Lecture 7 – Principles of haemodynamics

Question 1

Haemodynamics - define

Answer 1

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

Question 2

Factors important in blood flow (4)

Answer 2

• Cardiac output.
• Pressure difference.
• Arterial pressure:
o Profile through the circulation
o Systolic, diastolic
o Pulse pressure - stroke volume and compliance
o Mean arterial pressure
o Factors that regulate mean arterial pressure
• Resistance to flow. Flow is:
o Determined by arterial blood pressure and resistance
o Relationship to velocity
o Patterns - laminar, turbulent and bolus

Question 3

Factors in haemodynamics (6)

Answer 3

• Force - Cardiac contraction
• Work - Isovolumetric contraction / ejection
• Pressure - difference aorta to veins
• Compliance - arterial stretch
• Resistance - arterioles
• Flow velocity - slowing down blood flow in capillaries

Question 4

Darcys law (4)

Answer 4

Role of pressure energy in flow.
Flow = Pa - CVP / TPR
Q = P1 - P2 / R
According to this law blood will not go up/move if there is no pressure. No pressure= no movement.
Does not explain how blood can flow against the pressure e.g. low pressure at vena cava/ feet. Therefore, Bernoulli’s Law is needed.

Question 5

Bernoulli’s law (4)

Answer 5

Role of pressure, kinetic and potential energies in flow.
Blood is able to flow against pressure difference due to KE.
More pressure = more PE = Increase in flow.
Flow = Pressure (PV) + Kinetic (pV2/2) + potential (pgh)

KE = momentum of blood
Potential E = Effect of gravity
p = fluid mass
g = acceleration due to gravity

Question 6

Blood flow - define

Answer 6

Blood flow - Volume of blood flowing in a given time (ml/min).

Question 7

Perfusion - define

Answer 7

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

Question 8

Velocity of blood flow - define

Answer 8

Velocity of blood flow - Blood flow (cm/s) affected by the cross sectional area through which the blood flows, so flow may remain the same but velocity changes if there has been a change in cross sectional area.

Question 9

Blood flow - relationship with velocity (4)

Answer 9

Velocity of blood flow in aorta is high, decreasing as it gets further away.
Capillaries is lowest due to exchange occurring.
Arterioles resistance vessles, contract/dilate due to SNS.
Greater cross-sectional area the slower the flow.

Question 10

Patterns of blood flow (3)

Answer 10

Laminar
Turbulent
Bolus

Question 11

Patterns of blood flow - Laminar (3)

Answer 11

Most blood vessels.
Zero velocity at walls. Max in centre - RBCs.
Speeds up blood flow through narrow vessels.

Question 12

Patterns of blood flow - Turbulent (2)

Answer 12

Ventricles/Aorta/Atheroma.

DOES NOT flow lineraly/smoothly due to increases pressure and velocity.

Question 13

Patterns of blood flow - Bolus (3)

Answer 13

Capillaries.
RBCs too large, single file.
Uniform velocity.
Often plasma columns trapped between RBCs.

Question 14

Reynold’s number (3)

Answer 14

Describes laminar to tubular flow.
Turbulence occurs when the number is > 2000.
Re = pVD/u
Reynolds number = Density (p) x Velocity (V) x Diameter (D) / Viscosity (u).

Question 15

Arterial blood pressure and relationships (4)

Answer 15

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 16

Factors that affect arterial blood pressure (4)

Answer 16

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

Question 17

Arterial blood pressure - Role of aorta (5)

Answer 17

FLASHCARD

Question 18

Arterial blood pressure -During LV ejection (2)

Answer 18

60-80% of stroke volume is stored in aorta and arteries as these structures expand.
Energy stored in stretched elastin.

Question 19

Arterial blood pressure -During LV diastole (2)

Answer 19

Energy is returned to the blood as the walls of the aorta and arteries contract.
This sustains diastolic blood pressure and blood flow when heart is relaxed.

Question 20

Pulse pressure (2,6)

Answer 20

1) = Systolic pressure – Diastolic pressure
2) = Stroke Volume/ Compliance

1 – Ejection
2 – Peak Systolic
3 – Systolic decline
4 – Incisura/dicrotic notch (closure of aortic valve)
5 – Diastolic run off
6 – Peak Diastolic
• During systolic ejection phase (1) pressure in the aorta will rise and the aorta will stretch and give a peak systolic pressure (2) which will then decline during diastole (3) however, there will be a notch (4) where the aortic valve will close the pressure in the aorta will be higher than in the ventricles and cause the valve to slam shut.

Question 20

Arterial blood pressure - Pulse pressure (6)

Answer 20

FLASHCARD

Question 22

Compliance (4)

Answer 22

Compliance - How much the aorta is able to stretch and spring back again. Therefore, pulse pressure = stroke volume/compliance (how much the aorta is able to stretch).
If the aorta is not compliant at all, then the pressure in the aorta would be the same as in the ventricle:
o The more compliant the aorta, the lower the pulse pressure.
o The higher the stroke volume, the higher the pulse pressure

Question 23

Arterial blood pressure - Pulse pressure and SV (4,1) Rest vs Exercise

Answer 23

•	Rest vs Exercise:
o	Greater SV
o	Greater stretch of arteries 
o	Less compliant
o	Relatively greater systolic pressure

• 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. pulse pressure increases.

Question 24

What controls mean blood pressure? (7)

Answer 24

• Age
• Disease
• Distance along arterial tree
• Blood volume - SV, CO
• Exercise - SV, CO
• Emotion - stress, anger, fear, apprehension, pain. = centres connected to brain stem which will send messages to veins and arterioles to control where the blood will go and how much resistance it has.
• Wake/sleep - BP 80/50 mmHg