Lung tissue plasticity: morphometric analysis of anisotropic strain in liquid filled lungs

Lungs of rats were fixed at different inflation pressures (Ptp) during liquid filling with the pulmonary vessels tied to prevent vascular volume change after fixation had begun. Morphometric analysis showed that alveolar surface varied as a alveolar volume (Va) to the power 0.82, while the arithmetic mean tissue thickness varied as Va-0.2. This is interpreted as evidence for anisotropic expansion. Capillary volume (Vc) was found to increase from zero Ptp to a maximum at Ptp = 2 cm H2O then decrease as Va increased. Morphometric diffusion capacity of the membrane component increased as Va0.59 while that for whole lung (DL)paralleled the change in Vc. Alveolar capillary tissue unfolding is described as the main factor accounting for anistropic expansion of alveolar surface and for capillary configuration. The absolute values of Vc and DL were lower by 60% and 50%, respectively, compared with values obtained by standard instillation fixation methods and this is suggested could account for previous discrepancres between morphometric and physiologic estimates of DL.