Principles of Haemodynamics

Question 1

What is haemodynamics?

Answer 1

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

Question 2

What controls the force, work, pressure, compliance, resistance and flow velocity?

Answer 2

• 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 3

Effect of control of arterioles.

Answer 3

Control arterioles in different parts of the body and regulate the flow to different organs.

Question 4

If the heart is more compressed, what happens to contraction?

Answer 4

Increases

Question 5

How can the reservoir of venous blood increase cardiac output by Starling’s law?

Answer 5

When we squeeze the veins, and more blood arrives back to the right side of the heart, and when this reaches the left ventricles, the ventricles stretches and increases force of contraction (Starling’s law) – so soon balance both sides of the heart.

Question 6

What happens when there is reduced blood flow to one area of the CVS?

Answer 6

Reduced blood flow to one area increases pressure upstream and alters flow to other areas.

Question 7

What happens when we squeeze the muscular arterioles?

Answer 7

We can change resistance and direct flow wherever we want it. - blood spreads out and flows in various proportions around the body according to the amount of constriction and dilation in the arterioles.

Question 8

What is the difference between Darcy’s law and Bernoulli’s law? and why is Darcy’s law not sufficient?

Answer 8

• Darcy’s law - role of pressure energy in flow
• Bernoulli’s law - role of pressure, kinetic and potential energies in flow – NOT JUST PRESSURE.
• Darcy’s law explains on a simple level but not stuff like how pressure is higher in legs than from blood that just left the heart?

Question 9

Define blood flow.

Answer 9

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

Question 10

Define perfusion.

Answer 10

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

Question 11

What is velocity of blood flow?

Answer 11

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. (changes by constriction and dilation).

Question 12

Changes in velocity within blood vessels

Answer 12

• Velocity of blood flow in aorta is high
• Branching of arteries slows velocity
• Slowest flow in capillaries
• Velocity increases with veins coming together

Question 13

Volume flow equation

Answer 13

Volume flow (Q) = Velocity (V) x Area (A)
- The flow volume may remain the same, but area and velocity may chage, depending on vasodilation (inc. A, dec. V) or vasoconstriction (dec. A, inc. V).

Question 14

What are the 3 patterns of blood flow? Where are they common? and describe them.

Answer 14

1. Laminar
   – smooth BF
   – in most blood vessels
   – When blood flows through, the blood near the walls has more frictional contact w/ the edges and slows - maximum velocity at centre. ~ Concentric shells.
2. Turbulent
   – Ventricles (mixing),
   aorta (peak flow),
   atheroma (bruits)
   – Blood does not flow linearly and smoothly in adjacent layers
   – Due to increase pressure, velocity, viscosity or when there are some other problems like obstructions.
   – Sometimes you can get this in the heart, and you can hear it with a stethoscope.
3. Bolus
   – In capillaries when the size is the same or smaller than RBC.
   – RBC bends and slips through nicely in the capillary
   – smooth and low resistant flow
   – Fluid in front and fluid behind.

Question 15

What is Reynold’s number?

Answer 15

Reynold’s number – Describes what determines change from laminar to turbulent flow
• Turbulence occurs when Reynold’s number exceeds a critical value (>2000). By e.g. bruits, ejection murmur and increased blood velocity.
• As pressure increases, flow increases (as constant resistance), until it reaches turbulent flow, where increase in pressure doesn’t affect the flow that much.

Question 16

How can viscosity of the blood increase?

Answer 16

When stasis of the blood – we sometimes get minor clotting, which increases the viscosity of the blood.
When viscosity increases, pressure increases.

Question 17

What are the factors that affect arterial blood pressure?

Answer 17

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

Question 18

What are systolic, diastolic, pulse and mean blood pressure?

Answer 18

SYSTOLIC PRESSURE
•Pressure when ejecting
DIASTOLIC PRESSURE
•Pressure when relaxing
PULSE PRESSURE
• Difference between diastolic and systolic pressure
MEAN BLOOD PRESSURE
• Average pressure

Question 19

Role of the aorta in maintaining arterial blood pressure.

Answer 19

Recoil of elastic fibers of the aorta and large arteries helps to propel the blood into the circulation
DURING LV EJECTION
• 60-80% of stroke volume is stored in aorta and arteries (energy stored in stretched elastin) as these structures expand. - stretching means the aorta is accommodating.
DURING LV DIASTOLE
• 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

Arterial BP - rest vs exercise.

Answer 20

• Greater SV
• Greater stretch of arteries
• Less compliant
• Relatively greater systolic pressure

Question 21

How does SV effect pulse pressure during exercise?

Answer 21

During exercise as the stroke volume increases compliance curve gets very steep leading to very high pulse pressure.
Bigger pulse pressure when stroke volume raised

Question 22

What happens to the pulse pressure when there is a greater stretch of the arteries?

Answer 22

Greater stretch of the arteries as more blood is ejected causes less compliance and less recoil and the difference between systole and diastole increases ie pulse pressure increases.

Question 23

What is pulse pressure?

Answer 23

Difference between diastolic and systolic pressure. This is what the finger senses. Tells you about stroke volume and arterial compliance (stretchiness).

Question 24

Pulse pressure equation

Answer 24

Pulse pressure = SV / Compliance

Question 25

Compilance Equation

Answer 25

Compliance = (Change in volume) / (Change in pressure)

Question 26

What controls mean blood pressure?

Answer 26

Age 
Disease
Distance along arterial tree
Blood volume - SV, CO
Exercise - SV, CO 
Emotion - stress, anger, fear, apprehension, pain 
Wake/sleep -  dec. BP 80/50 mmHg

Question 27

Equation to find the mean BP

Answer 27

Mean BP = diastolic pressure + 1/3 (pulse pressure)

Question 28

Arterial pulse pressure and arterial tree

Answer 28

Pulse pressure at the aorta is relatively small but further down the arterial tree you it increases slightly because vessels become less compliant.
But you are still getting the effects of the aorta so it doesn’t increase all that much.

This one reason to measure it in the radial artery quite far away from the heart.

Question 29

Arterial pulse pressure and age

Answer 29

Age increases stiffness of vessels – particularly aorta – this means that large pulse pressure is present throughout arterial tree.

Question 30

Pulse pressure in the arterioles

Answer 30

Once you get into the arterioles the pulse pressure can’t be detected and the flow is more continuous.

Question 31

Affect of decreased arterial compliance on SV, and why is this important in the elderly?

Answer 31

Decreased compliance (steeper curve)
Stroke volume now increases systolic and pulse pressure disproportionally
- When SV is normal the pulse pressure is similar to younger person. – When CO and SV increases there is a much greater increase in pulse pressure, with the same SV. – so very high pressure per heart beat.

Increase in age – stiffer arteries (atherosclerosis) decreased compliance

Question 32

What would happen during exercise (increase in SV) for the elderly?

Answer 32

High pulse pressure and low compliance of the aorta – we are INCREASING AFTERLOAD (Laplace’s law) – so we are decreasing CO, so heart needs to work more, gets bigger, demands more O2… vicious cycle continues..