The first urge to defecate occurs when rectal pressure is increased to

The first urge to defecate occurs when rectal pressure is increased to 18 mmHg. When this pressure reaches 55 mmHg, the external and the internal sphincter relax, and there is reflex expulsion of the contents of the rectum. Defeacation reflex: Distension of the rectum and colon stimulates stretch receptors in the rectal wall. Then, the defecation reflexes travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation. Ref: Ganong&;s Review of medical Physiology 26th edition Pgno: 488