Surface tension induced by dipalmitoyl lecithin in vitro under physiological conditions.

1. The surface tension of 161 films of DPL have been measured on a Langmuir trough using a Wilhelmy balance under conditions controlled to simulate the state of the alveolar lining in vivo.2. The parameters controlled were temperature (maintained at 37 degrees C), humidity (100% at 37 degrees C), surfactant concentrations (encompassing the best available estimates), area changes (consistent with normal respiration), frequency adaptation to continuous cycling and composition and pH of the aqueous hypophase.3. Simultaneously maintaining all of these parameters within the best estimates of physiological limits, the relationships between surface tension and surface area showed appreciable differences from previous studies, our results showing higher minimum values of surface tension, appreciably less change in surface tension with compression and far less hysteresis between surface tension and surface area.4. The higher minimum values are consistent with original estimates of alveolar surface tension made by von Neergaard, namely 35-41 dyne cm(-1).5. Although appreciably smaller than hitherto reported, the change in surface tension with change in area is still adequate to impart alveolar stability.6. The reversibility between surface tension and surface area under physiological conditions is discussed in connection with compliance hysteresis which is considered to be more dependent upon geometric irreversibility of the alveolar surface than upon any intrinsic property of the surfactant.