Haemodynamics- Vascular Function Flashcards
CO = sum of…?
Sum of all local tissue blood flows
*tissue blood flow precisely controlled depending of tissue needs
What is the most important artery in terms of regulating blood flow/ where does biggest pressure drop occur
Small arteries and arteriole: these have muscular walls, can control lumen size and therefore resistance
High resistance > big pressure drop
Velocity is inversely proportional to cross-sectional area.
Q = (mean velocity) x (cross-sectional area) Q= v x A
v propor. 1/A
This is why flow DROPS in capillaries.
DRAW MAP flow diagram
…
Q = change P / R
P is the pressure gradient, normally this is 0
Blood flow (Q) equals
Cardiac Output (CO) which is ~5L/min at rest
*** so BF is directly proportional to the pressure gradient and to TPR
CO should equal (PA - PV)/TPR
Why is only MAP used?
Because USUALLY PV = 0mmHg
But in some situations (heart failure) it can be significant
Resistance in series
R(total) = R1 + R2 + R3
very uncommon in the body.
Increasing the R anywhere will increase the total R significantly.
Resistance in parallel
1/R(total) = 1/R1 + 1/R2 + 1/R3
Increasing the R at one point means other organ/tissue areas have little to no effect on TPR, but a big effect on local flow.
Draw the Jean Pouiseuille equation of flow
…
Draw Resistance equation
…
R is proportional to
1) tube length L
2) the viscosity of fluid (n)
incersely proportional to
3) radius
the r^4 factor
r=1 Q=1
r=2 Q=16
r=4 Q=256
So anything that alters vessel diameter has a huge effect!
Viscosity.
Determinants of?
A measure of d=friction between adjacent layers of fluid.
1) Temperature : more viscous with cold
2) Haematocrit
3) Shear rate (velocity)
4) Vessel diameter
Anaemia
low blood cells, lower viscosity, higher blood flow
What happens at slow shear rates (velocities)
- Viscosity no longer independent of shear rate
- viscosity increases as cells aggregate
Haematocrit
Red cell vol: total volume
in a sample of peripheral blood
How does vessel diameter affect viscosity
in very small blood vessels (less haematocrit> axial streaming/plasma skimming)
Poiseuilles equation assumes
1) steady laminar flow
2) Newtonian fluid
3) Rigid straight tube
these criteria don’t really fit!
How is the contractile state of vascular SM controlled?
1) Vascular endothelial cells (NO)
2) Mediators release locally from sympathetic nerve terminals
3) Circulating hormones (angio tensin II)
Shear Stress
as flow increases, shear stress on endothelial cells increase
-NO release: leads to vasodilation.
What factors result in turbulent Flow
- High fluid densities
- large tube diameters
- high flow velocities
- Low fluid viscosities
- abrupt variation in tubes
- Tube wall irregularities
How to predict turbulent flow with Re
if Re >2000 = turbulent
Re = pvD/ n
as we rarely know velocity we use
Re 4pQ/piDn
Stenosis
Turbulent flow in the area post the stenosis.
Turbulence generates sound waves (murmurs) that can be heard, these intensify with flow
Conservation of Energy: Bernoulli’s principle
Principle states: total energy of laminar flow without resistanct is constant and equals the sum of
Pressure energy (PxV)
Potential/gravitational energy (pgh)
Inertia/kinetic energy (1/2pv^2)
Total Energy:
E= PV + pgh + 1/2pv^2
In terms of Bernoulli’s Principle, what happens when there’s a narrowing in a vessel
- v increase
- KE increases
- and Total energy remains the same
so Pressure must decrease! (why P drops in the narrowest part)
Transmural Pressure
change P = Pinside - Poutside
Pressure difference across the wall of the vessel
tht tension is very much dependent on the radius.
Laplace Law
The larger the vessel radius the larger the wall tension required to withstand an internal pressure
proportional to radius
pressure
1/wall thickness
eg) giraffe
Giraffe
high pressure at top of neck = very very thick wall thickness.
