Lung bronchioles. Coloured X-ray of the bronchial tree (yellow) of a healthy human lung. This tree consists of bronchioles (lung airways) that branch out from a bronchus (upper right). Air from the mouth passes through the trachea (windpipe) and this lung's bronchus to reach its bronchioles. The air fills tiny sacs (alveoli) at the end of the bronchioles. In the alveoli, oxygen from the air is exchanged with carbon dioxide from the blood. The bronchial tree has been highlighted by bronchography: filling it with a radio-opaque medium to absorb the X-rays. The X-ray also shows some chest bones (white). The curved bones are some of the ribs that enclose the lungs. The ribs are connected to the bony column down right, which is the thoracic part of the backbone. The bone at top is the clavicle (collar bone).