S2: Haemodyamics I: Arteries Flashcards
What is Haemodynamics?
Relationship between blood flow, blood pressure and resistance to flow
What are the key factors in haemodynamics?
Force: Cardiac contraction
Work: Isovolumetric contraction/ejection
Pressure: Difference aorta to veins
Compliance: Arterial Stretch (increase volume under pressure)
Resistance: Arterioles (enables us to direct blood flow to one place and the other)
Flow: The vital parameter
Velocity: Slowing down blood flow in capillaries (good exchange of solutes and fluid)
What system is the CVS?
The CVS is a closed system and what happens in one part of CVS has a major impact on other parts.
Reduced blood flow to one area: Increases pressure above and alters flow to other areas
What is the role of blood in the venous system?
It is also known as the low pressure reservoir system.
The venous system vessels expand to hold blood and then recoil to push blood back into the heart.
The reservoir of venous blood can be used to increase cardiac output - Starlings law (what goes in goes out).
What is Darcy’s Law?
It shows the role of pressure in energy flow
Flow = Pa - CVP / TPR
(Pa-CVP) = Pressure difference
- CVP is usually very low so it is normally excluded from the equation.
How does dilation affect blood flow?
Dilation decreases TPR which increases blood flow
What is Bernoulli’s Law?
It is the role of pressure, kinetic and potential energies in flow
Flow = Pressure (PV) + Kinetic (pV2/2) + Potential (pgh)
Kinetic: Momentum of blood
Potential: Effect of gravity
p: 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 the cross sectional area through which the blood flows (cm/s).
What is the equation of velocity?
Velocity = Volume flow/ Area
Volume flow = Velocity x Area
Describe blood flow with velocity and cross sectional area
The bigger the cross sectional area, the same blood flow is going to produce a smaller velocity
- Velocity of blood flow in aorta is high
- Branching of arteries slows velocity
- Greater the cross sectional area= slower blood flow
- Velocity is slowest on the capillaries
- Velocity increases with veins coming together helping return of blood to the heart (as velocity increases= kinetic energy increases=momentum)
What are the 3 patterns of blood flow?
- Laminar
- Turbulent
- Bolus
Describe Laminar blood flow
In most arteries, arterioles, venules and veins
- Concentric shells
- Zero velocity at walls (molecular interactions between blood and wall cause friction) and maximum velocity as the centre which moves RBC towards the centre
- Viscosity of blood determines how much friction there is between concentric shells
- Speeds up blood flow through narrow vessels as RBC are in centre with maximum velocity
Describe turbulent blood flow
Ventricles (mixing), aorta (peak flow), atheroma (bruit)
- Turbulent blood flow causes heart murmurs
- Breaks down laminar blood flow –> velocity increases, viscosity decreases
- Blood does not flow linearly and smoothly in adjacent layers (whirlpools, eddies, vortices)
- Due to changes in velocity
Describe Bolus
Capillaries
- RBC have larger diameter than diameter of capillaries so they travel in single file
- Plasma columns (boluses of fluid) are trapped between RBC
- There is uniform velocity
- Little internal friction so there is low resistance
What is Reynold’s Number (Re)?
Describe what determines change from laminar to turbulent flow
Re = pVD/u
p = Density V= Velocity D= Diameter u = Viscosity
Turbulence occurs when Reynold’s number exceeds a critical value (Re>2000) e.g. Bruits, ejection murmur- increased blood velocity
Re breaks down relationship between pressure and flow
How is blood pressure measured?
- Pressure exerted by blood flow on vessel walls
This pressure is generated by left ventricular contraction
Factors affecting arterial blood pressure
- Cardiac Output (Starling’s/Laplace’s law, Contractility, Heart Rate)
- Properties of arteries - Aorta
- Peripheral resistance - Arterioles
- Blood viscosity- Haemocrit
What are the 4 key relationships that arterial blood pressure interacts with?
- Systolic pressure
- Diastolic pressure (maintained by aortic valve closed)
- Pulse pressure (what is measured)
- Mean blood pressure (determined from systolic and diastolic pressure)
Explain the role of the aorta
Recoil of elastic fibres of the aorta and large arteries help propel the blood into the circulation
- During left ventricle ejection most of the SV is stored in the aorta and arteries as these structures expand. Energy is stored in stretched elastin.
- During left ventricle diastole energy is returned to the blood as the walls of the aorta and arteries contract (recoil) when the aortic valve closes. This sustains diastolic blood pressure and constant blood flow when the heart is relaxed.
What is pulse pressure?
Systolic pressure - Diastolic pressure = Pulse pressure
Pulse pressure is the difference between systolic and diastolic pressure 120-80 = 40 mmHg
Pulse pressure is what the finger senses e.g. At the wrist (radial artery). It tells use about stroke volume (SV) and arterial compliance (stretchiness)
Pulse pressure = stroke volume / compliance
What are the 6 labels of pulse pressure?
- Ejection
- Peak Systolic
- Systolic decline due to compliance of aorta holding onto blood volume
- Incisura/dicrotic notch due to aortic valve closing causing recoil
- Diastolic run off
- Peak diastolic
Explain the relationship between pulse pressure and stroke volume
Greater SV = Greater stretch of arteries = Less compliant (graph –> not linear?) = Relatively greater systolic pressure
Changes in pulse pressure indicate changes in stroke volume
During exercise (increased SV) for elderly, huge changes in blood pressure would occur which could create afterload in the heart increasing load on the heart.
What is the equation of pulse pressure?
Pulse pressure = Stroke volume/Compliance
What is the equation of compliance?
Compliance = Change in volume / Change in pressure
Explain the relationship between pulse pressure and compliance
Arterial compliance
Decreased compliance = stroke volume now increases systolic and pulse pressure disproportionally
In the elderly an increase in age results in stiffer arteries (arteriosclerosis) which decreases compliance increasing pulse pressure.
Describe the relationship between pulse pressure, arterial tree and age
- Age increases stiffness of vessels (particularly aorta). This means that large pulse pressure is present throughout arterial tree
- More afterload means the heart works harder
Move away from aorta, pulse pressure increases:
- Tapering of vessels
- Increased stiffness of distal arteries
At arterioles pulse pressure disappears, a large drop of pressure means flow is more continuous
Describe pulse pressure of aortic stenosis
- Narrowing of the aortic valve
- Narrowing of exit from heart
- Slower upstroke
- Smaller peak (lower volume of blood ejected)
- Indicates poorer ejection
Describe pulse pressure of aortic regurgitation
- Leaky aortic valve
- Fast upstroke
- Larger peak
- Poor diastolic run off (less volume of blood delivered to aorta –> poor blood flow in systolic pressure)
- Indicates blood entering aorta/ventricles during diastole
Mean BP (with diastolic and pulse pressure)
Mean BP= Diastolic pressure + 1/3 Pulse Pressure
Mean BP is area under pulse pressure curve but its too complicated to usually work out
Factors controlling 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
