Lung surfactant: Film kinetics at the surface of an air bubble during prolonged oscillation of its volume

Abstract

These investigations were designed to provide a better understanding of the behaviour and action of surface-active materials in the alveolus of the lung. The bubble method was used to generate area/surface tension diagrams (ASD) for films of lung alveolar surfactants and of dipal-mitoyllecithin. If one rhythmically enlarges or reduces a bubble covered with a film of lung surfactant or lecithin, supplied in excess, the ASD changes as though surface-active substance was disappearing. This alteration in successive ASD's occurs very rapidly at first but later is much slower. After ca 1000 cycles practically no further change can be seen; this final or “equilibrium” ASD is approached asymptotically as the number of cycles increases. The equilibrium ASD is to a large extent independent both of the amount of surfactant applied and of whether the material has been applied to the bubble surface or to the hypophase. For films of lung surfactant, with oscillatory surface-area changes of 75% at 37 °C, the maximal surface tensions obtained with the largest area lie between 67–69 dyne/cm and the minimal values, with the smallest area, between 19–22 dyne/cm. For lecithin films under the same conditions, a maximal surface tension of, 70 dyne/cm and a minimal value of less than O.5 dyne/cm are found. If the bubble area is changed by less than 75%, the difference between maximal and minimal surface tension in the equilibrium ASD decreases, for both kinds of film, by an amount corresponding to the reduction in change of area, the minimal surface tension remaining the same. These results can be explained in part by an exchange of surfactant between the aqueous phase and the interface, and in part by the behaviour of the material at the interface durins ranid chanses of area.