Defecation
 |
Anatomy of the anus and rectum |
Defecation or
feceation is the act or process by which organisms eliminate solid or semisolid
waste material from the
digestive tract.
Humans remove waste anywhere from several times daily to a few times weekly;
sloths can go for a week or more without eliminating. 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 waste material. As the rectal walls expand due to the materials filling it from within, stretch receptors from the
nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the
colon where more
water is absorbed. If defecation is delayed for a prolonged period the fecal matter may harden, resulting in
constipation.
When the rectum is full, an increase in intrarectal 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 propel the faeces out of the rectum. The internal and external
sphincters of the anus allow the faeces to be passed by muscles pulling the
anus up over the exiting
feces.
During defecation the
chest muscles,
diaphragm, abdominal wall muscles, and
pelvic diaphragm all exert pressure on the digestive tract and
respiration temporarily ceases as the lungs push the chest diaphragm down in order to exert pressure.
Blood pressure rises throughout the body and the amount of blood pumped by the heart decreases. During this time, the body is effectively undergoing similar stresses to that of a
valsalva manoeuvre. 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).
Defecation may be involuntary or under voluntary control. Loss of control may be caused by physical injury (such as damage to the
anal sphincter muscles that may result from an
episiotomy), intense fright,
inflammatory bowel disease, impaired water absorption in the colon (see
diarrhea),
death, and
psychological or
neurological factors. See
fecal incontinence for a discussion of this.
The positions and modalities of
defecation are
culture-dependent. In some regions of the world, such as
East Asia, rural parts of the
Middle East, and parts of
Southern Europe, it is customary to squat (typically using
squat toilets), while in most of the
Western World sit-down
toilets are used. Similarly, the
anus and
buttocks may be cleared with
toilet paper or similar paper products, rags, leaves (including
seaweed), corn cobs or
sticks. In some cultures water is also used (e.g. as with a
bidet).
