Why is that when I get cold I’ve got to go to the bathroom? It doesn’t matter what I’ve been drinking or how long ago that I’ve taken a leek, the moment I get cold, my bladder says, “I’m full, you need to empty me now!” One would think that the body would want to protect against heat loss by keeping the urine and not expelling heat by voiding the bladder.
Just a WAG: maybe the bladder shrinks when it’s cold, causing it to be full more quickly. Then again, wouldn’t the urine shrink also, and isn’t the body temperature constant? So, to answer your question: I don’t know. 
One factor that may be at play is that ADH, a hormone that decreases the amount of water in the urine, is turned off by cold temperatures. So, the net effect is that cold temperature leads to less ADH which leads to more water in the urine, and thus more urine itself.
I didn’t know leeks were diuretics. And the reasoning that expelling something at body temperature somehow causes loss of heat to the rest of your body is wrong just like drinking water at body temperature will not make you any warmer.
I’m beginning to think that having to pee leads to being cold(er), rather than the other way around. I seem to warm up a bit after emptying the bladder.
I think it’s caused by the increased renal perfusion that results from peripheral vasoconstriction and shunting of blood centrally. When you’re cold, less of your blood flows through vessels near the surface, and more stays in the major central vessels (including the renal arteries/veins). The more blood that flows through the kidneys, the more is filtered to become urine.
YMMV,
PC
IANAphysiologist, so I don’t know the mechanism, but it is an adaptation to the cold. The more water you have swishing around in your system, the harder it is for you to keep warm (water requires a lot of energy to heat). So getting rid of that water (by peeing, rather than by sweating) allows one to keep warm more easily.
Good thinking, but the water is already warm. There was an interesting thread about weather you became colder or not by peeing, but peeing does not make you warmer. In fact, I would guess that having more water on board would tend to keep your temp from fluctuating too much. You are right about water requiring a lot of energy to heat, but I would expect it works in reverse, too, and once it’s warm, it can release it for a while.
Thanks PosterChild. Darn physiology professor, giving out wrong information!
http://www.sciencenet.org.uk/database/Biology/0106/b00957d.html
Basically, it says that you pee more because you don’t sweat as much. Also, because blood flow is cut off from your extremities, blood pressure to the kidneys increases, increasing urine production. Makes sense to me!
This makes a lot of sense. Still, if ADH levels were not turned off in parallel, you’d reabsorb the water from the increased filtrate (ie. the increased filtrate that occurred because of the shunting to the renal vasculature) and be left with a smaller volume of concentrated urine.
I’m just glad somebody figured this out, because a friend and I have been wondering about this for a long time, having suffered through many cold vacations in Lost Wages together. You don’t realize how long it takes to cross the streets on the Strip until you’ve got that cold-and-gotta-pee urge.