Rectum

The rectum is the final portion of the large intestine in some mammals, and the gut in others, terminating in the anus.

Medical procedures

For the diagnosis of certain ailments, a rectal exam may be done. Suppositories may be inserted into the rectum as a route of administration for medicine.

The rectum in human defecation

The rectum ampulla acts as a temporary storage facility 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. If defecation is delayed for a prolonged period constipation and hardened feces results.

When the rectum is full 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 and peristaltic waves propel the feces out of the rectum. The internal and external sphincters of the anus allow the feces to be passed by muscles pulling the anus up over the exiting feces.

See defecation for a fuller treatment.

Muscular anatomy of the human pelvis and rectum

The Levatores ani constrict the lower end of the rectum and vagina. They elevate and invert the lower end of the rectum after it has been protruded and everted during the expulsion of the feces. They are also muscles of forced expiration. The Coccygei pull forward and support the coccyx, after it has been pressed backward during defecation or parturition. The Levatores ani and Coccygei together form a muscular diaphragm which supports the pelvic viscera.

Sexual Stimuli

Due to the interfaced proximity of the anterior wall of the rectum to the vagina in females or to the prostate in males and the shared nerves thereof, stimuli can be resultant from phallic intrusion through the anus or disruption of some other form, including though ever so uncommon in adult humans, defecation itself. As a human sexual behavior, often this intrusion takes the form of anal intercourse.

See also: Gastrointestinal tract