3
$\begingroup$

It's a generally accepted fact that animals have only one heart, at least in vertebrates. Reality shows that duplicates certainly aren't necessary, but is there any reason why they aren't possible? There's plenty of fictional creatures with more than one heart, not always from genetic engineering or artificial additions. A second heart would offer redundancy if one failed, and there's probably some other benefits that could be had.

Edit for clarification:

  • I should make it explicit that I'm considering vertebrate animals here, not insects or worms, with a focus on redundancy as per the underlying premise detailed below (one heart failing can cause impairment, but it shouldn't be fatal). Mammalian examples are preferred, since I'm trying to attract answers easily applicable for other world-builders looking to design their own creatures; the majority of fictional creatures, at least the intelligent ones that get all the attention, do tend to be humanoids. Real-world examples of reptiles with multiple hearts are perfectly acceptable as well, however, given that there are plenty of other fictional creatures like dragons that fall into that category.

There is an underlying premise, for the purposes of this question: a self-regenerating creature (a vertebrate, not an earthworm). Having a second heart would allow it to survive the failure of one until it could heal or otherwise rebuild the tissue and restart that heart. Answers that also cover any other singular organs whose sudden failure would kill someone quickly (the liver, for instance) are appreciated but not required; the heart appears to be the biggest sticking point.

I am aware that the standard closed circulatory system seen in vertebrates like us humans would likely have problems with a second heart also pumping blood, so bear that in mind. I've thought of a hypothetical case of having twin (separate) circulatory systems, each with its own little heart, flowing through the whole body, wherein a failure would reduce one's capabilities but not be fatal or actively damaging in itself (duplication and appropriate shrinking of lungs, liver, windpipe, etc., optional). I currently have no idea whatsoever if that's actually feasible, though, so I'll leave it here as inspiration; if anyone wants to expand on this idea and prove it valid for an answer, go ahead!

I'm looking for an answer that can plausibly demonstrate how a creature might have two or more hearts; it doesn't need to be human, although something generally mammalian is preferable to make it more readily applicable for other world-builders. Bonus points for anyone who can come up with a good reason for this to evolve in nature and be favored over a single-heart system for that creature, but genetic engineering is a perfectly acceptable answer.

$\endgroup$
17
  • $\begingroup$ from commenter in my previous question, some say that having more than one heart in a standard circulatory system could easily cause a rupture if they do not beat in absolutely perfect synchrony. $\endgroup$ – Li Jun Nov 16 '20 at 23:52
  • 2
    $\begingroup$ In all honesty, how plausible it is can also depend on how you define a heart. Depending on the definition, worms can have either 0 or 10 hearts. In the real world, you can also find in animals "systems" that, while not hearts themselves, will aid the heart in pumping the blood throughout the body. An example? Your muscles. $\endgroup$ – ProjectApex Nov 17 '20 at 0:36
  • $\begingroup$ @ProjectApex I was thinking of what people usually think of: the standard kind of heart, like what you might see in a human, or a bird, or a cat, etc., not whatever worms or insects can do. $\endgroup$ – Palarran Nov 17 '20 at 2:05
  • 3
    $\begingroup$ @Palaran: What do people usually think of? When I read the title of the question my first reaction was to respond with the counter example of octopuses, which have three clearly distinct hearts: one for the systemic circulation and a pair for the respiratory circulation (pumping blood into the paired gills). If octopuses can do it, there is no reason why a modified mammal couldn't. $\endgroup$ – AlexP Nov 17 '20 at 12:42
  • 1
    $\begingroup$ Downvote: Lack of research. There are numerous examples of multi-heart organisms on Earth. $\endgroup$ – Frostfyre Nov 17 '20 at 13:37
6
$\begingroup$

First off, the heart is a pump. Nothing more or less.

Second, humans don't have one, they have two. They just happen to be together. One pumps to the lungs, and the other to the body as a whole. They are referred to as one because anatomists found one organ in the beast.

Third, as @ProjectApex pointed out, the musculature acts as a third set of pumps, using much the same system.

Fourth, if you think about it, I think you will find that a large number of hearts could be quite beneficial. In particular, pumps into and out of each limb, and otherwise around the body. I suspect this would be very useful for a large fast-moving creature.

It turns out a downside for the human pump system is that this one pump must get blood to every part of the body. This requires an overpressure system that leads to a variety of problems (clogged arteries, spurting wounds, ...). With a large number of pumps, these need not occur. Blood flow might be very smooth, with no concept of "pulse" or "blood pressure".

As for evolution, it could easily just be a different random configuration in the development of a circulatory system. Changing from a one heart/two pump system to a multiple system is harder, but not I think impossible. I think it would develop as auxillary pumps for distant parts of the body, and then add more and more, and finally decrease the role of a central organ.

$\endgroup$
4
$\begingroup$

There can definitely be more than one heart.

As David G pointed out, we mammals have 4 hears - our heart has 4 chambers. The right and the left heart are distinct from each other, operate at different pressures and do different jobs. The 2 chambers of the right heart push blood through the resistance of the pulmonary vasculature and then the left heart pumps that blood back out to the body.

Our distant cousins the hagfish have a similar sort of deal - one main heart to get blood out to the body and then helper hearts to overcome resistance from various high resistance vascular systems.

The vertebrate heart: an evolutionary perspective

Living agnathans have a circulatory system which consists of the main ‘systemic (or brachial)’ heart and three accessory hearts (Fig. 2). The ‘portal’ heart is used to pump blood from the intestines to the liver, the ‘cardinal’ heart pumps blood from the head to the body and ‘caudal’ heart pumps blood from the trunk and kidneys to the rest of the body

I find it interesting that hagfish have these extra hearts but are sluggish with low pressure systems. I guess if your main heart operates at a low pressure it needs a lot of help along the way. I have read it posited that sauropods had an extra heart to get blood up to the head but I dont think it has been proven. To be plausible, have the extra heart with a job to do such that the distant ancestors of your creature gained a fitness advantage by having this extra heart.

$\endgroup$
2
$\begingroup$

Since you specified vertebrae (insects have quite different circulatory systems), there is indeed an animal that has multiple hearts (kind of) - the hagfish. From wikipedia:

The hagfish circulatory system also consists of multiple accessory pumps throughout the body, which are considered auxiliary “hearts”.

$\endgroup$
0
$\begingroup$

Frame challenge.

Having multiple hearts on a vertebrate more complex than a fish or amphibian could be troublesome. The heart pumps blood by contracting and relaxing. You'd have to keep them in perfect synchrony to the millisecond. If they are not in sync you can cause peaks of high pressure, or moments of low pressure. For an animal with the level of activity we humans have, that would mean much more frequent cardiac arrests.

Worse - if your hearts are not in sync, very rapid changes in pressure between the two hearts would be equivalent to microscopic episodes of uncontrolled decompression, which would give the creature symptoms of decompression sickness.

The "auxiliary" hearts of the hagfish mentioned in Radovan's answer work because of the hagfish's very simple circulatory system and its very low pressure. More complex creatures would probably not benefit for it.


An edit to address this comment by Nuclear Hogie:

I'm not convinced synchrony is much of a problem, given that the human heart relies on a precise synchronization of the different chambers - we already have multiple independently controlled pumps that only work efficiently when synchronized. (...)

The four chambers of the heart are kept in sync by one single relay, the sinoatrial node. Having more hearts would require more nodes, which would make synchronization more complex. The alternative is to have the current existing heart have more chambers, which would make it bigger and require much more space.


Despite all I said above, it is not clear yet how sauropods managed to pump blood all the way to their heads. Some hypothesis involve auxiliary hearts in their long necks. This is still an ongoing discussion in paleontology. Maybe this could be a source of inspiration for you.

$\endgroup$
4
  • $\begingroup$ I'm not convinced synchrony is much of a problem, given that the human heart relies on a precise synchronization of the different chambers - we already have multiple independently controlled pumps that only work efficiently when synchronized. Adding distance between the pumps might make synchronization between the hearts a little trickier, but certainly not impossible. $\endgroup$ – Nuclear Hoagie Nov 17 '20 at 15:50
  • $\begingroup$ @NuclearHoagie good point, I edited the post to address it. $\endgroup$ – The Square-Cube Law Nov 17 '20 at 16:01
  • $\begingroup$ Pressure relief chambers. Voids next to some of the major blood vessels which are made of an elastic tissue. Normally collapsed, if the blood pressure goes too high it overcomes the elasticity of the walls, which allow blood to flow into them, thus controlling pressure surges. As the blood pressure decreases, the walls contract, returning the blood to the main circulatory system. $\endgroup$ – Keith Morrison Nov 19 '20 at 3:04
  • 1
    $\begingroup$ I'm not sure synchrony is a good thing. We do it for our four chambers of love because the handoff from the atria to the ventricles increases efficiency in our current design by allowing for each chamber to be largely empty when it is ready to next fill, increasing the volume moved in each stroke, so to speak. If you have multiple pumps in sequence, depending on design, you're going to be additively increasing pressure right after each beat, and decreasing pressure right before. If the beats were dispersed fairly evenly, like a multiphase signal, the maximum and minimum pressure would be lower $\endgroup$ – TheEnvironmentalist May 7 at 11:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.