Effect of increasing blood flow on distribution of pulmonary transit times in man

The architecture of the vascular network is a major determinant of the distribution of transit times in the organ. Changes in pulmonary blood volumes, pulmonary transit times and the heterogeneity (coefficient of variation) of pulmonary transit times in man were measured using the first-pass radiotracer technique both at rest and during muscular exercise. Dichotomously branching and dispersive flow models of the lung vasculature were applied. The results showed that total pulmonary blood volume increased by 29%, total transit time decreased by 55% and the heterogeneity of pulmonary transit times decreased by 31% when blood flow increased threefold. All changes obeyed power law functions. The changes were greatest when blood flow increased from baseline to intermediate levels. Further increases in total blood flow were associated with decreases in transit times but not with more homogeneous distribution of transit times. The simulated results suggest that the transit time through the arterial vessels and the large veins have a negligible heterogeneity. Most of the variability of transit times through the lungs is due to the heterogeneity of the transit times through the capillaries and venules.