Animals that do not use energy to maintain body temperature, like amphibians and many reptiles, can tolerate some mixing of oxygenated and deoxygenated blood. Therefore, they have three-chambered hearts.

Animals with high energy needs, like birds and mammals, require a constant and efficient supply of oxygen to maintain their body temperature, which is facilitated by the separation of the heart.

The separation prevents oxygenated and deoxygenated blood from mixing, ensuring an efficient supply of oxygen to the body.

Ventricles have thicker muscular walls because they need to pump blood with greater force to various organs in the body, whereas atria mainly facilitate blood collection and transfer to the ventricles.

When the right ventricle contracts, it pumps deoxygenated blood to the lungs for oxygenation.