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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) lung - alveoli. b) kidney - parietal layer of Bowman's capsule and thin limb of loop of Henle. c) lining blood vessels, chambers of the heart and lymphatic vessels. d) lining body cavities and free surfaces of organs found in those cavities.