The document describes the circulatory system and circulation of blood through the heart. It details the four chambers of the heart - right and left atria which receive blood, and right and left ventricles which pump blood. Deoxygenated blood enters the right atrium from the body, is pumped to the lungs by the right ventricle, receives oxygen in the lungs, and enters the left atrium. It then passes to the left ventricle and is pumped through the aorta to the body as oxygenated blood, completing the double circulation through the pulmonary and systemic circuits.