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Myth: The urge to pee in your wetsuit when immersed in water is purely psychological.

I defy anyone who's been diving for more than a nanosecond not to have succumbed to this at some stage, but it's not in the mind – far from it. It's all down to your kidneys, those bean-shaped organs that filter waste products from the blood and help regulate your blood pressure. The body's total blood volume varies considerably with such factors as hydration and fluid losses through sweating, diarrhoea etc. Various receptors therefore exist, that detect changes in blood volume and trigger compensatory mechanisms in order to help keep it as stable as possible.

When you are immersed in water, your peripheral blood vessels constrict in an attempt to minimise heat loss by shunting blood away from the skin's surface and extremities, towards the warmer body core and vital organs. Although the total volume of blood in the body has not changed, the volume of blood flowing through the body core (particularly the heart) increases. This causes stretching of the chambers in the heart, fooling the body into thinking it is fluid-overloaded; a chain of hormone releases is triggered, which results in an increase in the rate of urine production in the kidneys ("diuresis") – and an uncontrollable urge to pee.

So what can you do to dampen down this response? Stay warm by using a thicker wetsuit, or stay warm and dry by using a drysuit (preferably with a pee valve). Avoiding diuretic fluids such as tea and coffee a few hours before diving will reduce (but not eliminate) the urge to urinate underwater. You might assume that consuming less water before a dive will similarly reduce the need to pee, but this is not the case: the core blood volume will still increase, regardless of hydration level, and the dehydration will only serve to predispose the diver to dark, smelly wee and decompression sickness.

And finally, it's worth remembering that although not the most fragrant fluid in the world, urine is sterile – so not the health hazard it may seem to be, provided you give your wetsuit a good freshwater rinse at the earliest opportunity once it's been saturated with pee. Incidentally, because of urine's sterility, it's far safer to wee on your contact lenses than clean them with saliva, if you're ever caught short without lens solution...

London School Of Diving