5) Principles of Haemodynamics

Question 1

What is Haemodynamics?

Answer 1

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

Question 2

What are the key factors in haemodynamics?

Answer 2

• Force: Cardiac contraction
• Work: Isovolumetric contraction/ ejection
• Pressure: Difference between different vessels
• Compliance: Atrial stretch
• Resistance: In arterioles
• Flow velocity: Slowing down blood flow in capillaries

Question 3

Is the CVS an open or closed system?

Answer 3

• The CVS is a closed system.
• A change in one part of the system has an impact on other parts of the system

(For e.g.: reduced blood flow in one area increases pressure upstream and alters flow in other areas)

Question 4

How many capillary beds normally does blood go through?

Answer 4

1

Question 5

What is it called when blood goes through 2 capillary beds?

Answer 5

• A portal system

Question 6

Where is majority of the blood found in the CVS?

Answer 6

• In the veins

Question 7

How does venous blood affect cardiac output?

Answer 7

• It can be used to increase cardiac output (Starling’s law)
• Veins have a thin muscle layer and so contract slightly
• When they contract they send more blood back into the heart and so put more blood into arterial circulation
• This as a result increases cardiac output

Question 8

What is Darcy’s law of Haemodynamics?

Answer 8

• Role of pressure energy in flow
• Flow = (P1 - P2)/ R

P1 - P2: difference in pressure (from arterial to venous end)
R: Resistance to flow

Question 9

What is Bernoulli’s Law of Haemodynamics?

Answer 9

• Role of pressure, kinetic and potential energies in flow (not pressure alone)
• Kinetic energy refers to the momentum of blood
• Potential energy refers to the effect of gravity

- Flow = Pressure (P) + Kinetic (ρV^2/2) + Potential (ρgh)
ρ = Fluid mass
P = pressure
V = velocity
h = height
g = acceleration due to gravity

Question 10

What is blood flow?

Answer 10

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

Question 11

What is Perfusion?

Answer 11

• Blood flow per given mass of tissue (ml/min/g)

Question 12

What is velocity of blood flow?

Answer 12

• Speed of blood flow (cm/s) affected by the cross sectional area of the vessel in which blood flows.
• Flow may remain the same but velocity changes if there is a change in the cross sectional area.

Question 13

What is the relationship between cross-sectional area and velocity of blood flow?

Answer 13

• The greater the cross-sectional area the slower the blood flow. This is why blood flow is slowest in the capillaries
• When veins come together once again the velocity increases

Question 14

Explain the change in total area and velocity of vascular bed from aorta to the vena cavae

Answer 14

• In the aorta velocity is very high as there is a very small total area.
• As the aorta branches into arteries the total area increases causing the velocity to decrease.
• Arteries branch into arterioles and arterioles into capilliaries which have the largest area (as area keeps increasing with each branching) causing velocity to be slowest.
• The slow velocity in the capillaries allows for gas and nutrient exchange to take place.
• The capillaries at the end of the bed join together to form venules which have a smaller area and a larger velocity.
• Venules then join together to form veins and veins join together to the vena cavae
• At each joining the area gets smaller and the velocity gets bigger and bigger
• In the venae cavae we end with an area that is similar to the aorta but a lower velocity

Question 15

What is the equation for volume flow?

Answer 15

• Volume flow (Q) = Velocity (V) x Area (A)

Question 16

What are the three patterns of blood flow?

Answer 16

• Laminar flow (Arteries, arterioles, venules and veins): Flow in regular concentric (arch) shapes where velocity is slower at the walls due to friction and highest at the centre. Most red blood cells are found at the centre. This speeds up blood flow through narrow vessels
• Turbulent (ventricles, aorta, atheroma): Blood flows in whirlpools and vortices due to increased pressure and velocity. It has a high resistance to flow.
• Bolus (capillaries): Red blood cells have a larger diameter compared to that of the capillaries and so are able to move in a single file. Plasma columns are trapped between the RBCs. It has uniform velocity with very little internal friction and very low resistance to flow

Question 17

What is Reynold’s number?

Answer 17

• It describes what determines change from laminar to turbulent flow
• Turbulent flow occurs when Reynold’s number exceeds a critical value. Below this critical value flow will be laminar.
• Reynolds number = (Density x Velocity x Diameter)/ Viscosity
• In order to increase Reynolds number we need to increase density, diameter and velocity and we need to decrease Viscosity

Question 18

What generates the pressure exerted by blood on vessel walls?

Answer 18

• Left ventricular contraction

Question 19

Describe the relationship between distance from left ventricle and arterial pressure

Answer 19

• Arterial pressure falls as distance from left ventricle increases

Question 20

What type of vessels are arterioles and which type of nervous system are they controlled by?

Answer 20

• Arterioles are resistance vessels which are under sympathetic control

Question 21

What is systolic pressure?

Answer 21

• Pressure when ejecting blood

Question 22

What is diastolic pressure?

Answer 22

• Pressure when relaxing

Question 23

What is pulse pressure?

Answer 23

• Difference between diastolic and systolic pressure
• This is the “pulse” we feel with our fingers.
• It tells us about the stroke volume and arterial compliance (stretch)
• Pulse pressure = Stroke volume/ Compliance
• If compliance is low then pulse pressure is high. If stroke volume is high then pulse pressure is high

Question 24

What is compliance?

Answer 24

• The ability of a vessel wall to stretch.
• Arteries have high compliance which means they require a high pressure in order to increase their volume.
• This is due to the thick layer of smooth muscle they have in the arterial wall

Question 25

What is mean blood pressure?

Answer 25

- Average pressure between systolic and diastolic pressure

Question 26

What happens to the aorta and arteries during left ventricular systole?

Answer 26

- Arteries and aorta expand which stores energy within the stretched elastin

Question 27

What happens to the aorta and arteries during left ventricular diastole?

Answer 27

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

Question 28

Describe the changes in the arteries when going form rest to exercise.

Answer 28

- During exercise there is a greater stretch of the arteries as more blood is ejected (stroke volume). - This causes less compliance and less recoil. - Finally pulse pressure also increases

Question 29

How is arterial pressure affected by age?

Answer 29

- In older people their arteries become stiffer (atherosclerosis) causing a decrease in compliance - This means a large pulse pressure will be present throughout the arterial tree.

Question 30

How does arterial pressure change as we go down the arterial tree?

Answer 30

- Pulse pressure becomes more noticeable as we go down the arterial tree as vessels become less compliant - Pulse is measured at the radial artery as it is quite far from the heart however cannot be detected in the arterioles as flow is more continuous

Question 31

What controls mean blood pressure?

Answer 31

- Age (less compliance and atherosclerosis) - Disease (cardiovascular diseases) - Distance along arterial tree (less compliance further from the arterial tree) - Blood volume (Stroke volume and Cardiac Output) - Exercise (Stroke volume and Cardiac Output) - Emotions (Stress, anger, fear) - Awake or asleep