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GUTAbbrev. for grand unified theory. See fundamental forces.
in most animals, the digestive tube that starts with the mouth opening and ends with the anal opening; in organisms with a differentiated digestive tract, the section that comes after the stomach, called the intestine.
Morphology. The primitive gut forms during the gastrula stage of embryonic development as a cul-de-sac that communicates with the external environment through the primitive mouth, or blastopore. In most protostomes (all worms, mollusks, arthropods), the blastopore develops into the mouth of the adult animal. The anal opening forms at the site of the blastopore in echinoderms, chaetognaths, certain other invertebrates, and all chordates (that is, deuterostomes). The mouth develops anew at the opposite end of the body. The primitive gut is preserved virtually unchanged only in coelenterates (hydroid polyps and medusae); in higher coelenterates (anthozoans, scyphozoans, and ctenophores) and flatworms there is a foregut in addition to the primitive gut (usually called the midgut). A hindgut also develops in nemertines, roundworms, and representatives of all species of invertebrates that have an anal opening. In vertebrates and many higher invertebrates the gut is differentiated into specially organized sections. The digestive glands in some of these groups (arthropods, mollusks) are connected with the gut.
The digestive tube in all vertebrates is differentiated into several sections (in lower vertebrates, the boundaries between these divisions are not always clearly pronounced): the oral cavity; the pharynx, or gullet; and the gut, consisting of the foregut (esophagus and stomach), the midgut, or small intestine, and the hindgut, or large intestine (ending in the cloaca or anus). The gut wall consists mostly of smooth muscle and is innervated by the sympathetic nervous system and sensory spinal nerves. The efferent ducts of two large digestive glands, the liver and pancreas, empty into the initial division of the small intestine (called the duodenum in terrestrial vertebrates). The intestinal mucosa contains a large number of small digestive glands that secrete intestinal juice. The surface of absorption of the intestine in the lower fishes is increased by the formation of the spiral valve; in higher fishes and terrestrial vertebrates, both by the greater length of the intestine itself (forming loops) and by the formation of pyloric appendages and a system of smaller folds; in birds and mammals, similarly and, additionally, by the formation of numerous protrusions from the mucous membrane called villi.
The division of the intestine into small and large sections is found as early evolutionarily as many fishes; this division is even more pronounced in amphibians and reptiles which have an outpouching, or cecum, at the juncture between the small and large intestines. In birds, the large intestine is usually very short and furnished with two ceca. In mammals, the intestine is quite long and clearly differentiated. The initial section (the midgut, or small intestine), subdivided into the duodenum, the jejunum, and ileum, has numerous loops. It is separated from the following division (the hindgut, or large intestine) by an annular fold. The hindgut, which is especially long in herbivores, consists of the large intestine (with a cecum) and the rectum. In some rodents, carnivores, all lemuroids, and anthropoid apes, the end of the cecum forms the vermiform appendix. The rectum is not sharply demarcated from the large intestine. It ends in a cloaca in monotremes and in an anus in viviparous animals.
In man, the intestine is the part of the digestive tube from the outlet of the stomach to the anus. It consists of a small and large intestine. In the small intestine, the duodenum is distinguished from the mesentery that joins the jejunum to the ileum. The jejunum and ileum form loops in the middle abdomen and, partly, in the cavity of the lesser pelvis. Unlike the duodenum, the jejunum and ileum are movable, since they are suspended from the mesentery and invested with peritoneum. The duodenum begins at the pylorus and, bending around the head of the pancreas, passes into the mesenteric area at the level of the second lumbar vertebra. In the region of the right iliac fossa the small intestine passes into the large intestine, which consists of the cecum along with the vermiform appendix, the colon, and the rectum. The cecum is situated below the site of entry of the small intestine. Its continuation, the ascending colon, rises to the undersurface of the liver and curves into the transverse colon. The transverse colon forms the splenic flexure in the left hypochondrium and becomes the descending colon. The descending colon passes into the S-shaped sigmoid colon at the level of the left ilium. The sigmoid colon passes into the rectum in the cavity of the lesser pelvis.
There are four sheaths in the walls of the intestine: the mucous membrane, which lines the intestine from within; the tunica submucosa, which consists of areolar tissue; the muscular layer, which consists of an external, longitudinal layer of smooth muscle and an internal, circular layer of smooth muscle; and the serous membrane, or peritoneum. The mucous membrane is covered with epithelium and contains muscle plate, or myotome.
The structure of the intestinal walls varies from section to section. For example, the presence of microscopic digestive glands and absorptive apparatus, or villi, is characteristic of the small intestine. In the duodenal wall, some of the tubular glands are heavily branched. Efferent ducts of the liver and pancreas empty into the duodenal lumen. Masses of lymphoid tissue are scattered within the mucosal layer (especially of the ileum) as nodes called Peyer’s patches (lymphoid follicles and follicle aggregates). Layers of smooth muscle cells are evenly distributed in the muscular sheath of the small intestine; the circular layer is thicker. The mucous membrane of the large intestine forms numerous semilunar folds and long crypts. The inner layer of the muscular coat is continuous; the outer layer is divided into three bands stretching along the intestine. The serous membrane has a number of evaginations consisting of clusters of fatty tissue covered with mesothelium.
All of the intestinal layers contain blood and lymphatic vessels. The intestine is supplied with blood by branches of the aorta (the celiac and mesenteric arteries). Venous blood leaves the intestine through the mesenteric veins. The lymphatic vessels in the intestinal wall discharge lymph into the mesenteric nodes and from the nodes into the thoracic duct. The intestine is supplied with sympathetic innervation by the mesenteric, celiac, and hypogastric plexuses and with parasympathetic innervation by the vagus and pelvic nerves.
IA. L. KARAGANOV
Physiology. The intestine is the site of the main processes involved in breaking down the biopolymers of food, absorbing its organic and inorganic components, and absorbing most of the water, salts, and other substances in the digestive juices. The intestine also plays an important role in interstitial metabolism. The enzymic hydrolysis of proteins, fats, carbohydrates, and nucleic acids is carried out in the small intestine. Supramolecular aggregates and large molecules are hydrolyzed by enzymes in the pancreatic juice and the juice of the intestinal (Brunner’s) glands. Bile plays an important part in breaking down lipids. Food is further hydrolyzed by enzymes (carbohydrases, peptidases, esterases, Upases, nucleotidases, phosphatases) that are structurally bound to the membranes of the epithelial cells.
The composition of the enzymes depends on the kind of food to be digested. For example, the intestinal mucosa of a nursing infant contains lactase, which is necessary for breaking down milk sugar. Almost all of the enzymes are concentrated near the brush border formed by microvilli on the surface of the membranes of the intestinal epithelial cells, where they carry out so-called membranous, or parietal, digestion to support the intermediate and final stages of hydrolysis and the beginnning of absorption. The active transport of the final products of digestion across the intestinal cell membranes becomes a decisive factor in absorption.
About 80–90 percent of the peptide and glycoside bonds are hydrolyzed in the small intestine. It is in the small intestine too that amino acids and monosaccharides are absorbed and that triglycérides are broken down. The triglycérides are absorbed as monoglycerides, diglycerides, and fatty acids (the long-chain acids in the intestinal mucosa, once again esterified, enter the lymph; most of the short-chain acids, which are not resynthe-sized, enter the blood). The digestive and absorptive processes do not function at the same rates in different portions of the small intestine because of the uneven distribution of the enzymes involved in the cavitary stage of digestion (that is, distribution from the beginning to end of the small intestine) and because of the uneven distribution of the enteral enzymes responsible for membranous digestion (these, in the crypt-to-villus direction as well).
The small intestine’s digestive function is closely related to its barrier function, which protects the body against unassimilated polymers and oligomers (including antigens). The barrier function is virtually absent in newborns. In adult animals and man, the wall of the small intestine is impermeable to large molecules (the effective radius of the pores of the intestinal membranes being about 4 angstroms) and has, in addition, both an enzyme layer that hydrolyzes polymers and oligomers and an external mucopolysaccharide layer that creates a diffusion barrier.
In the large intestine, the digestive processes have a secondary role, although digestive enzymes are present. The dominant process is the reverse absorption (reabsorption) of water, minerals, and the organic component of the stomach contents (chyme). Electrolytes, glucose, and water (about 95 percent) and some of the vitamins and amino acids produced by intestinal flora are absorbed in the large intestine. Fecal matter is formed as the contents move forward and condense. The intestinal flora normally inactivates the enzymes passing to the large intestine from the small intestine. On the other hand, endogenous amino acids, proteins, phospholipids, and certain other compounds enter the intestinal cavity and are reabsorbed (interstitial metabolism).
The normal operation of all of the processes occurring in the intestine is due largely to the contractions of the smooth muscles, which mix the food and secretions, bring the chyme into contact with the inner surface of the intestine, and pass it forward. The automatism of the intestinal contractions is neurogenic in nature and dependent on intramural nerve plexuses. The rhythm of the contractions is determined by rhythm “pacemakers” located in the duodenal wall.
The movements of the small intestine are divided into peristalsis, pendular movement, and rhythmic segmentation. Peristalsis is accomplished by coordinated contractions of the longitudinal and transverse muscles at a wave rate of 1 cm/sec, periodically accelerating to 2–25 cm/sec. Antiperistaltic movements sometimes arise in the proximal portions of the small intestine, causing juices from the duodenum to be ejected into the stomach. Pendular movements and rhythmic segmentation ensure the mixing of the intestinal contents.
Intestinal function is regulated by nervous and humoral mechanisms. Some hormones, especially those of the hypophisis and adrenals, influence the synthesis of intestinal enzymes, absorption, secretion, and motor function. Hormones manufactured by cells of the small intestine (chiefly of the duodenum) also participate in the regulation of intestinal activity. When irritated or injured, certain divisions of the central nervous system and some nerve fibers alter the secretion of the intestinal juice and the absorptive process. The vagus nerves stimulate intestinal motor activity, and the sympathetic nerves inhibit it.
A variety of methods are used to study the intestine, including fluoroscopy (with the introduction of barium into the intestine as a contrast medium). The rectum is examined visually through a special instrument called a proctoscope. Enzymes and intestinal contents are studied by means of intubation. Specimens of the mucosa can be obtained by biopsy and studied histochemically, biochemically, and microscopically. The content of food substances and ordinary and tagged metabolites in the blood and feces can also be determined by load methods. In animal experiments, a portion of the intestine can be isolated and either or both of its ends drawn outside the body. External anastomoses (separate intestinal segments joined through special tubes), multiple fistulas, and angiostomy are other methods in current use. Research is also carried out on isolated segments of the intestine and its mucous membrane and on individual intestinal cells.
REFERENCESBabkin, B. P. Sekretornyi mekhanizm pishchevaritel’nykh zhelez. Leningrad, 1960.
Bogach, P. G. Mekhanizmy nervnoi reguliatsii motornoi funktsii tonkogo kishechnika. Kiev, 1961.
Ugolev, A. M. Pristenochnoe (kontaktnoe) pishchevarenie. Leningrad-Moscow, 1963.
Ugolev, A. M. Fiziologiia i patologiia pristenochnogo (kontaktnogo) pishchevareniia. Leningrad, 1967.
Shlygin, G. K. Fermenty kishechnika v norme i patologii. Leningrad, 1967.
Bockus, H. L. Gastroenterology, 2nd ed., vol. 2. Philadelphia-London, 1964.
Handbook of Physiology, vols. 2–4. Washington, D.C., 1967–68.
A. M. UGOLEV, N. N. IEZUITOVA, and N. M. TIMOFEEVA