Schematic cross-sectional diagram of terminal airways illustrating the transition from an opened to a closed airway. Contraction of smooth muscle cells favors airway constriction coupled with variable quantities of plasma proteins in the airway wall and the airway lumen, including fibrin deposition (see Figure 1 in Wagers et al. [ref. 6]). Normally there are tethering forces that will tend to expand terminal airways, while the quantity and quality of airway liquid (and mucus) favor airway obstruction. Although surface tension in the distal airways would be expected to play a minor role in decreasing patency under these conditions, administration of exogenous surfactant might help expel occluding material by dilution, dispersion, and rapid lowering of surface tension at the surfaces of the plugs.