The heart is in fact a double pump. The right side of the heart pumps blood through the lungs via the pulmonary circulation. The blood picks up oxygen in the lungs and is then returned to the left side of the heart.

The left side of the heart pumps the oxygenated blood into the systemic circulation, that is, to all the blood vessels of the body apart from those in the lungs. The blood from the systemic circulation drains into the right side of the heart, and the cycle is repeated.

This arrangement meets the body cells’ prime requirement for an unending supply of oxygen.

Press PLAY to see the video showing the blood flow through the systemic and pulmonary circulations.