How many chambers does the human heart have?

The heart is really two pumps side by side, each with two chambers, giving four in total. Blood returning from the body, low in oxygen, enters the right atrium, drops into the right ventricle and is pushed out to the lungs. There it picks up oxygen and releases carbon dioxide, then returns to the left atrium, passes into the left ventricle and is driven out through the aorta to the rest of the body. The atria are thin-walled collecting chambers that only have to nudge blood downwards; the ventricles do the real work, and the left ventricle has by far the thickest muscle because it must generate enough pressure to reach the toes and the brain. Valves between the chambers snap shut to stop backflow, and the sound of them closing is what a stethoscope picks up as the familiar double beat. This separation into two circuits keeps oxygen-rich and oxygen-poor blood from mixing.

Birds and crocodilians evolved four-chambered hearts separately from mammals, an example of convergent evolution.