[JumpCrisscross]

> don't understand how that would limit evolution

Cardiac tissue is a surface. Blood is a volume. I think they’re saying blue whale hearts are near the largest current biology can evolve. Which is interesting because it suggests if we could e.g. engineer whales with carbon-fiber hearts or whatever, they’d evolve to grow even bigger.

[observationist]

Blackheart the whale, lurking the oceans, ever devouring and growing. I think you've got a good Lovecraft/Pirates of the Carribean/Black Mirror mashup premise here.

[fluoridation]

Cardiac tissue is also a volume. It can be thicker or thinner as the animal requires.

[adrian_b]

Yes, but the force of a muscle does not increase with volume, but only with its section.

So a bigger heart would need to have thicker walls in comparison with its internal volume, which would increase disproportionately the volume of the heart walls, so for bigger and bigger hearts the useful internal volume that is filled with blood would be a smaller and smaller fraction of the total heart volume.

The lower efficiency of a bigger heart would require a higher contraction frequency to compensate, but the bigger a heart is the smaller is its maximum contraction frequency, due to the small speed of the propagation of an excitation through muscle cells and nervous cells.

[fluoridation]

>The lower efficiency of a bigger heart would require a higher contraction frequency to compensate

If that's the case, then how come rats have a higher heart rate than humans, who have a higher heart rate than whales? Do whales have hearts with thinner walls than humans, while rats have big lumps of muscle with teeny tiny chambers?

[JumpCrisscross]

> the force of a muscle does not increase with volume

It does. But it must still impart that force through a surface to a volume. And at a certain point, you hit the mechanical limits of that surface’s ability to hold itself together.

[boxed]

Or just a lot of smaller hearts.