General Anatomy and Digestive Processes

Objectives

When you have completed this section, you should be able to

• list the regions of the digestive tract and the accessory organs of the digestive system;
• list the functions and major physiological processes of the digestive system;
• distinguish between mechanical and chemical digestion; and
• describe the basic chemical process underlying all chemical digestion, and name the major substrates and products of this process.

Key Point Review

1. What must nutrients such as proteins be digested into before they can be used by the human body?

2. Which physiological process of the digestive system truly moves a nutrient from the outside to the inside of the body?

3. What one type of reaction is responsible for all chemical digestion?

4. Name some nutrients that are absorbed without being digested.

The Mouth Through Esophagus

Objectives

When you have completed this section, you should be able to

• describe the gross anatomy of the digestive tract from the mouth through the esophagus;
• describe the composition and functions of saliva;
• describe the neural control of salivation and swallowing; and
• describe the histological layers of the esophagus and explain the functional significance of each.

Think About It

Why is proprioception important in protecting the tongue from being bitten?

Key Point Review

5. List as many functions of the tongue as you can.

6. Imagine a line from the mandibular bone to the root canal of a tooth. Name the tissues, in order, through which this line would pass.

7. What is the difference in function and location between intrinsic and extrinsic salivary glands? Name the extrinsic salivary glands and describe their locations.

8. Describe the muscularis externa of the esophagus and its action in peristalsis.

9. Describe the mechanisms for preventing food from entering the nasal cavity and larynx during swallowing.

The Stomach

Objectives

When you have completed this section, you should be able to

• describe the gross and microscopic anatomy of the stomach;
• state the function of each type of epithelial cell in the gastric mucosa;
• describe the components of gastric juice and state their functions;
• explain how the stomach produces hydrochloric acid and pepsin;
• describe the contractile responses of the stomach to food; and
• describe the three phases of gastric function and how gastric activity is activated and subsequently inhibited.

Think About It

Contrast the epithelium of the esophagus with that of the stomach. Why is each epithelial type best suited to the function of its respective organ?

Key Point Review

10. Name four types of epithelial cells of the gastric glands and state what each one secretes.

11. Explain how the gastric glands produce hydrochloric acid and how this results in the alkaline tide of the hepatic-portal blood.

12. What positive feedback cycle can you identify in the formation and action of pepsin?

13. How does food in the duodenum inhibit motility and secretion in the stomach?

The Liver, Gallbladder, and Pancreas

Objectives

When you have completed this section, you should be able to

• describe the gross and microscopic anatomy of the liver, gallbladder, bile duct system, and pancreas;
• describe the digestive secretions and functions of the liver, gallbladder, and pancreas; and
• describe the effects of CCK and secretin on the secretion of bile and pancreatic juice.

Think About It

Make a diagram showing how secretin exerts negative feedback control over duodenal pH.

Key Point Review

14. What contribution does the liver make to digestion?

15. Trace the pathway taken by a molecule of bile salt from the liver and back. What is this pathway called?

16. Name two hormones, four enzymes, and one buffer secreted by the pancreas, and state the function of each.

17. What causes the duodenum to secrete cholecystokinin (CCK), and what effects does this have on other parts of the digestive system?

The Small Intestine

Objectives

When you have completed this section, you should be able to

• describe the gross and microscopic anatomy of the small intestine;
• state how the mucosa of the small intestine differs from that of the stomach, and explain the functional significance of the differences;
• define contact digestion and state where it occurs; and
• describe the types of motility seen in the small intestine.

Think About It

Why do you think the small intestine is so much longer in a cadaver than in a living person?

Key Point Review

18. What three structures increase the absorptive surface area of the small intestine?

19. Sketch a villus and label its epithelium, brush border, lamina propria, blood capillaries, and lacteal.

20. Distinguish between segmentation and the migrating motor complex of the small intestine. How do these differ in function?

The Large Intestine

Objectives

When you have completed this section, you should be able to

• describe the gross anatomy of the large intestine;
• contrast the mucosa of the colon with that of the small intestine;
• state the physiological significance of intestinal bacteria;
• discuss the types of contractions that occur in the colon; and
• explain the neurological control of defecation.

Key Point Review

21. How does the mucosa of the large intestine differ from that of the small intestine? How does the muscularis externa differ?

22. Name and briefly describe two types of contractions that occur in the colon and nowhere else in the alimentary canal.

23. Describe the reflexes that would cause defecation in an infant. Describe the additional neural controls that would function following toilet-training.

Chemical Digestion and Absorption

Objectives

When you have completed this section, you should be able to

• describe how polysaccharides, proteins, and fats are chemically digested, name the enzymes involved, and discuss the functional differences among these enzymes;
• describe how monosaccharides, amino acids, and lipids are absorbed by the small intestine; and
• describe the processing of nucleic acids, vitamins, minerals, and water by the digestive system.

Think About It

Explain why the right lymphatic duct does not empty dietary fat into the bloodstream.

Think About It

Young adult women have four times as many iron transport proteins in the intestinal mucosa as men have. Can you explain this?

Think About It

Magnesium sulfate (epsom salt) is poorly absorbed by the intestines. In light of this, explain why it has a laxative effect.

Key Point Review

24. What three polymers account for most of the dietary calories? What are the end products of enzymatic digestion of each?

25. What two nutrients are digested by saliva? Why is only one of them digested in the mouth?

26. Name as many enzymes of the intestinal brush border as you can, and identify the substrate or function of each.

27. Explain the distinctions between an emulsification droplet, a micelle, and a chylomicron.

28. What happens to digestive enzymes after they have done their job? What happens to dead epithelial cells that slough off the gastrointestinal mucosa? Explain.

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