DIGESTIVE SYSTEM, VERTEBRATE

any of the systems used by vertebrates for the process of digestion. Although the account here is based primarily on the human digestive system, other vertebrates are discussed as they illustrate important departures through specialization. The human digestive system consists of (1) the digestive tract, or the series of structures and organs through which food passes during its processing into forms absorbable into the bloodstream and also the structures through which solid wastes pass in the process of elimination, and (2) other organs that contribute juices necessary for the digestive process. The human digestive system as seen from the front. The digestive tract (Figure 1) begins at the lips and ends at the anus. It consists of the mouth, or oral cavity, with its teeth, for grinding the food, and its tongue, which serves to knead the food, mix it with saliva, and start it on its way to the stomach; the throat, or pharynx; the esophagus, or gullet; the stomach; the small intestine, consisting of the duodenum, the jejunum, and the ileum; and the large intestine, consisting of the cecum, a closed-end sac connecting with the ileum, the ascending colon, the transverse colon, the descending colon, and the sigmoid colon, which terminates in the rectum. Glands contributing digestive juices include the salivary glands, the gastric glands in the stomach lining, the pancreas, and the liver and its adjunctsthe gallbladder and bile ducts. Additional reading William T. Keeton, James L. Gould, and Carol Grant Gould, Biological Science, 4th ed. (1986), is a comprehensive study that includes an examination of digestion. Charles J. Flickinger et al., Medical Cell Biology (1979); and Walter Hoppe et al. (eds.), Biophysics (1983; originally published in German, 1977), examine the structure of cells and cell membranes, the molecular mechanics of peptides, and the function of enzymes. Specialized studies include H.J. Vonk and J.H.R. Western, Comparative Biochemistry and Physiology of Enzymatic Digestion (1984); Thomas H. Wilson, Intestinal Absorption (1962); D.H. Smyth (ed.), Intestinal Absorption, 2 vol. (1974); B.P. Babkin, Secretory Mechanisms of the Digestive Glands, 2nd rev. ed. (1950); A.S.V. Burgen and N.G. Emmelin, Physiology of the Salivary Glands (1961); Arnold V. Wolf, Thirst (1958); Geoffrey H. Bourne and George W. Kidder (eds.), Biochemistry and Physiology of Nutrition, 2 vol. (1953); E.F. Annison and D. Lewis, Metabolism in the Rumen (1959); and J.B. Jennings, Feeding, Digestion, and Assimilation in Animals, 2nd ed. (1972). Chapters on digestion can be found in such comprehensive texts as Henry Gray, Anatomy of the Human Body, 30th American ed., edited by Carmine D. Clemente (1985); B.I. Balinsky, An Introduction to Embryology, 5th ed. (1981); and Leslie Brainerd Arey, Developmental Anatomy, 7th rev. ed. (1974). Frank H. Netter, The Digestive System: A Compilation of Paintings on the Normal and Pathologic Anatomy, 3 vol. (195762), is a part of the Ciba Collection of Medical Illustration series. Specialized works include Leonard R. Johnson et al. (eds.), Physiology of the Gastrointestinal Tract, 2nd ed. (1981); Horace W. Davenport, Physiology of the Digestive Tract, 5th ed. (1982); Charles F. Code (ed.), Alimentary Canal, 5 vol. (196768); John Morton, Guts: The Form and Function of the Digestive System, 2nd ed. (1979); R.J. Last, Anatomy, Regional and Applied, 7th ed. (1984); Alfred Sherwood Romer and Thomas S. Parsons, The Vertebrate Body, 6th ed. (1986); C. Ladd Prosser (ed.), Comparative Animal Physiology, 3rd ed. (1973); J.A. Colin Nicol, The Biology of Marine Animals, 2nd ed. (1967); and Robert D. Barnes, Invertebrate Zoology, 4th ed. (1980). Anatomy and physiology Organ function Mouth and oral structures Little digestion of food takes place in the mouth; however, food is prepared in the mouth for transport through the upper alimentary canal, thus aiding the digestive processes that take place in the stomach and small intestine. Mastication, or chewing, is the first mechanical process to which food is subjected. Movements of the lower jaw in chewing are brought about by the muscles of mastication (the masseter, the temporal, the medial and lateral pterygoids, and the buccinator). The sensitivity of the peridontal membrane that surrounds and supports the teeth, rather than the power of the muscles of mastication, limits the force of the bite. Mastication is not essential for adequate digestion and nutrition. Chewing does aid digestion, however, by reducing food to small particles and mixing it with the saliva secreted by the salivary glands. The saliva lubricates and moistens dry food, while chewing distributes the saliva throughout the food mass. The movement of the tongue against the hard palate and the cheeks helps to form a rounded mass, or bolus, of food. Salivary glands The oral cavity has other functions in addition to those associated with mastication. It is there that food is tasted and mixed with saliva secreted by several sets of salivary glands. The saliva dissolves some of the food and acts as a lubricant, facilitating passage through the subsequent portions of the digestive tract. The saliva of some mammals, including humans, also contains a starch-digesting enzyme called amylase (ptyalin), which initiates the process of enzymatic hydrolysis; it splits starch (a polysaccharide containing many sugar molecules bound in a continuous chain) into molecules of the double sugar maltose. Many carnivores, such as dogs and cats, have no amylase in their saliva; hence their natural diet contains very little starch. The salivary glands are controlled by the two divisions of the autonomic nervous system (sympathetic and parasympathetic), and it is generally held that this innervation is exclusively responsible for regulation of the glands' secretory activity. No hormone appears to be involved. Normally secretion of saliva is constant, regardless of the presence of food in the mouth. When something touches the gums, the tongue, or some region of the mouth lining, or when chewing occurs, the amount of saliva secreted increases. The stimulating substance need not be fooddry sand in the mouth or even moving the jaws and tongue when the mouth is empty are effective in increasing the salivary flow. This coupling of direct stimulation to the oral mucosa with increased salivation is known as the unconditioned salivary reflex. When an individual learns that a particular sight, sound, smell, or other stimulus is regularly associated with food, that stimulus alone may suffice to stimulate increased salivary flow. This response is known as the conditioned salivary reflex. A variety of drugs are capable of increasing or decreasing salivary flow. Two types of drugs that increase the flow of saliva are sympathomimetic agents, or drugs that produce effects similar to those elicited by the sympathetic nervous system (e.g., epinephrine [adrenaline], norepinephrine [noradrenaline], and amphetamine), and parasympathomimetic agents, or drugs that mimic the effects of the parasympathetic nervous system (e.g., acetylcholine and pilocarpine). Among the drugs that decrease salivary flow are antagonists of epinephrine and norepinephrine (e.g., ergotamine and dibenzylchlorethamine) and antagonists of acetylcholine (e.g., atropine and scopolamine). The composition of saliva varies, but its principal components are water, inorganic ions similar to those commonly found in the blood plasma, and a number of organic constituents. The amount of saliva secreted by a human in 24 hours usually amounts to one to 1.5 litres. Although saliva is slightly acidic, the bicarbonates and phosphates contained within it serve as buffers and maintain the pH, or hydrogen ion concentration, of saliva relatively constant under ordinary conditions. The concentrations of bicarbonate, chloride, and sodium in saliva are directly related to the rate of flow. There is also a direct relation between the bicarbonate concentration and the partial pressure of carbon dioxide in the blood. The concentration of chloride varies from five millimoles per litre at low flow rates, to 70 millimoles per litre when it is high. The sodium concentrations in similar circumstances vary from five millimoles per litre to 100 millimoles per litre. The concentration of potassium is often higher than that in the blood plasma, up to 20 millimoles per litre, which accounts for the sharp and metallic taste of saliva when flow is brisk. Organic constituents of saliva consist of salivary proteins, free amino acids, specific blood group substances, and the enzymes lysozyme and amylase. Glucose is normally absent from saliva even in individuals with diabetes. The main functions of saliva are to initiate the digestion of starch, to keep the mucous membranes of the mouth moist, and to facilitate speech. The constant flow of saliva keeps the oral cavity and teeth comparatively free from food residues, sloughed epithelial cells, and foreign particles. By removing material that may serve as culture media, saliva inhibits the growth of bacteria. Lysozyme serves a protective function, for it has the ability to lyse, or dissolve, certain bacteria. Taste is mediated by chemical mechanisms, and substances must be in solution for the taste buds to be stimulated. Saliva provides the solvent for food materials. The secretion of saliva also provides a mechanism whereby certain organic and inorganic substances can be excreted from the body, including mercury, lead, potassium iodide, bromide, morphine, ethyl alcohol, and certain antibiotics, such as penicillin, streptomycin, and chlortetracycline. Although saliva is not essential to life, its absence results in a number of inconveniences, including dryness of the oral mucous membrane, poor oral hygiene because of bacterial overgrowth, a greatly diminished sense of taste, and difficulties with speech. Features of the gastrointestinal tract General features of digestion and absorption There are four means by which digestive products are absorbed: active transport, passive diffusion, facilitated diffusion, and endocytosis. Active transport involves the movement of a substance across the membrane of the absorbing cell against an electrical or chemical gradient. It is carrier-mediated, that is, the substance is temporarily bound to another substance that transports it across the cell membrane, where it is released. The process requires energy and is at risk of competitive inhibition by other substances; that is, other substances with a similar molecular structure can compete for the binding site on the carrier. Passive diffusion requires neither energy nor carrier; the substance merely passes along a simple concentration gradient from an area of high concentration of the substance to an area of low concentration, until a state of equilibrium exists on either side of the membrane. Facilitated diffusion also requires no energy, but it involves a carrier, or protein molecule located on the outside of the cell membrane that binds the substance and carries it into the cell. The carrier may be competitively inhibited. Endocytosis takes place when the material to be absorbed, on reaching the cell membrane, is enfolded into it. That part is then pinched off into the cell interior. This process is similar to phagocytosis. Absorption of all food by the small intestine occurs principally in the jejunum; however, the duodenum, although the shortest portion of the small intestine, has an extremely important role. The duodenum receives not only chyme saturated with gastric acid, but pancreatic and liver secretions as well. It is in the duodenum that the intestinal contents are rendered isotonic with the blood plasma; i.e., the pressures and volumes of the intestinal contents are the same as those of the blood plasma, so that the cells on either side of the barrier will neither gain nor lose water. The bicarbonate secreted by the pancreas neutralizes the acid secreted by the stomach. This brings the intestinal contents to the optimal pH, allowing the various enzymes to act on their substrates at peak efficiency. A number of important gastrointestinal hormones regulate gastric emptying, gastric secretion, pancreatic secretion, and contraction of the gallbladder. These hormones, along with neural impulses from the autonomic nervous system, provide for autoregulatory mechanisms for normal digestive processes. Most salts and minerals, as well as water, are readily absorbed from all portions of the small intestine. Twelve to 25 grams of sodium are present in the average daily diet. The sodium is absorbed by an active process, and the necessary metabolic energy is provided by the epithelial cells of the mucosa of the small intestine. Sodium is moved from the lumen of the intestine across the mucosa against a concentration gradient (i.e., a progressive increase in the concentration of sodium) and an electrochemical gradient (i.e., a gradual increase in the concentration of charged ions). Sodium ions are absorbed more readily from the jejunum than from other parts of the small intestine. Potassium is absorbed at about 5 percent of the rate of sodium. It is thought that potassium moves across the intestinal mucosa passively or by facilitated diffusion as a consequence of water absorption. Chloride is readily absorbed in the small intestine and probably takes place as a consequence of sodium absorption. The absorption of water appears to be secondary to the absorption of electrolytes (substances that dissociate into ions in a solution). Water absorption occurs throughout the small intestine, though chiefly in the jejunum. The upper small intestine absorbs approximately 95 percent of a 50-gram sample within 10 minutes. Water moves freely across the intestinal mucosa both ways, but tends to move in the direction of the hypertonic solution (the solution into which a net flow of water occurs), and away from the hypotonic solution (one from which a net flow of water occurs). Thus, if the contents of the lumen are hypotonic, water moves rapidly from the lumen to the blood. If the contents of the intestinal lumen are hypertonic, water moves more rapidly from the blood into the lumen. This two-way movement of water tends to maintain the intestinal contents in an isotonic state. Specific features of digestion and absorption Carbohydrates Carbohydrates are absorbed as monosaccharides (simple sugars that cannot be further broken down by hydrolysis) or disaccharides (carbohydrates that can be hydrolyzed to two monosaccharides). The absorption of glucose and galactose is dependent on the presence of sodium and uses active transport to move against a concentration gradient. Amylose, a starch polysaccharide (a carbohydrate that contains many monosaccharides), accounts for 20 percent of dietary carbohydrate. It consists of a straight chain of glucose (sugar) molecules bound to their neighbours by oxygen links. The bulk of the starch is amylopectin, which has a branch chain linked in after every 25 molecules of glucose on the main chain. Only a small amount of starch is digested by salivary amylase; most is rapidly digested in the duodenum by pancreatic amylase. But even this enzyme has little effect on the branch chains of amylopectin, and even less on the linkages in cellulose molecules. This accounts for the inability of humans to break down cellulose. Pancreatic amylase changes amylose to maltose (a disaccharide) and the amylopectins to dextrins. In the brush border (comprising ultrafine microvilli) and the surface membrane of the epithelial enterocytes are the disaccharidase enzymes, lactase, maltase, sucrase, and trehalase, which hydrolyze maltose and the dextrins to the monosaccharides glucose, galactose, and fructose. Fructose appears to be absorbed by simple diffusion, but glucose and galactose are transported by an energy-using process, probably binding it to a specific protein carrier with attached sodium ions; the sugar is released inside the enterocyte, sodium is pumped out, and the sugars then diffuse into the circulation down a concentration gradient.