MANFRED KAGE / SCIENCE PHOTO LIBRARY MANFRED KAGE / SCIENCE PHOTO LIBRARY
False-colour scanning electron micrograph of a section of a human lung, showing capillaries surrounding the alveolar sacs (the large dark spaces). The latter form the final divisions of the bronchial tree (the trunk of which is the trachea, the windpipe). The rough-surfaced spheres are white blood cells and red blood cells can be seen in the lumens of several capillaries. Alveoli are numerous (millions per lung) & adjacent ones share the same blood capillary & connective tissue septum (brighter areas). It is via this extensive capillary bed of the pulmonary alveoli that the exchange of gases between air & blood occurs. Magnification: x830 at 6x7cm size.
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