Lung tissue. False-colour scanning electron micrograph (SEM) of a section of lung tissue. The image shows several alveolar ducts and alveolar sacs (large, irregular dark areas) together with a few alveoli (smaller round dark areas). The alveoli are separated by a porous membrane known as the interalveolar septum. The tiny openings in the interalveolar septum are sectioned capillaries and it is in this intricate network of capillaries that the exchange between carbon dioxide and oxygen takes place. Many red blood cells are scattered throughout the frame. Magnification: x110 at 6x7cm size. x170 at 4x5ins