Atria are the two upper chambers of the heart, known as the left atrium and right atrium. They receive blood returning to the heart from the body and lungs. The right atrium collects deoxygenated blood from the body through the superior and inferior vena cavae, while the left atrium receives oxygenated blood from the lungs via the pulmonary veins. The primary function of the atria is to pump blood into the lower chambers of the heart, called the ventricles. The right atrium sends blood to the right ventricle, which then pumps it to the lungs for oxygenation. The left atrium sends blood to the left ventricle, which distributes it throughout the body.