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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.
A substance that decreases the surface tension in the alveoli, allowing the lungs to expand. The substance in the fluid lining the alveoli that prevents their collapse exhalation the phase of pulmoary ventilations in which air is expelled from the alveoli