This muscular tube is made up of the ascending colon, the transverse colon and the descending colon which ends at the rectum and the anus. The colon's most important job is to store, process and get rid of waste. The colon also absorbs some nutrients and water. Key to this process are the hundreds maybe thousands of. A children's educational website with educational information on how your body works both inside and outside. 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 function of the anus. These are the internal anal sphincter and the external anal sphincter, which are circular muscles that normally maintain constriction of the orifice.