L14 Circulation- Hemodynamics Flashcards
Explain Poiseuille’s law.
Applies to laminar flow of Newtonian fluids through uniform cylindrical tubes (blood is non-Newtonian and circulatory system isn’t uniform)
- Flow is direction proportional to pressure gradient and inversely proportional to resistance
- Flow is also directly proportional to radius^4= diameter double give flow increase by 16
How does Poiseuille’s law relate to blood flow?
It puts flow in relationship to resistance, pressure gradient and radius of vessel
How does Poiseuille’s law related to BP?
Flow is directly proportional to pressure gradient
How does Poiseuille’s law related to resistance?
Flow is indirectly proportional to resistance
Define viscosity.
Internal frictional resistance between adjacent layers of fluid
-viscosity= shear stress/ shear rate (pressure/velocity)
How does viscosity change with vessel diameter?
Increasing vessel diameter increases viscosity b/c of increase in hematocrit
How does hematocrit change viscosity?
Increase in hematocrit= increase in viscosity
How does hematocrit change with vessel diameter?
Hematocrit is lesser in smaller vessels than in larger b/c of plasma skimming and axial streaming
Define laminar blood flow.
Fluid moves in parallel concentric layers within a tube
-Laminar flow is silent
Define turbulent blood flow.
Fluid moves in a disorderly pattern
- Murmurs, bruit, thrombi
- Korotkoff sounds- based on changes in V of blood flow
Define Reynold’s number.
Dimensionless number indicating propensity of turbulent blood flow
- Higher Reynold’s number= greater chance for turbulent blood flow
- N(R)= (density x Diameter x velocity)/ viscosity
- Larger diameter = higher chance for turbulence (aorta, smokers)
Define Bernoulli’s principle.
In a constant flow system, total energy (potential + kinetic) remains constant
- Energy conserved by velocity of flow (kinetic) or lateral pressure (potential)
- Total energy (E)= potential energy (P) + velocity of blood flow (pv^2 / 2)
- When velocity of blood flow increases, lateral pressure decreases
How does Bernoulli’s apply to the circulation system?
- Blood flow moves from higher to lower total energy, not from higher to lower pressure
- Abrupt decreases in vessel size (stenosis) converts potential energy into kinetic= lateral pressure decreases and velocity of blood flow increases
Define the Laplace relationship.
Wall Tension= Pressure x Radius / Wall Thickness
How does wall tension affect fxn of dilated hearts?
Large radius= high wall tension, more systolic work/higher oxygen consumption to overcome tension
-High wall tension opposes shortening
How does wall tension affect arteriolar vasoconstriction?
Large wall thickness/lumen diameter ratio= low wall tension, provides greater control of vessel diameter and blood flow
What is the relationship between velocity of blood flow and vessel cross sectional area?
As total cross-sectional area increases, velocity of blood flow decreases
-Velocity is slowest in capillaries to allow for nutrient reabsorption
Explain series resistances in circulation.
1
Explain parallel resistances in circulation.
1
Define how velocity of blood flow and pressures change throughout circulation in relation to changes in total cross-sectional area.
1
Define shear stress
Resistance to movement between laminae (pressure)
Define shear rate
Relative velocities between laminae (velocity of blood flow)
Explain the shearing laminae in blood vessels.
The shearing laminae of blood are concentric cylinders that move with different velocities.
- Inner most moves with highest velocity, outermost moves slowest
- Velocity profile becomes parabolic with maximum velocity at central axis
- Lower viscosity = sharper parabolic profile
Define Newtonian vs. non-Newtonian fluids.
- Newtonian fluid has constant viscosity over a range of shear rates and stress, homogenous fluid (water)
- non-Newtonian fluid has viscosity changes over ranges of shear rate and stress, non-homogenous fluid (blood)
How does velocity change blood viscosity?
As velocity of blood flow increases, viscosity decreases due to loss of interactions
Why do higher altitudes lead to higher hematocrit?
Increased erythopoietin (EPO) occurs to increase RBCs in blood to carry more oxygen= increase in hematocrit= increased viscosity
Define plasma skimming.
Tendency of smaller vessels to have more plasma and less RBCs due to axial streaming
-As blood flow slows down from larger to smaller vessels, viscosity should increase, but it doesn’t b/c of axial streaming
Define axial streaming.
Tendency of RBCs to accumulate in axial laminae at high shear rates
- Important b/c it changes your hematocrit
- Highest velocity in a vessel is in the center
How does wall tension affect fxn of capillaries?
Small radius = low wall tension, can withstand very large amounts of transmural pressures
How does wall tension affect fxn of aneurysms?
Large radius= high wall tension, can’t withstand transmural pressures and eventually rupture
