False-colour scanning electron micrograph (SEM) of a section of the human lung, showing a blood capillary (centre) containing red blood cells, surronded by a network of alveoli (air sacs). The alveoli are the final branches of the bronchial tree, the network of airways which begins with the trachea (windpipe). The alveolar network constitutes a respiratory area in which exchange of gases (oxygen and carbon dioxide) takes place between blood (in the capillaries) and air, through the thin epithelium that separates adjacent alveoli. Magnification: x350 at 35mm size.