Renormalized random walk study of oxygen absorption in the human lung

Gas exchange at the acinar level involves several physico-chemical phenomena within a complex geometry. A gas transport model, which takes into account both the diffusion into the acinus and the diffusion across the alveolar membrane, is used to understand gas mixing in realistic systems. The behaviour of the system, first computed on model geometries in 3D finite elements method, can in fact be derived from the topological structure of the acinus only. A new efficient method based on random walks on a lattice is thus used to compute gas diffusion in structures taken from real morphological data. This approach shows that, at rest, the human acinus efficiency is only of 30-40%. These results provide a new evidence of the existence of diffusional screening at the acinar level, which means permanent spatial inhomogeneity of oxygen and carbon dioxide partial pressure. Application of these results to CO2 clearance provides for the first time a theoretical support to the empirical relation between the O2 and CO2 partial pressures.