Principles of Haemodynamics Flashcards
What is haemodynamics?
Haemodynamics is the relationship between blood flow, blood pressure and resistance of flow
List the key factors of haemodynamics?
Force- cardiac contraction
Work- isovolumetric contraction/ ejection
Pressure- difference aorta to veins
Compliance- arterial stretch
Resistance- arterioles
Flow velocity- slowing down blood flow in capillaries
What is Darcy’ law?
Darcy’s law- role of pressure energy inflow
P1-P2= pressure difference (arterial- venal)
R= resistance to flow
Flow= P1 - P2/R
Flow= Pa - CVP/ TPR
What is Bernoulli’s law?
Bernoulli’s law- role of pressure, kinetic and potential energies in flow; not just pressure
Flow= Pressure (P) + Kinetic (rV2/2) + Potential (rgh)
Kinetic energy: momentum of blood
Potential energy: effect of gravity
ρ = fluid mass: P = pressure
V = velocity; h = height
g = acceleration due to gravity
What are the fundamental blood flow definitions? (blood flow, perfusion, velocity)
Blood flow- volume of blood flowing in a given time (ml/min)
Perfusion- blood flow per given mass of tissue (ml/min/g)
Velocity of blood flow- blood flow (cm/s) 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.
What relationship does velocity have with blood flow? (hint there is an equation)
Velocity of blood flow in aorta is high
Branching of arteries slows velocity
Blood flow (Q) remains constant at 10ml/s
Total volume flow (ml/min) stays the same
Volume flow (Q)= Velocity (V) x Area (A)
V= Flow/Area
Describe the laminar pattern of blood flow
Most arteries, arterioles, venules, veins
Concentric shells
Zero velocity at walls (molecular interactions)
Maximum velocity at centre
Moves RBCs towards the centre
Speeds up blood flow through narrow vessels
Increased velocity or pressure or imperfections in the walls cause…
Describe the turbulent pattern of blood flow
Ventricles (mixing), aorta (peak flow), atheroma (bruits)
Blood does not flow linearly and smoothly in adjacent layers (whirlpools, eddies, vortices) due to increased pressure and velocity
High resistance to flow
Describe the bolus pattern of blood flow
Capillaries
RBCs have a larger diameter than diameter of capillaries so move in single file
Plasma columns are trapped between RBC
Uniform velocity, little internal friction and very low resistance- very efficient flow rate
What is Reynold’s number?
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
Re= pVD/u
Turbulence occurs when Reynold’s number exceeds a critical value (>2000) e.g. bruits, ejection murmur, increased blood velocity
How does arterial blood flow/ how is it pumped?
Pressure exerted by blood on vessel walls and generated by left ventricular contraction
Highest in aorta, 120mmHg during systole, 80 mmHg during diastole- arteries recoil during diastole pushing back to stop pressure from reaching zero
Arterial pressure falls steadily in systemic circulation with distance from left ventricle
Arterioles are the resistance vessels under sympathetic control
What are the four key relationships of arterial blood pressure?
Arterial blood pressure involves interactions between four key relationships:
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 role of the aorta in arterial blood pressure
During left ventricular systole:
60-80% of stroke volume is stored in aorta and arteries as these structures expand
Energy 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 heart is relaxed
What is the pulse pressure?
Pulse pressure is what the finger senses e.g. at the wrist (radial artery)
Tells you about stroke volume and arterial compliance (stretchiness)
Pulse pressure= stroke volume/compliance
If compliance is low the pulse pressure will be high
How does exercise affect arterial blood pressure
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 i.e. pulse pressure increases
