Blood flow is determined by the pressure difference and the resistance of the vessel. Pressure difference is the driving force for blood flow, and resistance is impediment to flow. The relationship of flow (Q), resistance (R), and pressure difference (∆P) is expressed by Ohm’s law (Q=∆P/R). The magnitude of blood flow is directly proportional to the pressure difference. The direction of blood flow is determined by the direction of the pressure gradient from high to low pressure. Blood flow is inversely proportional to resistance, and a change in resistance is the major mechanism for changing blood flow, particularly in arterioles. Resistance can either be measured for the entire systemic vascular system, called total peripheral resistance, or measured in a single organ.
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