For those of us without the luxury of a drysuit, there are two types of wetsuit people. Those who have peed in there wetsuit, and those who lie about it. Only the strongest of bladders (and wills) are resilient enough to last the dive duration without urination but why is it that we often have the urge to pee when diving?
When we first learn to dive, we hear about how pressure increases as we descend deeper and this pressure also compresses our internal veins in our arms and legs. This restriction means they push the blood they carry back to the body’s core and redistributes blood from our extremities to the larger veins in our chest.
More blood is then arriving in the right atrium in the heart and the muscular wall stretches. The increase in blood pressure sets off receptors which trigger release of a vasodilator (blood vessel relaxant) which has a side effect of water loss.
More blood to the core also means more blood to the kidneys, filtering even more urine, which results in us needing the bathroom!
This physiological pathway is actually far more complex than the basic run-down above and there’s a lot more to find out in the field of dive-medicine but the summary is: diving dehydrates us and it’s important that we make sure we stay hydrated because dehydration increases the risk of decompression sickness. So continue peeing underwater in your wetsuit if you must, just make sure you drink up before diving in.