firedog is right! without getting into the details, the 'salt content' of urine is just the right concentration that it wont prompt the body to turn on thirst mechanisms...if you drink the piss of your friend however, it might make you more thirsty.
Interestingly, if you are trapped out at sea, drink your piss before you drink the sea water!
It makes sense. The urine is sterile. Untill it comes out that is.
Thanks to this thread, I can now tell people that I wouldn't piss on them if they were dying of thirst in the middle of the desert!! Because I like a little variety in my sayings..