Flow Patterns and the Localization of Vascular Disease in the Circulation

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 sites are confined mainly to regions of geometrical irregularity where vessels branch, curve, and change diameter and where blood is subjected to sudden changes in velocity, direction, or both. In such regions, flow is disturbed and separation of streamlines from the wall with formation of eddies are likely to occur. We describe here 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, to understand the mechanics of flow in branching, expanding, and curved vessels, it is first necessary to deal with some basic fluid dynamic concepts. It is particularly necessary, as there is a common misunderstanding among physicians and surgeons that the formation of eddies at sites of disturbed flow represents turbulent flow. As explained below, this is usually not the case. As is the custom in many textbooks of medical physiology, we begin our lesson in fundamental hydrodynamics by defining the steady laminar flow of a liquid through a circular cylindrical tube, known as Poiseuille flow.