Lung bronchus cross-section, coloured scanning electron micrograph (SEM). This longitudinal freeze-fracture has exposed the inner epithelial surface of a bronchus (pink). The different ciliated and secretory cells are just visible. The bronchus is surrounded by pale pink lung tissue consisting mainly of alveolar elements (alveoli). Two blood vessels (orange lumens) are at upper left and top right. Magnification: x150 when printed at 10 centimetres across.