The atrium is a chamber in the heart that serves as a receiving area for blood returning from the body and lungs. There are two atria: the right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs. The atria contract to push blood into the ventricles, which then pump it out to the rest of the body. Overall, the atria play a crucial role in the heart's function of circulating blood.