Nutrition 330 Introductory Nutrition

Study Guide: 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 first objective 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.

3.1 Digestion

Introduction

As the term gastrointestinal implies, the stomach (gastrus) 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. The chewing and mixing of food with saliva in the mouth initiates digestion in the GI tract.

The study of digestion and absorption requires that you learn many new terms, especially if you have not studied human physiology. The best approach is to review the anatomy of the GI tract and follow the sequence by which food passes through it to relate the functions of the different segments of the tract to the digestive process.

Objectives

After completing this section, you should be able to

• 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.
• briefly describe the functions of the anatomical structures in the first objective as food passes through the tract.
• define digestion, identify the two forms of digestion, and explain how the involuntary muscles function in mechanical digestion.
• 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.

Key Terms

After completing section 3.1, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 3, “Digestion, Absorption, and Transport,” pages 67–71
• Carefully examine the glossary of GI anatomy terms on page 68. It will help you understand the definitions and functions of the anatomical features of the GI tract.

Digestion

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

Mechanical digestion produces physical changes in food. It begins with the chewing and mixing of food with saliva in the mouth; this process prepares the bolus for swallowing. As the bolus enters the stomach, further mixing and kneading by the strong muscular wall produces the chyme, a suspension of tiny food particles in a semi‑liquid form that can allow digestive juices greater access to nutrients. Throughout the GI tract (with the exception of the mouth) are involuntary muscles, which are responsible for the two main types of propulsion, peristalsis and segmentation, which facilitate further mechanical digestion. Peristalsis is the alternating contractions of the circular and longitudinal muscles, which create a wave‑like motion that propels intestinal contents along the tract. Segmentation is the periodic squeezing of the circular muscles in the segments of the intestine, which creates a rhythmic motion that allows a thorough mixing of intestinal contents with digestive juices, as well as better contact with the absorbing surface of the intestinal wall.

Chemical digestion refers to the chemical breakdown of nutrients into basic absorbable units by the digestive enzymes secreted by specialized glands in the mouth, stomach, and pancreas, and by the epithelial cells of the small intestine.

Reading Assignment

• Chapter 3: “The Secretions of Digestion” and “The Final Stage,” pages 71–76 (up to “Absorption”)

Digestive Enzymes

You may find the following definitions of terms used for some components of nutrients helpful in understanding the digestive actions.

• monosaccharide: a single‑sugar molecule (e.g., glucose, fructose)
• disaccharide: a pair of monosaccharides linked together (e.g., maltose)
• polysaccharide: numerous (up to thousands) monosaccharides 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 (ten 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 triglyceride
• 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 and three fatty acids; the most 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)

For this unit, enzymes have been categorized into four major classes: amylase, protease, lipase, and disaccharidase. Further differentiation of enzymes in each class will be discussed in later units, as each nutrient is studied. The following table outlines the production sites of the body secretions that contain the different classes of enzymes and identifies the digestive actions on nutrients.

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

Hydrochloric acid is produced in the stomach. It creates the acid conditions needed by the enzyme pepsin, a protease enzyme secreted by the stomach. Hydrochloric acid is a strong acid (pH 2 or below) that can kill most bacteria entering the body with food. To protect the stomach from auto‑digestion by gastric juice, the cells of the stomach wall secrete a thick, slimy mucus that coats the wall and lubricates the food.

Bicarbonate is present in pancreatic juice. It neutralizes chyme to a neutral or slightly alkaline pH, at which pancreatic and intestinal enzymes work best.

Bile is produced by the liver and is stored in the gallbladder. Fat in the diet stimulates the secretion of a gastrointestinal hormone, which stimulates the gallbladder to squirt bile into the duodenum. Bile is not an enzyme but an emulsifier, suspending fat 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, a process that will be discussed in Unit 5.

Study Questions

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3.2 Absorption

Introduction

Absorption can be defined as the passage of simple nutrients, produced by digestion, through the epithelial cells of the small intestine into the bloodstream or the lymphatic system.

Carbohydrates, fats, and proteins are broken down into the basic building units to be absorbed. Vitamins, minerals, water, and alcohol are absorbed without being changed by the digestive enzymes.

Absorption occurs mainly in the absorptive surface of the small intestine. The structural features of the mucosa, the inner lining of the intestine, constitute a remarkable absorptive surface. This surface is made up of numerous villi and microvilli, which can recognize, select, and regulate the absorption of the nutrients the body needs. Your textbook gives give a clear description of the components of the inner lining of the small intestine, relating in simple terms how they work together in absorption.

Objectives

After completing this section, you should be able to

• 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.

Key Terms

After completing section 3.2, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 3: “Absorption,” pages 76–78

Study Questions

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3.3 Transport

Introduction

The transport of nutrient molecules, after they are absorbed by the mucosal cells of the intestine, involves two circulatory systems: the vascular and lymphatic systems. Water‑soluble nutrients, such as 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 by the portal vein 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 they reach the thoracic duct, which terminates in a vein that leads to the heart. As a result, 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.

Objective

After completing this section, you should be able to identify the two circulatory systems that deliver nutrients from the gastrointestinal tract to other parts of the body.

Key Terms

After completing section 3.3, you should be able to define and use the following terms in context:

portal vein
thoracic duct
chylomicron

Reading Assignment

• Chapter 3: “The Circulatory Systems,” pages 78–81

  Note: You will not be tested on the detailed information in these pages.

The Circulatory Systems

Your textbook presents a general review of the vascular and lymphatic systems. For this course, you are required to remember that only the water‑soluble nutrients (see introduction) are absorbed directly into the bloodstream. Fat‑soluble nutrients are absorbed by the lymphatic system as chylomicrons and are then released into the bloodstream. After nutrient molecules are absorbed into the capillaries of the intestine, the blood leaves through the portal vein. This vein branches into capillaries in the liver, where nutrients are prepared for use before being transported to various parts of the body. In contrast, lymph that is carrying nutrients bypasses the liver and collects in the thoracic duct, which connects to a vein that leads to the heart.

Study Questions

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3.4 Common Digestive Problems

Introduction

With an understanding of the functions and anatomical structures of the gastrointestinal tract, we can appreciate the ingenious coordination of digestion and absorption. Such coordination involves intricate control by gastrointestinal hormones and nerve pathways. However, malfunctions occur from time to time. Common digestive problems include choking on food, vomiting, diarrhea, constipation, gas, heartburn, and ulcers. In this section, we shall look at the causes of these problems and at ways of remedying them.

Objective

After completing this section, you should be able to

• discuss the following digestive problems: choking, vomiting, diarrhea, constipation, gas, heartburn, and ulcer.

Reading Assignment

• Chapter 3 “Highlight 3: Common Digestive Problems,” pages 87–93

  Note: You will not be tested on the detailed information in these pages. Read through this section and try to relate the situations to your own experience or to that of someone you know.

Constipation

The textbook states that a person is not necessarily constipated, even if several days pass between bowel movements. This situation is unlikely on a high-fibre diet. Normal bowel function should include not only bowel movements free of discomfort, but also once‑a‑day regularity.

Study Questions

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