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A popular "well-known fact" is that all creatures on Earth consist mostly of water (i.e. H2O). Indeed, a liquid called "normal saline solution" is just a solution of 0.9% sodium chloride in distilled water. It is generally considered the "safe" liquid to be injected into human body in relatively high volumes without any bad effects.

I know that the same 0.9% solution is also used in vet practice for at least cats and dogs. As far as I understand, that's because blood circulation structure and basic blood regeneration specs are more or less the same for cats and dogs as for humans.

But what about other animals? My wild guess is that same 0.9% solution would be ok for all mammals (probably because their blood tract is similar), but what about others? Birds? Fish? Vertebrae? Invertebrae? Animals? Plants?

If 0.9% solution is not universal, what are the other possible variants? (i.e. could you provide examples of other compositions for some other species?)

If 0.9% is indeed universal, I'd be glad to hear any pointers on why does it happen (i.e. is it related to evolution of life in Earth history, is it related to chemical composition of bodies, related to some specific cells, etc).

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    $\begingroup$ Not saying that it is universal, but the reason the that we use 0.9% saline is so that osmosis doesn't cause all of our cells to suddenly try to draw in water to create an equal concentration on both sides of the cell membrane. It all has to do with the "stuff" inside our cells. $\endgroup$ – SolarLunix Jul 14 '15 at 5:06
  • $\begingroup$ Great insight, thanks! Now I just need to dig in through how cell membranes evolved and which creatures started having these "modern" 0.9%-level membranes in their cells... $\endgroup$ – GreyCat Jul 14 '15 at 7:22
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    $\begingroup$ Try looking into concentration of ocean water. If we have all evolved from the cells that swam the ocean there is a good chance there is something to it. Also look into fresh water fish too. If there is going to be a difference in osmosis with the 0.9% solution it will most likely be in fish. $\endgroup$ – SolarLunix Jul 14 '15 at 9:57

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