One of the 2 main organs of the respiratory system. The lungs supply the body with the oxygen needed for aerobic metabolism and eliminate the waste product carbon dioxide. Air is delivered to the lungs via the trachea (windpipe); this branches into 2 main bronchi (air passages), with 1 bronchus supplying each lung. The main bronchi divide again into smaller bronchi and then into bronchioles, which lead to air passages that open out into grape-like air sacs called alveoli (see alveolus, pulmonary). Oxygen and carbon dioxide diffuse into or out of the blood through the thin walls of the alveoli. Each lung is enclosed in a double membrane called the pleura; the two layers of the pleura secrete a lubricating fluid that enables the lungs to move freely as they expand and contract during breathing. (See also respiration.)
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