Anus

"The anus is the end of the large intestine, below the rectum, through which stool (solid waste) leaves the body. The anus is formed partly from the outer skin layers of the body and partly from the intestine. Two ring-like muscles, called sphincter muscles, open and close the anal opening to let stool pass out of the body. The anal canal, the part of the anus between the rectum and the anal opening, is about 1½ inches long.
The skin around the outside of the anus is called the perianal area. Tumors in this area are skin tumors, not anal cancer."

Hemorrhoids

Hemorrhoids or haemorrhoids, are vascular structures in the anal canal which help with stool control. They become pathological or piles when swollen or inflamed. In their physiological state, they act as a cushion composed of arterio-venous channels and connective tissue. Hemorrhoids are very common, especially during pregnancy and after childbirth. They result from increased pressure in the veins of the anus. The pressure causes the veins to swell, making them painful, particularly when you are sitting. The most common cause is straining during bowel movements. Hemorrhoids may be caused by:
1. Straining during bowel movements
2. Constipation
3. Sitting for long periods of time
4. Anal infections
5. Certain diseases, such as liver cirrhosis
6. Hemorrhoids may be inside or outside the body.
7. Internal hemorrhoids occur just inside the anus, at the beginning of the rectum.
8. External hemorrhoids occur at the anal opening and may hang outside the anus.