Transmission Electron Micrograph (TEM) of a section of alveolar wall, showing the gaseous diffusion barrier between blood and alveolar air; attenuated cytoplasm of pneumocyte on outside; common basement membrane, thin cytoplasm of capillary endothelial cell on inside. Alveoli are located in the respiratory zone of the lungs, at the distal termination of the alveolar ducts and atria, forming the termination point of the respiratory tract. Magnification: 30,000x when printed at 4x7".