Stress-strain relationships during uniform and non uniform expansion of isolated lungs.

Stress-strain relationships in various directions were determined on isolated rabbit and cat lungs both during unrestrained and restrained expansion, the latter approximating the condition of cylindrical deformation. Under this condition the distortion due to the weight supported at the lung surface was also studied. Experimental data did not fit the equilibrium equations for a continuum solid, except for relatively small distortions of lung parenchyma. On the contrary, over the full range of lung distortions they fitted a model made up by a three-dimensional network of interconnected springs. While distortions produced even at relatively low lung volumes caused substantial changes of surface pressure distribution, the distorting effects of the weight were negligible under this respect. These results support the conclusion that pleural surface pressure distribution in rabbits, cats and dogs is mainly due to the lung distortion caused by the mismatching between lung and chest wall inherent shapes, while that caused by the lung weight plays only a minor role.