WebMD provides a medical diagram of the anus and explains its function and problems that can affect the anus.
The human anus is the external opening of the rectum. Two sphincters control the exit of feces from the body during an act of defecation, which is the primary.
Most causes of a swollen anus are temporary and harmless, but some require medical attention. Depending on the cause of the swelling, people may have additional symptoms, such as pain, itching, burning, or bleeding around the anus. Damage to the lining of the anus or overstretching.
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.