Fish have a two-chambered heart, and blood is oxygenated in the gills before directly circulating through the body, resulting in a single passage through the heart per cycle, unlike other vertebrates with double circulation.

Amphibians and many reptiles have three-chambered hearts, allowing some mixing of oxygenated and deoxygenated blood, as their body temperature depends on the external environment.

Birds and mammals have a separated heart to prevent the mixing of oxygenated and deoxygenated blood, ensuring efficient oxygen supply for their high energy needs.

De-oxygenated blood from the body enters the right atrium, which contracts to transfer the blood to the right ventricle. The right ventricle then pumps this blood to the lungs for oxygenation.

The left atrium collects oxygen-rich blood from the lungs, and upon contraction, transfers it to the left ventricle. The left ventricle then pumps the oxygenated blood to the rest of the body.