Blood Flow: Laminar vs Turbulent
Blood Flow
is the quantity of blood that passes a given point in circulation in a unit time.
e.g. renal blood flow is about 1000 ml/min.
In adults total blood flow through the entire systemic circulation is about 5000 ml/min under resting conditions.
Nature of Blood Flow
Laminar Blood Flow
occurs when blood flows...
at a steady rate.
through a long and smooth blood vessel.
is silent.
Blood flows in streamlines.
A layer of blood close to the vessel wall is almost stagnant due to cohesive forces between the inner surface of vessel wall and blood.
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Inner concentric cylindrical layer of blood slips over this outer layer → moves faster than the first layer
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The third layer slips over the second one → its velocity is even higher
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Thus the velocity of blood layers increases as we go from wall to the center of the vessel.
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The resultant velocity profile is a bullet-like parabola with the maximum velocity at the center.
Turbulant blood flow
occurs when...
blood flow is very high.
there is an obstruction in the vessel.
the blood makes a sharp turn.
the blood flows over a rough surface.
Blood flows in all directions, usually forming whorls and continuously mixing within the vessel.
Increases resistance to blood flow.
Can be auscultated as murmers e.g.
Korotokoff sound during measuring blood pressure
Murmers of stenosis, shunt, cardaic valvular lesions etc.
Very intense turbulance can be felt with touch as thrills.
Reynold's Number
Predicts whether blood flow would be laminar or turbulent.
The higher the Renold's number → the more the chance of turbulent blood flow.
Below 2000 flow is usually leminar.
Above 3000 flow is mostly turbulent.
d = diameter of the blood vessel. Large diameter tends to cause turbulence. In smaller vessels, blood flow is almost always laminar.
v = velocity of blood. High velocity tends to cause turbulence e.g. high cardiac output.
ρ = density of blood. High density tends to cause turbulence.
η = viscocity of blood. High viscocity tends to cause laminar flow.