Rectum

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.: 397 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 connects with the sigmoid colon at the level of S3, and connects with the anal canal as it passes through the pelvic floor muscles.: 397 Supports of the rectum include: 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. For the diagnosis of certain ailments, a rectal exam may be done. These include faecal impaction, prostatic cancer and benign prostatic hypertrophy in men, faecal incontinence, and internal haemorrhoids.: 179–180