![]() ![]() The small membranous and terminal bronchioles carry out conductive functions, whereas respiratory bronchioles and alveolar ducts can carry out both conducting and gas exchange functions. For the purposes of this review, distal airways will refer to airways less than 2 mm in diameter that consist of small membranous, terminal and respiratory bronchioles as well as alveolar ducts. Saclike structures measuring about 200 μm in diameter, alveoli can evaginate from respiratory bronchioles, alveolar ducts and alveolar sacs. Alveolar ducts open into atria that communicate with alveolar sacs, which terminate into alveoli. Each terminal bronchiole subdivides into two or more respiratory bronchioles that transition into alveolar ducts. Bronchioles are intralobular airways with diameters less than 5 mm that branch into five to seven terminal bronchioles. Eventually, bronchi enter a pulmonary lobule and are then termed a bronchiole. After entering the lungs the primary bronchi branch downward and outward repeatedly, giving rise to smaller bronchi, which results in a dramatic increase in the number of airways and a progressive decrease in the diameter of each airway (Figure 1). The trachea divides into two primary bronchi that enter the lung at each hilum. There are no proven specific treatments for distal airway dysfunction, and protective ventilation strategies to minimize distal airway injury may be the best therapeutic approach at this time. The presence of elevated airway resistance, intrinsic positive end-expiratory pressure or a lower inflection point on a pressure–volume curve of the respiratory system may indicate the presence of impaired distal airway function. In addition, injurious mechanical ventilation strategies may contribute to distal airway dysfunction. More recent data suggest that distal airway dysfunction plays a significant role in acute respiratory distress syndrome. The airways include both membranous bronchioles and gas exchange ducts, and have been referred to in the past as the 'quiet zone', in part because these structures were felt to contribute little to lung mechanics and in part because they were difficult to study directly. Distal airways are less than 2 mm in diameter, comprising a relatively large cross-sectional area that allows for slower, laminar airflow. ![]()
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