Direct determination of volume- and time-dependence of alveolar surface tension in excised lungs.

We measured alveolar surface tension directly by observing the spreading properties of fluid droplets placed by micropipette within individual alveoli. Alveolar surface tension in excised rat lungs at total lung capacity and 37 degrees was 29.7 +/- 1.4 mN-m-1. In lungs held at functional residual capacity, surface tension changed slowly, rising to 9 mN-m-1 at 30 min with a subsequent approximately linear rise to 16 mN-m-1 at 115 min. Thereafter it rose more rapidly (20 mN-m-1 at 140 min), suggesting that it was not asymptotically approaching an equilibrium value.