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As followup to this question: What would it need to make land animals (and humans) able to digest salt water? That would be very helpful for everyone travelling across the oceans.

There are millions of organisms that can live in the salt water, so why can't we?

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  • $\begingroup$ You should give this a read Osmosis $\endgroup$ – Chinu Aug 10 '16 at 8:27
  • $\begingroup$ It would take a lot because the sodium of salt water affects us down to cellular level. Sodium is one of the humans body's so called "electrolytes". And unless these are within tolerable ranges, you will become very ill and/or die. builtlean.com/2012/11/28/electrolytes So what @ChrisJ said: it is a whole lot easier to just desalinate the water. $\endgroup$ – MichaelK Aug 10 '16 at 8:32
  • $\begingroup$ @Chinu: I know the basics of osmosis, however that applies to every organism, including fish. $\endgroup$ – PMF Aug 10 '16 at 8:32
  • $\begingroup$ @PMF Not all fish can survive in salt water. Not all fish can survive in brackish water. Not all fish can survive in fresh-water. All organisms are evolved to live within a rather narrow range regaring the electrolyte balances. You would have to — pretty much — re-evolve the entire species to adapt them to such a radically different environment. $\endgroup$ – MichaelK Aug 10 '16 at 8:34
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    $\begingroup$ Perhaps you might want to read this post in Biology.SE. On "There are millions of organisms that can live in the salt water, so why can't we?" That is because we evolved to survive on fresh water and terrestrial ecosystems. The same way why fishes cannot survive in deserts. $\endgroup$ – WYSIWYG Aug 10 '16 at 8:59
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You need to remove the salt, there's plenty of ways of doing this without re-engineering the human race. However if you want to, I'm sure you can re-apply these to a biological system.

Check out Hypernatremia for all the symptoms that need to be overcome if excessive salt gets into the system. My thinking is filtering before it hits the rest of the system as it makes it much easier than dealing with all the other systems.

Also, even with the only water being the sea doesn't stop you getting fresh water from the air

You can also use the sun to desalinate water and there is good old fashioned hydrolysis if you still have power.

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  • $\begingroup$ Unfortunatelly, all known ways of desalinating water use massive amounts of energy and quite a bit of equipment. $\endgroup$ – PMF Aug 10 '16 at 8:53
  • $\begingroup$ @PMF depends on the scale, a hand pump to remove salt can be purchased for less than a $1,000 $\endgroup$ – Chris J Aug 10 '16 at 9:02
  • $\begingroup$ @PMF also, electrolysis of water can be done with a battery and two wires, burn the gases again and you have pure water $\endgroup$ – Chris J Aug 10 '16 at 9:04
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    $\begingroup$ I heard using these hand pumps is very physically demanding. You almost loose as much water by sweating that you generate. $\endgroup$ – PMF Aug 10 '16 at 9:27
  • $\begingroup$ @PMF - if it's hand driven, but no reason you can't hook it up to a motor, anything a person can do by hand I would rank as not a massive amount of energy. if you want zero energy, you can go the solar still method... there's plenty of ways to distill $\endgroup$ – Chris J Aug 10 '16 at 9:29
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If you are looking at modifications to the body, there a four possible routes:

  1. A separate digestive tract for only water/fluids with a kidney-like filtering organ connected to it. This would allow the body to extract usable water from the ingested salt water and expel the rest.
  2. Having the walls of the digestive tract develop a kidney-like function that regulates salt before it hits the bloodstream. This would likely increase the risk of infections, because kidney function depends on very delicate tissues that wouldn't do well when exposed to ingested objects or stomach acid.
  3. Extra kidneys or supercharged ones might be able to remove enough salt from the blood to maintain balance, but the tissues around the digestive tract would be exposed to much higher salt concentrations, disrupting vital processes at the slightest further increase.
  4. Rebalancing the entire body chemistry for higher salt concentrations, at which point sweet water becomes undrinkable because it will cause cells to burst when they absorb too much water.

Without changing anything, your answer is simple: Desalination.

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  • $\begingroup$ Errr... What is a kidney-filtering-Organ? $\endgroup$ – PMF Aug 10 '16 at 9:29
  • $\begingroup$ Kidneys filter our blood and through a complex process are able to reclaim water and some useful nutrients from it. I'm proposing a similar organ using the same process to extract sweet water from the salt water. $\endgroup$ – Cyrus Aug 10 '16 at 11:19
  • $\begingroup$ Oh, got it. I just didn't know the english word "kidney" for the organ and thought it was the name of some scientist that invented a filtering technology. $\endgroup$ – PMF Aug 10 '16 at 11:44
  • $\begingroup$ Point 4 isn't necessarily the case, if we had sufficiently capable Kidneys the salt can be removed fast enough to avoid unbalancing the body: We'd have to switch to drinking "little and often" to copy them, but cats can drink either freshwater or sea water indefinitely. Ref: goo.gl/Dmg4m0 $\endgroup$ – Jon Story Aug 10 '16 at 14:38
  • $\begingroup$ @JonStory Then point 3 has it covered. Point 4 is the last and least desirable or workable of the options, it's mostly there for completeness. $\endgroup$ – Cyrus Aug 10 '16 at 14:48

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