Resistance

- Poiseuille's equation gives factors that change the resistance of blood vessels.

tt r4

where:

R = resistance t) = viscosity of blood I = length of blood vessel r4 = radius of blood vessel to the fourth power

- Resistance is directly proportional to the viscosity of the blood. For example, increasing viscosity by increasing hematocrit will increase resistance and decrease blood flow.

- Resistance is directly proportional to the length of the vessel.

- Resistance is inversely proportional to the fourth power of the vessel radius. This relationship is powerful. For example, if blood vessel radius decreases by a factor of 2, then resistance increases by a factor of 16 (24), and blood flow accordingly decreases by a factor of 16.

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