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CHAPTER
23
The Digestive System
Objectives
Part 1: Overview of the Digestive System
Digestive Processes
2. List and define the major processes occurring during digestive system activity.
Basic Functional Concepts
3. Describe stimuli and controls of digestive activity.
Digestive System Organs: Relationships
4. Describe the location and function of the peritoneum.
5. Define retroperitoneal and name the retroperitoneal organs of the digestive system.
Part 2: Functional Anatomy of the Digestive System
The Mouth and Associated Organs
9. Describe the gross and microscopic anatomy and the basic functions of the mouth,
pharynx, and esophagus.
Digestive Processes: Mouth to Esophagus
12. Describe the mechanisms of chewing and swallowing.
14. Name the cell types responsible for secreting the various components of gastric juice and
indicate the importance of each component in stomach activity.
The Small Intestine and Associated Structures
18. Identify and describe structural modifications of the wall of the small intestine that
enhance the digestive process.
19. Differentiate between the roles of the various cell types of the intestinal mucosa.
The Large Intestine
26. List the major functions of the large intestine.
27. Describe the regulation of defecation.
Part 3: Physiology of Digestion and Absorption
28. List the enzymes involved in digestion; name the foodstuffs on which they act.
Developmental Aspects of the Digestive System
31. Describe embryonic development of the digestive system.
32. Describe abnormalities of the gastrointestinal tract at different stages of life.
Suggested Lecture Outline
Part 1: Overview of the Digestive System (pp. 850–855; Figs. 23.1–23.6)
A. Digestive system organs fall into two main groups: the alimentary canal and the
accessory organs (pp. 850–851; Fig. 23.1).
2. Accessory digestive organs aid digestion physically and produce secretions that break
down foodstuffs in the GI tract; the organs involved are the teeth, tongue, gallbladder,
salivary glands, liver, and pancreas.
I. Digestive Processes (pp. 851–852; Figs. 23.2–23.3)
A. Ingestion is the simple act of putting food into the mouth (p. 851; Fig. 23.2).
B. Propulsion moves food through the alimentary canal and includes both swallowing and
peristalsis (p. 851; Figs. 23.2–23.3).
II. Basic Functional Concepts (p. 852; Fig. 23.4)
A. The digestive system creates an optimal internal environment for its functioning in the
lumen of the GI tract, an area that is technically outside of the body (p. 852; Fig. 23.4).
1. Digestive activities within the GI tract are triggered by mechanical and chemical
stimuli.
III. Digestive System Organs: Relationships (pp. 852–855; Figs. 23.5–23.6)
A. Relationship of the Digestive Organs to the Peritoneum (pp. 852–854; Fig. 23.5)
1. The visceral peritoneum covers the external surfaces of most of the digestive organs,
and the parietal peritoneum lines the body wall of the abdominopelvic cavity.
2. The peritoneal cavity is located between the visceral and parietal peritoneums and is
filled with serous fluid.
B. The splanchnic circulation serves the digestive system and includes those arteries that
branch off the abdominal aorta to serve the digestive organs and the hepatic portal
circulation (p. 854).
C. Histology of the Alimentary Canal (pp. 854–855; Fig. 23.6)
1. Mucosa is the innermost, moist, epithelial membrane that lines the entire digestive
3. Muscularis externa typically consists of smooth muscle and is responsible for
peristalsis and segmentation.
4. Serosa, the protective outer layer of the intraperitoneal organs, is the visceral
peritoneum.
Part 2: Functional Anatomy of the Digestive System (pp. 856–892;
Figs. 23.7–23.31; Tables 23.1–23.3)
IV. The Mouth and Associated Organs (pp. 856–861; Figs. 23.7–23.11)
A. The mouth is a stratified squamous epithelial mucosa-lined cavity with boundaries of
the lips, cheeks, palate, and tongue (pp. 856–861; Figs. 23.7–23.11).
1. The lips and cheeks have a core of skeletal muscle covered externally by skin that
helps to keep food between the teeth when we chew.
2. The palate forms the roof of the mouth and has two parts:
a. The anterior hard palate, is formed by the palatine process of the maxilla and the
3. The tongue is made of interlacing bundles of skeletal muscle and is used to reposition
food when chewing, mix food with saliva, initiate swallowing, and help form conso-
nants for speech.
4. Major and minor salivary glands produce saliva, which cleanses the mouth, dissolves
5. The teeth tear and grind food, breaking it into smaller pieces.
a. The primary dentition, also called deciduous, or baby, teeth consists of 20 teeth that
are lost to make way for the permanent dentition.
b. The permanent dentition consists of 32 teeth, including the wisdom teeth, or third
molars.
V. The Pharynx (pp. 861–862)
A. The pharynx (oropharynx and laryngopharynx) provides a common passageway for food,
fluids, and air (pp. 861–862).
VI. The Esophagus (pp. 862–863; Fig. 23.12)
A. The esophagus provides a passageway for food and fluids from the laryngopharynx to the
stomach where it joins at the cardial orifice (pp. 862–863; Fig. 23.12).
VII. Digestive Processes: Mouth to Esophagus (pp. 863–864; Fig. 23.13)
A. Mastication, or chewing, begins the mechanical breakdown of food and mixes the food
with saliva (p. 863).
B. Deglutition, or swallowing, is a complicated process that involves two major phases
(p. 863; Fig. 23.13).
VIII. The Stomach (pp. 864–874; Figs. 23.14–23.20; Tables 23.1–23.2)
A. The stomach is a temporary storage tank where the chemical breakdown of proteins is
initiated and food is converted to chyme (pp. 864–865; Fig. 23.14).
1. The adult stomach varies from 15–25 cm long; its diameter and volume vary
depending on the amount of food it contains.
a. The major regions of the stomach include the cardial part, fundus, body, and the
B. Microscopic Anatomy (pp. 866–869; Figs. 23.15–23.16; Table 23.1)
1. The surface epithelium of the stomach mucosa is a simple columnar epithelium
2. The gastric glands of the stomach produce gastric juice, which may be composed of a
combination of mucus, hydrochloric acid, intrinsic factor, pepsinogen, and a variety of
hormones.
C. Digestive Processes in the Stomach (pp. 869–874; Figs. 23.17–23.20; Table 23.2)
1. Gastric secretion is controlled by both neural and hormonal mechanisms and acts in
three distinct phases: the cephalic phase, the gastric phase, and the intestinal phase.
4. The rate at which the stomach empties is determined by both the contents of the
stomach and the processing that is occurring in the small intestine.
IX. The Small Intestine and Associated Structures (pp. 874–887; Figs. 23.21–23.28;
Tables 23.2–23.3)
A. The small intestine is the site of the completion of digestion and absorption of nutrients
(pp. 875–878; Figs. 23.21–23.23).
1. It extends from the pyloric sphincter to the ileocecal valve where it joins the large
intestine. It has three subdivisions: the duodenum, the jejunum, and the ileum.
B. The liver and gallbladder are accessory organs associated with the small intestine
(pp. 878–882; Figs. 23.24–23.25).
1. The liver is the largest gland in the body and has four lobes.
2. The liver is composed of liver lobules, which are made of plates of liver cells
(hepatocytes).
3. The digestive function of the liver is to produce bile, which is a fat emulsifier.
C. The pancreas is an accessory gland that is retroperitoneal (pp. 882–884; Figs. 23.26–
23.28).
1. Pancreatic juice consists mainly of water and contains enzymes that break down all
categories of foodstuffs and electrolytes.
D. Digestive Processes in the Small Intestine (pp. 884–887; Tables 23.2–23.3)
1. Food takes 3–6 hours to complete its digestive path through the small intestine, the site
of virtually all nutrient absorption.
X. The Large Intestine (pp. 887–892; Figs. 23.29–23.31; Table 23.2)
A. The large intestine absorbs water from indigestible food residues and eliminates the latter
as feces (pp. 887–890; Figs. 23.29–23.30).
1. The large intestine exhibits three unique features: teniae coli, haustra, and epiploic
appendages, and has the following subdivisions: cecum, appendix, colon, rectum, and
anal canal.
B. Digestive Processes in the Large Intestine (pp. 890–892; Fig. 23.31)
1. The movements seen in the large intestine include haustral contractions and mass
movements.
Part 3: Physiology of Digestion and Absorption (pp. 892–901; Figs.
23.32–23.36)
XI. Digestion (pp. 892–895; Figs. 23.32–23.34)
A. Digestion is a catabolic process in which large food molecules are broken down to
chemical building blocks (monomers), which are small enough to be absorbed by the
GI tract lining (pp. 892–895; Figs. 23.32–23.24).
1. Digestion is accomplished by enzymes, secreted by intrinsic and accessory glands of
the alimentary canal, used in hydrolysis reactions.
2. Carbohydrates
a. Monosaccharides are simple sugars that are absorbed immediately (glucose,
galactose, and fructose).
3. Proteins digested into amino acids in the GI tract include not only dietary proteins but
also enzyme proteins secreted into the GI tract lumen.
a. Pepsin, secreted by the chief cells, begins the digestion of proteins in the stomach.
4. The small intestine is the primary site for lipid digestion.
a. Lipases are secreted by the pancreas and are the enzymes that digest fats after they
have been pretreated with bile.
5. Nucleic acids (both DNA and RNA) are hydrolyzed to their nucleotide monomers by
pancreatic nucleases present in pancreatic juice.
XII. Absorption (pp. 895–898; Fig. 23.35)
A. Absorption occurs along the entire length of the small intestine, and most of it is
completed before the chyme reaches the ileum (pp. 895–898; Fig. 23.35).
1. Absorption of Specific Nutrients
a. Glucose and galactose are transported into the epithelial cells by common protein
carriers and are then moved by facilitated diffusion into the capillary blood.
b. Several types of carriers transport the different amino acids before entering the
2. Malabsorption of nutrients can result from anything that interferes with the delivery of
bile or pancreatic juices, as well as factors that damage the intestinal mucosa.
XIII. Developmental Aspects of the Digestive System (pp. 898–901; Fig. 23.36)
A. Embryonic Development (pp. 898–899; Fig. 23.36)
1. The epithelial lining of the developing alimentary canal forms from the endoderm with
the rest of the wall arising from the mesoderm.
anus.
B. Aging (pp. 899, 901)
1. GI tract motility declines, digestive juice production decreases, absorption is less
efficient, and peristalsis slows, resulting in less frequent bowel movements and, often,
constipation.
Cross References
Additional information on topics covered in Chapter 23 can be found in the chapters listed below.
1. Chapter 1: Serous membranes
3. Chapter 3: Microvilli; membrane transport
4. Chapter 4: Simple columnar epithelium; areolar connective tissue; serous and mucous
glands
9. Chapter 14: Sympathetic and parasympathetic controls
10. Chapter 15: Papillae and taste buds
11. Chapter 16: Hormones
12. Chapter 17: Pernicious anemia
Lecture Hints
1. Emphasize that the digestive system is not only the alimentary (gastrointestinal) canal but
all organs and tissues that aid in the process of digestion.
3. Digestion is the process of breaking large particles into small particles. Emphasize that
4. Most students have difficulty with the serous coverings of the abdominal viscera. A
sealed garbage bag can be used to demonstrate the continuity of these membranes. Use
5. Spend some time with the hepatic portal system. This is another example of blood
6. When discussing the histology of the GI tract, ask the class: “What is the logical epithe-
lial choice for the mouth? For the esophagus?” Point out that the choice of columnar
epithelium for the mucosal layer of the GI tract is ideally suited to its function.
7. Emphasize that the esophagus is not covered by serosa, but instead has an adventitia as
8. Emphasize that the lower esophageal sphincter (gastroesophageal) is not a true sphincter.
9. As a point of interest, mention that heartburn is actually acid reflux into the lower portion
of the esophagus.
11. Have students note the difference between the way the mucosa in the stomach has a
12. Intrinsic factor is a stomach secretion necessary for vitamin B12 absorption; however,
13. As each cell of the stomach mucosa is described, relate the logical function of each type
to the overall function of the stomach.
15. When introducing the digestive function of the small intestine, lead into the topic by
asking the class: “What functions must occur as chyme enters the initial part of the small
intestine?” Using carefully led questioning, the class should respond: acid neutralization,
further digestion of carbohydrates and proteins, and initiation of lipid digestion.
16. Students have difficulty with the pathways of flow in the liver lobule. Use two-dimensional
17. Ask the class why the hepatic artery is necessary, since the liver is already supplied by
18. Emphasize that the pancreas is a dual function/structure gland, endocrine and exocrine.
20. Emphasize that the amount of time the contents of the large intestine are in contact with
21. Point out the logical names of digestive enzymes: The prefix usually indicates the sub-
22. Spend time on fat digestion and absorption, from emulsification to movement through
the bloodstream. Point out that carbohydrates and proteins take a different (vascular) path
to the liver than do the lipids.
Activities/Demonstrations
1. Audiovisual materials are listed in the Multimedia in the Classroom and Lab section of
this Instructor Guide (p. 387).
2. Have students calculate their total caloric intake over a 24-hour period by using a simple
3. Demonstrate the emulsification action of bile: first mix oil and water together and allow
the layers to separate out. Then add bile salts and shake vigorously. Point out that the
layer of oil has been dispersed into hundreds of tiny fat spheres by the action of the bile
salts.
4. Use a torso model and/or dissected animal model to exhibit digestive organs.
Critical Thinking/Discussion Topics
1. Discuss symptoms, treatment, and prognosis of a hiatal hernia.
2. Explain why it is important to chew food properly.
3. Explore the importance of the liver.
4. Discuss the cause, treatment, and prevention of ulcers.
Library Research Topics
1. Research the causes and treatment of ulcers.
2. Study the benefits of fiber in the diet.
3. Research liver transplants in terms of rationale for the transplant, procedure, and
prognosis.
8. What are the common cancers of the digestive system? Are they limited to the accessory
structures?
List of Figures and Tables
All of the figures in the main text are available in JPEG format, PPT, and labeled & unlabeled
format on the Instructor Resource DVD. All of the figures and tables will also be available in
Transparency Acetate format. For more information, go to www.pearsonhighered.com/educator.
Figure 23.1 Alimentary canal and related accessory digestive organs.
Figure 23.2 Gastrointestinal tract activities.
Figure 23.5 The peritoneum and the peritoneal cavity.
Figure 23.6 Basic structure of the alimentary canal.
Figure 23.7 Anatomy of the oral cavity (mouth).
Figure 23.8 Dorsal surface of the tongue, and the tonsils.
Figure 23.9 The salivary glands.
Figure 23.10 Human dentition.
Figure 23.11 Longitudinal section of a canine tooth within its bony
socket (alveolus).
Figure 23.17 Neural and hormonal mechanisms that regulate release of
gastric juice.
Figure 23.18 Mechanism of HCl secretion by parietal cells.
Figure 23.19 Peristaltic waves in the stomach.
Figure 23.20 Neural and hormonal factors that inhibit gastric emptying.
Figure 23.21 The duodenum of the small intestine, and related organs.
Figure 23.22 Structural modifications of the small intestine that
Figure 23.26 Structure of the enzyme-producing tissue of the pancreas.
Figure 23.27 Activation of pancreatic proteases in the small intestine.
Figure 23.28 Mechanisms promoting secretion and release of bile and
pancreatic juice.
Answers to End-of-Chapter Questions
Multiple-Choice and Matching Question answers appear in Appendix H of the main text.
Short Answer Essay Questions
18. A drawing of the organs of the alimentary canal with labels can be found on page 850,
Figure 23.1.
19. The digestive system does contain local nerve plexuses known as the local (enteric)
nervous system or the “gut brain.” This is essentially composed of nerve plexuses in the
20. See pp. 854–855 and Fig. 23.6.
The basic alimentary canal wall structure consists of four tunics: the mucosa, submucosa,
muscularis, and serosa. The mucosa consists of a surface epithelium overlying a small
amount of connective tissue called the lamina propria and a scanty amount of smooth
21. The mesentery is a double peritoneal fold that suspends the small intestine from the
posterior abdominal wall. The mesocolon is a special dorsal mesentery that secures the
22. The six functional activities of the digestive system are ingestion, propulsion, mechanical
digestion, digestion, absorption, and defecation. (pp. 851–852)
23. a. The boundaries of the oral cavity include the lips, cheeks, tongue, palate, and
oropharynx.
b. The epithelium is stratified squamous epithelium because the walls have to withstand
considerable abrasion. (pp. 856–857)
24. a. The normal number of permanent teeth is 32; deciduous teeth, 20.
b. Enamel covers the crown; cement, the root.
25. The two phases of swallowing are as follows (pp. 863–864):
a. Buccal (voluntary) phase of swallowing: organs involved—tongue, soft palate;
26. The parietal cells secrete hydrochloric acid and intrinsic factor. Chief cells produce pep-
sinogen. Mucous neck cells produce mucus that helps shield the stomach wall from dam-
age by gastric juices. Enteroendocrine cells secrete hormones into the lamina propria.
(p. 866)
27. Gastric secretion is controlled by both neural and hormonal mechanisms. The stimulation of
gastric secretion involves three distinct phases: the cephalic, gastric, and intestinal phases.
The cephalic phase occurs before food enters the stomach and is triggered by the sight,
aroma, taste, or thought of food. Input is relayed to the hypothalamus, which stimulates
the vagal nuclei of the medulla oblongata, causing motor impulses to be sent via vagal
28. a. The cystic and common hepatic ducts fuse to form the bile duct, which fuses with the
pancreatic ducts just before entering the duodenum. (p. 875; Fig. 23.21)
b. The point of fusion of the common bile duct and pancreatic duct is called the
hepatopancreatic ampulla. (p. 875)
29. The absence of bile (which causes fat emulsification) and/or pancreatic juice (which
30. The stellate macrophages function to remove debris such as bacteria from the blood. The
hepatocytes function to produce bile, in addition to their many metabolic activities. (p. 878)
31. a. Brush border enzymes are intestinal digestive enzymes; these are secreted by intestinal
absorptive cells and remain bound to the plasma membrane of the microvilli. (p. 877)
32. Activation of the pancreatic enzymes in the small intestine illustrates the “wisdom of the
33. Common inflammatory conditions include appendicitis in adolescents, ulcers and
gallbladder problems in middle-aged adults, and constipation in old age. (p. 899)
34. The effects of aging on digestive system activity include declining mobility, reduced
production of digestive juice, less efficient absorption, and slowing of peristalsis.
(pp. 899, 901)
Critical Thinking and Clinical Application Questions
1. If the agent promotes increased bowel motility without providing for increased bulk,
diverticulosis is a possibility, because the rigor of the colonic contractions increases
2. This patient has the classical symptoms of a gallbladder attack in which a gallstone has
lodged in the cystic duct. The pain is discontinuous and colicky because it reflects the
3. The baby’s blood would indicate acidosis due to the intestinal juice passing through the
4. a. Most gastric ulcers are found to be caused by infection with Helicobacter pylori.
This drug regimen successfully eradicates the infection, while also soothing the
5. An endoscope is an instrument used to visually inspect any cavity of the body and is
6. Along with the risk of dehydration, severe diarrhea can result in loss of potassium, which
could lead to an electrolyte imbalance that affects his neuromuscular function. His severe
weakness may be a symptom of this. (p. 898)
7. The circle of tonsils around the opening of the pharynx is protective because air, food,
and liquids, none of which is typically sterile, pass the tonsils almost immediately. This
Suggested Readings
Barary, Nathan, and Leonard I. Zon. “Endothelium—Chicken Soup for the Endoderm.”
Science 294 (5542) (Oct. 2001): 530–531.
Boroom, Ken. “Getting to the Bottom of Irritable Bowels.” New Scientist 198 (2651)
(April 2008): 13.
Ferber, Dan. “Cracking Gut Bugs’ Cell-Skewing Strategy.” Science 294 (5550) (Dec. 2001):
2269.
Harder, Ben. “Germs That Do a Body Good.” Science News 161 (5) (Feb. 2002): 72–74.
Seppa, N. “Immune Cells Rush to Gut in Food Allergy.” Science News 159 (14) (April 2001):
214.
