Lung, respiratory ducts and alveoli
Individual alveoli ("air sacs") are interconnected by passageways called "respiratory ducts" or "respiratory sacs".
Note that the structure of alveolar walls cannot be readily resolved at this magnification.
At the entrance to each alveolus, a "knob" along the alveolar wall contains smooth muscle fibers which allow the size of the opening to be adjusted. Each alveolar wall is lined on either surface by simple squamous epithelium, with capillaries sandwiched in between.
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Last updated: 30 March 2012 / dgk