HOW LUNGS WORK
Each time you breathe, air is drawn into your nose or mouth, down through your throat and into your windpipe (trachea). This windpipe is a tube about ten centimetres long (in adults), which splits into two smaller air tubes called the bronchi, one going to the left lung and the other to the right lung.
The air passes down the bronchi which divide another 15 to 25 times into thousands of smaller airways, called bronchioles, until the air reaches the alveoli (gas exchanging air sacks).
Inside the alveoli, oxygen moves across paper-thin walls to the capillaries (tiny blood vessels) and into the blood. It is then picked up by chemicals (haemoglobin) in the red blood cells that carry it around the body. At the same time, waste products from the body, in the form of carbon dioxide, come out of the capillaries back into the alveoli, ready to be breathed out.
Freshly oxygenated blood is carried from the lungs to the left side of the heart which pumps blood around the body through the arteries. Once the oxygen has been used up, the blood returns, through the veins, to the right side of the heart. From there it is pumped to the lungs so that the carbon dioxide can be removed and more oxygen taken on board.
Breathing out is usually just a matter of relaxing the diaphragm (large dome shaped muscle below the lungs) and the muscles between the ribs. This pushes the air out and the lungs return to their resting size.