Why do our tears and sweat have salt in them. Why aren’t they just be water?
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Two reasons ultimate (amongst many proximate others):
1) most fluids in the body have a similar osmolarity, meaning similar amount of dissolved ions and molecules, around 300 mOsm. Sure some fluids (like in your inner ear) can have a dramatically different composition than your plasma or your cerebrospinal fluid or whatever, but the osmolarity is very similar. And the reason for that is because it is very difficult to compartmentalize fluids of different osmolarity, you’d have to separate them with a water tight and molecule/ion tight barrier, otherwise they will drive each other to equilibrium. So when you produce tears or sweat, it makes sense that these fluids are similar to other internal fluids, otherwise it would be insanely expensive (bioenergetically speaking) to produce pure water as a fluid. To do so you’d have to take a normal bodily fluid and remove everything in it, which means a bunch of things have to be pumped out against their concentration and probably electrical gradients.
2) why do athletes drink Gatorade? Why are you recommended drinks with an osmolarity of about 300 mOsm when you have excessive diarrhea or vomitting? Because if you take in pure water, it dilutes your blood a bit, which is a systemic fluid, meaning a tiny change in electrolyte concentrate can mess up with a lot of organs. If you do take just pure water, your body actually quickly responds by telling the kidney to get rid of that water to ensure constant concentrations of stuff in your system. Then your rehydration plan would be useless. Now think the other way around, if you had to produce pure water for tears and sweat, you’d be increasing the concentration of everything in your body making you need rehydration asap or your kidney will have to work extra hard to get rid of the excess ions in your blood.
BTW, this is a highly simplified answer, I even had to sacrifice some accuracy for it to be so. But feel free to ask any follow up questions or ask for more detail!
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