if humans need salt, why does seawater dehydrate us?

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I’ve come to understand that the reason humans shouldn’t drink seawater is that it requires more water to get rid of that salt through urine. But if salt is a necessary nutrient, why would the body need to get rid of it at all? If your body were low on salt, what it be good for you to drink seawater?

Edit: added follow-up hypothetical question

In: Biology
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As far as I know it is mainly because the amount of salt differs. You have a body salt percentage of about 0.9%. Sea water has a salt percentage of about 1.5%(?).
It’s more than your body needs or can handle and so the “overdose” needs to be getten rid of. Your body does that by washing out what’s to much. And that’s pretty much why drinking sea water dehydrates you.

Please correct me, if I am mistaken in any way.

The body needs SOME salt, but like every chemical your body requires for continued life, it needs it in moderation.

The issue with salt is that water flows towards it, which causes your blood to do the same. if the saturation is too high that salt gets stored and draws blood from where it’s needed

What the body needs is to get rid of toxic and unnecessary substances that come from the metabolism of other substances. We generate a lot of molecular waste during the metabolic processes inside our cells. The way our body has to get rid of all that molecular waste, is to eliminate it dissolved in water, that’s urine basically. So in order to eliminate that molecular waste, we need to eliminate water.

That water is depleted from our blood. But if we were to only eliminate that waste and water, then all the salts and ions dissolved in the blood would increase their concentration (potassium, sodium, calcium, magnesium, etc). So we need to eliminate part of those ions in the urine to keep the concentrations within the normal range. So to replenish everything, we need to consume new water, new salts, new ions, everything, but in the right proportions.

Sea water for example, has way too much salt in relation to water; and within the intestines, water will be sucked from the inside of the cells towards the cavity of the intestines via osmosis, producing a diarrhea.

Saltwater has too much salt, more than your body needs and more than your kidneys can filter out. So if you drink nothing but seawater, the salt levels in your blood rise too high, which kills you.

To answer the hypothetical, no. The reasoning is that water always follows salt. So the salt concentration in your body is too low vs the seawater, thus more water is sucked out than absorbed. The absorption of water in many animals is passive via this process

Ever heard of “too much of a good thing”?

Salt regulates our blood pressure and water in our cells along with potassium. Too much salt pulls water out of our cells and can increases blood pressure to dangerous levels. Without fresh water to dilute this, our kidneys can’t function and cells shrivel up and you die.

To much water will dilute sodium and cause trouble as well because it is needed in certain amounts for our nervous system and cells to function properly.

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