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Distinguish|Deification (disambiguation)About||sugar processing|Sugarcane|the death metal band|Defecation (band)Redirect|Poopy Pants|the Captain Underpants villain|Pippy PoopypantsCleanup|date=October 2010 Defecation (from late Latin defecatio ) is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material ( feces ) from the digestive tract via the anus . Waves of muscle|muscular contraction known as peristalsis in the walls of the colon (anatomy)|colon move fecal matter through the digestive tract towards the rectum . Undigested food may also be expelled this way in the process called egestion.
The defecation cycle
In the adult human, the process of defecation, or the defecation cycle, is normally a combination of both voluntary and involuntary processes with force. 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 rectal 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.
If this urge is not acted upon, the material in the rectum is often returned to the colon (anatomy)|colon by reverse peristalsis where more water is absorbed, thus temporarily reducing pressure and stretching within the rectum. The additional fecal material is stored in the colon 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 and Autolysis (biology)|autolyze , resulting in constipation .
Once the voluntary signal to defecate is sent back from the brain, the final phase of the cycle begins.The abdominal muscles contract (straining)causing the intra abdominal pressure to increase. the perineal wall is lowered causing the ano rectal angle to decrease from 90 deree to less than 15 degrees (almost straight)and the external anal sphincter relaxes. The rectum now contracts and shortens in peristalsis|peristaltic waves , thus forcing fecal material out of the rectum and out through the anal canal. The internal anal sphincter|internal and External anal sphincter|external anal sphincters along with the puborectalis muscle allow the feces to be passed by pulling the anus up over the exiting feces in shortening and contracting actions.
Muscular aspects
Defecation is often 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 (anatomy)|diaphragm , abdominal wall muscles, and pelvis|pelvic diaphragm exert pressure on the digestive tract.Citation needed|date=April 2010
Cardiovascular aspects
During defecation, the thoracic blood pressure rises, http://www.britannica.com/EBchecked/topic/155613/defecation Defecation, Encyclopædia Britannica 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 terminating the Valsalva maneuver, blood pressure falls; this, often coupled with standing up quickly to leave the toilet, results in a common incidence of Syncope (medicine)|fainting .
Neurological aspects
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 be contracted only together, not individually, and that they both show relaxation during urination. This explains why defecation is frequently accompanied by urination , and why urination is frequently accompanied by flatulence .
Defecation may be involuntary or under voluntary control. Young children learn voluntary control through the process of toilet training . Once this has been achieved, loss of control causing fecal incontinence may be caused by physical injury & ndash; such as damage to the External anal sphincter|anal sphincter that may result from an episiotomy , intense frightCitation needed|date=September 2011, excessive pressure placed upon the abdomen, inflammatory bowel disease , impaired water absorption in the colon ( diarrhea ), and psychological Citation needed|date=February 2009 or neurological factors.
The loss of voluntary control of defecation is experienced frequently by those undergoing a terminal illness.Joanne Lynn, MD. Merck. October 2007. http://www.merck.com/mmhe/sec01/ch008/ch008f.html Symptoms During a Fatal Illness. Uploaded 3/1/09.
Posture aspects
Main|Defecation posture The positions and modalities of defecation are Culture#Cultures by region|culture-dependent . The natural and instinctive method used by all primates, including humans for defecation, is the squatting position .Kira A. The Bathroom. Harmondsworth: Penguin, 1976, revised edition, pp.115,116. Squat toilet s, sometimes referred to as 'natural-position toilets', are still used by the vast majority of the world, including most of Africa and Asia. The widespread use of seated-position toilet s in the Western World is a recent development, beginning in the 19th century with the advent of indoor plumbing.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 in 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 ...Bockus. Gastroenterology. p. 754 2nd ed. Saunders, Philadelphia and London, 1964
Cleaning
Main|Anal cleansing The anus and buttocks may be anal cleansing|cleansed with toilet paper , similar paper products, or other absorbent material. In some cultures water is used (e.g. as with a bidet or Lota (vessel)|lota ) either in addition or exclusively. In South Korea, some toilets known as Toilets in Japan|washlets are designed to wash and dry the anus of the user after defecation. This is very common in households, or public washrooms.
See also
Portal|BiologyCommons category
Coprophilia
Laxative
References
Notes
Reflist
Refimprove|date=August 2009
Bibliography
Widmaier, Raff, Strang (2006). "Vanders Human Physiology, the mechanisms of body function. Chapter 15. McGraw Hill.
