Defecation (colloquially known as number two, shitting, a bowel movement or bm) is the final act of digestion, by which organisms eliminate solid, semisolid, and/or liquid waste material (feces) from the digestive tract via the anus.

Humans expel feces with a frequency varying from a few times daily to a few times weekly.[citation needed] Waves of muscular contraction (known as peristalsis) in the walls of the colon move fecal matter (called faeces or feces) through the digestive tract towards the rectum. Undigested food may also be expelled this way, in a process called egestion.

Physiology[edit]

The rectum ampulla (anatomically also: ampulla recti) acts as a temporary storage facility for the unneeded material. As the rectal walls expand due to the material filling it, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. This urge to defecate arises from the reflex contraction of rectal muscles, relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external anal sphincter. If the urge is not acted upon, the material in the rectum is often returned to the colon by reverse peristalsis, where more water is absorbed and the faeces is stored until the next mass peristaltic movement of the transverse and descending colon. If defecation is delayed for a prolonged period the fecal matter may harden, resulting in constipation. If defecation occurs too fast, before excess liquid is absorbed, diarrhea may occur.[1]

When the rectum is full, an increase in intra-rectal 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 push the feces out of the rectum. The internal and external anal sphincters along with the puborectalis muscle allow the feces to be passed by muscles pulling the anus up over the exiting feces.[citation needed]

Defecation is normally assisted by taking a deep breath and trying to expel this air against a closed glottis (Valsalva maneuver). This contraction of expiratory chest muscles, diaphragm, abdominal wall muscles, and pelvic diaphragm exerts pressure on the digestive tract. Ventilation at this point temporarily ceases as the lungs push the chest diaphragm down to exert the pressure. Thoracic blood pressure rises and as a reflex response the