Figure 1: Image on the left depicts a hypothetical airway: dashed lines on the left and right images circumscribe the outer and inner dimensions prior to various changes which can impact airway lumenal area (AL) and airflow. (a) Thickening of the basal lamina, epithelial cell hyperplasia, edema formation, and/or bronchial vascular dilation all lead to a swelling or thickening of the innermost layer(s) of the airway: outer diameter is unchanged, but AL is reduced, leading to decreased airflow. On the other hand, the ASM layer itself might become thickened: this can be directed outwardly such that AL and airflow are unchanged (b) or directed inwardly such that AL and airflow are both reduced (c). Finally, the ASM can actively constrict, leading to a reduction in outer diameter, AL and airflow (d).