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If I remember correctly, one of the kaiju problems, especially the bipedal standing one, regards oxygen flow or circulation or respiratory issues (but correct me if I am mistaken regarding the oxygen uptake, and if it actually regarding the oxygen in the blood I imagine to use whale blood mechanism instead, which may be for another question).

So I wonder, would having multiple lungs can be a solution for it, or will there be any bad side effect to it?

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    $\begingroup$ You can find solutions, but please remember that biology is all about efficiency. Such organisms may survive, but they are still not economic $\endgroup$
    – Raditz_35
    Sep 26, 2020 at 12:54
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    $\begingroup$ @JBH what? show me where the pacific rim in it? i dont watch the movie btw but i dont see any plot or movie element in my question. $\endgroup$
    – Li Jun
    Sep 27, 2020 at 7:42
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    $\begingroup$ @JBH the Kaiju is a mythical large monster. You might as well say that the OP is talking about Godzilla which a Kaiju as well. Extremely large monsters is a staple of many series and stories, and this site is filled with a million-and-one questions about them. Using "Kaiju" as a term just gives you a rough idea of what he's talking about. $\endgroup$
    – Demigan
    Sep 27, 2020 at 16:47
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    $\begingroup$ @JBH again, Kaiju is a term for an entire movie genre and for a specific type of large monster. en.m.wikipedia.org/wiki/Kaiju. You might as well complain about someone asking for Warp Drives, also a relatively common topic on this site. Because clearly they are from <name one of the hundred different sci-fi settings> and that is a 3rd party thing right? Or magic? Or a detective? And his question can easily be about focussing on his own world but with he hopes more realistic Kaiju. Even if it is for a specific 3rd party world the question can be valuable for others who do build theirown $\endgroup$
    – Demigan
    Sep 29, 2020 at 5:56
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    $\begingroup$ @JBH but you cannot look into the OP's mind either, and since he didnt mention specific 3rd party things its ridiculous to invoke a rule to close it based on the CHANCE that he might be. Also enforcing the letter of the law without considering any other factors, like the possibility that even if it is a 3rd party world that the question can still be valuable, is just bad for everyone involved. $\endgroup$
    – Demigan
    Sep 30, 2020 at 5:42

1 Answer 1

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The problem is surface area versus volume. Also called the square cube law.

Lungs require surface area to gather oxygen. They do this by making thin walls in the lungs where blood is on one side and oxygen on the other, giving oxygen a chance to get bound to the red bloodcells. Whenever you double the size of the creature a rough estimate gives you 4x more surface area in the lungs. Despite lungs being 3D the actual place where the oxygen passes into the blood is still a 2D surface. The body uses ingenious methods to increase that surface area but its about at its limit.

The rest of the creature is made up out of cells. Each extra cell requires extra oxygen to live. These cells occupy a volume. If you double the size of a creature it roughly receives 8x more cells to feed oxygen to.

So with each time you double its size, you need to make the lungs 4x as big to still give the body the same amount of oxygen to live as its not-doubled counterpart.

In other words, by the time you reach Kaiju size you need a whole lot more than 1 extra lung, you are going to need several dozen extra lungs. But don't worry, your Kaiju will also create more heat and only have the surface area of its skin to lose it effectively, so it might overheat and die before it can suffocate!

As for whales: they swim (relatively) slowly and use a lot of tricks to reduce their oxygen use when they dive for long periods of time. But if you were to make them move and fight as much as a Kaiju would their oxygen use would go through the roof again.

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  • $\begingroup$ i am not sure what you mean, ignoring the heat problem, do you mean the lungs would be to many to fit inside the kaiju body to be feasible solution? because several dozen of extra lungs is what i expect would happen regarding the multiple lungs for the kaiju, but if that simply what you imply, does that mean it feasible way to not suffocate or not worth it? $\endgroup$
    – Li Jun
    Sep 26, 2020 at 11:41
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    $\begingroup$ @LiJun the question is going to be where you'll fit all the extra lungs. Each of the dozen extra sets of lungs is going to be as big as a normal set of lungs, and it's not like a humanoid shape has room to hold those. $\endgroup$
    – Erik
    Sep 26, 2020 at 12:21
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    $\begingroup$ @LiJun yes. Lets scale up a human for example. We make a man of 2m into a man of 4m tall. You would need to give that man not 4 lungs, but 8 lungs somewhere on his body to survive. 4 lungs would mean nothing changes for the bodyplan as you have 2x the size, 4 lungs means you now need to displace the other organs. Now try to scale that up to 40m, which I guess is about the range your Kaiju would be at. You would now need to fit +/-300 lungs in a body that would, without any other organs, fit at most 50 lungs inside. $\endgroup$
    – Demigan
    Sep 26, 2020 at 17:59
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    $\begingroup$ @LiJun That was with human size, as it would be weird to talk about other sizes. Now you would normally increase the size of the lungs rather than stick a few extra on there, but I hoped it would get the point across: You would need to fit +/-300 human lungs inside the Kaiju. That may not seem much but that is 1800 Liters of air capacity. Those Liters have to pass through the surface area though! A human has 6 liters of total lung volume versus +/-75 liters body volume total (12,5 body liters versus 1 air Liter). 1/2 $\endgroup$
    – Demigan
    Sep 27, 2020 at 8:23
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    $\begingroup$ A very rough estimate gives you 2700 Liters total body for a 40m humanoid Kaiju, meaning my initial guess of 50 lungs was dead wrong. However 1800 Liters of air for 2700 Liters of body leaves little room for anything else, 1,5 Liters of body needs to be fed by 1 Liter of air because that volume of air has to go through a lung surface area that has grown with a factor of 4 each time while the body volume it has to feed has grown at a factor of 8. 2/2 $\endgroup$
    – Demigan
    Sep 27, 2020 at 8:28

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