Relations among alveolar surface tension, surface area, volume, and recoil pressure

For pulmonary structure-function analysis excised rabbit lungs were fixed by vascular perfusion at six points on the pressure-volume (P-V) curve, i.e. at 40, 80, and 100% of total lung capacity (TLC) on inflation, at 80 and 40% TLC on deflation, and at 80% TLC on reinflation. Before fixation alveolar surface tensions (gamma) were measured in individual alveoli over the entire P-V loop, using an improved microdroplet method. A maximal gamma of approximately 30 mN/m was measured at TLC, which decreased during lung deflation to about 1 mN/m at 40% TLC. Surface tensions were considerably higher on the inflation limb starting from zero pressure than on the deflation limb (gamma-V hysteresis). In contrast, the corresponding alveolar surface area-volume (SA-V) relationship did not form a complete hysteresis over the entire volume range. There was a considerable difference in SA between lungs inflated to 40% TLC (1.49 +/- 0.11 m2) and lungs deflated to 40% TLC (2.19 +/- 0.21 m2), but at 80% TLC the values of SA were essentially the same regardless of the volume history. The data indicate that the gamma-SA hysteresis is only in part accountable for the P-V hysteresis and that the determinative factors of alveolar geometry change with lung volume. At low lung volumes airspace dimensions appear to be governed by an interplay between surface and tissue forces. At higher lung volumes the tissue forces become predominant.