Principles of haemodynamics

Question 1

What is haemodynamics?

Answer 1

The relationship between blood flow, blood pressure and resistance to flow

Question 2

What are the 6 main factors in haemodynamics and what is responsible for each factor?

Answer 2

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

Question 3

What type of system is the CVS? closed or open

Answer 3

closed system

Question 4

What is the significance of the CVS being a closed system?

Answer 4

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

Question 5

Where is the majority of the blood in the CVS?

Answer 5

Venous system- low pressure reservoir system: this reservoir can be used to increase cardiac output (Starling’s Law)

Question 6

What is the relation between Darcy’s law and blood flow?

Answer 6

Darcy’s law: flow = (Pa - CVP) / TPR

It takes into account the role of pressure energy in blood flow

Question 7

What is the relation between Bernoulli’s Law and blood flow?

Answer 7

Flow= pressure + kinetics + potential

It takes into account the role of pressure, kinetic and potential energies in blood flow

Question 8

What is potential energy?

Answer 8

Effect of gravity

Question 9

What is kinetic energy?

Answer 9

Momentum of blood

Question 10

Define blood flow

Answer 10

Volume of blood flowing in a given time (ml/min)

Question 11

Define perfusion

Answer 11

Blood flow per given mass of tissue (ml/min/g)

Question 12

Define velocity of blood flow

Answer 12

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

Question 13

Explain why the velocity of blood flow in for example capillaries and arterioles is high compared to aortic blood flow velocity

Answer 13

Capillaries and arterioles are highly branched = increased cross sectional area = decreased velocity of blood flow

Question 14

What is the equation used to calculate volume flow?

Answer 14

Volume flow (Q) = velocity (V) x area (A)

Question 15

What are the three types of blood flow?

Answer 15

Laminar flow: in most arteries, arterioles, venules and veins

Turbulent flow: ventricles, aorta, atheroma

Bolus flow: capillaries

Question 16

Describe lamina flow

Answer 16

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

Question 17

Describe turbulent flow

Answer 17

The blood flows in whirlpools, eddies and vortices due to the increased pressure and velocity or obstructions

Question 18

Describe bolus flow

Answer 18

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

Question 19

What is meant by reynolds number?

Answer 19

Re = pVD/u
Describes what determines change from laminar to turbulent flow
Turbulence occurs when Reynold’s number exceeds critical value of 2000

Question 20

What factors increase Reynolds number?

Answer 20

Density
Velocity
Diameter

Question 21

What factor decreases Reynolds number?

Answer 21

Viscosity

Question 22

Give an example of when Re > 2000

If Re> 2000 then laminar flow —–> turbulent flow

Answer 22

Bruits

Ejection murmur

Increased blood velocity

Question 23

Arterial blood pressure involves interaction between which 4 key pressures?

Answer 23

Systolic pressure: pressure when ejecting

Diastolic pressure: pressure when relaxing

Pulse pressure: difference between diastolic and systolic pressure

Mean blood pressure: average pressure

Question 24

Name the factors that can affect arterial blood pressure

Answer 24

Cardiac output (SV, HR)
Properties of arteries
Peripheral resistance
Blood viscosity

Question 25

Describe the role of the aorta and arteries in blood pressure

Answer 25

Aorta and large arteries contain elastic fibres

Elastic fibres help to propel the blood into the circulation

Question 26

What is the role of the aorta and arteries in left ventricular systole and diastole?

Answer 26

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

Question 27

What is meant by pulse pressure?

Answer 27

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

Question 28

How can you calculate pulse pressure from systolic and diastolic pressure?

Answer 28

Pulse pressure (mm Hg) = systolic - diastolic

Question 29

What causes a higher pulse pressure during exercise?

Answer 29

• Stroke volume increases
  
  • More blood ejected = greater stretch of arteries
  • Less compliance
  • Less recoil

Difference between systole and diastole increases = increased pulse pressure

Question 30

What is used to calculate compliance?

Answer 30

Compliance = change in volume/change in pressure

Question 31

Describe how the arteries change with age and how this affects pulse pressure

Answer 31

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

Question 32

Describe the compliance of vessels as they move further away from the heart

Answer 32

Further away from heart = less compliant = higher pulse pressure

Question 33

How is the mean BP calculated?

Answer 33

Mean BP= Diastolic pressure + 1/3 pulse pressure

Question 34

List some factors that affect the mean blood pressure

Answer 34

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)