MANFRED KAGE / SCIENCE PHOTO LIBRARY MANFRED KAGE / SCIENCE PHOTO LIBRARY
False-colour scanning electron micrograph of a section of a human lung, showing the capillaries surrounding alveolar ducts and air sacs (large dark areas). The latter form the final divisions of the bronchial tree (the trunk of which is the trachea, the windpipe). Red blood cells can be seen in the lumens of several capillaries. Rough-surfaced white blood cells are present (there is one in the top left corner). There are millions of alveoli in each lung, adjacent ones share the same blood capillary and connective tissue septum (brighter areas). It is via this extensive capillary bed that the exchange of gases between air and blood occurs. Magnification: x275 at 6x7cm size.
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