Anus is a tube, about 40 mm long, which leads from the rectum through the circular muscle (sphincter) to the anal opening.
Healthy people after their early childhood normally manage a sphincter, keeping it closed, or opening it when the stool is released. The structure and function of this part of the digestive tract are relatively simple, and it rarely involves the problems. The only common illness are hemorrhoids.
One of the causes of the problems in anus is a food with a small fiber content, because the stool after these food are small and hard and can damage the walls of the sphincter during passage. Additionally, any somewhat difficult passage of the stool presenta a strain on the anus. Furthermore, hemorrhoids are often the result of over-preoccupation (in many people) with a stool; namely, many often force the muscles of the sphincter to the actions they would normally do by themselves if we left them to perform their normal function.