Lung lining. Coloured scanning electron micrograph (SEM) of mucus-producing cells (orange, round) and cilia (yellow, hair-like) lining an airway of the lung. Mucus secreted here traps bacteria, dust and other particles. Rhythmic movements of the cilia serve to move the mucus and the trapped particles away from the gas-exchanging parts of the lung, and towards the throat, where they can be expelled. Magnification: x1500 when printed 10 centimetres wide.