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Is blood that decreases in viscosity as it increases in temperature plausible? It's supposed to be a biomodification that decreases the amount of energy the heart needs to put in during strenuous physical activity; I'm just not sure what biochemical mechanisms there might be IRL that would enable such a thing to exist.

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    $\begingroup$ I'm pretty sure the viscosity of most liquids decreases as you increase the temperature. If not all of them! $\endgroup$
    – Hearth
    Jul 26 at 20:10
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That the viscosity of a liquid decreases with increasing temperature it is very possible. Wikipedia has a section dedicated to the various possible models and relations to calculate the change based on the temperature.

However I think you are looking at it from the wrong side. The core of a living body is usually at a constant temperature, while the extremities fluctuate more, usually being at a lower temperature than the core. This would mean that the viscosity would increase, which, topped with the reduced section of the blood vessels at the far end of the body, would result in an increased effort for the heart.

If any, you want your blood to not vary much its viscosity with temperature. And don't forget that a couple degrees is already a large variation in temperature for a living being.

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    $\begingroup$ Good points both. If your fingers skin gets to 30C thats ok. If your core temp drops to 30 C you are dead. Similar for heat, I can wash the dishes in that 50C water (it hurts a bit but ok), but if my core temp reaches 42C I'll be rushed to the hospital and injected anti fever meds $\endgroup$
    – Hobbamok
    Jul 26 at 13:47
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    $\begingroup$ From a fluid dynamics point of view, blood isn't well described by a single coefficient of viscosity (it's a viscoelastic fluid made up of a Newtonian fluid in the form of plasma with cells in it). To change the effective viscosity of the fluid as a function of outside temperature (i.e. keeping the body core at ~37), you could suggest some chemical process from the brain that instructs the blood cells to change properties (i.e. be more spongy/stiff) which would change effective viscosity. $\endgroup$
    – Ed Smith
    Jul 26 at 16:29
  • $\begingroup$ @EdSmith I think I'll do that - thanks. $\endgroup$
    – KEY_ABRADE
    Jul 26 at 19:11
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Maybe, but it seems implausible.

Fats and oils can have these sorts of properties, where viscosity decreases as temperature increases. However, they tends to have a melting point slightly below human body temperature, and as a result, if the body temperature dipped slightly below that, the organism would face the risk of their blood congealing into a solid mass.

Additionally, different sorts of chemicals dissolve in oils when compared to those that dissolve in water, so the being in question would have to have an entirely different oil-based biochemistry, rather than a water-based one.

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    $\begingroup$ Even water has this property. Just look at this data - going from 15 to 80 degrees C drops the viscosity by over two thirds! $\endgroup$
    – user253751
    Jul 26 at 14:05
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This is a "just in case" comment, meant to cover a possible scenario where viscosity isn't technically what you were looking for, as this is a common misconception outside health sciences. Of course you as an author decide what crazy worlds we would explore, but I'd just like to note that many languages talk casually about "blood thinner" medications and use terminology which only superficially seems to overlap with the idea of physical viscosity. Such medications and physiological events nonetheless are about a biochemical reactions which affect the process of blood coagulation due to complex interactions between many proteins and other factors – it's a not a simple temperature-dependent change in viscosity. The extreme case is clotting, which can look pretty terrifying.

Hopefully this is actually a good treatise of physical (non-chemical) properties of blood, though. d(^_^)

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