Abstract
Atherosclerotic plaques occur predominantly at specific regions in the neighborhood of bends or junctions. These findings prompted many investigators to speculate that local hemodynamic forces are closely related to the pathogenesis and proliferation of atherosclerosis. The formation mechanism of thrombi in the cardiovascular system has also been considered in connection with changes in hemodynamic circumstance. Up to the present, however, no definite conclusion has been obtained in regard to the mode of participation of blood flow in the devlopment of atherosclerosis. The reason is that detailed information about the fluid dynamic features of blood flow has not been acquired owing to the difficulties in measuring the spatial and temporal distributions of velocities and pressures in the three-dimensional flow field inside the narrow blood vessel lumen. By means of the flow visualization technique, we have investigated flow patterns in various blood vessel models made of glass or metacrylate resin and reached a conclusion that complicated flow patterns seen in tubes with a stenosis, bifurcation, or branchings can be understood in terms of the occurrence of a secondary flow, named the horseshoe vortex.
The generation of the horseshoe vortex in a flow through the tube with a bluff protuberance into the boundary layer can be explained as follows. A radial pressure gradient toward the tube wall is produced along the upstream surface of the protuberance because of the interaction between the viscous sheared flow and the wall of the tube. This pressure gradient makes fluid particles turn round downward directly before the obstacle. Then they curled round on themselves and formed a bound vortex tube, the horseshoe vortex, which in turn passes round the front of the protuberance in both directions. In a tube with a Y-shaped bifurcation or a rectangular side branch, the flow divider at the branching site acts in place of the protuberance to produce a vortex tube similar in pattern to the horseshoe vortex. The vortex tube extends from the high pressure region, i. e. the apex of the flow divider, to the low pressure, i. e. the lateral margin of the branch orifice and generates swirling secondary flows in the main and branched tubes. From the results of the present model experiments, it will be suggested that the following mechanical factors may initiate or facilitate the atherogenesis and thrombogenesis: collision of blood vessel walls and the blood cells captured by the horseshoe vortex, the interaction of the walls and blood cells due to turbulence, and localized high wall shear stresses.
