A novel mathematical model derived from fundamental engineering principles for simulating the spatial and temporal gas diffusion process within the alveolar region of the lung was presented recently by Koulich et al. . The model depends on a physical property of the alveolar region termed effective diffusivity, function of the diffusivity, solubility, and interface geometry of each alveolar constituent. Unfortunately, the direct determination of the effective diffusivity of the alveolar region is impractical because of the difficulty in describing the internal geometry of each alveolar constituent. However, the transient solution of the macroscopic model can be used in conjunction with the lung diffusing capacity (measured in laboratory via the single-breath technique) to determine the effective diffusivity of the alveolar region. With the effective diffusivity known, the three-dimensional effects of red blood cell distribution on the lung diffusing capacity can be predicted via numerical simulations. The results, obtained for normal (random), uniform, center-cluster, corner-cluster, and several chain-like distributions, unveil a strong relationship between the type of cell distribution and the lung diffusing capacity.