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So in my fantasy world, I have giants, they aren't super massive, but they average 9' which is still gigantic. But there is a problem, with that size, the heart has to work extra hard to pump all that blood, so to solve this problem, my giants have two hearts to split the load of pumping blood.

But anatomically speaking, I have no idea where to put the second heart.

Would I put it in the same place as the first heart but on the other side? Although that would take up lung space so they wouldn't have as much stamina.

So the question stands, where do I fit the second heart?

One note: I need this to be science-based, my world does have magic, but I don't want magic to be a crutch that is required for a race to survive.

Also, I need the giant to still look mostly like a human, except giant of course.

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  • $\begingroup$ Well, if a human became that tall, I would assume that lungs, heart, organs, etc. would grow with the external body, so you would not need a second heart. $\endgroup$
    – user11111111111
    Dec 9, 2020 at 17:47
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    $\begingroup$ A 9' human is about the size of some species of bear, which get along fine with just one heart... Ditto giraffes and elephants... $\endgroup$
    – Matthew
    Dec 9, 2020 at 17:48
  • $\begingroup$ @Ian54 except in human examples of very large humans such as Andre the Giant have heart problems due to their gigantic human proportions. Just an increase in size isn't enough, which is why I employ a second heart. Also a second heart is kinda cool $\endgroup$
    – Firestryke
    Dec 9, 2020 at 17:54
  • $\begingroup$ If evolved from humans or hominids, likely the only place it could happen would be the lower aorta. But I don't know that this gives you much of a performance increase. Maybe some smaller pumps in the thighs and upper arms is sufficient. $\endgroup$
    – John O
    Dec 9, 2020 at 17:57
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    $\begingroup$ @JohnO woah woah there isn't any evolution involved, I am a creationist, and my worlds are all created as well. $\endgroup$
    – Firestryke
    Dec 9, 2020 at 17:59

4 Answers 4

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You want two hearts? The Doctor has two hearts! Let's get two hearts!

  • Smaller lungs, meaning diminished air capacity unless the lungs have greater or more efficient oxygen transference.

  • Smaller stomach. This isn't as silly as it might sound. Humans today employ bariatric surgery to make the stomach smaller as a weight-loss procedure. In fact, humans (at least, Americans... right?) can easily live on less food than they do. At worst, your Goliaths would need to eat a bit more often (Elevenses, anyone?)

  • Smaller intestines. This might be the most believable of the lot. While a small increase in the efficiency of the intestines, particularly the large intestine, would allow you to chop a chunk of them off, I believe it's entirely realistic based on the fact that what makes the intestine efficient is the surface area inside the intestine. A larger person might imply a larger intestine — which you can interpret as either normal-diameter but longer or greater diameter but the same length compared to a normal human. In other words, I think it's entirely believable for the space you need to exist without expecting the intestines to work differently at all.

  • Your giants never had an "appendix" in the first place. Unlike humans, that space was given over to a second heart. I've changed my mind — I think this is the best choice.

However...

Splitting the load isn't as simple as adding a second pump. If you think about it, two pumps in series means (simplistically) twice the blood pressure in the rest of the system. That's most efficient from a need-for-internal-space perspective, but it has other problems you might consider. Capillaries must be designed for that pressure (or your giant's eyes are perpetually blood shot or simply blood red). You might need a thicker dermis to ensure capillaries and veins don't burst through the skin or serious (and I mean serious) varicose veins. Yes, you can design (or declare) your Goliath to handle blood pressure. Or...

You can put the pumps in parallel, meaning one of two things: (a) you either have a more complex cardio-vascular system such that, say, blood to one half of the body goes through one heart and blood for the other half of the body through the other heart, or (b) blood pressure increases.

Series vs. parallel solves different kinds of problems. Series is useful when you need to have the same pressure over a long distance. Parallel is useful when you need more pressure. A series solution will not drain the proverbial pool faster, but it will let you push the drainage a greater distance. Parallel will drain the pool twice as fast, but you can't significantly push the water any further than with one pump (putting the output of the two pumps in a single, small pipe increases the pressure, but decreases the volume).

You see, blood pressure is a tricky thing. It's not completely understood by doctors today, but they understand a lot of it. Blood pressure is affected by at least:

  • blood vessel diameter
  • blood vessel wall permittivity
  • the volume of blood in the system
  • the volume of water in the system
  • the force of fluids pumped through the system
  • receiving cell permittivity
  • obstructions
  • chemical control/feedback of the process

Which means "why" might be important

So, you want two hearts! No problem! But you might want to step back for a moment and see if there's room in your story for why you need two hearts. The answer to that question has a fair impact on how the hearts are configured, which will impact how much space you need for the two hearts.

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  • $\begingroup$ Some cephalopods have multiple hearts, and they seem split to do two things: pump blood through gills to more efficiently oxygenate blood (in some ways performing the same function that expanding and contracting lungs does), and another heart to then pump the oxygenated blood to where it needs to go. So maybe giant hearts are split similarly... $\endgroup$
    – Garrett Motzner
    Dec 9, 2020 at 18:49
  • $\begingroup$ This was very informative, thanks! I will probably go with two separate cardiovascular systems with one heart for each, I will decrease the intestine size, then have one heart in the normal position that we have it, and position the other in the intestinal area. One heart supplies blood to the upper half of the body, the other the lower half. One lung for each system. $\endgroup$
    – Firestryke
    Dec 9, 2020 at 18:54
  • $\begingroup$ This will minimize the "stand up too fast and faint." problem. $\endgroup$
    – Firestryke
    Dec 9, 2020 at 18:55
  • $\begingroup$ @Firestryke Disconnected circulatory systems will have you run into a lot of self-regulation problems, since immune and hormonal signals are carried by the blood. Basically, you'll require a duplicate of every endocrine system organ in the body. If not, the upper half of the body won't get any insulin or testosterone, while the lower half will have all sorts of growth and immune problems. $\endgroup$
    – Nuclear Hoagie
    Dec 10, 2020 at 18:37
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    $\begingroup$ @NuclearHoagie Or just have the systems set to where the glands feed into both systems, it might be a bit wonky, but due to the extra-large size it could feasibly fit $\endgroup$
    – Firestryke
    Dec 10, 2020 at 18:55
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Frame challenge:

Fully grown giraffes stand 4.3–5.7 m (14.1–18.7 ft) tall [...] The circulatory system of the giraffe has several adaptations for its great height. Its heart, which can weigh more than 11 kg (25 lb) and measures about 60 cm (2 ft) long, must generate approximately double the blood pressure required for a human to maintain blood flow to the brain. As such, the wall of the heart can be as thick as 7.5 cm (3.0 in).

Giraffes have unusually high heart rates for their size, at 150 beats per minute.[49]:76 When the animal lowers its head the blood rushes down fairly unopposed and a rete mirabile in the upper neck, with its large cross sectional area, prevents excess blood flow to the brain. When it raises again, the blood vessels constrict and direct blood into the brain so the animal does not faint.

The jugular veins contain several (most commonly seven) valves to prevent blood flowing back into the head from the inferior vena cava and right atrium while the head is lowered. Conversely, the blood vessels in the lower legs are under great pressure because of the weight of fluid pressing down on them. To solve this problem, the skin of the lower legs is thick and tight, preventing too much blood from pouring into them.

Your giants can do well with one heart, as the giraffe does

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  • $\begingroup$ again, the giraffe has a drastically different physiology than my giants, which have similar proportions and physiology as normal humans, but with 2 hearts $\endgroup$
    – Firestryke
    Dec 9, 2020 at 18:00
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    $\begingroup$ @Firestryke a large elephant rearing up on its hind legs and reaching above its head with its trunk to grab tree branches can be momentarily twenty, fwenty five, or possibly thirty feet tall overall, and then drop back to a normal shoulder height of ten to twelve feet. A large whale in a vertical position can have a much greater vertical extension than any giraffe or elephant. Thus adaptations for great height are common among widely different mammals which aren't closely related. Some humans have been over eight feet tall, did all of them haveheart problems. $\endgroup$
    – M. A. Golding
    Dec 9, 2020 at 22:07
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It's simple: It's about placement. One of my gripes about the first Spongebob Squarepants movie is why the heck the Krusty Krab 2 is right next to the original. We know Mr. Krabs to be a shrewd businessman and a die-hard capitalist, so the placement in the movie would make the Krusty Krab 2 a big waste of money. If it was at the other side of Bikini Bottom, that means that customers would have to travel a shorter distance to get Krabby Patties, and make Mr. Krabs money, which would be a win-win scenario. In a similar way, an second heart should be placed a good distance from the first heart, for maximum effectiveness. Not only it prevents a predator from taking out both hearts at once, but it helps with circulation, which improves thermoregulation and transport of nutrients, oxygen and waste.

The secondary heart is best near an organ that needs a lot of nutrients and oxygen, such as the brain or the liver. But if you want the same brain capacity, and have the second heart near the brain, you will need a bigger skull with more space. And that would require stronger support muscles and bones, but that can easily be done. Or, you could place the second heart where the appendix would be, and having 1-2 less feet of intestine shouldn't affect overall health too much.

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You could use horses, which in a sense have 5 hearts, as a model. The frog in each hoof acts as a pump to send blood back up to the body: https://en.wikipedia.org/wiki/Circulatory_system_of_the_horse#Frog So your giants would have 3 hearts: when they walk or run - which is when the extra pumping is needed - the foot pumps come into action.

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