FUNCTION OF THE DIGESTIVE SYSTEM
All cells of the body require nutrients. These nutrients are derived from the intake of food that contains proteins, fats, carbohydrates, vitamins and minerals, and cellulose fibers and other vegetable matter of no nutritional value. The primary digestive functions of the GI tract are the following:
• To break down food particles into the molecular form for digestion
• To absorb into the bloodstream the small molecules pro-duced by digestion
• To eliminate undigested and unabsorbed foodstuffs and other waste products from the body
After food is ingested, it is propelled through the GI tract, coming into contact with a wide variety of secretions that aid in its digestion, absorption, or elimination from the GI tract.
The process of digestion begins with the act of chewing, in which food is broken down into small particles that can be swallowed and mixed with digestive enzymes. Eating—or even the sight, smell, or taste of food—can cause reflex salivation. Saliva is se-creted from three pairs of glands: the parotid, the submaxillary, and the sublingual glands. Approximately 1.5 L of saliva is secreted daily. Saliva is the first secretion that comes in contact with food. Saliva contains the enzyme ptyalin, or salivary amylase, which begins the digestion of starches (Table 34-1). Saliva also contains mucus and water, which help to lubricate the food as it is chewed, thereby facilitating swallowing.
Swallowing begins as a voluntary act that is regulated by a swal-lowing center in the medulla oblongata of the central nervous sys-tem. As food is swallowed, the epiglottis moves to cover the tracheal opening and prevent aspiration of food into the lungs. Swallowing, which propels the bolus of food into the upper esoph-agus, thus ends as a reflex action. The smooth muscle in the wall of the esophagus contracts in a rhythmic sequence from the upper esophagus toward the stomach to propel the bolus of food along the tract. During this process of esophageal peristalsis, the lower esophageal sphincter relaxes and permits the bolus of food to enter the stomach. Subsequently, the lower esophageal sphincter closes tightly to prevent reflux of stomach contents into the esophagus.
The stomach stores and mixes the food with secretions. It secretes a highly acidic fluid in response to the presence or anticipated in-gestion of food. This fluid, which may have a pH as low as 1, de-rives its acidity from the hydrochloric acid (HCl) secreted by theglands of the stomach. The function of this gastric secretion is two-fold: to break down food into more absorbable components Aorta and to aid in the destruction of most ingested bacteria. The stomach can produce about 2.4 L per day of these gastric secretions.Gastric secretions also contain the enzyme pepsin, which is important for initiating protein digestion. Intrinsic factor is also secreted by the gastric mucosa. This compound combines with dietary vitamin B12 so that the vitamin can be absorbed in the ileum. In the absence of intrinsic factor, vitamin B12 cannot be absorbed and pernicious anemia results. Peristaltic contractions in the stomach propel its contents toward the pylorus. Because large food particles cannot pass through the pyloric sphincter, they are churned back into the body of the stomach. In this way, food in the stomach is agitated mechanically and broken down into smaller particles. Food remains in the stomach for a variable length of time, from a half-hour to several hours, depending on the size of food particles, the composition of the meal, and other factors. Peristalsis in the stomach and contractions of the pyloric sphincter allow the partially digested food to enter the small intestine at a rate that permits efficient absorption of nutrients. This food mixed with gastric secretions is calledchyme. Hormones, neuroregulators, and local regulators found in the gastric secretions control the rate of gastric secretions and influence gastric motility (Table 34-2).
The digestive process continues in the duodenum. Secretions in the duodenum come from the accessory digestive organs—the pancreas, liver, and gallbladder—and the glands in the wall of the intestine itself. These secretions contain digestive enzymes and bile. Pancreatic secretions have an alkaline pH because of high concentrations of bicarbonate. This neutralizes the acid entering the duodenum from the stomach. The pancreas also secretes di-gestive enzymes, including trypsin, which aids in digesting pro-tein; amylase, which aids in digesting starch; and lipase, which aids in digesting fats. Bile (secreted by the liver and stored in the gallbladder) aids in emulsifying ingested fats, making them easier to digest and absorb.
The intestinal glands secrete mucus, hormones, electrolytes, and enzymes. The mucus coats the cells and protects the mucosa from injury by HCl. Hormones, neuroregulators, and local regulators found in these intestinal secretions control the rate of intestinal se-cretions and also influence GI motility. Intestinal secretions total approximately 1 L/day of pancreatic juice, 0.5 L/day of bile, and 3 L/day of secretions from the glands of the small intestine. Ta-bles 34-1 and 34-2 summarize the actions of digestive enzymes and GI regulatory substances.
Two types of contractions occur regularly in the small intestine: segmentation contractions and intestinal peristalsis. Segmentationcontractions produce mixing waves that move the intestinal con-tents back and forth in a churning motion. Intestinal peristalsis pro-pels the contents of the small intestine toward the colon. Both movements are stimulated by the presence of chyme.
Food, initially ingested in the form of fats, proteins, and carbo-hydrates, is broken down into absorbable particles (constituent nu-trients) by the process of digestion. Carbohydrates are broken down into disaccharides (eg, sucrose, maltose, galactose) and mono-saccharides (eg, glucose, fructose). Glucose is the major carbohy-drate that the tissue cells use as fuel. Proteins are broken down into amino acids and peptides. Ingested fats are emulsified into mono-glycerides and fatty acids. These smaller molecules are then ready to be absorbed. Chyme stays in the small intestine for 3 to 6 hours, allowing for continued breakdown and absorption of nutrients.
Small, finger-like projections called villi are present through-out the entire intestine and function to produce digestive enzymes as well as to absorb nutrients. Absorption is the primary function of the small intestine. Vitamins and minerals are not di-gested but rather absorbed essentially unchanged. Absorption be-gins in the jejunum and is accomplished by both active transport and diffusion across the intestinal wall into the circulation. Ab-sorption of different nutrients takes place at different locations in the small intestine. Iron and calcium absorption takes place in the duodenum. Fats, proteins, carbohydrates, sodium, and chloride are absorbed in the jejunum. Vitamin B12 and bile salts are ab-sorbed in the ileum. Magnesium, phosphate, and potassium are absorbed throughout the small intestine (Society of Gastroentero-logic Nursing and Associates, 1998).
Within 4 hours after eating, residual waste material passes into the terminal ileum and passes slowly into the proximal portion of the colon through the ileocecal valve. This valve, which is nor-mally closed, helps prevent colonic contents from refluxing into the small intestine. With each peristaltic wave of the small intes-tine, the valve opens briefly and permits some of the contents to pass into the colon.
Bacteria make up a major component of the contents of the large intestine. They assist in completing the breakdown of waste material, especially of undigested or unabsorbed proteins and bile salts. Two types of colonic secretions are added to the resid-ual material: an electrolyte solution and mucus. The electrolyte so-lution is chiefly a bicarbonate solution that acts to neutralize the end products formed by the colonic bacterial action. The mucus protects the colonic mucosa from the interluminal contents and also provides adherence for the fecal mass.Slow, weak peristaltic activity moves the colonic contents slowly along the tract. This slow transport allows efficient reabsorption of water and electrolytes, which is the primary purpose of the colon. Intermittent strong peristaltic waves propel the contents for con-siderable distances. This generally occurs after another meal is eaten, when intestine-stimulating hormones are released. The waste materials from a meal eventually reach and distend the rectum, usually in about 12 hours. As much as one fourth of the waste ma-terials from a meal may still be in the rectum 3 days after the meal was ingested.
Feces consist of undigested foodstuffs, inorganic materials, water, and bacteria. Fecal matter is about 75% fluid and 25% solid ma-terial. The composition is relatively unaffected by alterations in diet, because a large portion of the fecal mass is of nondietary ori-gin, derived from the secretions of the GI tract. The brown color of the feces results from the breakdown of bile by the intestinal bacteria. Chemicals formed by intestinal bacteria (especially indole and skatole) are responsible in large part for the fecal odor. Gases formed contain methane, hydrogen sulfide, and ammonia, among others. The GI tract normally contains approximately 150 mL of these gases, which are either absorbed into the portal circulation and detoxified by the liver or expelled from the rectum as flatus.
Elimination of stool begins with distention of the rectum, which reflexively initiates contractions of the rectal musculature and relaxes the normally closed internal anal sphincter. The in-ternal sphincter is controlled by the autonomic nervous system; the external sphincter is under the conscious control of the cere-bral cortex. During defecation, the external anal sphincter volun-tarily relaxes to allow colonic contents to be expelled. Normally, the external anal sphincter is maintained in a state of tonic con-traction. Thus, defecation is seen to be a spinal reflex (involving the parasympathetic nerve fibers) that can be inhibited voluntarily by keeping the external anal sphincter closed. Contracting the ab-dominal muscles (straining) facilitates emptying of the colon. The average frequency of defecation in humans is once daily, but the frequency varies among individuals.