Fig. 1.

Schematic cross-section of an airway and surrounding lung parenchyma. The airway is held open by the tethering stress of the lung parenchyma (P_p) and is constricted by the airway smooth muscle tension (τ_muscle). Other forces that can either open or close the airway result from the differences between the alveolar pressure (P_alv) and airway lumen pressure (P_lumen), and the passive tension from stretching or compressing the airway wall tissue (τ_wall).