Haemodynamics (I & II)

Question 1

What is Haemodynamics?

Answer 1

Relationship between blood flow, blood pressure and resistance to flow.

Question 2

What are some of the key factors in Haemodynamics?

Answer 2

• Force = Strength of cardiac contraction
• Work = energy put in for isovolumetric contraction and ejection (this produces PRESSURE)
• Pressure = Pressure difference between aorta and veins
• Compliance = Extent of arterial stretch (important in creating diastolic pressure)
• Resistance - Produced by arterioles
• Flow
• Velocity - Changes depending on where in circulation, slows down in capillaries

Question 3

What does Darcy’s law state?

Answer 3

Q = (P1 – P2)/R
Where:
Q = Flow
P1-P2 = pressure difference
R = Resistance to flow

Question 4

What is Bernoulli’s law?

Answer 4

Shows the role of pressure, kinetic and potential energies in flow.
Flow = Pressure + Kinetic + Potential energies (pgh)
Pressure = Pa - CVP
Kinetic: Momentum of blood
Potential: Effect of gravity

Question 5

Define blood flow, blood velocity and perfusion.

Answer 5

• Blood flow = The volume of blood flowing in a given time (ml/min)
• Velocity = Blood flow divided by the cross-sectional area through which the blood flows (cm/s)
• Perfusion = Blood flow per given mass of tissue (ml/min/g)

Question 6

What is the equation used for velocity and how does this change as the vessel becomes more narrow?

Answer 6

Velocity = Volume flow / area
Velocity of blood flow through the aorta is high, because there is a large volume flow of blood (CO) going through a small cross-sectional area.
As it branches and we get further branching of blood vessels, we have the same CO but it’s being spread out over a larger surface area, thus velocity slows.
Velocity increases with the veins coming together, it is under a lower pressure (so there is less drive to the heart) but velocity has still increased.

Question 7

What are the 3 different types of blood flow?

Answer 7

Laminar
Turbulent (in ventricles, high velocity)
Bolus (in capillaries)

Question 8

What does Reynolds number show?

Answer 8

Re = (pVD)/u
p = Density
V = Velocity
D = Diameter
μ = Viscosity
As pressure increases so does laminar flow. However after a certain point the flow starts to become turbulent. Reynolds number is an arbitrary number which shows us the point at which laminar flow becomes turbulent. i.e. anything above 2300.

Question 9

How would you calculate blood flow using arterial pressure and what are the factors which effect arterial BP?

Answer 9

Blood Flow = Arterial blood pressure / Total Peripheral Resistance
Factors:
Cardiac output – (Starling’s/Laplaces’s laws, Contractility, HR)
Properties of arteries – aorta
Peripheral resistance – Arterioles
Blood viscosity – Haemocrit

Question 10

What are the 4 key interactions which produce arterial BP?

Answer 10

Systolic pressure
Diastolic pressure
Pulse pressure
Mean blood pressure 
🔑 🔑

Question 11

What is pulse pressure?

Answer 11

This is what the finger senses. The pulse pressure tells you about stroke volume and arterial compliance (stretchiness, how good they are). This is because Pulse Pressure = SV/ Compliance

Question 12

What happens to pulse pressure as stroke increases?

Answer 12

Relationship is relatively linear at first (not too steep) as you stretch your arteries more you get a relatively small change in BP.
As the volume gets greater however, the curve becomes more steep, so small increases in stroke volume will cause larger increases in BP (this would be seen in exercise).

Question 13

How does a decrease in compliance effect the pulse pressure and stroke volume? Why does compliance tend to decrease as you get older?

Answer 13

It causes small changes in stroke volume to have a larger effect on pulse pressure earlier than it should thereby making it disproportionate. Decrease in compliance occurs in elderly due to stiffer arteries.

Question 14

Why does pulse pressure disappear at arterioles?

Answer 14

Decreased velocity

Question 15

What is the equation used to calculate mean BP?

Answer 15

Mean BP = diastolic pressure + 1/3 [pulse pressure]

We would have to work out the area under the BP curve, but this is long and tedious.

Question 16

What controls mean BP?

Answer 16

Age (as compliance drops as you get old)
Disease (constant vascular tone -> increase in BP)
Distance along arterial tree (BP is different in different parts of circulation)
Posture –transient drop in BP
Blood volume
Exercise

Question 17

What controls TPR?

Answer 17

Poiseulle’s law
Myogenic response
Blood viscosity

Question 18

What is Poiseuilles law?

Answer 18

Poiseuille’s law describes parameters that govern vascular resistance, it is building on Darcy’s law (which just gives TPR, in Poiseuille’s law we are looking at the factors that effect TPR)

Question 19

What is the combined Darcy’s law and Poiseuilles law equation?

Answer 19

Blood flow = P1 – P2 x π r4/
8 η L
r = Radius of vessel
η = Blood viscosity
L = Vessel length
We can see that increased viscosity (more RBCs, more platelets) will decrease flow/increase resistance this is due to it making the flow less laminar. A longer vessel length also decreases flow.

Question 20

Explain, using Poiseuilles law, how arterioles are the main site of resistance to blood flow.

Answer 20

We know vascular resistance is controlled by radius4, the pressure difference across vessels (P1-P2) and length (L).
Well, arteriole radius is tightly controlled by sympathetic nerves, meaning they can contract/dilate a lot thus change the vascular resistance and alter blood flow.
Arterioles also have the largest pressure drop across them (i.e. start pressure – end is largest), this is usually 40-50mmHg.
Added together the arterioles also have a relatively small length.

Question 21

Capillaries have a much smaller radius than arterioles but they do not control TPR. Why is that?

Answer 21

Capillaries do not receive sympathetic innervation (they are only one cell thick) so cannot alter radius (cannot alter flow), bolus flow in the capillaries reduces viscosity (due to less friction between the molecules) and hence reduces resistance (from Poiseuille’s law), so blood moves quicker.
Capillaries are also arranged in parallel, so have a low total resistance as the total resistance, RTotal is spread across all the capillaries in that capillary bed (1/R1 + 1/R2)
In contrast arterioles are in series, you have arteries -> arterioles -> capillaries. Therefore the total resistance is the sum of the resistances in arterioles that are in series.

Question 22

What is the Bayliss myogenic response?

Answer 22

A sort of auto regulation. Whereby an increase in pressure will distend the vessel initially leading to a large increase in flow. But then if you carry on increasing pressure your blood vessels will constrict and blood flow will not increase much more at all under the changing pressure. This is a very important local way of controlling blood flow.

Question 23

Define viscosity. What are the factors that contribute to viscosity?

Answer 23

“Viscosity is a measure of internal friction opposing the separation of the lamina.”
• Velocity of blood: Centre is less viscous because it is faster
• Vessel diameter
• Haematocrit: Concentration of cells in blood plasma. The more cells, the more interactions that can occur (causing friction).

Question 24

What are the 3 ways blood returns to the heart through veins?

Answer 24

Pressure gradient (although small) that helps blood get back to the heart from the feet. So, Venous return = Venous Pressure – Pressure in RA/Venous resistance.
Thoracic pump, when we inhale our thoracic cavity expands which increases our abdominal pressure, this forces blood upwards towards the heart. This means a greater RV SV. Through thats how we know blood flows faster with inhalation.
Skeletal muscle pumps - contraction of leg muscles will return blood to the RA and any retrograde (back) flow is prevented by our venous valves.