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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.
Gas exchange. The air sacs (alveoli) have a very large total surface area and a very good blood supply. There is an exchange of gases between the air sacs and their surrounding capillary blood vessels. Oxygen diffuses from the air sac into the blood.Carbon dioxide diffuses from the blood into the air sac. These four features are particularly important to our lungs efficient for gas exchange.as exchange. The air sacs (alveoli) have a very large total surface area and a very good blood supply. There is an exchange of gases between the air sacs and their surrounding capillary blood vessels. Oxygen diffuses from the air sac into the blood.