Figure 1.

Flow separations at an arterial branch can predispose or contribute to pathogenesis. Flow separation and the flow disturbance that occurs in the separated region are proatherogenic—complex, transient vortices form and dissipate (but not completely) throughout each heartbeat. The primary characteristics of disturbed flow are low average shear stress, constantly changing gradients of shear stress, oscillatory flow (and shear stress) because of flow reversal, and multifrequency, multidirectional, secondary flows. High shear stress protects against atherosclerosis as long as it remains below levels that detach the endothelium (estimated >40 N/m² ; rare). In pulsatile flow within arteries, the separation region expands and contracts with the cardiac cycle. Excellent illustrations of computational imaging of human arterial hemodynamics¹⁹ can be viewed online.⁸⁰