A novel mathematical model derived from fundamentalengineering principles for simulating the spatial and temporal gasdiffusion process within the alveolar region of the lung was presentedrecently by Koulich et al. . The model depends on a physicalproperty of the alveolar region termed effective diffusivity, function ofthe diffusivity, solubility, and interface geometry of each alveolarconstituent. Unfortunately, the direct determination of the effectivediffusivity of the alveolar region is impractical because of thedifficulty in describing the internal geometry of each alveolarconstituent. However, the transient solution of the macroscopic modelcan be used in conjunction with the lung diffusing capacity (measuredin laboratory via the single-breath technique) to determine theeffective diffusivity of the alveolar region. With the effectivediffusivity known, the three-dimensional effects of red blood celldistribution on the lung diffusing capacity can be predicted vianumerical simulations. The results, obtained for normal (random),uniform, center-cluster, corner-cluster, and several chain-likedistributions, unveil a strong relationship between the type of celldistribution and the lung diffusing capacity.