Unit 3
Digestion, Absorption, and Transport

The human gastrointestinal (GI) tract is a 26-foot long, muscular, flexible tube that begins with the mouth as one opening and ends with the anus as the other. Its main function is to digest and absorb nutrients from food. In this unit, we review the anatomical features of the GI tract and study how the involuntary muscles and enzymes function in the mechanical and chemical digestion of food. The processes of digestion, absorption, and transport involve specially controlled systems and mechanisms; this unit presents only an overview of these complicated processes. We shall elaborate further as we discuss individual nutrients in later units. Finally, in this unit, we briefly discuss some common digestive problems, such as choking, vomiting, diarrhea, constipation, heartburn and acid indigestion, and peptic ulcers.

This unit consists of four sections:

3.1—Digestion
3.2—Absorption
3.3—Transport
3.4—Common Digestive Problems

Objectives

After completing this unit you should be able to

  1. identify the following anatomical structures on a diagram of the human gastrointestinal tract: mouth, salivary glands, epiglottis, esophagus, cardiac sphincter, stomach, pyloric sphincter, gallbladder, pancreas, duodenum, jejunum, ileum, ileocecal valve, colon, rectum, and anus.
  2. briefly describe the functions of the anatomical structures in objective 1 as food passes through the tract.
  3. define digestion, identify the two forms of digestion, and explain how the involuntary muscles function in mechanical digestion.
  4. identify the four major classes of enzymes found in saliva, gastric juice, and the pancreatic and intestinal juices; state their digestive actions on nutrients; and discuss the importance of hydrochloric acid, bicarbonate, and bile in the digestion of nutrients.
  5. identify the following structures on a diagram of the absorptive surface of the intestinal cells: villi, microvilli, capillaries, and lymph ducts. Briefly describe how these structures are important in absorption.
  6. identify the two circulatory systems that transport nutrients from the gastrointestinal tract to other parts of the body.
  7. discuss the following digestive problems: choking, vomiting, diarrhea, constipation, gas, heartburn, and ulcer.