Intermediate 8 minutes
The circulatory system underwater
Why you feel the urge to pee on every dive, and what that has to do with decompression
Almost every diver has noticed the same thing: a few minutes after entering the water, the urge to pee shows up, even if you'd just gone right before. It's not suggestion, and it's not just the cold. It's a concrete physiological mechanism, and it has direct consequences for decompression risk.
In this article you'll see how the circulatory system transports gases, what happens to it when you enter the water, why that causes dehydration, and what role circulation plays in absorbing and eliminating nitrogen.
Circulation is the system that decides how much inert gas your body absorbs and how quickly it eliminates it. It's the physiological piece that connects directly to decompression.
Two circuits and one transport system
The heart works as two coupled pumps. The pulmonary circuit sends blood to the lungs to pick up oxygen and drop off carbon dioxide; the systemic circuit distributes it through the rest of the body, delivers that oxygen to the tissues, and collects waste.
Oxygen travels almost entirely bound to the hemoglobin in red blood cells, with only a smaller fraction dissolved in the plasma. Carbon dioxide uses several routes, most of them dissolved or chemically transformed.
Inert gases, on the other hand (the nitrogen in the air you breathe) don't bind to anything or transform: they simply dissolve in the blood and tissues according to the pressure they're under, following the principle you'll see formulated as Henry's Law in the diving physics category.
That detail is the key to everything that follows: nitrogen travels passively, and how much goes in and comes out depends entirely on how much blood reaches each tissue.
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