Haemodynamics Flashcards
blood flow is analogous to give comparison
electrical current V = IR v = Voltage I = current R = resistance /_\P = QR /_\ P = Pressure gradient Q = flow R = Resistance
Ohm’s law
V =IR
V =voltage
I = current
R = Resistance
Darcy’s law
perfusion or flow = pressure gradient / resistance
Q = /_\P/R
how can Darcy’s law be applied to the whole CVS system
flow = pressure gradient / resistance – CO = MAP ÷ TPR – cardiac output (CO) – mean arterial pressure (MAP) – total peripheral resistance (TPR)
what do we mean by mean arterial pressure
the average pressure across the cardiac cycle
where is the pressure generated
in heart during systole
cardiac output determines
systolic pressure
increased CO = increased systolic pressure
pressure of aorta
~100mmHg
pressure of capillaries
~25mmHg
pressure of great veins
~2 mmHg
vascular resistance influenced by
viscosity - thicker =faster
length = shorter = faster
vessel radius = wider = faster
how is poiseuille’s law applied to resistance to flow
R = ηL / r^4
– η is viscosity, L is length and r is vessel radius
vessel radius on resistance
tiny change to radius can have big impact
inversely proportional to power of 4
site of most vascular resistance
arterioles
why are resistance greater in small arteries than large arteries
• Large elastic conductance arteries have limited capacity to vary diameter
– unlike small muscular arteries and arterioles
• Thus resistance is largely generated in these small arteries and arterioles
small changes in diameter of blood vessels lead to
big changes in flow
blood will go where resistance is less - control perfusion to allow blood to go to tissue where nutrients is most in demand
what is viscosity proportional
proportional to resistance
inversely proportional to flow
why is blood thicker than water
2.5 -3 times more viscous than water
plasma proteins and red blood cells
haematocrit - HCT
the proportion of blood volume occupied by red cells
haematocrit of women
37-47%
haematocrit for men
40-54% - more viscous than women
how will haematocrit affect viscosity
anaemia - lower HCT - less viscous
polycythaemia - higher HCT increase viscosity
why slight decrease in haematocrit in capillaries and smaller vessels
Capillaries and smaller vessels (<300 μm) tend to have (slightly) lower
haematocrit due to axial accumulation
rbc cells in centre of flow - branches get more plasma than rbx
do we excpet radius or viscosity to have a greater impact on resistance and therefore flow
– r
4 has much more effect on flow than
– it is just slightly faster than it would expected
blood flow is
laminar
laminar speed
speed of blood slows as we move closer to vessel wall - silent smooth and continuous
highest velocity in the centre
if blood flow is laminar what factor must be important
size
how is flow changed in small arteries
Since small arteries have a large area to volume, most blood is near the wall and there is a large effect on flow
– here increased viscosity slows flow
how is flow changed in larger arteries
In larger arteries, there are more layers and so increased velocities
– this diminishes the effect of viscosity
consequence of speed
increased chance of turbulence
where does non laminar flow tend to occur
in ascending aorta and around branch points - narrowed points where velocity increases
basis of measuring turbulent bloodflow
he basis of Korotkoff sounds in measuring BP
– the basis of bruit in atheroma
– and also bronchial breathing
transmural pressure
• The transmural pressure is the pressure that either distends or collapses blood vessels
transmural pressure formula
pressure pushing in - pressure pushing out
• If the pressure outside (Po) exceeds the pressure inside (Pi) what happens
• If the pressure outside (Po) exceeds the pressure inside (Pi) then the vessel will collapse
what is transmural pressure related to
the wall tension and the radius
– The Law of LaPlace
law of laplace transmural pressure
wall tension divided by radius
– PTM = T ÷ r or rearranged to T = PTM x r
wall tension
how “thick” the vessel wall needs to be
wall tension needed for vessels with small radius
less wall tension is required to balance
out the distending pressure
wall tension needed for larger radius
more wall tension is required to
balance the distending pressure
which vessels are more distensible veins or arteries
veins x 8
compliance
the change in volume for a given change in pressure and
so relates to a vessels distensibility
– C = ΔV ÷ ΔP so can be rearranged to ΔV = C x ΔP
what vessels are more compliant
veins x 20 more compliant than arteries
