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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.
What is the first characteristic that assists the alveoli in diffusion? the wall of each alveolus is made up of a single layer of cells and so are the capillaries around it, this means that between the blood in the capillaries and the air in the alveolus there is a barrier.