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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 basic function of alveoli is exchange of gases. Its structure is the site where the gaseous exchange during respiration takes place. These structures are surrounded by capillaries carrying blood. The exchange of carbon dioxide in the blood from these capillaries occurs through the walls of alveolus.