Pulmonary surface film stability and composition.

Stability of pulmonary alveoli at end expiration requires a very low air-water surface tension (e.g., less than 10 mN.m-1). Another important requirement is that the surface film maintain this low surface tension for a sufficiently long time at fixed lung volume. We measured monolayer collapse rates at 37 degrees C of lung surface-active material (SAM) and certain lipids found in this material and compared them with alveolar monolayer collapse rates calculated from published lung compliance changes. We found collapse rates for purified SAM or a mixture of dipalmitoyl lecithin (DPPC):monoenoic lecithin (PC):cholesterol (CHOL) (3.03:1.65:1 molar ratios) to be much greater than collapse rates of alveolar films estimated from indirect measurements. Monolayers of pure DPPC or DPPC with 10 mol% monoenoic PC and/or CHOL had collapse rates equal to or less than those estimated from lungs. We conclude that the alveolar monolayer is enriched in DPPC to the extent of 90 mol% or greater. Enrichment may exclude more mobile components from the monolayer during expiration when surface tension reaches verry low values.