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Lung airways and mediastinum, 3D coloured computed tomography (CT) scan. The lung airways narrow as they branch from the trachea to the bronchi to the bronchioles, terminating in the alveoli where gas exchange takes place with the blood. The blood is oxygenated in the lungs to provide oxygen for respiration, while carbon dioxide, the main waste product of respiration, is removed from the blood in the lungs. The mediastinum is the area between the lungs that contains the heart, backbone and other major chest structures. (KEYSTONE/SCIENCE PHOTO LIBRARY/VSEVOLOD ZVIRYK/SCIENCE PHOTO LIBRARY)