Principles of haemodynamics Flashcards
Heamodynamics
How blood flows
Haemodynamics is the relatioship between blood flow, blood pressure and resistance to flow.
Factors in haemodynamics
- Force: cardiac contraction
- Work: isovolumetric contraction and ejection
- Pressure: difference aorta to veins
- Compliance: arterial stretch
- Resistance: arterioles
- Flow velocity: slowing down blood flow in capillaries
CVS - closed system
Since the CVS is a closed system, whatever happens on one part of the CVS has an impact on the other parts. - Reduced blood flow to one area increases pressure upstream and alters flow to other areas.
Venous system
- Majority of blood is in the venous system - low pressure reservoir system.
- Starling’s law states that this reservoir cant be used to increased cardiac output.
Pressure within vessels
Blood flow: Darcy and Bernoulli laws
Darcy’s law - role of pressure, kinetic and potential energies in flow.
Flow = P1 - P2 / R
* P1-P2 is the pressure difference.
* R ia resistance to flow.
Flow = Pa -CVP / TPR
Blood flow fundamental definitions
- Blood flow: volume of blood flowing in a given minute (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 velovity changes of there’s a change in cross sectional area
Blood flow: relationship with velocity
Velocity of blood flow in aorta is high, branching of arteries slows it.
- Greater the cross sectional area - the slower the flow of blood, slowest in capillaries.
- Velocity increases with veins coming together.
- Vasodilation = velocity 1cm/s and area 10cm^2
- Vasoconstriction = velocity 10cm^2 and area 1cm^2
Volume flow(Q) = Velocity (V) x Area(A)
V= Flow/Area
Blood flow(Q) remains constant at 10ml/s and total volume flow(ml/min) stays the same.
3 patterns of blood flow
- Laminar - Most arteries, arterioles, venules and veins
* Concentric shells.
* Zero velocity at walls (molecular interactions)
* Max velocity at centre.
* Moves RBC to centre, speeds up blood flow through narrow vessels. - Turbulent - Ventricles (mixing), aorta (peak flow), atheroma (bruits)
* Blood doesn’t flow linearly and smoothly in adjacent layers due to increased pressure and velocity.
* High resistance to flow. - Bolus - Capillaries
* RBC have larger diameter than capillaries, so they move along in a single file.
* Plasma columns are trapped between RBC.
* Uniform velocity, little internal friction and low resistance.
Laminar flow diagram
Blood flow: Reynold’s number (Re)
Describes what determines change from laminar to turbulent flow.
Poiseuille’s law is only valid for conditions of laminar flow. At some critical velocity, the flow becomes turbulent with the formation of eddies and chaotic motion which do not contribute to the flow rate.
- Re= ρVD/μ
ρ=density V=velocity D=diameter μ=viscosity
Turbulence occurs when Reynold’s number exceeds a critical value (>2000), eg bruits, ejection murmur, increased blood velocity.
Arterial blood flow
- Pressure exerted by blood on vessel walls, generated by left ventricular contraction.
- Highest in aorta (120mmHg during systole and 80mmHg during diastole)
- Arterial pressure falls steadily in systemic circulation with distance from left ventricle.
- Arterioles are the resistance vessels under sympathetic control.
Arterial blood pressure
- Systolic pressure: Pressure when ejecting
- Diastolic pressure: Pressure when relaxing
- Pulse pressure: Difference between diastolic and systolic pressure
- Mean blood pressure: Average pressure
Arterial blood pressure - role of the aorta
- 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.
Arterial blood pressure: pulse pressure
Pulse pressure is what the finger senses, at the wrist (radial artery)
Pulse pressure = stroke volume / compliance
If complianceis low the pulse pressure will be high.
Arterial blood pressure: pulse pressure and stroke volume
Rest vs Excercise:
* 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, pulse pressure increases.
Arterial blood pressure: pulse pressure and compliance
- Arterial compliance: decreased compliance, stroke volume increases systolic and pulse pressure disproportionally.
- Important in the elderly: increase in age - stiffer arteries (atherosclerosis), decreased compliance.
Arterial pulse pressure - arterial tree and age
- Pulse pressure becomes more noticeable further down the arterial tree because vessels become less compliant.
- This is why we shiuld measure it in the radial artery, far away from the heart.
- Age increases stiffness of vessels - particulary in the 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 .
Means arterial blood pressure
What control mean blood pressure?
- Age
- Disease
- Distance along the arterial tree
- Blood volume - affects SV and CO
- Excercise - increased SV and CO
- Emotion - stress, anger, fear, apprehension, pain
- Wake/sleep - decreases BP 80/50 mmHg
