Dynamic surface tension properties of mixed lecithin-cholesterol films related to the respiratory mechanics.

A bubble method was used to record dynamic area/surface tension diagrams of mixed lecithin-cholesterol films prepared at different weight ratios. The lecithin/cholesterol weight ratios were: 1:2, 1:1, 4:1, 6:1, 8:1 and 12:1. The minimum surface tension is about 20 dyn/cm for all weight ratios at amplitudes of 75, 25, 12.5, 6 and 3% area oscillation. The maximum surface tension depends on both the amplitude of area oscillation and the lecithin/cholesterol weight ratio. The maximum surface tension increases with increasing amplitude. At the amplitudes 25, 12.5, 6 and 3% the maximum surface tension is significant higher at the weight ratios 1:2 and 1:1 compared to the other ratios. In addition, equilibrium surface tensions during interruptions of the oscillation at maximum and minimum area were recorded. The equilibrium surface tension increases with increasing amplitude up to 25% area oscillation. A static hysteresis seems to remain. Some consequences regarding the pulmonary respiratory mechanics are discussed.

The significance of alveolar geometry and surface tension in the respiratory mechanics of the lung.

It is customary to describe alveolar respiratory mechanics in terms of a bubble-shaped model alveolus, using the Laplace equation. The coexistence of alveoli of different radii cannot be satisfactorily explained by this model, even if hypotheses as yet unconfirmed by experiment are taken into account. Moreover, this model requires the assumption of extremely low surface tensions, near 0 dny/cm, to explain the absence of atelectasis with very small alveolar radii and the maintenance of alveolar fluid balance. On the basis of investigations of the dynamic surface tension of lung alveolar surfactant from rat lungs, however, the minimal surface tension of alveoli is 18-20 dny/cm. In addition, the lung does not consist of isolated and spherical alveoli but a dense packing of polyhedral air spaces separated by alveolar septa. This paper is an attempt to analyze the recoil forces of the lung due to surface tension on the basis of the polyhedral alveolar structure and the minimum surface tension mentioned above. It is demonstrated that several geometrical parameters are able to guarantee the stability of the alveolar structure of the lung to a greater extent than a variable surface tension. It is concluded that not a single (and fictive) alveolus but the septal intersections and the "peripheral" septa (limiting only one air space) are the smallest morphological units of the respiratory mechanics. Some consequences concerning the pressure-volume behavior of the whole lung, the above mentioned coexistence problem and the alveolar fluid balance are discussed.