Your body attempts to keep it at a constant level. This is much easier if you drink more than you need to; it's easy to remove excess water, harder to remove excess sodium especially if you're about to drink more water and would then be sodium-deficient. But in general, serum sodium shouldn't vary much unless you're at least a little dehydrated.
I try to always keep some water handy, since if I don't, I have a tendency not to go grab it until a bit later than I really should have.
If the second glass of water has no salt in it, then salt will start crossing the membrane and equalising the salt levels in both glasses. However, if the second glass of water already has some salt in it, then the first glass will lose less salt through osmosis to the second glass.
Now, imagine the first glass is your blood, and the second glass is your waste water. By measuring the salt levels in the first glass, we can estimate the concentration of salt in the second glass, and therefore how much water is being cycled through.
You can also measure serum sodium in urine, but urine tests are more significantly impacted by recent consumption (e.g. if I drink a big glass of water a few hours before the test, likely the serum sodium levels will be significantly lowered), regardless of general trends. This means they're normally used as a secondary diagnosis tool in case blood sodium is abnormal to validate kidney function.
Is that correct? Normally in osmosis water will cross the membrane to equalize concentration of ions right?
Say sodium and water passes equally well, there will be a net flow of water one way, and a net flow of sodium the other way. Notice that the flow of water will make the water rise on the flow-to side, then pressure there will rise and oppose the flow.
humans are big membrane enclosed bags of mostly water, but also fats, proteins, etc. And some salts including sodium-salt. According to what I keep reading, none of that other stuff matters, only the sodium-salt and the water. Add a little salt to the big-garbage-bag-that-is-a-human, we are told, then you must add a ton of water to keep the salinity the same. How much? Till you are pissing clear, you're not done. Your body magically keeps every thing else in balance, but you need to manually intervene by overconsuming water. And I just don't believe this could be the case, it's just the "drink lots of water" meme repackaged.
"what is the harm" or "why" is never addressed.
