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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.
Medical Definition of Gas exchange. Gas exchange: The primary function of the lungs involving the transfer of oxygen from inhaled air into the blood and the transfer of carbon dioxide from the blood into the exhaled air. Last Editorial Review: 5/13/2016.