Scanning electron micrograph (SEM) of a section of the human lung, showing the network of alveolar ducts and alveoli (air sacs) that form the final divisions of the bronchial tree - the trunk of which is the trachea, the windpipe. Alveoli are numerous (millions per lung) with adjacent ones sharing the same blood capillary & connective tissue septum (white-grey). It is here that the exchange of oxygen and carbon dioxide with the blood circulation occurs. Magnification: X 68 at 35mm size.
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