Structure and Function of the Digestive System

Large Intestine

The large intestine is primarily responsible for absorbing water from undigested materials coming from the small intestine and eliminating the waste as feces.

The major function of the large intestine is to absorb water from undigested material and form and eliminate the feces. It also houses bacteria that synthesize biotin, B12, and K vitamins, which can be absorbed by the lining of the large intestine. The large intestine receives contents from the small intestine through the ileocecal sphincter, a ring of smooth muscle that when relaxed, allows contents to pass. Most of the nutrients and water have been absorbed by the time the digestive material reaches the large intestine. The amount of water absorbed determines the consistency of the feces. The walls of the large intestine lack villi, the projections that increase surface area, and do not secrete many enzymes, unlike the cells of the small intestine. The large intestine does contain many goblet cells, mucus-secreting columnar cells found in the epithelium. The mucus provides a protective layer to the intestinal lining against acids and gases and facilitates the movement of feces.

The longitudinal muscles of the large intestine are arranged in three bands called teniae coli. From their normal muscle tone, the teniae coli pull the lining of the large intestine to form pouches called haustra. As the haustra fill, the muscular layer contracts approximately every 25 minutes to move the material to the next haustra. Parts of the large intestine include the cecum, appendix, colon, rectum, and anus. The large intestine begins in the lower right quadrant of the abdominal cavity with a large sac-like structure called the cecum, inferior to where it meets the small intestine via the ileocecal valve. Protruding from the cecum is the appendix, a narrow pocket of lymphatic tissue that houses bacteria and contains tissues to help with immunity. The colon travels up from the cecum to the superior region of the abdominal cavity as the ascending colon, crosses the abdomen as the transverse colon, and travels down the abdomen on the left side as the descending colon. The colon then extends to the pelvic cavity as the sigmoid colon where it enters the rectum. The large intestine terminates at the anal canal and feces is eliminated through two anal sphincters, the internal anal sphincter and the external anal sphincter. During defecation, the internal anal sphincter opens involuntarily and the external anal sphincter is under voluntary control.
The large intestine is the primary site of water absorption and feces production. The amount of water absorbed determines the consistency of the feces. Goblet cells facilitate the movement of feces and protect the intestinal lining from acids and gases.