Take it away, Michael Byrne:
The phenomenon of needing to pee in the cold has a name: cold diuresis. It remains to be definitively accounted for by doctors, but several explanations seem fairly reasonable.
When you get cold, your blood vessels, particularly those in your fingers and toes, constrict because of something called vasoconstriction. In an effort to maintain a warm core temperature, the body tries to keep blood away from more susceptible extremities. Simply: the body lets less warmth out, and less cold in, and one way to do that is by not allowing blood into colder places. This is accomplished by limiting the space that blood could occupy. I’ve know people with Raynaud’s disease, a disorder in which the body exaggerates this effect. Their hands would turn all sorts of blue and pink and be really cold (to them and to the touch). The effect is startling.
How this theoretically works for your bladder is this. Because you’re sending less blood out to the extremities by reducing the volume it can occupy in those extremities, you have more blood elsewhere. The same total amount, but less space – this, naturally, equals higher blood pressure. To regulate that, your kidneys move to pull liquid out of the body, which leads to more liquid in your bladder than there would normally be. Sup, pee.
Update from a reader:
When I used to camp a lot, I was taught by my elders to pee before sacking out for the night because keeping all that piss warm inside your body takes a lot of energy. Made sense from a physics perspective – water (piss) has a very high specific heat, i.e. the energy needed to raise a gram of material (handily this = 1 milliliter of water) 1 degree Celsius, or alternatively the energy needed to keep it at a given temperature against the various forces conspiring to draw heat from it (such as the frigid mountain air). Though just as plausibly, the annoying and literally chilling effect of having to de-sleeping-bag and take a leak in the middle of the night could have been the real reason. Anyway, it’s good advice.