The passage of our bodily waste is controlled by circular muscles called sphincters. The external sphincters are under our control. The sphincter around the urethra is smaller than the one around the anus, so when you decide to urinate you can relax it without relaxing the whole pelvic floor. This means you can pass urine without needing to pass stool at the same time.
When you do pass stool however, the relaxation of the stronger anal sphincter also decreases tension in the weaker urinary sphincter, allowing urine to pass at the same time. But this isn’t always the case – it is possible, but difficult, to do one without doing the other.
Asked by: Kate Selby, Dundee
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Dr Nish Manek is a GP in London. She completed her medical degree at Imperial College and was runner-up in the University of London Gold Medal. Manek has also developed teaching courses for Oxford Medical School, and has penned articles for The Guardian and Pulse magazine.