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In the alveoli, oxygen from the air is absorbed into the blood. Carbon dioxide, a waste product of metabolism, travels from the blood to the alveoli, where it can be exhaled. Between the alveoli is a thin layer of cells called the interstitium, which contains blood vessels and cells that help support the alveoli.
The alveoli are only one cell thick, allowing the relatively easy passage of oxygen and carbon dioxide between the alveoli and the capillary. One cubic millimeter contains 170 alveoli and the area of alveoli is on average 70 square meters. The number of alveoli differs between people and larger lungs have more alveoli.