When you breathe in, air passes from the nose or mouth through the windpipe (trachea) which divides into two air passages, one going to each lung

The two air passages are known as the right and left bronchus and they form smaller tubescalled bronchioles, which carry air through the lungs

At the end of the bronchioles are millions of tiny air sacs or alveoli. It is here that oxygen is absorbed from the air and passes into the bloodstream to be circulated around the body

Carbon dioxide is the waste product that must be removed from the body. It passes from the bloodstream into the alveoli and is expelled by the lungs when you breathe out