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How would human appearance be different from what it is now if our circulatory system used Hemocyanin for oxygen transport instead of Hemoglobin?

Limitations, restrictions, additional details, etc. :

Hemoglobin can still be present for any functions, other than oxygen transport, it might perform currently, but only in the absolute minimum quantities needed for those functions, and only if Hemocyanin cannot perform those same functions instead. Hemoglobin should be removed to the maximum extent possible without compromising critical biological functions other than oxygen transport, and Hemocyanin should replace any and all Hemoglobin possible, in as many functions as it can perform in the stead of Hemoglobin.

Oxygen transport functions should be taken over utterly and completely by Hemocyanins, in whatever quantities needed to match the oxygen transport capacities of the now missing Hemoglobin. In other words, any difference in efficiency should be made up for in quantity.

This question is about appearance changes, but bonus points for any other unusual side effects this type of change might cause.

If a reason for the change is needed, let's say Aliens have been abducting humans to keep as pets, and are using combinations of selective breeding, transfusions, and direct gene manipulation to achieve their desired aesthetic in their pet population (like how humans breed dogs and cats, etc.). Or handwavium, whichever you prefer.

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  • $\begingroup$ Hemoglobin is the oxygen-carrying pigment in the entire lineage of vertebrates. Replacing it with hemocyanin would require massive re-engineering of just about the entire biochemistry; the result is very unlikely to be similar to a human. $\endgroup$
    – AlexP
    Nov 2, 2019 at 20:53

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As Willk said, hemoglobin is a very widespread protein and just "swapping it out" in humans (or other vertebrates) is pretty much impossible, without turning your test subject into something that is most definitely not whatever you started out with.

If you want to handwavium the fact that vertebrates have been using hemoglobin for the past X million years, here are some important elements to take into account:

  • Hemocyanine isn't contained inside specific cells, it's directly in solution in the animal's hemolymph. This would have consequences on how blood transfusions would work, and on how some vital statistics are measured. I don't think it would be the most relevant for your story, but it does need to be kept in mind for medical issues.

  • Hemocyanine is generally weaker than hemoglobin at transporting oxygen through a body, because not all hemocyanines bind it in a cooperative manner. However,

  • Hemocynanin has been found to be more efficient at transferring oxygen throughout a body in low temperature and high pressure conditions, such as the ocean floor where crabs and bivalves live. Your Cyanohumans may be more resistant to these factors due to this. But spiders and scorpions that live in hot deserts also do fine with hemocyanine.

  • Hemocyanin will give a blue to purple hue to your hemolymph, depending on which form it takes. I imagine this is one of the effects you wanted to be described (as in, where will this hue change be visible), so here are some pointers. Your lips, tongue, mouth and pretty much all mucous membranes are red due to their increased blood content, and would therefore be bluish if their hemolymph contained hemocyanine instead of hemoglobin. The same goes for other sensitive body parts, like your hand and feet, the whites of your eyes, or skin that has a rash, even small. Your Cyanohumans will therefore look somewhat like undead people, especially if they're being kept as indoor pets by aliens.

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Hemoglobin is well integrated into the organism. Swapping in foreign stuff would cause the organism to crash.

There are lots of aspects of the organism that depend on the structure of hemoglobin. How it is carried in the red cell. Iron metabolism. Copper metabolism. Handling degradation products. Oxygen affinity. Really a bucketload of integral stuff. Hemoglobin has been in our lineage a really long time and is at the root of a huge array of life sustaining processes.

Swapping in hemocyanin for hemoglobin is like swapping in the code for a Mac CPU register for the same code in Windows. It's a CPU register, right? You can imagine how that would work.

Swapping in hemocyanin for hemoglobin seems a nonstarter for me. But a vertebrate that could make and use hemocyanin for specialty applications, extreme temperature backup, or maybe simple copper chelation is a whole other thing. That could totally work. Certain hemocyanin are, despite being antigenically every foreign, somehow not immunogenic - I am thinking of keyhole limpet hemocyanin which is used in vaccine manufacture. Your engineered humans might have loads of KLH in them, maybe preferentially in their skin.

And the KLH plus oxygen would be a beautiful "opalescent blue" which I know is what you really wanted for an answer.

blue klh https://twitter.com/StellarBiotech/status/1009127848756457472

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