Computational Fluid Dynamics Study of Transit Time through Pulmonary Arterial Trees

The structure-function relationship in vascular systems has been studied in an attempt to understand the various functional design criteria for these networks. For arterial tree networks, some of the functional design criteria that have been studied includes minimization of the cost of maintaining a network, equalization of shear stress, cancellation of reflected pressure waves, and damping of pulsatile flow. For the pulmonary arterial tree, control of the input to the capillaries might also be a functional design criterion. Recent work in this area has revealed that blood flow transit times between the pulmonary arterial inlet and descendent arteries are correlated to diameter of the descendent arteries independent of the path length to those arteries. Consequently, blood entering the pulmonary arterial inlet reaches descendent arteries of equivalent diameter at approximately the same time, regardless of the distance traveled. This observation implies that factors other than the distance traveled control the transit time distribution in the pulmonary arterial tree. The motivation for this thesis was to begin the process of understanding how the structure of the tree might influence transit time. One hypothesis that might explain this observation is that velocity profile and the geometry of the pulmonary arterial tree interact to produce the correlation between transit time and vessel diameter. A velocity profile along the radius of a tube often takes the form of a parabola, where velocity is maximal at the.center of the tube and zero at the wall of the tube. In other words, the velocity of a fluid streamline depends on its proximity to the wall. Streamlines that enter a relatively small daughter vessel at a bifurcation are generally very close to the wall, and have a low velocity through the parent vessel to the daughter. Consequently, the streamlines that enter the daughter may have a similar transit time as streamlines that travel well beyond that bifurcation, through the larger sibling vessel, because the streamlines that flow through the sibling are traveling faster. This could explain why transit time is dependent on the diameter of the descendent vessel, independent of distance traveled..