Greetings from America’s fourth-best city, Seattle, where I’m attending the STOC’2006 conference. I arrived here yesterday from America’s third-best city, Boston, where I visited MIT for a week and gave a talk about The Learnability of Quantum States. (I’ll leave the best and second-best cities as exercises for the reader.)
Since tomorrow’s my birthday, I’ll consider myself free to blog about whatever I feel like today (as opposed to most days, when I blog about whatever the invisible space antelopes tell me to). So without further ado, here’s a question that bugged me for years: why do we need to urinate on a regular basis?
I mean, I understand solid waste perfectly well, and I also understand the need to get rid of urea and the other waste products in urine. But why constantly excrete water, something that humans and other animals regularly die from not having enough of? Why not store the water in the body until the next time it’s needed? From a Darwinian perspective, a regularly-vacating bladder would seem to make as little sense as a toothless vagina.
And yet, after minutes of diligent Wikipedia research, I’ve pieced together what I believe is a complete solution to this pee versus in-pee puzzle.
The short answer is that conserving water, rather than just pissing it away (so to speak), is exactly what our bodies try to do. But one needs to remember that, while feces comes directly from the digestive tract, urine is collected from waste products in the bloodstream. In particular, the kidneys contain permeable membranes whose job is to let wastes like urea through, while keeping the useful stuff (like red blood cells) out. However, as with any other filtration process, it’s difficult or impossible to keep all the water on one side of the barrier.
So what the body does instead is to let the water through, then slowly absorb it back into the bloodstream as needed. That’s why your urine is darker (more concentrated) if you’re dehydrated than if you aren’t. At some point, though, it presumably becomes infeasible to extract more water from the bladder without also letting the toxic wastes back into the bloodstream.
Now, I know what you’re thinking. You’re thinking, “why isn’t my urine always dark? In other words, why don’t I always absorb as much water as possible back into my bloodstream, whether I’m dehydrated or not? Why not save the water for a (non) rainy day?”
Aha, I’ve got an answer to that one too. Besides excreting wastes, another function of urine is to maintain a homeostatic balance between water and sodium in the blood. If there’s too much water (say, because you just drank six beers), your blood will be too thin, which can cause brain damage (completely apart from the other effects of the beer). Ideally, your body would store the excess water separately from the blood — and again, that’s exactly what it tries to do, but your bladder is only so big.
In summary, if you think through what my “in-pee” solution would actually entail, it turns out to be almost identical to the “pee” solution that Nature actually adopted. One might even say that pee = in-pee.
[Note for harping relatives: now do you understand why I didn't go to medical school?]