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I'm designing an alien species, whose blood is highly acidic and can easily cause someone's hand to melt down. It's pretty similar to the Xenomorphs from Ridley Scott's Alien, except for the color maybe.

Now my question is, could something like that exist? Could a creature naturally evolve acidic blood as a defense mechanism? If yes, how would it work and what would it look like?

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    $\begingroup$ Welcome to WorldBuilding.SE! You may be interested in these two existing questions, that both ask about a similar premise. Let me know if either of them answer your question, and if so, we can close this as a duplicate. $\endgroup$ – F1Krazy Jun 14 at 12:32
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    $\begingroup$ Two details... one, does the blood have to be corrosive, or is it just required that injuring them release a corrosive substance? and two, does it have to be acidic, or would a strongly caustic material work too? $\endgroup$ – Starfish Prime Jun 14 at 12:54
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    $\begingroup$ Highly acidic isn't a problem: highly corrosive is. Logic dictates that if the blood can melt human skin but not alien flesh, then the alien must be much, much less chemically reactive than the human. The challenge isn't making acidic blood, but making a living alien entirely out of platinum, teflon and argon. If they could evolve to be that resistant to acid, why didn't life on earth evolve that way? Maybe there are rare elements on their planet, but still it's a difficult explanation. $\endgroup$ – Adrian Hall Jun 14 at 17:09
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Yes, this should be possible. Nature is capable of creating and withstanding acid fluids (most obvious example: the stomach), so having acid blood developed as a defense mechanism sounds not that strange. The blood would most likely still be red if not alternative molecules for the transport of oxygen are used.

Only problem I see here is that the whole body of the creature should be able to withstand acid, not just the veins, else bleeding would lead to certain death.

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    $\begingroup$ Acids are problematic due to their tendency to react with stuff instead of merely dissolving it without chemical modification. Neutral, water-based blood can carry all sorts of useful materials without unduly affecting them, which is a very valuable property. $\endgroup$ – Starfish Prime Jun 14 at 12:55
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    $\begingroup$ I could imagine Acid-proof carriercells. I would be ineffective, but possible. There are several molecules already transported by carrier-molecules or carrier-cells, Oxygen, ATP/ADP, etc... $\endgroup$ – miep Jun 14 at 13:05
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To amplify StarfishPrime's comment, a natural defense biomechanism which releases (and possibly generates at the time of injury) acid or alkali fluid is a lot easier to justify than the blood itself.
Related capabilities in Terran animals: snakes which spit venom, frogs whose epidermis contains toxins.

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possibly, instead of having acid blood you could have a separate system to the circulatory system which would be exclusively for defense. It could also be used as a sort of stomach spread around the body.

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