Healthy lung. Coloured bronchogram x-ray of a side (lateral) view of a healthy human lung. The vertebrae of the spinal column are seen at left. At lower right, the bottom two ribs of the rib cage are visible as diagonal bands. From the trachea (not seen) air passes into each lung via a bronchus. This tube branches into many bronchioles (centre, red), to deliver air to every part of the lung. The bronchioles divide to form tiny air sacs called alveoli (not seen) where gaseous exchange occurs. Bronchography renders the airways visible on X-ray film by prior injection of a radio-opaque contrast medium into the windpipe.