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Why Do You Need to Pee as Soon as You Get in a Swimming Pool?

It feels like a fundamental design flaw that a pleasant dip is always so swiftly interrupted by a desperate need to urinate

You usually know when you’re about to go swimming. Cellphones have made high-spirited pushings-in a thing of the past, while statistically speaking, there are a lot more successful trips to the gym than accidental “oh shoot, I’ve driven off a cliff” incidents. So, normally, when you get into a pool, you know what’s coming, and you’ve had a chance to pee.

But, shortly after getting in, it’s not unusual to feel the ol’ piss-wolf knocking at the urethra door, leaving you with the choice of getting out and clomping back into the locker room (leaving anyone paying attention in no doubt whatsoever that you’re off to take a leak), or just going for it in the pool, which, even though you know the blue-cloud thing is unlikely, still feels rude at best (seriously, where does the piss even go?). Also, anyone too near you will feel a big warm wave and know you’re a filthy pool-pisser, so there’s that.

What’s happening here is probably something called cold diuresis, a reaction by your body to the water’s temperature being lower than it could do with — it’s also why jogging in the cold necessitates more pee breaks than a summer workout. Your body wants to keep your core nice and warm, so it restricts blood flow to the surface of the skin and extremities (as this is where heat is lost), constricting blood vessels in a bid to deal with your dropping temperature. This process, called vasoconstriction, also raises your blood pressure, which isn’t ideal, so your kidneys expel the fluid within them to your bladder to balance this out and lower it again. 

That liquid? Good old-fashioned piss

(While the effects are thought to lessen with frequency, scuba divers experience a more extreme version of this, due to being chest-down in deep water for a long time with reduced gravitational effects on their limbs. Peeing in a wetsuit is fine, as it just passes through the fabric, but peeing in a drysuit is another matter — don’t ever hug someone right after they unzip a drysuit. Drysuit divers who spend lengthy periods underwater take desmopressin acetate pills, as prescribed to bedwetters, to reduce the need to constantly empty themselves.)

There isn’t total consensus about cold diuresis, as oddly enough, scientists seem to have more important things to investigate than yellow warmth seeping through Speedos. Other ideas include cold-induced aquaporin inhibition (meaning cells that normally absorb water from your blood are stopped from doing so, leading this excess water to be redirected to the kidneys to maintain blood pressure) and hydrostatic pressure — the water around you pressing on your body — causing fluids to move towards the center of your body with pretty much the same result.

Whichever of these it is (and it could well be a combination of the three), the end result is the same: wee-wee. 

Happy swimming

Don’t forget to rehydrate afterwards, and enjoy your chip sandwich.