False-colour scanning electron micrograph (SEM) of a section of human lung, showing alveoli (bottom & left) & a bronchiole (top). In humans, some 700 million alveoli, with a total surface area the size of a tennis court, perform the lungs' primary role of gas exchange. The walls of the alveoli are usually composed of just one layer of cells. Fine blood capillaries run alongside them, & oxygen & carbon dioxide readily diffuse across the 0.3 micrometre thick barrier between them. The bronchiole is one of the finer divisions of the air passages of the lungs; its corrugated inner surface is due to its highly folded epithelial lining. Magnification: x74 at 35mm size. Reference: MICROCOSMOS, figure 2.34, page 33.