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Magic lets you take a lot of shortcuts when it comes to fighting, and occasionally do things that normal people may refer to as 'cheating', such as installing a second, redundant heart. In the event the first heart goes down, (suppose someone stabs you clean through it, for instance) the second heart kicks in and keeps the blood pumping through your body.

The redundant heart itself is somewhat magical in nature to let it actually work a bit better. It's a dormant organ, that upon the demise of the first heart, sprout and injects itself into the vein and artery network of a human being. It can also, regardless of position, supply blood to and from the lungs. Also, being magical, it has no problem doing this and is capable of matching the original heart's flow regardless of where it is on the body, because magic. (The downside is that this redundant heart only works for 24 hours, at which point the original heart must take back over.)

Now, the question is: where should this redundant heart be placed? It must take up room, after all. A bit of organ rearrangement is fine, and the heart can be placed anywhere there's tissue. You can even grow a place for it, for instance, if you want to have it placed in your hand, you can have it grown as a bulge in the palm of the hand. Presumably you'd just use that arm as a shield arm. (Which isn't a bad idea, but since that hinders the use of the hand outside combat, it's not really what I'm looking for.)

The goal is to place the heart somewhere that, during the course of a normal battle, won't be stabbed by accident, but also won't inhibit normal motions.

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    $\begingroup$ The heel, obviously... $\endgroup$ – Nate White Dec 3 at 3:39
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    $\begingroup$ It goes in the heel. Your warrior can go to war then and get stabbed in the (real) heart and survive. But some guy named Paris then shoots the redundant heart and instead of "two hearts" you live in legends as an "Achiles' heel". $\endgroup$ – VLAZ Dec 3 at 6:31
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    $\begingroup$ "In the event the first heart goes down, (suppose someone stabs you clean through it, for instance) the second heart kicks in and keeps the blood pumping through your body ..." ... hmmm ... did you mean keeps the blood pumping through your body and gushing out through the first perforated heart ! You might want a quarantine mechanism of the first heart before the second one kicks in! $\endgroup$ – Hoki Dec 3 at 14:41
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    $\begingroup$ @Graham only certain soldiers in certain specific situations have made that trade-off. If the magical new heart just sprouts up as a replacement, why not have the damaged heart simultaneously shrink? Heck, KISS and have the magic repair the existing heart!! $\endgroup$ – RonJohn Dec 3 at 17:50
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    $\begingroup$ Pak Protectors have 2nd heart in groin afair. Note supply of Thalium Oxide needed to ensure tree of life viability. :-). $\endgroup$ – Russell McMahon Dec 4 at 2:46

13 Answers 13

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I'm going to issue a framing challenge here, because I think you're asking the wrong question.

If you're trying to make it more likely for someone to survive battlefield injuries, a redundant heart isn't really what you need. Damage to the heart isn't what causes most people to die on the battlefield. What kills them is exsanguination. It doesn't matter if there's a spare heart in there if there's no blood to pump.

So, if you really want to get creative and keep people alive, what you want is a redundant CIRCULATORY SYSTEM.

I'm thinking a smaller, completely separate network of blood vessels that ONLY supply the vital organs. Have the vessels run along the inside of bones of the chest, spine, and skull so that you can't have major vessels cut open and bleed out. Put the second heart at the base of the skull. In the event of someone opening up an artery, the secondary system would keep just enough oxygen going to keep heart, lungs, brain, etc going long enough for someone to get to you and seal you up, maybe get some fresh blood in you and get the whole business started back up again.

This wouldn't help for anything REALLY traumatic like having your skull cracked open or your chest crushed, but the majority of the time it's the major arteries in your legs and abdomen that get opened and dump all the blood out, so if you've got a way to keep that blood loss from impacting your brain and heart, you're a LOT more likely to survive.

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    $\begingroup$ But... we already have that. The body can isolate parts of the circulatory system under certain conditions. You could use the same system in a more controlled manner to stop blood flow to e.g. an arm that was cut off. This still leaves the core arteries vulnerable, but I don't think you can really work around that anyway. Don't forget that major vessels never really end - they just branch and branch and branch until they're within reach of every cell that needs stuff from blood; how do you fit two systems of supply without also having them leak into one another? $\endgroup$ – Luaan Dec 3 at 9:26
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    $\begingroup$ @Luaan If a major artery in your arms or legs is cut open, the first part of treating that is either plugging the hole or binding the whole limb off. But the point is, both of these things need to be done from the outside, it is not something your body can do on its own. This answer basically gives the body a way to fix this issue for a few hours or a day, hoping that by then either you yourself or someone else can give you the necessary outside help. $\endgroup$ – quarague Dec 3 at 9:56
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    $\begingroup$ @quarague I don't think morris is trying to say that. They're saying that in the event of an arm getting cut off you will lose all your blood in your first blood system. But you have a separate secondary system that will keep your organs supplied with blood. Hopefully for a long enough time that you can seal the open arteries and produce/acquire more blood to replenish what you lost. $\endgroup$ – Aequitas Dec 4 at 5:13
  • $\begingroup$ these things need to be done from the outside, it is not something your body can do on its own. It's not something your body can do. But it is something your body could do. It seems easy to imaging that, had evolution taken different turns, the body would have strong valves in some of the major arteries that seal off in the case of very serious injury. $\endgroup$ – cowlinator Dec 5 at 1:38
  • $\begingroup$ @cowlinator I mean, we SORT of already do. That's what it means when you "go into shock". Your body tries to pull all the blood out of your extremities and into your vital organs to keep you alive. Sometimes it does more harm than good though. $\endgroup$ – Morris The Cat Dec 5 at 1:47
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In the event the first heart goes down, (suppose someone stabs you clean through it, for instance) the second heart kicks in and keeps the blood pumping through your body.

So what you mean is, instead of dying in ten seconds or so as the blood pressure in your brain drops to zero, instead your magic super soldier get to live for a minute or so going "augh! argh! oh my god there's a goddamn sword sticking out of my chest oh sweet zombie mithras it hurts so much, hurgh" and then mercifully passing out because the magical extra heart has just efficiently exsanguinated them through the one or more gaping holes through the biggest vessels in their circulatory system.

(You're gonna need to apply your magical skills to marketing this, you know.)

Backup hearts are useful for some kinds of poison (foxglove, perhaps) and medical rather than traumatic cardiac arrests (eg. heart attack). Cardiac arrest due to anoxia (eg. drowning or choking) will remain fatal, because there's no oxygenated blood to pump around. Similarly, it can't fix strangling because you can pump all you like but blood ain't getting to that brain, and you'll stay unconscious whilst it dies. There's a small subset of traumatic cardiac arrests it will work for (say, cardiac tamponade) but for everything else it just won't help.

For battlefield effectiveness, Morris the Cat has some good suggestions, but unfortunately a core circulatory system won't help you dust yourself off and either run away or get stuck back in. You just have to lie there til someone finds you, hopefully within 24 hours, and magically restores your original heart. If you don't have healing magic, that wonderful clever auxiliary circulatory system will just let you contemplate your life choices for a bit before dying.

For combat effectiveness you will need to:

  • Stop thinking about your heart. Hearts are a single point of failure. Screw hearts! Get you some peristaltic major blood vessels instead.
  • Fix the problem of acute traumatic coagulopathy. You need it to carry on working when you've been badly beaten up, and regular human blood just doesn't.
  • Do a better job of, y'know, not bleeding. Blood vessels that do a better job of sealing themself up when severed, sort of thing. Autotourniquet muscles. Evolved systems aren't great at this, but hey! magic!
  • Keep a small emergency store of oxygen. I don't care what you keep it in, but even the equivalent of a lungful is enough to beat potential drownings, chokings and strangulations. Maybe a magical spleen?

There's a bunch more I could suggests to make your super soldiers super at staying up and fighting, instead of super at dramatic deaths, but it starts drifting out of the scope of the question. I'm sure you get the idea though...

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monty Wild Dec 3 at 23:17
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The lower abdomen / pelvic region.

In order for the secondary heart to be effective there has to be damage which causes cardiac arrest without major haemorrhaging. This would include blunt trauma to heart or questionably minor puncture wounds which would manage to stop the heart. In this cause the use of a secondary heart could be beneficial if it appeared in the lower abdominal region, preferably about the pelvis bone to provide protection. The main advantage of this location would be easy access to the abdominal aorta and inferior vena cava, two major circulatory pathways that could handle the throughput required by the body (assuming the SVC blood flow could passthrough the atrium). Of course this would require 'magic' to somehow prevent blood from entering the damaged heart (keep valves closed?) and to connect the redundant heart to pulmonary circulation.

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    $\begingroup$ I second this uncomplicated answer. The pelvis is essentially a secondary ribcage, just open at the front. It won't stop a stab, but the ribcage won't necessarily stop one either. What it will do is stop a swung blade at your hip. So it's the next best thing to the upper torso for keeping a secondary heart safe. $\endgroup$ – Ruadhan Dec 3 at 15:22
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Immediately behind the genitals.

If you get maimed there you're not gonna want to live anyway.

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  • $\begingroup$ Hah! I came here hoping to upvote "Sleeve" and "throat". This will have to do. $\endgroup$ – msouth Dec 5 at 17:46
  • $\begingroup$ This comment made me laugh uncontrollably. And decided to join this community. $\endgroup$ – Kiran Subbaraman 2 days ago
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Two redundant hearts, one under each foot.

This placement capitalizes on energy that would otherwise be wasted, by using the fall of the body onto the foot to compress the heart and drive blood effectively back up from the lowest part of the body. The accessory hearts would pressurize the venous system and facilitate blood return, improving blood supply and oxygenation to the cramp-prone leg muscles.

The sole-mounted hearts would work the best and provide the most benefit during that most important part of any battle: running your precious ass away.

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  • $\begingroup$ There's not much wasted energy in walking or running. When walking, there's little vertical movement happening, while when running, the calf/ankle/foot system forms a spring that recovers around 90% of the energy of vertical movement. Adding a heart to each foot would make movement harder, as you now go "squish" rather than "boing" with each step. $\endgroup$ – Mark Dec 3 at 22:35
  • $\begingroup$ @Mark Yes but/and. You lift on one foot and the other swings through the now just large enough gap. You roll forward onto the now front foot which in turn lifts slightly allowing the other leg in turn to swing through, with leg shortened slightly by knee bend. Even without energy recovery the actual motion requires only about lift x mass x g Joules opr say 80 kg x 0.02m x 10 (g==10 :-) ) =~ 16 Joules. At 2 leg swings / second =~ 30 Watts at about 1.5+ m/s = 5+ km/h = v fast walk. energy recovery is a bonus. [[I may be wrong :-) ]]. [Comment welcome]. $\endgroup$ – Russell McMahon Dec 4 at 2:52
  • $\begingroup$ This has a problem of oxygenation, though. The heart is close to the lungs in part to speed oxygenation. If it's far away, it's not going to get as much, and since these are far away, the blood may end up spending too much time in the lungs due to low regional blood pressure. $\endgroup$ – NetworkLlama Dec 4 at 22:45
  • $\begingroup$ This would not only not help a person to survive battlefield injuries, it would also make it less likely for a person to survive while simply walking barefoot over any kind of hazard. $\endgroup$ – cowlinator Dec 5 at 3:23
  • $\begingroup$ The victim starts dancing after you stab them in the heart because the beating of their new one depends on the rate of them stepping. $\endgroup$ – Muuski 2 days ago
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It's a dormant organ, that upon the demise of the first heart, sprout and injects itself...

In this case, it isn't really a redundant heart, right? It is a replacement heart. It would make sense to keep it near the old one. That way, it can sprout into place and you won't have a gaping hole in your chest. As the other answer notes, the main problem isn't keeping the blood flowing. It is keeping the blood flowing inside the body.

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  • $\begingroup$ What if you get stabbed or shot in the heart area, Andrew Jackson style (He almost died from that) if you shot in the heart area and it hit both hearts that would be pretty pointless. $\endgroup$ – John Lewis Dec 3 at 19:25
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The problem there, as others have said, is that this will still be pumping blood around, past whatever hole was made to damage the original heart, and if that heart stopped beating because of blood loss, then it won't help at all. The only time it'd actually really be helpful is if the heart stopped beating... in which case the fix would want to be a second, backup muscle that could grow around the existing heart.

But assuming you can handwave that away... everybody who has ever died (according to a Trivial Pursuit question in the 80s, at least) has died from "lack of oxygen to the brain". Kind of by definition.

If your brain gets smashed, then you're dead anyway.

But if you had a heart (and, OK, some vestigial lungs or gills or magical way to get the blood oxygenated) in your head, then even decapitation would be potentially curable: the only thing that would kill you is brain-smushing. It wouldn't keep you fighting in the event of decapitation, but it'd help survivability.

If the heart seed sprouting was triggered by the brain blacking out, rather than on the destruction of the heart, then it would also protect against things like strangulation, acceleration-induced unconsciousness, physical shock, throat-slashing, etc. For these things, it WOULD keep you up and fighting, especially if it also doped the blood with painkillers direct to the brain.

It would be like a "second wind": you'd be about to go down for the count, then pow, you're up and with more energy... at least in your head.

It would not even need to be the size of a heart, since it's only delivering blood to the head, not the whole body; it can even have only two lobes, if it is pushing blood only to the brain.

To realistically remain fighting for more than a few extra seconds, though, it'd need to also deliver to the rest of the body, or the muscles would be starved of oxygen. But even without that, a brain-heart might give you that last "hail mary" chance to win (or flee!) when the foe drops their guard believing you to be defeated.

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The diaphragm.

There is already some space there to allow the muscles of the diaphragm to expand and contract with your breath. The presence of another heart in this area would mean the abdomen would not contract as fully on an exhale; perhaps magic users can be spotted by a bit of a protruding belly on an otherwise slender frame. Plus the aorta already runs through this area, so plugging into the circulatory would be relatively easy compared with other, more peripheral areas of the body. Plus you get the protection of a thick wall of muscle and the lowest part of the ribcage.

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There's quite a lot of plumbing to make the blood flow through the lungs and then through the body. If you put the 'standby' heart anywhere other than the chest cavity you end up running major high pressure blood vessels from the 'standby' to your chest. There's the veinous return from the body of oxygen depleted blood, a semi high pressure feed to the lungs, a low pressure return from the lungs of oxygenated blood and a very high pressure feed of oxygenated blood out to the body into a complex manifold branching off the Aorta. So your magic 'standby' is somewhere protected inside a bony box, suddenly there's a load of new plumbing required! And if it's not in the chest, it's high pressure plumbing through soft tissue protected spaces. A slash to the throat or belly and your second heart is cut off.

And as others have said... exsanguination will take you out pretty quickly. If your primary heart has been hacked up then there is an open hole into the high pressure system (the Aorta) so you can expect blood to fountain from the chest wound for a few seconds until the system runs dry and then you die even though your 'standby' heart has picked up and is running at full load. You'd need some pretty spectacular valving systems to cut off flows to open holes to stop that.

You'd probably find that some form of 'zombie' power would be better. After all Zombies can walk with their chests blown apart, clearly no blood flow going on there. Arm yourself with some mechanism that allows your body to switch from a blood transported Oxygen powered system to something that can run in the absence of Oxygen. Stored magic energy? Cold fusion? Fission? Just as long as it's not other people's brains or your fellows will finish you off before repairing you.

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The most "realistic" thing that I could imagine would be for the redundant heart to really be a kind of "heart seed", maybe the size of a hazelnut or walnut which would be located for example on the aortic arc, or somewhere nearby. There's enough fatty tissue in the mediastinum so you could place it there.

Not only is being stabbed in the heart actually quite unlikely to happen. The problem with being stabbed in the heart is manyfold. First of course, the heart may stop beating (but not necessarily, even!). So a second heart looks like it might be a plus.

But then, there is (obviously!) a hole, so keeping circulation going is only the second best thing you can do, unless you can cork the hole, or something (actually, nature tries to do just that when an artery is severed, but the fact that most people still die from a reasonably-sized open artery proves that it isn't a very effective measure).

Next, the location of the one heart that we have (including the "plumbing" and "wiring") is not entirely random. It is exactly where it is for a good reason. Put it in a different location, and it won't do its job. Note that there are two independent circuits to serve, not just one (with different requirements). So, without proper placement and connectivity, another heart is pretty useless.

So, the really best solution would be that the "heart seed" sprouts the moment the original heart stops beating, and then magically grows a new heart within, say a number, 10 or 20 seconds. Doing so, it absorbs and replaces the damaged heart which finally crumbles to dust, or to "nothing" (in a Body Snatcher alike manner, only faster and in the subject's favour). The heart seed is thereafter gone, and the new, hole-free heart starts beating.

It'll still be a challenge for a warrior to stay on his feet during that time, but 10 seconds is within the realm of "possible" (that's why Mozambique Drill exists). So, seeing how it's magic, why not give a bit of leeway.

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I chellange your premises, that a second heart will help if the first heart is stabbed.

You will lose many liters of blood thru that open wound gushing high pressure blood. In a minute you will faint, in five you will be dead.

And your second heart make things worse by giving the enemy another place to stab and drain you dry.

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The real problem is discrete parts.

What you need is the ability for specialized cells to quickly change specialty. So that any cell in your body could more or less instantly become any other kind of cell based on need. Maybe even redundant, distributed memory storage throughout the body so that the brain's contents could be rebuilt.

Then it's really more just a question of total mass.

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In a "Bag of Holding"

Since we're using magic anyway, let the redundant heart, as well as some sort of reservoir of extra blood, an extra lung or two, with a large supply of oxygen to supply them, and while we're at it a supply of glucose that can be quickly added to supply immediate energy needs; all be inside a magical container that is larger on its inside than its outside, because its inside is actually just outside our spacetime "brane". This container is located between the hemispheres of the brain, for the reasons Morris the Cat lays out so well.

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  • $\begingroup$ Nope. Extradimensional storage is too advanced a power here, and the question specifically noted that the heart must take up room. $\endgroup$ – Halfthawed Dec 3 at 19:14
  • $\begingroup$ It takes up room. Just not as much as all of the things inside it. $\endgroup$ – Monty Harder Dec 3 at 22:06

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