The human heart has 4 chambers: two atria and two ventricles. The atria are the upper receiving chambers, while the ventricles are the lower pumping chambers.
The right atrium receives oxygen-poor blood from the body and passes it to the right ventricle, which pumps it to the lungs through the pulmonary artery. After the blood picks up oxygen, it returns to the left atrium through the pulmonary veins. The left ventricle then pumps it through the aorta to the rest of the body.
A muscular wall called the septum separates the right and left sides, helping prevent oxygen-rich and oxygen-poor blood from mixing. The heart also has four valves—tricuspid, pulmonary, mitral and aortic—that keep blood moving in one direction.
A common mix-up is confusing chambers with valves: the human heart has four of each. Some animals have different heart structures; for example, most fish have a two-chambered heart, while amphibians generally have three chambers.