Laminar flow

This can be considered as being represented by a series of concentric layers or laminae sliding past each other, the outer ones nearly stationary, the inner ones moving faster. All motion is parallel to the walls of the tube. The maximal flow velocity is in the central axis of the vessel, resulting in a parabolic velocity profile. This difference creates a shearing effect greatest at the wall of the vessel. The shear rate is the rate of change of velocity of flow between concentric laminae of blood. The shear stress on the endothelial surface is directly proportional to the shear rate and inversely proportional to the viscosity of the fluid. Flow is directly proportional to pressure in laminar flow.

Turbulent flow £

In turbulent flow, the velocity of flow varies rapidly with respect to space and <_

time, and some of the fluid energy is dissipated as heat. Flow separation occurs, S

with the axial flow stream being separated from flow in areas more peripherally, y as in aneurysms, and in segments just beyond arterial stenoses.

The tendency to turbulent flow is directly proportional to velocity of blood flow and to the diameter of the blood vessel, and inversely proportional to the viscosity of blood divided by its density. Turbulent flow is thus related to high flow rates, changes in vessel diameters, angles and branching points in the circulation.

Reynolds number = v.d/(]/p) where v — velocity of blood flow (cm/s) d — diameter of the vessel 1 — viscosity of blood in poises p — density of blood

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