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Last Updated: Dec 15, 2016

The alveolus is a small, balloon like sac at the end of the small air passages in the lungs (the bronchiole). Oxygen is inhaled and absorbed into the bloodstream through the thin wall of each alveolus, and carbon dioxide is passed the other way (from the blood to the lungs) to be exhaled. There are about 300 million alveoli in each lung.