Lung surface tension and air space dimensions from multiple pressure-volume curves.

Pressure-volume (PV) curves of excised cat, dog, rabbit, and rat lungs were determined in a sequence of three conditions: 1) normal-surface, air-filled; 2) saline-filled; and 3) polyoxyethylene (20) sorbitan monolaurate-(Tween 20) surface, air-filled. Since the surface tension of lung washings containing 2% Tween 20 is constant, the Tween-surface air-filled lungs presumably exhibit the pressure-volume behavior of lungs with constant surface tension. These data along with the assumption of equivalent geometry in the three conditions permit calculation of the variation of surface tension in the normal lung as a function of volume without assuming a specific surface area vs. volume function or a maximum surface tension. The calculated surface tension dropped during deflation from a high of 50 dyn/cm total lung capacity (TLC) to a low of 4 dyn/cm (less than 25% TLC) with the species being roughly similar. The PV behavior of Tween-surface lungs appears to fit a simple model of alveolar expansion. Air dimensions calculated for the four species on the basis of this model are ordered in the same sequence as morphological measurements, but larger in magnitude.