bowel movement

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16th century drawing of a human defecating in squatting position outside.

A seagull defecating in flight

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 usually defecate from three times a week, up to three times a day. According to the Mayo Clinic, the healthy rate of normal defecation is one bowel movement per day, less or more frequent movements probably indicate an unhealthy condition, such as lack of fiber or toxic constipation^[1] 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 defecation cycle

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.

If this urge is not acted upon, the material in the rectum is often returned to the 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 autolyze, resulting in constipation.

Once the voluntary signal to defecate is sent back from the brain, the final phase of the cycle begins. The rectum now contracts and shortens in peristaltic waves, thus forcing fecal material out of the rectum and out through the anal canal. The internal and 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 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.

Cardiovascular aspects

During defecation, the thoracic blood pressure rises^[2], 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 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^{[citation needed]}. 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 (such as damage to the anal sphincter that may result from an episiotomy), intense fright, excessive pressure placed upon the abdomen, inflammatory bowel disease, impaired water absorption in the colon (see diarrhea), and psychological^{[citation needed]} or neurological factors.

The loss of voluntary control of defecation is experienced frequently by those undergoing a terminal illness.^[3]

Posture aspects

Defecation in squatting position

Defecation in sitting position

The positions and modalities of defecation are culture-dependent. The natural and instinctive method used by all primates, including humans for defecation, is the squatting position. Squat toilets, sometimes referred to as 'natural-position toilets', are still used by the vast majority of the world, including most of Africa, Asia and the Middle East.^[4] The widespread use of seated-position toilets in the Western World is a recent development, beginning in the 19th century with the advent of indoor plumbing.^[5]

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 ..."^[6]

Other considerations

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).

References

Wikimedia Commons has media related to: Defecation

• 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.