'Used’ blood is low in oxygen and high in waste carbon dioxide. When this blood enters the capillaries in the linings of the lungs it is only one cell’s thickness away from fresh air.

Carbon dioxide in the used blood readily passes out through the capillaries to the air in the lungs. At the same time, oxygen passes in to the blood where it is taken up by the red blood cells. The oxygen-rich blood then flows back to the heart, where it is pumped to the rest of the body.