Why does it hurt when you are "holding it in" when needing to use the restroom?

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In the adult human, the process of defecation, or the defecation cycle, is normally a combination of both voluntary and involuntary processes. The defecation cycle is the interval of time between the completion of one defecation, and the completion of the following defecation. At the start of the cycle, the rectum ampulla (anatomically also: ampulla recti) acts as a temporary storage facility for the unneeded material. As additional fecal material enters the rectum, the rectal walls expand. A sufficient increase in fecal material in the rectum causes stretch receptors from the nervous system located in the rectal walls to trigger the contraction of rectal muscles, relaxation of the internal anal sphincter and an initial contraction of the skeletal muscle of the external sphincter. The relaxation of the internal anal sphincter causes a signal to be sent to the brain indicating an urge to defecate. http://en.wikipedia.org/wiki/Defecation Jun 05 09, 11:34 PM