Resistance to blood flow is an important property of blood vessels. Resistance can be determined by the ratio of mean pressure difference over and mean flow through a blood vessel (R = ΔP/Q, Ohm’s law). For a single uniform vessel Poiseuille’s law can predict its resistance, but in practice we are dealing with a vascular bed consisting of many blood vessels. The resistance although depending on the vascular geometry and (local) blood viscosity, can be calculated directly from measurements of mean pressure difference, ΔP, and mean blood flow, Q using Ohm’s law. Rules for addition of resistances are discussed. Resistance should always be calculated from a mean arterial-venous pressure difference and mean flow both averaged over the entire heartbeat in the steady state. In the systemic circulation venous pressure is usually much lower than aortic pressure and can be omitted. However, this is not the case in the pulmonary circulation. The total vascular resistance is mainly determined by the small arteries and arterioles, often called resistance arteries. This means that the mean pressure in all large, conduit arteries, is almost the same. The arterioles regulate flow to the local tissue.
Poiseuille Ohm’s law Pressure distribution Waterfall model Starling resistor
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