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Digestive System

Anatomy of the Digestive System

1. The digestive system consists of the alimentary canal (a hollow tube extending from mouth to anus) and several accessory digestive organs (Figure 14.1). The wall of the alimentary canal has four main tissue layers mucosa, submucosa, muscularis externa, serosa (Figure 14.2). The serosa (visceral peritoneum) is continuous with the parietal peritoneum, which lines the abdominal cavity wall.

2. Organs of the alimentary canal:

   1. The mouth or oral cavity contains teeth and tongue and is bounded by lips, cheeks, and palate. Tonsils guard its posterior margin.

   2. The pharynx is a muscular tube that provides a passageway for food and air.

   3. The esophagus is a muscular tube that completes the passageway from the pharynx to the stomach.

   4. The stomach is a C-shaped organ located on the left side of the abdomen beneath the diaphragm (Figure 14.3). Food enters it through the cardio-esophageal sphincter and leaves it to enter the small intestine through the pyloric sphincter. The stomach has a third oblique layer of muscle in its wall that allows it to perform mixing or churning movements. Gastric glands produce hydrochloric acid, pepsin, renin, mucus, gastrin, and intrinsic factor. Mucus protects the stomach itself from being digested.

   5. The tubelike small intestine is suspended from the posterior body wall by the mesentery. Its subdivisions are the duodenum, jejunum, and ileum (Figure 14.4). Food digestion and absorption are completed here. Pancreatic juice and bile enter the duodenum through a sphincter at the distal end of the bile duct. Microvilli, villi, and circular folds increase the surface area of the small intestine for enhanced absorption.

   6. The large intestine frames the small intestine. Subdivisions are the cecum; appendix; ascending, transverse, and descending colon; sigmoid colon; rectum; anal canal (Figure 14.5). The large intestine delivers undigested food residue (feces) to the body exterior.

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3. Many accessory organs duct substances into the alimentary tube (Figure 14.6).

   1. The pancreas is a soft gland lying in the mesentery between the stomach and small intestine. Pancreatic juice contains enzymes (pancreatic amylase: acts on starch; trypsin: acts on protein; lipase: acts on lipids) in an alkaline fluid.

   2. The liver is a four-lobed organ overlying the stomach. Its digestive function is to produce bile, which it ducts into the small intestine.

   3. The gallbladder is a muscular sac that stores and concentrates bile. When fat digestion is not occurring, the continuously made bile backs up the cystic duct and enters the gallbladder.

   4. Salivary glands (three pairs - parotid, submandibular, sublingual) secrete saliva into the oral cavity. Saliva contains mucus and serous fluids. The serous component contains salivary amylase.

4. Two sets of teeth are formed. The first set consists of 20 deciduous teeth that begin to appear at 6 months and are lost by 12 years. Permanent teeth (32) begin to replace deciduous teeth around 7 years. A typical tooth consists of crown covered with enamel and root covered with cementum. Most of the tooth is bonelike dentin. The pulp cavity contains blood vessels and nerves.

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Functions of the Digestive System

1. Foods must be broken down to their building blocks to be absorbed. Building blocks of carbohydrates are simple sugars, or monosaccharides, Building blocks of proteins are amino acids. Building blocks of fats, or lipids, are fatty acids and glycerol.

2. Both mechanical (chewing) and chemical food breakdown begin in the mouth. Saliva contains mucus, which helps bind food together into a bolus, and salivary amylase, which begins the chemical breakdown of starch. Saliva is secreted in response to food in the mouth, mechanical pressure, and psychic stimuli. Essentially no food absorption occurs in the mouth.

3. Swallowing has two phases: The buccal phase is voluntary; the tongue pushes the bolus into the pharynx. The involuntary pharyngeal-esophageal phase involves the closing off of nasal and respiratory passages and the conduction of food to the stomach by peristalsis.

4. When food enters the stomach, gastric secretion is stimulated by vagus nerves and by gastrin (a local hormone). Hydrochloric acid activates the protein-digesting enzyme pepsin, and chemical digestion of proteins begins. Food is also mechanically broken down by the churning activity of stomach muscles. Movement of chyme into the small intestine is controlled by the enterogastric reflex.

5. Chemical digestion of fats, proteins, and carbohydrates is completed in the small intestine by intestinal enzymes and, more importantly, pancreatic enzymes. Alkaline pancreatic juice neutralizes acid chyme and provides the proper environment for the operation of enzymes. Both pancreatic Juice (the only source of lipases) and bile (formed by the liver) are necessary for normal fat breakdown and absorption. Bile acts as a fat emulsifier. Secretin and cholecystokinin, hormones produced by the small intestine, stimulate release of bile and pancreatic juice. Segmental movements mix foods; peristaltic movements move foodstuffs along the small intestine. Most nutrient absorption occurs by active transport into the capillary blood of the villi (Figure 14.7). Fats are absorbed by diffusion into both capillary blood and lacteals in the villi.

6. The large intestine receives bacteria-laden indigestible food residue. Activities of the large intestine are absorption of water and salts and of vitamins made by resident bacteria. When feces are delivered to the rectum by peristalsis and mass peristalsis. the defecation reflex is initiated.

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Nutrition and Metabolism

1. Metabolism includes all chemical breakdown (catabolic) and building (anabolic) reactions needed to maintain life (Figure 14.8).

2. Carbohydrates, most importantly glucose, are the body's major energy fuel. As glucose is oxidized, carbon dioxide, water, and ATP are formed. The sequential pathways of glucose catabolism are glycolysis, which occurs in the cytosol, and the Krebs cycle and electron transport chain (in the mitochondria). During hypergiycemia, glucose is stored as glycogen or converted to fat. In hypoglycemia, glycogenolysis, gluconeogenesis, and fat breakdown occur to restore normal blood glucose levels.

3. Fats insulate the body, protect organs, build some cell structures (membranes and myelin sheaths), and provide reserve energy. When carbohydrates are not available, more fats are oxidized to produce ATP. Excessive fat breakdown causes blood to become acidic. Excess dietary fat is stored in subcutaneous tissue and other fat depots.

4. Proteins form the bulk of cell structure and most functional molecules. They are carefully conserved by body cells. Amino acids are actively taken up from blood by tissue cells; those that cannot be made by body cells are called essential amino acids. Amino acids are oxidized to form ATP mainly when other fuel sources are not available. Ammonia, released as amino acids are catabolized, is detoxified by liver cells that combine it with carbon dioxide to form urea.

5. The liver is the body's key metabolic organ. Its cells remove nutrients from hepatic portal blood. It performs glycogenesis (glucose converted to and stored as glycogen), glycogenolysis (glycogen broken down into glucose), and gluconeogenesis (formation of glucose from proteins and fats) to maintain homeostasis of blood glucose levels. Its cells make blood proteins and other substances and release them to blood. Fats are burned by liver cells to provide some of their energy (ATP); excesses are stored or released to blood in simpler forms that can be used by other tissue cells. Phagocytic cells remove bacteria from hepatic portal blood. Most cholesterol is made by the liver; cholesterol breakdown products are secreted in bile. Fats and cholesterol are transported in the blood by lipoproteins. LDL's transport cholesterol to body cells; HDL's carry cholesterol to the liver for degradation. Cholesterol is used to make functional molecules and for some structural purposes; it is not used for energy.

6. A dynamic balance exists between energy intake and total energy output (heat + work + energy storage). Interference with this balance results in obesity or malnutrition leading to body wasting.

7. When the three major types of foods are oxidized for energy, they yield different amounts of energy, Carbohydrates and proteins yield 4 kcal/gram; fats yield 9 kcal/gram. Basal metabolic rate (BMR) is the total amount of energy used by the body when one is in a basal state. Age, sex, body surface area, and amount of thyroxine produced influence BMR.

8. Total metabolic rate (TMR) is number of calories used by the body to accomplish all ongoing daily activities. It increases dramatically as muscle activity increases. When TMR equals total caloric intake, weight remains constant.

9. As foods are catabolized to form ATP, more than 60 percent of energy released escapes as heat, warming the body. The hypothalamus initiates heat-loss processes (radiation of heat from skin and evaporation of sweat) or heat-promoting processes (vasoconstriction of skin blood vessels and shivering) as necessary to maintain body temperature within normal limits. Fever (hyperthermia) represents body temperature regulated at higher-than-normal levels.

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Developmental Aspects of the Digestive System and Metabolism

1. The alimentary tract forms as a hollow tube. Accessory glands form as outpocketings from this tube.

2. Common congenital defects include cleft palate, cleft lip, and tracheoesophageal fistula, all of which interfere with normal nutrition. Common inborn errors of metabolism are phenylketonuria (PKU) and cystic fibrosis.

3. Various inflammatory conditions plague the digestive system throughout life. Appendicitis is common in adolescents, gastroenteritis and food poisoning can occur at any time (given the proper irritating factors), ulcers and gallbladder problems increase in middle age. Obesity and diabetes mellitus are bothersome during later middle age.

4. Efficiency of all digestive system processes decreases in the elderly. Gastrointestinal cancers, such as stomach and colon cancer, appear with increasing frequency in an aging population.