The Circulatory System The circulatory system is the course taken by the blood through the arteries, capillaries, and veins and back to the heart. In humans, the heart is made up of four chambers the right and left auricles, or atria, and the right and left ventricles. The right side of the heart pumps oxygen-poor blood from the cells of the body back to the lungs for new oxygen; the left side of the heart receives blood rich in oxygen from the lungs and pumps it through the arteries to the various parts of the body. Circulation begins early in fetal life. It is estimated that a given portion of the blood completes its course of circulation in approximate