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External sphincter

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Defecation is body's method for removing waste, called feces, from the digestive tract. Defecation is usually voluntary for most. However, a number of factors can contribute to the loss of control, either by incontinence or extreme constipation.


Incontinence is the involuntary elimination of waste. Urinary incontinence is the inability to stop urination, and fecal incontinence is the inability to stop defecation. Fecal defecation can have a number of causes.

  • diarrhea
  • nerve damage
  • anal surgery
  • radiation
  • dementia
  • constipation


When constipated, feces is either difficult and uncomfortable to pass, or simply impassable. As seen above, constipation can cause anal leakage. This occurs when the dry, hard mass of feces in the rectum and colon stretch the intestines to the point that fluid waste from farther up the system leaks around the plug and out through the anus. Even should anal leaking not occur, constipation is still uncomfortable at best. At worst, it can prove fatal. Constipation can be caused by a number of things.

  • too much calcium or aluminum (such as from antacids)
  • dietary or activity changes
  • colon cancer
  • eating disorders
  • too much dairy
  • irritable bowel syndrome
  • nerve damage
  • inactivity
  • overuse of laxatives
  • pregnancy
  • underactive thyroid
  • stress
  • certain medications


The colon, or large intestine, extends from the cecum at the small intestine to the rectum. The inner wall of the colon consists of a mucous membrane that absorbs the fluids from the mostly liquid material received from the small intestine, and secretes mucous to lubricate the waste materials as the water is removed. Waste is pushed down the length of the colon by peristalsis. During peristalsis, the smooth muscles of the colon contract in sequence to from a peristaltic wave, which pushes waste down into the rectum.


The rectum is a muscular tube that temporarily holds feces as it exits the colon. As the rectum fills, it stretches out to accommodate the feces. Stretch receptors in the rectal walls stimulate the need to defecate. If the urge to defecate is not acted upon (voluntarily or involuntarily) the urge to defecate subsides and the feces is pushed back into the colon by reverse peristalsis. Should defecation occur, the pressure forces the walls of the anal canal apart and the rectum shortens and pulls back over the feces to expel it.


The anus is the terminal end of the anal canal. There are two muscular constrictors, the internal and external sphincters, that control the anus. As feces exits, the anus is drawn up over the passing waste. The muscles of the pelvic diaphragm does this pulling to prevent the anal canal from pushing out of the body.

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