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Large intestine, posterior section of the vertebrate intestine, consisting typically of four regions: the cecum, colon, rectum, and anal canal (qq.v.). The large intestine is wider and shorter than the small (in man, approximately 5 feet, or 1.5 metres, in length as compared to 23 feet for the small intestine) and has a smooth inner wall. In the proximal, or upper, half of the large intestine, enzymes from the small intestine complete the digestive process, and bacteria produce the B vitamins: Bb2, thiamin, and riboflavin, as well as vitamin K. The large intestine’s primary function, however, is absorption of water and electrolytes from digestive residues (which in man usually takes about 24-30 hours) and storage of fecal matter until it can be expelled. Churning movements of the intestine gradually expose digestive residue to the absorbing walls. A progressive and more vigorous type of movement known as peristalsis, which occurs only two to three times daily, propels the material toward the anus.
In animals that are primarily vegetarian the large intestine is usually longer. The immature frog (or tadpole), for example, eats mainly plant matter and has a long, highly coiled large intestine. As the frog matures and begins to eat mostly insects, its intestine becomes considerably shorter.
High-protein food, such as meat, can readily be digested by the small intestine; it requires much more chemical action and agitation, however, to reduce the tough cellulose fibres of plant cells. The large intestine performs this function with its slow digestive process. Birds have specialized areas in the large intestine called the coeea. These are two saclike structures that are thought to absorb water; most other digestive functions of birds occur in the small intestine. Vitamin Bb2 helps rabbits digest food. As it is produced only in the large intestine, the animal must eat its fecal material so that the vitamin can help with further digestion.
In newborn human beings, the large intestine does not contain the bacteria essential for production of vitamin K, lack of which may cause hemorrhaging (excessive bleeding). Infant diets should be supplemented with this vitamin for a few weeks until the infant is capable of producing its own supply. Common afflictions of the large intestine include inflammation,.such as colitis, and abnormal growths such as diverticulosis and benign or malignant tumours.
