Anus

The anus (also called the anal opening) is the external opening at the end of the digestive tract (the digestive tract is the long tube through which food passes from the mouth to the outside). It is located at the center of the buttocks, between the two muscles of the buttocks. It is where the intestines and gases produced by digestion exit.

The anus is connected to the rectum, which is the final part of the large intestine that stores stool until it is passed. The short passage between the rectum and the external anal opening is called the anal canal, which is only several centimeters long.

Structure

The anal canal is surrounded by two rings of muscle called sphincters. The internal sphincter is made of involuntary muscle, meaning it works automatically. The external sphincter is voluntary muscle that a person can control through conscious effort. The coordination of these two muscles determines when stool is passed.

The tissue surrounding the anus has small folds that allow it to stretch. The upper surface of the canal contains permanent ridges called the columns of Morgagni, and a boundary called the dentate line, which separates the lower part with high sensitivity from the upper part with lower sensitivity.

Function

The primary function of the anus is to control the passage of stool. When stool fills the rectum, the walls stretch and send a signal to the brain, where a person can decide to hold it in or let it out. This system of control is something children learn in early life.

Health conditions

Common conditions affecting the anus include hemorrhoids, which are swollen veins; anal fissures, which are tears in the tissue that cause pain and bleeding; and anal warts. Anal cancer can also occur, and is associated with certain types of HPV. Persistent pain or bleeding typically requires medical evaluation.