Present descriptions of nitric oxide (NO) transport in the lungs use two compartment models: airway compartment without mixing and alveolar compartment with perfect mixing. These models neglect NO molecular diffusion in the airways. To assess the impact of axial diffusion on expired NO profile, we solved a transport equation that incorporated diffusion, convection, and NO sources in the symmetrical Weibel model of the lung. When NO parameters computed from experimental data with the two

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... ent models are used in our model as NO sources, simulated end-expired NO is 29-45 and 64-78% of experimental values at expiratory flows of 50 and 2,000 ml/s, respectively. These lower values are because of NO axial diffusion: During expiration, NO back diffusion (opposed to convection) prevents some NO from being expired, so a two-to fivefold increase of airway NO excretion is necessary to simulate end-expired NO consistent with experimental data. We conclude that, insofar as a significant amount of NO is produced in small airways, models neglecting NO axial diffusion underestimate excretion in the airways.