Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Gastrointestinal System

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other forms of dietary fibers, and unabsorbed sugars. Fermentation of these residues is a major source of energy for the microorganisms in the colon. The met- abolic end point is the generation of short-chain fatty acids, which play a major role in intestinal epithelial cell growth and differentiation. Colonic microorganisms also play a role in vitamin synthesis and in absorption of calcium, magnesium, and iron. Vitamin K, for example, is synthesized by the intestinal flora. Lastly, the resident gut flora provides a crucial line of resistance to coloni- zation by exogenous microbes, and therefore is highly protective against invasion of tissues by pathogens. The administration of broad-spectrum antibiotics can dis- rupt the microbial balance and allow overgrowth of spe- cies with potential pathogenicity, such as Clostridium difficile (see Chapter 29). ■■ Digestion is the process of dismantling foods into their constituent parts, and absorption is the process of moving nutrients and other materials from the GI tract into the internal environment. ■■ Carbohydrates must be broken down into monosaccharides before they can be absorbed, a process that involves both salivary and pancreatic amylase and the brush border enzymes of the small intestine. ■■ Digestion of proteins begins in the stomach with the action of pepsin and continues in the small intestine facilitated by the action of the proteolytic pancreatic enzymes, which split the protein molecules into dipeptides, tripeptides, or amino acids that are absorbed through the wall of the intestine. ■■ Fats are emulsified by bile into small droplets, which are broken down by pancreatic lipase into triglycerides containing medium- and long-chain fatty acids. Bile salts form micelles that transport these substances to the surface of intestinal villi, where they are absorbed by lymphatics. ■■ Fluid not absorbed during movement of a meal through the small intestine is absorbed in the colon, and food residues are fermented by the intestinal flora, resulting in short-chain fatty acids, which support epithelial cell proliferation and differentiation, and protects the colonized host against invasion by pathogenic organisms. Colonic microorganisms also play a role in vitamin synthesis and in absorption of calcium, magnesium, and iron and the synthesis of vitamins, including vitamin K. SUMMARY CONCEPTS

Anorexia, Nausea, and Vomiting Anorexia, nausea, andvomitingarephysiologic responses common to many GI disorders. These responses are pro- tective to the extent that they signal the presence of dis- ease and, in the case of vomiting, remove noxious agents from the GI tract. They also can contribute to impaired intake or loss of fluids and nutrients. Anorexia Anorexia represents a loss of appetite. Several factors influence appetite. One is hunger, which is stimulated by contractions of the empty stomach. Appetite or the desire for food intake is regulated by the hypothalamus and other associated centers in the brain. Smell plays an important role, as evidenced by the fact that appetite can be stimulated or suppressed by the smell of food. Loss of appetite is associated with emotional factors, such as fear, depression, frustration, and anxiety. Many drugs and disease states cause anorexia. For example, in uremia the accumulation of nitrogenous wastes in the blood contributes to the development of anorexia. Anorexia often is a forerunner of nausea, and most con- ditions that cause nausea and vomiting also produce anorexia. Nausea and Vomiting Nausea is an ill-defined and unpleasant subjective sensa- tion. It stimulates the vomiting center in the brain stem and often precedes or accompanies vomiting. Anorexia usually precedes nausea, and stimuli such as foods and drugs that cause anorexia in small doses usually pro- duce nausea when given in larger doses. A common cause of nausea is distention of the duodenum or upper small intestinal tract. Nausea frequently is accompa- nied by autonomic nervous system manifestations such as watery salivation and vasoconstriction with pallor, sweating, and tachycardia. Nausea may function as an early warning signal of a pathologic process. As a basic physiologic protective mechanism, vomit- ing limits the possibility of damage from ingested nox- ious agents by emptying the contents of the stomach and portions of the small intestine. Nausea and vomit- ing may also represent a total-body response to drug therapy, including overdose, cumulative effects, toxicity, and side effects. The act of vomiting consists of taking a deep breath, closing the airways, and producing a strong, forceful contraction of the diaphragm and abdominal muscles along with relaxation of the gastroesophageal sphinc- ter. Respiration ceases during the act of vomiting. Vomiting may be accompanied by dizziness, lighthead- edness, a decrease in blood pressure, and bradycardia. Retching is the term used to describe the rhythmic spas- modic movements of the diaphragm, chest wall, and abdominal muscles without the expulsion of vomitus (dry heaves).