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Defecation is the final act of digestion by which organisms eliminate solid, semisolid 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. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum. Undigested food may also be expelled this way; this process is called egestion.
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 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.
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.
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 exert pressure on the digestive tract. Ventilation at this point temporarily ceases as the lungs push the chest diaphragm down in order to exert the pressure. Thoracic blood pressure rises and as a reflex response the amount of blood pumped by the heart decreases. Death has been known to occur in cases where defecation causes the blood pressure to rise enough to cause the rupture of an aneurysm or to dislodge blood clots (see ]thrombosis]). Also, in release of the Valsalva maneuver blood pressure falls, this coupled often with standing up quickly to leave the toilet results in a common incidence of blackouts in this situation.
When defecating, the external sphincter muscles relax. The anal and urethal sphincter muscles are closely linked, and experiments by Dr. Harrison Weed at the Ohio State University Medical Center have shown that they can only be contracted together, not individually, and that they both show relaxation during urination[How to reference and link to summary or text]. This explains why defecation is frequently accompanied with urination.
Defecation may be involuntary or under voluntary control. Young children learn voluntary control through the process of toilet training. Once trained, loss of control causing fecal incontinence may be caused by physical injury (such as damage to the anal sphincter that may result from an ]episiotomy]), intense fright, inflammatory bowel disease, impaired water absorption in the colon (see diarrhea), and psychological or neurological factors.
In certain cases, residual neurological impulses can cause defecating. For example, when men were hanged at the gallows, they were made to wear certain clothing in order to prevent the feces from spreading.
The anus and buttocks may be cleansed with toilet paper, similar paper products, or other absorbent material. In some cultures water is used (e.g. as with a ]bidet]) either in addition or exclusively. In Japan and South Korea, some toilets known as ]washlets] are designed to wash and dry the anus of the user after defecation (see anal cleansing).
- Main article: Defecation posture
The positions and modalities of defecation are culture-dependent. The natural and instinctive method used by all primates, including humans, is the squatting position. Squat toilets are still used by the vast majority of the world, including most of Africa, Asia and the Middle East. The widespread use of sit-down toilets in the Western World is a recent development, beginning in the Nineteenth Century with the advent of indoor plumbing.
Bockus Gastroenterology, the standard textbook on the subject, states:
"The ideal posture for defecation is the squatting position, with the thighs flexed upon the abdomen. In this way the capacity of the abdominal cavity is greatly diminished and intra-abdominal pressure increased, thus encouraging expulsion ..."
- Kira A. The Bathroom. Harmondsworth: Penguin, 1976, revised edition, pp.115,116.
- A History of Technology, Vol.IV: The Industrial Revolution, 1750-1850. (C. Singer, E Holmyard, A Hall, T. Williams eds) Oxford Clarendon Press, pps. 507-508, 1958
- Bockus. Gastroenterology. p. 754 2nd ed. Saunders, Philadelphia and London, 1964
- Deeb, Benjamin (2004). "Healthy to the Core: How to Measure Effective Defecation." Greenwood Press.
- Widmaier, Raff, Strang (2006). "Vanders Human Physiology, the mechanisms of body function. Chapter 15. McGraw Hill.
Digestive system, physiology: gastrointestinal physiology
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