The Rectum and Anus:
Anatomy and functions of the rectum and anus:
The anus is the terminal portion of the large intestine, extending from the sigmoid colon to the anal canal. The anus is expandable, conforming to the size and shape of the fecal content, and it normally varies from 1.2 to 3.5 cm in diameter during the act of defecation. The anus is surrounded by the subcutaneous muscle which aids in defecation.
The rectum (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 may result.
When the rectum is full the increase in 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.
Common disorders of the rectum and anus:
| University of Missouri Health Care | This information is not intended to replace the advice of a doctor. |
