End segment of the large intestine (see digestion) in which feces accumulate just prior to discharge. It is 5–6 in. (13–15 cm) long and lined with mucous membrane. One set of muscles separates it from the anal canal; another shortens it to expel feces. The rectal walls distend as feces enter, which stimulates the urge for defecation.
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The rectum intestinum 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 becomes 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 sphincter allow the feces to be passed by muscles pulling the anus up over the exiting feces.
Dosimetric evaluation of rectum and bladder using image-based CT planning and orthogonal radiographs with ICRU 38 recommendations in intracavitary brachytherapy.(Original Article)(International Commission on Radiation Units)(computed tomography)
Jan 01, 2008; Byline: Swamidas. Jamema, Sherly. Saju, Umesh. Mahantshetty, S. Pallad, D. Deshpande, S. Shrivastava, K. Dinshaw The purpose is...