Nutrition 331: Nutrition for Health

Study Guide: Unit 4

Body Systems and Digestion


To understand how food works in the body, you need a basic understanding of the major body systems and of the process of digestion itself. These topics form the subject of this unit.


After completing this unit you should be able to

  1. identify and describe, briefly, the major body systems listed below.
    • cellular
    • cardiovascular
    • immune
    • hormonal
    • nervous
    • excretory
  2. describe chemical and mechanical digestion, starting at the mouth and finishing at the small intestine.
  3. identify the anatomy and organs of the gastrointestinal tract.
  4. describe the large intestine’s role in digestion.
  5. describe mechanical digestion.

Section 1 The Cellular, Cardiovascular, Immune, Hormonal, and Nervous Systems

Reading Assignment

Read pages 78–87 of Chapter 3: “The Remarkable Body” (to “The Digestive System.”)

Note: This reading is optional. It serves as a review for students who have not studied physiology recently.

Section 2 Digestion, Absorption, Transport, and Excretion

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 section, 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 section presents only an overview of these complicated processes. We elaborate further as individual nutrients are discussed in later units.

As the term gastrointestinal implies, the stomach (Greek gastr, related to the belly) and the intestines are the two principal organs of the digestive tract. The mouth is also an important organ of the GI tract, as digestion—both mechanical and chemical—begins there.

Study Tip: The study of digestion and absorption requires that students, especially students who have not done advanced biology or physiology courses, learn many new terms. The best approach is to review the anatomy of the GI tract and relate the functions of the different segments of the tract to the digestive processes.

Reading Assignment

Read “The Digestive System,” pages 87–97 of Chapter 3: “The Remarkable Body.”

Optional Reading: “A Letter from Your Digestive Tract” (pp. 98–100).

Anatomy of the Gastrointestinal Tract

In addition to the organs shown in Figure 3.8 on page 89 of the textbook, you should also be aware of the structures described below.

Epiglottis—a flap located at the start of the esophagus which prevents food from entering the lungs.

Sphincter—a circular muscle surrounding, and able to close, a body opening. The stomach has two sphincters: one at the start of the stomach and one at the end.

Duodenum—the first part of the small intestine, the site at which foods are subjected to the action of bile and pancreatic juices, and where the breakdown of proteins, carbohydrates, and fats begins.

Jejunum—the first two-fifths of the small intestine beyond the duodenum; its principal function is absorption of digested material.

Ileum—the last segment of the small intestine that precedes the large intestine and functions in the absorption of nutrients.

Mechanical Digestion

Digestion can be defined as the breakdown of food into absorbable forms, through mechanical and chemical action.

The text describes mechanical digestion on pages 88–91. Figure 3.9 (p. 90) shows a swallowed food mass. This is also called a bolus.

Segmentation is a second type of mechanical digestion not discussed in the text. This is the periodic squeezing of the circular muscles (see Fig. 3.9, p. 90) in the segments of the intestine. This squeezing creates a rhythmic motion allowing a thorough mixing of the intestinal contents with digestive juices and enabling better contact with the absorption surface of the intestinal wall.

Chemical Digestion

Chemical digestion refers to the chemical breakdown of nutrients into basic absorbable units by digestive enzymes. These enzymes are secreted by specialized glands in the mouth, stomach, and pancreas, and by the epithelial cells of the small intestine. The textbook describes this process (pp. 92–96).

You may find the following definitions helpful:

Enzymes—specialized proteins that facilitate the conversion of one substance into another without the enzyme being altered. They are vitally important in thousands of chemical reactions in the body. Digestive enzymes break down food into small molecules that can be absorbed.

Monosaccharide—a single-sugar molecule (e.g., glucose, fructose).

Disaccharide—a pair of monosaccharides linked together (e.g., maltose, lactose, sucrose).

Polysaccharide—numerous monosaccharides (up to thousands) linked together (e.g., starch).

Disaccharidase—enzyme involved in hydrolyzing (breaking down) a specific disaccharide into its monosaccharide components.

Amino acid—the building block of proteins.

Dipeptide—two amino acids bonded together.

Tripeptide—three amino acids bonded together.

Polypeptide—many (10 or more) amino acids bonded together; polypeptides form proteins.

Fatty acid—the building block of fats.

Glycerol—the “backbone” to which fatty acids are attached to produce a glyceride.

Monoglyceride—a molecule of glycerol with one fatty acid attached.

Diglyceride—a molecule of glycerol with two fatty acids attached.

Triglyceride—a molecule of glycerol with three fatty acids attached; triglycerides are the predominant form of fat in the diet and in the body.

Emulsified fat—fat droplets dispersed and stabilized in a watery solution with the help of emulsifiers (e.g., bile).

pH—a measure of how acidic or alkaline a solution is. Acids have a low pH (about 0.5 for a strong acid to 4 for a weak acid). Neutral solutions have a pH of 7. Alkaline solutions have a pH from 9 (weak alkali) up to about 13.5 (strong alkali).

Digestive enzymes may be categorized into four major classes: protease, lipase, amylase, and disaccharidase. Note that the suffix ase indicates that the compound is an enzyme. Enzymes of each class are further differentiated in later units, as each nutrient is studied.

The following outline summarizes the digestive actions of the various enzymes.

Mouth—the saliva contains amylase, which starts the breakdown of starch to smaller polysaccharides and maltose.

Stomach—the stomach produces gastric juice, which contains pepsin, an enzyme that breaks protein down into small polypeptides.

Pancreas and small intestine—these organs produce pancreatic juice and intestinal juice, which contain a variety of enzymes. Protease, specifically trypsin, breaks polypeptides down into peptides, dipeptides, tripeptides, and free amino acids.

Lipase breaks emulsified fat down into monoglycerides, free fatty acids, and glycerol.

Amylase breaks starch and polysaccharides down into maltose; and disaccharidases break disaccharides down into monosaccharides.

Three other non-enzymatic secretions important to digestion are hydrochloric acid, bicarbonate, and bile.

Hydrochloric acid is produced in the stomach. It functions primarily to assist in the hydrolysis, or chemical breakdown, of protein to smaller peptides by creating the acid conditions needed by the enzyme pepsin, a protease enzyme, which is also secreted by the stomach. Hydrochloric acid is a strong acid (pH 2 or below) that can kill most food-borne bacteria entering the body. To protect the stomach from auto-digestion by acidic gastric juice, the cells of the stomach wall secrete a thick, slimy mucus that coats the wall. Mucus also lubricates the food.

Bicarbonate, secreted by the pancreas, neutralizes stomach acid that reaches the small intestine. It is described in the text on page 93.

Bile is produced by the liver and stored in the gallbladder. Fat in the diet stimulates the gallbladder to squirt bile into the duodenum. Bile is not an enzyme but an emulsifier, causing fat to form a suspension in the watery medium of the GI tract so that enzymes can break fat into its component parts. Bile is also essential for the absorption of fat by the intestinal mucosa.

Note: Chemical digestion is also discussed in Unit 5: Carbohydrates, Unit 6: Lipids, and Unit 7: Protein and Amino Acids.


Now that nutrients have been broken down to their basic building blocks, absorption may occur. Absorption can be defined as the passage of simple nutrients through the epithelial cells of the small intestine and into the blood stream or the lymphatic system.

Carbohydrates, fats, and proteins are broken down into the basic building units to be absorbed. Vitamins and minerals are absorbed with virtually no change by the digestive enzymes. Water and alcohol are absorbed directly into the blood.

Absorption is described in the textbook on pages 96–97.


The transport of nutrient molecules, after absorption by the mucosal cells of the intestine, involves two circulatory systems: the blood or vascular system (Fig. 3.3, p. 81) and the lymphatic system (pp. 80–82). Water-soluble nutrients—monosaccharides, amino acids, short and medium-chain fatty acids (14 or fewer carbons), water-soluble vitamins, and minerals—are released directly into the bloodstream via the capillaries surrounding the small intestine and then transported to the liver. Fatty acids with long hydrocarbon chains (16 or more carbons) and fat-soluble vitamins cannot gain access directly into these capillaries; they cluster together in the intestinal cells to form large fat molecules to which special protein carriers are attached, forming a lipoprotein complex called a chylomicron. The chylomicrons are then taken up by the lymphatic system, in which they are transported through the lymph spaces, eventually reaching the thoracic duct, which terminates in a vein that leads to the heart. Ultimately, all nutrients end up in the bloodstream to be distributed to various parts of the body where they are metabolized, stored, or excreted according to need.


Water reabsorption occurs in the colon. The colon also harbours a massive concentration of bacteria. Before food residue is expelled as feces, it is fermented by these bacteria. Bacteria may comprise a significant part of the fecal weight. One effect of bacterial fermentation is gas production. The bacteria perform several useful functions:

  • They partially digest fibre. The body obtains some food energy as a result.
  • They produce some vitamins. A part of the body’s vitamin K requirement is met by colonic bacteria. However, the body is able to absorb only a fraction of the vitamins produced in the colon.
  • They prevent the growth of pathogenic bacteria.

Normally the bacteria in the colon co-exist well within the body, doing no harm. Occasionally, however, the “wrong” type of bacteria gain a foothold; a common result is diarrhea.

There has been great interest in recent years in the activity of the intestinal bacteria. The microbiome, as it is known, is believed to be strongly involved in many disease processes, either in beneficial or harmful ways.


Digestion, absorption, and transportation are three different but important steps in converting food into usable energy. A fourth step, metabolism, will be discussed in Unit 8.

Review Questions

See the textbook, page 112, “Self-Check” questions 3, 4, 5, 7, 8, 9, and 10.