How do lungs work?

Breathing is driven mainly by the diaphragm, a dome-shaped muscle below the lungs. When it contracts and flattens, and rib muscles lift the chest, the chest volume increases and air flows in. Air travels down the windpipe into branching bronchi and bronchioles, ending in around 480 million alveoli. Each alveolus is wrapped in capillaries, and the walls are so thin that oxygen diffuses into the blood and carbon dioxide diffuses out. When the diaphragm relaxes, the lungs recoil and air is breathed out. Adults at rest breathe about 12 to 20 times a minute.