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The air that enters the body goes down the windpipe into tubes in the lungs called bronchial tubes.
Within the lungs, the bronchial tubes branch into thousands of smaller, thinner tubes called bronchioles. These tubes end in bunches of small round air sacs called alveoli.
Small blood vessels run through the walls of the air sacs. When air reaches the air sacs, the oxygen in the air passes through the air sac walls into the blood in the capillaries. At the same time, carbon dioxide moves from the capillaries into the air sacs. This process is called gas exchange.
The airways and air sacs are elastic. When you breathe in, each air sac fills up with air like a small balloon. When you breathe out, the air sacs deflate and the air goes out.