In any event, drinking one's own urine is hardly unheard of; in some cultures, in fact, it's fairly common (sadhus in India, among others, drink it). Hippocrates advocated it 2,500 years ago. And in some homeopathic circles a sip of urine is seen as a way to stimulate the immune system. Moreover, there's nothing inherently dangerous or "wrong" with it. Fresh urine is sterile—no bacteria or viruses after the kidneys have filtered out all that crud. It does contain uric acid, which is what caused Ralston's gums to become sore.
However, if one is in an emergency pinch and begins to "save" urine in a bottle for later use, then the stuff can begin to breed bacteria (question: If you're in such dire straits, does it really matter?). It's also true that you're going to gain very little real benefit from drinking urine to stave off death by thirst—you'll be producing such a tiny amount, it's not apt to make any real difference. You really need close to a gallon of water a day to maintain good bodily health; more if you're working hard. And there is that law of diminishing returns; after all, urine is a waste product designed to rid the body of the materials it doesn't need.
But—and here's the nub of your question—what about filtering? My response: Why not? It may help remove any bacteria that have developed, and if it's a carbon-based filter such as the MSR MiniWorks ($79; www.msrcorp.com) that will help remove the acid component and even improve the, um, flavor.
Excuse me. I have this sudden urge to get a glass of clean, cool water...
Read "Trapped", an exclusive excerpt from Aron Ralston's new book featured in the September 2004 issue of Outside.