small intestine(redirected from intestinum tenue)
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intestine, muscular hoselike portion of the gastrointestinal tract extending from the lower end of the stomach (pylorus) to the anal opening. In humans this fairly narrow (about 1 in./2.5 cm) tubelike structure winds compactly back and forth within the abdominal cavity for about 23 ft (7 m), and is known as the small intestine. It is not only an organ of digestion (for that part of the process not completed by the stomach) but is the chief organ of absorption. By contraction of its muscular walls (peristalsis) the food mass is propelled onward and, as it is carried along, it is subject to the digestive action of the secretions of the intestinal lining as well as to that of bile and pancreatic juice which enter the upper intestine (duodenum) from ducts leading from the liver and pancreas. Innumerable minute projections (villi) in the intestinal mucous lining absorb the altered food for distribution by the blood and lymphatic systems to the rest of the body. Food continues to pass into the middle (jejunum) and end (ileum) of the small intestines.
The small intestine joins the large intestine, or colon, at the cecum in the right lower abdominal cavity. Here, also, is the appendix, a blind pouch projecting from the cecum. The large intestine is wider in diameter. Its direction as it leaves the cecum is upward (ascending colon), across the abdominal cavity (transverse colon) beneath the stomach, and then downward (descending colon) on the left side of the abdominal cavity, making a sharp turn in the left lower portion (sigmoid) to merge with the rectum. In all, the large intestine is about 5 ft (1.5 m) long. Bacteria, the indigestible residue of food, and mucus form the bulk of matter in the large intestine. The water content of the bulk is absorbed through the walls of the large intestine, and the solid matter is excreted through the rectum.
See digestive system.
the portion of the gut of vertebrate mammals, including man, that is situated between the stomach and the large intestine. The final digestion of food, the absorption of nutrients, and the movement of chyme occur in the small intestine.
In mammals, including man, the small intestine is divided into three sections: the duodenum, the jejunum, and the ileum. The length of the small intestine differs according to the nature of the digestive process: in man it is 6–7 m long, in cattle 27–49 m, in dogs 1.8–4.8 m, in cats 0.8–1.3 m, and in swine 16–21 m. In man the diameter of the small intestine is 48 mm in the initial section and 27 mm at the place where the ileum empties into the large intestine.
The walls of all sections of the small intestine are composed of mucous, submucous, and serous membranes and a muscular coat. Each section varies in the microscopic and submicroscopic characteristics of the intestinal wall, which reflect differences in function. The surface area of the small intestine’s mucous membrane is greatly increased by folds and protuberances: examples are the spiral valve in the mucous membrane of some fishes, and the villi, folds, and crypts in the small intestine of birds, mammals, and man.
Food is digested both in the intestinal cavity itself and by means of membrane-mediated (parietal) digestion. Cavitary digestion is achieved by the flow of pancreatic juice, bile, and intestinal juice into the lumen of the small intestine. During this process, supramolecular aggregates and the large molecules of proteins, carbohydrates, and fats in the food mass are hydrolyzed by the proteolytic, amylolytic, and lipolytic enzymes in the small intestine. The products of intermediate hydrolysis are adsorbed on the surface of the mucous membrane of the small intestine, where the final stages of hydrolysis and transition to absorption take place.
Parietal digestion is achieved by means of enzymes that are structurally bound to the cell membranes of the intestinal epithelium. The digestion and absorption of food in the small intestine are accompanied by contractions of the intestinal walls that mix and stir the chyme and propel it through the intestine.
IA. L. KARAGANOV