Principles of haemodynamics Flashcards
What is haemodynamics?
The relationship between blood flow, blood pressure and resistance to flow
What are the 6 main factors in haemodynamics and what is responsible for each factor?
Force: cardiac contraction
Work: isovolumetric contraction/ejection
Pressure: difference between aorta an veins
Compliance: arterial stretch
Resistance: arterioles
Flow velocity: slowing down blood flow in capillaries
What type of system is the CVS? closed or open
closed system
What is the significance of the CVS being a closed system?
It means that what happens in one part of the CVS has a major impact on the other parts. For example, reduced blood flow to one area increases the pressure upstream and alters flow to other areas
Where is the majority of the blood in the CVS?
Venous system- low pressure reservoir system: this reservoir can be used to increase cardiac output (Starling’s Law)
What is the relation between Darcy’s law and blood flow?
Darcy’s law: flow = (Pa - CVP) / TPR
It takes into account the role of pressure energy in blood flow
What is the relation between Bernoulli’s Law and blood flow?
Flow= pressure + kinetics + potential
It takes into account the role of pressure, kinetic and potential energies in blood flow
What is potential energy?
Effect of gravity
What is kinetic energy?
Momentum of blood
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) affected by cross sectional area through which the blood flows. Blood flow may stay the same BUT velocity can change if there has been a change in cross sectional area
Explain why the velocity of blood flow in for example capillaries and arterioles is high compared to aortic blood flow velocity
Capillaries and arterioles are highly branched = increased cross sectional area = decreased velocity of blood flow
What is the equation used to calculate volume flow?
Volume flow (Q) = velocity (V) x area (A)
What are the three types of blood flow?
Laminar flow: in most arteries, arterioles, venules and veins
Turbulent flow: ventricles, aorta, atheroma
Bolus flow: capillaries
Describe lamina flow
Occurs in most arteries, arterioles, venules and veins
The blood flow is in concentric shells, with near zero velocity at the walls and maximum velocity near the centre. This moves RBC’s towards the centre an speeds up blood flow through narrow vessels
Describe turbulent flow
The blood flows in whirlpools, eddies and vortices due to the increased pressure and velocity or obstructions
Describe bolus flow
RBCs have a larger diameter than the capillary diameter, so they move in single file.
There are plasma columns trapped between RBCs; here there is uniform velocity, little friction and VERY low resistance
What is meant by reynolds number?
Re = pVD/u
Describes what determines change from laminar to turbulent flow
Turbulence occurs when Reynold’s number exceeds critical value of 2000
What factors increase Reynolds number?
Density
Velocity
Diameter
What factor decreases Reynolds number?
Viscosity
Give an example of when Re > 2000
If Re> 2000 then laminar flow —–> turbulent flow
Bruits
Ejection murmur
Increased blood velocity
Arterial blood pressure involves interaction between which 4 key pressures?
Systolic pressure: pressure when ejecting
Diastolic pressure: pressure when relaxing
Pulse pressure: difference between diastolic and systolic pressure
Mean blood pressure: average pressure
Name the factors that can affect arterial blood pressure
Cardiac output (SV, HR)
Properties of arteries
Peripheral resistance
Blood viscosity
Describe the role of the aorta and arteries in blood pressure
Aorta and large arteries contain elastic fibres
Elastic fibres help to propel the blood into the circulation
What is the role of the aorta and arteries in left ventricular systole and diastole?
Left ventricular diastole
60-80% of the stroke volume is stored in aorta and arteries as these structures expand. The energy is stored in the stretched elastin
Left ventricular diastole
Energy is returned to the blood as the walls of the aorta and arteries contract
This sustains the diastolic blood pressure and blood flow when the heart is relaxed
What is meant by pulse pressure?
Pulse pressure is what the finger senses, for example, at the wrist you can feel the radial artery for the pulse. It tells you about the stroke volume and the arterial compliance (stretchiness)
Pulse pressure = stroke volume/compliance
How can you calculate pulse pressure from systolic and diastolic pressure?
Pulse pressure (mm Hg) = systolic - diastolic
What causes a higher pulse pressure during exercise?
- Stroke volume increases
- More blood ejected = greater stretch of arteries
- Less compliance
- Less recoil
Difference between systole and diastole increases = increased pulse pressure
What is used to calculate compliance?
Compliance = change in volume/change in pressure
Describe how the arteries change with age and how this affects pulse pressure
Older = stiffer arteries= less compliance
Less compliance means that the increased stroke volume will increases systolic pressure disproportionally = higher pulse pressure
Higher pulse pressure in elderly could increase afterload during exercise (higher pulse pressure during exercise) which could lead to heart failure
Describe the compliance of vessels as they move further away from the heart
Further away from heart = less compliant = higher pulse pressure
How is the mean BP calculated?
Mean BP= Diastolic pressure + 1/3 pulse pressure
List some factors that affect the mean blood pressure
Age Disease Distance along arterial tree (further = higher) Blood volume Exercise Emotion (eg- fear, anger, stress, anxiety) Pain Waking up/Going to sleep (lower BP)
