Haemodynamics Flashcards
What is haemodynamics?
physical laws governing pressure/flow relationships in blood vessels
fluid mechanics principles can apply to the CVS, but what is different?
blood is not simple ‘Newtonian fluid’ - contains red and whtie blood cells, platelets, lipds
blood vessels are not uniform, straight, rigid tubes
Equation for flow
flow = change in p / resistance
p = pressure gradient between arteries and veins created by pumping heart action
General structure if blood vessel
adventitia
media (smooth muscle, elastin, collagen)
intima (endothelium)
lumen
How is there active control over the blood vessels?
smooth muscle contracts and relaxes, expanding and narrowing the lumen
What is normally found between the media and adventitia?
sympathetic nerves which innervate the smooth muscle
causes vasoconstriction
How does internal diameter and wall thickness change between blood vessels?
aorta - arteries - arterioles - capillaries - venules - veins - vena cava
decreases to capillaries then increases to VC
How does elasticity and smooth muscle of arteries differ?
elasticity decreases with size
smooth muscle in areries and arterioles more
Which blood vessels have no smooth muscle and is not elastic?
capillaries and venules
Compare the total cross sectional areas with all the blood vessels
aorta < arteries < arterioles < capillaries
capillaries > venules > veins
What can be used to see CSA?
change in velocity of the blood
increased velocity = decreased CSA
What are the determinants of flow?
Darcy’s
Flow – directly related to the pressure difference (Pi-Po)
Flow – inversely related to the resistance (R)
What are the determinants of flow?
Poiseuille’s
Flow – directly related to pressure difference (Pi-Po)
Flow – inversely related to length of tube (l)
Flow – inversely related to viscosity of fluid ()
Flow – directly related to radius of tube (r)4
Key point for flow being directly related to pressure difference
greater the pressure gradient, greater the flow
Key point for flow being inversely related to length of tube
longer the vessel, lower the flow
Key point for flow being inversely related to the viscosity
increased viscosity, lower flow
Key point for flow being directly related to the radius of the tube
greater radius of vessel, greater flow
In real body, what is resistance (flow) actually determined by?
radius of the blood vessels
and pumping of the heart
What is the exact relationship between relative radius and relative flow?
F ∝ r^4
slight increase in radius means large increase in flow
Which vessels have the greatest capacity to change radius?
small arteries and arterioles
‘resistance vessels’
Which vessels have the largest drop in pressure? why? and why is this important?
small arteries and arterioles
increased resistance due to change in radius
to decrease pressure before entering capillaries
Equation for total peripheral resistance
arterial - venous P / cardiac output
What also occurs in blood when flow changes?
smoothing of pulsatile blood to steady
Equation for resistance
not necessary to remember
pressure difference / flow
Why will individual blood flows be less than the total flow?
cardiac output is distributed to vascular beds (usually arranged in parallel)
How is total peripheral resistance affected by parallel vascular beds?
less than individual resistance
What is laminar blood flow?
velocity is max in the centre
straight flow
What is turbulent flow?
lack of laminar movement
need higher dirving pressure to achieve same flow
Why does turbulent flow occur?
when there is branches in the blood vessels
atherosclerosis - artificially narrows the vessels
How do the vessels make blood flow as laminar as possible?
Vessels are not rigid tubes
Elasticity conferred by elastin + collagen
Elastic conduits (arteries) and high resistance arterioles have a smoothing effect on blood flow
What is compliance in relation to blood vessels?
change in volume for a given change in pressure
Arterial role in relation to compliance
provides filtering and smoothing
Venous role in relation to compliance
provides capacity for storage
If a blood vessel is compliant, what does this mean?
it is less distensible
What is venous return affected by?
venomotor tone (constriction)
venous valve competence
skeletal muscle pump - muscle contraction in legs
respiration - increased inspiration, decreases intra-thoracic pressure, increases intra-abdominal pressure, provides pressure gradient
