Laminar flow is the characteristic flow pattern in all blood vessels except the ventricles and capillaries. The resistance to flow is the result of friction between the laminas rather than between the fluid and the vessel walls since the vessel is effectively lined by a non-slip lamina. Vessel geometry, by affecting fluid velocity, will affect vascular resistance. At higher fluid velocities there is more shearing between the laminas and so greater internal friction. As velocity is given by flow divided by cross-sectional area, the narrower the vessel the greater is the velocity and so the greater is the internal friction or vascular resistance.
Resistance to steady laminar flow along a straight tube is proportional to its length L and the fluid viscosity h, and inversely proportional to the tube radius to the power 4. Two constants are also involved so that the exact relationship is
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