Alveolar size as a determinant of pulmonary distensibility in mammalian lungs.

Pulmonary distensibility depends on surface tension and tissue elastic forces, but the relative contribution of each to total lung recoil remains incompletely defined. By applying an exponential analysis to static pressure-volume curves obtained from the excised lungs of cats, dogs, and rats with air and saline filling, the exponential constant K (an index of lung distensibility) was related to the mean linear intercept Lm (a morphometric estimate of the mean size of peripheral air spaces at maximal inflation). K for air filling (Ka) was unrelated to K for saline filling (Ks), and Ks was unrelated to Lm, but similar highly significant regressions of Ka on Lm were found in each species, and the common regression was similar to that described previously for human lungs. Approximately 86% of the variance in lung distensibility (Ka) within and between species was explained by Lm. Because Lm determines the size of air spaces, and therefore the alveolar surface-to-volume ratio, the findings indicate that the density of surface forces is the major determinant of lung distensibility in the air-filled state.