Lung surfactant causes the surface tension, γ, in the alveoli to drop to nearly zero on exhalation; in the upper airways γ is ∼30 mN/m and constant. Hence, a surface tension gradient exists between alveoli and airways that should lead to surfactant flow out of the alveoli and elimination of the surface tension gradient. However, the lung surfactant specific protein SP-C enhances the resistance to surfactant flow by regulating the ratio of solid to fluid phase in the monolayer, leading to a jamming transition at which the monolayer transforms from fluidlike to solidlike. The accompanying three orders of magnitude increase in surface viscosity helps minimize surfactant flow to the airways and likely stabilizes the alveoli against collapse.
Bibliographical noteFunding Information:
Financial support was provided from National Institutes of Health grant HL-51177 and the University of California Tobacco Related Disease Research Program, grant 11RT-0222.