Lecture 3: haemodynamics

Question 1

What is Darcy’s law?

Answer 1

Flow = Pressure difference/resistance

Q = ΔP/R

Question 2

What is perfusion pressure?

Answer 2

Difference in pressure between the arteries that supply a region and the veins that drain it.

ΔP = P_A - P_v

Question 3

Why is arterial blood pressure considerred to be almost equal to perfusion pressure?

Answer 3

Pressure in venous system is very low

Question 4

What is the relationship between CO, BP and vascular resistance?

Answer 4

CO = BP/VR

Question 5

What is meant by laminar flow?

Answer 5

For any Newtonian fluid the velocity profile is parabolic as the stream directly next to the vessel wall has a higher resistance than the liquid at the centre and so travels at a slower velocity

Question 6

How does the velocity profile of water compare to that of blood?

Answer 6

More blunt in blood

Question 7

What influences resistance to laminar flow?

Answer 7

• Friction in vessel wall
• radius of the tube
• length of the tube
• viscosity of the fluid

Question 8

What is Poiseuille’s law?

Answer 8

R α (Ln)/r⁴

Resistance α (length of vessel x viscosity of blood) / radius⁴

Question 9

How does resistance change as the length of a vessel is increased?

Answer 9

It increases

Question 10

How does resistance change as blood viscosity increases?

Answer 10

It increases

Question 11

How does resistance change as the radius of a vessel increases?

Answer 11

It decreases dramiatically

Question 12

Why do capillaries have a low resistance despite their small radius?

Answer 12

They are short (<1mm)

They are connected in parallel

Question 13

How is blood viscosity determined?

Answer 13

The ratio of red blood cells to plasma, calculated by the haematocrit (a percentage)

Question 14

What factors affect blood viscosity?

Answer 14

Haematocrit increases at high altitude and decreases with anaemia

Question 15

What does the Fahræus-Lindqvist effect describe?

Answer 15

An effect where the viscosity of a fluid changes with the diameter of the tube it travels through

Question 16

What happens to viscosity as tube diameter decreases?

For what range of diameter is this valid?

Answer 16

Viscosity decreases

10-300µm

Question 17

Why does the Fahræus-Lindqvist occur?

Answer 17

Plasma cell-free layer forms as RBCs move to centre of vessel and plasma is left around vessel walls. The cell-free layer has a lower viscosity and so lowers the resistance to blood flow

Question 18

What is vasomotion?

Answer 18

Changes in the diameter of a vessel

Increase = vasodilatation

Decrease = vasoconstriction

Question 19

What factors affect vasomotion?

Answer 19

Nerves

Hormones

Local factors

Question 20

What are the physical characteristics of sympathetic nerves?

Answer 20

Short pre-ganglionic fibre and long post-ganglionic fibre

Question 21

What is the neurotransmitter released from the pre-ganglionic fibre of a sympathetic nerve?

Answer 21

Acetylcholine

Question 22

What is the neurotransmitter released from the post-ganglionic fibre of a sympathetic nerve?

Answer 22

Noradrenaline

Question 23

What is the main regulator of total peripheral resistance?

Answer 23

The sympathetic nervous system

Question 24

What is the neurotransmitter released to cause vasoconstriction?

Answer 24

Noradrenaline

Question 25

Which type of vessel(s) have vasoconstrictor nerves?

Answer 25

Arterioles and venules

More abundant in arterioles than venules

Question 26

What is the result of increased vasoconstriction of arterioles?

Answer 26

Increased TPR

Increased APB

Question 27

What is the result of increased constriction of venules?

Question 28

What is the most efficient method of vasodilatation?

Answer 28

Inhibition of sympathetic tone

Question 29

What is the effect of adrenaline on vasomotion?

Answer 29

In most peripheral systems it causes vasoconstriction

May cause vasodilatation in skeletal muscle

Question 30

What did Furchgott and Zawadski discover in 1980?

Answer 30

A vasodilator response to acetylcholine changed to a vasoconstrictor response if the endothelial lining was rubbed away

Question 31

Name a local factor which has an effect on vasomotion

How does this affect blood vessels?

Answer 31

Nitric Oxide

Acetylcholine stimulates to emdothelium to synthesise NO which promotes vasodilatation

Question 32

How is nitric oxide formed in blood vessels?

Answer 32

• Acetylcholine stimulates endothelium
• Cleavage of arginine by an endothelial membrane enzyme called No synthase
• Enzyme activity regulated by Ca2+- calmodulin complex so that ACh increases rate of No synthesis by promoting Ca²⁺ entry into cells

Question 33

How is blood pressure measured?

Answer 33

Sphymomanometer

Inflatable cuff around brachial artery inflated higher than systolic pressure

Pressure gradually released

Systolic BP: pressure when korotkoff sounds start

Diastolic BP: pressure at which sounds stop

Question 34

What is arterial stiffness?

Answer 34

A measure of the rigidity of blood vessels

Question 35

What causes arterial stiffness to increase?

Answer 35

Ageing and disease leads to calcium and collagen deposition which increases rigidity

Question 36

How can an increase pulsatility be calculated?

Answer 36

Increased systolic to diastolic ratio

Increased pulsatility index (PI), S:D/mean

Question 37

What does an increased pulsatile index signify?

Answer 37

Increased vascular resistance