Principles of haemodynamics Flashcards
Define haemodynamics
Haemodynamics is the relationship between blood flow, blood pressure and resistance to flow.
• Force - cardiac contraction
Define each of the terms below or what they do:
- Work - isovolumetric contraction / ejection
- Pressure - difference aorta to veins
- Compliance - arterial stretch
- Resistance - arterioles
- Flow velocity - slowing down blood flow in capillaries
cardiac contraction isovolumetric contraction / ejection difference aorta to veins arterial stretch arterial stretch slowing down blood flow in capillaries
What does darcys law state?
What is the equation for blood flow?
Role of pressure energy in flow
Flow = pressure difference divide by TPR
therefore…
Flow = Pressure in arteries - CVP divided by TPR
What does Bernoulli’s law state?
Role of pressure, kinetic and potential energy in flow - not just pressure.
Therefore Flow = pressure + kinetic (pV^2/2) +poteintial (pgh)
Kinetic energy: momentum of blood Potential energy: effect of gravity p = fluid mass: P = pressure V = velocity: h = height g = acceleration due to gravity
Define blood flow, perfusion, and velocity of blood
Blood flow - volume of blood flowing in a given time (ml/min).
Perfusion - Blood flow per given mass of tissue (ml/min/g).
The velocity of blood flow - Blood flow (cm/s) affected by the cross-sectional area through which the blood flows, so the flow may remain the same but velocity changes if there has been a change in cross-sectional area.
Describe the relationship between blood flow and velocity
On image - write up
What are the three patterns of blood flow
Laminar - Most arteries, arterioles, venules and veins
Concentric shells, zero velocity at walls (molecular interactions), the maximum velocity at the centre.
Moves RBCs towards the centre. Speeds up flow through narrow vessels.
Turbulent flow - ventricles (mixing), aorta (peak flow), atheroma (bruits)
Blood does not flow linearly, in smooth adjacent layers due to increased pressure and velocity.
High resistance to flow
Bolus - capillaries
RBCs have a large diameter than the diameter of the capillaries so move in a single file. Plasma columns are trapped between RBCs.
Uniform velocity, little, internal friction and very low resistance
What is Reynold’s number (Re)? and describe the graph
Describes what determines change from laminar to turbulent flow
Flow descriptions such as Poiseuille’s law 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.
Turbulence occurs when Reynold’s number exceeds a critical value (>2000)
eg. bruits, ejection murmur, increased blood velocity.
Re = pVD/ u
Describe arteriole blood flow
- Pressure exerted by blood on vessel walls & generated by left ventricular contraction.
- Highest in aorta, 120 mmHg during systole, 80 mmHg during diastole.
- Arterial pressure falls steadily in systemic circulation with distance from left ventricle
What factors affect arteriole blood pressure?
- Cardiac output (SV, HR)
- Properties of arteries
- Peripheral resistance
- Blood viscosity
Define systolic, diastolic, pulse and mean blood pressures
Where do these show on the graph
Systolic pressure - Pressure when ejecting
Diastolic pressure - Pressure when relaxing
Pulse pressure - Difference between diastolic and systolic pressure
Mean blood pressure - Average pressure
What is the purpose of recoil of elastic fibres in the aorta?
Recoil of elastic fibers of the aorta and large arteries helps to propel the blood into the circulation
• During LV ejection
60-80% of stroke volume is stored in aorta and arteries as these structures expand.
Energy stored in stretched elastin.
• During LV 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 heart is relaxed.
What is pulse pressure and what does it tell you?
Pulse pressure is what your finger senses (radial artery)
It tells you about the stroke volume and arterial compliance (stretchiness)
What is the equation for pulse pressure?
Pulse pressure = systolic pressure - diastolic pressure
What is the equation for pulse pressure?
Pulse pressure = stoke volume - compliance
Describe changes in Arterial blood pressure - pulse pressure and stroke volume when exercising compared to rest
- 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.
- Bigger pulse pressure when stroke volume raised
- Changes in pulse pressure indicate changes in stroke volume
Describe the compliance curve and equation
On image
What is decreased compliance?
A steeper curve, stroke volume increases systolic and pulse pressure disproportionally
In the elderly, we get stiffness in arteries leads to decreased compliance
Describe Arterial pulse pressure - arterial tree and age (5)
- Pulse pressure at the aorta is relatively small but further down the arterial tree it increases slightly because vessels become less compliant.
- But the effects of the aorta are still present so it doesn’t increase all that much.
- 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
What controls mean blood pressure?
- Age
- Disease
- Distance along arterial tree
- Blood volume - SV, CO
- Exercise - SV, CO
- Emotion - stress, anger, fear, apprehension, pain
- Wake/sleep - BP 80/50 mmHg
What is the equation for mean blood pressure?
Describe the graph
Diastolic pressure + 1/3 of pulse pressure
