Chapter 17: The Digestive System

Chapter Synopsis

In this chapter, digestion is described as a process by which food can be changed into substances that can be absorbed and utilized by the body cells. The digestive system has two major contributing components, the alimentary canal and the accessory organs. The alimentary canal starts at the mouth, follows through to the pharynx, esophagus, stomach, small intestine, large intestine, and finally ends at the anal canal. Foods undergo three processes in the body: digestion, absorption, and metabolism, utilizing both mechanical and chemical digestion. Finally fats, proteins, and carbohydrates are discussed, as well as the action of enzymes and gastric juices in the process of digestion.

Chapter Objectives and Notes

1.      List in sequence each of the component parts or segments of the alimentary canal from the mouth to the anus and identify the accessory organs of digestion.

a.       The component parts of the alimentary canal from mouth to anus are mouth, pharynx (throat), esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine (cecum, colon, rectum, anal canal).

b.      Accessory organs of digestion include teeth and tongue, salivary glands (parotid, submandibular, sublingual), gallbladder, pancreas, and vermiform appendix.

2.      List and describe the four layers of the wall of the alimentary canal. Compare the lining layer in the esophagus, stomach, small intestine, and large intestine.

            The four layers that make up the wall of the alimentary canal include

                (1) mucosa or mucous membrane,

                (2) submucosa,

                (3) muscular coat or muscularis, and

                (4) adventitia or serosa.

Although the same four tissue coats form all organs of the alimentary tract, their structures vary in different organs. The mucosa of the esophagus is composed of tough, stratified abrasion-resistant epithelium. The mucosa of the remainder of the tract is a delicate layer of simple columnar epithelium designed for absorption. In the stomach region, the mucosa is lined with thousands of microscopic gastric glands that secret gastric juice and hydrochloric acid. When the stomach is empty, its mucous lining lies in folds called rugae. The mucous lining of the small intestine, like that of the stomach, contains thousands of microscopic glands. These are called intestinal glands, and they secrete intestinal digestive juice. The intestinal lining is made up of multiple circular folds called plica. These folds are covered with thousands of tiny “fingers” called villi. Inside each villus lies a rich network of blood capillaries that absorb the products of digestion. No villi are present in the mucosa of the large intestine. As a result, less surface area is available for absorption and the efficiency and speed of movement of substances through the wall of the large intestine is much lower than in the small intestine.

            The submucosa is a connective tissue layer below the mucosa. It contains nerves and blood vessels. The two layers of muscle tissue called the muscularis move food through the digestive tube by means of contraction. Contraction also mixes food with digestive juices and mechanically breaks down food. The outermost covering called the serosa anchors digestive organs to the wall of the abdominal cavity.

3.      List and describe the major disorders of the digestive organs.

            The stomach is the potential site of numerous diseases and conditions.

            Pylorospasm is a fairly common condition in infants and occurs when the pyloric muscle fibers do not relax normally. As a result, food is not allowed to leave the stomach and is instead vomited. Pyloric stenosis is an obstructive narrowing of its opening.

            An ulcer is an open wound or sore in an area of the digestive system that is acted on by acid gastric juice.

            Stomach cancers, usually adenocarcinomas, are characterized by chronic indigestion, vomiting, anorexia, heartburn, stomach pain, and blood in the feces.

4.      Discuss the basics of protein, fat, and carbohydrate digestion and name the end products of each of these processes.

a.                   Protein digestion starts in the stomach. Two enzymes (rennin and pepsin) found in gastric juice cause giant protein molecules to break up into simpler compounds. Then in the intestine, other enzymes (trypsin in pancreatic juice and peptidase in intestinal juice) complete protein digestion. Every protein molecule is made up of many amino acids joined together. When enzymes have split up the large protein molecule into its separate amino acids, protein digestion is completed.

b.                  Fat digestion may begin in the stomach when an enzyme called gastric lipase is secreted. However, most fats go undigested until they are emulsified by bile in the duodenum of the small intestine. After this takes place, the pancreatic enzyme (pancreatic lipase) splits fat molecules into their building blocks, fatty acids, and glycerol.

c.                   Very little carbohydrate digestion occurs before food reaches the small intestine. Salivary amylase found in saliva begins the chemical breakdown of carbohydrates, but it has little time to work because food is usually swallowed quite fast. Gastric juices contain no carbohydrate-digesting enzymes. Once food reaches the small intestine, pancreatic and intestinal juice enzymes digest starches and sugars. A pancreatic enzyme (amylase) starts the process by changing starches into a sugar called maltose. Three intestinal enzymes—maltase, sucrase, and lactase—digest sugars by changing them into simple sugars, namely glucose. Maltase digests maltose (malt sugar), sucrase digests sucrose (cane sugar), and lactase digests lactose (milk sugar). The end products of carbohydrate digestion are simple sugars.

5.      Define and contrast mechanical and chemical digestion.

            Mechanical digestion breaks food into tiny particles, mixes these particles with digestive juices, moves them along the alimentary canal, and finally eliminates the digestive wastes from the body. Chewing, swallowing, peristalsis, and defecation are the main types of mechanical digestion.

            Chemical digestion breaks down large, nonabsorbable food molecules into smaller, absorbable molecules that are able to pass through the intestinal mucosa into blood and lymph.

6.      Define: peristalsis, bolus, chyme, jaundice, ulcer, and diarrhea.

a.       Peristalsis—the movement of food through the digestive tube. It is brought about as a result of contractions of the muscular coat.

b.      Bolus—a small, rounded mass created after food has been chewed. It allows food to be swallowed.

c.       Chyme—a semisolid mixture that comes to exist in the stomach after food has been acted on by gastric juice.

d.      Jaundice—a yellowish skin discoloration that occurs when excessive amounts of bile are absorbed into the blood.

e.       Ulcer—an open wound or sore in an area of the digestive system that is acted on by acid gastric juice. Two common sites for ulcers are the stomach and the duodenum. Left untreated, ulcers cause persistent pain and may actually perforate the wall of the digestive tube, causing hemorrhage and inflammation of the abdominal cavity.

f.        Diarrhea—a state in which fecal material has become fluid.

Chapter 17 Review

Short Answer

Complete the following objectives. Write your answers on a separate sheet of paper.

1.                  List in sequence each of the component parts, or segments, of the alimentary canal from the mouth to the anus and identify the accessory organs of digestion.

2.                  List and describe the four layers of the wall of the alimentary canal. Compare the lining layer in the esophagus, stomach, small intestines, and large intestines.

3.                  Discuss the basics of protein, fat, and carbohydrate digestion and give the end products of each process.

4.                  Define and contrast mechanical and chemical digestion.

5.                  Define peristalsis, bolus, chyme, jaundice, ulcer, and diarrhea.

Completion

Complete the statements below by writing in the missing words.

6.                  Pancreatic cells secrete pancreatic juice into the pancreatic ducts; however, the main duct empties into the __________.

7.                  The liver secretes __________.

8.                  Once food is digested, the next step in the utilization of food is __________.

9.                  The largest gland in the body is __________.

10.              The three areas of the small intestine are (a) , (b) , empties into the (c) __________.

11.              Food enters the stomach by passing through the at the end of the esophagus.

12.              Once food has been mixed thoroughly with gastric juices, it is broken down into a semisolid mixture called __________.

13.              This exocrine gland produces enzymes that digest the three major kinds of food. This gland is called the __________.

14.              Yellowish discoloration of the skin is referred to as __________.

15.              Wavelike, rhythmic contractions are called __________.