This study is a critical evaluation of the concept of Milic-Emili et al. (J. Appl. Physiol 1966, 21: 749-759) that lung density has an overwhelming influence on the regional distribution of alveolar expansions and transpulmonary pressures, at least in healthy subjects in quasi-static conditions. Our measurements showed, indeed, that these distributions have a vertical, gravitational gradient, and that almost mirror images of these were obtained after 180 degrees body inversion. These inversions of distributions were not accompanied by interfering changes in shape of chest or lung. This implies that the lung, although being a structured network and thus essentially subjected to stress-strain interactions, behaves mechanically as a liquid in a gravitational field, at least within the conditions of our investigations.
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