Human and rat lungs were degassed and filled with air and physiological salt solution for determination of pressure-volume relationships. Extracts were prepared from the specimens and examined for their surface film-forming activity. Both the aeration of the alveolar structure of the lungs at functional residual pressure and the surface activity of the extracts showed wide variation. Numerical ratios were defined to express these two properties quantitatively, and a high degree of correlation was observed between them. The range of correlation was extended by treating the rat lungs with nonionic detergents. These findings support the hypothesis that the stability of the pulmonary alveolar structure is dependent on intrinsic surface-active material and have encouraged further attempts at formulating a theory of alveolar mechanics, taking surface forces into account. The theory is presented, and some of its weaknesses are pointed out. Submitted on October 21, 1960

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Reduction of the surface area of 24 extracts of rabbit lungs by four-fifths decreased the surface tension to a minimum value of 0–5 dynes/cm at 18–22 C. However, minimum surface tension was above 15 dynes/cm if 1) the temperature was raised to 42 C, 2) the extract was prepared with distilled water, 3) phospholipase C was incubated with the extract, and 4) cholesterol or oleic acid was added to the surface. If blood or serum was added during the extraction, minimum surface tension was usually (although not invariably) elevated. Rinsing diluted rat serum or chylomicrons through the airways increases elastic recoil of excised rat lungs. Other reports show that heating a lung above 42 C or rinsing a solution of phospholipase C through the airways also increases elastic recoil of excised rat lungs. Therefore, these conditions alter the surface tension of lung extracts and the pressure-volume characteristics of the lungs concordantly. In addition, we found that the surface tension of lung extracts was not stable below 24 dynes/cm. Similar instability of the surface within the lung should lead to gradual atelectasis if a low transpulmonary pressure is maintained. elastic recoil; atelectasis; compliance; lung surface Submitted on January 21, 1965

The surface tension of the lung decreases markedly on compression resulting from deflation, as calculated from P-V data, and follows a force-area path very similar to that of several mucus surfaces. With changes in surface area of less than 50%, lung extract and mucus bubble surfaces are mechanically reversible. For decrease in area greater than 50%, surface tension approaches a lower limiting tension of 10–15 d/cm. On re-expansion of the surface, an upper limiting tension of 40–50 d/cm is approached. Hysteresis of considerable magnitude occurs and indicates a major alteration of the surface materials compressed beyond 50%. Estimates of lung surface area based on these considerations accord with histologic estimates. Submitted on December 4, 1958

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