Surface pressure hysteresis of mixed lipid/protein monolayers: applications to the alveolar dynamics.

Experimental data on surface pressure-surface area hysteresis of mixed serum albumin/dipalmitoyl lecithin/sphingomyelin monolayers in the Langmuir trough are presented. Several possible physicochemical mechanisms of the hysteresis are discussed: Marangoni effect, surface pressure relaxations, bulk-to-surface diffusion interchange, and collapse. Depending on the concrete conditions each of these mechanisms can be important. Possible applications of these results to the alveolar dynamics are presented and discussed on the basis of the balloon model of the alveolus. The main conclusions of biological importance are that 1) the alveolar stability depends on the DPL/SM ratio as well as on the protein content. Under normal breathing conditions the surface pressure hysteresis is small and does not play a decisive role in the alveolar dynamics. 2) At large extent of compression the collapse predominates in determining the hysteretic behavior of the alveolar surface.