More clearly: Why do you have to pee more during and after swimming?

Diuresis is the medical term for increased urination. Cold immersion diuresis is common to cold water swimmers and is the strong desire to pee after (and sometimes during) swimming in cold water.

As we have now discussed many times, swimming in cold water leads to peripheral vasoconstriction, by the cold of the water leaching away the heat of the blood vessels closer to the surface. The blood vessels constrict resulting in reduction of blood flow in the body’s extremities to conserve heat in the cold.

This reduction in blood flow to the periphery therefore actually leads to a quick increase in blood pressure. The body attempts to compensate for this increase in arterial blood pressure by relieving itself of liquid elsewhere. The easiest, quickest and least costly expenditure from a metabolic (energetic) point of view is urine. So you will often find as cold swimmers do, that walking into cold water even before you are fully immersed, you will develop a sudden urge to urinate caused by this blood pressure increase. And during a swim liquid also builds up in the bladder but without practice, they are unable to urinate in really cold water while swimming, many swimmers, even very experienced cold water swimmers will have to stop or momentarily pause to urinate. Also when you exit the water the demand to urinate can reach quite powerful levels as the muscles finally relax.

A consequence of this increased urination often forgotten is mild dehydration. Marathon swimmers in cold water, such as the English Channel are taking most of their food as liquid carbohydrates. The volume of water needed or used is generally close to one litre an hour, and because of cold diuresis, more of the liquid processed by the kidney, instead of being absorbed back as is normal, goes to the bladder. Part of this mechanism is that the cold suppresses the production of ADH, aka vasopressin, the anti-diuretic hormone that suppresses diuresis (urination). The swimmer urinates more, so the swimmer needs more liquid to compensate for the mild dehydration. And you have a self-sustaining cycle as long as the swimmer is immersed in tolerable cold water.

Thus … well-nourished high weeds near many regular open water swimming locations such as the Sandycove Island car park.

The desire to pee after swimming doesn’t just afflict cold water swimmers but also pool swimmers. The reasons are the same. Pools, though warmer than the sea are still colder than normal human temperature. Experienced pools swimmers will have noted, probably without realising it, that the desire to pee will be brought on after jumping into a pool that feels cool or after a swim. More commonly, leaving a warm pool for a cooler changing area or showers that are running cool or cold, or exposure to cold draughts (drafts) across the skin which cause goose-pimpling also cause a sudden blood pressure increase due to peripheral vaso-constriction, and result in that desire to pee.

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