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 (A_L) 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 A_L is reduced, leading to decreased airflow. On the other hand, the ASM layer itself might become thickened: this can be directed outwardly such that A_L and airflow are unchanged (b) or directed inwardly such that A_L and airflow are both reduced (c). Finally, the ASM can actively constrict, leading to a reduction in outer diameter, A_L and airflow (d).