STUDENT LEARNING OBJECTIVES
At the completion of this chapter, you should be able to do the following:
1.Outline the organization of the digestive system, discussing its main components.
2.Describe the primary layers of the typical gastrointestinal wall.
3.Describe the functions of the following by making a table: oral cavity, tongue, salivary glands, teeth, pharynx, esophagus, stomach, small intestine, large intestine, and rectum.
4.Describe the functions of the peritoneum and its mesenteries.
5.Outline the functions of the following organs: liver, gallbladder, pancreas.
6.Describe the overall process of mechanical digestion.
7.Discuss the roles of peristalsis and segmentation throughout the gastrointestinal tract.
8.Explain how motility of food is regulated.
9.Explain the functions of digestive enzymes, discussing the role of proenzymes.
10.Making a table, outline the digestion of carbohydrates, fats, and proteins.
11.Explain the roles that saliva, gastric juice, pancreatic juice, bile, and intestinal juice play in digestion.
12.Explain how digestive gland secretions are controlled.
13.Outline the process of absorption and elimination.
LANGUAGE OF SCIENCE AND MEDICINE
Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.
absorption
(ab-SORP-shun) [absorp- swallow, -tion process]
alimentary canal
(al-eh-MEN-tar-ee kah-NAL) [aliment- nourishment, -ary relating to, canal channel]
amylase
(AM-eh-layz) [amyl- starch, -ase enzyme]
anal canal
(AY-nal kah-NAL) [an- ring (anus), -al relating to, canal channel]
ascending colon
(ah-SEND-ing KOH-lon) [ascend- climb, colon large intestine]
bile
(byle)
bile salt
(byle)
body
(BOD-ee)
brush border
cardia
(KAR-dee-ah) [cardia heart]
cementum
(sih-MEN-tem) [cementum rough stone]
cephalic phase
(seh-FAL-ik fayz) [cephal- head, -ic relating to]
cheek
chemical digestion
(KEM-ih-kal dye-JES-chun)
chief cell
cholecystokinin (CCK)
(koh-leh-sis-toh-KYE-nin) [chole- bile, -cyst- bladder, -kin- move, -in substance]
chyme
(kyme) [chym- juice]
chymotrypsin
(kye-moh-TRIP-sin) [chymo- juice, -tryps- pound, -in substance]
colon
(KOH-lon) [colon large intestine]
constipation
(kon-stih-PAY-shun) [constipa- crowd together, -tion process]
crown
deciduous teeth
(deh-SID-yoo-us) [decid- fall off, -ous relating to]
defecation
(def-eh-KAY-shun) [de- remove, -feca- waste (feces), -tion process]
deglutition
(deg-loo-TISH-un) [deglut- swallow, -tion process]
dentin
(DEN-tin) [dent- tooth, -in substance]
dentition
(den-TISH-en) [dent- tooth, -tion state]
descending colon
(dee-SEND-ing KOH-lon) [descend- move downward, colon large intestine]
diarrhea
(dye-ah-REE-ah) [dia- through, -rrhea flow]
digestive system
(dye-JES-tiv SIS-tem) [digest- break down, system organized whole]
digestive tract
(dye-JES-tiv TRAKT) [digest- break down, tract trail]
digestion
(dye-JES-chun) [digest- break down, -tion process]
duodenum
(doo-oh-DEE-num) [shortened from intestinum duodenum digitorum intestine of 12 finger-widths] pl., duodena or duodenums (doo-oh-DEE-nah, doo-oh-DEE-numz)
elimination
(ee-lim-ih-NAY-shun) [e- out, -limen- threshold, -ation process]
emulsified
(ee-MULL-seh-fyde) [e- out, -muls- milk, -i- combining form, -fy process]
enamel
(eh-NAM-el) [enamel hard glossy coating]
endocrine cell
(EN-doh-krin sell) [endo- within, -crin- secrete, cell storeroom]
esophagus
(eh-SOF-ah-gus) [es- will carry, -phagus food] pl., esophagi (eh-SOF-ah-gye)
feces
(FEE-seez) [feces waste]
fundus
(FUN-duss) [fundus bottom] pl., fundi (FUN-dye)
gastric juice
(GAS-trik) [gastr- stomach, -ic relating to]
gastric phase
(GAS-trik) [gastr- stomach, -ic relating to]
gastrin
(GAS-trin) [gastr- stomach, -in substance]
gastrointestinal (GI) tract
(gas-troh-in-TES-tin-ul TRAKT) [gastr- stomach, -intestin- intestine, -al relating to, tractus trail]
glucagon
(GLOO-kah-gon) [gluca- sweet (glucose), -agon lead or bring]
greater omentum
(oh-MEN-tum) [omentum fatty covering of intestines] pl., omenta (oh-MEN-tah)
hard palate
(PAL-et)
hepatic lobule
(heh-PAT-ik LOB-yool) [hepa- liver, -ic relating to, lobus- pod, -ule small]
hiatal hernia
(hye-AY-tal HER-nee-ah) [hiat- gap, -al relating to, hernia rupture] pl., herniae or hernias (HER-nee-ee, HER-nee-ahz)
hydrolysis
(hye-DROHL-ih-sis) [hydro- water, -lysis loosening]
ileum
(IL-ee-um) [ileum groin or flank] pl., ilea (IL-ee-ah)
ingestion
(in-JES-chun) [in- within, -gest- carry, -tion process]
insulin
(IN-suh-lin) [insul- island, -in substance]
intestinal juice
(in-TES-tih-nal) [intestin- intestine, -al relating to]
intestinal phase
(in-TES-tih-nal) [intestin- intestine, -al relating to]
intrinsic factor
(in-TRIN-sik FAK-tor) [intr- inside or within, -insic beside]
jejunum
(jeh-JOO-num) [jejunus empty]
lacteal
(LAK-tee-al) [lact- milk, -al relating to]
lecithin
(LES-ih-thin) [lecith- yolk, -in substance]
left lobe
lesser omentum
(oh-MEN-tum) [omentum fatty covering of intestines]
lip
lipase
(LYE-pays) [lip- fat, -ase enzyme]
lower esophageal sphincter (LES)
(eh-SOF-eh-JEE-ul SFINGK-ter) [es- will carry, -phag- food (eat), -al relating to, sphinc- bind tight, -er agent]
mastication
(mas-tih-KAY-shun) [masticat- chew, -tion process]
mechanical digestion
(dye-JES-chun) [digest- break down, -tion process]
mesentery
(MEZ-en-tair-ee) [mes- middle, -enter- intestine]
micelle
(my-SELL) [mic- grain, -elle small]
motility
(moh-TIL-ih-tee) [mot- move, -il- relating to, -ity state]
mucosa
(myoo-KOH-sah) [muc- slime, -osa relating to] pl., mucosae (myoo-KOH-see)
mucus
(MYOO-kus) [mucus slime]
muscularis
(mus-kyoo-LAIR-is) [mus- mouse, -cul- little, -aris relating to] pl., musculares (mus-kyoo-LAIR-eez)
neck
nuclease
(NOO-klee-ayz) [nucle- nut or kernel (nucleic acid), -ase enzyme]
oral rehydration therapy (ORT)
(OR-al ree-hye-DRAY-shun THER-ah-pee) [or- mouth, -alis relating to, re- back again, -hydra- water, -ation process]
pancreatic islet
(pan-kree-AT-ik EYE-let) [pan- all, -creat- flesh, -ic relating to, islet island]
pancreatic juice
(pan-kree-AT-ik) [pan- all, -creat- flesh, -ic relating to]
parietal cell
(pah-RYE-ih-tal sell) [paires- wall, -al relating to, cell storeroom]
parotid
(peh-RAH-tid) [par- beside, -ot- ear, -id relating to]
pepsin
(PEP-sin) [peps- digestion, -in substance]
pepsinogen
(pep-SIN-oh-jen) [peps- digestion, -in- substance, -o- combining form, -gen produce]
peptidase
(PEP-tyd-ayz) [pept- digestion, -ide- chemical, -ase enzyme]
peristalsis
(pair-ih-STAL-sis) [peri- around, -stalsis contraction]
peritoneum
(pair-ih-toh-NEE-um) [peri- around, -tone- stretched, -um thing] pl., peritonea (pair-ih-toh-NEE-ah)
proenzyme
(proh-EN-zyme) [pro- first, -en- in, -zyme ferment]
propulsion
(proh-PUL-shen) [pro- in front, -pul- drive, -sion process]
protease
(PROH-tee-ayz) [prote- protein, -ase enzyme]
pulp cavity
(KAV-ih-tee) [pulp flesh, cav- hollow, -ity condition]
pyloric sphincter
(pye-LOR-ik SFINGK-ter) [pyl- gate, -or- guard, -ic relating to, sphinc- bind tight, -er agent]
pylorus
(pye-LOR-us) [pyl- gate, -orus guard]
rectum
(REK-tum) [rect- straight, -um thing]
retropulsion
(ret-roh-PUL-shen) [retro- backward, -pul- drive, -sion process]
right lobe
root
saliva
(sah-LYE-vah)
secretion
(seh-KREE-shun) [secret- separate, -tion process]
segmentation
(seg-men-TAY-shun) [segment- cut section, -ation process]
serosa
(see-ROH-sah) [ser- watery fluid, -osa relating to] pl., serosae (see-ROH-see)
sigmoid colon
(SIG-moyd KOH-lon) [sigm- sigma (Σor σ) eighteenth letter of Greek alphabet (Roman S), -oid like, colon large intestine]
soft palate
(PAL-et) [palat- palate (roof of mouth)]
sublingual gland
(sub-LING-gwall) [sub- under, -lingua- tongue, -al relating to, gland acorn]
submandibular gland
(sub-man-DIB-yoo-lar) [sub- under, -mandibul- chew (mandible or jawbone), -ar relating to, gland acorn]
submucosa
(sub-myoo-KOH-sah) [sub- under, -muc- slime, -osa relating to] pl., submucosae (sub-myoo-KOH-see)
tongue
transverse colon
(trans-VERS KOH-lon) [trans- across, -vers- turn, colon large intestine]
trypsin
(TRIP-sin) [tryps- pound, -in substance]
upper esophageal sphincter (UES)
(eh-SOF-ah-JEE-ul SFINGK-ter) [es- will carry, -phag- food (eat), -al relating to, sphinc- bind tight, -er agent]
vermiform appendix
(VERM-ih-form ah-PEN-diks) [vermi- worm, -form shape, append- hang upon, -ix thing] pl., appendices (ah-PEN-dih-seez)
villus
(VIL-us) [villus shaggy hair], pl., villi (VIL-eye)
zymogen
IT took only 3 seconds. Sangetha turned away from her sewing basket to pick up her cell phone. When she looked back, she saw a handful of buttons disappear into her 24-month-old daughter's mouth. She quickly dropped down and removed the buttons from her daughter Jinder's mouth—but Sangetha had no way of knowing how many had gone in. Had any already gone down Jinder's throat? She was coughing slightly and drooling. Sangetha immediately took Jinder to the emergency department, thankfully a very short drive away, where they took radiographs of Jinder's neck and chest. On the x-ray film, they found one button in Jinder's lower esophagus.
Before you read this chapter, try to hypothesize what will happen to the button in Jinder's digestive system. Will it pass through the entire digestive system and be safely expelled, or is there another, potentially more serious scenario? What would you do? See if you can answer the questions concerning Jinder's story at the end of this chapter.
Remember Jinder's button swallowing from the Introductory Story? See if you can answer the following questions about her now that you have read this chapter.
1. With what layer of the esophageal lining would the button be in contact?
a. Serosa
b. Muscularis
c. Submucosa
d. Mucosa
Because the button was rounded, with no sharp points, the decision was made to let it “pass” through Jinder's system naturally.
2.As the button makes its way through Jinder's alimentary tract, it will go through which sequence of sphincters and valves?
a.Lower esophageal, pyloric, ileocecal, anal
b.Anal, pyloric, lower esophageal, ileocecal
c.Lower esophageal, ileocecal, pyloric, anal
d.Pyloric, ileocecal, anal, lower esophageal
3.Imagine a camera is attached to the button. As the button travels through Jinder's small intestines, the viewing monitor shows many tiny projections that look like gently moving, fuzzy fingers in the lining of the small intestine. What are these projections?
a.Plicae
b.Rugae
c.Villi
d.Crowns
To solve these questions, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.
This chapter deals with the anatomy and physiology of the digestive system. Working together, the organs of the digestive system modify food in both chemical composition and physical state so that nutrients can be absorbed and used by the body cells. This process is called digestion. The primary purpose of the digestive process is to bring essential nutrients into the internal environment so that they are available to each cell of the body. In this chapter, we will investigate the structures and functions of the digestive system.
ORGANIZATION OF THE DIGESTIVE SYSTEM
Organs of Digestion
The digestive system comprises the digestive tract and other organs that aid digestion. The main organs of the digestive tract (Figure 21-1) form a hollow tubelike system, open at both ends, which passes through the ventral cavities of the body. The digestive tract is often referred to as the alimentary canal or gastrointestinal (GI) tract (though this technically only refers to the stomach and intestines). Ingested food passing through the lumen of the GI tract is actually outside the internal environment of the body. In fact, you can think of the body as a tube within a tube: The outer skin “tube” folds inward through the mouth and again meets the outer skin at the anus, thus making the “inside” of the GI tract continuous with the external environment!
In Box 21-1 we list the main organs of the digestive system as well as the accessory organs located in the main digestive organs or opening into them. Organs such as the larynx, trachea, diaphragm, and spleen are
FIGURE 21-1 Location of the major digestive organs.
also labeled in Figure 21-1, but they are not digestive organs. They are shown to assist your understanding of the orientation of the digestive organs with respect to other important body structures.
Wall of the GI Tract
Layers
The GI tract is fashioned from four layers of tissues: (1) an inner layer of mucosa, (2) a submucous coat of connective tissue that contains the main blood vessels of the tract, (3) a muscular layer, and (4) an outer fibroserous layer (Figure 21-2). Blood vessels and nerves travel through the mesentery to reach the digestive tube throughout most of its length.
Mucosa
The innermost layer of the GI wall—the layer facing the lumen (open space) of the tube—is called the mucosa or mucous layer. Note in Figure 21-2 that the mucosa is made up of three layers—an inner mucous epithelium, a layer of loose connective tissue called the lamina propria, and a thin layer of smooth muscle called the muscularis mucosae.
Submucosa
The submucosa layer of the digestive tube is composed of connective tissue that is thicker than the mucosal layer. It contains numerous small glands, blood vessels, and parasympathetic nerves that form the submucosal plexus.
Muscularis
The muscularis—or muscular layer—is a thick layer of muscle tissue that wraps around the submucosa. This portion of the wall is characterized by an inner layer of circular and an outer layer of longitudinal smooth muscle. Like the submucosa, the muscularis contains nerves organized into a plexus called the myenteric plexus.
Serosa
The serosa—or serous layer—is the outermost layer of the GI wall. The serosa is actually the visceral layer of the peritoneum— the serous membrane that lines the abdominopelvic cavity and covers its organs. The lining attached to and covering the walls of the abdominopelvic cavity is called the parietal layer of the peritoneum. The fold of serous membrane shown in Figure 21-2 that connects the parietal and visceral portions is called a mesentery. Notice that nerves and vessels servicing the wall of the GI tract are distributed within the mesentery.
BOX 21-1 Organs of the Digestive System
Segments of the Gastrointestinal Tract
Mouth
Oropharynx
Esophagus
Stomach
Small intestine
▪Duodenum
▪Jejunum
▪Ileum
Large intestine
▪Cecum
▪Colon
▪Ascending colon
▪Transverse colon
▪Descending colon
▪Sigmoid colon
▪Rectum
Anal canal
Accessory Organs
Salivary glands
▪Parotid
▪Submandibular
▪Sublingual
Tongue
Teeth
Liver
Gallbladder
Pancreas
Vermiform appendix
Modifications of Layers
Although the same four tissue layers form the various organs of the digestive tract, their structures vary in different regions of the tube throughout its length. Variations in the epithelial layer of the mucosa, for example, range from stratified layers of squamous cells that provide protection from abrasion in the upper part of the esophagus to the simple columnar epithelium, designed for absorption and secretion, throughout the remainder of the tract.
1. What is the basic function of the digestive tract?
2. Name the four layers of the digestive tract wall.
FIGURE 21-2 Wall of the GI tract. The wall of the gastrointestinal (GI) tract is made up of four layers, shown here in a generalized diagram of a segment of the GI tract. Notice that the serosa is continuous with a fold of serous membrane called a mesentery. Notice also that digestive glands may empty their products into the lumen of the GI tract by way of ducts.
MOUTH
Structure of the Oral Cavity
The mouth or oral cavity (Figure 21-3) is created by the following structures: (1) the lips, which surround the orifice of the mouth and form the anterior boundary of the oral cavity, (2) the cheeks (side walls), and (3) the hard palate and soft palate (roof). The tongue and its muscle foundation also create part of the oral cavity.
The lips are covered externally by skin and internally by mucous membrane that continues into the oral cavity. Besides keeping food in the mouth while it is being chewed, the lips help sense the temperature and texture of food before it enters the mouth. They also participate in forming many sounds of speech.
The mucous membrane lining of the cheeks forms the lateral boundaries of the oral cavity and is continuous with the lips in front. Our cheeks are also lined by mucous membrane that is reflected onto the gingiva, or gums, and the soft palate. The bulk of the cheeks is formed by the buccinator muscle. This large muscle is sandwiched (along with a considerable amount of adipose tissue) between the outer skin and mucous membrane lining. Numerous small mucus-secreting glands are located between the mucous membrane and the buccinator muscle; their ducts open opposite the last molar teeth.
The hard palate consists of portions of four bones: two maxillae and two palatines (see Chapter 9, Figure 9-5). The soft palate, which forms a partition between the mouth and nasopharynx (see Chapter 20, Figure 20-3), is formed from muscle arranged in the shape of an arch. The opening in the arch leads from the mouth into the oropharynx. Suspended from the midpoint of the posterior border of the arch is a small cone-shaped process, the uvula.
Tongue
The tongue, which forms the base of the oral cavity and whose free end protrudes into it, is a solid mass of skeletal muscle (intrinsic muscles) covered by a mucous membrane. The tongue has a blunt root, a tip, and a central body. The upper, or dorsal, surface of the tongue is normally moist, pink, and covered by rough elevations, called papillae, which possess sensory organs called taste buds (Figure 21-4).
The lingual frenulum is a fold of mucous membrane in the midline of the ventral surface of the tongue that helps anchor the tongue to the floor of the mouth. The floor of the mouth and ventrum (lower surface) of the tongue are richly supplied with blood vessels. In this region, many vessels are superficial and covered only by a very thin layer of mucosa. This explains why some soluble drugs, such as aspirin or nitroglycerin (used during a heart attack), are absorbed into the circulation rapidly if placed under the tongue.
The tongue is also important for moving food material during mastication (chewing) and deglutition (swallowing), as well as for speech—especially during the formation of syllables. These activities are accomplished both by intrinsic muscles making up the tongue as well as by extrinsic muscles that insert into the tongue (but have their origins on the hyoid bone or one of the bones of the skull).
Salivary Glands
The salivary glands are typical of the accessory glands associated with the digestive system. They are located outside the alimentary canal and deliver their exocrine secretions by way of ducts from the glands into the lumen of the tract (Figure 21-5). The mucous and serous cells found in the
FIGURE 21-3 The oral cavity.
FIGURE 21-4 Dorsal surface of tongue. Sketch showing the three divisions of the tongue.
FIGURE 21-5 Salivary glands. Location of the salivary glands.
glands together secrete a mixture of fluids that are then modified by the duct cells on their way out of the salivary gland.
Three pairs of major salivary glands (see Figure 21-5)—the parotid, submandibular, and sublingual glands—secrete a major portion (about 1 liter) of the saliva produced each day.
The parotids are the largest of the paired salivary glands. Shaped generally like pyramids, they are located between the skin and underlying masseter muscle in front of and below the external ear. The parotids produce a watery, or serous, type of saliva containing enzymes but not mucus. The parotid ducts penetrate the buccinator muscle on each side and open into the mouth opposite the upper second molars.
Submandibular glands contain both enzyme and mucus-producing elements. These glands are located just below the mandibular angle. The submandibular gland is irregular in form and about the size of a walnut. The ducts of the submandibular glands open into the mouth on either side of the lingual frenulum.
Sublingual glands are the smallest of the main salivary glands. They lie in front of the submandibular glands, under the mucous membrane covering the floor of the mouth. Each sublingual gland is drained by 8 to 20 ducts that open into the floor of the mouth. Unlike the other salivary glands, the sublingual glands produce only a mucous type of saliva.
Teeth
The teeth are designed to cut, tear, and grind ingested food so that it can be mixed with saliva before it is swallowed. During the process of mastication, food is ground into small bits, which increases the surface area that can be acted on by the digestive enzymes.
Typical Tooth
A typical tooth (Figure 21-6) consists of three main parts: crown, neck, and root. The crown is the exposed portion of a tooth. It is covered by enamel—the hardest and chemically most stable tissue in the body. Enamel consists of approximately 97% calcified (inorganic) material and only 3% organic material and water. Its color varies from light yellow to grayish white. The thickness of enamel is typically greatest on the cusp and least at its border near the gum. The neck of a tooth is the section that connects the crown with the root below it.
The root of a tooth fits into the socket of the alveolar process of either the upper or lower jaw. The root of a tooth may be a single peglike structure or may comprise two or three separate conical projections. The root is not rigidly anchored to the alveolar process but is suspended in the socket by the fibrous periodontal membrane.
In addition to enamel, the outer shell of each tooth is composed of two additional dental tissues—dentin and cementum (see Figure 21-6). Dentin makes up the greatest proportion of the tooth shell. It is covered by enamel in the crown and by cementum in the root area. The dentin contains a pulp cavity that consists of connective tissue, blood and lymphatic vessels, as well as sensory nerves.
FIGURE 21-6 Typical tooth. A molar tooth sectioned to show its bony socket and details of its three main parts: crown, neck, and root. Enamel (over the crown) and cementum (over the neck and root) surround the dentin layer. The pulp contains nerves and blood vessels.
FIGURE 21-7 Dentition. In the deciduous set, where letters are used in place of numbers, there are no premolars and only two pairs of molars in each jaw. Generally, the lower teeth erupt before the corresponding upper teeth. The photo inset is a “Panorex” dental x ray. It displays the full dentition in a single “flattened-out” image. Arrows show the third molars or “wisdom teeth” that have not yet erupted.
Types of Teeth
Dentition refers to the type, number, and arrangement of teeth in the jaws. Human dentition is made up of two growth stages: 20 deciduous teeth (baby teeth) that appear early in life, replaced later by 32 permanent teeth (Figure 21-7). The first deciduous tooth usually erupts at about 6 months of age. The remainder follow at the rate of one or more a month until all 20 have appeared. There is, however, great individual variation in the age at which teeth erupt. Deciduous teeth are generally shed between the ages of 6 and 13 years. The third molars (“wisdom teeth”) are the last to appear, and usually erupt sometime after 17 years of age.
3. Describe the boundaries of the oral cavity.
4. Where are taste buds located?
5. What are the names of the three types of salivary glands?
6. How do the names of the salivary glands describe their locations?
7. What are the three main parts of a typical tooth?
PHARYNX
The act of swallowing moves a rounded mass of food, called a bolus, from the mouth to the stomach. As the food bolus passes from the mouth, it enters the oropharynx (the second division of the pharynx) and then passes down the digestive tube proper—the portion of the digestive tract that serves only the digestive system. The anatomy of the pharynx is discussed in more detail on page 458, Chapter 20.
ESOPHAGUS
The esophagus—a collapsible, muscular, mucosa-lined tube about 25 cm (10 inches) long—extends from the pharynx to the stomach and pierces the diaphragm in its descent from the thoracic cavity to the abdominal cavity (Figure 21-8, A). It lies posterior to the trachea and heart and serves as a muscular passageway for food, pushing the food toward the stomach.
The esophagus is the first segment of the digestive tube proper, and the four layers that form the wall of the GI tract tube can be identified there (Figure 21-8, B). The esophagus is normally flattened, and thus the lumen is practically nonexistent in the resting state. The inner circular and outer longitudinal layers of the muscular layer are striated (voluntary) in the upper third, mixed (striated and smooth) in the middle third, and smooth (involuntary) in the lower third of the tube. You have control over swallowing in the upper third of your esophagus, but once the bolus passes the middle third of your esophagus, it cannot be voluntarily regurgitated.
Each end of the esophagus is encircled by muscular sphincters that act as valves to regulate passage of material. The upper esophageal sphincter (UES) in the cervical part of the esophagus helps prevent air from entering the esophagus during respiration. Relaxation of the UES is what permits belching (or burping), which is the sudden escape of air trapped in the stomach and esophagus.
The lower esophageal sphincter (LES) is also called the cardiac sphincter. The esophageal hiatus is an opening in the diaphragm located near the junction between the terminal portion of the esophagus and the stomach. The esophageal hiatus may become enlarged to the point where part of the stomach pushes upward through the diaphragm and into the chest. This condition is called a hiatal hernia (Box 21-2).
FIGURE 21-8 Esophagus. A, Diagram showing the major features of the esophagus. B, Microscopic appearance of the esophageal wall. The four layers of the gastrointestinal (GI) wall are easily identified.
BOX 21-2 Diagnostic Study
Upper Gastrointestinal X-Ray Study
An upper GI (UGI) study consists of a series of radiographs of the lower part of the esophagus, stomach, and duodenum. Barium sulfate is usually used as the contrast medium. The test is used to detect ulcerations, tumors, inflammation, or anatomical malpositions such as hiatal hernia (portion of stomach pushed through the hiatus [opening] of the diaphragm). Obstruction of the upper GI tract is also easily detected.
In this test the patient is asked to drink a flavored liquid containing barium sulfate. As the contrast medium travels through the system, the lower portion of the esophagus, gastric wall, pyloric channel, and duodenum are each evaluated for defects. Benign peptic ulcer is a common pathological condition affecting these areas. Tumors, cysts, or enlarged organs near the stomach can also be identified by an anatomical distortion of the outline of the upper GI tract. Can you identify the outline of the stomach in the figure?
X-ray film of the lower part of the esophagus, stomach, and duodenum.
STOMACH
Size and Position of the Stomach
Just below the diaphragm, the digestive tube dilates into an elongated pouchlike structure, the stomach (Figure 21-9). The size of the stomach varies according to several factors, notably the amount of distention. For some time after a meal, the stomach is enlarged because of distention of its walls, but as food passes out of the stomach, the walls partially collapse, leaving the organ about the size of a large sausage. In adults, the stomach usually holds a volume of up to 1 to 1.5 liters.
Divisions of the Stomach
The fundus, body, and pylorus are the major divisions of the stomach. The fundus is the enlarged portion to the left and above the opening of the esophagus into the stomach. The body is the central part of the stomach, and the pylorus is its lower portion (see Figure 21-9). The small collar or margin of the stomach at its junction with the esophagus is often called the cardia or cardiac part.
FIGURE 21-9 Stomach. A, A portion of the anterior wall has been cut away to reveal the muscle layers of the stomach wall. Notice that the mucosa lining the stomach forms folds called rugae. B, Divisions of the stomach.
Sphincter Muscles
Sphincter muscles regulate passage of material at both stomach openings. Sphincter muscles consist of circular fibers arranged to form an opening in the center of them when they are relaxed and no opening when they are fully contracted. The lower esophageal sphincter (LES), or cardiac sphincter, controls the opening of the esophagus into the stomach, and the pyloric sphincter controls the opening from the pyloric portion of the stomach into the first part of the small intestine (duodenum).
Stomach Wall
Each of the four layers of the stomach wall suits the function of this organ, as shown for you in Figures 21-9 and 21-10. Of particular interest are the modifications to the stomach mucosa and muscularis, both of which are briefly described below.
Gastric Mucosa
The epithelial lining of the stomach consists largely of folds, called rugae. Within these folds are many marked depressions called gastric pits. Numerous coiled tubular-type glands, or gastric glands, are found below the level of the pits,
FIGURE 21-10 Gastric pits and gastric glands. Gastric pits are depressions in the epithelial lining of the stomach. At the bottom of each pit is one or more tubular gastric glands. Chief cells produce the enzymes of gastric juice, and parietal cells produce stomach acid. Endocrine cells secrete the appetite-boosting hormone ghrelin.
particularly in the fundus and body of the stomach. Figure 21-10 illustrates the anatomical relationship of the gastric pits and gastric glands. The glands secrete most of the gastric juice, a mucous fluid containing digestive enzymes and hydrochloric acid (HCl).
In addition to the mucus-producing cells that cover the entire surface of the stomach and line the pits, the gastric glands contain three major secretory cells—chief cells, parietal cells, and endocrine cells (see Figure 21-10). Chief cells (zymogenic cells) secrete the enzymes of gastric juice. Parietal cells secrete hydrochloric acid and are also thought to produce the important substance known as intrinsic factor. Intrinsic factor binds to vitamin B12 molecules to protect them from digestive juices until they reach the small intestine—and then facilitates the absorption of B12. Endocrine cells secrete ghrelin (GHRL)—a hormone that stimulates the hypothalamus to secrete growth hormone and increase appetite—and gastrin, which influences digestive functions.
Gastric Muscle
The thick layer of muscle in the stomach wall—the muscularis—is made of three distinct sublayers of smooth muscle tissue. As Figure 21-9, A illustrates, there is the usual layer of longitudinal muscles and circular muscles, as well as an underlying oblique layer. The crisscrossing pattern of smooth muscle fibers formed by this arrangement gives the stomach wall the ability to contract strongly at many angles—thus making mixing very efficient.
Function of the Stomach
The stomach performs the following functions:
▪Serves as a food reservoir until the food can be partially digested and moved farther along the gastrointestinal tract.
▪Secretes gastric juice, which aids in the digestion of food.
▪Churns food, breaking it into small particles and mixing them well with the gastric juice.
▪Secretes intrinsic factor, important for vitamin B12 absorption.
▪Performs a limited amount of absorption.
▪Produces the hormones gastrin, which helps regulate digestive functions, and ghrelin (GHRL), which increases appetite.
▪Helps protect the body by destroying pathogenic bacteria swallowed with food or with mucus from the respiratory tract.
8. Describe the passage of the esophagus from the oral cavity to the stomach.
9. What are the three main divisions of the stomach?
10. What is the function of gastric pits?
SMALL INTESTINE
Size and Position of the Small Intestine
The small intestine is a tube measuring about 2.5 cm (1 inch) in diameter and 6 meters (20 feet) in length. Its coiled loops fill most of the abdominal cavity (see Figure 21-1). It is divided into the duodenum, the jejunum, and the ileum. The duodenum is the uppermost division and the part to which the pyloric end of the stomach attaches. It is about 25 cm (10 inches) long and is shaped roughly like the letter C. The duodenum becomes the jejunum at the point where the tube turns abruptly forward and downward. The jejunal portion continues for approximately the next 2.5 meters (8 feet), at the end of which it becomes the ileum, but without any clear line of demarcation between the two divisions. The ileum is about 3.5 meters (12 feet) long.
Wall of the Small Intestine
Notice in Figure 21-11 that the intestinal lining has circular plicae (folds) that have many tiny projections called villi. Millions of these projections, each about 1 mm in height, give the intestinal mucosa a velvety appearance. Each villus contains an arteriole, venule, and lymph vessel (lacteal).
FIGURE 21-11 Wall of the small intestine. Note folds of mucosa are covered with villi and each villus is covered with epithelium, which increases the surface area for absorption of food.
Mucus-secreting goblet cells are found in large numbers on villi. Endocrine cells that produce intestinal hormones are also found in the villi. The presence of villi and microvilli greatly increases the surface area of the small intestine hundreds of times. The increase in surface area allows this organ to efficiently digest and absorb most of the nutrients we ingest.
11. What are the three main divisions of the small intestine?
12. What are intestinal villi? What is their function?
LARGE INTESTINE
Size of the Large Intestine
The lower part of the alimentary canal bears the name large intestine because its diameter is noticeably larger than that of the small intestine. Its length, however, is much less, being about 1.5 to 1.8 meters (5 or 6 feet). Its average diameter is approximately 6 cm (2.3 inches), but the diameter decreases toward the lower end of the tube. Box 21-3 describes a common test used for imaging the large intestine.
Divisions of the Large Intestine
The large intestine is divided into the cecum, colon, and rectum (Figure 21-12). The first 5 to 8 cm (2 or 3 inches) of the large intestine is the cecum. It is a blind pouch located in the lower right quadrant of the abdomen (see Figure 21-1).
The colon is divided into the following portions: ascending, transverse, descending, and sigmoid (see Figure 21-12).
▪The ascending colon lies in a vertical position, on the right side of the abdomen, and extends up to the lower border of the liver. The ileum joins the large intestine at the junction of the cecum and ascending colon (see Figure 21-12). The ileocecal valve permits material to pass from the ileum into the large intestine, but not usually in the reverse direction.
▪The transverse colon passes horizontally across the abdomen, below the liver, stomach, and spleen. Note that this part of the colon is above the small intestine (see Figure 21-1). The transverse colon extends from the hepatic flexure (near the liver) to the splenic flexure (near the spleen). At these two points the colon bends on itself to form roughly 90-degree angles.
FIGURE 21-12Divisions of the large intestine. Illustration showing divisions of the large intestine and adjacent vascular structures.
BOX 21-3Diagnostic Study
Barium Enema Study
The barium enema (BE) study, or lower GI series, consists of a series of x-ray films of the colon that are used to detect and locate polyps, tumors, and diverticula (abnormal “pouches” in the lining of the intestine). Abnormalities in organ position can also be detected.
The test begins with an enema of approximately 500 to 1,500 ml of fluid containing barium sulfate. The patient is placed in various positions, and the progress of the barium's flow through the intestine is monitored on a fluoroscope. Small polyps and early changes in ulcerative colitis are more easily detected with an air-contrast barium enema study. In this study, after the bowel is outlined with a thin coat of barium, air is added to enhance the contrast and outline of small lesions. After the x-ray films are taken, the patient is allowed to expel the barium.
The figure shows a colorized radiograph of a barium enema produced by a special technique that produces a clear image of the large intestine and its position relative to the skeleton. Can you identify the divisions of the large intestine in the figure?
Colorized radiograph of a barium enema.
▪The descending colon lies in the vertical position, on the left side of the abdomen, and extends from a point below the stomach and spleen to the level of the iliac crest.
▪The sigmoid colon is the portion of the large intestine that courses downward below the iliac crest. It is called sigmoid (meaning “S-shaped”) because it forms an S-shaped curve.
The last 17 to 20 cm (7 or 8 inches) of the intestinal tube is called the rectum (Figure 21-13). The terminal inch of the rectum is called the anal canal. The opening of the canal to the exterior is guarded by two sphincter muscles—an internal one of smooth muscle and an external one of striated muscle. The opening itself is called the anus. The anus is directed slightly posteriorly and is therefore at almost a right angle to the rectum (Figure 21-14).
FIGURE 21-13 The rectum and anus. Lumen of GI tract.
Wall of the Large Intestine
One of the most notable modifications of the GI wall in the large intestine is the presence of intestinal mucous glands. These glands produce the lubricating mucus that coats the feces as they are formed (see Figure 21-2). Although cells lining the large intestine have microvilli, these cells do not form villi like those that appear in the lining of the small intestine.
Another notable feature of the wall of the colon is the uneven distribution of fibers in the muscle layer. The longitudinal muscles are grouped into tapelike strips called taeniae coli, and the circular muscles are grouped into rings that produce pouchlike haustra between them (see Figure 21-12). In the rectum, rings of circular muscle form the rectal valves seen in Figure 21-13.
VERMIFORM APPENDIX
The vermiform appendix (from vermis “worm,” forma “shape”) is a wormlike tubular organ. It averages 8 to 10 cm (3 to 4 inches) in length and is most often found just behind the cecum or over the pelvic rim. The lumen of the appendix communicates with the cecum 3 cm (about 1 inch) below the ileocecal valve, thus making it an accessory organ of the digestive system (see Figure 21-12). Its functions are not fully understood, but recent research suggests that the appendix serves as a sort of “breeding ground” for some of the nonpathogenic intestinal bacteria thought to aid in the digestion or absorption of nutrients.
FIGURE 21-14 Peritoneum. Sagittal view of the abdomen showing the peritoneum and its reflections. Intraperitoneal spaces are shown in yellow and extraperitoneal spaces in green. The portion of the extraperitoneal space along the posterior wall of the abdomen is often called the retroperitoneal space.
PERITONEUM
The peritoneum is a large, continuous sheet of serous membrane that covers most of the organs just described and holds them loosely in place. It lines the walls of the entire abdominal cavity (parietal layer) and also forms the serous outer coat of the organs (visceral layer), as you can see in Figure 21-14. All the space outside the parietal peritoneum is called extraperitoneal space.
In several places the peritoneum forms reflections, or extensions, that bind the abdominal organs together (see Figure 21-14). The mesentery is a fan-shaped projection of the parietal peritoneum that extends from the lumbar region of the posterior abdominal wall. The mesentery allows free movement of each coil of the intestine and helps prevent strangulation of the long tube.
The greater omentum is a continuation of the serosa of the greater curvature of the stomach and the first part of the duodenum to the transverse colon. It is a fold of two layers of peritoneal serous tissue that largely fuse together by adulthood. This ventral fold of the greater omentum hangs down as an “apron” in front of the abdominal organs (see Figure 21-14) and can become filled with fatty deposits. In cases of localized abdominal inflammation such as appendicitis, the greater omentum envelops the inflamed area, walling it off from the rest of the abdomen. The lesser omentum attaches from the liver to the lesser curvature of the stomach and the first part of the duodenum. Take a few moments to examine the relationships of peritoneal extensions in Figure 21-14.
13. What are the four main divisions of the colon?
14. What are haustra? How do they differ from taeniae coli?
15. Where is the vermiform appendix located?
16. What are some of the functions of the greater omentum?
LIVER
Structure of the Liver
The liver is the largest gland in the body. It weighs about 1.5 kg (3 to 4 pounds), lies immediately under the diaphragm, and occupies most of the right hypochondrium and part of the epigastrium (see Figure 21-1).
The liver consists of two lobes separated by the falciform ligament (Figure 21-15). The left lobe forms about one sixth of
FIGURE 21-15 Gross structure of the liver. Diagrams of a normal liver. A, Anterior view. B, Inferior view.
the liver, and the right lobe makes up the remainder. The right lobe has three parts: the right lobe proper, the caudate lobe, and the quadrate lobe. Each lobe is divided into numerous lobules by small blood vessels and by fibrous strands that form a supporting framework called the perivascular fibrous capsule.
The hepatic lobules (Figure 21-16), the anatomical units of the liver, are tiny hexagonal or pentagonal cylinders about 2 mm high and 1 mm in diameter. Blood vessels—including portal veins carrying blood from the GI tract—arranged around a lobule feed blood into sinusoids that extend through the liver tissue, which processes the blood. The blood then drains into a central vein, which in turn drains into hepatic veins. Meanwhile, bile secreted by liver tissue collects in tiny bile ducts that surround each lobule.
Bile Ducts
The small bile ducts within the liver join to form two larger ducts that emerge from the undersurface of the organ as the right and left hepatic ducts. These ducts immediately join to form one common hepatic duct. The common hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct (Figure 21-17), which opens into the duodenum in a small raised area called the major duodenal papilla. This papilla is located 7 to 10 cm (2 to 4 inches) below the pyloric opening from the stomach.
Function of the Liver
The liver is one of the most vital organs of the body. Here, in brief, are its main functions:
▪Liver cells detoxify various substances.
▪Liver cells carry on numerous important steps in the metabolism of all three kinds of foods—proteins, fats, and carbohydrates.
▪Liver cells store several substances—iron, for example, and vitamins A, B12, and D.
▪The liver produces important plasma proteins and serves as a site of hematopoiesis (blood cell production) during fetal development.
▪Liver cells secrete about a pint of bile a day.
Bile Secretion by the Liver
The main components of bile are bile salts, bile pigments, and cholesterol. Bile salts (formed in the liver from cholesterol) are an essential part of bile. They aid in the absorption of fats and then are themselves absorbed in the ileum. Eighty percent of bile salts are recycled in the liver to again become part of “new” bile. Bile also serves as a pathway for elimination of bile pigments, which are breakdown products of erythrocytes.
FIGURE 21-16 Microscopic structure of the liver. A, This diagram shows the location of liver lobules relative to the overall circulatory scheme of the liver. B, Enlarged views of several lobules show how blood from the hepatic portal veins and hepatic arteries flows through sinusoids toward a central vein in each lobule (black arrows).
FIGURE 21-17 Ducts that carry bile from the liver and gallbladder. Obstruction of either the common hepatic or the common bile duct by a stone or spasm prevents bile from being ejected into the duodenum.
GALLBLADDER
The gallbladder is a pear-shaped sac 7 to 10 cm (3 to 4 inches) long and 3 cm broad at its widest point (see Figure 21-17). It can hold 30 to 50 ml of bile. The gallbladder lies on the undersurface of the liver and is attached there by areolar connective tissue.
The gallbladder stores bile that enters it by way of the hepatic and cystic ducts. During this time, the gallbladder concentrates bile 5-fold to 10-fold. Later, when digestion occurs in the stomach and intestines, the gallbladder contracts and ejects the concentrated bile into the duodenum.
PANCREAS
Structure of the Pancreas
The pancreas is a grayish pink–colored gland about 12 to 15 cm (6 to 9 inches) long, weighing about 60 g. It resembles a fish with its head and neck in the C-shaped curve of the duodenum, its body extending horizontally behind the stomach, and its tail touching the spleen (see Figure 21-17).
The pancreas is composed of both exocrine and endocrine glandular tissue (Figure 15-18, p. 342). The exocrine tissues form acinar units that release their collective secretions into microscopic ducts that unite to form larger ducts. Eventually these ducts join the main pancreatic duct, which extends throughout the length of the gland. The pancreatic duct empties into the duodenum at the same point as the common bile duct.
Embedded between the exocrine units of the pancreas, like little islands, lie clusters of endocrine cells called pancreatic islets. Although there are about a million islets, they constitute only about 2% of the total mass of the pancreas. Several kinds of cells make up the islets. They are secreting cells, but their secretion passes into blood capillaries rather than into ducts. Thus the pancreas is a dual gland—an exocrine, or duct, gland because of the acinar units and an endocrine, or ductless, gland because of the pancreatic islets.
Function of the Pancreas
The pancreas has a number of important functions that affect how nutrients are processed by the body.
▪The acinar units of the pancreas secrete the digestive enzymes found in pancreatic juice. Hence the pancreas plays an important part in digestion.
▪Beta cells of the pancreatic islets secrete insulin, a hormone that exerts a major control over carbohydrate and fat metabolism.
▪Alpha cells of the pancreatic islets secrete glucagon, a hormone that also helps control carbohydrate and fat metabolism.
The endocrine functions of the pancreas are also discussed for you in detail in Chapter 15, page 342.
17. Where is the liver located? How many lobes does it have?
18. Name three of the many functions of the liver.
19. Trace the route of bile from the gallbladder to the duodenum.
20. What is the function of the acinar units of the pancreas?
OVERVIEW OF DIGESTIVE FUNCTION
The digestive system uses various mechanisms to make nutrients available to each cell of our bodies (Table 21-1). Complex foods must first be taken in—a process called ingestion and then broken down into simpler nutrients in the process of digestion. To physically break large chunks of food into smaller bits and to move it along the tract, movement (or motility) of the gastrointestinal (GI) wall is required. Chemical digestion—that is, breakdown of large molecules into small molecules—requires secretion of digestive enzymes into the lumen of the GI tract. After being digested, nutrients are ready for the process of absorption, or movement through the GI mucosa into the internal environment. The material that is not absorbed must then be excreted to make room for more material—a process known as elimination. Of course, all these activities must be coordinated, which we have already learned is the process of regulation. Some of the major digestive processes are summarized in Table 21-1 and Figure 21-18.
TABLE 21-1 Primary Mechanisms of the Digestive System
MECHANISM
DESCRIPTION
Ingestion
Process of taking food into the mouth, starting it on its journey through the digestive tract
Digestion
A group of processes that break complex nutrients into simpler ones, thus facilitating their absorption; mechanical digestion physically breaks large chunks into small bits; chemical digestion breaks molecules apart
Motility
Movement bythe muscular components of the digestive tube, including processes of mechanical digestion; examples include peristalsis and segmentation
Secretion
Release of digestive juices (containing enzymes, acids, bases, mucus, bile, or other products that facilitate digestion); some digestive organs also secrete endocrine hormones that regulate digestion or metabolism of nutrients
Absorption
Movement of digested nutrients through the gastrointestinal (GI) mucosa and into the internal environment
Elimination
Excretion of the residues of the digestive process (feces) from the rectum, through the anus; defecation
Regulation
Coordination of digestive activity (motility, secretion, etc.)
DIGESTION
After food is ingested (taken into the mouth), the process of digestion begins immediately. Digestion is the overall name for all the processes that mechanically and chemically break complex foods into simpler nutrients that can be easily absorbed. Let's begin our discussion with an overview of mechanical digestion and then move on to a discussion of chemical digestion.
Mechanical Digestion
Mechanical digestion consists of all movement (motility) of the digestive tract, including mastication, deglutition, peristalsis, and segmentation. These mechanical actions are described below.
Mastication
Mechanical digestion begins in the mouth when the particle size of ingested food material is reduced by chewing movements, or mastication. The tongue, cheeks, and lips play an important role in keeping food material between the cutting or grinding surfaces of the teeth when a person is biting off or chewing food. In addition to reducing particle size, chewing movements serve to mix food with saliva in preparation for swallowing.
Deglutition
The process of swallowing, or deglutition, involves three main steps, or stages, that may be divided into the formation
FIGURE 21-18 Overview of digestive functions. Several important digestive functions are summarized in these diagrams. Note that the digestive tract is an extension of the external environment—extending like a tunnel through the body.
and then movement of a food bolus from the mouth to the stomach (Figure 21-19):
1.Oral stage (mouth to oropharynx)
2.Pharyngeal stage (oropharynx to esophagus)
3.Esophageal stage (esophagus to stomach)
First, the oral stage, which is voluntary and under control of the cerebral cortex, involves the formation of a food bolus that is to be swallowed. During the oral stage, the bolus
FIGURE 21-19 Deglutition. A, Oral stage. During this stage of deglutition (swallowing), a bolus of food is voluntarily formed on the tongue and pushed against the palate and then into the oropharynx. Notice that the soft palate acts as a valve that prevents food from entering the nasopharynx. B, Pharyngeal stage. After the bolus has entered the oropharynx, involuntary reflexes push the bolus down toward the esophagus. Notice that upward movement of the larynx and downward movement of the bolus close the epiglottis and thus prevent food from entering the lower respiratory tract. C, Esophageal stage. Involuntary reflexes of skeletal (striated) and smooth muscle in the wall of the esophagus move the bolus through the esophagus toward the stomach.
is pressed against the palate by the tongue and then moved back into the oropharynx. The involuntary pharyngeal and esophageal stages that follow both consist of movement of food from the pharynx into the esophagus and, finally, into the stomach.
To propel food from the pharynx into the esophagus, three openings must be blocked: mouth, nasopharynx, and larynx. Continued elevation of the tongue seals off the mouth. The soft palate, including the uvula, is elevated and tensed, causing the nasopharynx to be closed off. Food is prevented from entering the larynx by muscle action that causes the epiglottis to block this opening. As a result, the bolus slips over the back of the epiglottis to enter the laryngopharynx. Contractions of the pharynx and esophagus compress the bolus into and through the esophageal tube.
Peristalsis and Segmentation
After food enters the lower portion of the esophagus, smooth muscle tissue in the wall of the GI tract is responsible for its movement forward. The motility produced by smooth muscle is of two main types: peristalsis and segmentation.
Peristalsis is often described as a wavelike ripple of the muscle layer of a hollow organ. The diagram in Figure 21-20 shows step by step how peristalsis occurs. A bolus stretches the GI wall, triggering a reflex contraction of circular muscle that pushes the bolus forward. This, in turn, triggers a reflex contraction in that location, pushing the bolus even farther. This process continues as long as the stretch reflex is activated by the presence of food.
Segmentation is most easily described as mixing movements. Segmentation occurs when digestive reflexes cause a forward-and-backward movement within a single region, or
FIGURE 21-20 Peristalsis. Peristalsis is a progressive type of movement in which material is propelled from point to point along the gastrointestinal (GI) tract. A, A ring of contraction occurs where the GI wall is stretched, and the bolus is pushed forward. B, The moving bolus triggers a ring of contraction in the next region that pushes the bolus even farther along. C, The ring of contraction moves like a wave along the GI tract to push the bolus forward.
FIGURE 21-21 Segmentation. Segmentation is a back-and-forth action that breaks apart chunks of food and mixes in digestive juices. A, Ringlike regions of contraction occur at intervals along the gastrointestinal (GI) tract. B, Previously contracted regions relax and adjacent regions now contract, effectively “chopping” the contents of each segment into smaller chunks. C, The location of the contracted regions continues to alternate back and forth, chopping and mixing the contents of the GI lumen.
segment, of the GI tract (Figure 21-21). These mixing movements help mechanically break down food particles, mix food and digestive juices thoroughly, and bring digested food in contact with intestinal mucosa to facilitate absorption.
Peristalsis and segmentation can occur in an alternating sequence. When this happens, food is churned and mixed as it slowly progresses along the GI tract.
Regulation of Motility
Gastric Motility
The process of emptying the stomach takes about 2 to 6 hours after a meal, depending on the amount and content of the meal. Food is churned with gastric juices in the stomach to form a thick, milky material known as chyme, which is ejected about every 20 seconds into the duodenum. As you can see in Figure 21-22, chyme is continually being pushed toward the pyloric sphincter by waves of peristaltic contractions—a process called propulsion. Because the pyloric sphincter remains closed most of the time, however, the chyme is forced to move backward—a process called retropulsion. Thus, because the chyme is temporarily “trapped,” peristalsis creates a sort of “back-and-forth” movement that helps mix the chyme and gastric juice. Eventually, the contraction force of the pyloric sphincter decreases, allowing a little of the chyme to pass through to the duodenum.
Intestinal Motility
Intestinal motility includes both peristaltic contractions and segmentation. Segmentation in the duodenum and upper jejunum mixes the incoming chyme with digestive juices from the pancreas, liver, and intestinal mucosa. This mixing
FIGURE 21-22 Gastric motility. Mixing actions in the stomach include both propulsion (forward movement) and retropulsion (backward movement). As peristaltic contractions become stronger, some of the liquid chyme squirts past the pyloric sphincter (which has decreased its muscle tone) and into the duodenum. The stomach continues to mix the chyme as it is gradually released into the duodenum.
action also allows the products of digestion to contact the intestinal mucosa, where they can be absorbed into the internal environment. Peristalsis continues as the chyme nears the end of the jejunum—moving the food through the rest of the small intestine and into the large intestine. After leaving the stomach, chyme normally takes about 5 hours to pass all the way through the small intestine.
A list of definitions of the different processes involved in mechanical digestion, along with the organs that accomplish them, is presented in Table 21-2.
21. What is meant by the term motility?
22. Is deglutition a voluntary or involuntary process? Explain.
23. What is the purpose of peristalsis and segmentation?
Chemical Digestion
Chemical digestion entails all the changes in chemical composition that transform foods during their travel through the digestive tract. These changes result largely from the hydrolysis of foods. Hydrolysis is a chemical process in which a compound unites with water and then splits into simpler compounds (see Chapter 2). Numerous enzymes in the various digestive juices catalyze the hydrolysis of foods.
TABLE 21-2 Processes of Mechanical Digestion
ORGAN
MECHANICAL PROCESS
NATURE OF PROCESS
Mouth (teeth and tongue)
Mastication
Chewing movements-reduce size of food particles and mix them with saliva
Deglutition
Swallowing-movement of food from mouth to stomach
Pharynx
Deglutition
Esophagus
Deglutition
Peristalsis
Rippling movements that squeeze food downward in digestive tract; a constricted ring forms first in one section, then the next, etc., causing waves of contraction to spread along entire canal
Stomach
Churning
Forward and backward movement of gastric contents, mixing food with gastric juices to form chyme
Peristalsis
Wave starting in body of stomach about three times per minute and sweeping toward closed pyloric sphincter; at intervals, strong peristaltic waves press chyme past sphincter into duodenum
Small intestine
Segmentation (mixing contractions)
Peristalsis
Forward and backward movement within segment of intestine; serves to mix food and digestive juices thoroughly and to bring all digested food into contact with intestinal mucosa to facilitate absorption; purpose of peristalsis, on the other hand, is to propel intestinal contents along digestive tract
Large intestine
Colon
Segmentation
Peristalsis
Churning movements within haustral sacs
Descending colon
Mass peristalsis
Entire contents moved into sigmoid colon and rectum; occurs three or four times a day, usually after a meal
Rectum
Defecation
Emptying of rectum, so-called bowel movement
Digestive Enzymes
Although our interest until now has primarily concerned intracellular enzymes (see Chapter 2), our current discussion focuses on extracellular digestive enzymes as well. In the following paragraphs we briefly review enzymes in general and outline some characteristics of digestive enzymes in particular.
Recall that enzymes can be defined simply as “organic catalysts”—that is, they are organic compounds (proteins) that accelerate chemical reactions without being “consumed” by the reactions themselves.
As with any type of enzyme, digestive enzymes are specific in their action; that is, they act only on a specific substrate. This is attributed to a “key-in-a-lock” kind of action—the configuration of the enzyme molecule fitting the configuration of some part of the substrate molecule.
Digestive enzymes are continually being destroyed or eliminated from the body and therefore have to be continually synthesized, even though they are not used up in the reactions they catalyze. However, it is important to note that most digestive enzymes are synthesized and secreted as inactive proenzymes. Enzymes that break apart proenzymes and thus convert them to active enzymes are often called kinases. For example, enterokinase is a kinase that changes inactive trypsinogen into active trypsin.
Although we eat six main types of chemical substances (carbohydrates, proteins, fats, vitamins, mineral salts, and water), only the first three have to be chemically digested to be absorbed.
Carbohydrate Digestion
Carbohydrates are compounds composed of carbon, hydrogen and oxygen that ordinarily occur in the ratio 1:2:1 and exist both as monosaccharides and more complex forms. Monosaccharides are called “simple sugars” because they are each a single saccharide group. Simple sugars with 6 carbon atoms, such as glucose (C6H12O6), are called hexoses. Other important hexoses include fructose and galactose. Disaccharides or “double sugars” include sucrose (glucose + fructose) and lactose (glucose + galactose). Polysaccharides, notably starches and glycogen, are long chains or polymers of linked simple sugars.
Polysaccharides are hydrolyzed to disaccharides by enzymes known as amylases, found in saliva and pancreatic juice. The enzymes that catalyze the final steps in carbohydrate digestion are sucrase, lactase, and maltase (Figure 21-23).
FIGURE 21-23 Carbohydrate digestion. Amylase in saliva and pancreatic juice hydrolyzes polysaccharides into disaccharides. Brush border disaccharidases in the lining of the small intestine then promote hydrolysis of the disaccharides into monosaccharides.
These enzymes are located in the cell membrane of epithelial cells covering the villi and, therefore, lining the intestinal lumen. The resulting end products of digestion, mainly glucose, are thus conveniently made at the site of absorption (and are not floating around somewhere in the lumen).
Protein Digestion
Protein compounds have very large molecules made up of folded or twisted chains of amino acids, often hundreds in number. Enzymes called proteases catalyze the hydrolysis of proteins first into a variety of intermediate compounds called proteoses and peptides, which are simply shorter strands of amino acids. Then finally, proteases break these shorter molecules into individual amino acids (Figure 21-24).
The main proteases are pepsin in gastric juice, trypsin and chymotrypsin in pancreatic juice, and peptidases of the intestinal brush border (Box 21-4). Peptidases are also present within each intestinal cell, where they break apart dipeptides and tripeptides absorbed into these cells. Each kind of protease catalyzes the breaking apart of a specific kind of peptide bond. Because different amino acid combinations within a protein or polypeptide can have slightly different kinds of peptide bonds holding them together, a whole arsenal of different proteases is needed for efficient protein digestion.
Fat Digestion
Because fats are insoluble in water, they must be emulsified—that is, dispersed into very small droplets—before they can be digested. Two substances found in bile, lecithin and bile salts, emulsify dietary oils and fats in the lumen of the small intestine.
Lecithin is a phospholipid similar to other phospholipids that make up the bulk of cellular membranes. Lecithin mixes
FIGURE 21-24 Protein digestion. Gastric juice protease (pepsin) and pancreatic juice proteases (trypsin and chymotrypsin) hydrolyze proteins into proteoses and peptides. Protein digestion is then completed by pancreatic proteases, which hydrolyze proteoses into amino acids, and by intestinal peptidases, which hydrolyze peptides into amino acids.
with lipids and water, forming tiny spheres called micelles. The tiny micelles permit fat components of foods to be soluble in water. Bile salts, which are derived from the lipid cholesterol, emulsify fats by forming micelles in the same manner.
The mechanical process of emulsification facilitates chemical digestion of fats by breaking large fat drops into small droplets. This process provides a greater contact area between fat molecules and pancreatic lipases, the main fat-digesting enzymes (Figure 21-25). The final products of fat digestion are fatty acids and glycerol (see Chapter 2).
You can find a summary of chemical digestion in Table 21-3.
Residues of Digestion
Certain components of food cannot be digested and are eliminated from the intestines in the feces. Among these residues of digestion are cellulose (a carbohydrate, also known as “dietary fiber”) and undigested connective tissue from meat (mostly collagen). These substances remain undigested because humans lack the enzymes required to hydrolyze them. Residues of digestion also include undigested fats. Some fat molecules also have minerals such as calcium and
FIGURE 21-25 Fat digestion. Hydrolysis by the enzyme lipase is facilitated by prior emulsion of fats by bile (lecithin and bile salts). Though not pictured, colipase is needed to anchor lipase molecules to the inner face of each micelle.
magnesium, which render the fats indigestible. In addition to these wastes, feces consist of bacteria, pigments, water, and mucus.
TABLE 21-3 Chemical Digestion
DIGESTIVE JUICES AND ENZYMES
SUBSTANCE DIGESTED (OR HYDROLYZED)
RESULTING PRODUCT*
Saliva
Amylase
Starch (polysaccharide)
Maltose (a double sugar, or disaccharide)
Gastric Juice
Protease (pepsin) plus hydrochloric add
Proteins
Partially digested proteins
Pancreatic Juice
Proteases (e.g., trypsin)†
Proteins (intact or partially digested)
Peptides and amino acids
Lipases
Fats emulsified by bile
Fatty acids, monoglycerides, and glycerol
Amylase
Starch
Maltose
Nucleases
Nucleic adds (DNA, RNA)
Nucleotides
Intestinal Enzymes ‡
Peptidases
Peptides
Amino adds
Sucrase
Sucrose (cane sugar)
Glucose and fructose§ (simple sugars, or monosaccharides)
Lactase
Lactose (milk sugar)
Glucose and galactose (simple sugars)
Maltase
Maltose (malt sugar)
Glucose
Nucleotidases and phosphatases
Nucleotides
Nucleosides
* Substances in boldface type are end products of digestion (that is, completely digested nutrients ready for absorption).
† Secreted in inactive form (trypsinogen); activated by enterokinase, an enzyme in the intestinal brush border.
‡ Brush border enzymes.
§ Glucose Is also called dextrose; fructose is also called levulose.
BOX 21-4 The Brush Border
The term brush border refers to the microvilli on the epithelial mucous cells that line the small intestine, visible in the figure. These microvilli are on the apical surfaces of the epithelial cells—the surfaces that face the interior of the intestinal lumen. Because, when viewed under high magnification, the microvilli look like the bristles of a brush, the surface of the intestinal mucosa was nicknamed the “brush border.”
The brush border represents the boundary between the external environment (the lumen of the alimentary canal) and the internal environment of the body. It is across this border that molecules must pass if they are going to be absorbed into the body.
The brush border possesses such an incredibly large surface area because the microvilli increase the apical surface area. There are usually 2,000 to 3,000 microvilli on each cell! Of course, the presence of intestinal villi, circular folds (plicae circulares), and numerous loops also adds to the intestinal surface area (see Figure 21-11).
The large surface area of the brush border provides sites for digestive enzymes on the plasma membranes of intestinal cells—the brush border enzymes. The efficiency of the last stages of digestion is thus enhanced by having more surface area for more digestive enzymes.
The large surface area also provides more opportunities for the absorption of digested nutrients. More surface area allows for more phospholipid bilayer to absorb fats and more carrier molecules to carry amino acids, peptides, monosaccharides, and other nutrients.
Intestinal epithelium. The diagram shows intestinal epithelial cells joined by tight junctions and covered with microvilli on their apical (lumen-facing) surfaces. RER, Rough endoplasmic reticulum; SER, smooth endoplasmic reticulum.
24. What type of reaction do all digestive enzymes catalyze?
25. What is a proenzyme?
26. Name the final digestive products of the following: carbohydrates, proteins, fats.
SECRETION
Saliva
Saliva is secreted by the salivary glands (see Figure 21-5), and as with all digestive secretions, is mostly water. However, it also contains amylase and lipase. Water helps mechanically digest food as it moves through the digestive tract by helping to liquefy the food in the process of making chyme. Mixed in the water is a combination of other important substances such as mucus, which has the primary functions of protecting the digestive mucosa and lubricating food as it passes through the alimentary canal.
Gastric Juice
Gastric juice is secreted by exocrine gastric glands. Gastric juice contains not only the basic water and mucus mixture of most other digestive juices but also a unique combination of other substances.
Chief cells in the gastric glands secrete the enzymes in gastric juice. Primary among the gastric enzymes is pepsin, which is secreted as the inactive proenzyme pepsinogen.
Pepsinogen is converted to pepsin by hydrochloric acid (HCl), which is produced by parietal cells of the gastric glands.
Besides secreting acid, parietal cells also produce intrinsic factor. Intrinsic factor binds to molecules of vitamin B12, protecting them from the acids and enzymes of the stomach. Intrinsic factor remains attached to B12 until it reaches the lower small intestine, where it facilitates the absorption of B12 across the intestinal wall.
Pancreatic Juice
Pancreatic juice is secreted by the exocrine acinar cells of the pancreas. As with other digestive secretions, pancreatic juice is mostly water. However, pancreatic juice also contains various digestive enzymes. All of these enzymes are secreted as zymogens—inactive proenzymes. For example, protein-digesting trypsin is first released as the zymogen trypsinogen, which is converted to active trypsin. After it is activated, trypsin can then activate other enzymes such as chymotrypsin (and other protein-digesting enzymes), amylase, various lipases, and nucleases (RNA- and DNA-digesting enzymes). This process changes each molecule's shape to the active enzyme form. The advantage of this system is that the enzymes do not digest the cells that make them.
Bile
Bile is an interesting mixture of many different substances that is secreted by the liver and stored and concentrated by the gallbladder.
Bile contains several substances that aid in digestion, specifically lecithin and bile salts. As we saw earlier, both of these substances break down large drops of fat into smaller droplets, thus making the fats more easily digestible. Both lecithin and bile salts wrap a hydrophilic shell around the droplets, making them water soluble and therefore able to move freely through the watery chyme in the lumen of the gastrointestinal tract. Bile also contains a small amount of sodium bicarbonate, which as with the sodium bicarbonate secreted by pancreatic duct cells, helps neutralize chyme. Excreted substances in bile include cholesterol, products of detoxification, and bile pigments.
We will discuss the diverse functions of the liver further in the next chapter.
Intestinal Juice
The term intestinal juice refers to the sum of intestinal secretions, rather than to a premixed combination of substances, that enters the gastrointestinal lumen by way of a duct. Most intestinal cells produce a water-based solution of sodium bicarbonate that aids in buffering. Goblet cells of the intestinal mucosa also produce a watery solution of mucus. Thus intestinal juice is a slightly basic, mucous solution that buffers and lubricates material in the intestinal lumen.
Intestinal juice is largely a product of the small intestine, but goblet cells in the mucosa of the large intestine produce some lubricating mucus.
We've listed the various secretions of the digestive tract and their components for you in Table 21-4. Take a few moments to review this reference table before you continue.
27. Discuss the functions of the primary components of saliva.
28. Name the components of gastric juice. Which type of cell produces each component?
29. What components of bile are considered to be excretions? Why?
CONTROL OF DIGESTIVE GLAND SECRETION
Exocrine digestive glands secrete when food is present in the digestive tract or when it is seen, smelled, or imagined. Integrated nervous and hormonal reflex mechanisms control the flow of digestive juices in such a way that they appear in proper amounts when and for as long as needed.
TABLE 21-4 Digestive Secretions
DIGESTIVE JUICE
SOURCE
SUBSTANCE
FUNCTIONAL ROLE*
Saliva
Salivary glands
Mucus
Lubricates bolus of food; facilitates mixing of food
Amylase
Enzyme; begins digestion of starches
Sodium bicarbonate
Increases pH (for optimum amylase function)
Water
Dilutes food and other substances; facilitates mixing
Gastricjuice
Gastric glands
Pepsin
Enzyme; digests proteins
Hydrochloric acid
Denatures proteins; decreases pH (for optimum pepsin function)
Intrinsic factor
Protects and allows later absorption of vitamin B12
Mucus
Lubricates chyme; protects stomach lining
Water
Dilutes food and other substances; facilitates mixing
Pancreatic juice
Pancreas (exocrine portion)
Proteases (trypsin, chymotrypsin, collagenase, elastase, etc.)
Enzymes; digest proteins and polypeptides
Lipases (lipase, phospholipase, etc.)
Enzymes; digest lipids
Colipase
Coenzyme; helps lipase digest fats
Nucleases
Enzymes; digest nucleic acids (RNA and DNA)
Amylase
Enzyme; digests starches
Water
Dilutes food and other substances; facilitates mixing
Mucus
Lubricates
Sodium bicarbonate
Increases pH (for optimum enzyme function)
Bile
Liver (stored and concentrated in gallbladder)
Lecithin and bile salts
Emulsify lipids
Sodium bicarbonate
Increases pH (for optimum enzyme function)
Cholesterol
Excess cholesterol from body cells, to be excreted with feces
Products of detoxification
From detoxification of harmful substances by hepatic cells, to be excreted with feces
Bile pigments (mainly bilirubin)
Products of breakdown of heme groups during hemolysis, to be excreted with feces
Mucus
Lubrication
Water
Dilutes food and other substances; facilitates mixing
Intestinal juice
Mucosa of small and large intestine
Mucus
Lubrication
Sodium bicarbonate
Increases pH (for optimum enzyme function)
Water
Small amount to carry mucus and sodium bicarbonate
* Boldface type indicates a chemical digestive process; italic type indicates a mechanical digestive process.
Control of Salivary Secretion
As far as is known, only reflex mechanisms control the secretion of saliva. Chemical, mechanical, olfactory, and visual stimuli initiate afferent impulses to centers in the brainstem that send out efferent impulses to the salivary glands, stimulating them. Chemical and mechanical stimuli come from the presence of food in the mouth. Olfactory and visual stimuli come, of course, from the smell and sight of food.
Control of Gastric Secretion
Stimulation of gastric juice secretion occurs in three phases that are controlled by reflex and chemical mechanisms. Because stimuli that activate these mechanisms arise in the head, stomach, and intestines, the three phases are known as the cephalic, gastric, and intestinal phases, respectively. As you read the description of each phase, glance at the diagrams shown in Figure 21-26.
The cephalic phase is also spoken of as the “psychic phase” because psychic (mental) factors activate the mechanism. For example, the sight, smell, taste, or even thought of food that is pleasing to an individual activates control centers in the medulla oblongata from which parasympathetic
FIGURE 21-26 Phases of gastric secretion.
fibers of the vagus nerve conduct efferent impulses to the gastric glands. Vagal nerve impulses also stimulate the production of gastrin, which stimulates gastric secretion, thus prolonging and enhancing the response.
During the gastric phase of gastric secretion, products of protein digestion in foods that have reached the pyloric portion of the stomach stimulate its mucosa to release gastrin into the blood within stomach capillaries. When it circulates to the gastric glands, gastrin greatly accelerates their secretion of gastric juice, which has a high pepsinogen and hydrochloric acid content. Gastrin release is also stimulated by distention of the stomach (caused by the presence of food).
The intestinal phase of gastric juice secretion is less clearly understood than the other two phases. Various different mechanisms seem to adjust gastric juice secretion as chyme passes to and through the intestinal tract. Experiments show that gastric secretions are inhibited when chyme containing fats, carbohydrates, and acid (low pH) is present in the duodenum.
Gastric secretion—and thus chemical digestion in the stomach—can be slowed when the duodenum becomes full. This prevents the stomach from finishing its task before the small intestine is ready to receive the chyme.
Control of Pancreatic Secretion
Several hormones released by intestinal mucosa are known to stimulate pancreatic secretion. Secretin induces the production of pancreatic fluid low in enzyme content but high in bicarbonate (HCO3−). This alkaline fluid acts to neutralize the acid (chyme) entering the duodenum.
The other intestinal hormone, known as cholecystokinin (CCK), was originally thought to be two separate substances. It has now been identified as one chemical with several important functions: (1) it causes the pancreas to increase exocrine secretions high in enzyme content; (2) it opposes the influence of gastrin on gastric parietal cells, thus inhibiting hydrochloric acid secretion by the stomach; and (3) it stimulates contraction of the gallbladder so that bile can pass into the duodenum.
Control of Bile Secretion
Bile is secreted continually by the liver and is stored in the gallbladder until needed by the duodenum. The hormones secretin and CCK, as described, stimulate ejection of bile from the gallbladder.
Control of Intestinal Secretion
Our knowledge about the regulation of intestinal exocrine secretions continues to grow. We know that intestinal secretions contain bicarbonate, which along with pancreatic bicarbonate, neutralizes acid from the stomach. Bicarbonate secretion is regulated by a reflex sensitive to changes in pH of the chyme. Neural mechanisms also help control the secretion of intestinal juice.
30. How is salivary secretion controlled?
31. Name the three phases of gastric secretion.
ABSORPTION
Process of Absorption
Absorption is the passage of substances (notably digested foods, water, salts, and vitamins) through the intestinal mucosa into the blood or lymph. As stated earlier, most absorption occurs in the small intestine, where the large surface area provided by the intestinal villi and microvilli (see Figure 21-11) facilitates this process.
Mechanisms of Absorption
Absorption of water is simple and straightforward: osmosis. Some substances depend on other active or passive transport mechanisms (or both) for absorption. Movement of sodium and glucose out of the GI tract is a good example. Primary active transport of sodium by an ATP-driven sodium pump in intestinal cells creates a concentration gradient that drives the passive co-transport of glucose along with sodium in a process called secondary active transport. Amino acids and several other compounds are thought to also be absorbed by such a secondary active transport mechanism.
Fatty acids, monoglycerides (products of fat digestion), and cholesterol are transported with the aid of lecithin and bile salts from fat droplets in the intestinal lumen to absorbing cells on villi. Lecithin and bile salts form microscopic spheres called micelles, which contain simple lipids (see Figure 21-25).
As Figure 21-27 illustrates for you, water-soluble micelles formed in the lumen of the intestine approach the brush border of absorbing cells. There, simple lipid molecules are released to pass through the plasma membrane by simple diffusion. After they are inside the cell, fatty acids are rapidly reunited with monoglycerides to form triglycerides (neutral fats).
Vitamins A, D, E, and K, known as the “fat-soluble vitamins,” also depend on bile salts for their absorption. Some water-soluble vitamins, such as certain of the B group, are small enough to be absorbed by simple diffusion; however, most require carrier-mediated transport. Many drugs (sedatives, analgesics, antibiotics) appear to be absorbed by simple diffusion because they are lipid soluble.
Note that, after absorption, most nutrients do not pass directly into the general circulation. Lacteals conduct fats along a series of lymphatic vessels and through many lymph nodes before releasing them into the venous blood flowing through the subclavian veins. Nutrients that are absorbed into the blood, such as amino acids and monosaccharides, first travel by way of the hepatic portal system to the liver.
FIGURE 21-27 Absorption of fats. Fats such as triglycerides are chemically digested within emulsified fat droplets, yielding fatty acids, monoglycerides, and glycerol (left). Fatty acids and other lipid-soluble compounds (such as cholesterol) leave the fat droplets in small spheres coated with bile salts (micelles). When a micelle reaches the plasma membrane of an absorptive cell, individual fat-soluble molecules diffuse directly into the cytoplasm. The endoplasmic reticulum of the cell resynthesizes fatty acids and monoglycerides into triglycerides. A Golgi body within the cell packages the fats into water-soluble micelles called chylomicrons, which then exit the absorptive cell by exocytosis and enter a lymphatic lacteal. IF, Interstitial fluid.
ELIMINATION
The process of elimination is simply the expulsion of the residues of digestion—feces—from the digestive tract. Formation of feces is the primary function of the colon. The act of expelling feces is called defecation.
Defecation is a reflex brought about by stimulation of receptors in the rectal mucosa. Normally, the rectum is empty until mass peristalsis moves fecal matter out of the colon into the rectum. This distends the rectum and produces the desire to defecate. Also, it stimulates colonic peristalsis and initiates reflex relaxation of the internal sphincters of the anus. Voluntary straining efforts and relaxation of the external anal sphincter may then follow as a result of the desire to defecate. Note that this is a reflex partly under voluntary control. If one voluntarily inhibits defecation, the rectal receptors soon become depressed and the urge to defecate does not usually recur until hours later, when mass peristalsis again takes place.
Constipation occurs when the contents of the lower part of the colon and rectum move at a rate that is slower than normal. Extra water is absorbed from the fecal mass, producing a hardened, or constipated, stool.
Diarrhea may occur as a result of increased motility of the small intestine. Chyme moves through the small intestine too quickly, reducing the amount of absorption of water and electrolytes. Diarrhea may also result from bacterial toxins that damage the water reabsorption mechanisms of the intestinal mucosa. The large volume of material arriving in the large intestine exceeds the limited capacity of the colon for absorption, so a watery stool results. Prolonged diarrhea can be particularly serious, even fatal, in infants because they have a minimal reserve of water and electrolytes (Box 21-5).
32. Explain the term secondary active transport.
33. Describe how fatty acids are absorbed by cells of the GI mucosa.
34. What triggers the defecation reflex?
BOX 21-5 Health Matters
Infant Diarrhea
Severe diarrhea caused by a rotavirus, an intestinal infection, kills more than 600,000 infants and young children worldwide each year. Death results from severe dehydration caused by 20 or more episodes of diarrhea in a single day. More than three million U.S. children suffer symptoms of rotavirus intestinal infection annually, and 65,000 require hospitalization. Good medical care in this country has limited the number of U.S. infant deaths caused by the disease each year to about 50.
Unfortunately, in developing countries, rotavirus-induced diarrhea remains one of the leading causes of infant mortality. The first major attempt at a rotavirus vaccine provided good protection against the virus but was withdrawn from the world market at the close of the twentieth century because of side effects. New rotavirus vaccines are now available and others are in the final stages of testing. Some have already been licensed outside the United States.
Until safe and effective vaccines become widely available and administered, one of the best treatment options available in many areas of the world involves oral administration of liberal doses of a simple, easily prepared solution containing sugar and salt. Called oral rehydration therapy (ORT), the salt-sugar solution replaces nutrients and electrolytes lost in diarrheal fluid. Because the replacement fluid can be prepared from readily available and inexpensive ingredients, it is particularly valuable in the treatment of infant diarrhea in developing countries.
The BIG Picture
The digestive system's primary contribution to overall homeostasis is its ability to maintain a constancy of nutrient concentration in the internal environment. It accomplishes this by breaking large, complex nutrients into smaller, simpler nutrients so they can be absorbed (see figure). The digestive system also provides the means of absorption—the cellular mechanisms that operate in the absorptive cells of the intestinal mucosa. The digestive system also provides some secondary, less vital functions. For example, the teeth and tongue aid the nervous system and respiratory system in producing spoken language. Also, acid in the stomach assists the immune system by destroying potentially harmful bacteria. Some of the various vital and nonvital roles played by the different organs that make up the digestive system are summarized in the figure.
The digestive system requires functional contributions by other systems of the body. Regulation of digestive motility and secretion requires the active participation of both the nervous system and the endocrine system. The oxygen needed for digestive activity requires the proper functioning of both the respiratory system and the circulatory system. The body's framework (integumentary and skeletal systems) is required to support and protect the digestive organs. The skeletal muscles must function if ingestion, mastication, deglutition, and defecation are to occur normally. As you've seen, the digestive system cannot operate alone—nor can any other system or organ, for that matter. The body is truly an integrated system, not a collection of independent components.
Summary of digestive function.
Cycle of LIFE
Significant changes in both the structure and function of the digestive system can occur at different times in the human life cycle. Such changes result in numerous diseases or pathological conditions and may occur in any segment of the intestinal tract from the mouth to the anus. In addition, life cycle changes also involve the accessory organs of digestion, such as the teeth, salivary glands, liver, gallbladder, and pancreas.
Because of the immaturity of intestinal mucosa in young infants, some types of intact proteins can pass through the epithelial cells that line the tract. The result may be an early allergic response caused by the protein triggering the baby's immune system. Lactose intolerance is another age-related example of a common digestive system problem. Intestinal lactase, needed for the digestion of lactose, or milk sugar, is almost always present at the time of birth but may level off rapidly or slowly, leaving many adults largely lactose intolerant.
Many other changes in the digestive system occur throughout our lives. Inflammation of the parotid salivary gland (mumps) is a common disease of children, whereas appendicitis occurs more frequently in adolescents. The incidence of appendicitis then decreases with age because the size of the opening between the appendix and the intestinal lumen decreases, thus reducing the incidence of infection. In contrast, gallbladder disease and ulcers are primarily problems of middle age. In more elderly individuals, a decrease in volume of digestive fluids coupled with a slowing of peristalsis and reduced physical activity often results in constipation and diverticulosis.
MECHANISMS OF DISEASE
A number of physiological, metabolic, and nutritional disorders can affect human digestion and nutrition. These range from genetic conditions and abnormalities of digestive tract development to metabolic disorders that undermine your body's ability to maintain homeostasis. Disorders of the digestive system abound, including disorders of the mouth and esophagus, tooth decay, gingivitis, and periodontitis. Gastroesophageal reflux disease also is a common and serious problem for many Americans.
Beyond common afflictions such as ulcers, diarrhea, constipation, and other more serious diseases, such as a variety of cancers of organs within the digestive system, many people suffer from eating disorders such as anorexia nervosa and bulimia. Obesity, especially childhood obesity, is now a national scourge, afflicting up to 40% of America's children—one of the highest rates in the world. And yet, protein-calorie malnutrition and vitamin deficiencies, especially of vitamins C and D, are also commonplace.
Find out more about these digestive, metabolic, and nutritional diseases online at Mechanisms of Disease: Digestive System.
CHAPTER SUMMARY
To download an MP3 version of the chapter summary for use with your iPod or other portable media player, access the Audio Chapter Summaries online at http://evolve.elsevier.com.
Scan this summary after reading the chapter to help you reinforce the key concepts. Later, use the summary as a quick review before your class or before a test.
ORGANIZATION OF THE DIGESTIVE SYSTEM
A. The digestive system comprises the digestive tract and other organs that aid digestion
B. The main organs of the digestive tract form a hollow tubelike system, which passes through the ventral cavities of the body (Figure 21-1)
C. Walls of the GI tract—fashioned from four layers of tissues (Figure 21-2):
1. Mucosa—inner mucous lining, innermost layer of the GI wall
2. Submucosa—layer of connective tissue that contains the main blood vessels of the tract
3. Muscularis—muscular layer characterized by an inner layer of circular and an outer layer of longitudinal smooth muscle
4. Serosa—also called serous layer, outer fibrous layer
D. Modifications of layers—layers vary in different regions of the tube throughout its length
MOUTH
A. Structure of the oral cavity
1. Lips—covered externally by skin and internally by mucous membrane
2. Cheeks—form the lateral boundaries of the oral cavity and mucous membrane lining is continuous with the lips in front
a. Bulk of the cheeks is formed largely by the buccinator muscle
b. Contain mucus-secreting glands
3. Hard palate—consists of portions of four bones; two maxillae and two palatines
4. Soft palate—forms a partition between the mouth and nasopharynx; made of muscle arranged in an arch (Figure 23-3)
5. Tongue—solid mass of skeletal muscle covered by a mucous membrane; forms the base of the oral cavity
a. Tongue has a blunt root, a tip, and a central body
b. Covered by rough elevations (papillae); possess sensory organs called taste buds (Figure 21-4)
c. Lingual frenulum—fold of mucous membrane in the midline of the ventral surface of the tongue; helps anchor the tongue to the floor of the mouth
d. Important for mastication (chewing) and deglutition (swallowing)
6. Salivary glands—located outside the alimentary canal; deliver their exocrine secretions by way of ducts from the glands into the lumen of the tract (Figure 21-5)
a. Three major pairs: parotid, submandibular, and sublingual glands
7. Teeth—designed to cut, tear, and grind ingested food
a. Typical tooth—consists of three main parts: crown, neck, and root (Figure 21-6)
(1) Crown—exposed portion of a tooth; covered by enamel
(2) Neck—section that connects the crown with the root below it
(3) Root—fits into the socket of the alveolar process
b. Types of teeth (Figure 21-7)
(1) Deciduous teeth (20 baby teeth)
(2) Permanent teeth (32)
PHARYNX
A. Tube through which a food bolus passes when moved from the mouth to the esophagus
ESOPHAGUS
A. Collapsible, muscular, mucosa-lined tube; extends from the pharynx to the stomach (Figure 21-8)
1. First segment of the digestive tube proper
2. Four layers form the wall (Figure 21-8, B)
3. Each end of the esophagus is encircled by muscular sphincters
a. Upper esophageal sphincter (UES)
b. Lower esophageal sphincter (LES) (cardiac sphincter)
STOMACH
A. Size and position of the stomach (Figure 21-9)
1. Just below the diaphragm
2. Size of the stomach varies; notably the amount of distention
B. Divisions of the stomach
1. Fundus—enlarged portion to the left and above the opening of the esophagus into the stomach
2. Body—central part of the stomach
3. Pylorus—lower portion
4. Cardia—small collar or margin of the stomach at its junction with the esophagus
C. Sphincter muscles—regulate passage of material at both stomach openings
1. Consist of circular fibers arranged to form an opening in the center of them when they are relaxed; no opening when they are fully contracted
a. Lower esophageal sphincter (LES) (cardiac sphincter)—controls the opening of the esophagus into the stomach
b. Pyloric sphincter—controls the opening from the pyloric portion of the stomach into the first part of the small intestine
D. Stomach wall
1. Epithelial lining of the stomach consists largely of folds (rugae)
2. Gastric pits—marked depressions within the rugae
3. Gastric glands—secrete most of the gastric juice (Figure 21-10)
4. Three major secretory cells:
a. Chief cells—secrete the enzymes of gastric juice
b. Parietal cells—secrete hydrochloric acid and intrinsic factor
c. Endocrine cells—secrete ghrelin (GHRL); hormone that stimulates the hypothalamus to increase appetite and release gastrin
5. Gastric muscle
a. Muscularis is made of three sublayers of smooth muscle tissue (Figure 21-9)
(1) Longitudinal muscles
(2) Circular muscles
(3) Oblique muscles
E. Functions of the stomach
1. Serves as a food reservoir
2. Secretes gastric juice
3. Breaks up food into small particles and mixes it with the gastric juice
4. Secretes intrinsic factor
5. Performs a limited amount of absorption
6. Produces the hormones gastrin and ghrelin
SMALL INTESTINE
A. Size and position of the small intestine
1. Tube measuring about 2.5 cm (1 inch) in diameter and 6 meters (20 feet) in length
2. Its coiled loops fill most of the abdominal cavity (Figure 21-1)
3. Divisions of the small intestine:
a. Duodenum—uppermost division; part to which the pyloric end of the stomach attaches; 25 cm (10 inches) long
b. Jejunum—point where the tube turns abruptly forward and downward; 2.5 meters (8 feet)
c. Ileum—3.5 meters (12 feet) long
B. Wall of the small intestine
1. Intestinal lining has plicae with villi (Figure 21-11)
2. Villi—tiny projections on the intestinal mucosa
a. Each villus contains an arteriole, venule, and lymph vessel (lacteal)
b. Contain mucus-secreting goblet cells
c. Each villus is covered by microvilli
d. Presence of villi and microvilli greatly increases the surface area of the small intestine
LARGE INTESTINE
A. Size of the large intestine
1. Diameter is noticeably larger than that of the small intestine
2. Length is about 1.5 to 1.8 meters (5 or 6 feet) (Box 21-3)
B. Divisions of the large intestine (Figure 21-12)
1. Cecum
2. Colon (Figure 21-12)
a. Ascending
b. Transverse
c. Descending
d. Sigmoid
3. Rectum—last 17 to 20 cm (7 or 8 inches) of the intestinal tube
a. Anal canal—terminal inch of the rectum
b. Anus—opening of the canal (Figure 21-14)
C. Wall of the large intestine
1. Intestinal glands—produce the lubricating mucus that coats the feces as they are formed (Figure 21-2)
2. Uneven distribution of fibers in the muscle layer
VERMIFORM APPENDIX
A. Wormlike tubular organ; 8 to 10 cm (3 to 4 inches) in length
B. Most often found just behind the cecum or over the pelvic rim
C. Communicates with the cecum 3 cm (about 1 inch) below the ileocecal valve
PERITONEUM
A. Large, continuous sheet of serous membrane that covers most of the digestive organs
B. Forms reflections, or extensions, that bind the abdominal organs together (Figure 21-14)
1. Mesentery—fan-shaped projection of the parietal peritoneum that extends from the lumbar region of the posterior abdominal wall
2. Greater omentum—continuation of the serosa of the greater curvature of the stomach and the first part of the duodenum to the transverse colon
3. Lesser omentum—attaches from the liver to the lesser curvature of the stomach and the first part of the duodenum
LIVER
A. Structure of the liver
1. Largest gland in the body
2. Lies immediately under the diaphragm; occupies most of the right hypochondrium and part of the epigastrium (Figure 21-1)
3. Consists of two lobes separated by the falciform ligament (Figure 21-15)
a. Left lobe—forms about one sixth of the liver
b. Right lobe—divides into right lobe proper, caudate lobe, and quadrate lobe
c. Each lobe is divided into numerous lobules by small blood vessels and by fibrous strands (perivascular fibrous capsule)
d. Hepatic lobules—anatomical units of the liver (Figure 21-16)
B. Bile ducts
1. Small bile ducts form right and left hepatic ducts
2. These ducts immediately join to form one common hepatic duct
a. Common hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct (Figure 21-17)
C. Functions of the liver
1. Detoxify various substances
2. Carry on numerous important steps in the metabolism of all three kinds of foods
3. Store several substances such as iron and vitamins
4. Produce important plasma proteins
5. Secrete about a pint of bile a day
D. Bile secretion by the liver
1. Main components of bile are bile salts, bile pigments, and cholesterol
GALLBLADDER
A. Pear-shaped sac 7 to 10 cm (3 to 4 inches) long and 3 cm broad at its widest point (Figure 21-17); lies on the undersurface of the liver
B. Serous, muscular, and mucous layers compose the wall of the gallbladder; mucosal lining has rugae
C. Stores bile that enters it by way of the hepatic and cystic ducts
PANCREAS
A. Structure of the pancreas
1. Grayish pink–colored gland about 12 to 15 cm (6 to 9 inches) long, weighing about 60 grams; its body extends horizontally behind the stomach with its tail touching the spleen
2. Composed of exocrine and endocrine glandular tissue
3. The pancreatic duct empties into the duodenum at the same point as the common bile duct; major duodenal papilla
4. Pancreatic islets—clusters of endocrine cells, making the pancreas a dual gland (exocrine, or duct, gland and endocrine, or ductless, gland)
B. Functions of the pancreas
1. Acinar units of the pancreas secrete the digestive enzymes
2. Beta cells of the pancreas secrete insulin
3. Alpha cells secrete glucagon
OVERVIEW OF DIGESTIVE FUNCTION
A. Digestive system uses various mechanisms to make nutrients available to each cell of our bodies (Table 21-1)
1. Ingestion—food is taken in
2. Digestion—breakdown of food into simpler nutrients
3. Motility of the GI wall—physically breaks large chunks of food into smaller bits and moves it along the tract
4. Secretion of digestive enzymes into the lumen of the GI tract
5. Absorption—movement through the GI mucosa into the internal environment
6. Elimination—excretion of material not absorbed
7. Regulation—coordination of various functions of the digestive system
DIGESTION
A. Processes that chemically and mechanically break complex foods into simpler nutrients that can be easily absorbed
B. Mechanical digestion—movement (motility) of the digestive tract (Table 21-2)
1. Mastication—chewing movements
a. Reduces size of food particles
b. Mixes food with saliva in preparation for swallowing
2. Deglutition—process of swallowing; three stages (Figure 21-19)
a. Oral stage (mouth to oropharynx)
b. Pharyngeal stage (oropharynx to esophagus)
c. Esophageal stage (esophagus to stomach)
3. Peristalsis and segmentation—motility produced by smooth muscle in the GI tract
a. Peristalsis—wavelike ripple of the muscle layer of a hollow organ (Figure 21-20)
b. Segmentation—mixing movements (Figure 21-21)
4. Regulation of motility
a. Gastric motility—emptying the stomach takes about 2 to 6 hours after a meal
(1) Food is churned with gastric juices in the stomach to form a thick, milky material; chyme
(2) Chyme is churned (propulsion and retropulsion) and mixed with gastric juices (Figure 21-22)
b. Intestinal motility—includes both peristaltic contractions and segmentation
(1) Segmentation in the duodenum and upper jejunum mixes chyme with digestive juices from the pancreas, liver, and intestinal mucosa
(2) Peristalsis continues as the chyme nears the end of the jejunum
C. Chemical digestion—changes in chemical composition that transform foods during their travel through the digestive tract; changes are a result of hydrolysis
1. Hydrolysis—chemical process in which a compound unites with water and then splits into simpler compounds
2. Digestive enzymes
a. Enzymes—“organic catalysts”; organic compounds that accelerate chemical reactions
b. Specific in their action (Figure 21-5)
c. Continually being destroyed or eliminated from the body
d. Most digestive enzymes are synthesized and secreted as inactive proenzymes (Figure 21-5)
3. Carbohydrate digestion
a. Carbohydrates are compounds made of saccharides
b. Polysaccharides are hydrolyzed to disaccharides by enzymes known as amylases
c. Enzymes that catalyze the final steps in carbohydrate digestion are sucrase, lactase, and maltase (Figure 21-23)
4. Protein digestion
a. Protein compounds have very large molecules made up of folded or twisted chains of amino acids
b. Proteases catalyze the hydrolysis of proteins into individual amino acids (Figure 21-24)
c. Main proteases: pepsin, trypsin, chymotrypsin, and peptidases (Box 21-4)
5. Fat digestion
a. Fats must be emulsified by bile
(1) Emulsification facilitates chemical digestion of fats by breaking large fat drops into small droplets
b. Lipase is the main fat-digesting enzyme (Figure 21-25)
6. Residues of digestion—certain components of food cannot be digested and are eliminated from the intestines in the feces
SECRETION (TABLE 21-4)
A. Saliva—secreted by the salivary glands (Figure 21-5)
1. Mucus—lubricates food and protects the digestive mucosa
B. Gastric juice—secreted by exocrine gastric glands (Figure 21-10)
1. Gastric juice contains the basic water and mucus mixture of most other digestive juices and combinations of other substances
2. Pepsin—primary gastric enzyme; secreted by chief cells
a. Secreted as the inactive proenzyme pepsinogen
b. Pepsinogen is converted to pepsin by hydrochloric acid (HCl), which is produced by parietal cells
3. Parietal cells also produce intrinsic factor
a. Intrinsic factor—binds to molecules of vitamin B12, protecting them from the acids and enzymes of the stomach; later facilitates its absorption
C. Pancreatic juice—secreted by the exocrine acinar cells of the pancreas
1. Pancreatic juice contains various digestive enzymes; secreted as zymogens (proenzymes)
a. Trypsin—protein-digesting enzyme
b. Amylase
c. Lipase
d. Nuclease—RNA- and DNA-digesting enzyme
D. Bile—mixture of many different substances that is secreted by the liver; stored in gallbladder
1. Lecithin and bile salts—break down large drops of fat into smaller droplets; makes the fats more easily digestible
2. Contains a small amount of sodium bicarbonate; neutralizes chyme
E. Intestinal juice—sum of intestinal secretions that enters the gastrointestinal lumen by way of a duct
1. Largely a product of the small intestine
2. Goblet cells in the mucosa of the large intestine produce some lubricating mucus
CONTROL OF DIGESTIVE GLAND SECRETION
A. Control of salivary secretion
1. Reflex mechanisms control the secretion of saliva
2. Chemical and mechanical stimuli come from the presence of food in the mouth
3. Olfactory and visual stimuli come, of course, from the smell and sight of food
B. Control of gastric secretion—three phases (Figure 21-26)
1. Cephalic phase (“psychic phase”)—mental factors activate the mechanism
2. Gastric phase—when products of protein digestion reach the pyloric portion of the stomach, they stimulate release of gastrin
a. Gastrin greatly accelerates their secretion of gastric juice
3. Intestinal phase—various mechanisms seem to adjust gastric juice secretion as chyme passes to and through the intestinal tract
C. Control of pancreatic secretion—stimulated by several hormones; secretin, CCK
D. Control of bile secretion—secreted continually by liver and stored in the gallbladder; secretin and CCK stimulate ejection of bile
E. Control of intestinal secretion—little is known about the regulation of intestinal exocrine secretions
ABSORPTION
A. Process of absorption
1. Absorption—passage of substances through the intestinal mucosa into the blood or lymph
2. Most absorption occurs in the small intestine
B. Mechanisms of absorption
1. Absorption of water is simple and straightforward: osmosis
2. Some substances depend on more complex mechanisms to be absorbed
a. Secondary active transport
3. Fatty acids, monoglycerides, and cholesterol are transported with the aid of lecithin and bile salts (Figure 21-27)
ELIMINATION
A. Elimination—expulsion of the residues of digestion (feces) from the digestive tract
B. Defecation is a reflex brought about by stimulation of receptors in the rectal mucosa
C. Constipation—occurs when the contents of the lower part of the colon and rectum move at a rate that is slower than normal; extra water is absorbed
D. Diarrhea—occurs as a result of increased motility of the small intestine; may also result from bacterial toxins that damage the water reabsorption mechanisms of the intestinal mucosa
REVIEW QUESTIONS
Write out the answers to these questions after reading the chapter and reviewing the Chapter Summary. If you simply think through the answer without writing it down, you won't retain much of your new learning.
1. List the component parts or segments of the GI tract and the accessory organs of digestion.
2. Name and describe the four tissue layers that form the wall of GI tract organs.
3. Identify the structures that form the mouth.
4. Define the following terms associated with the mouth: hard palate and soft palate, uvula, lingual frenulum.
5. Define the term papillae.
6. List and give the location of the paired salivary glands.
7. What type of saliva is produced by the parotid glands?
8. Describe a typical tooth.
9. Compare and contrast deciduous and permanent teeth.
10. List the divisions of the stomach. What is the difference between gastric pits and gastric glands?
11. Identify the three major cell types of the gastric glands. What cell type produces hydrochloric acid? Gastric enzymes? Gastrin? Ghrelin?
12. Describe the seven functions of the stomach.
13. List the divisions of the small intestine from proximal to distal.
14. In what area of the GI tract do you find villi? Haustra? Taeniae coli?
15. List the divisions of the large intestine.
16. What is believed to be the function of the vermiform appendix?
17. Discuss the peritoneum and its reflections.
18. Discuss the anatomy of a typical liver lobule.
19. Identify the ducts of the liver.
20. Explain the functions of the gallbladder.
21. Differentiate between endocrine and exocrine functions of the pancreas.
22. Define the actions of mechanical digestion: mastication, deglutition, peristalsis, and segmentation.
23. Discuss the process of chemical digestion. Describe carbohydrate, protein, and fat digestion.
24. Differentiate between the different digestive secretions: saliva, gastric juice, pancreatic juice, bile, and intestinal juice.
25. Compare and contrast absorption and elimination.
CRITICAL THINKING QUESTIONS
After finishing the Review Questions, write out the answers to these items to help you apply your new knowledge. Go back to sections of the chapter that relate to items that you find difficult.
1. Explain the role of the tongue's intrinsic and extrinsic muscles.
2. Describe the unique muscular layer of the esophagus.
3. Increasing the interior surface area of the small intestine allows it to absorb nutrients more efficiently. What examples can you find that add to the interior surface area of the small intestine?
4. If an elderly patient had abdominal pain, why would it be unlikely that it is caused by appendicitis?
5. How would you explain the two types of processes within the digestive system?
6. Why do you think emulsification is an example of mechanical digestion rather than chemical digestion to emulsify fats?
Lyslys4
Disgestion.docx
0
