Lungs. Computer graphic of a wire-frame head and torso, showing the respiratory system. Air breathed into the nose and mouth passes down the pharynx before entering the windpipe (trachea). The two lungs (green) are the main organs of the respiratory system, lying in the chest within the ribcage. They supply the body with the oxygen needed for aerobic metabolism and eliminate the waste product carbon dioxide. Air entering the lungs passes through a system of canals known as bronchi and bronchioles, which lead into tiny air sacs called alveoli. Gases diffuse into or out of the blood through the thin walls of the alveoli.
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