Flow and Vascular Geometry

Fluid mechanical factors play an important role in the localization of sites of atherosclerosis, the focal deposition of platelets resulting in thrombosis, and the formation of aneurysms in the human circulation. The localization is confined mainly to regions of geometrical irregularity where vessels branch, curve and change diameter and where blood is subjected to sudden changes in velocity and/or direction. In such regions, flow is disturbed and separation of streamlines from the wall, with formation of eddies, is likely to occur. We shall describe the flow patterns and fluid mechanical stresses at these sites and consider their possible involvement in the genesis of the above mentioned vascular diseases. However, in order to understand the relationship between vessel geometry and the observed flow patterns, it is first necessary to deal with some aspects of the mechanics of flow in branching, expanding and curved vessels. Such a treatment will also serve to dispel the notion, common among physicians and surgeons, that the formation of eddies at sites of disturbed flow represents turbulent flow.