Physiology of Defecation Animation / Defecation reflex / Constipation

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Physiology of Defecation Animation / Defecation reflex / Constipation

Physiology of Defecation
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Defecation is the term given for the act of expelling feces from the digestive tract via the anus. It is a complex function that requires coordinated involvement from the gastrointestinal system, the nervous system, as well as the musculoskeletal system. The frequency of defecation within a 24-hour period varies depending on age and diet, but most people tend to have a bowel movement 1 to 3 times daily.

Organ Systems Involved
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The colon is responsible for propelling feces toward the rectum and beginning the urge to defecate. The external anal sphincter and the puborectalis muscle relax to allow the passage of feces out of the rectum. Valsalva maneuver and abdominal muscle contraction are performed to increase intra-abdominal pressure and expel feces more rapidly. Rectal afferent nerves are responsible for the sensation of rectal fullness and the urge to defecate. Sacral nerves S2-S4 supply innervation to the muscles most involved in the act of defecation via the pudendal nerve.

Mechanism
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Colonic mass movements and peristalsis move intestinal contents distally into the rectum. Rectal filling activates mechanoreceptors in the rectal wall causing awareness of the need to defecate. As stool reaches the rectum, a small amount is allowed to pass through to the anal canal by an involuntary relaxation of the internal anal sphincter. This action, known as the rectoanal inhibitory reflex, is necessary for anal sampling, which is the process of determining if the rectal contents are of the gaseous, solid, or liquid form. At this time, if defecation is not socially acceptable or convenient, the rectal wall relaxes, and the need to defecate subsides temporarily. If it is a proper time to defecate, the person generally either sits or squats depending on their environment. Next, contraction of the abdominal muscles and performing the Valsalva maneuver while simultaneously relaxing the external anal sphincter and puborectalis muscle will expel feces from the body due to the pressure gradient generated between the rectum and anal canal. After fecal expulsion, the closing reflex occurs, which involves the external anal sphincter regaining its tone to maintain continence at rest.

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