Chapter 15 Digestive System. I. The Gastrointestinal Tract. a. The digestive system mechanically and chemically breaks down food into molecules that can be absorbed into the bloodstream or lymph. Residues are then eliminated at the end of the GI tract. Fig. 15.1. i. Mechanical digestion physically breaking the food up into smaller pieces. ii. Chemical digestion enzymatic breakdown of food molecules by breaking chemical bonds. These enzymes are secreted by the GI tract organs themselves, or by accessory glands/organs. b. Table 15.1 provides a general overview of the GI tract organs and their functions. c. The digestive system includes a long tube called the gastrointestinal (GI) tract that runs from the mouth to the anus, and a collection of glands/organs that add secretions to the food for digestion. d. The space inside the GI tract is called the lumen. e. Stretched out, the GI tract of an adult human can be as long as 30 feet. Mucous epithelium lines the lumen. f. Histologically, the GI tract is composed of the following layers: Fig. 15.2. i. Mucosa the innermost layer that comes into direct contact with the food; lined with an epithelium. Mucous secretion helps to lubricate the lumen for the passage of the food and protect the mucosal tissue from abrasion and autodigestion by digestive enzymes. Contains a variety of structures that act to increase surface area for absorption of digestion products. ii. Submucosa consists of connective tissues, blood vessels, lymphatic vessels, and nerves. iii. Muscularis externa a layer of smooth muscle that serves to mix (segmental contractions) and propel the food (peristaltic contractions) along the length of the GI tract. In most GI tract organs, this layer is subdivided into an inner circular layer and an outer longitudinal layer, but there are exceptions iv. Serosa a thin layer of epithelia and connective tissue that makes up the outer margin of the GI tract organs. It secretes serous fluid to lubricate its surface so that the GI tract organs can glide over one another without causing abrasion. g. Some GI tract organs contain sphincters circular arrays of smooth muscle that can close off sections of the GI tract to control food movement. h. The digestive system is assisted by accessory organs, which secrete enzymes and other substances into the GI tract. II. Specialized Compartments for Food Processing. a. Mouth. Begins the process of chemical and physical digestion. i. Teeth. Fig. 15.3. [Discuss anatomy]. 1. Adults have 32 (permanent). 2. Young children have 20 (deciduous). 3. Chewing breaks down food: a. Physically into smaller pieces with greater surface area. b. Chemically by mixing food with saliva, which contains the enzyme salivary amylase, chemical breakdown of starches begins. ii. Saliva secreted by three main pairs of glands: Fig. 15.4. 1. Parotid salivary gland. 2. Submandibular salivary gland. 3. Sublingual salivary gland. a. In addition to amylase, salivary glands also a watery serous fluid, and a thicker mucous. iii. Tongue contributes to speech, manipulation of food for chewing and swallowing, and contains taste receptors.
1. Taste buds are microscopic, and reside on the sides of large projections on the tongue called papillae, which are visible to the naked eye. 2. Bolus formed by the tongue and palate, it passes into the pharynx when swallowed; it then passes into the esophagus on its way to the stomach. (These tubes also secrete mucous for lubrication). iv. Pharynx passageway for air and food; commonly called the throat. Fig. 15.5. 1. Swallowing begins with voluntary muscle contractions within the pharynx. a. As the bolus moves down the pharynx, stretch receptors there trigger the swallowing reflex involuntary muscle contractions (peristalsis) within the lower pharynx and esophagus. i. This prevents the bolus from entering the nose, and causes the epiglottis to close; you can t breathe when swallowing. b. When the bolus reaches the end of the esophagus, it passes through the inferior esophageal (cardiac) sphincter, and enters the stomach. Fig. 15.6. III. Stomach muscular expandable sack. Fig. 15.7. a. Stores and mixes food (segmentation). i. There are three layers of smooth muscle within the muscularis externa of the stomach: 1. Inner oblique layer. 2. Middle circular layer. 3. Outer longitudinal layer. b. Secretes enzymes and chemicals that chemically break down the food, liquefying it. c. Controls the passage of food into the small intestine. d. The epithelium of the stomach contains many gastric glands, which secrete: i. Hydrochloric acid (HCl). ii. Mucus. iii. Pepsinogen (precursor of pepsin, which enzymatically breaks down proteins). iv. Intrinsic factor (a protein needed for vitamin B 12 absorption in the small intestine). v. The substances above, along with water, make up gastric juice. e. Gastric juice is highly acidic, which dissolves food, kills some bacteria and some parasites, and causes heartburn. Food + gastric juice = chyme. i. The acidity denatures proteins, exposing their peptide bonds so they can be chemically broken by enzymes; ph is about 2. ii. The acidity also converts pepsinogen to the enzyme pepsin, which breaks peptide bonds. iii. Accumulating protein fragments trigger secretion of gastrin (in the stomach and small intestine), a hormone that stimulates additional secretion of pepsinogen and HCl. iv. Safeguards against autodigestion of the stomach: 1. A thick layer of alkaline mucous is secreted by the gastric glands. 2. Pepsinogen is inactive until it mixes with HCl within the lumen. 3. Only small amounts of digestive juice are produced when food is not present. 4. Stomach lining replaces itself every 3 days. f. Little absorption of food molecules occurs in the stomach; exceptions include alcohol, aspirin, and certain drugs. g. The stomach empties by peristaltic waves. i. It periodically squirts small amounts of chyme through the pyloric sphincter and into the small intestine. ii. This takes 2-6 hours, depending on how much food is in the stomach, and the kind of food present. IV. Small intestine. Fig. 15.8. a. Is about 20 feet long. b. Is subdivided into three segments: i. Duodenum. About 1 foot long. ii. Jejunum. About 8 feet long.
iii. Ileum. About 12 feet long. c. Digestion within the small intestine depends on secretions from the small intestine itself, as well as the pancreas, liver, and gallbladder. i. Enzymes secreted by the small intestine itself include: 1. Maltase breaks the disaccharide maltose into 2 glucose molecules (glucose is a monosaccharide). 2. Sucrase breaks the disaccharide sucrose into the monosaccharides glucose and fructose. 3. Lactase breaks the disaccharide lactose into the monosaccharides glucose and galactose. d. Chyme entering the duodenum stimulates the pancreas to release pancreatic juice, which contains: Fig. 15.10. i. Bicarbonate ions (HCO 3 - ) to buffer the acidic chyme (HCl) exiting the stomach, so pancreatic enzymes can function. ii. Mucous. iii. Enzymes that break down carbohydrates, fats, proteins, and nucleic acids. 1. Pancreatic amylase breaks polysaccharides into disaccharides. 2. Trypsin breaks down proteins and polypeptides down into smaller peptide fragments. 3. Chymotrypsin breaks down proteins and polypeptides down into smaller peptide fragments. 4. Carboxypeptidase breaks polypeptides into amino acids. 5. Lipase breaks triglycerides into fatty acids and glycerol. e. The liver secretes bile; bile is stored in the gallbladder when food is not present in the duodenum, and is secreted into the duodenum when food is present. i. Bile emulsifies fats, exposing a greater surface area to digestive enzymes. ii. Cholesterol is used in the formation of bile salts, and the liver also secretes cholesterol directly into the bile for elimination through the GI tract. V. Absorption in the small intestine. a. Each day, approximately 9 L of fluids are secreted by the digestive organs, which enters the small intestine; 95% of these fluids are re-absorbed along with the nutrients, water, salts, and vitamins released by digestion. b. The vast majority of nutrients are absorbed through the small intestine. Fig. 15.8. i. The lumen of the small intestine has a large surface area that is specialized for absorption. 1. Circular folds (plicae circulars). 2. Villi. 3. Microvilli (brush border). ii. Plasma membrane proteins of the brush border actively transport the following substances directly into the epithelial cells, through the extracellular fluid, and into the blood vessels: Fig. 15.9. 1. Monosaccharides. 2. Amino acids. 3. Mineral ions. 4. Water also crosses by osmosis. iii. Bile salts emulsify fats, and combine with glycerol and fatty acids to form micelles. 1. When micelles contact the epithelial cells, lipids within them diffuse into the cells. 2. There, fatty acids and glycerol recombine to form triglycerides. 3. The triglycerides then combine with cholesterol, phospholipids, and proteins to form chylomicrons within the cell. 4. Chylomicrons leave the cell by exocytosis and enter a lacteal. 5. Lacteals (lymphatic vessels) eventually drain into the venous blood supply.
VI. Accessory Organs: Pancreas, Liver, and Gallbladder. Fig. 15.10. a. Pancreas. i. Is both and endocrine organ and an exocrine organ. 1. Endocrine functions: secretion of the hormones insulin and glucagon. a. Insulin lowers blood sugar levels b. Glucagon raises blood sugar levels. 2. Exocrine functions: secretion of mucous, buffers (bicarbonate ions), and the enzymes discussed above. ii. Table 15.2 summarizes relevant digestive enzymes. b. Liver functions in digestion. i. Produces bile. ii. Nutrient rich blood from the intestines passes through the hepatic portal system. Fig. 15.11. 1. Within the liver, excess blood glucose is removed and stored as glycogen. 2. Proteins are attached to lipids to form lipoproteins, which facilitate lipid transport within the blood. 3. Stores iron, vitamins A, D, E, K, B 12, and folate. 4. Removes toxins and drugs from the blood. 5. Converts amino groups to urea for excretion by the kidneys. c. Gallbladder stores and concentrates bile, and secretes bile for fat emulsification. Fig. 15.12. i. Secretion of the hormone cholecystokinin (CCK) by the duodenum stimulates contraction of the gallbladder, causing the release of bile into the duodenum. ii. Precipitation of crystals within the bile can lead to the formation of gallstones. VII. Large intestine. Fig. 15.13. a. Anything not absorbed by the small intestine moves into the large intestine. b. Leftovers are concentrated, salts are actively transported out of the lumen, and water follows by osmosis. i. About 90% of the water, sodium ions, and potassium ions that remain at this point get absorbed by the large intestine. c. Anything left is eliminated by the body as feces. i. 30% of feces (dry weight) is made up of bacteria (including E. coli, which produces fatty acids and some vitamins which we absorb). ii. The remainder is water and indigestible materials. 1. Dietary fiber = cellulose. d. The large intestine is about 5 feet long, and is subdivided into: i. Cecum. ii. Appendix (lots of lymphatic tissue). iii. Colon. 1. Ascending. 2. Transverse. 3. Descending. 4. Sigmoid. 5. Rectum. iv. Anal canal. e. When feces accumulate and distend the rectal wall, stretch receptors activate the defecation response, which is mediated by the nervous system. i. Two anal sphincters must relax for defecation to occur. 1. The internal anal sphincter is involuntarily controlled, and relaxes as part of the defecation reflex. 2. The external anal sphincter is under voluntary control. VIII. Nerves and Hormones in Digestion.
a. The nervous system controls production of saliva; salivation can begin with the mere thought, sight, or smell of food. i. Salivation is further stimulated by the flavor and presence of food in the mouth. b. Neural digestive reflexes associated with chewing cause the stomach to begin secreting gastric juice and mucous c. Stretching of the stomach and the presence of protein there stimulates the secretion of the hormone gastrin, which circulates through the blood, back to the stomach, to cause an increase in gastric juice production. d. The arrival of chyme in the small intestine causes local nervous reflexes to release enzymes from both the small intestine and pancreas; it also causes the small intestine to release hormones that stimulate the release of additional digestive enzymes from the pancreas and bile (under the influence of CCK). e. A large number of hormones are involved in digestion; in addition to the hormones discussed in the sections above, Table 15.5 lists some additional examples. Study suggestions for this chapter: In the textbook at the end of the chapter, the sections entitled 1) Highlighting the Concepts, 2) Recognizing Key Terms, and 3) Reviewing the Concepts are all good for you to gauge your comprehension and focus your study efforts.