Principles of Haemodynamics Flashcards
What are the key parameters considered in haemodynamics?
- Force: Cardiac contraction
- Work: Isovolumetric contraction/ ejection
- Pressure: Difference between different vessels
- Compliance: Atrial stretch
- Resistance: In arterioles
- Flow velocity: Slowing down blood flow in capillaries
What is haemodynamics?
The relationship between blood flow, blood pressure and resistance to flow
Is the CVS an open or closed system?
Closed
What is the main effect of the CVS being a closed system?
- Change in one part of the system impacts other parts of the system
- Reduced blood flow in one area causes changes in flow in other areas by reducing upstream pressure
How many capillary beds normally does blood go through?
1
What is it called when blood goes through 2 capillary beds?
Portal system
Where is majority of the blood found in the CVS?
Veins/venous system
How does venous blood affect cardiac output?
- Can increase cardiac output ( RECAP - Starling’s law)
- Have a thin muscle layer and contract slightly
- Upon contraction, send more blood back into the heart
- More blood moves into arterial circulation
- Therefore increases cardiac output
What is Darcy’s law of haemodynamics?
Flow = (P1 - P2)/ TPR
P1 - P2: difference in pressure (arterial pressure - CVP)
What is Bernoulli’s law of haemodynamics?
Flow = Pressure (P) + Kinetic (ρV^2/2) + Potential (ρgh)
ρ = Fluid mass
P = pressure
V = velocity
h = height
g = acceleration due to gravity
In Bernoulli’s Law of haemodynamics, what does kinetic energy refer to?
Momentum of blood
In Bernoulli’s Law of haemodynamics, what does potential energy refer to?
Effect of gravity
What is blood flow?
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 velocity of blood flow?
Speed of blood flow (cm/s)
What is velocity of blood flow affected by?
Changes in cross-sectional area of vessels
RECAP - Why is blood velocity at the aorta high?
- Only one aorta through which cardiac output passes
- This aorta has a relatively low cross sectional area ∴ velocity is high
RECAP - As the blood spreads out into other arteries from the aorta, there is a reduction in velocity. Suggest why.
- Greater total cross sectional area (lots of arteries will have a greater collective area than a single aorta)
RECAP - Why is there a further reduction in blood velocity through the arterioles and capillaries?
- Individually, each arteriole and capillary has a low cross sectional area.Collectively, total cross sectional area is high ∴ blood velocity is low
- Cardiac output is transferred into the high number of arterioles and capillaries. The arterioles can also influence resistance to blood flow.
RECAP - Why is blood velocity in the venous system high?
- Blood funnelled back into the venous system which has fewer vessels compared to capillaries
- Lower total cross sectional area - blood returns to heart in a reasonable timeframe
What is the equation for velocity of blood flow?
Blood flow/cross-sectional area
Describe laminar blood flow
- Flow in regular concentric (arch) shapes
- Zero velocity at the walls due to friction and highest at the centre.
- Most red blood cells are found at the centre. This speeds up blood flow through narrow vessels
- Usually occurs in arteries, veins etc.
Describe turbulent blood flow
- Blood flows in whirlpools and vortices due to increased pressure and velocity.
- Has a high resistance to flow.
- Occurs in aorta and vessels affected by atheroma
Describe bolus blood flow
- Red blood cells have a larger diameter compared to that of the capillaries and so move in a single file. Plasma columns found before and after each red blood cell
- Has uniform velocity with very little internal friction and very low resistance to flow
- Allows maximum access for gas and nutrient exchange.
What does Reynold’s number determine?
- Determines change from laminar to turbulent flow
Refeering to Reynold’s number, when might blood flow transition from laminar to to turbulent blood flow?
- Turbulent flow occurs when Reynold’s number exceeds a critical value (usually 2000)
- Below this critical value flow will be laminar.
What is the mathematical equation for Reynold’s number?
Reynolds number = (Density x Velocity x Diameter)/ Viscosity
In order to increase Reynolds number increase density, diameter and velocity and decrease viscosity
What generates the pressure exerted by blood on vessel walls?
Left ventricular contraction
Describe the relationship between distance from left ventricle and arterial pressure
As distance from left ventricle increases, arterial pressure will decrease
What type of vessels are arterioles and which type of nervous system are they controlled by?
Arterioles are resistance vessels which are under sympathetic control
What is systolic pressure?
Pressure when contracting and ejecting blood into circulation e.g used by left ventricle to eject oxygenated blood into systemic circulation
What is diastolic pressure?
Pressure when relaxing
What is pulse pressure?
- Difference between diastolic and systolic pressure
- This is the “pulse” we feel with our fingers.
How is pulse pressure related to stroke volume and compliance?
Pulse pressure = Stroke volume/ Compliance
If compliance is low then pulse pressure is high. If stroke volume is high then pulse pressure is high
What is compliance?
The ability of a vessel wall to stretch.
Why do arteries generally have high compliances?
This is due to the thick layer of smooth muscle they have in the arterial wall
What is the effect of a blood vessel having a high compliance?
Require a high pressure in order to increase their volume.
What is mean blood pressure?
Average pressure between systolic and diastolic pressure
What happens to the aorta and arteries during left ventricular systole?
Arteries and aorta expand
- This causes energy to be stored within the stretched elastin
What happens to the aorta and arteries during left ventricular diastole?
- Energy returns back to the blood as the walls of the aorta and arteries contract.
- Sustains diastolic blood pressure and flow when the heart is relaxed
Describe the changes in the arteries when going form rest to exercise.
- Greater stretch of the arteries as more blood is ejected (stroke volume).
- Causes less compliance and less recoil.
- Pulse pressure also increases
How is arterial pressure affected by age?
- In older people their arteries become stiffer (atherosclerosis) causing a decrease in compliance
- Large pulse pressure will be present throughout the arterial tree.
How does arterial pressure change as we go down the arterial tree?
- Pulse pressure becomes more noticeable as vessels become less compliant
- Pulse is measured at the radial artery as it is quite far from the heart however cannot be detected in the arterioles as flow is more continuous
What controls mean blood pressure?
- Age (less compliance and atherosclerosis)
- Disease (cardiovascular diseases)
- Distance along arterial tree (less compliance further from the arterial tree)
- Exercise (Stroke volume and Cardiac Output)
- Emotions (Stress, anger, fear)
What is the effect of vasoconstriction of the veins?
- Increases volume of blood that is returning to the heart
- Increase the stretch on the wall of the heart.
- Increased force of contraction and greater stroke volume, thereby increasing cardiac output.
Name one flaw of Darcy’s Law
- Darcy’s law accounts for pressure determining flow, but in reality there are other factors involved.
- There are times when flow can be against a small pressure gradient so the pressure difference between CVP and the heart could be almost zero, yet still get good filling of the heart
During an auscultation, what sounds may represent turbulent blood flow?
- Ventricular filling
- Contraction of the aorta during ejection
- Bruits - occur when blood flow through occluded blood vessels
With reference to Reynold’s Number, describe the relationship between flow and pressure.
- Initially linear with laminar flow
- Above critical value, becomes turbulent - greater energy required to drive blood flow.
Would anaemia cause turbulent or laminar blood flow?
Turbulent
- due to reducing blood viscocity
Atherosclerosis contributes to turbulent blood flow. Suggest why.
- Narrower blood vessels so diameter reduces
- Cross-sectional area is lower
- Causes an increase in velocity
What is the formula for mean blood pressure?
Diastolic pressure + 1/3(pulse pressure)
