Why do you urinate when you defecate?

Answer 1

The primary muscles that function to hold in stool are the internal and external anal sphincters. The internal sphincter is a completely involuntary organ, and as stool passes through this sphincter, the muscle slowly relaxes. The stool then contacts the external sphincter, which is a completely voluntary organ. At this point, we sense that there is an increased pressure in our rectum, and we likely feel the need to defecate. If a toilet or other commode is close by, we sit down and voluntarily relax our external anal sphincter, and the stool passes through. If, however, we would rather not have a bowel movement at that moment, we voluntarily tighten our external anal sphincter, and the stool is retained in the rectum.