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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 function of the alveoli is to get oxygen into the blood stream for transport to the tissues, and to remove carbon dioxide from the blood stream. In the lungs, air is diverted into smaller and smaller microscopic branches called respiratory bronchioles, which connect to the alveolar ducts.