Heart - Double Pump

Double Pump 
The heart is a double pump. The right side of the heart sends oxygen-poor blood, which is also high in carbon dioxide, to the lungs, and the left side sends oxygen-rich blood throughout the body. Therefore, the blood actually travels in two circuits: (1) from the heart to the lungs and back to the heart, and (2) from the heart to the body and back to the heart. The right side of the heart is a pump for the first circuit, and the left side is a pump for the second. The left ventricle has the harder job of pumping blood to all parts of the body; therefore, its walls are thicker than those of the right ventricle.
The right side of the heart pumps oxygen-poor blood to the lungs, and the left side pumps oxygen-rich blood to the tissues.