2. When you breathe in, air enters your lungs.

3. Inside the lungs are many tiny air sacs called alveoli.

5. Alveoli provide a large surface area for gas exchange.

11. Because it is all folded up, an alveolus can have a large surface area but still be very small.

16. Your lungs contain 300 million alveoli. If these could be spread out flat...

23. ... they would cover a tennis court!

26. In the lungs, oxygen moves into the blood and carbon dioxide moves out.

27. This is called gas exchange.

28. Inside the lungs are tiny air sacs called alveoli, with walls only one cell thick.

31. The alveoli are surrounded by tiny blood vessels that also have very thin walls.

32. Gases such as oxygen and carbon dioxide are able to pass through these thin walls.

35. Carbon dioxide diffuses from the blood into the alveoli.

37. Oxygen diffuses out of the alveoli into the blood.

39. Blood vessels carry oxygen to all the cells in the body.

40. In your body, cells take oxygen out of the blood...

43. ... and carbon dioxide moves into the blood.