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So when we swim, we can make water not enter through our nose and mouth, simply by holding our breath. However, the mouth and the nose aren't the only cavities. What about the anus and the genitals? They obviously can't "hold breath", so how do they stop water from entering?

I thought the reason is gravity, but then, people would be killed when diving because your butt is facing up, against the gravity.

Another explanation might be that the internal pressure is higher than the external one, but I fail to recall any organ that creates the pressure..

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    $\begingroup$ It is worth noting that some small amount of water can get in through these openings as there are a few parasites that rely on this as their transmission vector. $\endgroup$ – RBarryYoung Sep 11 '14 at 15:11
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    $\begingroup$ "We can make water not enter through our nose . . ." this is not as simple as you make it sound. I rely on a noseclip. Conversely, the other orifices you mention at least have a sphincter. $\endgroup$ – peterG Sep 12 '14 at 0:03
  • $\begingroup$ @RBarryYoung the Candiru is a good read. For the nose, Naegleria fowleri $\endgroup$ – user8915 Sep 12 '14 at 1:17
  • $\begingroup$ @RBarryYoung and then we could discuss the iris sphincter muscle admitting nightmares! $\endgroup$ – user8915 Sep 12 '14 at 17:33
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For the same reason that when you urinate or use the toilet, you have to exert pressure and urine or faeces just does not simply leak out when you stand up. The urethra seals shut when the urethral sphincter muscles relax. Similarly Sphincter ani externus muscle seals shut in the anal canal when there is no antagonistic force. So water can't just simply enter our body in such huge quantities that we die. It is possible to get more water through your anus with some pressure which is what happens when you get an enema.

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