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New method of calculating alveolar CO2 transport

Cruz proposed that the expirogram of CO2 does not represent the process of gas exchange (FASEB J. 21:609.4, 2007). It represents a simple alveolar gas dilution by the tidal volume that does not reach the residual volume (RV). Caucha et al. showed that phase II is formed by the distribution of regional dead spaces (FASEB J. 22:763.6, 2008). However, this latter model could not predict which anatomic Weibel's model generations were involved. Thus, here, we modify Caucha's model by separating the convection‐diffusion process (method 1) in three different steps (method 2): 1. Convection. 2. Convection‐diffusion and 3. Diffusion. As a result, new alveolar CO2 concentrations at the apex and base regions of the lung were found (see figure, interrupted and continuous lines for method 1 and 2, respectively). This picture was obtained after searching the generations involved: the first 8 generations (step 1), generations 9th‐19th (step 2) and the last 5 generations (step 3), which produced the simulated expirogram similar to the experimental curve. In conclusion, the analysis presented follows our theory of alveolar gas mixing; during inspiration, the alveolar CO2 is diluted by the tidal volume (figure), and during expiration, the uneven emptying of parallel regions of the lung generates the expirogram.