Why does endothermic vertebrates have a greater requirement for a four-chambered heart than fish or frogs?

Question

Why does endothermic vertebrates have a greater requirement for a four-chambered heart than fish or frogs?

Answer 1

One of the reasons humans have a four chambered heart as opposed to the 1,2 or 3 chambers is because of the way in which our blood is oxygenated.  One atrium and ventricle are dedicated to getting the oxygen poor blood from the body and taking the blood to the lungs.  Whereas the other atrium and ventricle are dedicated to getting the blood from teh lungs and getting it out to the rest of the body.  We have to have oxygen rich blood in order to maintain a fast-paced of movement, thinking, metabolism, etc.  Without enough oxygen we don't make enough ATP and we would ultimately slow down drastically or die.  This system, allows for a rich oxygenation of blood with mixing.

Because fish breath through their gils, they don't have to do a looped sort of pumping to and from the heart.  The blood just picks up the oxygens there and continues to the rest of their bodies without having to go back to the heart.  

Some frogs can breathe via their skin (not all but some), but their overall size can handle the fact that their blood is not super oxygenated.  Also, the way in which the blood gets dumped into the heart and then carried back out to the body and lungs (through vessel branching) is favorable that most oxygen rich blood goes to the body and most oxgyen poor blood goes to the lungs.

Answer 2

Endothermic vertebrates generate much heat to maintain body temperature and this comes from a faster metabolic rate which requires an efficient delivery of oxygenated blood to respiring tissue.  A 4 chambered heart which separates oxygenated and deoxygenated blood provides such an efficient delivery.