The vertebrate heart consists of muscular chambers capable of strong contractions that circulate blood through the body. Thecomplete separation of oxygenated and deoxygenated blood, which is made possible by the four chambered hearts, assuresthat blood that reaches the tissues has the highest oxygen content.

Mammalian hearts consist of two separate pumps, each with two chambers. In each pump, an atrium receives and briefly storesthe blood, passing it to a ventricle that propels it through the body. One pump is for pulmonary circulation and consists of theright atrium and ventricle. Oxygen depleted blood from the body empties into the right atrium through a large vein, the superior
vena cava. The right atrium contracts, transferring the blood to the right ventricle. Contraction of the right ventricle sends theoxygen-depleted blood to the lungs via pulmonary arteries. The other pump, consisting of the left atrium and ventricle, powerssystemic circulation. Newly oxygenated blood from the lungs enters the left atrium through pulmonary veins and is passed to theleft ventricle. Strong contractions of the left ventricles, the heart's most muscular chamber, send the oxygenated blood coursingout through the aorta to the rest of the body.