04 Blood Flow

Question 1

Give the equation that defines MABP

Answer 1

MABP = CO x TPR

Question 2

What is Poiseuille’s equation

Answer 2

R = 8Ln / pi x r^4    
n = viscosity, R = resistance and r = radius of the vessel

Question 3

Define laminar and turbulent flow

Answer 3

Laminar: streamline flow of blood and the blod velocity is the same at any one point
Turbulent: irregular flow characterised by whirlpool

Question 4

Compare the biochemical consequences between laminar and turbulent flow

Answer 4

• Laminar: endothelial quiescence and survival; anti-coagulation and vasodilation
• Turbulent: endothelial apoptosis and proliferative; coagulation, platelet aggregation and vasoconstriction

Question 5

How is MABP measured

Answer 5

MABP = diastolic pressure + 1/3 pulse pressure (systolic - diastolic)

Question 6

Why does the aortic pressure falls slowly after the closure of aortic valve

Answer 6

This is because the aorta and large arteries act to buffer the change in pulse pressure, the distention keeps aortic pressure high - Windkessel effect

*When the aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation

This is related to their compliance. As compliance reduces (eg ageing), the pulse pressure will increase

Question 7

Define compliance

Answer 7

It is the relationship between transmural pressure and vessel volume

Question 8

How can the vein compliance be affected

Answer 8

Smooth muscle contraction - compliance reduces and reduces venous volume and increase venous pressure

Question 9

True or false: veins are 10 or 20 times more compliant than arteries

Answer 9

True

Question 10

Why do we usually not faint from lying down to standing up

Answer 10

• Increased sympathetic activity:
  1. vasoconstriction of arteries
  2. vasoconstriction of the veins to stiffen the wall to reduce venous volume
  3. increased HR and contractility to increase CO
• Myogenic venoconstrction: autoregulation of the response to increase venous pressure to stiffen the veins more
• Skeletal pump and respiratory pump

Question 11

What are the other problems with standing

Answer 11

Incompetent valves: varicose veins

Prolonged increase in venous pressure in the leg: oedema