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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.
At each cell in your body, oxygen is exchanged for a waste gas called carbon dioxide. Your bloodstream then carries this waste gas back to the lungs where it is removed from the blood stream and then exhaled. Your lungs and respiratory system automatically perform this vital process, called gas exchange. In addition to gas exchange, your respiratory system performs other roles important to breathing.