Why do our tears and sweat have salt in them. Why aren’t they just be water?
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Osmosis. Fluid without any salt would be just sucked back in instantly and it is pretty easy for a cell to poop some salt out
Your body needs a balance of salt and water. If you were to only sweat water you’d have problems that come from high sodium like blood pressure spikes.
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!
A human body is 0.4% salt by mass. It is not equally distributed because it is dissolved in water where it is 0.9% by mass, like in the blood. Tears are water from inside your body that is excreted and it will have the same salt content as the water in the body.
So others have the same salt content as the body. If you do not have the same salt concentration you get osmosis with the tissues in contact and water would be pulled into the cells of the eyes until the same salt level is reached.
It is quite complicated for the body to change salt level, the kindies in specialized in that to manage the concentration in our bodies. This is why sweat also have the same concentration, it would require work to change it.
Your body struggles to move water across membranes. It’s easier for the body to pump salt from the cells in tear glands out into the ducts, and then let osmosis (the passive movement of water across a permeable membrane to create equal water potential on both sides) passively fill the tear duct with water to create a tear.
Common way to demonstrate this done in schools is get chunks of potato and weight them. Put one in pure water and one in very salty water. After a few hours, the one in salty water will weigh less, as water has moved from the relatively un-salty potato into the very salty water. The potato in the pure water will have gained weight – water has moved from the unsalty water into the relatively high salty tissue of the potato.
N.B. The body can move water via “aquaporin” channels, but these are only found on select cells, and as a rule in the body – if it wants to move water, it moves salt and the water will follow.
Rule 2 forbids straightforward questions.
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