This is an interesting question many of our patients ask. There are many muscles (both voluntary and involuntary) that are involved in helping one to both urinate and pass stool. These muscles have dual functions — holding urine and stool in, and passing urine and stool to the outside world. The muscles that help to hold in our stool are stronger and larger than the muscles that hold in our urine.

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.

Control of urine is similar. The muscles are smaller, but the concept is the same. There is both an involuntary internal urethral sphincter and a voluntary external urethral sphincter. The internal urethral sphincter opens when pressures inside the urinary bladder are great enough; we can hold back the urine by tightening our external urethral sphincter, or we can urinate by relaxing this muscle.

So why do we often urinate when we defecate, but not vice versa? The main reason is that the muscles of the pelvic floor play a role in defecation and urination. The pelvic floor muscles relax when we defecate. However, they will not necessarily have to fully relax when we urinate. But when the pelvic floor musculature does relax, in addition to allowing stool to pass, it decreases the tension in our urinary sphincters, allowing urine to flow. Because our anal sphincters are stronger than our urinary sphincters, it is easier for us to have control over our bowels than our urine.

Of course, we all don’t have normal control over our bowels or bladder. There are many conditions that can adversely affect our bowel or bladder continence. Also, the control of urine and stool differ in men and women because the anatomy of the male and female pelvis are so different.

Lisa Edwards, MOTR/L, BSRS

References:

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