ABSTRACT
I. Introduction 173
II. Metastable Films of Pulmonary Surfactant 174
A. Classical Model 174
Selective Adsorption 175
Selective Exclusion 176
B. Supercompressed Fluid Model 179
III. Conclusions 185
Acknowledgments 186
References 186
I. Introduction
The most fundamental question concerning pulmonary surfactant is how the
interfacial films that coat the alveoli achieve and sustain the very low surface ten-
sions observed in the lungs. The low magnitude of the surface tension is now well
established. Estimates based on the pressure-volume characteristics of excised
lungs indicate that surface tensions reach values as low as 2-3 mN/m (1). Direct measurements, using the contact angle of fluorocarbon droplets deposited
in the peripheral alveoli (2), confirm surface tensions of 1 mN/m (3). These low surface tensions occur only in partially deflated lungs in which the surfactant
films have been compressed by the shrinking alveolar surface area. After a single
deflation, surface tensions remain essentially constant at these low values in static
lungs for tens of minutes (1,4). A continuous compression of the films, therefore,
is unnecessary to maintain the low surface tensions. These physiological
observations define the unusual characteristics of the surfactant films in situ.
