The large intestine is about 1.5 m (5 feet) long and is characterized by the following components:
- The cecum is a dead‐end pouch at the beginning of the large intestine, just below the ileocecal valve.
- The appendix (vermiform appendix) is an 8‐cm (3‐inch) long, fingerlike attachment to the cecum that contains lymphoid tissue and serves immunity functions.
- The colon, representing the greater part of the large intestine, consists of four sections: the ascending, transverse, descending, and sigmoid colons. At regular distances along the colon, the smooth muscle of the muscularis layer causes the intestinal wall to gather, producing a series of pouches called haustra. The epithelium facing the lumen of the colon is covered with openings of tubular intestinal glands that penetrate deep into the thick mucosa. The glands consist of absorptive cells and goblet cells. The absorptive cells absorb water and the goblet cells secrete mucus. The mucus lubricates the walls of the large intestine to smooth the passage of feces.
- The rectum is the last 20 cm (8 inches) of the large intestine. The mucosa in the rectum forms longitudinal folds called anal columns.
The functions of the large intestine include the following:
- The anal canal, the last 3 cm (1 inch) of the rectum, opens to the exterior at the anus. An involuntary (smooth) muscle, the interior anal sphincter, and a voluntary (skeletal) muscle, the external anal sphincter, control the release of the feces through the anus.
- Peristalsis: Rhythmic contractions of the large intestine produce a form of segmentation called haustral contractions in which food residues are mixed and forced to move from one haustrum to the next. Peristaltic contractions produce mass movements of larger amounts of material.
- Bacterial digestion: Bacteria that colonize the large intestine digest waste products. They break down indigestible material by fermentation, releasing various gases. Vitamin K and certain B vitamins are also produced by bacterial activity.
- Absorption: Vitamins B and K, some electrolytes (Na + and Cl –), and most of the remaining water is absorbed by the large intestine.
- Defecation: Mass movement of feces into the rectum stimulates a defecation reflex that opens the internal anal sphincter. Unless the external sphincter is voluntarily closed, feces will be evacuated through the anus.