The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about 12 centimetres (4.7 in) long, and begins at the rectosigmoid junction, the end of the sigmoid colon, at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used. Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal which is about 4 centimetres (1.6 in) long, before the gastrointestinal tract terminates at the anal verge. The word rectum comes from the Latin rectum intestinum, meaning straight intestine.
The rectum is a part of the lower gastrointestinal tract. The rectum is a continuation of the sigmoid colon, and connects to the anus. The rectum follows the shape of the sacrum and ends in an expanded section called the rectal ampulla, where feces are stored before their release via the anal canal. An ampulla is a cavity, or the dilated end of a duct, shaped like a Roman ampulla.
Unlike other portions of the colon, the rectum does not have distinct taeniae coli. The taeniae blend with one another in the sigmoid colon five centimeters above the rectum, giving rise to a layer of longitudinal muscle that surrounds the rectum on all sides for its entire length.
The rectum acts as a temporary storage site for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed from the feces. If defecation is delayed for a prolonged period, constipation and hardened feces results.
When the rectum becomes full (if the internal and external sphincters are relaxed) the increase in intrarectal pressure forces the walls of the anal canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal. Although peristalsis in the colon delivers material to the rectum, laxatives such as bisacodyl or senna that induce peristalsis in the large bowel do not appear to initiate peristalsis in the rectum. They induce a sensation of rectal fullness and contraction that frequently leads to defecation, but without the distinct waves of activity characteristic of peristalsis. The anal longitudinal muscle also participates in defecation by everting the anus.
By their definitions, suppositories are inserted, and enemas are injected, via the rectum. Both of these may be used for the delivery of drugs or to relieve constipation; enemas are also used for a variety of other purposes, medical and otherwise.
One cause of constipation is faecal impaction in the rectum, in which a dry, hard stool forms. Manual evacuation is the use of a gloved finger to evacuate faeces from the rectum, and, after the application of stool softeners, is utilised in acute constipation. It is also in the long-term management of neurogenic bowel, seen most frequently in people with a spinal cord injury or multiple sclerosis. Digital rectal stimulation, the insertion of one finger into the rectum, may be used to induce peristalsis in patients whose own peristaltic reflex is inadequate to fully empty the rectum.
In the context of mesenteric ischemia, the upper rectum is sometimes referred to as Sudak's point and is of clinical importance as a watershed region between the inferior mesenteric artery circulation and the internal iliac artery circulation via the middle rectal artery and thus prone to ischemia. Sudak's point is often referred to along with Griffith's point at the splenic flexure as a watershed region.