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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.
The arteries deliver the oxygen-rich blood to the capillaries, where the actual exchange of oxygen and carbon dioxide occurs. The capillaries then deliver the waste-rich blood to the veins for transport back to the lungs and heart. Veins carry the blood back to the heart.