Color enhanced transmission electron micrograph (TEM) of a section of the alveolar wall. Alveoli are tiny sacs at the end of the bronchial tree in the lungs. The alveolar walls are lined with a one-cell thick layer of epithelium, gas exchanging tissue, that oxygen must pass through to reach the blood. Seen here: pneumocytes, capillaries and capillary endothelial cells. Magnification: 2,000x. Enhancement of 9N0777.