We have two lungs – one on each side of the chest. When we breathe in, air passes from our nose or mouth down through a tube called the windpipe (trachea). The windpipe divides into two tubes (airways), one going to each lung. These tubes are known as the right and left bronchus. They divide to form smaller tubes called bronchioles, which carry air through the lungs.
At the end of the bronchioles are millions of tiny air sacs called alveoli. In the alveoli, oxygen is absorbed from the air we breathe in. This oxygen passes into the bloodstream to be circulated around the body.
Carbon dioxide is a waste gas that needs to be removed from the body. It passes from the bloodstream into the alveoli and is then breathed out by the lungs.
Just below the lungs is a sheet of muscle called the diaphragm. The diaphragm and the muscles between the ribs contract and relax in order to suck air into and push air out of the lungs. They are the main muscles used for breathing when you’re relaxed.
During heavy exercise, the muscles in your shoulders and upper chest can also help with breathing. These muscles are not designed to work for long periods of time and get tired easily.
The lungs are covered by a lining (membrane) called the pleura, which has an inner and outer layer. The inner layer covers the lungs. The outer layer lines the ribcage and the diaphragm. The pleura produces a fluid that acts as a lubricant, which allows the lungs to move in and out smoothly.