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exchange in the lungs occurs in millions of small air sacs. In mammals and reptiles, these are called alveoli, and in birds, they are known as atria. These microscopic air sacs have a rich blood supply, bringing the air into close contact with the blood. A system of airways, or hollow tubes, allow the air sacs to interface with the external environment; the largest of these is the trachea, which branches in the middle of the chest

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