Principles of Haemodynamics

Question 1

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

Answer 1

• 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 2

How does Darcy’s Law come into blood flow?

Answer 2

• The Darcy’s law takes into account the role of pressure energy in blood flow.
• Flow = (Pa - CVP) / TPR.
• Where,
  
  • Pa :- arterial pressure.
  • CVP: - central venous pressure.
  • TPR :- Total peripheral resistance.
  • (Pa - CVP) is the pressure difference between the arteries and veins.

Question 3

How does Bernoulli’s Law come into blood flow?

Answer 3

• It takes into account the role of pressure, kinetic and potential energies in blood flow (So not just pressure like Darcy’s law).
• Flow = Pressure (PV) + Kinetic (ρV²/2) + Potential (ρgh).
• Where,
  
  • Kinetic energy: momentum of blood.
  • Potential energy: effect of gravity.
  • ρ = fluid mass.
  • P = pressure.
  • V = velocity.
  • h = height.
  • g = acceleration due to gravity.
• [This law gives a reason as to why the blood in the vena cava still enters the right atrium even though the pressure is the same in both regions].

Question 4

What is perfusion?

Answer 4

Perfusion is measured as the rate at which blood is delivered to tissue, or volume of blood per unit time per unit tissue mass (ml/min/g).

Question 5

What is the relationship between blood flow and velocity?

Answer 5

• Cross-sectional area brings blood flow and veolcity together.
• The greater the cross-sectional area, the slower the flow (remember the equation for blood velocity from lecture 1).
• For example, the velocity of the blood flow in the aorta is high. The branching of the arteries slows the blood velocity. The velocity is the slowest in the capillaries. The velocity increases with the veins coming together.

Question 6

Describe the three types of blood flow.

Answer 6

1. LAMINAR FLOW: IN MOST ARTERIES, ARTERIOLES, VENULES AND VEINS.
   
   • The blood flow is in concentric shells, with near zero velocity at the walls (due to molecular interactions) and maximum velocity near the centre.
   • This moves RBCs towards the centre and speeds up blood flow through narrow vessels (as they do not brush up against the walls).
2. TURBULENT FLOW: VENTRICLES (MIXING), AORTA (PEAK FLOW), ATHEROMA (BRUITS).
   
   • Here, the blood does not flow linearly and smoothly in adjacent layers.
   • There are whirlpools, eddies and vortices due to the increased pressure and velocity, or obstructions.
3. BOLUS FLOW: CAPILLARIES.
   
   • RBCs have a larger diameter than the diameter of the capillaries, so they move in a single file.
   • There are plasma columns trapped between RBCs.
   • Here, there is uniform velocity, little internal friction and very low resistance.

Question 7

What is Reynold’s number?

Answer 7

• It describes what determines the change from laminar to turbulent flow.
• Turbulence occurs when Reynold’s number exceeds a critical value (> 2000), eg. bruits, ejection number, increased blood velocity.
• Re = (ρVD) / μ
• Where,
  
  • ρ - density.
  • V - velocity.
  • D - diameter.
  • μ - viscosity.
• Reynold’s number does not have any units.

Question 8

What are the factors that affect arterial blood pressure?

Answer 8

1. Cardiac output (SV, HR).
2. Properties of arteries.
3. Peripheral resistance - (according to Darcy’s law).
4. Blood viscosity - (according to Reynold’s number).

Question 9

Arterial blood pressure involves interactions between four key (pressure) relationships. What are they?

Answer 9

1. SYSTOLIC PRESSURE: pressure when ejecting.
2. DIASTOLIC PRESSURE: pressure when relaxing.
3. PULSE PRESSURE: difference between diastolic and systolic pressure.
4. MEAN BLOOD PRESSURE: average pressure.

Question 10

Describe the role of the aorta and arteries in blood pressure.

Answer 10

• The recoil of the elastic fibers of the aorta and large arteries helps to propel the blood into the circulation.
• DURING LV SYSTOLE:
  
  • 60-80% of the stroke volume is stored in the aorta and arteries as these structures expand.
  • The energy is stored in the stretched elastin.
• DURING LV 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 11

Describe pulse pressure.

Answer 11

• Pulse pressure = systolic pressure - diastolic pressure.
• It is usually 40mmHg.
• Pulse pressure is what the finger senses, for example, at the wrist (radial artery).
• It tells you about the stroke volume and the arterial compliance (stretchiness).
• Pulse pressure = stroke volume / compliance

Question 12

How is pulse pressure and stroke volume affected by exercise?

Answer 12

Rest vs exercise

• During exercise as the stroke volume increases and the compliance curve gets very steep leading to very high pulse pressure.
• There is a 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. the pulse pressure increases.

[There is a bigger pulse pressure when the stroke volume is raised].

Question 13

How is pulse pressure and compliance affected by exercise?

Answer 13

• Compliance = Change in volume / Change in pressure
• There is a decreased compliance (causing a steeper curve)
• The stroke volume now increases systolic and pulse pressure disproportionally.
• As you increase in age the stiffer arteries (atherosclerosis) decreased compliance.

Question 14

What does age have to do with arterial pulse pressure?

Answer 14

• Age increases the stiffness of vessels – particularly the aorta – this means that a large pulse pressure is present throughout arterial tree.
• Also, the further a vessel is from the aorta, the compliance decreases and there is higher pulse pressure.

Question 15

List some factors that affect the mean blood pressure.

Answer 15

• Age.
• Disease.
• Distance along the arterial tree.
• Blood volume (SV, CO).
• Exercise (SV, CO).
• Emotion (anger, stress, fear, etc.)
• Wake/sleep (decreased pressure during sleep)