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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 the 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 feces out of the rectum. The internal and external sphincters of the anus allow the feces to be passed by muscles pulling the anus up over the exiting feces.

During defecation the chest muscles, diaphragm, abdominal wall muscles, and pelvicThe pelvis is the bony structure located at the base of the spine (properly known as the caudal end). The pelvis incorporates the socket portion of the hip joint for each leg (in bipeds) or hind leg (in quadrupeds). It forms the lower limb (or hind-limb) 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 pressureBlood pressure or arterial blood pressure is the pressure (force per unit area) exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to the pressure in the large arteries, such as the brachial artery (i rises throughout the body and 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 aneurysmAn aneurysm (or aneurism (from Gr. a dilatation) is a localized dilation or ballooning of a blood vessel. Aneurysms most commonly occur in the arteries at the base of the brain (the circle of Willis) and in the aorta (the main artery coming out of the hea or to dislodge bloodBlood is a circulating tissue composed of fluid plasma and cells ( red blood cells, white blood cells, platelets). Medical terms related to blood often begin in hemo or hemato ( BE: haemo and haemato from the Greek word for "blood". Blood of different spe clots (see thrombosisThrombosis is the formation of a clot or thrombus inside a blood vessel, obstructing the flow of blood through the circulatory system. Thromboembolism is a general term describing both thrombosis and its main complication: dislodgement of a clot and embol).

Defecation may be involuntary or under voluntary control. Loss of control may be caused by physical injury, intense emotions, impaired water absorption in the colon (see diarrheaDiarrhea in American English, (spelt diarrhoea elsewhere) is a condition in which the sufferer has frequent and watery bowel movements. This condition can be a symptom of injury, disease or foodborne illness and is usually accompanied by abdominal pain, a), and psychological or neurological factors. See fecal incontinence for a discussion of this.

The positions and modalities of defecation are culture-dependent . In some countries, like Turkey and Japan, it is customary to squat (thus 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, the hand, or with water (compare bidet).