Your heart is always pumping blood around your body. Mammals have a double circulatory system, pumping oxygenated blood out on the left side and deoxygenated blood to the lungs on the right. Learn about the heart's chambers, function of the lungs, arteries, arterioles, capillaries, veins and their valves, pulmonary arteries, and pulmonary veins. Animated graphics present concepts effectively.