Principles of Haemodynamics Flashcards
What is haemodynamics
The relationship between blood flow, blood pressure and resistance to flow
Describe what the following refer to
- force
- work
- compliance
- pressure
- resistance
- flow velocity
Force - cardiac contraction
Work - isovolumetric contraction and ejection
Compliance - arterial stretch
Pressure - difference between aorta and veins
Resistance - in arterioles
Flow velocity - slowing down blood flow in capillaries
What does the CVS mostly consist of?
The CVS is a closed system with the blood only going to one capillary bed before returning to the heart
- What happens in one part of CVS has a major impact on other parts
- Reduced blood flow to one area increases pressure upstream and alters flow to other areas
What is the distribution of blood in different vessels
Describe Darcy’s law
The role of pressure energy in flow
Describe Bernoulli’s law
The role of pressure, kinetic and potential energies in flow
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 (cm/s) is affected by the cross sectional area through which the blood flows, so flow may remain the same but velocity changes if there has been a change in cross sectional area
Describe how velocity is related to blood flow
Velocity of blood flow in the aorta is high;
- Branching of the artery slows velocity
- The greater the cross sectional area, the slower the flow, slowest in capillaries
- Velocity increases with veins coming together
Velocity = flow / area
Describe the three patterns of blood flow
Laminar - found in most arteries, arterioles, venules, veins - concentric shells - zero velocity at walls - maximum velocity at the centre - moves red blood cells towards the centre - speeds up blood flow through narrow vessels
Turbulent - found in ventricles when mixing, aorta at peak flow and atheroma - blood does not flow linearly and smoothly in adjacent layers due to increased pressure and velocity - there is a high resistance to flow
Bolus - found in capillaries - RBCs have a larger diameter than the capillary diameter so move in single file - plasma columns are trapped between the RBCs - uniform velocity, little friction and very low resistance
Describe Reynold’s number (Re)
- This number describes what determines the change from laminar to turbulent flow
- On a graph of flow (y axis) against pressure (x axis) reynolds number is when the graph starts to plateau
- Turbulent flow occurs when reynolds number exceeds the critical value of 2000 e.g. increased blood velocity
Describe Poiseuille’s law
- Flow descriptions are valid only for conditions of laminar flow
- At some critical velocity, the flow will become turbulent with the formation of eddies and chaotic motion which do not contribute to the flowrate
What is arterial blood flow and how does it change
- Pressure exerted on blood by vessel walls generated by left ventricular contraction
- Highest in the aorta at 120 mmHg at systole (contraction), 80 mmHg at diastole (relaxation)
- Arterial pressure falls steadily in systemic circulation with distance from the left ventricle
- Arterioles are the resistance vessels under sympathetic control
Describe the four elements that determine arterial blood pressure (hint: pressure)
- Systolic pressure - pressure when ejecting
- Diastolic pressure - pressure when relaxing
- Pulse pressure - difference between diastolic and systolic pressure
- Mean blood pressure - average pressure
Describe the role of the aorta in arterial blood pressure - what happens in ventricular systole and diastole
During left ventricular systole -
- 60-80% of stroke volume is stored in the aorta and arteries as these structures expand
- Energy is stored in stretched elastin
During left ventricular diastole -
- Energy is returned to the blood as the walls of the aorta and arteries contract
- This sustains diastolic blood pressure and blood flow when the heart is relaxed
What is pulse pressure
- Pulse pressure is what the finger senses, eg. at the wrist (radial artery)
- Tells you about stroke volume and arterial compliance (stretchiness)
- If compliance is low the pulse pressure will be high
During exercise and rest, what happens to pulse pressure and stroke volume
Rest v Exercise
* Greater stroke volume
* Greater stretch of arteries
* Less compliant
* Relatively greater systolic pressure
During exercise greater stretch of the arteries as more blood is ejected causes less compliance and less recoil and the difference between systole and diastole increases ie pulse pressure increases
Describe what happens during exercise when compliance is decreased and why it is significant in older people
Arterial compliance:
- Decreased compliance (steeper curve)
- Stroke volume now increases systolic and pulse pressure disproportionally
Important in the elderly:
- Increase in age – stiffer arteries (atherosclerosis)
- Decreased compliance
- This increases afterload which reduces the heart’s efficiency and cardiac output
Describe how pulse pressure changes throughout the arterial tree and how it relates to age
- Pulse pressure becomes more noticeable further down the arterial tree because vessels become less compliant
- This one reason to measure it in the radial artery quite far away from the heart
- Age increases stiffness of vessels – particularly aorta – this means that large pulse pressure is present throughout arterial tree
- The pulse pressure can’t be detected by the time blood gets to the arterioles and the flow is more continuous
How can mean arterial blood pressure be found
- This is the area under the curve
- It can also be found by calculating the diastolic pressure + 1/3rd of the pulse pressure
What controls mean blood pressure
- Age - it rises as there is less compliance
- Disease - cardiovascular diseases
- Distance along the arterial tree
- Blood volume as it affects stroke volume and cardiac output
- Exercise as it increases stroke volume and cardiac output
- Emotion - stress, anger, fear, apprehension, pain
- Wake/sleep - decreases during sleep to 80/50 mmHg
