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This is a partner question to How do centaurs get enough calories to live?

Centaurs are a common mythological figure, with the body of a man where a horses head would be. They usually exist in fantasy worlds but let's say they are not sustained by magic but instead by biological processes.

This has obvious advantages in terms of speed, maneuverability, visibility, etc. There is however one obvious problem, they have the small mouth and throat of a human trying to support the energy and oxygen requirements of the massive body of both a human and a horse.

How would a running centaur be able to take in enough oxygen to support the massive horse body as well as their human part during intensive exercise.

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  • $\begingroup$ The horse's heart would be located roughly around the pelvis region of the person. So it seems likely that a centaur might have two hearts... $\endgroup$
    – user3334690
    Commented Jan 8, 2015 at 14:37
  • $\begingroup$ And where did the horse's lungs go in all this? If they've got the body of each combined, I see no reason that their organs couldn't be in some way linked. That gives them two sets of lungs, two hearts etc... one small set of lungs+heart for rapid gas exchange while sprinting, one large set for stamina and forcing blood around that big combobody $\endgroup$
    – Jon Story
    Commented Jan 8, 2015 at 16:01
  • $\begingroup$ The size of your mouth isn't what limits how much you breathe, but your lung capacity. Centaurs have both the lungs of a horse and of a person! $\endgroup$
    – JamesRyan
    Commented Jan 8, 2015 at 16:02

6 Answers 6

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A large part of getting Oxygen is the passage the air flows through and how restricted it is. Larger noses and sinuses are very important. Horses have large nostrils and their sinuses are most of the length of their skull. Sinuses not only warm and dampen the air they also filter it to help protect the lungs from a host of possible problems.

So a centaur is likely to have a disproportionately larger nose for its human body. (The Harry Potter movies have already portrayed them like this, as well as wide mouths and large teeth). So because the head can only have so much sinus it is unlikely that a Centaur will have the same top speed as a horse for the same amount of time. On top of that a Centaur is not built with the same aerodynamics as the horse and will be fighting resistance more. The Centaur would be more for power. If you add in Scott's answer with increased hemoglobin, then you could get some great sprinting bursts out of them or some other impressive feats. But they would have to recover to do them again. Kind of like anaerobic sprinting for us.

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  • $\begingroup$ Many related questions refer to the duplication of lungs. Would it be at all reasonable for something analogous to a horses larger sinuses to be located in the "human" half's "chest cavity"? Would things work out if centaurs had to be mouth breathers at all times? $\endgroup$
    – T.J.L.
    Commented Jan 22, 2016 at 19:57
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Borrow from Yaks and other high altitude adapted species.

Increase the amount of hemoglobin in the centaur's blood, thereby increasing oxygen transmission and the blood's oxygen storage capacity.

It would mean that centaurs could sprint for a fair length of time but not continuously: might that be why they developed archery?

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    $\begingroup$ So they would build up oxygen reserves in their blood and tissues that they used for a sprint but then would need to recover? $\endgroup$
    – Tim B
    Commented Jan 6, 2015 at 12:21
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    $\begingroup$ I like the way this answer contributes loosely to further explaining the other traits of the centaur. $\endgroup$
    – Smithers
    Commented Jan 6, 2015 at 17:14
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    $\begingroup$ @TimB - Extra hemoglobin wouldn't increase reserves, it would increase the O2 carrying capacity of blood. Sprinting would use ATP, replenished by the Kreb's cycle, and is a short burst, high intensity activity. $\endgroup$
    – JohnP
    Commented Jan 6, 2015 at 17:45
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    $\begingroup$ There's a limit as to how much haemoglobin you can pack into the red blood cells, and how many red blood cells you can put into the bloodstream, and a human or horse's levels of each is pretty much on the limit already - adding more RBCs can have negative consequences. $\endgroup$
    – Monty Wild ♦
    Commented Jan 6, 2015 at 20:54
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    $\begingroup$ @MontyWild But in a centaur, there's more bloodstream. The part below the waist would presumably have nearly as much blood volume (and as much hemoglobin) as a horse. $\endgroup$
    – Damian Yerrick
    Commented Jan 7, 2015 at 5:59
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Borrow from birds;

Airflow through the respiratory system in birds in a single direction and uses air sacs as bellows. You can dedicate the entire human torso to breathing with some sacs in the horse torso.

This will require extra ribs on the human torso to support the sacs though.

In full gallop the lower sacs can be powered by the movement of the front legs automatically increasing airflow.

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  • $\begingroup$ As an avid birder, I find this a terrible solution, even for a fantasy species. I don't see why a combination of two mammals would borrow from avian physiology. $\endgroup$
    – JYelton
    Commented Jan 8, 2015 at 17:42
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    $\begingroup$ @JYelton Convergent Evolution. Structures evolve based off of what is useful. Bats, for example, have very avian like wings. $\endgroup$
    – user458
    Commented Jun 2, 2015 at 18:37
  • $\begingroup$ @JYelton if you can make lungs that extract virtually all the oxygen from the air, yiu reduce the volume you need. Why's that a terrible solution? $\endgroup$
    – JDługosz
    Commented Jun 19, 2016 at 4:42
  • $\begingroup$ You don't need extra sacks in the human torso because you can use the horse lungs as sacks. Air can pass in both directions through human lungs while entering and exiting horse lungs. $\endgroup$
    – Pere
    Commented Sep 12, 2016 at 15:30
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tldr; you need to stop thinking of this as a human + horse and think of it as a totally unique animal that is neither.

There is however one obvious problem

...No, there are many, many problems.

  1. Ribcages. A ribcage exists to house and protects vital organs and regulate thoracic pressure. How and why would an animal exist with two? It wouldn't. Even if somehow responsibility was shared between organs in both thoraces, there's absolutely no way to fix the problem of...
  2. Oxygenation and circulation. The horses lungs are useless. The only way to move oxygenated blood to the horse extremities is by totally redoing blood flow started in the human torso. You would need much more powerful (and sizeable) lungs and blood vessels, i.e., a much bigger human torso and much smaller horse torso (since we've removed the lungs). So now we need to rethink...
  3. Muscles and skeleton. Since you've now totally changes the weight distribution on the skeleton, all of that is up in the air as well. The front legs would need to be much stronger, which means thick bones... much thicker than even a workhorse's front legs and certainly much thicker than your femur. With such an odd gait, a centaur would be much worse at short-distance running than a horse and much worse at long-distance running than a human.

To me, these are the most glaringly obvious problems, not to mention all the other systems in the body that wouldn't make sense.

There is a reason this animal doesn't exist in nature: it does not make sense. Instead of trying to merge a human and a horse, I would suggest creating a new animal that could exist and might pass as a centaur.

In the same way as a rhino might pass as a unicorn ;)

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    $\begingroup$ Real unicorns have curves! $\endgroup$
    – PTwr
    Commented Jan 8, 2015 at 8:41
  • $\begingroup$ You need proofreading $\endgroup$
    – JDługosz
    Commented Jun 19, 2016 at 4:45
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    $\begingroup$ Ribs from skull to pelvis is the norm, mammals are weird ones for having multiple section of the torso with no ribs. (mammal lost a lot of bones) Why not just have the human torso house only lungs while everything else in in the horse torso. $\endgroup$
    – John
    Commented Sep 28, 2019 at 2:21
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There are a few possible ways that centaurs would adapt to the increased O2 demands.

  • Higher utilization: Humans only use about 25% of the O2 that we inhale. We inhale 21% O2 (normal air percentage), and we exhale approx 14-16% O2. A higher utilization would support the increased demand (See next point)
  • Neovascularization: This is when the body grows new blood vessels into existing tissue to support increased demand. You will see this in athletes. In centaurs, it would be there from the start as they grow from colts. This would allow more blood to reach more tissue for higher demand.
  • Increased partial pressure - The alveoli (sacs in the lungs where oxygen exchange takes place) work on pressure differences. Blood has a higher CO2 pressure, lungs have higher O2 pressure. Osmosis flows from high to low pressure, so O2 flows to the blood, CO2 flows to the lungs for exchange.
  • As noted above, increased carrying capacity/volume: Humans at altitude cannot saturate hemoglobin (The oxygen binding component in blood) as well, so they compensate by producing more red blood cells. Centaurs either have more RBC's than normal, or they have adapted to be able to saturate the existing hemoglobin more. This would also support the partial pressure adaptation.
  • And, as always, there is the trite "Because magic".
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  • $\begingroup$ I think your partial pressure point needs more detail: how would the higher partial pressure be achieved? Particularly since increasing the overall pressure on one side would increase the partial pressure of the thing it's supposed to receive, (partially) undoing increased pressure on the other side. $\endgroup$
    – KRyan
    Commented Jan 6, 2015 at 18:43
  • $\begingroup$ @KRyan using the bird-based respiratory and reverse flow of blood vs the airflow $\endgroup$
    – ratchet freak
    Commented Jan 6, 2015 at 19:20
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There is an interesting example of centaurs in the first of the "World of Tiers" books by Philip José Farmer.
It's been a few years since I read them, but if memory serves they have a largish torso and head allowing for a large mouth/nostrils.
The lungs are in the horse body and the torso mainly serves as air-conduit.

As for the eating part in the other question (placed a comment there too): Large mouth and they are carnivorous as well.

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  • $\begingroup$ The problem with that is that the air has to go a long way to reach the lungs, I guess it's no harder than for a giraffe though. $\endgroup$
    – Tim B
    Commented Jan 6, 2015 at 19:59
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    $\begingroup$ @TimB, yes, simply a larger tidal volume. It would work better with two sets of lungs, though. $\endgroup$
    – Monty Wild ♦
    Commented Jan 6, 2015 at 20:56
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    $\begingroup$ That book came to my mind as well - in that case they had an extra breathing orifice roughly where the human torso transitioned into the horse torso. Having a very large nostril in their chest wasn't in keeping with appearances, but was necessary for breath volume. $\endgroup$
    – pluckedkiwi
    Commented Jan 6, 2015 at 21:48
  • $\begingroup$ @pluckedkiwi Interestingly enough that was an idea that occurred to me too. I think it would work but some of the other suggestions here are more elegant. $\endgroup$
    – Tim B
    Commented Jan 6, 2015 at 22:28
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    $\begingroup$ "Having a very large nostril in their chest wasn't in keeping with appearances" - you could say it is where the human navel is. $\endgroup$
    – Random832
    Commented Jan 7, 2015 at 20:41

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