The physical origin of sigmoidal respiratory pressure–volume curves: Alveolar recruitment and nonlinear elasticity

An important unsolved problem in medical science concerns the physical origin of the sigmoidal shape of pressure–volume curves of healthy (and some unhealthy) lungs. Conventional wisdom holds that linear response, i.e., Hooke’s law, together with alveolar overdistention play a dominant role in respiration, but such assumptions cannot explain the crucial empirical sigmoidal shape of the curves. Here, we propose a theory of alveolar recruitment together with nonlinear elasticity of the alveoli. The proposed model surprisingly and correctly predicts the observed sigmoidal pressure–volume curves. We discuss the importance of this result and its implications for medical practice.