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Bronchioles are the smallest branch of the respiratory system. They consist of larger bronchioles and terminal bronchioles, which are narrower, and lead directly to the Alveoli for gas exchange.


Bronchioles are the first and only airway vessel in the respiratory system not to contain glands or tissues other then smooth muscle and endothelial tissue. They are comprised of Smooth muscle cells, Endothelial cells, and in some cases bronchiolar exocrine cells. This is in part due to their extremely minimal size, having a diameter of only 0.5 - 1 mm. Even though they do not contain glands, some cells exist in the bronchioles to allow for the release of surfactant. These are the aforementioned bronchiolar exocrine cells, and are found in the highest proportion in the cells contacting alveoli, as the surfactant is used to reduce the amount of surface tension in the alveoli[1].


The bronchioles are subject to many infections and diseases including Bronchiolitis obliterans-organizing pneumonia (BOOP)[2].


  1. Papadakos PJ, Burkhard Lachmann. Mechanical Ventilation: Clinical Applications and Pathophysiology. Rotterdam, The Netherlands: Saunders Elsevier, 2008
  2. Pardo J, Panizo A, Sola I, Queipo F, Martinez-Peñuela A, Carias R.. (2013). Prognostic value of clinical, morphologic, and immunohistochemical factors in patients with bronchiolitis obliterans-organizing pneumonia. Human pathology. 44 (5), 718-24
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