Human Biology
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
Types of Tissues
Human tissues are classified into four groups. Epithelial tissue covers the body and lines its
cavities. The different types of epithelial tissue (squamous, cuboidal, and columnar) may be
stratified and have cilia or microvilli. Also, columnar cells can be pseudostratified. Epithelial
cells sometimes form glands that secrete either into ducts or into the blood.
Connective tissues, in which cells are separated by a matrix, often bind body components
together. Connective tissues have both white and yellow fibers and might also have fat (adipose)
cells. Loose fibrous connective tissue supports epithelium and encloses organs. Dense fibrous
connective tissue, including that of tendons and ligaments, consists of closely packed collagen
fibers. Adipose tissue stores fat. Both cartilage and bone have cells within lacunae, but the matrix
for cartilage is more flexible than that for bone, which contains calcium salts. In bone, the
lacunae lie in concentric circles within an osteon (or Haversian system) around a central canal.
Blood is a connective tissue in which the matrix is a liquid called plasma.
Muscular tissue is of three types. Both skeletal and cardiac muscle are striated; both cardiac
and smooth muscle are involuntary. Skeletal muscle is found in muscles attached to bones, and
smooth muscle is found in internal organs. Cardiac muscle makes up the heart.
Nervous tissue has one main type of functioning cell, the neuron, and various types of
neuroglial cells. Each neuron has dendrites, a cell body, and an axon. The brain and spinal cord
contain entire neurons, while the nerves contain only neuron fibers. Axons are specialized to
conduct nerve impulses.
Body Cavities and Body Membranes
The internal organs occur within cavities. The thoracic cavity contains the heart and lungs; the
abdominal cavity contains organs of the digestive, urinary, and reproductive systems, among
others. Membranes line body cavities and internal surfaces of organs. For example, mucous
membrane lines the tubes of the digestive system, while serous membrane lines the thoracic and
abdominal cavities and covers the organs they contain.
Organ Systems
The digestive, cardiovascular, lymphatic, respiratory, and urinary systems perform processing
and transporting functions that maintain the normal conditions of the body. The skeletal system
and muscular system support the body and enable movement. The nervous system receives
sensory input from sensory receptors and directs the muscles and glands to respond to stimuli.
The endocrine system produces hormones, some of which affect the functioning of the
reproductive system, which allows humans to reproduce. The skin and its accessory organs
comprise the integumentary system.
The accessory organs include nails, hair, and glands. Skin protects underlying tissues from
physical trauma, pathogen invasion, and water loss. Skin helps regulate body temperature, and
because it contains sensory receptors, skin also helps us to perceive our environment.
Skin has two layers. The epidermis consists of basal cells that produce new epithelial cells that
become keratinized as they move toward the surface. The dermis, a largely fibrous connective
tissue, contains epidermally derived glands and hair follicles, nerve endings, and blood vessels.
Sensory receptors for touch, pressure, temperature, and pain are also present in the dermis. A
subcutaneous layer, which is made of loose connective tissue containing adipose cells, lies below
the dermis.
Homeostasis
Homeostasis is the relative stability of the internal environment. Negative feedback
mechanisms maintain the environment relatively stable. When a sensor detects a change above
and/or below a set point, a regulatory center activates an effector that reverses the change and
brings conditions back to normal again. In contrast, a positive feedback mechanism brings about
rapid change in the same direction as the stimulus. However, positive feedback mechanisms are
useful under certain conditions such as when a baby is born.
The internal environment includes blood and tissue fluid. All organ systems contribute to the
stability of tissue fluid and blood. Specific contributions are made by the liver, which maintains
blood glucose steady, and the kidneys, which regulate the pH. The nervous and endocrine
systems regulate the other systems.
The Digestive System
The salivary glands send saliva into the mouth, where the teeth chew the food and the tongue
forms a bolus for swallowing. The air passage and food passage meet in the pharynx. When a
person swallows, the air passage is usually blocked off, and food must enter the esophagus where
peristalsis begins. The stomach expands and stores food. While food is in the stomach, it churns,
mixing food with the acidic gastric juices. The walls of the small intestine have fingerlike
projections called villi where nutrient molecules are absorbed into the cardiovascular and
lymphatic systems. The large intestine consists of the cecum, the colon which includes the
ascending, transverse, descending, and sigmoid colon, and the rectum, which ends at the anus.
The large intestine does not produce digestive enzymes; it absorbs water, salts, and some
vitamins.
Three Accessory Organs
Three accessory organs of digestion—the pancreas, liver, and gallbladder—send secretions to
the duodenum via ducts. The pancreas produces pancreatic juice, which contains digestive
enzymes for carbohydrate, protein, and fat. The liver produces bile, which is stored in the
gallbladder. The liver receives blood from the small intestine by way of the hepatic portal vein. It
has several vital functions, and any malfunction of the liver is a matter of significant concern.
Digestive Enzymes
Digestive enzymes are present in digestive juices and break down food into the nutrient
molecules glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed
into the blood capillaries of the villi. Fatty acids and glycerol are rejoined and repackaged as
lipoprotein droplets which enter the lacteals of the villi. Digestive enzymes have the typical
enzymatic properties. They are specific to their substrate and accelerate specific reactions at
optimum body temperature and pH.
Homeostasis
The digestive system works with the other systems of the body in the ways described in
“Human systems work together” on page 94.
Nutrition
The nutrients released by the digestive process must provide us with an adequate amount of
energy, essential amino acids and fatty acids, and all necessary vitamins and minerals. The
majority of the diet should be carbohydrates (e.g., bread, pasta, and rice) and fruits and
vegetables. These are low in saturated fatty acids and cholesterol molecules, whose consumption
is linked to cardiovascular disease. The vitamins A, E, and C are antioxidants that protect cellular
contents from damage caused by free radicals.
Blood, which consists of formed elements and plasma, has various functions. It transports
hormones, oxygen, and nutrients to the cells and carbon dioxide and other wastes away from
cells. It fights infections and has various regulatory functions. It maintains blood pressure,
regulates body temperature, and keeps the pH of body fluids within normal limits. All of these
functions help maintain homeostasis.
All blood cells are produced within red bone marrow from stem cells, which are ever capable
of dividing and producing new cells.
The Red Blood Cells
Red blood cells are small, biconcave disks that lack a nucleus. They live about 120 days and
are destroyed in the liver and spleen when they are old or abnormal. The production of red blood
cells is controlled by the oxygen concentration of the blood. When the oxygen concentration
decreases, the kidneys increase their production of erythropoietin, and more red blood cells are
produced. Red blood cells contain hemoglobin, the respiratory pigment, which combines with
oxygen and transports it to the tissues.
The White Blood Cells
White blood cells are larger than red blood cells, have a nucleus, and are translucent except
stained. Like red blood cells, they are produced in the red bone marrow. White blood cells are
divided into the granular leukocytes and the agranular leukocytes. The granular leukocytes have
conspicuous granules; in eosinophils, granules are red when stained with eosin, and in basophils,
granules are blue when stained with a basic dye. The granules in neutrophils don’t take up either
dye significantly. Neutrophils are the most abundant of the white blood cells, and they are
capable of phagocytizing pathogens. Many neutrophils die within a few days while they are
fighting an infection. The agranulocytes include the lymphocytes and the monocytes, which
function in specific immunity. Occasionally, the monocytes used to be large phagocytic cells of
great importance. They engulf old red blood cells and pathogens at a ferocious rate.
Blood Clotting
When there is a break in a blood vessel, the platelets clump to form a plug. Blood clotting
itself requires a series of enzymatic reactions involving blood platelets, prothrombin, and
fibrinogen. In the final reaction, fibrinogen becomes fibrin threads, entrapping cells. The fluid
that escapes from a clot is called serum and contains plasma minus fibrinogen.
Plasma
Plasma is mostly water (92%) and the plasma proteins (8%). The plasma proteins, most of
which are produced by the liver, occur in three classes: albumins, globulins, and fibrinogen. The
plasma proteins maintain osmotic pressure, help regulate pH, and transport molecules. Some
plasma proteins have specific functions: the gamma globulins, which are antibodies produced by
B lymphocytes, function in immunity, and fibrinogen is necessary for blood clotting. Small
organic molecules like glucose and amino acids are dissolved in plasma and serve as nutrients
for cells; the gas oxygen is needed for cellular respiration, and carbon dioxide is a waste product
of this process.
Capillary Exchange
At the arterial end of a cardiovascular capillary, blood pressure is greater than osmotic
pressure; therefore, water leaves the capillary. In the midsection, oxygen and nutrients diffuse
out of the capillary, while carbon dioxide and other wastes diffuse into the capillary. At the
venous end, osmotic pressure created by the presence of proteins exceeds blood pressure,
causing water to enter the capillary. Retrieving fluid by osmotic pressure is not completely
efficient. There is always some fluid that is not picked up at the venous end of the cardiovascular
capillary. This excess tissue fluid enters the lymphatic capillaries. Lymph is tissue fluid
contained within lymphatic vessels. The lymphatic system is a one-way system, and lymph is
returned to blood by way of a cardiovascular vein.
Blood Typing
The red blood cells of an individual are not always accepted easily by another person. For
instance, the membranes of red blood cells may contain type A, B, AB, or no antigens. In the
plasma, there are possible antibodies: anti-A or anti-B. If the corresponding antigen and antibody
are brought together, clumping, or agglutination, occurs; in this way, the blood type of an
individual can be determined in the laboratory. After determination of the blood type, it is
theoretically possible to determine who can donate blood to whom. For this, it is necessary to
consider the donor’s antigens and the recipient’s antibodies. Another important antigen is the Rh
antigen. This particular antigen must also be considered in the transfusing of blood, and it is
important during pregnancy because an Rh− mother may form antibodies to the Rh antigen when
carrying or after the birth of a baby who is Rh+. These antibodies can cross the placenta to
destroy the red blood cells of any subsequent Rh+ baby.
