The Alveoli
When air enters your lungs, it goes through a maze of smaller and smaller tubes until it reaches tiny sacs called alveoli. The sacs look like bunches of grapes at the end of the bronchial tubes. The alveoli are  where the oxygen from the air enters your blood, and the carbon dioxide from your body goes into the air. Alveoli are very tiny, but you have a lot of them in your lungs. In fact, you have 300,000,000 alveoli in each lung. That's six hundred million in total - and your body needs them all to get enough oxygen into your blood!
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