Theoretical capillary gas exchange models available in the literature are limited to either two-dimensional or axisymmetric configurations. The present study investigates shortcomings of a two-dimensional capillary CO-gas transfer model by comparing it to a more realistic three-dimensional configuration. Numerical results from both models, compared to measurements obtained experimentally via the rebreathing technique, demonstrate that CO-diffusion model results are strongly affected when a third dimension is included. The effect of including a third dimension is to increase the gas exchange from the alveolar region into the RBCs, due to a larger gas diffusion surface within the capillary, leading to a significant increase in the resulting CO diffusing capacity. The results also show a better agreement between the experimental results and the numerical results obtained with the three-dimensional model than with the two-dimensional model. Finally, the strong sensitivity of the results vis-a`-vis the domain lengths (keeping the capillary volume constant) highlight the importance of considering the three-dimensional capillary morphology very carefully.

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