Comparison of alveolar stability among mammals: The roles of surface tension and alveolar size

The independent roles of surface tension (T) and alveolar size were investigated in different mammalian species. Alveolar size was estimated from the mean linear intercept (L) of fixed lungs. Since the lung transpulmonary pressures (P) in different species are similar, we predicted according to the LaPlace equation (p=4T/L) that surface tension should vary in proportion to alveolar radius. The surface tension reducing property of surfactant was assayed using 2 methods. First, the in vitro assay utilized a modified Wilhelmy balance and Langmuir trough to measure minimum surface tension of lung extracts. The second method was an in situ assay which involved determinations of quasi-static pressure-volume curves. The volume remaining on deflation to 10 cmH₂O normalized to TLC, (volume at 30 cmH₂O). %V₁₀, provided an index of deflation stability which has been previously correlated to the amount of active surfactant in lungs within a species. Our results indicated that despite a range in L between 31-112um (mouse-dog), minimum surface tension was similar among species. Similarly, %V₁₀ demonstrated no proportional change with L across species. These results are inconsistent with predictions based on the simple LaPlace equation. We conclude that the traditional concept of alveolar stability based on the LaPlace relation should be critically re-examined and that other factors in promoting stability across species need to be explored. (Supported by the American Lung Association).