eli5 why does our bodies feel an urge to pee when we enter a body of water
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“…physicians call cold diuresis. It’s a side effect of constricting blood vessels and the resulting increase in blood pressure as the same amount of blood has a smaller space available to travel through your body.”
When you enter a body of water, the pressure exerted on the body is higher than when you are in air (ie the normal environment for us land-borne creatures).
The pressure compresses on compressible parts of our body – primarily the veins in skin, and our fleshy bits like the legs. There’s a increase return of blood/fluids to the central circulation, which causes the system (specifically kidneys, heart as the sensors) to perceive an “increase” to the volume of blood.
This causes the kidneys to filter out the perceived excess volume, as pee, which fills a bladder gradually to create the urge to pee. This prevents the system (specifically the heart, lungs) from getting excessively filled with this excess volume.
Cold water temperatures have an additive effect on the veins constricting.
Oppostive effect when one enters a hot environment like a sauna. Steam causes blood vessels to dilate, and less remains in the central system. So then some folks get light headed – due to redistribution of fluids outside this central system, with less going to the brain.
Fun fact – your question came out for distinction vivas for my university’s year 1 exam
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