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Light micrograph of a secion of lung tissue showing numerous alveoli (small air spaces) and alveolar ducts (larger spaces). Air enters the lungs via the bronchi, which divide repeatedly to form airways of ever-decreasing diameter. The terminal branches of this respiratory tree, the alveolar ducts, open into the blind-ended alveoli, which are the sites of gaseous exchange. The walls of the alveoli contain a fine network of tiny capillaries, carrying deoxygenated blood from the right side of the heart. The blood absorbs oxygen & releases carbon dioxide through the thin alveolar walls. Magnification: x25 at 6x7cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/ASTRID AND HANNS-FRIEDER MICHLER)