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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.
Background. The surface of the tiny air sacs of the lungs (alveoli), where oxygen goes into the bloodstream and carbon dioxide comes out, is coated in a thin watery layer that contains water and pulmonary surfactant.