The pressure-volume relationship of lungs subjected to repeated compression and expansion is studied in detail. The investigation was motivated by an attempt to understand why lungs are frequently injured by compression waves; hence the lung was compressed to a degree greater than normally encountered in physiological conditions. Attention was focused on the collapse of the lung at a critical strain and the reopening of the trap at a critical stress. We found that when a rabbit lung is compressed, about one-half to one-quarter of its gas may be trapped in the alveoli because of the closure of airways. Reopening of the trap occurs at a pressure higher than the critical pressure for collapsing. The difference of the critical pressures of collapsing and reopening is influenced by the rate of strain and the strain history, especially by the maximum compressive stress imposed on the lung. The stress-strain relationship of the lung tissue, which resembles the PV curves, depends strongly on the strain history.

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