Lungs. Coloured 3D magnetic resonance imaging (MRI) scan of healthy lungs, showing the bronchial tree (brown). The bronchial tree consists of bronchioles (lung airways) that branch out from one of two bronchi (at centre, not seen). Air from the mouth passes through the trachea (windpipe) and the bronchi to reach the bronchioles, where it fills tiny sacs (alveoli). In the alveoli, oxygen from the air is exchanged with carbon dioxide from the blood. Some chest bones can also be seen (white), such as the clavicle (collarbone, across top), spine (running down centre) and ribs (surrounding the lungs).