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If you're stuck in the middle of the ocean, why can't you drink the salt water?

Left to their own devices, salt and water diffuse until the salt concentration in the water is uniform. A familiar instance of this is that saltwater will dehydrate your skin, by a process called osmosis, because water flows out from your skin cells (where the salt concentration is lower) and into the saltwater (where the salt is more concentrated).

If your bloodstream has a higher concentration of salt than the cells that it's in contact with, those cells will similarly dehydrate. When it comes to the cells in your internal organs (and the cells in the blood itself), it's clear why this can kill you pretty quickly. To remain alive, vertebrates need to keep the salt content in their blood to about 0.9%.

It's possible to separate the salt from the water, but only by processes that consume energy. Such are the laws of thermodynamics! There are, in fact, industrial desalination plants that take the salt out of seawater, but this is often too expensive to be an economically viable source of water for human consumption or agricultural irrigation.

Your kidneys are little desalination plants that can remove salt from your bloodstream and expel it in your urine, but only up to a point. The human kidneys cannot produce urine that is more than about 2% salt. Since seawater is about 3.5% salt, if you drank it the concentration of salt in your bloodstream would increase, disrupting your metabolic processes and, if you kept it up for more than a little while, killing you.

Other mammals have more powerful kidneys, which can concentrate salt in the urine to more than 3.5% and can therefore survive drinking seawater. But this seems to be sufficiently costly that even sea-dwelling mammals like seals and manatees will avoid it if they can, preferring to get water from their food, either directly or by a metabolic reaction. See "How can sea mammals drink saltwater?"

Saltwater fish, of course, can expel salt efficiently enough to keep the concentration in their blood at the 0.9% level. Freshwater fish need to do the opposite, excreting very pure water in order to keep their blood saltier than the water in which they swim. A few fish, like salmon, can pull of the trick of surviving in both fresh and saltwater.


Diagram showing the flow of salt (ions) and pure water for a saltwater fish. Source: Wikipedia.


Some evolutionary biologists believe that the original vertebrates all lived in the sea, which back then might have been about 0.9% salt. As the sea's salt content started to increase, some vertebrates escaped to dry land or fresh water, carrying the imprint of their seafaring origins in their blood. The vertebrates that stayed in the sea would've had to develop powerful mechanisms of "osmoregulation"; see, e.g., "Vertebrate & Fish Evolution".


This information was taken from Quora. Click here to view the original post.

Do you think humans will ever be able to drink salt water? Will evolution lead us in this direction?

#Science #animals #Quora

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What are your thoughts on this subject?
38 Comments
James Webb
Samuel Taylor Coleridge, Water Water everywhere and not a drop to drink. From the Rime of the Ancient Mariner.
5
Dec 13, 2018 7:43PM
birthdayboy
Converse , if our blood salt level falls below 0.9%, we would begin to feel dizzy. and possibly pass out. Workers who are exposed to high temperatures are urged to take salt tableys
2
Jan 12, 2020 5:09PM
robert karel
Good thing it rains fresh water!
2
Feb 25, 2019 1:39AM
David Holmes
The information is really interestin
1
Feb 17, 2019 4:18PM
Greigg Graf
grew up on Long Island Sound, been in Florida for 30 years. It doesn't taste that great anyways
1
Jan 29, 2019 9:38PM
txfl
It was interesting, but he t DDS answer the original question.
1
Jan 17, 2019 4:54PM
Alan Oswald
Very good trivia question
1
Jan 2, 2019 1:26AM
Paul Chalk
Good Lord, did we all get through an entire question without abusive and insulting comments? Well done us!
2
Dec 31, 2018 4:50AM
Don Racette
Betty Lester You only knew it because I told you.
1
Dec 26, 2018 11:39PM
Darlene Davidson
James Webb, This is the salient fact of the article.
0
Dec 15, 2018 5:09PM
Keven Myers
Irma Rivera, Absolutely, we are fearfully and wonderfully made.....created.
1
Dec 5, 2018 4:07AM
Irma Rivera
I got the word correctly the first-time
1
Dec 1, 2018 6:16PM
Irma Rivera
I meant compex!!!
1
Dec 1, 2018 6:11PM
Irma Rivera
Don Toombs you're totally correct!!! Evolution is just a theory not proven!!! How can the big bang explains the complexity of the vast universe or how perfect we are created???
3
Dec 1, 2018 5:53PM
vasily tchaikovsky
Thanks ! Good information for everybody Urine can has 2,5 % of sult all other cells contaminated
0
Nov 30, 2018 10:47PM

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