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Situation:

The extraterrestrial vertebrate lifeforms have milky white blood rich in aqueous HCl, using an enzyme centered on aluminum oxychloride nanoparticles to transport oxygen, releasing chlorine atoms at high-oxygen concentration and oxygen at a high-chlorine concentration. However, the enzyme cannot transport CO2, and I am unsure if CO2 is soluble in HCl.

Question:

What methods are there to release CO2 from a body with HCl-rich blood? I also want to know the methods' effects (if any) on the respiration process and blood color.

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    $\begingroup$ Is your organism's blood composed of aqueous HCl? $\endgroup$
    – rubpy32
    Aug 5 at 4:34
  • $\begingroup$ @rubpy32 Yes, sorry if it's vague. Editing it to be clear $\endgroup$
    – E.UCIT
    Aug 5 at 4:36
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    $\begingroup$ Pure HCl boils at -85 °C. Hydrochloric acid is generally no more concentrated than 38%, as the HCl evaporates rapidly above that. So is your organism's blood HCl in water (or some other solvent), or actual HCl? $\endgroup$ Aug 5 at 4:37
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    $\begingroup$ I can't find references that address this particular issue directly, so no answer yet, but CO2 and HCl can react to form unstable chloroformic acid, similar to the CO2+water reaction which forms carbonic acid and is responsible for CO2 solubility in water, so I suspect that it should be soluble in aqueous HCl just fine. $\endgroup$ Aug 5 at 4:49
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    $\begingroup$ @LoganR.Kearsley the Wikipedia article indicates it's too unstable to be used in chemistry...though perhaps the esters mentioned could be formed biologically and broken down in the lungs, if this proves necessary. $\endgroup$ Aug 5 at 12:00

2 Answers 2

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It'll be different...

When CO2 dissolves in human blood, it becomes carbonic acid. The carbonic acid releases H+, acidifying the blood a little (but a very clinically relevant amount), and most of the CO2 circulates as bicarbonate ion (HCO3-). Acidifying any bicarbonate solution is famously fizzy as the CO2 is released in the reverse reaction.

Some (10%) of CO2 does move through human blood as dissolved gas. But it's nonpolar, and adding HCl to blood should if anything make the aqueous solution even more polar, so I would hazard a guess you'll have even less than 10% solubility of gas.

Logan Kearsley pointed out that adding HCl across one of the carbon dioxide bonds gets you chloroformic acid, but I think the equilibrium concentration of this compound is low, and it will have little effect on bulk CO2 transport. Similarly, it is possible to protonate CO2, but low stability means it should only be a curiosity unless some advanced biochemical means is evolved to change that.

Now in human blood some of the CO2 travels attached to hemoglobin (carboxyhemoglobin), but I think we're well into the range of acid hydrolysis of proteins, so let's scratch that one also.

What that leaves us is inorganic acid-stable catalysts which have evolved to sort of take over from proteins. The PROTOTYPE of these might be aluminum oxide, which adsorbs CO2 on its surface. But we're talking about a living being, which can presumably work wonders with its internal alumina. Even in inorganic industrial applications, alumina has many possible structures and catalytic activities, and your organisms will be assembling it atom by atom in a controlled way. So I'm going to go with alumina moving the CO2 at high efficiency in a manner fairly analogous to how hemoglobin carries oxygen.

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  • $\begingroup$ So an enzyme that uses alumina to transfer CO2? How does this affect blood color, if at all, or the respiration process, considering two different enzymes transporting CO2 and O2 separately throughout the body? $\endgroup$
    – E.UCIT
    Aug 5 at 23:15
  • $\begingroup$ It's a hard call. We've never seen life evolve "inorganic" materials the way they do enzymes - while there are many proteins with, say, molybdenum-containing cofactors, there is no genetic code they can randomly alter to move around how the Mo atoms are arranged. Having two enzymes seems inefficient, but there may be ways to make up for that. $\endgroup$ Aug 7 at 2:33
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I'm not aware of any issues that would keep the CO2 from just dissolving in the aqueous part of the blood as it does in humans. The blood contains less water, but a moderate reduction in solubility might actually make excretion of CO2 more effective. You don't need to dissolve much, human blood only contains tens of millimoles of CO2 per liter of blood. Its role in controlling pH would likely have to be taken by something else, though, considering the HCl present.

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