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There are several reasons why gas exchange takes place in the alveoli. First, although the alveoli are small, there are many of them so they have a huge surface area in which the gas exchange can take place. This surface area is moist and thin so the gases can be easily passed through the cells. The alveoli are full of capillaries (the smallest blood vessels) so that it is easy for oxygen to pass through the alveoli into the capillaries and the blood stream and carbon dioxide can easily pass out. While the bronchioles are the tubes that bring the air to the alveoli, they do not have the surface area or the capillaries to make the exchange of gases possible.
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