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Blood Flow & Velocity

Blood Flow & Velocity

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Blood flow
Blood Flow: The amount of blood that passes by a given point in a given amount of time.
Calculated using a variant of Ohm's Law of electricity: Q = Change in driving pressure/resistance of vessel wall.
Average total blood flow, at rest, is approximately 5 L/min, and is equal to cardiac output; cardiac output is the volume of blood pumped to the aorta per minute.
Change in blood flow
Blood flow to target organs is constantly readjusted to accommodate their metabolic needs.
The most efficient way to achieve this is to change the radius of blood vessels, and, therefore, the resistance to blood flow.
Vasoconstriction shrinks vessel radius, so resistance increases, which causes blood flow to decrease.
Vasodilation widens vessel radius, so resistance decreases, and blood flow increases.
Clinical correlation:
Atherosclerosis, which is the build up of fats, cholesterols, and other materials on the vessel wall in the form of plaque, which reduces the vessel, increases resistance, and restricts blood flow to tissues, starving them of necessary nutrients.
  • If atherosclerotic plaque detaches from the vessel wall, it can become lodged within smaller vessels and completely obstruct blood flow (such as in stroke).
Blood flow velocity
Velocity of blood flow refers to the linear distance blood travels in a given amount of time.
Velocity = Q/A; Q refers to blood flow, A refers to the cross-sectional area of the vessel (area = pi multiplied by the radius squared).
Laminar vs. Turbulent flow
Laminar flow
Aka, streamlined, flow refers to normal, linearly flowing layers of blood.
The layers of laminar-flowing blood create a parabolic profile because the velocity of blood flow is highest in the central layers, and lowest at the vessel wall.
Turbulent flow
Blood layers mix and run radially and axially.
Turbulent flow results from irregularities within the vessels, such as valves or clots that alter blood velocity, or from changes in blood viscosity.
Such blood flow can result in reduced tissue perfusion; the body may try to compensate by increasing blood pressure.
Clinical correlation:
Turbulence produces sounds that can be auscultated (with a stethoscope). Bruits refer to arterial murmurs, and can be indicative of vessel shunts or stenoses (narrowing).

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