Hemodynamics Flashcards
Relationship between velocity and flow
V=Q/A
Q = flow (cm^2/s)
A = cross-sectional area
If no branching…
And the cross sectional area increases…
The flow will
Stay the same
(Velocity decreases)
**if branching…flow can change
Cross-sectional area is largest in the ?
Capillaries…
So velocity is much lower in capillaries compared to the aorta and bigger arteries
Turbulent flow
Disordered, inefficient flow
Not ideal
Can do damage to the endothelium
Occurs at branch points and points with vessel narrowing
Laminar flow
Most efficient type of flow
Concentric layers of flow
Layers closer to the walls = slower due to friction
Reynolds number
Re = pDv/n
P = density D = diameter V = velocity N = viscosity
Greater than 200 = turbulent
Factors that would increase reynolds number and turbulency
Increased blood density, velocity, and vessel diameter
Factors that decrease reynolds number and turbulency
Increased blood viscosity
Anemia effecy on Re
Decreases viscosity —> increases Re
Increased CO —> Re?
Increases velocity —> increases Re
Decreased vessel diamter —> Re?
Increases velocity —> increases Re
Dramatic increase in systolic pressure in the external or internal carotid in comparison to the common and other carotid = sign of what
Stenosis of one of the carotid branches…thus directing much of the blood flow to the other…instead of splitting the flow between the two after bifurcation
Pulse pressure =
Difference between systolic and diastolic pressures
Mean arterial pressure
Diastolic pressure + 1/3*pulse pressure
Is the area under the graph of arterial pressure over the period of one cardiac cycle
Distensibility
Ability to change dimension in response to force
Ensure steady flow
Reduces the amount of work the heart has to do
Rigid vessels over time can lead to heart failure
Compliance
Change in volume for a given change in pressure (deltaV/deltaP)
Measure of how distensible vessels are
Elasticity
Ability to return to normal size after being distended
Lost over time/age
Which causes an increase in systolic pressure, and since diastolic pressure remains the same…pulse pressure increases with age
Ohm’s law for blood flow
Q = delta P/R
Q = flow. (Was current)
P = pressure (was voltage)
Poiseuille’s law
Replaces R with an equation in Ohm’s
R = 8(viscosity)(length)/pi*radius^4
Sub into Ohm’s law
Effect of the following on flow
- Decrease viscosity
- Increase viscosity
- Decreased radius
- Increased radius
- Decreased length
- Increased length
- Up (anemia)
- Down (polycythemia)
- Down (vasoconstriction)
- Up (vasodilation)
- Up (rare)
- Down (rare)
Total resistance for vessels in a series =
Resistances added up
Benefit = allows laminar flow
But otherwise cannot just have series because of increasing resistance…blood wouldn’t get back to the heart
Total resistance with vessels in parallel
Inverse of total = inverse of each individual
Overall resistance is lowered
Benefit = can shunt blood from one vessel to another depending on the resistance
Does viscosity of blood constant?
No
Not a Newtonian fluid…
Why?
- Blood viscosity increases with increasing hemacrit
- Fahraeus-Lingqvist effect = small diameter vessels, red blood cells move toward the center of … so outer part being more fluid…becomes less viscous and less friction
- Blood viscosity decreases with increased flow
Compare primary functions of arteries, veins, and capillaries
Relate those differences in structure
- Arteries:
- lots of smooth muscle to handle force of blood pumping through it
-adventitia secures arteries in place
-ENDOTHELIAL LAYER
—> separates blood from smooth muscle…receives signals and sends messenges that cause smooth to constrict or dilate
1A. Arterioles
- responsible for regulating total peripheral resistance
- vasoconstriction = increased pressure upstream and decreased downstream…
1B. Metaarterioles
- connects arterioles to venules
- bypasses flow directly from arteriole to venule
- allows completely shunt blood away from capillaries or just some
- Veins
- just enough of SmM for constriction and dilation
Valves allow for unidirectional flow and compartmentalization
- highly distensible - allows them to hold high volume for reservoir purposes - Capillaries
- single layer of endothelial cells
- sites for exchange with surrounding tissue
Why the drastic drop in pressure moving from muscular arteries to capillaries
Compensates for the increase in resistance in the capillaries
Law of LaPlace
T = P * r
T = wall tension P = transmural pressure R = radius
**capillaries can withstand a elevated “P” due to a small radius
Aneurysms
—> arteries remodel themselves to cope with an increasing wall tension - bulging to make a sphere shape
In order for diffusion to happen…the substance must be what is nature?
Hydrophobic
Fick’s Law
Diffusion in the capillary follows this law
J = -PS (Co - Ci)
J = movement of solute over time P = each substance diffusing across has a different eas of diffusion S = surface area, as you increase S...easier to get inside C = concentrations inside/out (gradient...most important factor)
Edema affect on diffusion across capillary
You have liquid that is pushing on the tissue …adding distance between the capillary and tissue
Once this happens…diffusion becomes diffusion limited…you have to wait for material to make its way to the tissue
Filtration
Capillary —> interstitial fluid
Capillaries have a higher pressure, fluids push into fluid
Driven by hydrostatic pressure…but it decreases as you approach the venule
Absorption
As you get to the venules…hydrostatic pressure diminishes and now the dominant pressure is oncotic pressure
Which is formed by plasma proteins in the plasma (mostly albumin) in the capillary…these proteins will start to pull water into capillary
Sterling equation
Summarizes the flow of fluid is equal to the difference in hydrostatic pressure minus the difference in oncotic pressure
The drop is pressure from arteriole to venule serves as a change from filtration —> absorption
If difference in HSP > difference OP = filtration (positive #)
Goal of vasodilation?
Vasoconstriction?
Vasoconstriction:
Causes pressure to decrease DOWNSTREAM in the capillaries
Leads to a DECREASE in filtration and INCREASE in absorption —> leads to restoration of blood volume
Vasodilation:
Causes pressure to increase DOWNSTREAM in the capillaries
Leads to INCREASE filtration and DECREASE absorption —> leads to edema
Pinocytosis
Method by which you get large lipid insoluble molecules into the cell
Effects of vasodilation of arterioles on filtration and absorption
Vasoconstriction of arterioles
Increased pressure on arterilar side…
Increased filtration and decreased absorption
—> more fluid entering into the interstitial space.
Constriction = opposite
Function of lymphatic system…
Vessels intercalated by capillary bed
Pressure within bed is lower than pressure within the capillaries…so fluid can flow into them
Main importance = helps resotre blood to its normal volume
Prevents edema
Deep vein thrombosis (DVT)
Caused by clot forming in the venous side…restricting blood flow
Messing up the flow —> increase in venous pressure —> similar effect as if you vasodilated the arterioles
Increase filtration and decrease absorption
Decrease pressure on arteriolar side
Effect?
Enhanced reabsorption
Cause = vasoconstriction
Increased pressure on the arteriolar side
Effect?
Enhanced filtration/edema
Cause = vasodilation
Increased pressure on venous side
Effect = enhanced filtration/edema (same as increasing arteriolar pressure)
Cause = heart failure, venous clots
Decreased pressure on venous side
Effect =?
Effect = enhanced reabsorption (same as decreased pressure on arteriolar side)
Cause = hemorrhage
Need to restore blood volume
Why does tension remain low in capillaries? Veins?
Capillaries have a small radius
Veins have little pressure
Refer to Law of LaPlace: T = P x R
What characteristic of veins allow them to make good ‘reservoirs’ of blood?
They have high compliance
Takes a small change in pressure to instill a large change in volume
