You have two lungs, one in each side of your chest, enclosed by an airtight box. Your ribs and the muscles that join them form the box, together with a tough sheet of muscle called the diaphragm.
Inside the lungs are tiny air sacs called alveoli, surrounded by surrounded by capillaries. The walls of the alveoli and capillaries are so thin that oxygen and carbon dioxide can pass through them. The alveoli of an adult have a total surface area of 70m, and the whole breathing apparatus is designed to bring fresh air as close as possible to the blood. Your lungs fill with air when you breathe in, and empty when you breathe out. They go up and down rather like balloons, but they aren’t just hollow bags. They are spongy organs made up of tightly packed tissue, nerves and blood vessels.