Your lungs are a pair of soft, spongy bags inside your chest.
When you breathe in, air rushes in through your nose or mouth, down your windpipe and into the millions of branching airways in your lungs.
The two biggest airways are called bronchi (singular bronchus), and they both branch into smaller airways called bronchioles.
The surface of your airways is protected by a slimy film of mucus, which gets thicker to protect the lungs when you have a cold
At the end of each bronchiole are bunches of minute air sacs called alveoli (singular alveolus).
Alveoli are wrapped around with tiny blood vessels, and alveoli walls are just one cell thick — thin enough to let oxygen and carbon dioxide seep through them.
There are around 300 million alveoli in your lungs.
The large surface area of all these alveoli makes it possible for huge quantities of oxygen to seep through into the blood. Equally huge quantities of carbon dioxide can seep back into the airways for removal when you breathe out.