Integrating pulmonary surfactant into lung mechanical simulations: A continuum approach to surface tension in poromechanics

Abstract

Surface tension arising in the air–liquid interface of alveoli is a fundamental mechanism in lung physiology that explains lung recoil and hysteresis during breathing. However, pulmonary surface tension is typically neglected in continuum models of the lungs, possibly due to their complex multiscale physicochemical nature. In this study, we formulate a poromechanical framework that incorporates the effect of surfactant-dependent surface tension in porous media for the prediction of lung hysteretic response. Using an internal variable formalism, we apply the Coleman–Noll procedure to establish an expression for the stress tensor that includes surface tension akin to the Young–Laplace law. Based on this formulation, we construct a non-linear finite-element model of human lungs to simulate pressure–volume curves and lung response during mechanical ventilation. Our results show that surfactant-dependent surface tension notably modulates pressure–volume curves and lung mechanics. In particular, our model captures the influence of surfactant dynamics on lung hysteresis and compliance, predicting the transition from an insoluble reversible regime to a dissipative one governed by Langmuir kinetics. We envision that our continuum framework will enable lung simulations where surfactant-related phenomena are directly considered in predictions, with important applications to modeling respiratory disease and lung response to mechanical ventilation.