6.2 The body, the lungs and oxygen
The figure shows a simple image of how the lungs absorb oxygen from the air.
Air contains several different gases but about twenty per cent of it is oxygen. Oxygen is a relatively small molecule, and when air is drawn into the lungs, oxygen molecules pass through the tissue on the walls of the lungs and is absorbed into the bloodstream. As we've seen, red blood cells carry oxygen around the bloodstream to all parts of the body.
Blood that passes through the lungs is greatly changed by the oxygen absorption process. The blood that enters the lungs has been around the body and doesn't have much oxygen left in it, since it's been used elsewhere. After passing through the lungs, however, the blood becomes full of oxygen again. Essentially, the blood is ‘recharged’ by passing through the lungs.