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3D numerical simulations of oxygen diffusion in the pulmonary acinus

The oxygen transfer from air to blood in the human lung takes place at the terminal part of the bronchial tree, named pulmonary acinus: it comprises four processes: convection, molecular diffusion in air, diffusion through the alveolar membrane into blood and binding to hemoglobin. To be efficient, the system must be built, in such a way that oxygen can reach the most peripheral airways even if it is absorbed along its pathway. A model is proposed, which focuses on the diffusion steps, in order to study the effect of the interplay between diffusivity, membrane permeability and geometrical complexity of the acinus. Our aim is to evaluate, through numerical simulations in realistic 3D geometries, acinus efficiency both at rest and at exercise, pointing out, namely, the role of "diffusional screening". Screening is found to exist at rest, the system efficiency being reduced to approximately 40%.