What is the rectum?

Quick Answer
The pouch at the end of the gastrointestinal tract that collects and stores food wastes just before defecation.
Expert Answers
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Structure and Functions

The rectum is the pouch at the end of the gastrointestinal tract, between the sigmoid colon and the anal canal. It is about 6 inches (15 centimeters) in length, dilated slightly at the end (the rectal ampulla), and lined by an orange-red tissue with mucous glands, much like the colon.

The rectum remains empty until just before defecation, when wastes pass into it from the colon. Three lateral folds in the rectum, called Houston’s valves, hold the feces in place until a sufficient quantity is collected and the walls of the rectum are distended. Nerve receptors in the walls detect the stretching and prompt the urge to defecate. Waves of muscular contraction, known as peristaltic waves, move the feces into the anal canal and through the anus.

Defecation depends in part on a person’s conscious relaxation of the anus in response to the defecation urge. If the urge is resisted, then the feces may return to the colon, where water may be absorbed from them. If defecation is postponed repeatedly, the feces may harden and cause constipation.

Disorders and Diseases

Like the anus, the rectum is subject to a variety of malformations caused by infection or injury. The rectum may develop an abscess—a cavity in its wall that is filled with pus—when bacteria infest a mucous gland. The abscess may open a channel, or fistula, to another part of the rectum, the anus, or an entirely different organ, such as the vagina; fistulas can also be the product of other diseases, such a diverticulitis or injury to the rectum. When the rectum is made to protrude through the anus and outside the body, usually from straining at stool, the condition is called rectal prolapse. Proctitis is inflammation of the rectal lining from a variety of infectious diseases, such as ulcerative colitis or a sexually transmitted disease.

About a quarter of colorectal cancers, the most common of the gastrointestinal tract, occur in the rectum, about equally in men and women. They are adenocarcinomas, growing in the lining of the rectum, and if untreated often spread to the liver or less commonly to the lungs and bones. Rectal cancer usually grows slowly and can be surgically removed.

Bibliography

Ahuja, Nita, and Brenda S. Nettles. Johns Hopkins Medicine Patients' Guide to Colon and Rectal Cancer. Burlington, Ma.: Jones and Bartlett Learning, 2014.

Beers, Mark H., ed. The Merck Manual of Medical Information: Second Home Edition. London: Pocket Books, 2004.

Keshav, Satish. The Gastrointestinal System at a Glance. 2d ed. Oxford: Wiley-Blackwell, 2013.

Levin, Bernard, et al., eds. American Cancer Society’s Complete Guide to Colorectal Cancer. Atlanta: American Cancer Society, 2006.

Parker, Steve. The Human Body Book. 2d ed. New York: Dorling Kindersley, 2013.

Thibodeau, Gary A., and Kevin T. Patton. Structure and Function of the Human Body. 14th ed. St. Louis, Mo.: Mosby/Elsevier, 2012.