Fig. 5.

Mechanisms of alveolar micromechanics during the deflation limb of a pressure–volume curve. Four mechanisms have been suggested (Gil et al. 1979): (1) Alveolar derecruitment, (2) Isotropic (balloon-like) destretching, (3) Shape changes and (4) Folding of alveolar walls. In vivo, the lung volume usually does not drop below the functional residual volume which is above the inferior infliction point. The occurrence of alveolar derecruitment is unlikely in this range of pressures but can be observed at very low lung volumes, e.g. with negative airway opening pressures. The other 3 mechanisms are likely to occur throughout the partial PV relationship above FRC although there is good evidence that folding dominates at lower volumes while destretching is most prominent at larger volumes. Shape changes have been described to be very dominant at intermediate volumes. This depiction is based on the observations of Gil et al. (1979), Bachofen et al. (1987), Tschumperlin and Margulies (1999) and Knudsen et al. (2018). The light microscopic images were taken from histological sections of a previous study (Knudsen et al. 2018). The description provided in this image is based on evaluations of lungs fixed at different pressures during the PV loop. Isolated phenomena occurring in the septal walls such as folding, shape change or stretching have never been observed in exactly the same alveolus directly. Scale bar 50 µm