Defecation and continence

Continence at rest depends on several mechanisms working together. The internal anal sphincter provides most of the resting pressure. The external sphincter and puborectalis add tonic activity, and puborectalis holds the anorectal angle at about 90°. The anal cushions form a seal. The rectum is compliant, so it can stretch and accommodate contents. Intact anorectal sensation lets a person tell gas, liquid and solid apart.

Defecation sequence. A mass movement carries faeces into the rectum, and wall stretch is detected. The intrinsic myenteric reflex produces weak peristalsis in the descending colon, sigmoid and rectum, and relaxes the internal sphincter (the rectoanal inhibitory reflex). The parasympathetic defecation reflex through the sacral cord (S2–S4, pelvic splanchnic nerves) greatly strengthens these waves and the sphincter relaxation, and afferent signals reach consciousness as the urge to defecate.

If the moment is suitable, voluntary inhibition of pudendal output relaxes the external sphincter and puborectalis. The anorectal angle straightens, the pelvic floor descends, and a Valsalva manoeuvre (closed glottis, contraction of diaphragm and abdominal muscles) raises intra-abdominal pressure. Squatting helps by straightening the angle further. If the moment is not suitable, the external sphincter and puborectalis contract voluntarily, the rectum accommodates, the urge fades, and contents may move back into the sigmoid until the next mass movement.