system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can drinking your urine in a survival situation save you if your body has already discarded it as waste?
gpt
Urine is almost entirely water.  Other than water, it has a very small amount of urea and a measurable but insignificant amount of other stuff, like salt, in it.

Urea is the big thing that your body tries to get rid of by peeing.  Your body produces urea, which is harmless at low concentrations but poisonous if there is too much in your blood.

Your kidneys have the ability to process a huge amount of urea - far more than your body normally produces.  When you drink urine you reabsorb that urea and your kidneys have to filter it back out of your blood.  But because of how much excess capacity your kidneys have, they're able to keep the amount of urea in your blood low despite drinking a few urinations worth of urine.

That being said, every time you drink urine the concentration of urea in your urine increases.  Eventually you will get to the point where the urea in your urine is so concentrated that your kidneys can't deal with it and it rises to poisonous concentrations in your blood.  

Urine is also essentially sterile, and so its otherwise safe to drink your own urine.  In fact, one of the ways that people cheat drug tests is by injecting urine from someone who doesn't do drugs into their bladder.  

Its not necessarily sterile if the person who it came from has a bladder infection or certain diseases, such as hepatitis, so while its normally safe to drink urine its not a good idea if you don't know the person who it came from.