Effect of flow on transpulmonary circulatory transport functions.

Lung transport functions (distributions of circulatory transit times across the lung) were characterized in four anesthetized dogs at various levels of mean pulmonary blood flow. The central circulation was found to approximate a mathematically linear, time-invariant system when respiratory frequencies were maintained at 40/min or more. Lung transport functions were obtained from 144 pairs of lung-input and lung-output dilution curves using a lumped-parameter model and an iterative convolution technique. Average relative dispersion (standard deviation of the transport function divided by mean transit time) was 0.46, about twice that found previously for segments of arteries. The relative dispersion tended to increase as the mean transit time increased, suggesting that the dispersing mechanism of the lung is dependent on the mean transit time (volume/blood flow). Differences between these results and those of single-vessel transport function studies can be resolved by considering the lung as a parallel-pathway system. It is hypothesized that, as total pulmonary blood flow increases, the pathways become more equally perfused and the relative dispersion of the lung decreases.