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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 two pleural membranes surround the lungs, which rest on the diaphragm. Each is a double membrane. The outer surface of the pleural membrane is called the visceral pleura, and the membrane that lines the body wall is called the parietal pleura. In between the two layers of the pleural membranes there is pleural liquid.