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This is for an alien life form with similar biology to mammalian Earth life. I already know about hemerythrin. I don't like the disadvantages it has. What are different feasible ways for blood to be purple(dark purple would be best) without it being toxic or having any other disadvantages in comparison to red blood?

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    $\begingroup$ Does this answer your question? What problems will a person with purple blood (hemerithrin instead of hemoglobin) have? $\endgroup$
    – sphennings
    Sep 27 at 3:11
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    $\begingroup$ @sphennings from what I learned earlier today, hemerithrin isn't as efficient in carrying oxygen as hemoglobin, so I wanted to know what my other options were, but this is still helpful. $\endgroup$ Sep 27 at 3:38
  • $\begingroup$ Does this answer your question? Is bi-metallic blood possible to develop organically in a species?. BTW, The science of life on Earth is fairly well known while the science of life outside of Earth is utterly unknown. So you're kinda asking for the moon - just to set an expectation. $\endgroup$ Sep 27 at 5:12
  • $\begingroup$ In humans? In Earth-based mammalian life? Earth-based life in general? Science fiction life from another planet? Modified/augmented humans? Augmented with technology? With magic? For at least some versions of this question, it's not even an interesting question. $\endgroup$
    – John O
    Sep 27 at 13:55
  • $\begingroup$ i'd go with the toxic options for alien lifeforms. something that is toxic to you is just something that you neglected to have a resistance for. Oxygen itself is already incredibly toxic. so much so that the single largest die-off on earth happened because of a sudden increase in Oxygen levels. it is called the great origination event. $\endgroup$ Sep 27 at 17:35
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Red and Blue Make Purple:

Your species has two parallel oxygen-carrying systems: one using hemoglobin, one using hemocyanin. While I won't try to justify how it got that way, there could be some good uses for this. Hemocyanin is found as free proteins in the blood, so these smaller molecules than red cells may help with peripheral perfusion into tissues where the blood vessels are extremely small. Or perhaps your species regularly encounters toxins that affect hemoglobin, like cyanide, that don't affect hemocyanin. And while carbon monoxide will react with hemocyanin, it's reaction with CO is WEAKER than with oxygen, rather than stronger like hemoglobin. So two parallel oxygenation systems (while a bit competitive, potentially) allow greater flexibility against environmental toxins. (Cyanide also affects the mitochondria, so other adaptations would be needed for that).

The result is that oxygenated blood will be purple, while oxygen-poor blood will be red.

Or you can bypass the fancy reasoning and simply have a blue compound in the blood that causes the red blood to appear purple.

color chart for blood

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    $\begingroup$ +1 just for the really good chart. $\endgroup$ Sep 27 at 5:09
  • $\begingroup$ Mutual coexisance maybe? I can imagine, many millions of years ago, two organisms, parasite and prey, co-existed in a sort of mutually beneficial relationship. Being two totally different species, they could have different blood. Now, after 10 million years of co-evolution, the two species merged as one. The two blood systems are the archaic left over of the merger. This kind of biological absorption is what is suspected happened to create our mitochondria. $\endgroup$
    – Sonvar
    Sep 27 at 18:44
  • $\begingroup$ But what would this blue compound be and what could its purpose be? $\endgroup$ Sep 27 at 21:17
  • $\begingroup$ @Chickenpeep So, weird idea - your person has hereditary methhemoglobinemia en.wikipedia.org/wiki/Cyanidehttps://en.wikipedia.org/wiki/… which can turn them blue (including the blood) but their body also can compensate by producing methylene Blue dye en.wikipedia.org/wiki/Methylene_blue naturally (the treatment for methhemoglobinemia) although too much dye is toxic eventually as well. $\endgroup$
    – DWKraus
    Sep 27 at 23:16
  • $\begingroup$ @Chickenpeep There's also Prussian Blue en.wikipedia.org/wiki/Prussian_blue , which is colloidal and non-toxic, a treatment for thallium and cesium heavy metal poisoning. I could envision a biochemical system where red blood cells contain enzymes to react cyanide to Prussian blue, or contain it as a means of dealing with environmental thallium poisoning. $\endgroup$
    – DWKraus
    Sep 27 at 23:30
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I guess I should make my comment above an answer. I've been thinking of it and it can be a solution, built upon the useful information provided by @DWKraus

Co-Evolved Creatures

In this case, 10's of millions of years ago, a parasite and host developed a mutual co-existence relationship. The host provided sustenance and transportation, and the parasite provided something, maybe a form of protection (defensive venomous stingers), increased energy or a better method of metabolism (such as what mitochondria does now.) Some mechanism that made it that the two cannot live separate from each other.

Now the host could be a mammal, having red blood and the parasite being a type of cephalopod, having blue blood. After many millions of years of co-evolution, the host began to absorb the parasite and integrate its body into itself, becoming an endosymbiont. In this method, many different species would develop, creating many variations to finally creating a successful body plan.

Now, in this 10-50 million years of evolution, it is not enough time for the full conversion to be completed. Some of the necessary organs of the parasite have yet to be fully powered by the host's metabolism and the two blood systems cannot power the organs of both systems, so a type of separate circulatory system had to develop to power the organs of both creatures.

Now, the blood is not purple as you would like, but if you cut this creature, as the answer above states, red and blue make purple. The two bloods spill out and mix, making it look purple.

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  • $\begingroup$ Neat, but not right for this particular creature. $\endgroup$ Oct 2 at 18:36

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