Intestine, also bowels, in higher animals, the portion of the digestive tract between the stomach and anus. In humans the intestine is divided into two major sections: the small intestine, which is about 6 m (20 ft) long, where the most extensive part of digestion occurs and where most food products are absorbed; and the large intestine, which has a larger diameter and is about 1.5 m (5 ft) long, where water is absorbed and from which solid waste material is excreted (see Digestive System; Feces).
The small intestine, which is coiled in the center of the abdominal cavity (see Abdomen), is divided into three sections. The upper portion includes the pylorus, the opening at the lower part of the stomach, through which the contents of the stomach pass into the duodenum. The duodenum is a horseshoe-shaped section surrounding part of the pancreas and the pancreatic duct, as well as ducts from the liver and gall bladder that open into it. The middle part of the small intestine, extending from the duodenum to the ileum, is called the jejunum, and the terminal portion is the ileum, which leads into the side of the first part of the large intestine, the cecum. The lining membrane, or mucosa, of the small intestine is especially suited for the purpose of digestion and absorption. The mucosa is folded; the folds are covered with minute mucosal projections called villi. Each villus is a small tube of epithelium surrounding a small lymphatic vessel, or lacteal, and many capillaries. Tiny glandular pits, called the crypts of Lieberkühn, open at the bases of the villi; these pits secrete the enzymes necessary for intestinal digestion. Digested carbohydrates and proteins pass into the capillaries of the villi and then to the portal vein, which enters the liver; digested fats are absorbed into the lacteals in the villi, and they are transported through the lymphatic system into the general bloodstream. The lining of the small intestine also secretes a hormone called secretin, which stimulates the pancreas to produce digestive enzymes.
The large intestine is divided into the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum. The cecum is a swollen sac located in the lower right-hand portion of the abdominal cavity; it is very large in herbivorous animals. The two important parts of the cecum in humans are the vestigal vermiform appendix (see Appendicitis), which often becomes diseased; and the ileocecal valve, a membranous structure between the cecum and the small intestine that regulates the passage of food material from the small intestine to the large intestine and also prevents the passage of toxic waste products from the large intestine back into the small intestine. The ascending colon rises along the right side of the abdominal cavity; the transverse colon runs across the body to the left side, where the descending colon travels downward. The sigmoid colon is the S-shaped portion of the large intestine as it enters the pelvic cavity. The rectum, about 15 cm (6 in) long, is the almost straight, terminal portion of the large intestine. At the exit of the rectum, called the anus, is a round muscle, the anal sphincter, that closes the anus. The large intestine has a smooth mucosal lining (only the rectum has folds) that secretes mucus to lubricate the waste materials.
Food and waste material are moved along the length of the intestine by rhythmic contractions of intestinal muscles; these contractions are called peristaltic movements. The entire intestine is held in place in the abdominal cavity by membranes called mesenteries.
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