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The lungs are the site of gas exchange. Oxygen from the air diffuses into the blood and carbon dioxide gas from the blood is given off into the air. For this reason, the lung contains many air spaces (the alveoli) as well as blood capillaries. <br> <br> The lungs are much like a sponge with blood capillaries surrounding the alveoli. The total surface area of all alveoli of a human lung is quite large and can be about 70 square meters, which is the size of a small apartment.