The anus is the opening at the end of the digestive tract where stool leaves the body. The rectum is the section of the digestive tract above the anus where stool is held before it passes out of the body through the anus.

The anus is formed partly from the surface layers of the body, including the skin, and partly from the intestine. The rectal lining consists of glistening red tissue containing mucus glands—much like the rest of the intestinal lining. The lining of the rectum is relatively insensitive to pain, but the nerves from the anus and nearby external skin are very sensitive to pain.

The veins from the rectum and anus drain into the portal vein, which leads to the liver, and then into the general circulation. The lymph vessels of the rectum drain into lymph nodes in the lower abdomen. The lymph vessels of the anus drain into the lymph nodes in the groin.

A muscular ring (anal sphincter) keeps the anus closed. This sphincter is controlled subconsciously by the autonomic nervous system (see Biology of the Nervous System: Autonomic Nervous System). However, the lower part of the sphincter can be relaxed or tightened at will.

Disorders of the anus and rectum include anal cancer (see Tumors of the Digestive System: Anal Cancer), anal fissure, anal itching, anorectal abscess, anorectal fistula, foreign objects in the anus and rectum, hemorrhoids, levator syndrome, pilonidal disease, proctitis, colorectal cancer (see Tumors of the Digestive System: Colorectal Cancer), and rectal prolapse. To diagnose disorders of the anus and rectum, a doctor inspects the skin around the anus for any abnormality. With a gloved finger, the doctor probes the rectum. For women, this is often done along with a manual examination of the vagina.

Next, a doctor looks into the anus and rectum with a 3- to 10-inch (about 7- to 25-centimeter) rigid viewing tube (anoscope or proctoscope). A longer, flexible tube (sigmoidoscope—see Diagnosis of Digestive Disorders: Endoscopy) may then be inserted so that the doctor can observe as much as 2 or more feet of the large intestine. An anoscopy or sigmoidoscopy is generally uncomfortable but not painful. However, if the area in or around the anus is painful because of an abnormal condition, the doctor may give a local, regional, or even general anesthetic before proceeding with the examination. Sometimes a cleansing enema to rid the lower part of the large intestine of stool is given before sigmoidoscopy. Tissue and stool samples for microscopic examination and cultures may be obtained during sigmoidoscopy. A barium enema x-ray may also be done.

Last full review/revision May 2012 by Parswa Ansari, MD