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In the alveoli Alveolus (also called air sac) is the capillary-rich sac in the lungs where the exchange of oxygen and carbon dioxide takes place by diffusion.
Words to Know. Alveoli: Tiny air-filled sacs in the lungs where the exchange of oxygen and carbon dioxide occurs between the lungs and the bloodstream. Bronchi: Two main branches of the trachea leading into the lungs. Bronchial tree: Branching, air-conducting subdivisions of the bronchi in the lungs.