Comment by manwe150

Some animals have multiple hearts, for instance cephalopods, like octopuses & squids, have 3 hearts.

However, for vertebrates it would be difficult to evolve to have extra hearts, because they have relatively rigid bodies.

A heart needs a space in which to expand, so it is not enough for the muscles in the wall of a blood vessel to increase in size and become capable of periodic contractions. You also need for all the space around it to evolve in providing an extra cavity inside which the new heart will be able to move without interaction with the surrounding organs, like the pericardium provides for the heart. In vertebrates, it was possible to evolve the pericardium because it has not evolved from nothing. It has evolved from a cavity (the so-called coelom) that existed in the ancestors of vertebrates long before having a skeleton and long before having a dorsal chord and even before having a blood vascular system. The modification of the coelom into a pericardium was a simple change, while the creation of a new internal cavity would be a very complex change. Moreover, also the nervous system requires changes, to be able to control the new heart.

So such changes are very unlikely in an animal like a vertebrate, because any intermediate stages would result in an animal that is disadvantaged in comparison with its competitors, only the final stage, with a functional second heart would be an improvement.

In general the evolution of animals does not happen at a constant rate. When an animal reaches a well optimized structure, it can keep that structure unchanged for hundreds of millions of years, because any deviation would result in a less competitive animal, which would be eliminated without descendants. For instance, there are a few species of sharks that have changed very little since Triassic, more than 200 million years ago.

Great changes in the structure of an animal happen only when there is no competition, which allows the intermediate worse forms to survive and produce descendants. Such lack of competition happens when a natural catastrophe kills most competitors or when an animal succeeds by chance to arrive in a new place, where nothing like it lived before, e.g. when passing accidentally to a different island or continent.

> any reason it couldn’t have mutated to have 2 hearts servicing each side of the body

There are probably no hard reasons. It is most likely that the path of incremental changes leading to that solution is either unlikely, or does not convey an advantage to propagation of genes.

Klingon whales, now?

There are measurements suited to purpose, then there are "technically you could do that" measurements, and it's the former we'd want to use when measuring what sorts of power and pressure and material properties of the vascular system and cardiac tissue of a whale. Enormous amounts of blood are being pumped around, and I'd have to imagine you're in the million miles of arteries and veins and capillaries ballpark, so there's a lot of pressure holding that mass back.

That'd be a fun model to figure out for a weekend project - what sorts of forces are we talking about - how efficient is it compared to say, a hummingbird, or a human, or an earthworm heart?

A binary vascular system? Do you want regenerating whales in the time vortex?