According to Poiseuille's law, a change in which parameter would have the greatest influence on...

According to Poiseuille's law, the flow rate of a fluid through a vessel is directly proportional to the fourth power of the vessel's radius. Therefore, a small change in the radius of the vessel has a much larger effect on blood flow compared to changes in pressure gradient, length of the vessel, or viscosity of the fluid.

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Poiseuille's law in medical physics and hemodynamics literature.