FIG 1 .

Overview of the respiratory airways. Tan coloring indicates mucus. Cyan dashed arrows indicate the direction of possible oxygen transport. In CF lungs, mucus can aggregate in different scenarios. In each scenario, “d” indicates the diameter of the airway and “x” indicates the mucus thickness. (A) Although less common in CF, the small airways, the alveolar sacs, can clog with mucus entirely (d ≤ 200 µm). (B) The surface layer of the bronchioles becomes dehydrated, and mucus thickening begins (300 µm ≤ d ≤ 1,000 µm). (C) Mucus aggregates can dislodge from lower airways and become lodged in larger, upper airways (the d value depends on the airway the aggregate is dislodged into; 300 µm ≤ d ≤ 5,000 µm).