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 Autonomic nervous system). However, the lower part of the sphincter can be relaxed or tightened at will.