The anus is an orifice between the cheeks of the buttocks which empties from the rectum.
The anus is a sphincter, a muscle that closes an orifice when it is at rest. In order to open the anus, it must be contracted. This can be done voluntarily, but can also happen involuntarily if the rectum is full of feces or gas.
Several veins run through the anus, and can be damaged when the sphincter is overly strained. This gives rise to hemorrhoids (which are effectively varicose veins), which can become inflamed or infected by the passage of feces.
Bleeding from the anus is a serious symptom and requires medical attention as soon as possible.