The rectum is located at the end of the colon and is about 5 inches in length. The rectum is normally empty, except when stool is propelled by the upper colon into the rectum just prior to a bowel movement. At that time, stool is ready to be excreted through the anal canal. The anal canal has two muscular "valves", called the internal and external sphincters, through which the stool must pass. The sphincters allow us to retain stool until we are ready to have a bowel movement, at which time the sphincters relax, releasing the stool.
The large intestine , also known as the large bowel , is the last part of the gastrointestinal tract and of the digestive system in vertebrates. Water is absorbed here and the remaining waste material is stored as feces before being removed by defecation. The colon  is the largest portion of the large intestine, so many mentions of the large intestine and colon overlap in meaning whenever precision is not the focus. Most sources define the large intestine as the combination of the cecum , colon, rectum , and anal canal. In humans, the large intestine begins in the right iliac region of the pelvis , just at or below the waist , where it is joined to the end of the small intestine at the cecum, via the ileocecal valve. It then continues as the colon ascending the abdomen , across the width of the abdominal cavity as the transverse colon , and then descending to the rectum and its endpoint at the anal canal.
The rectum is a chamber that begins at the end of the large intestine, immediately following the sigmoid colon, and ends at the anus see also Overview of the Anus and Rectum. Ordinarily, the rectum is empty because stool is stored higher in the descending colon. Eventually, the descending colon becomes full, and stool passes into the rectum, causing an urge to move the bowels defecate. Adults and older children can withstand this urge until they reach a bathroom. Infants and young children lack the muscle control necessary to delay bowel movement.
It is usually patients not doctors who balk at the sight of a medical instrument being unwrapped. But the flatus tube, used to treat a condition of the sigmoid colon, is one no surgeon relishes. Inflammatory bowel disease such as ulcerative colitis and Crohn's disease may occur here.