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The distribution of blood transit times within the pulmonary arterial tree has important implications with regards to overall lung function. Previously, we showed that the pulmonary arterial tree imparts little dispersion to an injected bolus, so that the bolus arrives at downstream arteries with a time delay, but little increase in variance. Furthermore, the arterial time delay is nearly the same for all pathways to arteries of the same diameter, independent of their pathway length. This small amount of dispersion was observed despite the velocity profile within the arterial tree and the substantial variation in arterial pathway lengths. Thus, we have begun to ask why velocity profile effects and pathway length heterogeneity within the pulmonary arterial tree have so little influence on bolus dispersion. X-ray angiography studies were used to visualize streamtube pathways within the pulmonary arterial tree. Full bolus injections were used to visualize all flow streamlines within the tree, while 'streamtube' injections labeled only about 1% of the inlet arterial cross-section. By changing the injector position within the arterial cross-section, different streamtubes were traced and found to remain intact downstream to vessels less than 200 micrometer in diameter. Thus, it appears that lower velocity streamtubes tend to peel off from the full velocity profile at arterial bifurcations, while flow streamtubes with higher average velocity travel down the main arterial trunk. The net result is that dispersive velocity profile effects are mitigated by the interaction between the distributed velocity profile and the branching pattern of the pulmonary arterial tree.
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