Flashcards in Hemodynamics Deck (22)
What are the constraints of Poiseulle's Law?
It only applies to the laminar flow of Newtonian fluids.
What is Poiseulle's Law?
F = [(Pi-Po)π(r^4)]/[8Ln]
Flow is proportional to the pressure gradient and the radius to the 4th power. It is inversely proportional to the viscosity.
What is the equation for resistance to flow?
R = [8Ln]/ [π(r^4)]
Resistance is proportional to the viscosity length and is inversely proportional to the radius to the 4th power.
What is viscosity?
Viscosity is the internal frictional resistance and it increases as the length and the hematocrit increases.
It is equal to: Shear Stress/Shear Rate (Pressure/Velocity)
What is the shear stress?
Resistance to movement between laminae (pressure)
What is the shear rate?
Relative velocities between laminae (velocity of blood flow)
What is laminar flow?
Fluid moves in parallel concentric layers.
What is turbulent flow?
Disorderly flow in the vessels.
What is Reynold's number equal to?
Rn = pDv/n
p = density
D = diameter
v = velocity
n = viscosity
At what value of the Reynold's number does turbulent flow occur?
What can turbulent blood flow cause?
It can cause:
b) damage to endothelial lining
d) Korotkoff sounds
What is the Bernoulli Principle?
In a constant flow system, the total energy of summing the kinetic and potential energies, is constant.
What is the Bernoulli Principle important in?
It is important in stenosis as a decrease in the cross sectional area will result in an increase in the kinetic energy.
What is the Laplace relationship?
Tension = (Pressure x Radius)/Wall Thickness
How does the Laplace relationship apply to capillaries?
Capillaries have: small radius = low wall tension, so they can withstand very large transmural pressures.
How does the Laplace relationship apply to arteriolar vasoconstriction?
They have relatively large wall thickness/lumen diameter ratio = low wall tension; so it provides greater control of vessel diameter and blood flow.
How does the Laplace relationship apply to aneurysms?
Aneurysms have a large radius = high wall tension; so it cannot withstand transmural pressures and therefore will eventually rupture.
How does the Laplace relationship apply to dilated hearts?
Dilated hearts have large radius = high wall tension = higher afterload, results in more systolic work, and higher oxygen consumption to overcome higher wall tension.
What happens to velocity as cross sectional area increases?
Why does velocity decrease from the aorta to capillaries if the cross sectional area is decreasing?
The overall cross-sectional area of all of the capillaries is increasing much more than the arteries that it is coming from and so the velocity will decrease.
How is series resistance calculated?
R1 + R2 + R3 ...