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It’s an occasional trope in sci-fi where warrior species get two hearts to better survive combat. Like the Star Trek Klingons (apparently), Halo Sangheili and the 40K Space Marines.

However, two hearts seems more a recipe for increasing your chances of death when you get hit in the torso. The downsides:

  • you lose a ton of blood. When hit in one of the hearts you lose the blood in that heart, likely in the aorta leading up to that heart and the blood in the vena cava leading up to the heart. On top of that, you are likely in combat when it happens so your blood is rushing, causing more blood loss at the point of impact as you’ll likely be pumping 0,25 L of blood into that heart per second (based on around 30 L of blood per minute over both hearts).
  • The aorta of your damaged heart will likely be compromised in the same hit. The still intact heart will now pump blood into its own aorta and where these two connect you get blood flowing into the damaged heart. This can partially be remedied with another one-way valve at the point of merging of the two aortas.
  • the vena cava splits up to supply both Hearts, adding surface area where you can hit a vital artery (more vital than both leg arteries combined). Similarly you now have two aortas that can be hit that merge in the center.

The chances of two hearts being beneficial and you surviving because of it, as opposed to getting hit in an empty spot where a second heart and its infrastructure might be, seems ultimately small. They in fact seem to increase your chances of dying instead.

How can a dual heart system be set up that increases your chances of survival even if one heart or the infrastructure* around it is hit?

*Vena cava and aorta, also the pulmonary artery.

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    $\begingroup$ Whenever you design a hydraulic system with two or more pumps, either for redundancy or for performance, you must also design the appropriate valves and shunts which govern the operation of the hydraulic system when one or more pumps is damaged, or just not operational. In general, when you design any kind of system and survivability or graceful degradation are design requirements, you need to identify potential failure modes and mitigate them accordingly. $\endgroup$
    – AlexP
    Feb 16 at 11:19
  • $\begingroup$ Horse has 5 hearts: 1♥️+ 4 frogs(hooves) $\endgroup$
    – user6760
    Feb 16 at 12:03
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    $\begingroup$ @AlexP yes, the point of the question. $\endgroup$
    – Demigan
    Feb 16 at 15:14
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    $\begingroup$ Thats a question for a time lord. $\endgroup$
    – DonQuiKong
    Feb 16 at 23:01
  • $\begingroup$ @user6760, By that logic, humans have three. Your leg muscles act to push venous blood back to the core. $\endgroup$ Feb 19 at 0:29

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Vasoconstriction

The actual volume of blood in the heart itself isn't particularly large -- only about 0.25 L. A reasonable puddle, but it's not going to kill you to lose it. (It's less than a standard blood donation.)

The reason why open wounds to the heart & aorta are typically fatal is that it isn't just the blood in the heart which you lose. You potentially lose all the blood flowing through the heart, which for an average human ~5L per minute at rest. And during stress or exertion that can increase 3-5 times. So it isn't the blood which is draining out of the heart which kills you - it's the blood which is being actively squirted out the wound.

If a creature evolved multiple hearts for redundancy sake, then they're likely to have also evolved mechanisms for isolating a damaged heart. This can be easily accomplished by having muscles which clamp down on the vessels leading to and from the heart when it's damaged. (Existing animals have this to some extent or the other, but typically for distal body parts, rather than the heart.) Sure, you may lose some blood which is in the heart and aorta, but you stop the massive blood loss. If the circulatory system is cleverly arranged, then judicious closing of selected veins and arteries could re-route blood around gaping wounds, shunting flow to the back-up heart and preventing fatal blood loss until the wound heals. This wouldn't even need to be a conscious effort - autonomic feedback systems could recognize damage and close vessels as appropriate.

Of course, this potentially allows for interesting side effects. If the body has the ability to automatically cut off the heart, one could imagine that in certain circumstances it could be fooled. For example, a suitably large emotional shock could cause your Klingheli Marines to tense up the shutoff valves to both of their hearts, thinking them both damaged, resulting in them passing out or even dying from a "broken" heart.

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    $\begingroup$ And note that the body could be engineered to do a better job of cutting off circulation where there are breaches--it's just that in our primitive past that has been of no value. What good does it do to cut circulation if the femoral artery is cut because in our primitive past that was an unsurvivable would anyway. Evolution doesn't assign any value to live long enough to reach the ER. $\endgroup$ Feb 17 at 23:38
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Have two independent cardiovascular systems.

The simplest recipe is to have each pump blood through one side of the body. This is very scientifically plausible - symmetry is very common in nature, the chambers of the heart just need to not have merged. There could be a little bit of crossover for the internal organs (but they'll all likely be duplicated) and the brain.

A less obvious, but also plausible scenario is two independent systems covering the entire body. Perhaps they could have evolved for different purposes at first, e.g. one for oxygenation and the other for nutrient transport, but eventually crossed over in their functions.

Two independent systems could each be fairly simple: a pair of 2-chambered hearts, like in fish, as opposed to a 4-chambered mammalian heart.

enter image description here

The systems could also be separate in their coverage: one circulatory system powers the brain and the internal organs, circulating oxygen, high volume of nutrients, and hormones; the other supplies the limbs. The practical advantage is that limbs can benefit from high blood pressure and flow, to cope with strenuous exercise, while internal organs work best at a steady flow. A pack creature could thus survive a severe blood loss in the limbs, then rest and recover.

In a way, mammals already have two hearts, but working the same loop, which requires all chambers of the heart to be functional.

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  • $\begingroup$ I wonder if it could be possible to wire one (smaller) heart to provide blood from lungs to brain and second heart to distribute semi-oxidated blood after brain had it to the rest of the body. It seems like it would need to be a race that lives in more peaceful conditions (no need to chase prey or run from predators), that has very, very strong evolutionary inclination to develop intelligence - generally making the brain organ crucial for survival and reproduction, while not requiring a fit body to match. Possibly a giant race, on par with whales in terms of size? $\endgroup$ Feb 16 at 22:16
  • $\begingroup$ @Yksisarvinen Completely possible. But if it's the same loop, then any leak affects both the brain and the body. I actually think merging the lungs and the brain could be more effective for survival in that scenario. $\endgroup$
    – Therac
    Feb 17 at 5:03
  • $\begingroup$ No need to chase prey or run from predators and has strong evolutionary inclination to develop intelligence... I'm not sure what the evolutionary drive is to force the development of problem solving and intelligence if it's not based on fight or flight @Yksisarvinen $\endgroup$ Feb 18 at 19:53
  • $\begingroup$ @ChrisSchaller You're missing at least one "f": evolutionary pressure is about survival AND reproduction. Socialization has excellent survival benefits for the species as a whole, but intelligence for intra-social competition has excellent reproduction benefits for individuals within that species. $\endgroup$
    – Jay McEh
    Feb 20 at 19:27
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that's why they wear all that bulky armor, silly

now I can't be sure about Klingons, but sangheili and space Marines sure do have a lot of protective gear...

but in all seriousness, I don't think having a two heart system would be too great, especially for a humanoid, as it would require an expansion of the chest area which is already quite vulnerable on its own, requiring a ribcage. a hit to a larger chest would not only cause damage to the heart, but also collateral damage due to fragmentation of the larger ribs supporting it.

your idea of a one way valve is the only conclusion I could come to as well, but being hit would still result in incapacitation. maybe a strategy to combine with the valve system would be to pump blood to only vital organs if one heart is compromised.

tl;dr: heart damage induced Raynaud's phenomenon

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Put them in sequence, one usually in restmode, allowing just throughflow. In extreme stress, both hearts get active, by syncing up the heartbeats. But then the blood pressure waterhammer https://en.wikipedia.org/wiki/Water_hammer starts damaging organs and braintissue with it.

Means, it has the risk of eccmo https://en.wikipedia.org/wiki/Extracorporeal_membrane_oxygenation#Side_effects_and_complications with blockages and hemorraghes.

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    $\begingroup$ Water hammer happens when air is in the water, that would be a. Air embolism. I expect that not to be a problem. More that blood might flow the wrong direction as one heart might overpower the other (the one that is active most of the time). As for “flowhrough”, due to the chambers I’d expect a problem there, so maybe a bypass artery that passes by the backup heart until it’s necessary? $\endgroup$
    – Demigan
    Feb 16 at 15:12
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There are real animals with multiple hearts, various arrangements. No need to speculate.

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    $\begingroup$ These all solve a specific issue and do not form a separate system to keep pumping. Should one of these hearts be damaged by for example being shot then the creature dies anyway, which kind of defeats the purpose of having multiple hearts to survive more. $\endgroup$
    – Demigan
    Feb 19 at 9:13
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It's easy to get two hearts working together: you just need to create a 3 way junction in your hearts input veins and arteries and where blood is pumped into the lungs and body, have both blood vessels from both hearts connect to the lungs and the body as one and seal off both hearts pumping out with an alternating valve that only allows one to pump out at once and seal off the recovering heart, so you don't break the circulation with reverse flow.

If you do the plumbing right you can actually easily insert a second heart in the human biology.

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Frame challenge

Do you know what the universes of Klingons, Sangheili and Space Marines have in common (apart from multiple hearts)? They don't explain how multiple hearts are a combat advantage. And the reason they don't do that is because it is irrelevant. To be more precise, in world building, the how is irrelevant 99% of the time.

You are writing a story, not a biology paper. The important thing in a story is the characters, not how their anatomy works. Write those characters, and let your readers fill in the how blanks with their own imaginations.

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    $\begingroup$ You could say this to literally any question on Worldbuilding. If we accept that kind of answer, there's no point to the whole site. $\endgroup$ Feb 19 at 15:59
  • $\begingroup$ Incorrect. In world building, the "how" is irrelevant 99% of the time, and this is a "how" question. $\endgroup$
    – Ian Kemp
    Feb 20 at 13:32
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    $\begingroup$ A full, complete, encyclopedic "how" is unnecessary to provide to the reader 99% of the time, yes. But a well-considered "how" can inform interesting plot elements, or inspire thematic resonance, or simply help form the underlying structure that guides a story in being more than "sure, this might as well happen now I guess". An author doesn't need to know where each of their character's shoelaces are manufactured, but if they care enough to ask about it, then the answer is potentially interesting. $\endgroup$
    – Jay McEh
    Feb 20 at 19:23
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    $\begingroup$ What other kind of question would you prefer? "How" is 99% of the Worldbuilding StackExchange! $\endgroup$ Feb 21 at 4:13

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