Anatomy Of Fecal Continence - Fecal Incontinence: Bladder Conditions

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Anatomy of fecal continence

You rely on your digestive system to process the food you eat, to absorb vital nutrients into the bloodstream, and then to remove waste products and indigestible food components from your body. By the time food has traveled through your lengthy digestive tract to the end of your large intestine, or colon, it is made up of waste material that loses water and solidifies as it slowly moves along.

At the end of your digestive tract are areas crucial to fecal continence: the rectum and the anal canal. Muscles in the colon propel feces into the rectum, which has expandable walls to hold the stool. Two areas of muscle surround the last inch of the rectum, called the anal canal. The internal anal sphincter, which is not under your conscious control, stays contracted most of the time to prevent leakage. Surrounding it, the external anal sphincter is made of voluntary muscle that is also contracted most of the time.

How does your body know when to release stool? When stretch-detecting nerve endings in the rectum detect that it is full, the internal sphincter opens briefly and lets a tiny bit of the rectum's contents come in contact with the external sphincter, which is rich with nerve endings. In a rapid "sampling reflex," these nerves inform the brain about whether the rectal contents are intestinal gas or liquid or solid stool, allowing you to act accordingly. You might allow gas to escape, look for a bathroom right away if you have diarrhea, or decide whether to have a bowel movement or wait for a more convenient time or place.

To delay, you constrict (usually without thinking about it) the external anal sphincter tighter to hold feces inside. When attempting to hold in stool, you also contract the puborectalis muscle, a pelvic floor muscle that loops around the rectum. When contracted, it pulls the rectum so that it lies at a 90-degree angle to the anal canal rather than more directly above it, thus countering the pull of gravity. Contracting these muscles often curbs the urge to defecate, which returns when more feces enter the rectum.

Once in the bathroom, you relax the puborectalis muscle and both the internal and external sphincters. To propel the stool downward, you may increase pressure in the abdominal cavity through the Valsalva or "bearing down" maneuver — closing off the airway, tightening the abdominal muscles, and pushing the diaphragm down.

Last updated:	September 05, 2008