Principles of Haemodynamics Flashcards
What is haemodynamics?
the relationship between blood flow, blood pressure and resistance to flow
What is the force of the heart?
Cardiac contraction
What is the stroke work?
The isovolumetric contraction and ejection
What is the blood pressure caused by?
the pressure difference between the aorta and the ventricles
Where is the majority of the blood in our body?
Majority of the blood in our body is in the venous system - low pressure venous system
How does venous reservoir link to CO?
Reservoir of venous blood can be used to increase CO via Starling’s Law
How does the CVS affect the rest of the body?
Events occurring in one part of the CVS has major impacts on the rest of the body.
Reduced blood flow to one area, increases pressure upstream and alters flow to other areas
What is Darcy’s Law?
The role of pressure energy in flow
Flow = Pa - CVP / TPR
What is Bernoulli’s Law?
the role of pressure, kinetic and potential energies in flow
Flow = PV + (PV²/2) + (ｐgh)
ｐ- fluid mass g - acceleration due to gravity h - height V - velocity P - pressure
What is blood flow?
The volume of blood flowing in a given time (ml/min)
What is perfusion?
blood flow per given mass of tissue (ml/min/g)
What is blood velocity?
The blood flow (cm/s) affected by SA through which blood flows
flow may remain the same but velocity changes if there has been a change in SA
Describe the velocity in arteries and aorta?
The velocity of blood flow in the aorta is high. The branching arteries slows velocity
How does surface area affect blood flow?
The greater the cross sectional area, the slower the flow
=> flow is slower in capillaries, velocity increases as the
veins come together
Outline the calculation for Volume flow
Volume flow = Velocity x Area
Q = VA
What is the normal blood flow (Q) in the body?
Blood flow remains constant at 10 ml/s
Total volume flow (ml/min) remains the same
What are the 3 patterns of blood flow?
Laminar - fast smooth flow
Turbulent - vortices /eddies due to increased speed /
obstacles
Bolus - very efficient flow in capillaries
Outline the features of laminar blood flow
- in most arteries, arterioles, veins and venules
- concentric shells, zero velocity at walls, max. velocity at
centre - moves RBCs to centre, speeds up blood flow through
narrow vessels
Describe the features of turbulent blood flow
- in ventricles (mixing), aorta (peak flow), atheroma (bruits)
- Blood doesn’t flow linearly and smoothly in adjacent
layers (whirlpools, eddies & vortices) due to increased
pressure & velocity - obstacles - arthyromas
Describe the features of bolus blood flow
- in capillaries
- RBCs have a larger diameter than capillaries, so move in
single file. - plasma columns trapped between RBCs
- uniform velocity, little internal friction and very low
resistance
What is Reynold’s (Re) Number?
Reynolds number describes what determines change from laminar to turbulent flow
Outline the equation for Reynolds number (Re)
Re = pVD/µ
p = density V = velocity D = density µ = viscosity
When does laminar flow turn to turbulent flow?
Turbulence occurs when reynolds number exceeds the critical value (<2000)
e.g. bruits, ejection murmur, increased blood velocity
How does arterial blood flow occur?
Pressure is exerted by blood on the vessel walls, and is generated by left ventricular contraction
Describe the arterial blood pressure
Pressure is highest in the aorta at 120 mmHg during systole and 80 mmHg during diastole
Arterial pressure falls steadily in systemic circulation with distance from left ventricle
What factors affect arterial BP?
- CO
- artery properties
- peripheral resistance
- blood viscosity
What key relationships does arterial BP involve the interactions of?
- systolic pressure - during ejection
- diastolic pressure - relaxing
- pulse pressure - difference in diastolic - systolic
- mean blood pressure - average pressure
What is the role of the aorta in arterial BP?
Recoil of elastic fibres of the aorta and large arteries help to propel the blood into the circulation
What occurs to the arterial BP during LV ejection
60 - 80% of SV is stored in the aorta and arteries as these structures expand. Energy is stored in stretched elastin
How does LV diastole affect arterial BP?
Energy is returned to the blood as the walls of the aorta and arteries contract. This sustains diastolic blood flow & pressure when the heart is relaxed
How is pulse pressure calculated?
pulse pressure = systolic pressure - diastolic pressure
pulse pressure is what the finger detects at radial artery
How does pulse pressure change during contraction?
- ejection
- peak systolic
- systole decline
- incisura / dicrotic notch (closure of aortic valves)
- diastolic run off
- peak diastolic
What can the pulse pressure tell us?
Pulse pressure tells you what the SV & arterial compliance is
pulse pressure = SV / arterial compliance
How does pulse pressure affect SV?
There’s a bigger pulse pressure when SV is raised
changes in pulse pressure indicates changes in SV
Compare the pulse pressure & SV during rest and exercise
During exercise:
- There’s a greater SV
- Greater arterial stretch
- less compliant
- relatively greater systolic pressure
SV increases - compliance curve is steeper leading to a very high pulse pressure
Why is there a greater arterial stretch during exercise?
Greater stretch of the arteries, as more blood is ejected during exercise, causing less compliance and recoil
The difference between systole and diastole increases (pulse pressure)
Outline the equation used to calculate compliance
compliance = change in vol / change in pressure
What is the effect of arterial compliance?
Decreased compliance => steeper curve
SV now increases systolic & pulse pressure disproportionally
Why does compliance matter more with increased age?
Arterial compliance is significant in the elderly
Increase in age => stiffer arteries (atherosclerosis) = decreased compliance
How does pulse pressure change within the arterial tree?
Pulse pressure at the aorta is relatively small but further down the arterial tree, it increases slightly as vessels become less compliant
- aorta effects still achieved
Where is pulse pressure measured?
In the radial artery (quite far from the heart)
How does age affect pulse pressure?
Age increases the stiffness of vessels - particularly the aorta
- large pulse pressure is present throughout the arterial
tree
once in arterioles, pulse pressure can’t be detected and flow is more continuous
How can we calculate mean BP?
mean BP = diastolic pressure + 1/3 pulse pressure
What is the mean BP controlled by?
- age
- disease
- distance along arterial tree
- blood volume (CO, SV)
- emotion (stress, anger, pain, apprehension, fear etc.)
- wake / sleep increases BP (80/50 mmHg)
