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

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

Flow = (Pa - CVP)/ TPR

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.

Flow = Pressure (PV) + Kinetic (ρV²/2) + Potential (ρgh)

Kinetic Energy: momentum of blood
Potential Energy: effect of gravity

Question 4

What is perfusion?

Answer 4

It is the blood flow per given mass of tissue (ml/min/g).

The amount of blood flowing into an organ

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.

Flow may remain the same but velocity changes if there has been a change in cross sectional area

As total cross section increases as more branches form, the velocity of the blood decreases
The velocity of the blood doesn’t reach the same speed as it came from the heart as it loses some energy

Question 6

Describe the three types of blood flow.

Answer 6

• 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.
• 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.
• 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.
Turbulence occurs when Reynold’s number exceeds a critical value (>2000), eg. bruits, ejection number, increased blood velocity.

Re = (ρVD)/μ

(density x velocity x diameter) / viscosity

Question 8

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

Answer 8

• SYSTOLIC PRESSURE: pressure when ejecting
• DIASTOLIC PRESSURE: pressure when relaxing
• PULSE PRESSURE: difference between diastolic and systolic pressure
• MEAN BLOOD PRESSURE: average pressure

Question 9

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

Answer 9

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 10

Describe pulse pressure.

Answer 10

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

High compliance -> Low pulse pressure
vice versa

Question 11

What does age have to do with arterial pulse pressure?

Answer 11

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 higher the pulse pressure.

Question 12

List some factors that affect the mean blood pressure.

Answer 12

• 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)

Question 13

What is Haemodynamics ?

Answer 13

Haemodynamics is the relationship between:

• blood flow
• blood pressure
• Resistance to flow

Question 14

What are the key factors affecting haemodynamics?

Answer 14

• Force (cardiac contraction)
• Work (isovolumetric contraction / ejection)
• Pressure ( difference between arteriole and venous pressure)
• Compliance (arterial stretch )
• Resistance
• Flow velocity

Question 15

Where is most of the blood i the CVS SYSTEM?

Answer 15

In the Venous System - low pressure reservoir system

Question 16

What can the reservoir of venous blood be used for and how?

Answer 16

it can be used to increase cardiac output - via starling’s law as there is an increase in venous return

Question 17

What is Blood Flow?

Answer 17

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

Question 18

What is Arterial Blood flow/ Arterial Pressure?

Answer 18

Pressure exerted by blood on vessel walls ad generated by LV
Highest in Aorta - 120mmHg during systole
Arterial pressure falls steadily in systemic circulation wit distance from LV

Flow = Pa/TPR

Question 19

What are the factors that affect Arterial Blood Pressure ?

Answer 19

Cardiac Output (SV, HR)
Properties of Arteries
Peripheral Resistance
Blood viscosity

Question 20

When considering Arterial Blood pressure, Pulse pressure and Stroke Volume, what do you expect at rest?

Answer 20

• Greater SV
• Greater Stretch of arteries
• Less compliant
• Relative greater systolic pressure

Question 21

When considering Arterial Blood pressure, Pulse pressure and Stroke Volume, what do you expect when exercising?

Answer 21

• greater stretch if arteries as more blood is ejected
• higher SV according to Starling’s higher contraction
• less compliance and less recoil
• difference between systole and diastole pressures increases -> pulse pressure increases

Question 22

What is a compliance curve?

Answer 22

Compliance Curve - relationship between SV and Arterial pressure
as we age - less compliant in the aorta
if compliance is low, small change in volume results in a massive change in pulse pressure

Compliance = Change in volume / change in pressure