ceruminous glands, 317 Cervical (term), 40, 40f Cervical cap, 594b, 595b Cervical curvature of spine, 179f,

180 Cervical enlargement, 280f Cervical lymph nodes, 427f Cervical nerves, 280f, 282f, 285f Cervical plexus, 285f, 288t Cervical vertebrae, 179, 179f,

180–182, 181f

Cervix (uterine), 575f, 576 cancer of, 576 dilation of in labor, 591–592, 592f

Cesarean section (C-section), 593 CF. See Cystic fibrosis CFTR protein, 483 CH4. See Methane, formation of Channel-mediated facilitated

diffusion, 103, 104f Cheekbones (zygomatic bones),

172, 174f, 175f, 176f, 177 Cheeks, 491 Chemical(s)

blood pressure affected by, 410–411, 410f

in body defense, 431t, 432–436 breathing affected by, 480 structural organization at level of,

28, 29f Chemical bonds, 58–64, 59–62f. See

also specific type chemical reactions and, 58,

62–64, 63f, 64t Chemical buffers, acid-base balance

and, 68, 476, 480, 555–556, 556f

Chemical changes in matter, 51 Chemical digestion, 496–497,

503–504, 503f. See also Digestion Chemical energy, 51 Chemical equation, 57, 64 Chemical reactions, 58, 62–64,

63f, 64t decomposition reactions, 63, 63f exchange reactions, 63–64, 63f factors affecting rate of, 64, 64t neutralization reactions, 67 reversibility of, 64 synthesis reactions, 62–63, 63f water in, 65

Chemical senses, 324–327, 325f, 326f. See also Smell, sense of; Taste, sense of

developmental aspects of, 329 Chemistry, 50–87

chemical bonds and, 58–64, 59–62f

chemical reactions and, 58, 62–64, 63f, 64t

energy and, 51–52 living matter (biochemistry) and,

64–81. See also Biochemistry matter and, 50–51 matter composition and, 52–57 molecules/compounds and,

57–58, 57f Chemoreceptors

in digestion, 506 in respiration control, 480 for taste and smell, 324–327,

325f, 326f, 492 Chemotactic factors, in immunity,

450t Chemotaxis

positive, 368, 432, 434, 434f Chemotaxis chemicals, complement

activation and, 436 Chemotherapy, 131b Chewing (mastication), 492, 500,

503f, 506 muscles used in, 230, 230f

CHF. See Congestive heart failure Chief cells, 494f, 495 Childbirth (parturition), 591–593,

592f Chills, fever and, 527 Chlorine, 53t, 56t, 57, 57f

sodium chloride formation and, 57–58, 57f, 59, 60f

Choking, 466 Cholecystokinin, 356t, 497t, 510,

510f

636 Subject Index

Cholesterol, 71f, 72, 73t, 515, 521 in cell membrane, 72 metabolism/transport of, 521–522 in plasma membrane, 90, 91,

91f, 521 Cholinergic fibers, 292 Chondrocytes, 120, 121f, 193b Chordae tendineae, 386f, 387, 388f Chorea, Huntington’s, 270, 279b Chorion, 587f Chorionic gonadotropin, human

(hCG), 353–357, 585 Chorionic vesicle (blastocyst), 585,

586f Chorionic villi, 586, 587f Choroid, 308f, 309 Choroid plexus, 271, 271f, 273,

275f Chromatid(s), 110, 110f, 111 Chromatin, 90, 90f, 94f, 100t

in cell division, 110, 110f, 111 Chromium, 53t Chromosomes, 90, 110–111, 110f Chronic bronchitis, 473, 481, 481f Chronic obstructive pulmonary

disease (COPD), 481, 481f Churning food (stomach), 503, 503f Chyme, 495, 508, 509–511, 510f Chymotrypsin, 505f, 509 Cigarette smoking. See Smoking Cilia, 97, 99t, 100, 114

in body defense, 431t in nasal mucosa, 464 oocyte transport and, 574 in trachea, 466, 467f

Ciliary body, 308f, 309 accommodation and, 313

Ciliary glands, 305 Ciliary muscle, autonomic nervous

system affecting, 294t, 316 Ciliary zonule, 308f, 309, 312 Ciliated (pseudostratified) columnar

epithelium, 116, 118f Circle of Willis, 404f, 405 Circular folds (plicae circulares),

497, 498f Circular muscle layer, alimentary

canal, 210f, 492, 493f stomach, 494f

Circular muscles, 228, 229f Circulation. See also specific type

and Blood vessels for brain/circle of Willis, 402–405,

404f cardiac, 385f, 389 fetal, 415, 415, 416f hepatic portal, 405, 405f, 406f,

521 physiology of, 406–415 portal, hypothalamic hormones

released into, 339, 340f pulmonary, 386, 387f renal, 539, 540f systemic, 386, 387f, 399–400,

401f, 403f Circulatory shock, 411

in burn injury, 150 hypotension and, 411

Circumcision, 568 Circumduction (body movement),

225f, 226 Circumflex artery, 385f, 389 Circumvallate papillae, 326, 326f Cirrhosis, 502 Cisterna chyli, 427f Citric acid cycle, 516, 517, 517f,

519f Cl. See Chlorine Clavicle, 173f, 184, 185f, 200f Clavicular notch, 183f Cleavage, 585, 586f Cleavage furrow, 110f, 111

Cleft lip, 527, 527f Cleft palate, 177, 464, 527, 527f Clitoris, 575f, 576f, 577 Clonal selection, 441

B lymphocyte, 441, 441f T lymphocyte, 447

Clone, 441 Closed (simple) fracture, 170 Closed reduction of fracture, 170 Clot-dissolving agent, 413 Clotting, blood (coagulation),

373–374, 373f undesirable, 374, 374f

Clotting cascade, 373–374 Clotting factors/proteins, 363f, 364,

521 deficits of, 374–375

CNS. See Central nervous system CO. See Cardiac output; Cobalt Coagulation (blood clotting),

373–374, 373f undesirable, 374, 374f

Coagulation factors/proteins, 363f, 364, 521

deficits of, 374–375 Cobalt, 53t Coccyx, 178, 179f, 182, 182f, 189f Cochlea, 317f, 318, 322, 322f, 324f Cochlear duct, 322, 322f, 324f Cochlear nerve, 284f, 322, 322f Codons, 112, 113f Coenzymes, vitamins as, 516 Coitus interruptus, 594b, 595b Cold exposure, hypothermia and,

525 Cold sore (fever blister), 149–150,

150f sun exposure and, 142

Collagen, 76, 76f, 120, 121f, 122f, 143 wrinkles and, 143b

Collarbone (clavicle), 173f, 184, 185f, 200f

Collateral branch, axonal, 255, 256f Collateral ganglion, 292, 293f Collecting ducts, 539–542, 541f Collecting vessels, lymphatic, 425f,

426 Colloid(s), 65 Colon, 499, 499f. See also Large

intestine ascending, 490f, 499, 499f cancer of, 130, 131b descending, 490f, 499, 499f peristalsis in, 511–513 sigmoid, 490f, 499, 499f transverse, 490f, 495f, 499, 499f

Colon cancer, 528 development of, 130f, 131b

Colony stimulating factors (CSFs), in hematopoiesis, 372

Color blindness, 310–312 Color vision, 309–312

developmental aspects of, 328 Columnar epithelium, 115, 115f,

116f, 118f pseudostratified, 116, 118f simple, 116, 117f stratified, 119

Coma, 272, 296b brain injury causing, 277

Comminuted fracture, 171t Commissures, 268, 270f, 271f

eye (medial and lateral), 305, 305f

Common carotid arteries, 385f, 400, 401f

pulse at, 407f Common fibular nerve, 288t, 289f Common hepatic artery, 400 Common hepatic duct, 496f, 502

blockage of, 502 Common iliac arteries, 400, 401f

Common iliac veins, 402, 403f Communicating arteries, 404f, 405 Communicating (gap) junctions,

92, 92f Compact bone, 161, 161f, 164f, 165f Complement, 431t, 435–436, 437f,

445, 446f, 450t Complementarity, principle of, 89 Complementary bases, 79f, 80, 109,

109f in protein synthesis, 112, 113f

Complement fixation, 436, 445, 446f Complete (fused) tetanus, 218, 219f Complete proteins, 515 Complicated plaques, 413b Compound(s), 57–58, 57f

inorganic, 64, 65–68 organic, 64–39, 68–81

Compound (open) fracture, 170 Compression fracture, 171t Computed tomography (CT), 36b,

37f in brain lesion diagnosis, 296b

Computerized axial tomography (CAT). See Computed tomogra- phy (CT)

Concentration acid-base/hydrogen ion, 67, 67f chemical reaction rate affected

by, 64t tonicity/osmotic pressure and,

105b Concentration gradient, 103

active transport and, 106 diffusion and, 103

Conceptus, 583, 584f Conchae (nasal), 176f, 177, 178,

464, 465f Concussion, 277 Condom, 594b, 595b Conducting zone structures, 470 Conduction deafness, 323 Conduction system of heart,

intrinsic, 390–391, 390f Conductivity, neuron, 126, 260,

262–263, 263f Condylar joint, 197–198, 197f Condyle (bone marking), 166t Cones, 309–312, 310f, 311b Congestive heart failure, 395–396 Conjunctiva, 305, 305f, 306f Conjunctivitis, 306

congenital gonococcal, 328 Connective tissue, 29f, 119–124,

121–123f, 127f. See also specific type

adipose, 122f, 124 aging and, 129, 245 areolar, 122f, 123–124, 142, 142f blood, 123f, 124 bone, 120, 121f cartilage, 120, 121f common characteristics of, 119–120 dense/dense fibrous, 120–123,

122f, 127, 142, 142f in dermis, 142 extracellular matrix of, 120 fibrocartilage, 120, 121f loose, 122f, 123–124, 123f as muscle component, 208t, 209,

209f, 245 of nerve covering, 281–282, 283f reticular, 123f, 124 in tissue repair (fibrosis), 127,

128 Connective tissue membranes, 135,

136–138, 138f Connexons, 92, 92f Constant (C) region, 443–444, 444f Constipation, 513

pregnancy and, 590 Contact dermatitis, 150, 453

Contraception, 594–595b Contractility (muscle), 30, 207, 208t,

213. See also Muscle contraction cardiac muscle, 124, 126, 208t movement and, 30, 207, 211 skeletal muscle, 30, 124, 208t,

213. See also Skeletal muscle contraction

smooth muscle, 126, 208t, 210 Contractions, uterine, 591–592, 592f,

593 Braxton Hicks, 591

Contracture, 128 Control center, in homeostasis,

45, 46f Contusion, brain, 277 Convergence, 314, 316 Convergent muscles, 228, 229f COPD. See Chronic obstructive

pulmonary disease CO poisoning. See Carbon

monoxide poisoning Copper, 53t Copulation, 569 Coracoid process, 184, 185f Core body temperature, 525 Cornea, 308f, 309 Cornified layer of skin, 141–142, 141f Cornua (horns), hyoid bone, 178,

178f Corona radiata, 270f Coronal (frontal) section/plane,

41, 42f Coronal suture, 172, 174f, 176f Corona radiata (oocyte), 574f, 578,

579f Corona radiata (projection fibers),

270f “Coronary” (myocardial infarct),

389, 393b Coronary arteries, 385f, 389, 400,

401f Coronary artery disease/

atherosclerosis, 412–413b, 418 heart failure and, 395 hypertension and, 412–413b 415

Coronary sinus, 389 Coronary sulcus, 385f, 389 Coronoid fossa, 186f, 187 Coronoid process, 187 Corpora quadrigemina, 271f, 272 Corpus callosum, 268, 270f, 271f,

275f Corpus luteum, 573, 574f, 579–580,

581f, 585 Cortex

adrenal. See Adrenal cortex cerebral, 265, 266–268, 267f, 268f,

269t, 282f hair, 147, 148f lymph node, 428, 428f renal, 539, 540f

Corti, spiral organ of, 322, 322f aging affecting, 329

Cortical nephrons, 541f, 542 Cortical radiate arteries, 539, 540f,

541f, 543f Cortical radiate veins, 539, 540f,

541f, 543f Corticospinal (pyramidal) tract, 266 Corticosteroids, 73t, 347–349

homeostatic imbalances and, 348–349

production in adrenal cortex, 347f in stress response, 350f

Cortisol, 73t, 347 Cortisone, 347 Costal cartilage, 183f Costal facets, 181f, 182 Cough reflex/coughing, 466,

475t, 507 smoking and, 466

Subject Index 637

Covalent bonds, 59–60, 61f, 62f Covalent molecules, 59–60

nonpolar, 60 Covering and lining membranes,

135, 136, 137f Coxal (term), 40, 40f Coxal (hip) bones, 188, 189f, 200f CP. See Creatine phosphate (CP),

ADP phosphorylation for ATP production by

Cr. See Chromium Crackles, 475 “Cradle cap,” 145, 145f Cranial (term), 39t. See also under

Superior Cranial aponeurosis, 229–230, 230f Cranial cavity, 42–43, 43f Cranial nerves, 252f, 253, 283, 284f,

286–287t Cranial sutures, 172, 174f, 194f, 195 Craniosacral division of autonomic

nervous system, 292. See also Parasympathetic division of autonomic nervous system

Cranium, 172–177, 172, 173–176f growth/developmental aspects of,

199–200, 200f, 201f Creatine phosphate (CP), ADP

phosphorylation for ATP production by, 219, 220f

Creatinine, 545 C (constant) region, 443–444, 444f Crepitus, 198 Crest (bone marking), 166t Cretinism, 344 Crib death, 485 Cribriform plates, 175f, 177, 465f Cricoid cartilage, 465f Crista ampullaris, 319, 320f Cristae (mitochondrial), 93 Crista galli, 175f, 177 Cross bridges, 213

in sliding filament theory, 216–217, 217f, 218f

Cross matching, blood, 378 Cross (transverse) section/plane,

41, 42f Crown of tooth, 500–501, 501f Crural (term), 40, 40f Crying, 475t Cryptococcal meningitis, 454b Cryptorchidism, 596 C-section (cesarean section), 593 CSF. See Cerebrospinal fluid CSFs. See Colony stimulating factors

(CSFs), in hematopoiesis CT. See Computed tomography Cu. See Copper Cubital vein, median, 402, 403f Cuboidal epithelium, 115, 115f, 116f

simple, 116, 117f stratified, 119

Cuboid bone, 191f Cuneiform bones, 191f Cupula, 319–320, 320f Curvatures (spinal), 179–180, 200

abnormal, 180, 180f, 201, 203f growth/developmental aspects

and, 179, 179f, 180, 201, 203f vertebral collapse/osteoporosis

and, 201, 203f Curvatures (stomach), 493, 494f Cushing’s syndrome, 348–349 Cushioning function, of water, 65 Cutaneous glands, 145–147, 146f Cutaneous membrane, 135,

136, 137f, 138–156. See also Skin (cutaneous membrane/ integument)

Cutaneous sensory receptors/ cutaneous sense organs, 139t, 140f, 141f, 143, 258

Cutaneous vascular plexus, 140f Cuticle

hair, 147, 148f nail, 149, 149f

CVAs. See Cerebrovascular accidents (strokes)

Cyanosis, 144, 477, 480, 481 Cyclic AMP (cAMP/cyclic adenine

monophosphate), in hormone mechanism of action, 336, 336f

Cyclic GMP (cGMP/cyclic guanosine monophosphate), in hormone mechanism of action, 336

Cysteine, 74f Cystic duct, 496f, 502 Cystic fibrosis, 483, 528 Cystitis, 548, 559, 568

prostatic hypertrophy and, 568 Cytokines, 447–448, 447f, 450t

in delayed hypersensitivity, 453 Cytokinesis, 110, 110f, 111 Cytomegalovirus retinitis, in HIV

infection/AIDS, 454b Cytoplasm, 89, 90f, 93–97, 94f, 95f,

97f, 98–99t. See also specific structure

Cytoplasmic membrane. See Plasma membrane

Cytoplasmic organelles, 93–97, 94f, 98–99t. See also specific type

Cytosine (C), 78, 79f, 80, 109, 109f Cytoskeleton, 96–97, 97f Cytosol, 93, 94f, 98t

glycolysis in, 516–517, 517f Cytotoxic (killer) T cells, 447, 447f,

448f, 449f, 450t in delayed hypersensitivity, 452 toxic chemicals released by, 447,

451f

Dandruff, 142 Dark (A) bands, 212, 212f DaTscan, 297b Daughter cells, 90, 111

in cleavage, 585 types A and B, 569f, 570

Daughter nuclei, 110 DCT. See Distal convoluted tubule ddC, for HIV infection/AIDS, 455b Dead space volume, 474 Deafness, 323

aging and, 328–329 maternal rubella and, 328

Death, flat EEG and, 296b Deciduous teeth, 500, 500f Decomposition reactions, 63, 63f Decubitus ulcers (bedsores), 144,

144f Deep (term), 39t. See also under

Internal Deep artery of thigh, 400 Deep-brain (thalamic) stimulation,

for Parkinson’s disease, 279b Deep palmar arch, 401f Defecation, 503f, 504, 511–513 Defecation reflex, 511–513 Defense system. See Body defenses Defibrillator, 391 Degenerative joint disease

(osteoarthritis), 198, 203 Deglutition (swallowing), 503, 503f,

506–507, 507f Dehydration synthesis, 68–69, 68f

disaccharides formed by, 70 protein synthesis and, 112 of sucrose, 69f triglycerides formed by, 70, 71f

Delayed hypersensitivity, 452–453 Delta waves, EEG, 297f Deltoid (term), 40, 40f Deltoid muscles, 233f, 234, 234f,

240f, 241f, 242t, 243t

Deltoid tuberosity, 186, 186f Dementia

AIDS, 454b Alzheimer’s, 278b

Denaturation, of functional protein, 77

Dendrites, 255, 256f, 258f neuron classification and, 260,

260f Dendritic cells, 430, 440, 447f

epidermal, 141f, 142, 440 Dens, 181f, 182 Dense/dense fibrous connective

tissue, 120–123, 122f in dermis, 142, 142f in tissue repair (fibrosis), 127

Dentin, 501, 501f Deoxyribonucleic acid. See DNA Deoxyribose, 70, 79f, 80 Depolarization

muscle cell, 214, 215f neuron, 261f, 262

Depo-Provera, 595b Depressed fracture, 171t Depressions (bone marking), 163,

166t Dermal papillae, 140f, 142–143,

142f Dermatitis, 156

contact, 150, 453 Dermis, 136, 139, 140f, 141f,

142–144, 142f, 151 Descending colon, 490f, 499, 499f Descending (motor) pathways, 280,

282f Desmosomes (anchoring junctions),

92, 92f in epidermis, 141f

Detrusor muscle, 547 Deuterium, 55, 56f DEXA (dual-energy X-ray

absorption), 529 Dextrinase, 505f Diabetes insipidus, 340–341, 553 Diabetes mellitus, 351, 453

aging and, 357 renal disease and, 549b

Dialysis, 549b Diapedesis, 368, 433, 434f Diaphragm, 43, 43f, 44f, 463f, 468f,

471 movement of during breathing,

471, 472f neural control of breathing and,

478, 479f Diaphragm (contraceptive), 594b,

595b Diaphysis, 163, 164f Diarrhea, 513 Diarthroses, 193, 196t Diastole, 391, 392, 392f Diastolic pressure, 408, 408f, 411 Diencephalon (interbrain), 266f,

269t, 270–271, 271f development of, 266f

Diet blood pressure and, 411 cardiovascular diseases and, 418 weight loss/obesity and, 530b

Diffusion, 102–104, 103, 103f in absorption, 511 in capillary exchange, 413, 414f,

470, 475 membrane permeability and, 103,

104f Digestion, 35, 503–504, 503f, 505f.

See also specific digestive organ and nutrient

aging affecting, 528 autonomic nervous system affect-

ing, 293–294, 294t, 506, 507 hydrolysis in, 65, 503

in large intestine, 499, 503f, 511–513

liver in, 502 mechanical breakdown and, 503,

503f, 506 in mouth/pharynx/esophagus,

491, 492, 503f, 505f, 506–507, 507f

in small intestine, 496–498, 503f, 505f, 509–511, 510f

in stomach, 495, 503f, 505f, 507–509, 508f

teeth in, 500 Digestive cavity, 44 Digestive enzymes, 495, 496, 497,

501–502, 505f, 509–510 Digestive hormones, 497t Digestive system, 30–33, 32f,

489–513, 490f. See also specific structure

accessory organs, 490, 490f, 500–502

aging affecting, 528 alimentary canal (gastrointestinal

tract) organs, 490–500, 490f anatomy of, 489–502, 490f autonomic nervous system affect-

ing, 293–294, 294t, 506, 507 cardiovascular system and, 417b,

531b developmental aspects of,

527–528 endocrine system and, 358b, 531b functions of, 502–513, 503f, 505f homeostatic imbalances and,

491–492, 500, 502, 508, 509–510, 511, 513, 527–528

homeostatic relationships of, 34f, 531b

integumentary system and, 155b, 531b

lymphatic system/immunity and, 456b, 531b

muscular system and, 246b, 531b nervous system and, 298b, 531b pregnancy and, 590 processes/controls of, 503–506,

503f, 504f, 505f reproductive system and, 531b,

597b respiratory system and, 484b,

531b skeletal system and, 202b, 531b urinary system and, 531b, 560b

Digital (term), 40, 40f Digital arteries, 401f Digital veins, 403f Dilation stage of labor, 591–592,

592f Dimers, 444 Dipeptidase, 505f Direct gene activation, in hormone

mechanism of action, 335–336, 336f

Directional terms in anatomy, 38, 39t Disaccharides, 69f, 70, 504

digestion of, 504, 505f Discs, intervertebral, 179, 179f, 195

herniated (“slipped”), 179 Dislocation, joint, 196 Dissociation, 65, 66f

strong and weak acid, 556, 556f Distal (term), 39t Distal convoluted tubule, 541f, 542,

544f Disulfide bonds, antibody, 443, 444f Diuresis, 340 Diuretics, 340

for weight loss, 530b Diversity, cellular, 100–102, 101f,

128–129 Diverticula, 511

638 Subject Index

Diverticulitis, 511 Diverticulosis, 511, 528 DJD. See Degenerative joint disease DNA (deoxyribonucleic acid), 79f,

80, 89, 90, 111–112 mutations of in cancer, 130b,

131b in protein synthesis, 89, 111–112,

113f replication of, 109, 109f structure of, 79f, 80, 109, 109f

Docking proteins, in exocytosis, 107, 107f

L-Dopa, for Parkinson’s disease, 279b

Dopamine, deficiency of, in Parkinson’s disease, 278b

Dorsal (term), 39t, 40f. See also under Posterior

Dorsal body cavity, 42–43, 43f Dorsal column, 280 Dorsal (posterior) horns, 275f, 279,

281f Dorsalis pedis artery, 400, 401f

pulse at, 407f Dorsal metatarsal veins, 403f Dorsal rami, 283, 285f, 293f Dorsal respiratory group, 478 Dorsal root, 279, 281f, 285f, 293f Dorsal root ganglion, 279, 281f, 285f Dorsal venous arch, 402, 403f Dorsiflexion (body movement),

225f, 226 Double helix, DNA, 79f, 80, 109,

109f Double sugars (disaccharides), 69f,

70 Dowager’s hump, 201, 203f Drug abuse, HIV transmission and,

455b DSR. See Dynamic spatial

reconstruction Duchenne’s muscular dystrophy,

244 Ductless glands, 119, 338. See also

Endocrine system/glands Duct system

in females, 33, 573–576, 575f in males, 33, 565–567, 566f

Ductus arteriosus, 415, 416f Ductus (vas) deferens, 565, 565f,

566–567, 566f, 567f Ductus venosus, 415, 416f Duodenal papilla, 496f, 497 Duodenal ulcers, 512b Duodenum, 490f, 494f, 495f,

496–497, 496f Dural venous sinuses, 273, 274f,

276, 403f Dura mater

brain, 273, 274f, 275f spinal, 280f, 281f

“Dust cells” (alveolar macrophages), 470, 470f

Dwarfism pituitary, 341, 341f thyroid deficiency and, 344

Dynamic equilibrium, 319–320, 320f Dynamic spatial reconstruction

(DSR), 36b Dyspnea, 481

in COPD, 481 pregnancy and, 591

Dystocia, 593 Dystrophin, 244 Dysuria, 548

ē. See Electron(s) Ear, 316–324. See also Equilibrium;

Hearing anatomy of, 317–318, 317f congenital abnormalities of, 328

developmental aspects of, 328–329

homeostatic imbalances and, 318, 323–324, 328–329

Eardrum (tympanic membrane), 317, 317f

homeostatic imbalance and, 318 Earwax (cerumen), 317 Eating. See Ingestion EC. See Emergency contraceptive

pill Eccrine sweat glands, 140f,

146–147, 146f ECF. See Extracellular fluid ECG. See Electrocardiogram/

electrocardiography (ECG) Ectoderm, 585, 587f Edema, 124, 425

cerebral, 277 in heart failure, 340, 396 hypertonic solutions for, 105b in inflammation, 432, 433, 433f pulmonary, 396

EEG. See Electroencephalogram Effacement, in dilation stage of

labor, 592 Effector

in homeostasis, 45, 46f in nervous system response, 30

Effector organ, in reflex arc, 263, 264f

Effector T cells, 447 Efferent arteriole, 541f, 542, 543f Efferent (motor) division of periph-

eral nervous system, 252f, 253 Efferent lymphatic vessels, 428,

428f Efferent nerves/neurons/tracts.

See Motor (efferent) nerves; Motor (efferent) neurons; Motor (efferent) tracts

Efferent pathway, in homeostasis, 45, 46f

Eggs. See Ova Ejaculation, 565, 566 Ejaculatory duct, 566, 566f, 567,

567f Elastic cartilage, 120 Elastic fibers, 120, 122f, 143 Elasticity, muscle, 213 Elastic laminae, blood vessel, 396,

397f Elbow joint, 194f Electrical energy, 51 Electrical synapses, 262 Electrocardiogram/electro-

cardiography (ECG), 391, 393b Electrochemical event, signal trans-

mission at synapse as, 263, 263f Electroencephalogram (EEG), 296b,

297f Electrolyte(s), 66

fluid balance and, 551 salts as, 66, 365f

Electrolyte balance, 66, 544, 553–555, 554f

Electrolyte imbalances, heart rate and, 394

Electromagnetic spectrum, energy of (radiant energy), 51, 56

Electron(s), 52, 53, 54, 54f, 54t, 55, 55f

chemical bond formation and, 58, 59f

valence, 58 Electron cloud, 54, 54f Electron shells (energy levels), 58,

59f Electron transport chain, 516,

517–518, 517f, 519f Elements, 52, 53t

chemically inert, 58, 59f

chemically reactive, 58, 59f identifying, 54–57, 55f, 56t

Elephantiasis, 457 Embolus/emboli, 374 Embryo, 564, 583, 584f, 587f. See

also Fetus development of, 583–593, 584f,

586f, 587f Embryonic period, 583, 584f,

585–587, 586f, 587f, 588t Emergency contraceptive pill,

594–595b Emesis (vomiting)/emetic center,

509 Emmetropia/emmetropic eye, 315b Emotional factors

breathing and, 479–480 skin color and, 145

Emphysema, 474–475, 481, 481f Emulsification of fats, by bile salts,

502 Enamel, tooth, 501, 501f Encephalitis, 273 Endocarditis, 389 Endocardium, 384, 385f Endocrine system/glands, 30, 31f,

119, 334–362, 338f, 354–355t. See also specific gland and Hormones

cardiovascular system and, 358b, 417b

developmental aspects of, 129, 357

digestive system and, 358b, 531b functioning tumors and, 341 homeostatic relationships of, 358b integumentary system and, 155b,

358b lymphatic system/immunity and,

358b, 456b muscular system and, 246b, 358b nervous system and, 298b, 358b reproductive system and, 358b,

597b respiratory system and, 358b,

484b skeletal system and, 202b, 358b stimuli activating, 337–338, 337f urinary system and, 358b, 560b

Endocrinology, 335 Endocytosis, 107–108, 108f

in capillary exchange, 413, 414f Endoderm, 585, 587f Endolymph, 318, 320f

equilibrium and, 320, 320f hearing and, 322f

Endometrium, 575f, 576 in uterine (menstrual) cycle, 580,

581f Endomysium, 208t, 209, 209f, 210 Endoneurium, 281, 283f Endoplasmic reticulum, 90f, 93–95,

94f, 95f, 98t Golgi apparatus in packaging

products of, 95, 95f protein synthesis/export by, 93,

94, 95f Endosteum, 163, 164f Endothelium, blood vessel, 396,

397f Endurance exercise

heart affected by, 222 muscles affected by, 222, 222f

Energy, 51–52 from ATP, 51, 63f, 64, 70, 81,

81f, 519f in chemical bonds, 51 in decomposition reactions, 63,

63f for muscle contraction, 81, 81f,

219–220, 220f in synthesis reactions, 63

Energy balance, 523–527, 524t, 526f Energy conversions, 51–52 Energy intake, 523 Energy levels (electron shells), 58,

59f Energy output, 523 Energy value of food, 513, 523 Enteroendocrine cells, 494f, 495 Enterogastric reflex, 509 Enzyme(s)/enzyme activity, 77–78,

77t, 78f, 111 chemical reaction rate affected

by, 77–78 protein synthesis and, 112, 113f

Enzyme-substrate complex, 77, 78f Eosinophils, 365f, 369, 370t

formation of, 371f Ependymal cells, 254, 254f Ephedra supplements, for weight

loss, 530b Epicardium (visceral pericardium),

137f, 384, 385–386f Epicondyle (bone marking), 166t Epidermal dendritic cells (Langer-

hans cells), 141f, 142, 440 Epidermis, 136, 139, 140–142, 140f,

141f in body defense, 431t

Epididymis, 565, 565f, 566f, 567f Epididymitis, 596 Epigastric region, 43, 44f Epiglottis, 464, 465f, 466, 491f, 507f Epimysium, 208t, 209, 209f Epinephrine (adrenaline), 290f, 293,

337f, 338, 349, 355t, 410 blood pressure and, 410–411 heart rate and, 394, 395f, 410 stress response/sympathetic

stimulation and, 337f, 338, 349, 350f, 411

Epineurium, 281, 283f Epiphyseal line, 163, 164f Epiphyseal plate, 120, 163, 168, 168f Epiphyses, 163, 164f Epithalamus, 271 Epithelial cells, 100, 101, 101f Epithelial membranes, 135, 136, 137f Epithelial root sheath, 148f Epithelial tissue/epithelium, 29f,

114–119, 115f, 117–118f, 127f. See also specific type

aging and, 129 alimentary canal, 492, 493f of bladder wall, 118f classification of, 115–119, 115f columnar, 115, 115f, 116f, 118f cuboidal, 115, 115f, 116f glandular, 119 regeneration of, 115 simple, 115–116, 115f, 117f squamous, 115, 115f, 116f of stomach, 494f, 495 stratified, 115, 115f, 116–119 transitional, 116f, 118f, 119,

547–548 unique characteristics of, 114–115

Equilibrium, 304, 318–320, 319f. See also Ear

deficits/homeostatic imbalance and, 323–324

ER. See Endoplasmic reticulum Erectile tissue, 566f, 567f, 568–569 Erection, 569 Erector spinae muscles, 233–234,

234f, 243t ERV. See Expiratory reserve volume Erythema, 145 Erythrocyte(s) (red blood cells),

100, 101f, 123f, 364, 365f, 366–368, 366f, 367f, 370t

in anemia, 367, 368t formation of, 371–372, 371f, 372f

Subject Index 639

gas transport and, 476, 477f, 478 IV therapy and tonicity of, 105b in sickle-cell anemia, 367, 367f,

368t in urine, 546t

Erythrocyte antigens, blood groups and, 375, 377, 377t

transplantation and, 451 Erythropoiesis, 371–372, 372f Erythropoietin, 356t, 371–372, 372f,

537 Escherichia coli, urinary tract

infections and, 559 Esophageal arteries, 400 Esophageal sphincter, 507f Esophagitis, 508 Esophagus, 30, 32f, 464, 465f, 466,

467f, 468f, 490f, 491, 491f, 492 food propulsion and, 506–507,

507f Essential amino acids, 515, 515f Essential (primary) hypertension,

411 Estradiol, 579 Estriol, 579 Estrogen replacement therapy, 598 Estrogens, 73t, 348, 353, 355t,

579, 582 adrenal, 348 aging affecting, 357 bone health and, 201 labor initiation and, 591 in menopause, 596–598 in menstrual/ovarian cycles, 580,

581f placental, 357

Estrone, 579 Ethmoid bone, 172f, 174f, 175f,

176f, 177 Ethmoid sinus, 177f Eupnea, 478 Evaporation, heat loss via, 525,

526f Eversion (body movement), 225f,

226 Exchange reactions, 63–64, 63f Exchange vessels, 398 Excreta, 35 Excretion, 35 Excretory system. See Urinary system Exercise

blood pressure affected by, 409, 410, 410f

breathing affected by, 478 heart affected by, 222, 415–418 muscles affected by, 221–222,

222f, 244, 245 osteoporosis and, 201–203 in weight control, 530b

Exocrine glands, 119, 338 cutaneous glands as, 145, 146f skin and, 129

Exocytosis, 94f, 96f, 107, 107f in capillary exchange, 413, 414f

Exophthalmos, in Graves’ disease, 345, 345f

Exotoxins, neutralization of, 445, 446f

Expiration (exhalation), 471, 472f, 473, 473f

Expiratory reserve volume, 474, 474f

Expulsion stage of labor, 592–593, 592f

Extensibility, muscle, 213 Extension (body movement), 223,

224f, 227f Extensions (cellular), 97–100

neuron, 101 Extensor carpi radialis muscle, 241f,

243t Extensor carpi ulnaris muscle, 241f

Extensor digitorum longus muscle, 238, 239f, 240f, 242t

Extensor digitorum muscle, 241f, 243t Extensor muscles, back (erector

spinae), 233–234, 234f, 243t External (term), 39t. See also under

Superficial External acoustic (auditory) canal/

meatus, 172, 174f, 317, 317f External anal sphincter, 499, 499f External carotid artery, 400, 401f External (outer) ear, 317, 317f External (extrinsic) eye muscles,

306, 307f, 316 External genitalia

in female, 576–577, 576f homeostatic imbalances/

development and, 593 in male, 568–569

External iliac artery, 400, 401f External iliac vein, 402, 403f External intercostal muscles, 232,

471, 472f neural control of breathing and,

478, 479f External jugular vein, 402, 403f External oblique muscles, 232,

233f, 240f, 242t External respiration, 471, 475, 476,

476f, 477f External urethral orifice, 566f, 567,

567f, 576f External urethral sphincter, 547f,

548 incontinence and, 550

Extracellular fluid, 91, 91f, 102, 550, 551f

Extracellular matrix, 120 Extrinsic (external) eye muscles,

306, 307f, 316 Eye, 305–316. See also under Visual

accessory structures of, 305–306, 305f, 306f

anatomy of, 305–313, 305–308f, 310f. See also specific structure

autonomic innervation/effects and, 291f, 294t, 316

congenital disorders of, 327–328 developmental aspects of,

327–328 extrinsic (external) muscles of,

306, 307f, 316 homeostatic imbalances and, 306,

309, 327–328 internal structures (eyeball) of,

307–313, 308f, 310f, 312f pathway of light through,

313–314, 313f Eyeball, 307–313, 308f, 310f, 312f.

See also Eye developmental aspects of, 328

Eyebrow, 305f Eyelashes, 305, 305f, 306f Eyelids, 305, 305f, 306f

in myasthenia gravis, 244, 245f Eye movements, 306, 307f Eye reflexes, 316 Eyestrain, 316 Eyeteeth (canines), 500, 500f

F. See Fluorine Facet (bone marking), 166t Facial artery, pulse at, 407f Facial bones, 172, 173f, 177–178

growth/developmental aspects and, 199–200, 200f

Facial muscles, 229–230, 230f, 240f, 242t

Facial nerve (cranial nerve VII), 284f, 287t, 327

in taste sensation, 327 Facilitated diffusion, 103–104, 104f

Falciform ligament, 495f, 502 Fallopian (uterine) tubes, 32f, 33,

573–574, 575f False labor, 591 False pelvis, 188, 189f False ribs, 183f, 184 Falx cerebri, 273, 274f Farsightedness (hyperopia), 315–316b Fascia, renal, 539 Fascicles

muscle, 209, 209f, 228–229, 229f nerve, 281, 283f

Fat (adipose tissue), 101, 101f, 122f, 124

autonomic nervous system affecting, 294t

bone storage and, 161, 163 Fat(s), 35, 514t. See also Lipid(s)

absorption/digestion of, 504, 505f metabolism of, 518–520, 519f neutral (triglycerides), 70–72, 71f,

73t, 515 saturated and unsaturated, 70–72,

72f, 515 trans, 72

Fat capsule, perirenal, 539 Fat cells, 100, 101, 101f, 122f, 124,

530b Fat deposits, 520 Fat-soluble vitamins, 73t Fatty acids, 70, 71f

lipid digestion and, 504, 505f omega–3, 72 saturated and unsaturated, 70–72,

72f Fatty streak stage, 412b Fe. See Iron Feces, 33, 34f, 35, 498, 511–513 Feedback

erythropoietin secretion and, 372, 372f

feeding behavior and, 523 glucose regulation and, 350, 352f homeostasis and, 45–46, 46f hormone secretion and, 337 uterine contractions in labor and,

591, 592f Feet. See Foot Female reproductive system, 32f, 33

anatomy of, 573–577, 575f developmental aspects of, 593,

596–598 function of, 577–580, 581f homeostatic imbalances and, 574,

576, 596 mammary glands and, 32f,

580–583, 582f, 583f pregnancy/embryonic develop-

ment and, 583–593, 584f, 588–589t, 589f, 594–595b

Female sex hormones, 73t, 348 Femoral (term), 40, 40f, 41 Femoral artery, 400, 401f

pulse at, 407f Femoral nerve, 288t, 289f Femoral vein, 402, 403f Femur, 173f, 190–191, 190f, 200f Fenestra, 413, 414f Fenestrated capillaries, 413, 414f Ferritin, 371 Fertility awareness methods, of

contraception, 595b Fertilization, 35, 564, 570, 570f, 574,

584–585, 584f, 585f, 586f Fetal circulation, 415, 416f Fetal heart rate, 394 Fetal hemoglobin, 378 Fetal period, 583–584, 584f, 587,

588–589t 589f Fetal skull, 199–200, 200f Fetus, 564, 583–584, 584f, 587f,

588–589t 589f

bone development in, 199–200, 200f

HIV transmission to, 455b Fever, 436, 527

in body defense, 431t, 436 heart rate and, 395

Fever blister (cold sore), 149–150, 150f

sun exposure and, 142 Fiber (dietary), 504 Fibers/processes (neuron), 101, 126,

126f, 255, 256f, 258, 258f neuron classification and, 260,

260f Fiber tracts, association and projec-

tion, 268, 270f Fibrillation, cardiac, 391, 393b Fibrils (myofibrils), 212–213, 212f Fibrin, 373f, 374 Fibrin clot, 374, 374f Fibrinogen, 365f, 373, 373f Fibroblasts, 100, 101f, 120–123, 122f Fibrocartilage, 120, 121f Fibrocartilage callus, in fracture

repair, 171–172, 171f Fibrosis, 127, 128 Fibrous capsule, renal, 539, 540f Fibrous connective tissue, 120–123,

122f in dermis, 142, 142f in tissue repair (fibrosis), 127,

128 Fibrous joints, 193, 194f, 195, 196t Fibrous layer of eye, 309 Fibrous pericardium, 383, 385f Fibrous (atherosclerotic) plaques,

412b, 413b Fibrous (structural) proteins, 76,

76f, 77, 111 Fibrous sheath, hair follicle,

147–148, 148f Fibrous tissue, dense, 120–123, 122f

in dermis, 142 in tissue repair (fibrosis), 127,

128 Fibula, 173f, 190f, 191 Fibular (term), 40f, 41 Fibularis muscles (longus/brevis/

tertius), 238–239, 239f, 240f, 241f, 242t

Fibular nerve, 288t, 289f Fibular vein, 402 “Fight-or-flight” response/system,

292, 349. See also Sympathetic (thoracolumbar) division of autonomic nervous system

Filaments, in sperm tail, 570, 571f Filtrate, 104

glomerular, 543, 543f, 544f, 545 Filtration, 103, 104

epithelium and, 114, 115, 116 glomerular, 542–543, 543f, 544f

Filtration slits, 541f, 542 Fimbriae, uterine tube, 574, 575f Fingerprints, 143 Fingers, bones of (phalanges), 173f,

187, 187f First-degree burn, 150, 151f First heart sound, 392 First messenger, in hormone

messenger of action, 336, 336f Fissures

in bone, 166t in cerebral hemispheres, 265,

267f lung, 467, 468f

Fixators (muscle), 226 Flaccid muscle, 221 Flaccid paralysis, 280 Flagella, 97–98, 101f, 102 Flat bones, 161f, 162, 162f Flatus, 511

640 Subject Index

Flexion (body movement), 223, 224f, 226, 227f

Flexor carpi radialis muscle, 240f, 243t

Flexor carpi ulnaris muscle, 241f, 243t

Flexor digitorum superficialis muscle, 243t

Flexor (withdrawal) reflex, 264f, 265 Floating ribs, 183f, 184 Fluid balance, 550–553, 551f, 552f,

554f body fluids/fluid compartments

and, 550–551, 551f electrolytes and, 551 water intake/output and, 551–553,

552f, 554f Fluid compartments, 550–551, 551f Fluid loss, in burn injury, 150 Fluid mosaic model, 90–92, 91f Fluid (hydrostatic) pressure,

filtration and, 104 Fluorine, 53t fMRI. See Functional MRI “Foam cells,” 412b Foliate papillae, 326, 326f Follicle cells, ovarian, 573, 578f Follicles

hair, 140f, 146, 147–148, 148f lymph node, 428, 428f ovarian, 573, 574f, 577, 578, 578f,

581f thyroid, 343, 344f

Follicle-stimulating hormone, 340f, 343, 354t

in oogenesis, 343, 577 in spermatogenesis/testis

regulation, 343, 569, 572f in uterine (menstrual) cycle, 580,

581f Follicular phase of ovarian cycle, 581f Fontanels, in fetal skull, 199–200,

200f Food. See also Nutrients

energy value of, 513, 523 nutrition value/guides and,

513–514, 514t, 515f Food absorption. See Absorption Food breakdown

digestion, 503–504, 503f, 506, 509–510, 510f, 511. See also Digestion

mechanical, 503, 503f, 506 Food groups, 513–514, 514t Food guides, 513–514, 515f Food ingestion. See Ingestion Food propulsion. See Propulsion Foot. See also Lower limbs

bones of, 191–192, 191f muscles of, 238–239, 239f

Foot processes, 541f, 542 Foramen, 166t

infraorbital, 176f interventricular, 275f jugular, 174, 175f magnum, 174, 175f mental, 174f, 176f obturator, 188 ovale (fetal heart), 415, 416f ovale (skull), 174, 175f vertebral/intervertebral, 179f, 180,

180f, 181f Forced expiration, 473 Forearm. See also Upper limbs

bones of, 186f, 187 muscles of, 235, 240f, 241f, 243t

Foreskin (prepuce), 566f, 567f, 568 Formed elements of blood, 364,

365f, 366–371, 366f, 367f, 370t. See also Blood cells

Fornix, 270f Fossa (bone marking), 166t

Fossa ovalis, 386f, 415 Fourth ventricle, 271f, 272,

274–276, 275f Fourth-degree burns, 151 Fovea centralis, 308f, 309 Fractures, bone, 170–172, 171f,

171t pathologic, 203

Freckles, 142 Free edge, of nail, 149, 149f Free nerve endings (pain recep-

tors), 139t, 143, 258–259, 259f Free radicals, 96 Frenulum, lingual, 491, 491f

homeostatic imbalance and, 491–492

Frequency (urinary) aging and, 559 infection and, 548

Frontal (term), 40f, 41 Frontal bone, 172, 174f, 175f, 176f

in fetal skull, 200f Frontalis muscle, 229–230, 230f,

240f, 242t Frontal lobe, 266, 267f Frontal (coronal) section/plane,

41, 42f Frontal sinus, 177f, 465f Frostbite, 525 Fructose, 69, 504, 505f FSH. See Follicle-stimulating hormone Fuel efficiency theory, of obesity,

530b Full-thickness burn, 150, 151f Functional groups, 68 Functional layer of endometrium,

in uterine (menstrual) cycle, 580, 581f

Functional MRI, 37b Functional (globular) proteins,

76–77, 76f, 77t, 111, 366 denaturation of, 77

Fundus of eye, 312, 313f of stomach, 493, 494f of uterus, 575f, 576, 590f

Fungiform papillae, 326, 326f Fused (complete) tetanus, 218, 219f Fusiform muscles, 228, 229f Fusion inhibitors, for HIV infection/

AIDS, 455b

G. See Guanine Galactose, 69, 504, 505f Gallbladder, 490f, 495f, 496f, 502

autonomic innervation/effects and, 291f

Gallbladder attack, 502, 528 Gallstones, 502, 528 Gametes, 564, 570, 570f Gamma globulin, 443. See also

Antibodies passive immunity and, 443, 443f

Gamma interferon, 450t Gamma rays, 56–57 Ganglia, 258, 258f. See also specific

type Ganglion cells, retinal, 309, 310f Gap (communicating) junctions,

92, 92f Gardasil, 576 Gases (matter), 51 Gas exchange, 412–414, 414f, 469,

470, 475, 476, 476f Gas movements, nonrespiratory,

475, 475t Gas transport, 30, 34f, 471,

476–477, 477f Gastric artery, 400 Gastric bypass surgery, for obesity,

530b Gastric cancer, 528

Gastric glands, 494f, 495 autonomic nervous system

affecting, 294t Gastric inhibitory peptide, 497t Gastric juice, 495, 507, 508

in body defense, 431t, 432 pH of, 67f

Gastric pits, 494f, 495 Gastric ulcers, 508, 512b Gastric veins, 405, 406f Gastrin, 356t, 495, 497t, 507 Gastrocnemius muscle, 236f, 239,

239f, 240f, 241f, 243t Gastroenteritis, 528 Gastrointestinal (GI) tract

(alimentary canal), 490–500, 490f. See also specific structure and Digestive system

walls of, 492, 493f Gender

basal metabolic rate affected by, 524t

determination of, 593 Gene(s), 111

cancer and, 130–131b in hormone mechanism of action,

335–336, 336f nucleic acids composing, 78 obesity, 530b

Genitals autonomic innervation/effects

and, 291f female, 576–577, 576f homeostatic imbalances/

development and, 593 male, 568–569

German measles (rubella), maternal, 295, 328

congenital eye disorders and, 328 Germinal center, lymph node, 428,

428f Germ layers, primary, 585, 587f Gestation period, 583 GH. See Growth hormone Gigantism, 341, 341f Gingiva(e) (gums), 491f, 501, 501f GIP. See Gastric inhibitory peptide GI tract. See Gastrointestinal (GI) tract Gland(s)/glandular epithelia, 119.

See also specific type autonomic nervous system

affecting, 294t endocrine (ductless), 119, 338.

See also Endocrine system/glands exocrine, 119

Glans penis, 566f, 567f, 568 Glaucoma, 312 Glenoid cavity, 184, 185f Glia/glial cells (supporting cells),

126f, 253–255, 254f Gliomas, 255 Globin, 76f, 371 Globular molecule, tertiary protein

structure and, 75f, 76 Globular (functional) proteins,

76–77, 76f, 77t, 111, 366 denaturation of, 77

Glomerular (Bowman’s) capsule, 541f, 542, 543f, 544f

Glomerular filtration, 542–543, 543f, 544f

Glomerulonephritis, 453, 559 Glomerulus, 541f, 542, 543f Glossopharyngeal nerve (cranial

nerve IX), 284f, 287t, 327 in taste sensation, 327

Glottis, 466 Glucagon, 350, 355t

in blood glucose regulation, 351–352, 352f, 355t

pancreatic islets producing, 350 Glucoamylase, 505f

Glucocorticoids, 73t, 347–348, 347f, 355t

homeostatic imbalances and, 348–349

in stress response, 347, 349, 350f Gluconeogenesis, 521, 522f Glucose, 69, 70, 504, 505f, 516

in ATP formation, 63f, 64, 70, 81, 219–220, 220f, 516–518, 518f

in diabetes mellitus, 351 glycogen in regulation of, 350,

352f, 521, 522f metabolism of. See Glucose

metabolism pancreatic hormones in regulation

of, 350–352, 352f transport of, 103–104 in urine, 350, 546t

Glucose metabolism, 63, 63f, 70, 516–518, 517f, 519f

liver and, 516, 521, 522f thyroid affecting, 343

Glucose phosphate, formation of, 63f, 64

Gluteal (term), 40f, 41 Gluteal nerves (superior and

inferior), 288t, 289f Gluteal tuberosity, 190, 190f Gluteus maximus muscle, 236,

236f, 241f, 243t Gluteus medius muscle, 236f, 238,

241f, 243t Glycerol, 70, 71f

lipid digestion and, 504, 505f Glycine, 74f Glycocalyx, 91, 92 Glycogen, 70, 518, 521, 522f

breakdown of (glycogenolysis), 63, 63f, 521, 522f

glucose regulation and, 350, 352f, 521, 522f

Glycogenesis, 521, 522f Glycogenolysis, 63, 63f, 521, 522f Glycolipids, 73t, 91f Glycolysis, in cellular respiration/

ATP formation, 219–220, 220f, 516–517, 517f

Glycoproteins, 91–92, 91f Glycosuria (glucose in urine), 350,

546t GnRH. See Gonadotropin-releasing

hormone (GnRH), control of testis and

Goblet cells, 116 in large intestine, 499 in trachea, 466

Goiters, 343–344, 344f Golgi apparatus, 94f, 95, 95f, 96f, 98t Golgi tendon organ, 259f Gomphoses, 195 Gonad(s), 343, 353, 355t, 564. See

also Ovaries; Testes differentiation of, 593 gonadotropic hormones affecting,

343 hormones produced by, 353, 355t

Gonadal arteries, 400, 401f Gonadal veins, 402 Gonadotropic hormones/gonado-

tropins, 343. See also Follicle- stimulating hormone; Luteinizing hormone

in female cycles, 580, 581f human chorionic (hCG), 353–357,

585 Gonadotropin-releasing hormone

(GnRH), control of testis and, 572f

Gonorrhea, 574 congenital eye disorders and, 328 pelvic inflammatory disease and,

574

Subject Index 641

Gouty arthritis (gout), 199 Graafian (vesicular) follicles, 573,

574f, 578, 578f, 581f Graded potential, 262 Graded responses, in muscle

contraction, 217–219 Grafts (transplantation), 448–451

renal, 549b Granulations, arachnoid, 273, 274f,

275f, 276 Granulation tissue, 128, 171 Granulocytes, 369, 370t Granzymes, 432, 447, 448f, 451t Graves’ disease, 345, 345f, 453 Gray matter, 258

cerebral (cerebral cortex), 266–268, 267f, 268f, 269t, 282f

spinal cord/roots, 279–280, 281f, 282f

Great cardiac vein, 385f, 403f Greater curvature of stomach, 493,

494f Greater omentum, 493–495, 495f Greater sciatic notch, 188, 189f Greater trochanter, 190, 190f Greater tubercle, 186, 186f Greater vestibular glands, 575f,

576f, 577 Great saphenous vein, 402, 403f Green cones, 310–312, 310f Greenstick fracture, 171t Groove (bone marking), 166t Gross anatomy, 28

of long bone, 163, 164f, 166t of skeletal muscles, 229–244

Ground substance, 120 Growth, 35

growth hormone treatment and, 342b

Growth hormone, 77t, 340f, 341, 342b, 354t

bone growth and, 168, 340f, 354t homeostatic imbalance, 341

Growth (epiphyseal) plate, 120, 163 Guanine (G), 78, 79f, 80, 109, 109f Guanosine monophosphate, cyclic

(cGMP), in hormone mechanism of action, 336

Gullet. See Esophagus Gum(s) (gingiva), 491f, 501, 501f Gustatory area, 267f Gustatory (taste) cells, 326, 326f Gustatory hairs, 326–327, 326f Gyri (gyrus), 265, 267f

H. See Hydrogen H+. See Hydrogen ions Hair(s), 147, 148f

developmental aspects of, 156 Hair bulb, 148, 148f Hair bulb matrix, 147, 148f Hair cells

in equilibrium, 319, 319f, 320 in hearing, 322, 322f, 324f

Hair follicle(s), 140f, 146, 147–148, 148f

Hair follicle receptor (root hair plexus), 140f

Hair loss, 156 Hair root, 140f, 147, 148f Hair shaft, 140f, 147, 148f Hallucinations, olfactory, 326 Hamate, 187f Hammer (malleus), 317f, 318 Hamstring muscles, 236f, 238, 241f,

243t Hand, bones of, 187, 187f Hapten(s), 438 Hard palate, 175f, 464, 465f, 491,

491f Harvey, William, 396 Haustra, 499f, 500

Haustral contractions, 511 Haversian (central) canals, 121f,

163, 165f Haversian system (osteon), 163,

165f Hb. See Hemoglobin HbA. See Hemoglobin A HbF. See Fetal hemoglobin HbO2. See Oxyhemoglobin hCG. See Human chorionic

gonadotropin HCO3

?. See Bicarbonate ion HDLs. See High-density lipoproteins He. See Helium Head. See also Skull

arteries supplying, 400, 401f muscles of, 229–230, 230f, 240f,

242t, 243t trauma to, brain injury and, 277 veins draining, 402, 403f

Headache, sinus, 464 Head (bone marking), 166t

of femur, 190, 190f of humerus, 186, 186f of radius, 186f, 187

Head of sperm, 570, 571f Healthy Eating Pyramid, 513–514,

515f Hearing, 304, 322–324, 322f, 323f,

324f. See also Ear cortical areas in, 267f, 322 deficits/homeostatic imbalance

and, 323–324 developmental aspects of,

328–329 Hearing loss, 323

aging and, 328–329 Heart, 28, 29f, 30, 31f, 34f, 383–396.

See also under Cardiac anatomy of, 383–389, 383–384f

385–386f 387f autonomic innervation/effects

and, 291f, 292, 294, 294t, 390, 394, 395f

blood supply of, 385f, 389 cardiac cycle/heart sounds and,

391–392, 392f cardiac muscle in, 124–126, 125f,

208t, 210–211 cardiac output and, 393–396,

395f chambers of, 384–386, 385–386f

387f congenital disease of, 415 coverings/walls of, 383–384, 385f developmental aspects of,

415–418 great vessels and, 385–386f 386,

387f homeostatic imbalances and, 384,

389, 391, 392, 395–396, 415 intrinsic conduction system of,

390–391, 390f physiology of, 390–396 size/location/orientation of, 383,

383–384f, 468f valves of, 386f, 387–390, 388f, 392

“Heart attack” (myocardial infarct), 389, 393b

Heart block, 391 Heartburn, 508

pregnancy and, 590 Heart disease, congenital, 415 Heart failure, congestive, 395–396 Heart murmurs, 392 Heart rate, 393, 394–395, 395f

cardiac output and, 393, 395f fetal, 394 normal, 394 pulse rate and, 406 rapid/slow (tachy-/bradycardia),

391

regulation of, 390–391, 394–395, 395f

thyroxine affecting, 345, 394, 395f Heart sounds, 392 Heart valves, 386f, 387–390, 388f Heat

energy conversion and, 51–52 in inflammation, 432, 433f, 434 muscles generating, 211 regulation of. See

Thermoregulation Heat capacity, of water, 65 Heat exhaustion, 527 Heat-loss mechanisms, in body

temperature regulation, 139, 139t, 144, 525–527, 526f

Heat-promoting mechanisms, in body temperature regulation, 139t, 525, 526f

Heat stroke, 527 Heavy chains, 443, 444f Heimlich maneuver, 466 Helicobacter pylori, peptic ulcers

and, 512b Helium, 55f, 59f

atomic mass number of, 55 atomic structure of, 55, 55f as chemically inert element, 59f

Helper factors, in immunity, 450t Helper T cells, 447–448, 447f, 449f,

450t, 452 in HIV infection/AIDS, 454b

Hematocrit, 364 Hematoma, 145

in fracture repair, 170–171, 171f Hematopoiesis, 371–373, 371f, 372f

in bone, 161, 163 developmental aspects of, 378

Hematuria, 546t Hemianopia, 314 Hemiplegia, 277 Hemocytoblast, 371, 371f Hemodialysis, 549b Hemoglobin, 76f, 77, 366, 367

fetal, 378 oxygen content/transport in, 366,

367, 476, 477f sickle cell anemia and, 367, 367f,

368t skin color and, 144 in urine, 546t

Hemoglobin A, 379 Hemoglobinuria, 546t Hemolysis, Rh incompatibility and,

377–378 Hemolytic anemia, 368t Hemolytic disease of newborn, 378 Hemophilia, 375 Hemorrhagic anemia, 368t Hemostasis, 373–375, 373f, 374f,

374f disorders of, 374–375, 374f

Henle, loop of (nephron loop), 541f, 542, 544f

Heparin, 549b Hepatic artery, common, 400 Hepatic ducts, 496f, 502 Hepatic (right colic) flexure, 499,

499f Hepatic metabolism, 518, 519f,

520–522, 522f Hepatic portal circulation, 405,

405f, 406f, 521 Hepatic portal vein, 402, 403f,

405, 405f, 406f Hepatic veins, 402, 403f Hepatitis, 502 Hepatopancreatic ampulla/sphincter,

496f, 497 Herd immunity, 442–443 Hermaphrodites, 593 Hernia, hiatal, 508

Herniated (“slipped”) disc, 179 Herpes simplex. See Human

herpesvirus 1 Hiatal hernia, 508 Hiccups, 475t High blood pressure (hypertension),

411–412, 418 atherosclerosis and, 412–413b,

415 diet and, 411

High-density lipoproteins (HDLs), 73t, 522

High-energy phosphate bonds, 81 High heat capacity, of water, 65 Hilum

of kidney, 538f, 539, 540f of lung, 467, 468f of lymph node, 428, 428f

Hinge joint, 196, 197f Hip (coxal) bone, 188, 189f, 200f Hip muscles, 236–238, 236–237f

241f, 242t, 243t Hip replacement/prosthesis,

192–193b His bundle (atrioventricular bundle),

390f, 391 Histamine, 332, 369

in acute (immediate) hypersensitivity, 452, 452f

blood pressure and, 411 in digestion, 497t in inflammatory response, 432,

433f HIV infection/AIDS, 375, 453,

454–455b growth hormone for wasting and,

342b Homeostasis, 45–46, 46f

control mechanisms in, 45, 46f disturbances in (homeostatic

imbalances), 46. See also specific type or organ/system affected

lysosome rupture, 95 selective permeability and, 102 tissue repair and, 128

Homeostatic relationships of cardiovascular system, 34f, 417b of digestive system, 34f, 531b of endocrine system, 358b of integumentary system, 34f, 155b of lymphatic system, 456b of muscular system, 246f of nervous system, 298f of reproductive system, 597b of respiratory system, 34f, 484b of skeletal system, 202b of urinary system, 34f, 560b

Hooke, Robert, 88 Hormone replacement therapy, 598 Hormones/hormonal stimuli, 30, 76,

77t, 119, 334, 335–338, 336f, 337f, 354–355t. See also specific type

bone growth and, 168 chemistry of, 335 contraception and, 594–595b control of release of, 337–338, 337f developmental aspects of, 357 in digestion, 497t endocrine system activation and,

337, 337f heart rate regulation and, 394,

395f mechanisms of action of, 335–336,

336f non-endocrine tissues/organs

producing, 353–357, 356t Horns (cornua), hyoid bone, 178,

178f Horny (cornified) cells, in skin,

141–142, 141f

642 Subject Index

hPL. See Human placental lactogen HPV. See Human papillomavirus

(HPV), cervical cancer caused by

Human body, 27–49. See also under Body

anatomy overview and, 27–28 homeostasis and, 45–46, 46f imaging of (medical imaging),

36–37b language of anatomy and, 38–45,

39t, 40f, 42f, 43f, 44f levels of structural organization,

28–33, 29f maintenance of life and, 33–38, 34f organ systems/overview and, 28,

29–33, 29f, 31–32f physiology overview and, 28

Human chorionic gonadotropin (hCG), 353–357, 585

Human herpesvirus 1, 142 cold sore caused by, 142,

149–150, 150f sun exposure and, 142

Human immunodeficiency virus infection/AIDS, 375, 453, 454–455b

growth hormone for wasting and, 342b

Human papillomavirus (HPV), cervical cancer caused by, 576

Human papillomavirus (HPV) vaccine, 576

Human placental lactogen, 357 Humerus, 173f, 184–187, 186f, 200f Humoral (antibody-mediated)

immunity, 437, 441–446, 441–444f, 445t, 446f, 447f, 449f. See also Antibodies

active, 442–443, 443f passive, 443, 443f primary response, 441, 441f, 442f secondary response, 442, 442f

Humoral stimuli, endocrine system activation and, 337f, 338

Humors of eyeball, 307, 308f, 312 Hunger, feeding behavior and, 523 Huntington’s disease/chorea, 270,

279b Hyaline (articular) cartilage, 120,

121f, 163, 164f, 167–168 in bone formation/growth,

167–168, 167f, 168f laryngeal, 465f, 466 in synovial joint, 195, 195f tracheal, 121f, 466, 467f

Hydrocephalus, 276, 276f Hydrochloric acid, 66, 67f

body defense and, 432 in digestion, 507–508 ionization of, 66 parietal cell production of, 494f,

495 peptic ulcers and, 508 pH of, 67f

Hydrogen, 53t, 56t, 59f atomic mass number of, 55, 56t atomic structure of, 54, 55, 55f, 56t atomic weight of, 55, 56f, 56t as chemically reactive element,

59–60, 59f, 61f isotopes of, 55, 56f

Hydrogen bonds, 60–62, 62f in DNA, 79f in proteins, 62, 75–76, 75f, 77

Hydrogen gas, formation of, 57, 59, 61f

Hydrogen ions, 65 in acid-base balance, 67, 67f,

555–556, 556f acids releasing, 66

Hydrogen peroxide, 96

Hydrolases, 78 Hydrolysis/hydrolysis reactions, 65,

68f, 69 ADP formation from ATP and,

80f, 81, 81f in digestion, 65, 503 of sucrose, 69f

Hydronephrosis, 539 Hydrophilic molecules, in plasma

membrane, 91, 91f Hydrophilic phospholipids, 72 Hydrophobic molecules, in plasma

membrane, 91, 91f Hydrophobic phospholipids, 72 Hydrostatic (fluid) pressure,

filtration and, 104 Hydroxide ion, 65, 66–67

in acid-base balance, 67, 67f Hydroxides, 66

in acid-base balance, 66, 556 Hygiene hypothesis, 482b Hymen, 575f, 576 Hyoid bone, 178, 178f, 465f, 491f Hyperaldosteronism, 348 Hypercalcemia

bone remodeling and, 168 heart rate and, 394

Hypercalcemic hormone, PTH as, 345, 346f

Hyperextension (body movement), 223, 224f

Hyperglycemia, 518 Hyperglycemic hormones

glucagon as, 352 glucocorticoids as, 347

Hyperopia/hyperopic eye, 315–316b

Hyperparathyroidism, 345–346 Hyperplasia, 129

prostatic (prostatic hypertrophy), 550, 567–568

Hyperpnea, 478 Hypersensitivities, 451–453, 452f.

See also Allergies Hypertension, 411–412, 418

atherosclerosis and, 412–413b, 415

diet and, 411 renal disease and, 549b

Hypertrophy, prostatic, 567–568 Hyperthermia, 525–527 Hyperthyroidism, 345, 345f, 524 Hypertonic solutions, 105b Hyperventilation, 480 Hypoaldosteronism (Addison’s

disease), 348, 348f, 555 Hypocalcemia

bone remodeling and, 168 heart rate and, 394

Hypocalcemic hormone, calcitonin as, 345, 346f, 354t

Hypochondriac regions, 44, 44f Hypodermis (subcutaneous tissue),

124, 139, 140f Hypogastric (pubic) region, 43, 44f Hypoglossal nerve (cranial nerve

XII), 284f, 287t Hypoglycemia, 518 Hypoglycemic hormone, insulin

as, 350 Hypospadias, 559 Hypotension, 411

orthostatic, 296, 411 Hypothalamus/hypothalamic

hormones, 269t, 271, 271f, 337f, 338, 338f, 339f, 354t

in body temperature regulation, 295, 525, 526f, 527

developmental aspects of, 295 endocrine system activation and,

337f feeding behavior and, 523

pituitary relationship and, 271, 337, 337f, 339–343, 339f, 340f

in stress response, 350f in thirst regulation, 552, 552f, 554f

Hypothermia, 525 Hypothyroidism, 344–345, 524 Hypotonic solutions, 105b Hypoventilation, 480 Hypoxia, 374, 477 H zone, 212, 212f

I. See Iodine I (light) bands, 212, 212f, 213 IBS. See Irritable bowel syndrome ICF. See Intracellular fluid IgA, 444, 445t IgD, 444, 445t IgE, 444, 445t

in immediate hypersensitivity, 452, 452f

IgG, 444, 445t IgM, 444, 445t Igs (immunoglobulins), 77t,

443–444, 444f, 445t, 446f, 450t. See also Antibodies

Ileocecal valve, 496, 499f Ileum, 490f, 496, 499f Iliac arteries, 400, 401f Iliac crest, 188, 189f Iliac (inguinal) regions, 43, 44f Iliac spines, 188, 189f Iliacus muscle, 237f, 238 Iliac veins, 402, 403f Iliocostalis muscle, 233–234, 234f Iliopsoas muscle, 237f, 238, 240f,

242t Iliotibial tract, 236, 236f, 241f Ilium, 188, 189f Image formation, on retina, 313, 314f Imaging, medical, 36–37b Immediate (acute) hypersensitivity,

452, 452f Immune response. See Immunity/

immune response Immune serum, passive immunity

and, 443, 443f Immune system, 429, 430f. See also

Immunity/immune response allergies and hygiene hypothesis,

482b cells of. See specific type developmental aspects of,

455–457 homeostatic imbalances and,

428, 434, 438, 451–453, 452f, 454–455b

sun exposure and, 142 Immunity/immune response,

127, 430, 430f, 436. See also Immune system

active, 442, 443f adaptive (specific), 430, 430f,

436–454, 449f, 450–451t 457 artificially acquired, 442, 443, 443f cardiovascular system and, 417b,

456b cellular (cell-mediated), 437,

446–448, 447f, 448f, 449f developmental aspects and,

455–457 digestive system and, 456b, 531b disorders of, 451–454, 452f,

454–455b endocrine system and, 358b, 456b herd, 442–443 homeostatic imbalances and,

428, 434, 438, 451–453, 452f, 454–455b

humoral (antibody-mediated), 437, 441–446, 441–444f, 445t, 446f, 447f, 449f. See also Antibodies

innate (nonspecific), 430–436, 430f, 431t

integumentary system and, 155b, 456b

muscular system and, 246b, 456b naturally acquired, 442, 443, 443f nervous system and, 298b, 456b passive, 443, 443f reproductive system and, 456b,

597b respiratory system and, 456b, 484b skeletal system and, 202b, 456b urinary system and, 456b, 560b

Immunization, 442, 443f Immunocompetent lymphocytes,

439–440, 439f, 441 Immunodeficiencies, 453, 454–455b Immunoglobulins, 77t, 443–444,

444f, 445t, 446f, 450t. See also specific type under Ig and Antibodies

Immunological memory, 437, 442. See also Memory cells

Immunology, 437 Immunosuppressive therapy, after

transplant, 451, 549b Impacted fracture, 171t Impacted teeth, 500 Impaired oxygen transport, 477 Impetigo, 150, 150f Implant(s), contraceptive, 594b, 595b Implantation, 576, 585, 586f Inborn errors of metabolism, 527–528 Incisive fossa, 175f Incisors, 500, 500f Inclusions, cell, 93, 98t, 99t Incompetent heart valve, 389, 392 Incomplete antigen (hapten), 438 Incomplete (unfused) tetanus, 217,

219f Incontinence, 550

aging and, 559, 561 pregnancy and, 591

Incus (anvil), 317f, 318 Indifferent stage of gonadal

development, 593 Infantilism, sexual, 573 Infant respiratory distress syndrome

(IRDS), 483 Infarct

cerebral, 296b myocardial, 389, 393b

Infection burn injury and, 150 lymph nodes in, 427, 428 skin, 149–150, 150f

Inferior (term), 39t Inferior articular process, vertebral,

180, 181f Inferior gluteal nerve, 288t, 289f Inferior horn, 275f Inferior meatus, nasal cavity, 306f Inferior mesenteric artery, 400, 401f Inferior mesenteric vein, 403f, 405,

406f Inferior nasal conchae, 176f, 178,

465f Inferior oblique muscle, 307f Inferior rectus muscle, 307f Inferior vena cava, 385–386f 386,

402, 403f veins draining into, 402

Infertility, male, 568 Inflammation, homeostatic

imbalances and, 434 Inflammatory chemicals, 432, 433 Inflammatory joint disease, 198–199,

199f Inflammatory response/

inflammation, 127, 128, 431t, 432–435, 433f, 434f

glucocorticoids affecting, 348

Subject Index 643

Infraorbital foramen, 176f Infundibulum, uterine tube, 574,

575f Ingestion, 503, 503f, 506

obesity and, 529–530b regulation of, 523

Inguinal (term), 40f, 41 Inguinal (iliac) regions, 43, 44f Inguinal lymph nodes, 427f Inhibiting hormones, hypothalamic,

339 Injury, atherosclerosis onset and, 412 Inlet, pelvic, 188, 189f Innate (nonspecific) defense system,

430–436, 430f, 431t. See also Immunity/immune response

antimicrobial proteins in, 431t, 435–436, 437f

cells and chemicals in, 431t, 432–436

complement in, 431t, 435–436, 437f

fever in, 431t, 436 homeostatic imbalances and, 434 inflammatory response in, 431t,

432–435, 433f, 434f interferons in, 431t, 436 natural killer (NK) cells in, 431t,

432 phagocytes in, 431t, 435, 435f surface membrane barriers in,

431t, 432 Inner cell mass, 585, 586f Inner (internal) ear, 317f, 318 Inorganic compounds, 64, 65–68.

See also specific type Insertion, muscle, 223, 223f, 235 Inspiration (inhalation), 471–473,

472f, 473f Inspiratory reserve volume, 474,

474f Insulin, 77t, 350–351, 355t

aging affecting production of, 357 in blood glucose regulation,

350–351, 352f, 355t diabetes and, 351 pancreatic islets producing, 350,

351f Insulin resistance, 351 Integration, 252, 252f, 269t, 282f Integration center, in reflex arc,

263, 264f Integument, 138. See also

Integumentary system; Skin Integumentary system, 29, 31f,

138–156. See also Skin body defense and, 138, 139t,

431t, 432 boundary maintenance and, 33,

138, 139t cardiovascular system and, 155b,

417b developmental aspects of,

153–156 digestive system and, 155b, 531b endocrine system and, 155b, 358b functions of, 138–139, 139t homeostatic relationships of, 34f,

155b lymphatic system/immunity and,

155b, 456b muscular system and, 155b, 246b nervous system and, 155b, 298b reproductive system and, 155b,

597b respiratory system and, 155b, 484b skeletal system and, 155b, 202b urinary system and, 155b, 560b

Interatrial septum, 386 Interbrain (diencephalon), 266f,

269t, 270, 271f development of, 266f

Intercalated disks, 124–126, 125f, 208t, 211

Intercarpal joints of hand, 194f Intercellular clefts, 413, 414f Intercondylar eminence, 190f, 191 Intercondylar fossa, 190f, 191 Intercostal arteries, 400 Intercostal muscles, 184, 232, 240f,

468f, 471, 472f, 473 neural control of breathing and,

478, 479f Intercostal nerves, 283–285, 285f,

478 in respiratory control, 478, 479f

Intercostal spaces, 183f, 184 Interferons, 431t, 436, 450t Interleukin(s), in hematopoiesis, 372 Interleukin 2, 450t Interlobar arteries, 539, 540f Interlobar veins, 539, 540f Intermediate (term), 39t Intermediate cuneiform, 191f Intermediate filaments, 94f, 97,

97f, 99t Internal (term), 39t. See also under

Deep Internal acoustic meatus, 174, 175f Internal anal sphincter, 499 Internal capsule, 270, 270f Internal carotid artery, 174, 400,

401f, 404, 404f Internal (inner) ear, 317f, 318 Internal iliac artery, 400, 401f Internal iliac vein, 402, 403f Internal intercostal muscles, 232,

473 Internal jugular vein, 402, 403f Internal oblique muscles, 232–233,

233f, 240f Internal respiration, 471, 475, 476f,

477–478, 477f Internal urethral orifice, 547, 547f Internal urethral sphincter, 547f,

548, 567 Interneurons (association neurons),

258f, 259–260 in ascending and descending

pathways, 282f in reflex arc, 263, 264f, 265

Interosseous membrane in forearm, 186f, 187 in leg, 190f, 191

Interphase, 108–109, 110f DNA replication during, 109, 109f

Interstitial cells, of testis, 565 testosterone produced by, 565,

571 Interstitial (tissue) fluid, 34f, 102,

412, 414f, 550, 551f fluid movements at capillary beds

and, 414, 414f Intertrochanteric crest, 190, 190f Intertrochanteric line, 190, 190f Intertubercular sulcus, 186, 186f Interventricular coronary arteries,

385f, 389 Interventricular foramen, 275f Interventricular septum, 386, 386f,

387f Intervertebral discs/joints, 179,

179f, 195 herniated (“slipped”), 179

Intervertebral/vertebral foramen, 179f, 180, 180f, 181f

Intestinal gastrin, 356t, 497t. See also Gastrin

Intestinal juice, 509 Intestines. See Large intestine; Small

intestine Intracellular fluid, 91, 91f, 102,

550, 551f Intracranial hemorrhage, 277

Intramolecular bonds, hydrogen bonds as, 62

Intramuscular injections in deltoid muscle, 234, 234f in gluteus medius muscle, 236f,

238 in rectus femoris muscle, 238 in vastus lateralis, 237f, 238

Intraocular pressure, 312 in glaucoma, 312

Intrapleural pressure, 473 Intrapulmonary volume/pressure, in

breathing, 471–473, 473f Intrauterine device, 594b, 595b Intravenous drug use, HIV

transmission and, 455b Intravenous (IV) solutions, tonicity

and, 105b Intrinsic conduction system of

heart, 390–391, 390f Intrinsic factor, 495 Intrinsic nerve plexuses, alimentary

canal, 492, 493f Inversion (body movement), 225f,

226 Involuntary (internal) anal

sphincter, 499 Involuntary muscle, 210. See also

Cardiac muscle/cardiac muscle tissue; Smooth muscle/smooth muscle tissue

Involuntary nervous system, 253, 290. See also Autonomic nervous system

Iodine, 53t, 56t deficiency of/thyroid hormone

synthesis and, 343–344 Ion(s), 51, 53, 58

heart rate regulation and, 394 Ionic bonds, 58–59, 60f Ionizing radiation, 57 IRDS. See Infant respiratory distress

syndrome Iris, 305f, 308f, 309

autonomic nervous system affecting, 294t, 316

Iron, 53t, 56t Iron-deficiency anemia, 368t, 379 Irregular bones, 162, 162f Irritability (responsiveness), 34

muscle cell, 213 neuron, 34, 126, 260. See also

Nerve impulses Irritable bowel syndrome (IBS), 528 IRV. See Inspiratory reserve volume Ischemia, myocardial, 389, 391,

393b Ischial body, 189f Ischial spine, 188, 189f Ischial tuberosity, 188, 189f Ischium, 188, 189f Islets of Langerhans (pancreatic

islets), 349–352, 351f Isografts, 451 Isovolumetric contraction/relaxation,

392, 392f Isoleucine, 515f Isometric contractions, 221 Isometric (resistance) exercises,

muscles affected by, 222, 222f, 245

Isotonic contractions, 221 Isotonic solutions, 105b Isotopes, 55–57, 56f

radioactivity exhibited by, 56 Isthmus of thyroid gland, 343, 344f IUD. See Intrauterine device IV solutions. See Intravenous (IV)

solutions, tonicity and

Jaundice, 145, 502 physiologic, 379

Jaw. See Mandible; Maxillae ( maxillary bones)

Jejunum, 490f, 496, 496f JG apparatus. See Juxtaglomerular

( JG) apparatus Joint(s), 29, 31f, 138f, 160, 192–199

aging affecting, 198, 201–203 artificial, 192–193b cartilaginous, 193, 194f, 195, 196t classification of, 192–193, 194f,

196t dislocation of, 196 fibrous, 193, 194f, 195, 196t homeostatic imbalances and, 196,

198–199, 199f inflammatory disorders of,

198–199, 199f membranes of, 136–138, 138f muscles in stabilization of, 211 replacement, 192–193b synovial, 138f, 193, 194f, 195–199,

195f, 196t, 197f, 199f Joint cavity, 138f, 195f, 196 Joint replacement, 192–193b Jugular foramen, 174, 175f Jugular notch, 183, 183f Jugular veins, 174, 402, 403f Juxtaglomerular ( JG) apparatus,

541f, 554f, 555 Juxtamedullary nephrons, 541f, 542

K. See Potassium Kaposi’s sarcoma, in HIV infection/

AIDS, 454b kcal (kilocalories), 513, 523, 524 Keratin, 76, 139t, 140, 141

body defense and, 139t, 140, 431t in hair, 147 in skin, 139t, 140, 141

Keratinization, 140, 147 Keratinocytes, 140, 141f, 144 Ketoacidosis, 520. See also Acidosis Ketones, 351 Ketosis, 351 Kidney(s), 32f, 33, 537, 538–546,

538f. See also under Renal in acid-base balance, 557 artificial, 549b autonomic nervous system

affecting, 294t blood pressure and, 409–410,

410f blood supply of, 539, 540f developmental aspects of,

557–559 erythropoietin produced by, 372,

372f, 537 in fluid and electrolyte balance,

550, 551, 553, 554f functions of, 537–538 homeostatic imbalances and, 539,

543, 546–547, 548, 549b, 559 inflammation of (pyelonephritis/

pyelitis), 548, 559 location/structure of, 538–539,

538f, 540f polycystic, 559 pregnancy and, 591 urine formation by, 542–545, 543f,

544f Kidney failure, 545, 549b Kidney stones, 546, 547f Kidney transplant, 549b Killer (cytotoxic) T cells, 447, 447f,

448f, 449f, 450t in delayed hypersensitivity, 452

Kilocalories, 513, 523, 524 Kinetic energy, 51, 52

in diffusion, 103 Kinins, 432, 433f Kneecap (patella), 173f, 191 Knee-jerk (patellar) reflex, 264f, 265

644 Subject Index

Knee joint, muscles of, 236–237f 238

Knee replacement/prosthesis, 192–193b

Krebs cycle. See Citric acid cycle Kyphosis, 180, 180f, 201, 203f

Labia majora, 575f, 576f, 577 Labia minora, 575f, 576f, 577 Labia of mouth (lips), 491, 491f

cleft, 527, 527f Labor, 591–593, 592f

false, 591 oxytocin and, 339–340, 354t,

591, 592f Labyrinth

membranous, 318 osseous (bony), 318

Lacrimal apparatus, 306, 306f Lacrimal bones, 174f, 176f,

177–178 Lacrimal canaliculi, 306, 306f Lacrimal caruncle, 305, 305f Lacrimal fluid (tears), 306

in body defense, 431t, 432 Lacrimal gland ducts, 306, 306f Lacrimal glands, 306, 306f

autonomic nervous system affecting, 294t

developmental aspects of, 328 Lacrimal sac, 306, 306f Lactase, 505f Lactation, 582

oxytocin and, 339, 354t Lacteals, 497, 498f Lactic acid, ATP production and,

219–220, 220f Lactiferous ducts, 582, 582f Lactiferous sinus, 582, 582f Lactogen, placental, 357 Lactose, 70

digestion of, 504, 505f Lacunae, 120, 121f, 163 Lambdoid suture, 174, 174f, 176f Lamellae, 163, 165f Lamellar corpuscles (pressure

receptors), 139t, 140f, 143, 259f Laminae

elastic, blood vessel, 396, 397f vertebral, 180, 180f

Lamina propria, 124, 136 alimentary canal, 492, 493f

Langerhans, islets of (pancreatic islets), 349–352, 351f

Langerhans cells (epidermal dendritic cells), 141f, 142, 440

Lanugo, 153 Large intestine, 30, 32f, 44f, 490f,

491, 495f, 496, 498–500, 499f in digestion, 499, 503f, 511–513 homeostatic imbalances and, 511,

513 Laryngeal cartilages, 465f, 466 Laryngopharynx, 464, 465f, 491f,

492 Larynx, 30, 32f, 463f, 465–466, 465f Lateral (term), 39t Lateral aperture, 275f, 276 Lateral column, 280 Lateral commissure (canthus), 305,

305f Lateral condyle

femoral, 190f, 191 tibial, 190f, 191

Lateral cuneiform, 191f Lateral epicondyle, 186f, 187, 190f Lateral femoral cutaneous nerve,

288t, 289f Lateral horn, 281f, 293f Lateral longitudinal arch, 191, 191f Lateral malleolus, 190f, 191 Lateral rectus muscle, 307f

Lateral rotation (body movement), 224f

Lateral sulcus, 266, 267f Lateral ventricles, 270f, 274–276,

275f Latissimus dorsi muscles, 233, 234f,

241f, 243t Laughing, 475t LDLs. See Low-density lipoproteins L-dopa, for Parkinson’s disease,

279b Left colic (splenic) flexure, 499,

499f Left hypochondriac region, 44, 44f Left iliac (inguinal) region, 43, 44f Left lower quadrant, 43, 44f Left lumbar region, 44, 44f Left upper quadrant, 43, 44f Leg. See also Lower limbs

bones of, 190f, 191 muscles of, 235–236, 238–239,

239f, 240f, 241f, 242t, 243t Lens of eye, 308f, 312, 313f

developmental aspects of, 328 light refraction/visual focus and,

313, 313f Leptin, 356t Lesser curvature of stomach, 493,

494f Lesser omentum, 493, 495f Lesser trochanter, 190, 190f Lesser tubercle, 186, 186f Let-down reflex, oxytocin and, 339,

354t Lethal hit, 447 Leucine, 515f Leukemia, 369, 379 Leukocytes (white blood cells), 123f,

364, 365f, 366f, 368–369, 370t formation of, 371f, 372 in urine, 546t

Leukocytosis, 368 Leukopenia, 368–369 LH. See Luteinizing hormone Li. See Lithium, atomic structure of Licensed practical nurse, 558b Life

cellular basis of, 88–89 maintenance of, 33–38, 34f

Life cycle, cell, 108–109 Life functions, 33–35 Ligament(s), 122f, 123, 160

of foot, 191 in synovial joints, 195f, 196

Ligamentum arteriosum, 385f, 415 Light

cone sensitivities and, 309–310, 310f

pathway through eye/refraction and, 313, 313f

photopupillary reflex and, 316 as radiant energy, 51

Light (I) bands, 212, 212f, 213, 217f Light chains, 443, 444f Limbic system, 269t, 271

olfactory pathways and, 325 Limbs. See Lower limbs; Upper limbs Line (bone marking), 166t Lingual frenulum, 491, 491f

homeostatic imbalance and, 491–492

Lingual tonsils, 464, 465f, 491f, 492

Lip(s) (labia), 491, 491f cleft, 527, 527f

Lipase(s), 505f, 509, 510 Lipid(s), 70–74, 71f, 72f, 73t, 514t,

515. See also Fat(s) absorption/digestion of, 504, 505f in cell membrane, 72 dietary sources of, 514t, 515 metabolism of, 518–520, 519f

phospholipids, 71f, 72, 73t in plasma membrane, 90–91, 91f saturated and unsaturated, 70–72,

72f, 515 smooth endoplasmic reticulum in

metabolism of, 94 triglycerides (neutral fats), 70–72,

71f, 73t, 515 Lipoprotein(s), 73t, 521–522 Liposuction, 530b Liquids (matter), 51 Lithium, atomic structure of, 55,

55f Lithotripsy, for renal calculi, 546 Liver, 33, 44f, 490f, 495f, 502. See

also under Hepatic autonomic innervation/effects and,

291f, 294t bleeding disorders and, 374, 375 in digestion, 502 homeostatic imbalances and, 502 in metabolism, 518, 519f, 520–522,

522f Lobes

of breast, 582, 582f of cerebral hemispheres, 265,

267f of lungs, 467, 468f

Lobules of breast, 582, 582f of testis, 565, 565f

Local depolarization, neuron, 262 Lockjaw (tetanus), 218 Long bones, 162, 162f

stages of formation, 167f structure of, 163–167, 164f, 165f,

166t Longissimus muscle, 233–234, 234f Longitudinal arches, of foot, 191,

191f Longitudinal fissure, 265, 270f Longitudinal muscle layer, alimen-

tary canal, 210, 210f, 492, 493f large intestine, 499–500 stomach, 494f

Loop of Henle (nephron loop), 541f, 542, 544f

Loose connective tissue, 122f, 123–124, 123f

Lordosis, 180, 180f during pregnancy, 590

Lou Gehrig’s disease (ALS), 213–214

Low-density lipoproteins (LDLs), 73t, 521–522

Lower limbs. See also Foot; Leg; Thigh

arteries supplying, 400, 401f bones of, 173f, 190–192, 190f,

191f muscles of, 235–239, 236–237f

239f, 240f, 241f, 242t, 243t nerves of, 289f veins draining, 402, 403f

Lower respiratory tract, 463 LPN. See Licensed practical nurse Lumbar (term), 40f, 41 Lumbar arteries, 400 Lumbar curvature of spine, 179f,

180 Lumbar enlargement, 280f Lumbar nerves, 280f, 285f Lumbar plexus, 285f, 288t, 289f Lumbar (spinal) puncture, 276 Lumbar regions, 44, 44f Lumbar vertebrae, 179, 179f, 181f,

182 Lumen

alimentary canal, 492, 493f of blood vessels, 396

Lunate, 187f Lung(s), 30, 32f, 463f, 467–471,

468f, 469f, 470f. See also under Pulmonary and Respiratory

autonomic innervation/effects and, 291f, 294t

collapse of (atelectasis), 473 developmental aspects of, 483,

485 Lung cancer, 481–483 Lung capacities, 474–475, 474f Lung (respiratory) sounds, 475 Lung volumes, 474–475, 474f Lunule, 149, 149f Lupus erythematosus, systemic

(SLE), 453 Luteal phase of ovarian cycle, 581f Luteinizing hormone, 340f, 343,

354t, 571 in ovulation, 343, 578, 579 testosterone production and, 343,

571, 572f in uterine (menstrual) cycle, 580,

581f Lye (sodium hydroxide)

ionization of, 66 pH of, 67f

Lymph, 425, 425f, 426 Lymphatic collecting vessels, 425f,

426 Lymphatic duct, 425f, 426 Lymphatic system, 30, 32f,

424–429, 425f cardiovascular system and, 417b,

456b developmental aspects of, 455–457 digestive system and, 456b, 531b endocrine system and, 358b, 456b homeostatic imbalances and, 428,

457 homeostatic relationships of, 456b integumentary system and, 155b,

456b lymph nodes in, 425f, 426–428,

427f, 428f muscular system and, 246b, 456b nervous system and, 298b, 456b organs of, 30, 428–429, 429f reproductive system and, 456b,

597b respiratory system and, 456b,

484b skeletal system and, 202b, 456b urinary system and, 456b, 560b vessels of (lymphatics), 30, 32f,

425–426, 425f, 426f, 427f Lymphatic vessels (lymphatics), 30,

32f, 425–426, 425f, 426f, 427f afferent and efferent, 428, 428f developmental aspects of, 455 disorders of, 457

Lymph capillaries, 425, 425f, 426f Lymph node(s), 30, 32f, 425f,

426–428, 427f, 428f in infection, 427, 428

Lymph node capsule, 427, 428f Lymphocytes, 365f, 366f, 369, 370t,

424, 427, 430, 438–440 in adaptive defense system,

438–440 differentiation/activation of,

438–440, 439f, 457 formation of, 371f, 438–439, 457

Lymphoid organs, 30, 428–429, 429f

developmental aspects of, 455–457

Lymphoid stem cells, 371, 371f Lysine, 74f, 515f Lysosomes, 94f, 95, 96f, 99t, 435,

435f in endocytosis/phagocytosis, 95,

107, 108f rupture of, 95

Subject Index 645

Lysozyme, 306, 432 in body defense, 432 in lacrimal secretions, 306

MAC. See Membrane attack complex Macrophage(s), 101, 101f, 369, 427,

430, 435, 435f, 440, 449f. See also Phagocytes/phagocytosis

activated, 440 antigen presentation by, 440,

446–447, 449f in inflammation, 434

Maculae, 319, 319f Magnesium, 53t, 56t Magnetic resonance imaging (MRI),

36–37b, 42f in brain lesion diagnosis, 296b

Main (primary) bronchi, 463f, 466–467, 468f

Main pancreatic duct, 496f Major nutrients, 513. See also

Nutrients Male infertility, 568 Male pattern baldness, 156 Male reproductive system, 32f,

33. See also specific organ or structure

anatomy of, 565–569, 566–567f developmental aspects of, 593, 596 function of, 569–573 homeostatic imbalances and,

567–568, 570–571, 573, 593, 596 Male sex hormones, 73t, 348 Malignant melanoma, 152f, 153 Malignant neoplasms (cancer), 129,

130–131b. See also specific type or organ affected

Malleoli, medial and lateral, 190f, 191

Malleus (hammer), 317f, 318 MALT. See Mucosa-associated

lymphatic tissue Maltase, 505f Maltose (malt sugar), 70

digestion of, 504, 505f Mammary glands (breasts), 32f,

580–583, 582f, 583f developmental aspects of, 596 prolactin affecting, 340f, 341, 354t

Mammillary bodies, 271, 271f Mammogram, 36b, 37f Mammography, 583, 583f Mandible, 172, 174f, 176f, 178

in fetal skull, 200f Mandibular fossa, 175f Mandibular ramus, 174f Manganese, 53t Mantoux test, 453 Manubrium, 183, 183f Manus (hand), 40f MAP. See Morning-after pill Marginal artery, 385f, 389 Masculinization, 349 Masseter muscle, 230, 230f, 240f,

242t Mass movements, 511 Mass number (atomic), 55, 56t Mast cells, in acute (immediate)

hypersensitivity, 452, 452f Mastication, 492, 500, 503f, 506

muscles used in, 230, 230f Mastoid fontanel, 200f Mastoiditis, 172 Mastoid process, 172, 174f, 175f,

176f Mastoid sinuses, 172 Maternal antibodies, passive

immunity and, 443, 443f Matrix

bone, 163, 167, 168 extracellular (connective tissue),

120

hair bulb, 147, 148f nail, 149, 149f

Matter, 50–51 composition of, 52–57 living, 64–81. See also

Biochemistry Maxillae (maxillary bones), 174f,

175f, 176f, 177 Maxillary sinus, 177f Meatus (bone marking), 166t Mechanical breakdown (food), 503,

503f, 506 Mechanical energy, 51 Mechanoreceptors, 316

in digestion, 506 Medial (term), 39t Medial commissure (canthus), 305,

305f Medial condyle

femoral, 190f, 191 tibial, 190f, 191

Medial epicondyle, 186f, 187, 190f Medial longitudinal arch, 191, 191f Medial malleolus, 190f, 191 Medial rectus muscle, 307f Medial rotation (body movement),

224f Median aperture, 275f, 276 Median cubital vein, 402, 403f Median nerve, 288t, 289f Median sacral crest, 182, 182f Median (midsagittal) section/plane,

41, 42f Mediastinum, 43, 383, 384f, 467,

468f Medical imaging, 36–37b Medical transcriptionist, 154b Medulla

adrenal. See Adrenal medulla hair, 147, 148f lymph node, 428, 428f renal, 539 respiratory centers in, 478, 479f,

480 Medulla oblongata, 267f, 269t, 270f,

271f, 272 Medullary cavity of bone (yellow

marrow), 163, 164f Medullary cords, 428, 428f Medullary (renal) pyramids, 539,

540f Megakaryocytes, 369. See also

Platelets Meiosis, 570, 570f

homeostatic imbalances and, 596 in oogenesis, 570f, 577–578, 578f,

579 in spermatogenesis, 569f, 570,

570f Meissner’s corpuscles (touch

receptors), 259f Melanin, 141f, 142

protection from ultraviolet radia- tion/sun damage and, 139t, 142

skin color and, 144 Melanocytes, 141f, 142

cancer of (melanoma), 152f, 153 in hair bulb, 147, 148f

Melanoma, 152f, 153 Melanosomes, 144 Melatonin, 343, 354t Membrane(s). See also specific type

basement, 115, 117–118f body, 135–138, 137f, 138f,

153–156 boundary maintenance and, 33 connective tissue, 135, 136, 138f covering/lining, 135, 136, 137f cutaneous, 136, 137f. See also Skin epithelial, 135, 136, 137f mucous. See Mucous membranes

(mucosae)

nuclear, 89, 90f, 94f, 99t phospholipids in, 72, 90–91, 91f plasma, 90–92, 90f, 91f, 92f, 94f,

98t selective permeability/diffusion

and, 102, 103, 104f. See also Membrane transport

serous. See Serous membranes (serosae)

synovial, 136–138, 138f Membrane attack complex, 436,

437f Membrane (cell) junctions, 92, 92f Membrane proteins, 90–92, 91f

in receptor-mediated endocytosis, 108, 108f

Membrane transport, 91, 102–108, 103f, 104f, 107f, 108f

active processes in, 104–108,107f, 108f

passive processes in, 102–104, 103f, 104f

Membranous labyrinth, 318 Membranous urethra, 548, 566f,

567, 567f Memory, immunological, 437, 442 Memory cells, 442, 448, 450t

B cells, 442, 449f, 450t T cells, 448, 449f, 450t

Menarche, 596 Ménière’s syndrome, 323–324 Meningeal layer of dura mater, 273,

274f, 275f Meninges

brain, 273, 274f spinal, 278, 281f

Meningitis, 273 cryptococcal, 454b

Menopause, 357, 577, 596–598 osteoporosis and, 201

Menses, 576, 579, 579f onset of, 579, 596

Menstrual (uterine) cycle, 353, 579, 580, 581f

Menstrual phase of menstrual cycle, 580, 581f

Menstruation, 576, 580 onset of, 596

Mental (term), 40f, 41 Mental foramen, 174f, 176f Merkel cells, 141f, 142 Merkel discs, 142 Mesenteric arteries, 400, 401f Mesenteric veins, 403f, 405, 406f Mesentery, 492, 493f, 495f, 496,

499f Mesocolon, transverse, 499f Mesoderm, 585, 587f Messenger RNA, 80–81, 95f, 112

in transcription, 112, 113f in translation, 112, 113f

Metabolic phase, 109 Metabolic rate

age-related changes in, 523–524, 524t, 528

basal, 523–524, 524t thyroxine affecting, 343, 345, 354t,

524, 524t total, 524

Metabolism, 35, 516–527, 519f carbohydrate, 63, 63f, 70,

516–518, 517f, 518f, 519f cellular, autonomic nervous

system affecting, 294t developmental aspects of,

527–528 energy balance and, 519f,

523–527, 524t, 526f fat/lipid, 518–520, 519f homeostatic imbalances and, 524,

525–528 inborn errors of, 527–528

liver in, 518, 519f, 520–522, 522f protein, 519f, 520 thyroid hormone and, 343, 354t,

524, 524t Metacarpals, 173f, 187, 187f Metaphase, 110f, 111 Metaphase plate, 110f, 111 Metastasis, 130b Metatarsals, 173f, 191, 191f Metatarsal veins, dorsal, 403f Methane, formation of, 57, 61f Methionine, 515f Mg. See Magnesium MI. See Myocardial infarct Microcirculation, 398 Microfilaments, 96–97, 97f, 99t Microglia, 254, 254f Microscopic anatomy, 28

of bone, 163–167, 165f Microtubules, 94f, 97, 97f, 99t

in mitotic spindle, 97, 110f, 111 Microvilli, 92f, 100, 497, 498f

renal tubular, 542 Micturition (voiding), 548–550

developmental aspects of, 559 Midbrain, 269t, 271–272, 271f

development of, 266f Middle cerebral arteries, 404, 404f Middle ear (tympanic cavity), 45,

317f, 318 Middle nasal conchae, 176f, 177,

465f Midpiece of sperm, 570, 571f Midsagittal (median) section/plane,

41, 42f MIF. See Migration inhibiting factor Migration inhibiting factor (MIF),

450t Milia, 153 Milk ejection, oxytocin and, 339,

354t Milk production, prolactin and,

341, 354t Milk teeth, 500, 500f Mineralocorticoids, 347, 347f, 355t

in stress response, 350f Minerals, 35, 516

dietary sources of, 514t, 516 Minipill, 594b, 595b Minivalves, lymph capillary, 425,

426f Minor nutrients, 513. See also

Nutrients Miscarriage, 590 Mitochondria (mitochondrion), 93,

94f, 98t citric acid cycle in, 517, 517f in sperm, 570, 571f

Mitosis, 110–111, 110f, 152, 570, 570f, 578f, 585

Mitotic spindle, 97, 110f, 111 Mitral (bicuspid) valve, 386f, 387 Mittelschmerz, 578 Mixed nerves, 282, 283 M line, 212, 212f Mn. See Manganese Mo. See Molybdenum Molars, 500, 500f Mole (skin), 142 Molecular formula, 57 Molecule(s), 28, 29f, 57–58

compounds differentiated from, 57–58, 57f

covalent, 59–60 Molybdenum, 53t Monoclonal antibodies, 443 Monocytes, 123f, 365f, 369, 370t

formation of, 371f in inflammation, 433f, 434

Monoglycerides, 505f Monomer(s), 68–69, 68f, 120, 444 Monosaccharides, 69–70, 69f, 504

646 Subject Index

Monounsaturated fats, 70 Mons pubis, 575f, 576f, 577 Morbid obesity, 530b Morning-after pill, 594–595b Morning sickness, 590 Morula, 585, 586f Motor aphasia, 277 Motor area, primary, 266, 267f,

268f Motor (efferent) division of periph-

eral nervous system, 252f, 253 Motor homunculus, 266, 268f Motor (efferent) nerves, 282 Motor (efferent) neurons, 256f,

258f, 259 in muscle cell contraction,

213–216, 214f, 215f, 216f in reflex arc, 263–265, 264f

Motor output, 252, 252f Motor speech area (Broca’s area),

266, 267f Motor (efferent) tracts, 280, 282f Motor unit, 213, 214f Mouth (oral cavity), 30, 32f, 44,

463f, 490f, 491–492, 491f in digestion, 491, 492, 503f, 505f,

506–507, 507f taste buds in, 326–327, 326f,

492 Movement, 33–34. See also Body

movements muscular system and, 30, 33,

207, 211 skeletal system and, 33, 161

MRI. See Magnetic resonance imaging

mRNA. See Messenger RNA MRS. See Magnetic resonance

spectroscopy MS. See Multiple sclerosis Mucosa. See Mucous membranes Mucosa-associated lymphatic tissue

(MALT), 429 Mucous membranes (mucosae),

116, 136, 137f alimentary canal, 210f, 492, 493f in body defense, 431t, 432 nasal, 463–464 respiratory, 463–464, 467f of stomach, 494f, 495

Mucous neck cells, 494f, 495 Mucus

in body defenses, 431t, 432 in cystic fibrosis, 483 gastric mucosa producing, 495 goblet cells producing, 116, 499 mucous neck cells producing, 495

Multiaxial joints, 194f, 197f, 198 Multinucleate cells, 111 Multipennate muscles, 228–229, 229f Multiple sclerosis, 257, 453 Multipolar neurons, 260, 260f Mumps, 501 Murmurs, 392 Muscle(s)/muscle tissue, 28, 124–126,

125f, 127f, 207–211, 208t, 209f, 210f. See also specific type and Muscular system; Skeletal muscle(s)/skeletal muscle tissue

aging affecting, 129 functions of, 211

Muscle attachments, 222–223, 223f Muscle contraction, 30, 207, 208t,

213. See also Skeletal muscle contraction

ATP supplying energy for, 81, 81f, 217, 219–220, 220f

cardiac muscle, 124, 126, 208t frequency of stimulus and,

217–218, 219f homeostatic imbalance and, 221 movement and, 30, 207, 211

skeletal muscle, 30, 124, 208t, 213–222, 214–220f, 222f. See also Skeletal muscle contraction

smooth muscle, 126, 208t, 210 strength of stimulus and, 218 types of, 221

Muscle control, development of, 244

Muscle fatigue, 220–221 Muscle fibers/cells, 101, 101f, 124,

125f, 208t, 209–211, 209f stimulation/contraction of,

213–217, 214–218f Muscle insertion, 223, 223f, 235 Muscle spindle, 259f Muscle tone, 221 Muscle twitches, 217, 219f Muscular dystrophy, 244 Muscularis externa, alimentary

canal, 492, 493f large intestine, 499–500 stomach, 494f, 495

Muscular pump lymph transport and, 426 venous return and, 394, 395f,

398, 398f Muscular system, 30, 31f, 207–250.

See also Muscle(s)/muscle tis- sue; Skeletal muscle(s)/skeletal muscle tissue

activity of, 213–222, 214–220f 222f body movements and, 222–226,

223f, 223t, 224–225f cardiovascular system and, 246b,

417b developmental aspects of, 244–245 digestive system and, 246b, 531b endocrine system and, 246b, 358b functions of, 211 gross anatomy of, 229–244. See

also specific named muscle homeostatic imbalances and, 221,

244–244, 245f homeostatic relationships of, 246f integumentary system and, 155b,

246b lymphatic system/immunity and,

246b, 456b microscopic anatomy and,

211–213, 212f movement and, 30, 33, 207, 211 nervous system and, 246b, 298b reproductive system and, 246b,

597b respiratory system and, 246b,

484b skeletal system and, 202b, 246b steroid abuse and, 231b tissue types and, 207–211, 208t,

209f, 210f urinary system and, 246b, 560b

Musculocutaneous nerve, 288t, 289f Mutations, in cancer, 130b, 131b Myasthenia gravis, 244, 245f, 453 Myelin, 255–257 Myelination, 254f, 255–257, 256f,

257f, 295–296 Myelin sheaths, 254f, 255–257,

256f, 257f multiple sclerosis and, 257 nerve impulse propagation and,

262 Myeloid stem cells, 371, 371f Myeloid tissue, 371. See also Bone

marrow Myenteric nerve plexus, 492, 493f Myocardial infarct, 389, 393b Myocardium, 384, 385–386f

inadequate blood supply to (ischemia), 389, 391, 393b

Myofibrils, 212–213, 212f Myofilaments, 209, 212f, 213

Myometrium, 575f, 576 Myopia/myopic eye, 315b Myosin, 97, 213

in sliding filament theory, 216–217, 217f, 218f

Myosin (thick) filaments, 212f, 213 in sliding filament theory,

216–217, 217f, 218f Myosin heads, 213

in sliding filament theory, 216–217, 218f

MyPlate, 514, 515f Myringotomy, 318 Myxedema, 344–345

N. See Nitrogen n0. See Neutron(s) Na. See Sodium NaCl. See Sodium chloride Na+-K+ (sodium-potassium) pump,

106, 106f Nail(s), 149, 149f Nail bed, 149, 149f Nail body, 149, 149f Nail folds, 149, 149f Nail matrix, 149, 149f Nail root, 149, 149f Nares (nostrils), 306f, 463, 463f,

465f Nasal (term), 40f, 41 Nasal aperture, posterior, 464, 465f Nasal bones, 174f, 176f, 178 Nasal cavity/passages, 30, 32f, 44,

463, 463f, 464, 465f Nasal conchae, 176f, 177, 178,

464, 465f Nasal hairs, in body defense, 431t Nasal mucosa, 463–464

pregnancy and, 591 Nasal septum, 463 Nasolacrimal duct, 306, 306f, 464 Nasopharynx, 464, 465f, 491f, 492 Natriuretic peptide, atrial, 347, 348f Natural killer (NK) cells, 431t, 432 Naturally acquired immunity

active, 442, 443f passive, 443, 443f

Nausea and vomiting, pregnancy and, 590

Navicular, 191f Ne. See Neon, as chemically inert

element Nearsightedness (myopia), 315b Neck of bone

of femur, 190, 190f of humerus, anatomical and

surgical, 186, 186f of radius, 186f

Neck cells, mucous, 494f, 495 Neck muscles, 230–232, 230f, 234f,

240f, 241f, 242t, 243t Neck of tooth, 501, 501f Negative feedback, 45–46, 337

erythropoietin secretion and, 372, 372f

glucose regulation and, 350, 352f homeostasis and, 45–46, 46f hormone secretion and, 337

Neon, as chemically inert element, 59f

Neoplasms (tumors), 129, 130b benign, 129, 130b malignant (cancer), 129,

130–131b. See also specific type or organ affected

Nephron(s), 539–542, 541f Nephron loop, 541f, 542, 544f Nerve(s), 30, 31f, 258, 281–283,

283f classification of, 282 cranial, 252f, 253, 283, 284f,

286–287t

spinal, 252f, 253, 279, 280, 280f, 281f, 283–290, 285f, 288t

structure of, 281–283, 283f Nerve cells, 255–265. See also

Neuron(s) Nerve fiber. See Axon Nerve growth factor, 77t Nerve impulses, 30, 51, 260–263,

261f, 262, 263f homeostatic imbalances and, 262 initiation/generation/propagation

of, 260–262, 261f in muscle cell contraction,

213–216, 214f, 215f, 216f transmission at synapses and,

262–263, 263f Nerve plexuses

alimentary canal, 492, 493f spinal, 285, 285f, 288t

Nervous system, 30, 31f, 251–303, 252f. See also Nerve(s); Nervous tissue

autonomic, 252f, 253, 290–295, 290f, 291f, 294t

cardiovascular system and, 298b, 417b

central, 252f, 253, 265–281. See also Central nervous system

developmental aspects of, 295–299, 295f

diagnosis of problems and, 296–297b

digestive system and, 298b, 531b endocrine system and, 298b, 358b functional classification of, 252f,

253 functions of, 251–252, 252f homeostatic imbalances and, 257,

262, 295, 295f, 296–297b homeostatic relationships of, 298f integumentary system and, 155b,

298b involuntary, 253, 290. See also

Autonomic nervous system irritability/responsiveness and, 34,

126, 260 lymphatic system/immunity and,

298b, 456b muscular system and, 244, 246b,

298b organization of, 252–253, 252f peripheral, 252f, 253, 281–295,

290f. See also Peripheral ner- vous system

protection of, 273–277, 274f, 275f, 276f

reproductive system and, 298b, 597b

respiratory system and, 298b, 484b

skeletal system and, 202b, 298b somatic, 252f, 253, 280, 290–291,

290f structural classification of, 252f,

253 urinary system and, 298b, 560b voluntary, 253. See also Somatic

nervous system Nervous tissue, 126, 126f, 127f,

253–265. See also Nervous system

neurons, 255–265, 256f. See also Neuron(s)

supporting cells, 126, 126f, 253–255, 254f

Neural layer of retina, 309, 310f Neural stimuli

blood pressure and, 408–409, 410f digestion control and, 293–294,

294t, 506, 507 endocrine system activation and,

337f, 338

Subject Index 647

heart rate regulation and, 394, 395f respiratory control and, 478, 479f

Neural tissue. See Nervous tissue Neurilemma, 257, 257f Neurofibrillary tangles, in Alzheimer’s

disease, 278b Neurofibrils, 255, 256f Neuroglia (supporting cells), 126,

126f, 253–255, 254f Neuromuscular junctions, 213

in muscle contraction, 213–216, 215f

Neuron(s) (nerve cells), 101, 101f, 126, 126f, 255–265, 256f

anatomy of, 126, 126f, 255–258, 256f, 258f

association (interneurons), 258f, 259–260

classification of, 258–260, 258f, 259f, 260f

homeostatic imbalances and, 257, 262

irritability/responsiveness and, 34, 126, 260

motor (efferent), 256f, 258f, 259 in muscle cell contraction,

213–216, 214f, 215f, 216f nerve impulse transmission and,

260, 261f, 262–263, 263f physiology of, 260–265, 261f,

263f, 264f reflexes/reflex arcs and, 263–265,

264f retinal, 309, 310f. See also

Photoreceptors sensory (afferent), 258–259, 258f,

259f supporting cells and, 126, 126f,

253–255, 254f terminology related to, 257–258

Neuron processes, 101, 126, 126f, 255, 256f, 258, 258f

neuron classification and, 260, 260f Neurophysiology, 28 Neurosecretory cells, 339, 339f Neurotransmitters, 213, 255,

262–263, 263f in axonal terminals, 255, 262, 263f in neuromuscular junction, 213,

215f signal transmission at synapses

and, 262–263, 263f Neutral fats (triglycerides), 70–72,

71f, 73t, 515 Neutralization, by antibodies, 445,

446f Neutralization reaction, 67 Neutron(s), 52, 54f, 54t, 55, 55f

isotopes and, 55 Neutrophils, 123f, 365f, 366f, 369,

370t, 435. See also Phagocytes/ phagocytosis

formation of, 371f in inflammation, 433–434, 433f,

434f Newborn

conjunctivitis in (ophthalmia neonatorum), 328

hemolytic disease of, 378 HIV transmission to, 455b nutrition and, 528 physiologic jaundice in, 379 skull in, 199–200, 200f spinal curvature in, 179, 179f urinary system in, 559 vision in, 328

Nicotine, blood pressure and, 410f, 411

Night blindness, 309 Nipple, 582, 582f Nissl bodies/substance, 255, 256f Nitrogen, 53t, 56t

Nitrogenous waste products, 33, 34f, 544–545

NK cells. See Natural killer (NK) cells

Nocturia, 559 Nodal (intrinsic) conduction system

of heart, 390–391, 390f Nodes of Ranvier, 256f, 257 Noise, hearing loss and, 323, 329 Nonaxial joints, 196, 197 Nonpolar covalent molecules, 60 Nonrespiratory air (gas) movements,

475, 475t Nonself antigens, 438 Nonspecific defense system. See

Innate (nonspecific) defense system

Nonsteroid hormones, 335 mechanism of action of, 336

Nonstriated muscle. See Smooth muscle/smooth muscle tissue

Noradrenaline. See Norepinephrine Norepinephrine, 290f, 292, 293,

337f, 338, 349, 355t stress response/sympathetic stimu-

lation and, 337f, 338, 349, 350f Normal body temperature, 35 Nose, 463–464 Nostrils (nares), 306f, 463, 463f, 465f Notch (bone marking), 166t Nuclear envelope/membrane, 89,

90f, 94f, 99t Nuclear medicine, 36–37b Nuclear pores, 89, 90f Nucleases, 509 Nuclei (cell bodies), 257

basal. See Basal nuclei Nucleic acids, 78–81, 79f. See also

DNA; RNA Nucleoli (nucleolus), 90, 90f, 94f,

100t Nucleoplasm, 89 Nucleotides, 78–81, 79f, 109, 109f Nucleus (atomic), 54, 54f, 54t, 56 Nucleus (cell), 89–90, 90f, 94f,

99–100t daughter, 110

Nurse, licensed practical, 558b Nutrients, 34f, 35, 513, 514t. See

also specific type capillary exchange of, 412–414,

414f Nutrition, 513–516, 514t, 515f

aging and, 528 pregnancy and, 528, 590

O. See Oxygen OA. See Osteoarthritis Obesity, 529–530b Obesity genes, 530b Oblique muscles

abdominal, 232–233, 233f, 240f, 242t

gastric, 494f ocular, 307f

Obturator foramen, 188, 189f Obturator nerve, 288t, 289f Occipital (term), 40f, 41 Occipital bone, 174, 174f, 175f,

176f in fetal skull, 200f

Occipital condyles, 174, 175f Occipitalis muscle, 229–230, 230f,

241f, 243t Occipital lobe, 266, 267f, 271f, 314f Ocular fundus, 312, 313f Oculomotor nerve (cranial nerve

III), 284f, 286t Odor sensation, 304, 324–326, 325f

cortical areas in, 267f, 325 developmental aspects of, 329

Oil(s) (lipids), 70–72, 72f

Oil (sebaceous) glands, 140f, 145–146, 146f

developmental aspects of, 153 Olecranal (term), 40f, 41 Olecranon, 186f, 187 Olecranon fossa, 186f, 187 Olfactory area, 267f, 325 Olfactory auras/hallucinations, 326 Olfactory bulb, 325f Olfactory epithelium, 325, 325f Olfactory filaments, 325, 325f Olfactory hairs, 325, 325f Olfactory nerve (cranial nerve I),

284f, 286t, 325 Olfactory receptor cells, 325, 325f Olfactory receptors, 325, 325f, 463 Oligodendrocytes, 254f, 255

myelin sheaths formed by, 254f, 257

Oligosaccharides, 505f Oliguria, 543 Omega-3 fatty acids, 72 Omentum

greater, 493, 495f lesser, 493, 495f

Oncogenes, 130–131b Oocyte, 585 Oocytes, 101–102, 573, 577, 578f,

586f. See also Ova fertilization of, 35, 564, 570, 570f,

574, 584–585, 584f, 585f, 586f primary, 577, 578f secondary, 574f, 578, 578f transport of, 574

Oogenesis, 577–579, 578f, 579f Oogonia, 577, 578f Open (compound) fracture, 170 Open reduction of fracture, 170 Ophthalmia neonatorum, 328 Ophthalmoscope/ophthalmoscopy,

312, 313f Opposition of thumb (body

movement), 225f, 226 Opsin, 311b Opsonization, 436 Optic canal, 174, 175f, 176f Optic chiasma, 271f, 314, 314f Optic disc (blind spot), 308f, 309,

310f Optic nerve (cranial nerve II), 284f,

286t, 308f, 309, 310f, 314f Optic radiation, 314, 314f Optic tracts, 314, 314f Oral (term), 40f, 41 Oral cavity (mouth), 30, 32f, 44,

463f, 490f, 491–492, 491f in digestion, 491, 492, 503f, 505f,

506–507, 507f taste buds in, 326–327, 326f, 492

Oral cavity proper, 491 Orbicularis oculi muscle, 230, 230f,

240f, 242t Orbicularis oris muscle, 230, 230f,

240f, 242t Orbit(s) (orbital cavity), 45 Orbital (term), 40f, 41 Orbital fissure, superior, 174, 176f Orbital model of atom/orbitals,

54, 54f Orchitis, 596 Organ(s), 28, 29f, 114. See also

specific organ structural organization at level of,

28, 29f Organ of Corti, spiral, 322, 322f

aging affecting, 329 Organelles, 93–97, 94f, 98–99t. See

also specific type Organic compounds, 64–65, 68–81.

See also specific type Organism, 28, 29f. See also specific

system

structural organization at level of, 28–29, 29f

Organ systems, 28–33, 29f, 31–32f. See also specific system

interrelationships among, 33, 34f structural organization at level of,

28–29, 29f Organ transplants, 448–451

renal, 549b Origin, muscle, 223, 223f, 235 Oropharynx, 464, 465f, 491f, 492 Orthostatic hypotension, 296, 411 Osmoreceptors, 552, 552f, 553, 554f Osmosis, 103, 104f Osmotic pressure, 105b Ossa coxae (coxal/hip bones), 188,

189f Osseous (bony) labyrinth, 318 Osseous tissue. See Bone(s)

( osseous tissue) Ossicles, 317f, 318, 322, 324f Ossification, 167, 168f Osteoarthritis, 198, 203 Osteoblasts, 167, 168 Osteochondral grafting, 193b Osteoclasts, 168 Osteocytes, 120, 163, 165f, 170 Osteon (Haversian system), 163, 165f Osteoporosis, 201–203 Otitis/otitis media, 318, 328, 464

deafness and, 323 Otolithic membrane, 319, 319f Otoliths, 319, 319f Otosclerosis, 323, 329 Outer (external) ear, 317, 317f Outer segment, photoreceptor, 311b Outlet, pelvic, 188 Ova (eggs), 33, 101, 564, 570, 570f,

579, 586f fertilization of, 35, 564, 570, 570f,

574, 584–585, 584f, 586f production of (oogenesis),

577–579, 578f, 579f Oval window, 317f, 318, 318, 322,

324f Ovarian arteries, 400 Ovarian cycle, 577, 581f Ovarian follicles, 573, 574f, 578,

578f, 581f primary, 574f, 577, 578f, 581f vesicular (Graafian), 573, 574f,

578, 578f, 581f Ovarian ligament, 573, 575f Ovaries, 30, 31f, 32f, 33, 338f, 353,

355t, 564, 573, 574f, 575f aging and, 357 anatomy of, 573, 574f gonadotropic hormones affecting,

340f, 343 hormones of, 353, 355t, 579–580

Overweight, 529b Ovulation, 573, 574f, 577, 578,

578f, 579, 580, 581f, 586f awareness of, contraceptive

methods and, 595b Ovum. See Ova Oxidases, 78, 96 Oxidative phosphorylation, in ATP

production, 219, 220f, 517f, 518 Oxygen, 34f, 35, 53t, 56t, 59f

atomic structure of, 56t as chemically reactive element,

59f, 60, 61f erythropoietin release and, 372,

372f exchange/transport of, 34f, 366,

367, 476, 476f, 477, 477f, 478. See also Gas exchange; Gas transport

in respiratory control, 479f, 480 skin color and, 144 as survival requirement, 35

648 Subject Index

Oxygen deficit, 221 Oxygen gas, formation of, 59, 61f Oxyhemoglobin, 476, 477f Oxytocin, 339–340, 339f, 354t, 591

labor initiation and, 339–340, 354t, 591, 592f

Oxytocin receptors, 591

P. See Phosphorus p+. See Proton(s) p42t gene, in cancer, 131b p53 tumor suppressor gene, 131b Pacemaker

cardiac (sinoatrial node), 208t, 390f, 391

respiratory, medulla as, 478, 479f Pacemaker (artificial), 391 Pain, in inflammation, 432, 433,

433f Pain receptors (free/naked nerve

endings), 139t, 143, 258–259, 259f

Palate, 464, 465f cleft, 177, 464, 527, 527f hard, 175f, 464, 465f, 491, 491f soft, 464, 465f, 491, 491f

Palatine bones, 175f, 177 Palatine processes, 175f, 177 Palatine tonsils, 464, 465f, 491f, 492 Pallor, 145 Palmar arches, 401f Palm of hand, bones of

( metacarpals), 173f, 187, 187f Palpebral fissure, 305, 305f Pancreas, 30, 31f, 33, 338f, 349,

351f, 355t, 490f, 495f, 496f, 501–502

autonomic innervation/effects and, 291f

in digestion, 490f, 496, 501–502, 505f, 509–510, 510f

endocrine (islets), 349–352, 351f, 355t

homeostatic imbalance and, 509–510

Pandemic, 454b. See also AIDS Pancreatic amylase, 505f, 509 Pancreatic ducts, 496–497, 496f Pancreatic hormones, glucose

regulation by, 350–352, 352f Pancreatic islets (islets of

Langerhans), 349–352, 351f Pancreatic juice, 509, 510f Pancreatitis, 509–510 Pannus, in rheumatoid arthritis, 199 Papillae

dermal, 140f, 142–143, 142f hair, 147–148, 148f of tongue, 326, 326f, 492

Papillary layer of dermis, 140f, 142–143, 142f

Pap smear, 576 Papule, in squamous cell

carcinoma, 152f, 153 Parafollicular cells, 344f, 345 Parallel muscles, 228, 229f Paralysis

flaccid, 280 spastic, 280–281

Paranasal sinuses, 177, 177f, 464 Paraplegic, 281 Parasympathetic division of

autonomic nervous system, 252f, 253, 290f, 291

anatomy of, 292 in digestion, 293–294, 294t, 506,

507 function of, 293–294, 294t heart rate and, 294, 294t, 394,

395f Parathyroid glands, 30, 31f, 337f,

338f, 345–346, 346f, 354t

Parathyroid hormone (parathor- mone/PTH), 337f, 345, 354t

bone remodeling and, 168, 170 calcium regulation and, 337f, 338,

346f, 354t Parietal bones, 172, 174f, 175f, 176f

in fetal skull, 200f Parietal cells, 494f, 495 Parietal layer of serous membrane,

136, 137f pericardium, 383–384, 385f

Parietal lobe, 266, 267f Parietal pericardium, 137f, 383–384,

385f Parietal peritoneum, 137f, 492 Parietal pleura, 137f, 467, 468f Parieto-occipital sulcus, 267f Parkinson’s disease, 270, 278–279b,

297b Parotid glands, 490f, 501 Partial-thickness burns, 150, 151f Particle size, chemical reaction rate

affected by, 64t Parturition (childbirth), 591–593,

592f Passive immunity, 443, 443f Passive processes/transport,

102–104, 103f, 104f Patella, 173f, 191 Patellar (term), 40f, 41 Patellar ligament, 191 Patellar (knee-jerk) reflex, 264f, 265 Patellar surface, 190f, 191 Pathogens, 149, 430

attenuated, for vaccines, 442 Pathologic fractures, 203 PCT. See Proximal convoluted

tubules Pectoral (shoulder) girdle

bones of, 173f, 184, 185f muscles of, 233f, 234f, 240f, 241f,

242t, 243t Pectoral (term), 41 Pectoralis major muscle, 232, 233f,

240f, 242t Pectoralis minor muscle, 240f Pedal (term), 40f Pedicles, vertebral, 180, 180f Pelvic (term), 40f, 41 Pelvic brim, 189f Pelvic cavity, 43, 43f Pelvic girdle

bones of, 173f, 188, 189f muscles of, 235–238, 236–237f

240f, 241f, 242t, 243t Pelvic inflammatory disease, 574 Pelvic inlet, 188, 189f Pelvic outlet, 188 Pelvic splanchnic (pelvic) nerves,

292 in micturition, 548

Pelvis, 188, 189f developmental aspects of, 200 in male versus female, 188, 189f

Penicillin reaction, 438 Penile (spongy) urethra, 548, 566f,

567, 567f Penis, 32f, 33, 566f, 567f, 568

autonomic nervous system affecting, 294t

developmental aspects of, 596 Pennate muscles, 228–229, 229f Pentamers, 444 Pepsin, 77, 494f, 495, 505f, 508 Pepsinogens, 494f, 495, 507, 508 Peptic ulcers, 508, 512b, 528 Peptides, 505f Perforated ulcer, 512 Perforating (Volkmann’s) canals,

165f, 167 Perforating (Sharpey’s) fibers, 163,

164f, 165f

Perforins, 432, 447, 448f, 451t Pericardial cavity, 384, 385f Pericarditis, 384 Pericardium, 136, 137f, 383, 383f,

385f homeostatic imbalances and, 384

Perilymph, 318, 322, 322f, 324f Perimetrium, 575f, 576 Perimysium, 208t, 209, 209f Perineum, 576f, 577 Perineurium, 281, 283f Periodic table, 52 Periodontal membrane (ligament),

501, 501f Periosteal layer of dura mater

( periosteum), 273, 274f, 275f Periosteum (bone), 163, 164f, 165f Peripheral congestion, in heart

failure, 396 Peripheral nervous system, 252f,

253, 258f, 281–295, 290f autonomic nervous system, 252f,

253, 290–295, 290f, 291f, 294t cranial nerves, 252f, 253, 283,

284f, 286–287t myelinated fibers in, 254f,

255–257, 256f nerve structure and, 281–283, 283f spinal nerves and nerve plexuses,

252f, 253, 279, 280, 280f, 281f, 283–290, 285f, 288t

supporting cells in, 254f, 255 Peripheral resistance, 408, 410f Perirenal fat capsule, 539 Peristalsis, 126, 503, 503f, 504f

in large intestine, 511–513 in mouth/pharynx/esophagus,

492, 506–507 reverse (vomiting), 509 in small intestine, 511 in stomach, 508–509, 508f

Peritoneum, 136, 137f, 492, 493f, 495f

Peritonitis, 492 Peritubular capillaries, 541f, 542, 543f Peritubular capillary bed, 542 Permanent teeth, 500, 500f Permeability, selective, 102, 103,

104f. See also Membrane transport

Pernicious anemia, 368t, 379 Peroxisomes, 94f, 96, 99t Perspiration (sweat), 138–139, 139t,

146 thermoregulation and, 525, 526f

Petechiae, 374 PET scans. See Positron-emission

tomography Peyer’s patches, 429, 429f, 497 PF3, in coagulation, 373 PGs. See Prostaglandins pH, 67, 67f

acid-base balance and, 555–556 of blood, 67f, 68, 364, 476 body defense and, 431t, 432 protein denaturation and, 77 renal regulation of, 544, 544f, 557 respiratory control and, 476, 479,

479f, 480, 557 of semen, 568 of urine, 545, 557 of vagina, 568

pH scale, 67, 67f Phagocytes/phagocytosis, 101, 107,

108f, 424, 431t, 435, 435f in areolar tissue, 124 in inflammation, 433–434, 434f

Phagosome, 435f Phalanges

of foot (toes), 173f, 191, 191f of hand (fingers), 173f, 187, 187f

Pharmacy technician, 82b

Pharyngeal-esophageal phase of swallowing, 506

Pharyngeal tonsils (adenoids), 429, 429f, 464, 465f

Pharyngotympanic (auditory) tube, 317f, 318, 464, 465f

Pharynx, 30, 32f, 463f, 464–465, 465f, 490f, 491, 492

food propulsion and, 492, 506–507, 507f

Phenylalanine, 515f in PKU, 528

Phenylketonuria (PKU), 528 Phimosis, 596 Phlebotomy technician, 376b Phosphate bonds, high-energy, 81 Phosphate buffer system, 556 Phosphatidylcholine, 71f Phospholipids, 71f, 72, 73t

in cell membrane, 72 in plasma membrane, 90–91, 91f

Phosphorus, 53t, 56t Phosphorylation, in ATP

formation, 106 direct phosphorylation, 219, 220f oxidative phosphorylation, 219,

220f, 517f, 518 Photopupillary reflex, 316 Photoreceptors, 309–313, 310f, 311b Phrenic arteries, 400 Phrenic nerves, 288t, 478

in respiratory control, 478, 479f Physical changes in matter, 51 Physical factors, in respiratory

control, 479 Physical therapy assistant, 321b Physiological acidosis, 555 Physiologic jaundice, 379 Physiology, 28

cell, 98–100t 102–114 complementarity principle and, 89 relationship to anatomy and, 28

Pia mater brain, 273, 274f spinal, 281f

PID. See Pelvic inflammatory disease

Pigment(s) skin, 144 visual, 311b

Pigmented layer of retina, 309, 310f

Pineal gland, 30, 31f, 271, 271f, 338f, 343, 354t

developmental aspects of, 357 Pinkeye, 306 “Pink puffers,” 481 Pinna (auricle), 317, 317f Pinocytosis, 107–108, 108f Pisiform, 187f Pitocin. See Oxytocin Pituitary dwarfism, 341, 341f Pituitary gland, 30, 31f, 271, 338f,

339–343, 339f, 340f, 341f, 342b, 354t

developmental aspects of, 357 electrolyte and fluid balance and,

553, 554f hypothalamus relationship and,

271, 337, 337f, 339–343, 339f, 340f

Pivot joint, 196–197, 197f PKU. See Phenylketonuria Placenta, 353–357, 586–587

delivery of, 592f, 593 fetal nutrition via, 528, 586 hormones produced by, 353–357,

580, 587, 590, 591 substances crossing, 590

Placental stage of labor, 592f, 593 Plane joint, 196, 197f Planes (body), 41, 42f

Subject Index 649

Planetary model of atom, 54, 54f Plantar flexion (body movement),

225f, 226 Plantar nerve, 289f Plantar region, 40f, 41 Plaques

atherosclerotic, 412b, 413b senile, in Alzheimer’s disease,

278b Plasma, 124, 364, 365f, 550–551,

551f gas transport in, 476

Plasma cells, 441–442, 441f, 449f, 450t

in lymph nodes, 428 Plasma membrane, 89, 90–92, 90f,

91f, 92f, 94f, 98t boundary maintenance and, 33 selective permeability/diffusion

and, 102, 103, 104f. See also Membrane transport

Plasma proteins, 364, 365f Platelet(s), 364, 365f, 366f, 369–371,

370t deficiency of (thrombocytopenia),

374 formation of, 371f, 372

Platelet factor 3 (PF3), in coagulation, 373

Platelet plug formation, 373, 373f Platysma, 230f, 232, 240f, 242t Pleura, 136, 137f, 467, 468f

inflammation of (pleurisy), 469 Pleural cavity, 468f Pleural fluid, 467 Pleural space, 467

air in, 473 pressure in (intrapleural pressure),

473 Pleurisy, 469 Plexuses (nerve). See Nerve plexuses Plicae circulares (circular folds),

497, 498f Pneumocystis pneumonia, in HIV

infection/AIDS, 454b Pneumonia, 473, 474, 485 Pneumothorax, 473–474, 473f PNS. See Peripheral nervous system Podocytes, 541f, 542 Polar body, 578, 578f, 579 Polarity

of phospholipids, 72 of water, 60, 62f, 65

Polarized membrane (neuron), 260, 261f

Polar molecules, 60, 62f Pole(s), charged, in polar

molecules, 60 Polycystic kidney disease, adult,

559 Polycythemia, 367–368 Polycythemia vera, 367 Polydipsia, in diabetes, 351 Polymer(s), 68–69 Polyp(s), gastrointestinal, 528

colon cancer and, 130f, 131b, 528 Polypeptides, 74, 75f, 504, 505f Polyphagia, in diabetes, 351 Polysaccharides, 69f, 70, 504 Polyunsaturated fats, 70 Polyuria, 351, 555

in Addison’s disease, 555 in diabetes, 351

Pons, 267f, 269t, 270f, 271f, 272 respiratory centers in, 478, 479f

Popliteal (term), 40f, 41 Popliteal artery, 400, 401f

pulse at, 407f Popliteal vein, 402, 403f Pores

capillary, diffusion through, 413, 414f

nuclear, 89, 90f in plasma membrane (protein

channels), 91, 91f, 103, 104f sweat, 140f, 146, 146f taste, 326

Portal circulation hepatic, 405, 405f, 406f hypothalamic hormones released

into, 339, 340f Positional terms/anatomical position,

38, 39t, 40f Positive chemotaxis, 368, 432, 434,

434f Positive feedback

homeostasis and, 45, 46 uterine contractions in labor and,

591, 592f Positron emission tomography

(PET), 36b, 37f, 57 in brain lesion diagnosis, 296b

Postcapillary venule, 398, 399f Postcentral gyrus, 267f, 268f Posterior (term), 39t. See also under

Dorsal Posterior association area, 267f, 268 Posterior body landmarks, 40f, 41 Posterior cerebral arteries, 404,

404f Posterior communicating artery, 404f Posterior femoral cutaneous nerve,

289f Posterior fontanels, 200f Posterior (dorsal) horns, 275f, 279,

281f Posterior inferior iliac spine, 189f Posterior interventricular artery, 389 Posterior nasal aperture, 464, 465f Posterior pituitary gland, 339, 339f,

340f, 354t hormones of, 339–341, 339f, 354t

Posterior sacral foramina, 182, 182f Posterior (vitreous) segment of eye,

308f, 312 Posterior superior iliac spine, 188,

189f Posterior tibial artery, 400, 401f

pulse at, 407f Posterior tibial vein, 402, 403f Posterior trunk muscles, 233–234,

234f, 241f, 243t Postganglionic axon, 290f, 291, 292 Postpartum bleeding, 593 Postural changes, blood pressure

and, 410f, 411 Posture, skeletal muscles in

maintenance of, 211 Potassium, 53t

heart rate and, 394 in muscle contraction, 214, 215f in neuron depolarization/

repolarization, 260, 261f, 262 Potential energy, 51 Practical nurse, 558b Precapillary sphincter, 398, 399f Precentral gyrus, 266, 267f, 268f Precipitation, 445, 446f Preganglionic axon, 290f, 291 Preganglionic neuron, 291, 292 Pregnancy, 583–593, 590f, 592f

anatomical changes during, 574, 589–590, 590f

childbirth (parturition) and, 591–593, 592f

HIV transmission and, 455b homeostatic imbalances and, 590 physiologic changes during,

590–591 prevention of (contraception),

594–595b Rh blood groups and, 377–378

Pregnancy tests, hCG-based, 357 Prematurity

body temperature regulation and, 295

respiratory problems and, 483 Premolars, 500, 500f Premotor area, 267f Prepuce

of clitoris, 576f, 577 of penis (foreskin), 566f, 567f,

568 Presbycusis, 329 Presbyopia, 328 Pressoreceptors (baroreceptors),

blood pressure and, 408 Pressure gradient, 104

blood pressure and, 407–408, 408f breathing and, 471, 473, 473f glomerular filtration and, 543

Pressure points, 407, 407f Pressure receptors (lamellar

corpuscles), 139t, 140f, 143, 259f Primary germ layers, 585, 587f Primary humoral response, 441,

441f, 442f Primary (essential) hypertension,

411 Primary motor area, 266, 267f, 268f Primary oocytes, 577, 578f Primary ovarian follicle, 574f, 577,

578f, 581f Primary sex organs, 564. See also

Gonad(s) Primary somatic sensory area, 266,

267f, 268f Primary spermatocyte, 569f, 570 Primary spinal curvatures, 179, 179f Primary structure of protein, 75, 75f Prime mover, 226 Principle of complementarity, 89 PRL. See Prolactin Processes (bone markings), 163,

166t Processes (neuron), 101, 126, 126f,

255, 256f, 258, 258f neuron classification and, 260,

260f Product(s), 57 Progesterone, 73t, 353, 355t,

579–580 in contraception, 594b, 595b in menstrual/ovarian cycles, 580,

581f placental, 357

Projection (bone marking), 163, 166t

Projection fiber tracts, 268, 270f Prolactin, 340f, 341, 354t Proliferative phase of menstrual

cycle, 580, 581f Pronation (body movement), 225f,

226 Prophase, 110–111, 110f Proprioceptors, 258, 259, 259f Propulsion (food), 503, 503f,

506–507, 507f, 508–509, 508f, 511–513. See also Peristalsis

Prostaglandins, 73t, 335, 347, 356t initiation of labor and, 591

Prostate cancer, 568 Prostate gland, 32f, 547, 566f,

567–568, 567f Prostatic hypertrophy/hyperplasia,

550, 567–568 urinary retention and, 550, 561,

568 Prostatic urethra, 548, 566f, 567,

567f Prostatitis, 568, 596 Prostheses, for joint replacement,

192–193b Prosthetic heart valve, 389, 389f Protease inhibitors, for HIV

infection/AIDS, 455b

Protective function of bone, 161 of connective tissue, 119, 120,

123, 124 of epithelial tissue, 114, 116f of muscle, 211 of skin, 138, 139t

Protein(s), 35, 74–78, 74f, 75f, 76f, 77t, 78f, 514t, 515

absorption/digestion of, 504, 505f, 508, 509

complete, 515 denaturation of, 77 dietary sources of, 514t, 515 fibrous (structural), 76, 76f, 77,

111 globular (functional), 76–77, 76f,

77t, 111, 366 Golgi apparatus in modifying/

packaging of, 95, 95f, 96f membrane, 90–92, 91f metabolism of, 519f, 520 plasma, 364, 365f in receptor-mediated endocytosis,

108, 108f rough endoplasmic reticulum in

synthesis/export of, 94, 95f structural levels of, 75–76, 75f synthesis of. See Protein synthesis transmembrane, 92 in urine, 546t, 549b

Protein buffer system, 556 Protein channels, 91, 91f, 103, 104f Protein synthesis, 63, 63f, 111–114,

113f DNA in, 89, 111–112, 113f in ribosomes, 93, 95f, 113f RNA in, 111–112, 113f transcription, 112, 113f translation, 112, 113f

Proteinuria, 546t, 549b Proton(s), 52, 53, 54, 54f, 54t, 55,

55f acids releasing, 66, 556 pH scale and, 67

Proton acceptors, 66 Proton donors, 66 Proto-oncogenes, 130–131b Proximal (term), 39t Proximal convoluted tubules, 541f,

542, 544f Pseudohermaphrodites, 593 Pseudopods, 101, 101f, 107, 108f Pseudostratified ciliated columnar

epithelium, 116, 118f Pseudostratified columnar

epithelium, 116, 118f Psoas major muscle, 237f, 238 Psoriasis, 150, 150f PTH. See Parathyroid hormone Ptosis, renal, 539 Puberty, 564, 571, 577, 596 Pubic (term), 40f, 41 Pubic arch, 189f Pubic bone (pubis), 188, 189f Pubic (hypogastric) region, 43, 44f Pubic symphysis, 188, 189f, 195 Pubis (pubic bone), 188, 189f Pulmonary arteries, 385–386f 386,

387f, 468f Pulmonary capillaries, 470, 470f Pulmonary circulation, 386, 387f Pulmonary congestion, in heart

failure, 395–396 Pulmonary edema, 396 Pulmonary embolism, 399 Pulmonary (visceral) pleura, 137f,

467, 468f Pulmonary semilunar valve, 386f,

389 Pulmonary thrombosis, 374 Pulmonary trunk, 385f, 386, 468f

650 Subject Index

Pulmonary veins, 385–386f 386, 387f, 468f

Pulmonary ventilation (breathing), 471–474, 472f, 473f. See also Respiration

Pulp, 501 Pulp cavity, 501, 501f Pulse (arterial), 406, 407f

pregnancy and, 591 Pulse rate, 406 Pupil, 305f, 308f, 309 Pupillary reflexes, 263, 316 Purkinje fibers, 390f, 391 Pus, 434

in urine, 546t P wave, 393b Pyelitis, 548 Pyelonephritis, 545, 548, 559 Pyloric antrum, 493, 494f Pyloric sphincter/valve, 493, 494f,

495, 496, 508–509, 508f Pylorus, 493, 494f, 508f, 509 Pyramidal (corticospinal) tract, 266 Pyrogens, 436, 527 Pyuria, 546t

QRS complex, 393b Quadrants, abdominopelvic, 43, 44f Quadratus lumborum muscles, 234,

234f, 243t Quadriceps muscles, 237f, 238,

240f, 242t Quadriplegic, 281 Quaternary structure of protein,

75f, 76 Quickening, 244

RA. See Rheumatoid arthritis Radial artery, 400, 401f

pulse at, 406, 407f Radial fossa, 186f Radial groove, 186–187, 186f Radial nerve, 288t, 289f Radial tuberosity, 186f, 187 Radial vein, 402, 403f Radiant energy, 51 Radiate arteries, cortical, 539, 540f,

541f, 543f Radiate veins, cortical, 539, 540f,

541f, 543f Radiation

aging and, 129 heat loss via, 525, 526f ultraviolet. See Ultraviolet

radiation Radiation therapy, for cancer, 131b Radioactivity (radiation), 56 Radioisotopes, 56

for positron-emission tomography, 36b, 57

Radiologic technologist, 169b Radioulnar joints, 186f, 187 Radius, 173f, 186f, 187, 200f Rales, 475 Ramus/rami (bone), 166t

ischial, 189f mandibular, 174f, 178 of pubic bone, 188, 189f

Ramus/rami (nerve) communicans, 292, 293f dorsal, 283, 285f, 293f ventral, 283–285, 285f, 293f

Ranvier, nodes of, 256f, 257 RAS. See Reticular activating system RBCs. See Red blood cells Reabsorption, tubular, 543–544,

543f, 544f acid-base balance and, 557 fluid balance and, 553, 554f

Reactant(s), 57 water as, 65

Real image, 313, 314f

Receptor-mediated endocytosis, 108, 108f

Receptors, 258, 258f, 263. See also specific type

hormone action and, 335 in homeostasis, 45, 46f

Rectum, 30, 32f, 490f, 495f, 499, 499f

Rectus abdominis muscles, 232, 233f, 240f, 242t

Rectus femoris muscle, 237f, 238, 240f, 242t

Rectus muscles, ocular, 307f Red blood cells, 100, 101f, 123f,

364, 365f, 366–368, 366f, 367f, 370t. See also Erythrocyte(s)

IV therapy and tonicity of, 105b Red cones, 310, 310f Red marrow, 163, 378 Redness

in inflammation, 432, 433f of skin (erythema), 145

Reduction fracture, 170 joint, 196

Reflexes/reflex arc, 263–265, 264f eye, 316

Refraction (light), 313, 313f Regeneration, 127

by epithelia, 115 in tissue repair, 127

Regional terms in anatomy, 38–41, 40f Regulatory (suppressor) T cells,

448, 450t allergy/autoimmune disease and,

482b Rejection, transplant, 448–451 Relaxin, 357, 590 Releasing hormones, hypothalamic,

339, 340f Remodeling (bone), 168–170, 168f

in fracture repair, 171f, 172 Renal arteries, 400, 401f, 538f, 539,

540f Renal calculi, 546, 547f Renal calyces, 539, 540f Renal columns, 539, 540f Renal corpuscle, 542 Renal cortex, 539, 540f Renal failure, 545, 549b Renal fascia, 539 Renal hilum, 538f, 539, 540f Renal medulla, 539 Renal pelvis, 539, 540f Renal ptosis, 539 Renal (medullary) pyramids, 539,

540f Renal tubules, 542, 543f, 544f

reabsorption by, 543–544, 543f, 544f, 553, 554f, 557

secretion by, 543f, 544, 544f, 557 Renal veins, 402, 403f, 538f, 539,

540f Renin, 347, 537, 554f, 555

water balance/blood pressure and, 347, 348f, 410, 410f, 537, 554f, 555

Renin-angiotensin mechanism, 554f, 555

Rennin, 508 Repair

bone fracture, 170–172, 171f tissue (wound healing), 126–128,

128f Repolarization, neuron, 261f, 262 Reproduction, 35

cellular, 35 Reproductive system, 32f, 33,

35, 564–602. See also Female reproductive system; Male reproductive system

accessory organs of, 564

cardiovascular system and, 417b, 597b

developmental aspects of, 593–598

digestive system and, 531b, 597b endocrine system and, 358b, 597b female anatomy, 573–577, 575f female function/cycles, 577–580,

581f homeostatic relationships of, 597b integumentary system and, 155b,

597b lymphatic system/immunity and,

456b, 597b male anatomy, 565–569, 566–567f male function, 569–573 mammary glands, 32f, 580–583,

582f, 583f muscular system and, 246b, 597b nervous system and, 298b, 597b pregnancy/embryonic

development and, 583–593, 584f, 588–589t, 590f, 594–595b

respiratory system and, 484b, 597b

skeletal system and, 202b, 597b urinary system and, 560b, 597b

Residual volume, 474, 474f Resistance (peripheral), 408, 410f Resistance (isometric) exercises,

muscles affected by, 222, 222f, 245

Resistance stage of stress response, 349

Resistin, 356t Respiration, 471. See also Cellular

respiration basic rhythm of, 478, 479f control of, 478–480, 479f developmental aspects of,

483–485 external, 471, 475, 476, 476f, 477f fetal, 483 gas exchange and, 413, 414f, 469,

470, 475, 476, 476f gas transport and, 30, 34f, 471,

476–477, 477f homeostatic imbalances and, 475,

477, 478, 480, 482b internal, 471, 475, 476f, 477–478,

477f neural regulation of, 478, 479f nonneural factors affecting,

479–480 volition (conscious control) and,

479 Respiratory bronchioles, 469, 469f Respiratory burst, 369 Respiratory capacities, 474–475, 474f Respiratory distress syndrome,

infant (IRDS), 483 Respiratory failure

in COPD, 481, 481f in myasthenia gravis, 244

Respiratory gases. See under Gas Respiratory membrane (air-blood

barrier), 470, 470f Respiratory mucosa, 463–464 Respiratory pump

lymph transport and, 426 venous return and, 395f, 398

Respiratory rate aging affecting, 485 neural control of, 478, 479f nonneural factors affecting,

479–480 Respiratory sounds, 475 Respiratory system, 30, 32f, 462–

488, 463, 463f. See also specific structure and under Pulmonary

in acid-base balance, 476, 556–557

anatomy of, 462–471, 463f, 465f, 467–470f

cardiovascular system and, 417b, 484b

developmental aspects of, 483–485

digestive system and, 484b, 531b endocrine system and, 358b, 484b homeostatic imbalances/disor-

ders and, 464–465, 466, 469, 473–474, 475, 477, 478, 480–483, 481f, 482b, 485

homeostatic relationships of, 34f, 484b

integumentary system and, 155b, 484b

lymphatic system/immunity and, 456b, 484b

muscular system and, 246b, 484b nervous system and, 298b, 484b physiology of, 471–480, 472f,

473f, 474f, 475t, 476f, 477f, 479f pregnancy and, 591 reproductive system and, 484b,

597b skeletal system and, 202b, 484b urinary system and, 484b, 560b

Respiratory (bronchial) tree, 469, 469f

Respiratory volumes, 474–475, 474f Respiratory zone, 469–470, 469f Responsiveness (irritability), 34

muscle cell, 213 neuron, 34, 126, 260. See also

Nerve impulses Resting membrane, neuron, 260,

261f Rete testis, 565, 565f Reticular activating system, 272 Reticular cells, 123f, 124 Reticular connective tissue, 123f, 124 Reticular fibers, 120, 123f Reticular formation, 269t, 272 Reticular layer of dermis, 140f, 142f,

143 Reticulocytes, 371 Retina, 308f, 309–312, 310f

image formation on, 313, 314f Retinal (vitamin A), in rhodopsin,

311b Retinitis, cytomegalovirus, in HIV

infection/AIDS, 454b Retroperitoneal location of

pancreas, 501 Retropulsion, 508f, 509 Reverse transcriptase inhibitors, for

HIV infection/AIDS, 455b R-group, amino acid, 74, 74f Rh blood groups, 377–378 Rheumatic fever, 453 Rheumatoid arthritis, 198–199, 199f,

453 Rhinitis, 464 Rhodopsin, 311b RhoGAM, 378 Rhythm method, of contraception,

595b Rib(s)/rib cage, 173f, 183f, 184,

200f, 468f movement of in breathing, 471,

472f, 473 Ribonucleic acid. See RNA Ribose, 70, 80 Ribosomal RNA, 80, 81, 93, 112 Ribosomes, 90, 93, 94f, 95f, 98t

in protein synthesis, 111–112, 113f Rickets, 170, 170f Right colic (hepatic) flexure, 499,

499f Right hypochondriac region, 44, 44f Right iliac (inguinal) region, 43, 44f Right lower quadrant, 43, 44f

Subject Index 651

Right lumbar region, 44, 44f Right lymphatic duct, 426 Right upper quadrant, 43, 44f RNA (ribonucleic acid), 80–81, 112

messenger, 80–81, 95f, 112, 113f in protein synthesis, 111–112, 113f ribosomal, 80, 81, 93, 112 in transcription, 112, 113f transfer, 80, 81, 112 in translation, 112, 113f

Rods, 309, 310f, 311b Root

hair, 140f, 147, 148f nail, 149, 149f of penis, 567f tooth, 501, 501f

Root canal, 501, 501f Root hair plexus (hair follicle

receptor), 140f Rooting reflex, 528 Rotation (body movement), 223,

224f Rough endoplasmic reticulum, 90f,

93–94, 94f, 95f, 98t Golgi apparatus in packaging

products of, 95, 96f protein synthesis/export by, 94,

95f Round ligament, 574, 575f Round window, 317f, 318, 324f rRNA. See Ribosomal RNA Rubella, maternal, 295, 328

congenital eye disorders and, 328 Rugae, gastric, 493, 494f Rule of eights, 58 Rule of nines, 150, 151f

S. See Sulfur Sacral (term), 40f, 41 Sacral canal, 182, 182f Sacral crest, median, 182, 182f Sacral curvature of spine, 179, 179f Sacral foramina, posterior, 182, 182f Sacral hiatus, 182, 182f Sacral nerves, 280f, 285f Sacral plexus, 285f, 288t, 289f Sacroiliac joint, 182f, 188, 189f Sacrum, 173f, 178, 182, 182f, 189f Saddle joint, 197f, 198 “Safer sex,” HIV infection/AIDS

prevention and, 455b Sagittal section, 41, 42f Sagittal sinus, superior, 274f, 275f Sagittal suture, 172, 176f Saliva, 501, 506

in body defense, 431t, 432 Salivary amylase, 501, 505f, 506 Salivary glands, 490f, 501

autonomic innervation/effects and, 291f, 294t

Salivary reflex, 263 Salt(s), 59–60, 65, 365f

elements found in, 53t, 66 water balance and, 553

Salt (table). See Sodium chloride Saltatory conduction, 262 Salty receptors, 327 SA node. See Sinoatrial (SA) node Saphenous nerve, 289f Saphenous vein, 402, 403f Sarcolemma, 211–212, 212f Sarcomeres, 212–213, 212f

in sliding filament theory, 216–217, 217f

Sarcoplasm, 209 Sarcoplasmic reticulum, 213 Sartorius muscle, 237f, 238, 240f,

242t Satellite cells, 254f, 255 Saturated fats/fatty acids, 70–72,

72f, 515 SCA. See Sickle-cell anemia

Scala tympani/vestibuli, 322, 322f, 324f

Scaphoid, 187f Scapulae (shoulder blades), 173f,

184, 185f, 200f Scapular (term), 40f, 41 Scar tissue, 127, 128, 128f Schlemm, canal of (scleral venous

sinus), 308f, 312 Schwann cells, 254f, 255, 256f

myelin sheaths formed by, 254f, 255–257, 256f, 257f

Sciatic nerve, 288t, 289f intramuscular injections and, 236f,

238 Sciatic notch, greater, 188, 189f SCID. See Severe combined

immunodeficiency disease Sclera, 305f, 308f, 309 Scleral venous sinus (canal of

Schlemm), 308f, 312 Scleroses, in multiple sclerosis, 257 Scoliosis, 180, 180f Screening tests, for cancer, 131b Scrotum, 32f, 33, 353, 566, 566f, 568

developmental aspects of, 596 SCT. See Sickle-cell trait Se. See Selenium Sebaceous (oil) glands, 140f,

145–146, 146f developmental aspects of, 153

Seborrhea, 145–146, 145f Sebum, 145

in body defense, 432 Secondary humoral response, 442,

442f Secondary oocyte, 574f, 578, 578f Secondary ovarian follicle, 581f Secondary polycythemia, 368 Secondary sex characteristics, 567

in female, 579, 580, 596 in male, 571–573, 596

Secondary spinal curvatures, 179, 179f

Secondary structure of protein, 75, 75f

Second-degree burn, 151, 151f Second heart sound, 392 Second-messenger system, in

hormone mechanism of action, 336, 336f

Secretin, 356t, 497t, 510, 510f Secretion

epithelium and, 114, 115, 116 glandular, 119 tubular, 543f, 544, 544f, 557

Secretory IgA, 444 Secretory phase of menstrual cycle,

580, 581f Secretory vesicles, 95, 96f, 107f

in exocytosis, 107, 107f Sections (body), 41, 42f Segmental arteries, 539, 540f Segmentation, 503, 503f, 504f Selective permeability, 102. See also

Membrane transport diffusion and, 103, 104f

Selenium, 53t Self-antigens, 438

autoimmune disease and, 453 development of tolerance to, 439

Self-tolerance, 439 loss of (autoimmune disease),

453 Sella turcica, 174, 175f Semen, 567, 568 Semen analysis, 568 Semicircular canals, 317f, 318, 320f

in equilibrium, 319–320, 320f Semilunar valves, 386f, 388f, 389 Semimembranosus muscle, 236f,

238, 241f, 243t

Seminal fluid, 567 Seminal glands/vesicles, 32f, 566f,

567, 567f Seminiferous tubules, 565, 565f Semitendinosus muscle, 236f, 238,

241f, 243t Senile plaques, in Alzheimer’s

disease, 278b Senility, 297 Sense organs, 252f. See also Special

senses Sensorineural deafness, 323

aging and, 329 maternal rubella and, 328

Sensory aphasia, 277 Sensory area, primary somatic, 266,

267f, 268f Sensory (afferent) division of

peripheral nervous system, 252f, 253

Sensory homunculus, 266, 268f Sensory input, 252, 252f Sensory layer of eye (retina), 308f,

309–312, 310f Sensory (afferent) nerves, 252f,

253, 282 Sensory (afferent) neurons,

258–259, 258f, 259f in reflex arc, 263–265, 264f

Sensory receptors, 30, 31f, 258, 259f. See also specific type

cutaneous, 139t, 140f, 141f, 143, 258

in reflex arc, 263, 264f responsiveness and, 126, 260 spatial map of (sensory

homunculus), 266, 268f for special senses, 304

Sensory (afferent) tracts, 280, 282f Septa, testicular, 565, 565f Septum pellucidum, 275f Serosa. See Serous membranes Serotonin, in hemostasis, 373 Serous fluid, 136, 137f Serous membranes (serosae), 116,

136, 137f of alimentary canal, 492, 493f of stomach, 494f

Serous pericardium, 383, 384, 385f Serratus anterior muscle, 240f Serum, coagulation and, 374 Sesamoid bones, 162 Set point, in homeostasis, 45 Severe combined immunodeficiency

disease, 453 Sex (gender)

basal metabolic rate affected by, 524t

determination of, 593 Sex chromosomes, 593

homeostatic imbalances and, 596 Sex hormones, 73t, 348, 564–565.

See also Estrogens; Testosterone adrenal, 347f, 348 bone growth and, 168 gonadal development and, 596 hypersecretion of, 349

Sexual differentiation, 593 Sexual infantilism, 573 Sexually transmitted infection, 559

HIV as, 455b Shaft

bone (diaphysis), 163, 164f hair, 140f, 147, 148f of penis, 567f, 568

Sharpey’s (perforating) fibers, 163, 164f, 165f

Shinbone (tibia), 173f, 190f, 191 Shivering, 525, 526f, 527 Shock

anaphylactic, 452 circulatory, 150, 411

Short bones, 162, 162f Shoulder blades (scapulae), 173f,

184, 185f, 200f Shoulder (pectoral) girdle

bones of, 173f, 184, 185f muscles of, 233f, 234f, 240f, 241f,

242t, 243t Shoulder joint, 194f Si. See Silicon Sickle-cell anemia, 367, 367f, 368t Sickle-cell trait, 367 SIDS. See Sudden infant death

syndrome Sight, 304. See also Eye; Vision Sigmoid colon, 490f, 499, 499f Signet ring cells (fat cells), 101,

101f, 122f, 124 Silicon, 53t Simple diffusion, 103, 104f Simple epithelium, 115–116, 115f,

116f, 117f columnar, 116, 117f cuboidal, 116, 117f squamous, 116, 117f

Simple (closed) fracture, 170 Simple sugars (monosaccharides),

69–70, 69f Sinoatrial (SA) node, 208t, 390f,

391 Sinus (bone marking), 166t Sinuses

lymph node, 428, 428f paranasal, 177, 177f, 464

Sinus headache, 464 Sinusitis, 177, 464 Sister chromatids, 110f. See also

Chromatid(s) Skeletal muscle(s)/skeletal muscle

tissue, 30, 31f, 124, 125f, 127f, 208t, 209–210, 209f

activity/contraction of, 208t, 213–222, 214–220f, 222f. See also Skeletal muscle contraction

cardiovascular system and, 246b, 417b

connective tissue component of, 208t, 209, 209f, 245

developmental aspects of, 244–245

digestive system and, 246b, 531b endocrine system and, 246b, 358b exercise affecting, 221–222, 222f,

244, 245 fascicles/fascicle arrangement and,

209, 209f, 228–229, 229f functions of, 211 gross anatomy of, 229–244. See

also specific named muscle growth hormone affecting, 340f,

342b, 354t homeostatic imbalances and, 221,

244, 245f homeostatic relationships of,

246b integumentary system and, 155b,

246b interactions of, 226–228, 227f lymphatic system/immunity and,

246b, 456b microscopic anatomy of, 211–213,

212f movement and, 30, 33, 207, 211,

222–226, 223f, 223t, 224–225f naming, 228 nervous system and, 246b, 298b reproductive system and, 246b,

597b respiratory system and, 246b,

484b skeletal system and, 202b, 246b steroid abuse and, 231b urinary system and, 246b, 560b

652 Subject Index

Skeletal muscle contraction, 30, 124, 208t

energy for, 219–220, 220f frequency of stimulus and,

217–218, 219f graded responses and, 217–219 homeostatic imbalance and, 221 isotonic and isometric, 221 muscle fatigue/oxygen deficit and,

220–221 muscle tone and, 221 nerve stimulus/action potential in,

213–216, 214f, 215f, 216f of single muscle fiber, 213–217,

214–218f sliding filament theory and,

216–217, 217f, 218f strength of stimulus and, 218 types of, 221 of whole muscle, 217–222, 219f,

220f, 222f Skeletal muscle fibers/cells, 101,

101f, 124, 125f, 208t, 209–210, 209f

anatomy of, 211–213, 212f contraction of, 213–217, 214–218f homeostatic imbalance, 213–214

Skeletal system, 29–30, 31f, 160–206. See also specific component

bones, 160–172 cardiovascular system and, 202b,

417b cartilages, 160 developmental aspects of,

199–203, 200f, 201f, 203f digestive system and, 202b, 531b endocrine system and, 202b, 358b homeostatic relationships of, 202b integumentary system and, 155b,

202b joints, 160, 192–199 ligaments, 160 lymphatic system/immunity and,

202b, 456b movement and, 33 muscular system and, 202b, 246b nervous system and, 202b, 298b reproductive system and, 202b,

597b respiratory system and, 202b,

484b urinary system and, 202b, 560b

Skeleton, 160 appendicular, 160, 173f, 184–192 axial, 160, 172–184, 173f developmental aspects of,

199–203, 200f, 201f, 203f Skin (cutaneous membrane/

integument), 29, 31f, 135, 136, 137f, 138–156. See also Integumentary system

appendages of, 140f, 145–149, 145f, 148f, 149f

autonomic innervation/effects and, 291f

in body defense, 138, 139t, 431t, 432

boundary maintenance and, 33, 138, 139t

color of, 144–145 connective tissue in, 123, 124 contracture, 128 developmental aspects of, 129,

143b, 153–156 functions of, 138–139, 139t homeostatic imbalances and, 142,

144, 145–146, 149–153, 150f, 151f, 152f, 156

homeostatic relationships of, 34f, 155b

melanin in protection of, 139t, 142

structure of, 139–144, 140f, 141f, 142f

sun damage/cancer and, 142, 143b, 150, 151f, 152, 152f, 153, 156

in thermoregulation, 138–139, 139t, 144, 525–527, 526f

Skin cancer, 142, 152–153, 152f Skin pigments, 144 Skin (decubitus) ulcers, 144, 144f Skull, 172–178, 173f, 174f, 175f, 176f

fetal/growth of, 199–200, 200f, 201f

SLE. See Systemic lupus erythematosus

Sleep apnea, 485 Sleep cycle, melatonin and, 343 Sleeve gastrectomy, 530 Sliding filament theory, 216–217,

217f, 218f Small cell carcinoma, of lung, 483 Small intestine, 30, 32f, 33, 44f,

490f, 491, 495f, 496–498, 496f, 498f

in digestion, 496–498, 503f, 505f, 509–511, 510f

homeostatic imbalances/disorders and, 509–510, 512b

Small saphenous vein, 403f Smell, sense of, 304, 324–326, 325f

cortical areas in, 267f, 325 developmental aspects of, 329

Smoking COPD and, 481, 481f lung cancer and, 481 nervous system development

and, 295 skin affected by, 143b, 156 trachea affected by, 466

Smooth endoplasmic reticulum, 94–95, 94f, 98t

Smooth muscle/smooth muscle tissue, 29f, 125f, 126, 208t, 210, 210f

alimentary canal, 126, 492, 493f contraction of, 126, 208t, 210

Smooth muscle fibers/cells, 29f, 101, 101f, 125f, 126, 208t, 210, 210f

Sn. See Tin Sneeze, 475t Sodium, 53t, 56t, 57, 57f, 59f

atomic structure of, 56t as chemically reactive element, 59f fluid and electrolyte balance and,

553, 554f heart rate and, 394 in muscle contraction, 214, 215f,

216f in neuron depolarization/

repolarization, 260, 261f, 262 sodium chloride formation and,

57–58, 57f, 59, 60f Sodium bicarbonate. See also

Bicarbonate, in pancreatic juice; Bicarbonate ion

in bicarbonate buffer system, 556 Sodium chloride, 57–58, 57f, 59, 60f Sodium hydroxide

ionization of, 66 pH of, 67f

Sodium-potassium pump, 106, 106f, 261f, 262

Soft palate, 464, 465f, 491, 491f Soleus muscle, 239, 239f, 240f,

241f, 243t Solids (matter), 51 Solute(s), 65, 102

tonicity/osmotic pressure and, 105b

Solute pumping/solute pumps, 106, 106f

Solution(s), 65, 102

Solvent(s), 65, 102 water as, 65, 102, 550

Somatic nervous system, 252f, 253, 280, 290–291, 290f

autonomic nervous system compared with, 290–291, 290f

Somatic reflexes, 263 Somatic sensory area, primary, 266,

267f, 268f Somatic sensory (afferent) fibers,

253 Somatostatin, 497t Sørensen, pH scale devised by, 67 Sound waves, transmission of, 322,

323f, 324f Sour receptors, 327 Spastic paralysis, 280–281 Special sense receptors, 304. See

also specific type Special senses, 304–333. See also

specific type developmental aspects of,

327–329 ear (hearing and balance),

316–324 eye and vision, 305–316 homeostatic imbalances and, 326,

327–329 smell, 324–326, 325f taste, 326–327

Specific defense system. See Adaptive (specific) defense system

Specific gravity, of urine, 545 Speech area, 267f, 268

motor (Broca’s area), 266, 267f Sperm, 33, 101f, 102, 353, 564, 568,

570, 570f, 571f, 584–585, 585f abnormal, 571, 571f production of (spermatogenesis),

569–571, 569f, 572f Spermatic cord, 565, 565f Spermatids, 569f, 570 Spermatocytes, 569f, 570 Spermatogenesis, 569–571, 569f,

572f Spermatogonia, 569, 569f Spermicidal foams/gels/sponges,

594b, 595b Spermiogenesis, 569f, 570 Sphenoidal fontanel, 200f Sphenoidal sinuses, 177, 177f, 465f Sphenoid bone, 174–177, 174f,

175f, 176f Spina bifida, 295 Spinal cavity, 43, 43f Spinal cord, 30, 31f, 252f, 253, 267f,

278–281, 280f, 281f developmental aspects of, 295 gray matter of, 279–280, 281f,

282f homeostatic imbalances and,

280–281 protection of, 278, 281f transection/injury of, 280–281 white matter of, 280, 281f, 282f

Spinal curvatures, 179–180, 179f abnormal, 180, 180f, 201, 203f growth/developmental aspects of,

179, 179f, 180, 201, 203f vertebral collapse/osteoporosis

and, 201, 203f Spinalis muscle, 233–234, 234f Spinal nerve(s), 252f, 253, 279, 280,

280f, 281f, 283–290, 285f, 288t Spinal nerve plexuses, 285, 285f,

288t Spinal reflexes, 265, 278 Spinal (lumbar) puncture, 276 Spine (bone marking), 166t Spine (vertebral column), 173f,

178–182, 179–182f developmental aspects of, 295

Spinous process, vertebral, 179f, 180, 180f, 181f

Spiral fracture, 171t Spiral organ of Corti, 322, 322f

aging affecting, 329 Spirometer, 474 Splanchnic nerves, 292, 293f

pelvic, 292, 548 Spleen, 30, 428–429, 429f, 490f, 495f Splenic artery, 400 Splenic (left colic) flexure, 499, 499f Splenic vein, 403f, 405, 406f Spongy bone, 161, 161f, 164f, 165f Spongy (penile) urethra, 548, 566f,

567, 567f Sprains, 198 Squamous cell carcinoma

of lung, 483 of skin, 152f, 153

Squamous epithelium, 115, 115f, 116f

simple, 116, 117f stratified, 118f, 119

Squamous suture, 172, 174f SR. See Sarcoplasmic reticulum Stapes (stirrup), 317f, 318, 324f Staphylococcus aureus, carbuncles

caused by, 149 Starch, 69f, 70

digestion of, 504, 505f, 506 Starling’s law of the heart, 394 Static equilibrium, 319, 319f Statins, 413 Stem cells

hematopoietic, 371, 371f of immune system, 457 oogonia, 577, 578f regeneration, 193 spermatogonia, 569, 569f

Stenosis, valvular, 389, 392 Stents, coronary angioplasty and,

413b Stercobilin, 371 Sterility, 343, 573

gonadotropic hormone hyposecretion and, 343

testosterone deficiency and, 573 Sterilization, for contraception, 595b Sternal (term), 40f, 41 Sternal angle, 183, 183f Sternal puncture, 183 Sternoclavicular joint, 184 Sternocleidomastoid muscles, 230f,

232, 234f, 240f, 241f, 242t Sternum, 173f, 183–184, 183f Steroids/steroid hormones, 72, 73t,

335. See also Corticosteroids anabolic, 231b mechanism of action of, 335–336,

336f STI. See Sexually transmitted infection Stimuli

endocrine system activation and, 337–338, 337f

in homeostasis, 45, 46f muscle contraction and, 217–218,

219f nervous system/neuron response

to, 252, 252f Stirrup (stapes), 317f, 318, 324f Stomach, 30, 32f, 44f, 490f, 491,

493–495, 494f, 495f. See also under Gastric

autonomic innervation/effects and, 291f

in body defense, 432 cancer of, 528 in digestion, 495, 503f, 505f,

507–509, 508f homeostatic imbalances and, 508,

509 in newborn, 528

Subject Index 653

Storage function, of bone, 161 Strabismus, 143, 327, 328f Strata, of skin, 140, 141f Stratified epithelium, 115, 115f,

116–119, 116f columnar, 119 cuboidal, 119 squamous, 118f, 119 transitional, 116f, 118f, 119,

547–548 Stratum basale (stratum

germinativum), 140, 141f Stratum corneum, 141–142, 141f Stratum granulosum, 140, 141f Stratum lucidum, 140–141 Stratum spinosum, 140, 141f Strength training (resistance/

isometric exercise), muscles affected by, 222, 222f, 245

Streptococcal infection, glomerulonephritis and, 559

Stress incontinence, pregnancy and, 591

Stress response, 292–293, 349, 350f glucocorticoids in, 347, 349, 350f sympathetic nervous system/

adrenal medulla in, 292–293, 337f, 338, 349, 350f

Stretch receptors in digestion, 506 in micturition, 548 in respiratory control, 478

Striated muscle, 209. See also Cardiac muscle/cardiac muscle tissue; Skeletal muscle(s)/skeletal muscle tissue

Striations cardiac muscle, 124, 125f, 208t, 210 skeletal muscle, 124, 125f, 208t, 209

Stroke (cerebrovascular accident), 277, 374

diagnosis of, 296–297b Stroke (heat), 527 Stroke volume, 393, 394, 395f Stroma, 124

of lung, 470 Strong acids, 66, 556, 556f Strong bases, 67, 556 Structural (fibrous) proteins, 76,

76f, 77, 111 Styloid process, 172, 174f, 175f

radius/ulna, 186f, 187 temporal bone, 172–174, 174f, 175f

Subarachnoid space, 273, 275f Subatomic particles, 52–53, 54t Subclavian arteries, 385f, 400, 401f Subclavian vein, 402, 403f Subcutaneous tissue (hypodermis),

124, 139, 140f Sublingual glands, 490f, 501 Submandibular glands, 490f, 501 Submucosa, alimentary canal, 210f,

492, 493f small intestine, 498f tracheal, 467f

Submucosal nerve plexus, 492, 493f Substance abuse, HIV transmission

and, 455b Substantia nigra/substantia nigra

transplantation, Parkinson’s disease and, 278b, 279b

Substrate, 77, 78f Substrate-level phosphorylation,

517, 517f Sucking reflex, 528 Sucrase, 505f Sucrose, 70

dehydration synthesis/hydrolysis of, 69f

digestion of, 504, 505f Sudden infant death syndrome

(SIDS), 485

Sudoriferous (sweat) glands, 138–139, 139t, 140f, 146–147

autonomic nervous system affecting, 294t

mammary glands as, 582 Sugar. See Fructose; Galactose;

Glucose Sugar-phosphate backbone, 80 Sulci (sulcus), 265 Sulfur, 53t, 56t Summing, of muscle contractions,

217, 219f Sunburn, 150, 151f Sun exposure

aging and, 129 immune system affected by, 142 melanin in protection from, 139t,

142 skin damage/cancer caused by,

142, 143b, 150, 151f, 152, 152f, 153, 156

Superficial (term), 39t. See also under External

Superficial palmar arch, 401f Superficial temporal artery, pulse

at, 407f Superior (term), 39t Superior articular process, vertebral,

180, 180f, 181f Superior gluteal nerve, 288t, 289f Superior mesenteric artery, 400,

401f Superior mesenteric vein, 403f,

405, 406f Superior nasal conchae, 177, 465f Superior oblique muscle, 307f Superior orbital fissure, 174, 176f Superior rectus muscle, 307f Superior sagittal sinus, 274f, 275f Superior vena cava, 385–386f 386,

402, 403f veins draining into, 402

Supination (body movement), 225f, 226

Support function, of bone, 161 Supporting cells, nervous tissue,

126, 126f, 253–255, 254f Suppressor (regulatory) T cells,

448, 450t allergy/autoimmune disease and,

482b Suppressor factors, in immunity,

450t Suprascapular notch, 184, 185f Sural (term), 40f, 41 Sural nerve, 289f Surface area, basal metabolic rate

and, 523, 524t Surface epithelium, alimentary

canal, 492, 493f stomach, 494f, 495

Surface membrane barriers, in body defense, 431t, 432

Surface tension of water, 62, 62f Surfactant, 470, 483

cells secreting, 470, 470f Surgery, for obesity, 530b Surgical neck, of humerus, 186,

186f Survival needs, 35–38 Suspension(s), 65 Suspensory ligament

of breast, 582f of ovary, 573, 575f

Sutures (cranial), 172, 174f, 176f, 194f, 195

Swallowing (deglutition), 503, 503f, 506–507, 507f

Sweat, 138–139, 139t, 146 thermoregulation and, 525, 526f

Sweat (sudoriferous) glands, 139t, 140f, 146–147

autonomic nervous system affecting, 294t

mammary glands as, 582 Sweat pores, 140f, 146, 146f Sweet receptors, 327 Swelling, in inflammation, 432, 433,

433f Swollen glands, 427 Sympathetic (thoracolumbar)

division of autonomic nervous system, 252f, 253, 290f, 291

aging affecting, 296 anatomy of, 292, 293f blood pressure and, 296, 408–409,

410f, 411, 554f function of, 292–293, 294t heart rate and, 292, 294t, 394,

395f homeostatic imbalances and, 293 in stress response, 292–293, 337f,

338, 349, 350f Sympathetic trunk/chain, 292, 293f Sympathetic trunk ganglion, 292,

293f Symphyses, 194f, 195 Synapse, 255

neuronal communication at, 262–263, 263f

Synaptic cleft, 213, 255 in neuromuscular junction, 213,

214, 215f signal transmission at synapse

and, 263f Synaptic vesicles, 213, 215f

signal transmission at synapse and, 262, 263f

Synarthroses, 193, 196t Synchondroses, 194f, 195 Syndesmoses, 194f, 195 Synergists (muscle), 226 Synovial fluid, 138, 138f, 195f, 196 Synovial joints, 138f, 193, 194f,

195–199, 195f, 196t, 197f, 199f homeostatic imbalances and,

198–199, 199f types of, 196–199, 197f

Synovial membranes, 136–138, 138f, 195, 195f

Synthesis reactions, 62–63, 63f Systemic circulation, 386, 387f

arteries of, 399–400, 401f veins of, 400–402, 403f

Systemic lupus erythematosus (SLE), 453

Systole, 391–392, 392f Systolic pressure, 408, 408f, 411

T. See Thymine T3. See Triiodothyronine T4. See Thyroxine Tachycardia, 391 Tail of sperm, 570, 571f Talus, 191, 191f Target cells/organs, hormone, 335,

336, 336f Tarsal (term), 40f, 41 Tarsal bones/tarsus, 173f, 191, 191f Tarsal glands, 305, 306f Taste, sense of, 304, 326–327, 326f

developmental aspects of, 329 Taste buds, 326–327, 326f, 492 Taste (gustatory) cells, 326, 326f Taste pores, 326, 326f Tau protein, in Alzheimer’s disease,

278b T cells. See T lymphocytes Tears, 306

in body defense, 431t, 432 Tectorial membrane, 322, 322f Teeth, 500–501, 500f

development of, 528 Teething, 528

Telophase, 110f, 111 Temperature

blood vessels/pressure affected by, 410

body. See Body temperature chemical reaction rate affected

by, 64t protein denaturation and, 77

Temperature receptors, 139, 259f Template, in DNA replication, 109,

109f Temporal artery, superficial, pulse

at, 407f Temporal bones, 172–174, 174f,

175f, 176f in fetal skull, 200f

Temporalis muscle, 230, 230f, 240f, 242t

Temporal lobe, 266, 267f Tendon(s), 122f, 123, 209–210,

209f of foot, 191–192

Tendon sheaths, 138, 195f, 196 Teniae coli, 499f, 500 Tentorium cerebelli, 273, 274f Terminal arteriole, 398, 399f Terminal bronchioles, 469, 469f Terminal ganglion, 292 Tertiary structure of protein, 75f, 76 Testes, 30, 31f, 32f, 33, 338f, 353,

355t, 564, 565, 565f, 566f, 567f anatomy of, 565, 565f developmental aspects of, 596 failure to descend

( cryptorchidism), 596 gonadotropic hormones affecting,

340f, 343 homeostatic imbalances and, 596 hormonal control of, 571, 572f hormones of, 73t, 353, 355t, 565,

571–573, 572f rete, 565, 565f. See also

Testosterone Testicular arteries, 400 Testis. See Testes Testosterone, 73t, 353, 565,

571–573, 572f anabolic steroid use and, 231b gonadal development and, 593

Tetanus (lockjaw), 218 Tetanus (tetanic contraction)

fused (complete), 218, 219f unfused (incomplete), 217, 219f

Tetany, 345 TF. See Tissue factor, in coagulation TH. See Thyroid hormone; Thyroid-

stimulating (thyrotropic) hormone Thalamic (deep-brain) stimulation,

for Parkinson’s disease, 279b Thalamus, 269t, 270, 270f, 271f,

282f Thermal energy, 51–52 Thermoregulation, 35, 46, 525–527,

526f skin in, 138–139, 139t, 144,

525–527, 526f Theta waves, EEG, 297f Thick (myosin) filaments, 212f, 213

in sliding filament theory, 216–217, 217f, 218f

Thigh. See also Lower limbs bone in, 190–191, 190f deep artery of, 400 muscles of, 235, 236–237f 240f,

241f, 242t, 243t Thin (actin) filaments, 212f, 213

in sliding filament theory, 216–217, 217f, 218f

Third-degree burn, 151, 151f Third ventricle, 270, 270f, 271, 271f,

274, 275f Thirst center, 552, 552f

654 Subject Index

Thirst mechanism, 552, 552f Thoracic (term), 40f, 41 Thoracic aorta, 399, 401f

branches of, 400 Thoracic cage/bony thorax, 173f,

182–184, 183f. See also Thorax Thoracic cavity, 43, 43f, 467

changes in during breathing, 471, 472f

organs in, 468f Thoracic curvature of spine, 179,

179f Thoracic duct, 426 Thoracic nerves, 280f, 285f Thoracic vertebrae, 179, 179f, 181f,

182 Thoracolumbar division of

autonomic nervous system, 292. See also Sympathetic (thoraco- lumbar) division of autonomic nervous system

Thorax bony (thoracic cage), 173f,

182–184, 183f changes in during breathing, 471,

472f muscles of, 232, 240f

3tTC, for HIV infection/AIDS, 455b Threonine, 515f Throat. See Pharynx Thrombin, in coagulation, 373, 373f Thrombocytopenia, 374 Thrombophlebitis, 399 Thrombopoietin, 372 Thrombus, 374, 374f Thymine (T), 78, 79f, 80, 109, 109f Thymosin, 346, 355t

in lymphocyte differentiation/ activation, 346, 355t, 439

Thymus gland, 30, 31f, 338f, 346, 355t, 429, 429f

lymphocyte differentiation/activa- tion in, 346, 355t, 439, 439f, 457

Thyroid cartilage, 465f, 466 Thyroid follicles, 343, 344f Thyroid gland, 30, 31f, 338f,

343–345, 344f, 344f, 346f, 354t developmental aspects of, 357 metabolism and, 343, 345, 354t,

524, 524t thyroid-stimulating (thyrotropic)

hormone affecting, 340f, 343, 354t

Thyroid hormone, 343, 354t Thyroid-stimulating (thyrotropic)

hormone (TSH), 340f, 343, 354t deficiency of, 344

Thyrotropic hormone. See Thyroid-stimulating (thyrotropic) hormone

Thyroxine (T4), 343, 354t, 524 basal metabolic rate/metabolism

and, 345, 354t, 524, 524t heart rate and, 345, 394, 395f hyposecretion of, 344–345

TIA. See Transient ischemic attack Tibia, 173f, 190f, 191, 200f Tibial arteries, 400, 401f

pulse at, 407f Tibialis anterior muscle, 238, 239f,

240f, 242t Tibial nerve, 288t, 289f Tibial tuberosity, 190f, 191 Tibial veins, 402, 403f Tibiofibular joint, 190f Tidal volume, 474, 474f Tight junctions, 92, 92f Tin, 53t Tinea pedis (athlete’s foot), 149 Tine test, 453 Tissue(s), 28, 29f, 114–129, 127f.