Haemodynamics Flashcards
Define haemodynamics
relationship between blood flow, BP, resistance to flow
Steps of haemodynamics?
Force : Contraction
Work : Isovolumetric contraction / ejection
Pressure : Diff aorta to veins
Compliance : Arterial stretch
Resistance : Arterioles
Flow : Vital parameter
Velocity : slowing down blood flow in capillaries
Features of low pressure reservoir system?
majority of blood in the venous system + can be used to increase CO - Starling’s law
Define Darcy’s law
role of pressure energy in flow Q = P1 - P2 / R Q : flow P1 - P2 : pressure diff R : resistance to flow Flow = Pa - CVP / TPR Pa : arterial presure
Define Bernoulli’s law
Role of pressure, kinetic, potential energies in flow Flow = PV + (ρV²/2) + ρgh PV : pressure (Pa -CVP) ρV²/2 : kinetic (momentum of blood) ρgh : potential (effect of gravity) ρ : fluid mass
Define blood flow
Volume of blood flowing in a given time (ml/min)
Define perfusion
Blood flow per given mass of tissue (ml/min/g)
Define velocity of blood flow
Blood flow divided by cross sectional area where blood flows (cm/s)
Define velocity
velocity = volume flow/area
Define volume flow
velocity x area
Describe blood flow relationship with velocity
- high velocity of blood flow in aorta
- arteries branching slows velocity
- greater the cross sectional area, slower blood flow
- slowest in capillaries
- increasing velocity with veins coming together
Where’s laminar blood flow?
Most arteries, arterioles, venules, veins
Where’s turbulent blood flow?
Ventricles (mixing), aorta (peak flow), atheroma (bruits)
Where’s bolus blood flow?
Capillaries
Describe laminar blood flow
Concentric shells
0 velocity at walls (molecular interactions between blood + wall)
Max velocity at centre so🔴towards centre
Speeds up blood flow via narrow vessels
Describe turbulent blood flow
Blood doesn’t flow linearly + smoothly in adjacent layers –> whirlpools, eddies, vortices due to changes in velocity
Describe bolus blood flow
🔴diameter > capillaries – single file
Plasma columns trapped between 🔴
Uniform velocity
Little internal friction - low resistance
Define Reynold’s number (Re)
Describes what determines change from laminar to turbulent flow Re = ρVD/μ ρ : density V : velocity D : diameter μ = viscosity
What happens when Re > 2000?
Turbulence occurs when Reynold’s number exceeds critical value
eg bruits, ejection murmur – increased blood velocity
Equation for blood flow?
Arterial BP/ TPR
Equation for blood flow?
Arterial BP/TPR
What’s the highest systolic pressure in aorta?
120 mmHg
What’s the highest diastolic pressure in aorta?
80 mmHg
How does arterial BP change?
falls steadily in systemic circulation with distance from left ventricle
Factors that affect arterial BP?
CO : Starling’s/Laplaces’s laws, contractility, HR
Properties of arteries : aorta
Peripheral resistance : arterioles, Haem-arterioles+veins
Blood viscosity : haemocrit, Haem-arterioles+veins
What does arterial BP involve?
Systolic, diastolic, pulse + mean blood pressure
Why have elastic fibres in aorta?
makes aorta compliant - can stretch under p to propel blood into circulation
How much SV is stored in aorta during LV ejection?
60-80% as these structures expand
Energy stored in stretched elastin
What sustains diastolic BP + blood flow when heart’s relaxed?
in LV diastole energy returned to blood as aorta walls contract
Equations for pulse p
Systolic pressure-Diastolic pressure = Pulse pressure
120 - 80 = 40
Pulse pressure = SV/compliance
Describe pressure profile
1 – Ejection 2 – Peak Systolic 3 – Systolic decline (aorta compliance) 4 – Incisura/dicrotic notch (aortic valve closes) 5 – Diastolic run off, 6 – Peak Diastolic
What is pulse p + what does it tell you?
finger senses radial artery which tells you about SV + arterial compliance (stretchiness)
Describe relationship between volume of blood in elastic arteries vs arterial p
exponential
when SV raised bigger pulse p
Why’s there a greater systolic p when there’s a greater SV?
as eject more blood it stretches aortic wall more, finite amount can stretch, aorta loses compliance creating more pressure
Describe elderly graph of volume of blood in elastic arteries vs arterial p
steeper exponential because, decreased compliance, increase in afterload, to increase SV
Equation for compliance?
compliance = change in v/change in p
Why decreased compliance with age?
stiffer arteries (arteriosclerosis)
Why does pulse p increase down arterial tree?
Tapering of vessels + increased stiffness of distal arteries
What happens to pulse p at arterioles?
At arterioles pulse pressure disappears – drop of pressure means flow more continuous
What’s aortic stenosis?
narrowing of aortic valve which narrows exit from heart so slower upstroke, smaller peak indicating poorer ejection
What’s aortic regurgitation?
Leaky aortic valve
fast upstroke, larger peak, poor diastolic run off indicating blood entering aorta/ventricles during diastolic
How to calculate mean BP?
diastolic pressure + 1/3 [pulse pressure]
What controls mean BP?
age, disease, distance along arterial tree, blood volume (SV,CO), exercise (SV,CO), emotion (stress, anger, fear, apprehension, pain), wake/Sleep (80/50 mmHg)
Equation of blood flow?
blood flow = arterial blood p / TPR CO = (Pa - CVP) / TPR blood flow = Pa - CVP x G G because TPR = 1/conductance G : conductance CO : bloow flow, cardiac output Pa = blood pressure
Equation of blood pressure (Pa)?
Pa = CO x TPR
What’s used in Darcy’s law of flow states?
Arterial p (Pa – mean BP 90 mmHg)
TPR (Total Peripheral Resistance of vasculature)
CVP (Central Venous Pressure)
Role of TPR?
controls blood flow + BP so when increased resistance, increase in pressure to keep constant flow
What controls TPR?
Poiseuille’s law
Myogenic response
Blood viscosity
How does vasodilation affect blood flow?
lower BP in artery, lower TPR in arterioles so increased blood flow
How does constriction affect blood flow?
higher BP in artery, higher TPR in arterioles so decreased blood flow
How does hypertension happen?
over constriction of arterioles –> reduction in blood flow – harmful –> end organ damage
Why does pressure drop from arteries to capillaries?
increased resistance
Define Poiseuille’s law
describes parameters that govern TPR
Equation of resistance?
resistance = 8ηL / πr⁴ r = Radius of vessel η = Blood viscosity L = Vessel length
Equation of conductance?
conductance = πr⁴ / 8ηL r = Radius of vessel η = Blood viscosity L = Vessel length
What’s the equation of Darcy’s + Poiseullie’s law?
blood flow = Pa - CVP x (πr⁴ / 8ηL)
r = Radius of vessel
η = Blood viscosity
L = Vessel length
eg of r⁴ effect?
radius 1 vs radius 2
2 has increased flow by 16x + exerts 1/16th of the resistance of radius 1
What’s TPR in arterioles controlled by?
r⁴
pressure diff across vessels P1 - P2
length L
What’s arteriole radius controlled by?
ANS release NA - vascular tone
Why do arterioles control TPR and not capillaries?
-Radius
No sympathetic innervation/smooth muscle
-Pressure drop
Less pressure drop across capillaries (20-30mmHg) than arterioles (40-50 mmHg) ∵ less resistance to blood flow in capillaries
-Length short
Why’s there less resistance in capillaries?
-Bolus flow reduces viscosity (η)
-Capillaries arranged in parallel RTotal = 1/R1 + 1/R2
but arterioles in series with arteries, arterioles,capillaries
RTotal = R1 + R2
What’s local blood flow mainly controlled by?
changes in radius of arterioles supplying a given organ/tissue
eg of intrinsic control of arteriole radius?
Factors entirely within an organ or tissue:
neural, humoral
eg of extrinsic control of arteriole radius?
Factors outside the organ or tissue:
bayliss myogenic response, tissue metabolites, local hormones, endothelial factors
What’s bayliss myogenic response?
- Increase pressure
- distended vessel
- feedbacks to constrict
- controls flow
- Decrease pressure
- dilate vessel
- maintain blood flow in presence of low BP
When does the bayliss myogenic response not work?
critical point below 60mmHg
Importance of bayliss myogenic response?
- Maintains local blood flow when changes in local BP (renal, coronary, cerebral circulation)
- Protective – low BP, still good flow + high BP, less flow/damage
Define viscosity
measure of internal friction opposing the separation of the lamina
What does blood viscosity depend on?
Velocity of blood
Vessel diameter
Haematocrit
What’s normal haematocrit?
45%
What’s typical blood η relative to water?
4-5
Clinical implications of polycythaemia?
- high η
- high TPR/BP
- low BF
Clinical implications of anaemia?
- low η
- low TPR/BP
- high HR (baroreceptor reflex)
Clinical implications of tube diameter (Fahraeus-Lindqvist effect)?
- η falls in small vessels (< 100 µm) due to bolus flow
- low resistance
- high BF in microvessels
Clinical implications of 🔴deformability?
- high η
- low BF
- sickle cell crises (pain)
Clinical implications of blood velocity?
slow venous flow in immobile legs – increased η
How much blood volume is in systemic veins + venules?
60%
Describe veins
Thin-walled, collapsible, voluminous vessels
Contain 2/3ths of blood volume
Contractile – has smooth muscle, innervated by sympathetic nerves
Control radius
What does the contraction of veins do?
Expels blood into central veins
Increases venous return/CVP/end-diastolic volume
Increases stroke volume (Starling’s law)
What’s the typical venous pressure of limb vein, heart level?
5-10 mmHg
What’s the typical CVP?
0-5 mmHg
What’s the typical venous pressure of foot vein, standing?
90 mmHg
Why do veins collapse when hand above heart?
gravity
What happens when you stimulate sympathetic nerves of veins?
Venoconstriction Shifts blood centrally Increases venous return/CVP/end-diastolic pressure Increased SV (Starling’s law)
How do veins return blood back to heart?
pressure gradient
thoracic pump
skeletal muscle pump
How does pressure gradient cause venous return?
Pressure in veins between 10 (supine) – 90 (standing) mm Hg
IVC/SVC/right atrium < 5 mm Hg
Venous return = Venous Pressure – Pressure right atrium / Venous resistance
How does thoracic pump cause venous return?
- inhalation
- thoracic cavity expands
- increased abdominal pressure
- increased pressure on IVC (squeezes it)
- forcing blood upward towards heart
- increased right ventricular SV
- blood flows faster with inhalation
- take more O2
- reduce CVP in RA
- increases pressure diff
How does skeletal muscle pump cause venous return?
- leg muscles contract
- returns blood into RA
- retrograde flow prevented by valves
- reduce high local venous pressures when upright position
- reduces swelling of feet from lower venous pressures, lower capillary pressure, less filtration
- lower CVP + SV during exercise
Why can standing for a long time cause you to faint?
-pooling blood
-decrease starlings low
-decrease SV
-decrease CO
-poor perfusion of brain
gravity, heat-induced vasodilatation, lack of muscle use
How does blood flow with very little pressure difference?
- ejected blood has greater kinetic energy at heart than feet (more velocity, V) + potential energy (more height, h)
- greater kinetic/potential energies overcome pressure gradient to maintain flow
- pressure + potential gradients cancel each other but KINETIC ENERGY
What’s Bernoulli theory?
mechanical energy of flow is determined by
pressure, kinetic, potential energies
Pressure (PV) + Kinetic (ρV2/2) + potential (ρgh)
ρ: fluid mass
