L14 Hemodynamics Flashcards
Viscosity
The internal frictional resistance b/t 2 adjacent layers of fluid
∆P/velocity ———–> Shear stress/Shear rate
more hematocrit would make a fluid more viscous.
Vessel diameter - larger = less viscous
Layers move in diff speeds, which make a parabola - the sharper the parabola (the more layers) the LESS viscous
Larger diameter = more layers
Poiseuille’s Law
BF = ∆P/R = (P1-P2)πr^4 / 8L*viscosity
Blood flow is proportional to P, and inversely proportional to R
BF propor. to r^4 VERY IMPORTANT
BF inv. propor. to L and viscosity
most important: Pressure gradient and radius
Hematocrit and vessel diameter
Due to axial streaming, RBCs line up single file to go into a smaller vessel, making it less viscous.
Plasma skims the outer edges of the vessel, which is why more plasma gets into super small vessels than hematocrit.
With larger diameter, they can clump up and travel that way.
Laminar vs. turbulent blood flow
Laminar - blood travels in layers within the tube
Turbulent - blood is disorderly and whooshes past.
murmurs, bruits,
can cause damage to endothelial lining,
this is what Korotkoff sounds are
Reynold’s Number (NR)
NR is a measurement of how likely turbulence is to be achieved. (higher is more likely)
NR = (rho)Dv / viscosity
High Diameter and fast velocity = turbulence. ROOT OF AORTA!
Bernoulli’s Principle
In a constant flow system, total energy is always conserved.
KE+PE=TE
IE: High velocity (KE) in a stenotic region lowers the transmural P (PE) on the vessel
LaPlace relationship
WT = P*r / thickness WT is the force that keeps the vessel open
Capillaries have a small radius therefore a LOW WT and can withstand high P’s
Aneurysm: this is a large radius and thin thickness, which creates HIGH P and eventually ruptures.
Dilated Heart: larger radius give HIGH WT. This WT is an afterload that leads to more O2 consumption and CHF
Wall Tension : Lumen Diameter relationship
Vessels have more control over their size if the:
WT is HIGH
Lumen is SMALL (small lumen r)
Resistance in Series vs. Parallel
Series: R is additive. Raise local R, raises total R
Parallel: Open more vessels with R in parallel to decrease total R
(bc BF has more vessels as options)
Organ supply is in parallel bc not all are open at the same time
Velocity vs. Pressure vs. Cross Sectional Area
Capillaries have the lowest velocity because they have the largest CS Area
Veins hold the most volume of blood (mainly bc compliance)
low CS Area means faster velocity
