Air is moved in and out of the body by breathing. There are two stages to this: breathing in and breathing out. When we breathe in the muscles between the ribs pull on the ribs and make them go upwards and outwards. At the same time the muscle in the diaphragm pull it downwards. Both these actions make the inside of the chest larger and air rushes in to take up the extra space and fill the lungs. When we breathe out the ribs fall and move inwards and the diaphragm rises. Both these actions make the inside of the chest smaller and air is pushed out of the lungs.

The rate at which we breathe is controlled by the brain. When we are resting we need less oxygen than when we are active so the brain sends messages to the chest muscles to make us breathe more slowly. When we are active we need more oxygen so the brain sends out messages to make us breathe more quickly.

The air contains tiny solids which could be harmful. However, the body has a way to clean up the air as it moves through the nose and down the windpipe into the lungs. If dirt were to collect in the lungs it would block the path of oxygen as the body tried to take it into the blood. If bacterial spores landed in the warm moist surface of the lung they could break open and release the bacteria which could then cause infection. The body prevents dirt and spores reaching the lungs by trapping them. The hairs in the nose filter out larger dust particles and the lining of the nose produces a sticky liquid called mucus which traps smaller particles and spores. Any particles and spores which manage to get through the nose may be trapped by the mucus lining the windpipe. The lining is also covered in microscopic hairs. They beat to and fro and push the mucus up to the top of the windpipe where it is swallowed.