Blood Hemodynamics
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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.
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).
- Cardiac murmurs often refer to structural valvular disease.
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).