This describes the tendency for transition from laminar flow to turbulent flow. For blood the Reynolds number is around 1000. If the velocity of blood flow exceeds a critical value, or if there is an obstruction to blood flow, eddies start to form and turbulent flow may develop. When turbulent flow is fully developed, the velocity profile becomes rectangular in shape. Turbulent flow requires the expenditure of more energy than is required for laminar flow. Murmurs or bruits are audible over areas of turbulent flow.
Local flow disturbances leading to altered velocity profile and turbulent flow are related to curvature of vessels, branch points, bifurcations and the angle of branch take-off. A haemodynamically significant arterial stenosis is associated with cross-sectional area reduction by 75% and reduction in vessel diameter of at least 50%.
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