Haemodynamics

Question 1

What is haemodynamics?

Answer 1

• Haemodynamics: the study of the physical and physiological principles governing the movement of blood through the circulatory system

Question 2

What does the left ventricle eject into the aorta with each systolic contraction?

Answer 2

A fixed volume (bolus) of blood

Question 3

What is Ohm’s Law? What does it say an increase in aortal blood volume during systolic contraction should produce?

Answer 3

flow (litres/min) = pressure (mm Hg)/ flow resistance
This increase in volume in the aorta should produce an increase in pressure depending on the flow resistance in the aorta and downstream arteries.

Question 4

If flow resistance was fixed, what would happen to the flow and pressure curves? What actually happens, and what does this mean?

Answer 4

If the flow resistance was fixed (like an electrical resistor) the flow curve would have exactly the same shape as the pressure curve….it clearly doesn’t!
Between points A and B the flow decreases nearly to zero while the pressure only drops a little. This decrease in flow out of the aorta late in systole means that during systole more blood enters the aorta than leaves it.
In other words, it distends during systole. When the aortic valve is shut during diastole it shrinks back. This distensibility of the aorta is called its compliance.

Question 5

What is aortic compliance?

Answer 5

The distensibility of the aorta is called its compliance.

Aortic compliance means that flow out of the aorta continues during diastole

Question 6

When do the aorta and major arteries distend and when do they contract?

Answer 6

Aorta (and major arteries) distend during systole

Aorta and elastic arteries contract during diastole

Question 7

What are the characteristics of elastic arteries?

Answer 7

o Thick-walled arteries near the heart; the aorta and its major branches
o Large lumen allow low-resistance conduction of blood:
o Contain elastin in all three tunics: highly compliant
o Withstand and smooth out large blood pressure fluctuations:
o Serve as pressure reservoirs

Question 8

What are the characteristics of muscular arteries?

Answer 8

o Distal to elastic arteries; deliver blood to body organs
o Have thick tunica media with more smooth muscle
o Active in vasoconstriction: some sympathetic innervation

Question 9

What are the characteristics of arterioles?

Answer 9

o Smallest arteries
o Lead to capillary beds
o Control flow into capillary beds via vasodilation and constriction from sympathetic nervous supply.

Question 10

What happens to pressure in the aorta and elastic arteries due to their distention during systole? What does this ensure? How does it affected the afterload and work of left heart?

Answer 10

• Due to their distention during systole, the pressure in the aorta and elastic arteries does not drop to near zero (like that in the ventricle) during diastole, but only to about 80 mmHg.
  This ensures that flow down the arterial tree continues during diastole.
  The distention of the aorta and elastic arteries during systole reduces the afterload and thus reduces the work of the left heart.

Question 11

When can a pulse pressure be felt in the wrist? What happens to it in the arterioles and capillaries?

Answer 11

A - A pulse pressure between systole and diastole is present in the muscular arteries (and so can be felt at the wrist) but decreases in the arterioles and disappears in the capillaries as fluid exchange across the capillary wall occurs in the capillaries.

Question 12

What is systolic blood pressure? What is its value in the brachial artery? What are current guidelines for if it is higher than this?

Answer 12

Systolic blood pressure = maximum pressure in arteries
In brachial artery systolic pressure normally = 120 mmHg (16 kPa)
Current guidelines are:
>140 mmHg systolic marginal hypertension
>160 mmHg definite intervention threshold

Question 13

What is diastolic blood pressure? What is its value in the brachial artery? What are current guidelines for if it is higher than this?

Answer 13

Diastolic blood pressure = minimum pressure in arteries
In brachial artery systolic pressure normally = 80 mmHg (10.7 kPa)
Current guidelines are:
>90 mmHg systolic marginal hypertension
>100 mmHg definite intervention threshold

Question 14

What is pulse pressure? Why and how does it differ between the aorta and brachial artery?

Answer 14

Pulse pressure = difference between systole and diastole.

Decreases slightly from aorta to brachial artery as arteries are elastic.

Question 15

How do you calculate pulse pressure? How about mean arterial pressure (MAP)?

Answer 15

Pulse pressure = (systolic-diastolic) pressure
MAP = diastolic pressure + 1/3 of pulse pressure
(e.g. if systolic = 120 and diastolic = 90, then MAP = 100 mmHg, or 90 + 1/3 of 30)
REMEMBER HOW TO CALCULATE MAP!!!

Question 16

What happens to the arteries as a person gets older (or smokes cigarettes)? What does this do to the systolic pressure?

Answer 16

Their arteries gradually loose some of their elastin which is replaced by collagen.
The arteries loose elasticity or “harden”. This INCREASES the systolic pressure as the aorta cannot stretch as much to accommodate the stroke volume during systole.
(in a pressure-increase in volume graph, a steeper slope indicates a higher compliance, and so a younger age)

Question 17

What name is given to the effect of arterial compliance? Why is this?

Answer 17

The effect of arterial compliance is often called the “Windkessel effect”
This is because it is similar to the effect of having an air filled ‘buffer’ chamber called a Windkessel in a water pump.
This chamber evens out the flow of water in a sprayer.
The walls of the aorta and elastic arteries distend when the blood pressure rises during systole and recoil when the blood pressure falls during diastole.
There is a thus net storage of blood during systole which discharges during diastole.
The distensibility of the large elastic arteries is therefore analogous to a capacitor.

Question 18

How does peak systolic blood pressure change as a person ages? What does this cause?

Answer 18

As people age the peak systolic blood pressure increases due to the decreased arterial compliance; a high peak pressure puts extra strain on arteries and can lead to damage.

Question 19

Why do we need a blood pressure of 120/80 mmHg?

Answer 19

If the blood pressure is high, then the flow through a particular organ can be regulated by relaxing or constricting its input arterioles.
An arteriole is analogous to a water tap; if the water pressure is high, flow is proportional to tap opening; so flow is controlled by vasoconstriction.

Question 20

How is local flow calculated?

Answer 20

Local flow = pressure/local resistance

Question 21

What does a high blood pressure ensure?

Answer 21

Ensures that local dilation of arterioles is effective in increasing local blood flow

Question 22

Due to Poiseuille’s Law, what do small changes in arterial diameter produce?

Answer 22

They produce large changes in flow

Question 23

What is the equation for Poiseuille’s Law?

Answer 23

Q = (π * Δp * r4) / ( 8 * μ * l)
where:
Q - Flow rate (m³/s);
π - Pi
μ - Viscosity of blood (Pa * s) (also written as n)
r - Radius of the pipe;
L - Length of the tube; and
Δp - Pressure change (Pa).

Question 24

What is the meaning of Poiseuille’s law?

Answer 24

At a fixed pressure differential between the two ends of a rigid tube, flow is proportional to (radius of tube)^4
In other words if you keep pressure constant but double the radius you increase flow by 2⁴ (16) times the previous level.
In a rigid tube this equation applies to the flow at all times.
In a tube with compliance the average flow is controlled by Poiseuille’s law.
Small increases in radius increase flow as r⁴.
Similarly small decreases in diameter reduce flow as r⁴.

Question 25

What is an atheroma and how does it affect blood flow?

Answer 25

Atheroma = fatty deposit on inside of an artery
Because of Poiseuille’s Law, even a small atheroma can dramatically decrease blood flow and render tissue hypoxic
An advanced atheroma would reduce blood flow to perhaps 5% of normal

Question 26

What is cardiac output?

Answer 26

Cardiac output = heart rate x stroke volume
The total blood flow out of the heart (litres/min) is the cardiac output.
Cardiac output is a key parameter in heart function.
Measurement of C.O. to is essential to diagnose many heart problems.
Since the heart is a piston-type pump, its output will be the stroke volume multiplied by the heart rate

Question 27

Why did it used to be difficult to estimate stroke volume? How is it measured nowadays?

Answer 27

We can measure heart rate from the pulse or from the ecg, but In the past it was very difficult to get an estimate of stroke volume. Indirect methods in particular the Fick principle were used.
However nowadays we can measure stroke volume directly from echocardiography.

Question 28

What are typical values pf heart rate, stroke volume and cardiac output?

Answer 28

Heart rate = 70 bpm
Stroke volume = 70 ml
Therefore CO = 4900 ml/min

Question 29

What is another method to measure cardiac output to the body? How does it work?

Answer 29

Transoesophageal Doppler ultrasonography is another method to measure cardiac output to the body.
An ultrasound probe is inserted into the oesophagus to mid-thoracic level.
The probe can be used to measure blood velocity in the first (ascending) part of the aorta.
The cross-section size of the patient’s aorta can be measured directly by pulsed ultrasound or estimated using a nomogram based on patient age, height and weight.
If we know the blood velocity and the cross-sectional area of the aorta, we can work out the cardiac output.

Question 30

How can we calculate cardiac output from blood velocity and cross-sectional area of the aorta?

Answer 30

Cardiac output = Flow through aorta = (cross section area aorta) x blood velocity
E.g. aortic velocity 500 cm/min , aortic cross-section area 10 cm2, C.O. = 500 x 10 cm3/min =5000 cm3/min =5000 ml/min = 5 litres/min

Question 31

What are the benefits of Doppler ultrasound?

Answer 31

• Doppler ultrasound is non-invasive, accurate and inexpensive; it has high levels of reliability and reproducibility.

Question 32

What factors affect heart rate?

Answer 32

Autonomic innervation
Hormones
Fitness levels
Age

Question 33

What factors affect stroke volume?

Answer 33

Heart size
Fitness levels
Gender
Contractility
Duration of contraction
Preload (EDV - end diastolic volume)
Afterload (resistance)

Question 34

Where does the cardiac output go? How much to the brain, kidneys and heart? What percentage do these organs take up? How much is left and where does this go?

Answer 34

Your BRAIN needs about 700 ml per minute of blood (14% resting Cardiac Output)
Your KIDNEYS take about 1250 ml/min (25% resting C.O.).
Your HEART needs about 200 ml per minute of blood (4% resting C.O.)
These three organs at rest take up about 43 % C.O.
After heart, brain and kidneys have had their share of the cardiac output only about 2.7 L/min is available for the rest of the body.
Where this goes depends on your digestive state.
After a meal, at least 1.5 L/min goes to the gut (about 33% C.O.) to allow for digestion, leaving only about 24% (1.2 L/min) of the cardiac output for all the muscles and skin.

Question 35

What happens to cardiac output during exercise?

Answer 35

During exercise blood flow through the exercising muscles can increase more than 10 fold, and in the some athletes, up to 15- to 25-fold.
During exercise, cardiac output increases more than THREE fold to cope with increased demand for oxygen.

Question 36

What mechanisms increase cardiac output during exercise?

Answer 36

Since cardiac output = stroke vol. x heart rate, we can increase cardiac output either by increasing heart rate (up to about 2.5 times resting) and/or increasing stroke volume (up about 1.5 times, from about 70 ml to 100ml).
A normally fit young man or woman should be able to increase their resting output about (2.5 x 1.5), I.e. 3.75 times. 3.75 x 5 = (approx) 19 l/min
Some endurance athletes have a max C.O. >25 l/min

Question 37

What does Poiseuille’s Law state about the relationship of blood flow and viscosity?

Answer 37

Poiseuille’s law states that the flow is inversely proportional to the viscosity of the fluid

Question 38

What depends on both the viscosity of blood as well as diameter of the arterioles?

Answer 38

Afterload

Question 39

What mainly determines the viscosity of the blood?

Answer 39

The viscosity depends mainly on the haematocrit (proportion of red cells in blood, normally ~45%.
If the haematocrit is too high, the viscosity is too high and the heart has to work much harder to pump the blood around the body.
On the other hand if the haematocrit is too low, not enough oxygen is transported.
The viscosity also depends on the mechanical properties (mainly the deformability) of the red cells (erythrocytes)

Question 40

Why could red cells moving through capillaries be a problem?

Answer 40

Capillaries may be 5 µm or less, smaller than the diameter (7 µm) of the red cells, and so the red cells have to bend or deform to get through.

Question 41

What is crucial for the total peripheral resistance in the arterial tree to be kept low? What is the result of this not being right?

Answer 41

Flexibility of the erythrocytes
If the cells become less flexible then flow resistance goes up and the tissues can become poorly perfused and hypoxic.
Also if the cells are too large or malformed (eg as in sickle cell disease) they can block up the capillary and compromise oxygen delivery. They may also rupture and release haemoglobin into the plasma; this is haemolysis.

Question 42

What are the primary determinants of blood viscosity?

Answer 42

Haematocrit, red blood cell deformability, and to a lesser extent red blood cell aggregation and plasma viscosity

Question 43

One unit increase in haematocrit can cause what increase in blood viscosity? What happens to their relationship as haematocrit increases?

Answer 43

4%

As haematocrit increases, their relationship becomes more sensitive

Question 44

What happens when the haematocrit rises to 60-70%? What disease state does this happen with and what can it be caused by?

Answer 44

The blood viscosity can become as great as 10 times that of water, and its flow through blood vessels is greatly retarded because of increased resistance to flow.
This can lead to end organ failure.
This happens in polycythemia, which can be due to an excessive production of red blood cells (“absolute polycythemia”) or a decrease in the volume of plasma (“relative polycythemia”)

Question 45

What do arterioles’ thinner walls enable them to do? How do they manage to sustain the arterial pressures within these thin walls?

Answer 45

Enables them to contract or relax efficiently

Sustain pressure due to Laplace’s Law

Question 46

What does Laplace’s Law state?

Answer 46

The pressure that an elastic vessel can withstand depends on the tension produced in the walls by their elasticity divided by the radius (diameter) of the vessel.
In a CYLINDER (eg a blood vessel) the pressure withstood is proportional to T/R
In a SPHERE (eg an alveolus) the pressure withstood is proportional to T/2R

Question 47

What is the equation of Laplace’s Law? What does this mean?

Answer 47

P(withstood)=k(T/R)
The smaller the radius of a vessel, the greater the pressure that a given wall strength can withstand.
Thus small diameter arterioles only need thin walls to withstand normal arterial pressures.

Question 48

What are the characteristics of capillaries and why are they beneficial?

Answer 48

Small diameter means they can withstand fluid pressures of >30mm Hg using only the small tension generated by the basement membrane
No smooth muscle in their walls, as muscle would impede exchange of fluids and gases

Question 49

What happens if an artery wall weakens or gets a tear?

Answer 49

Its radius increases and so the balancing pressure (from Laplace’s law) that the elastic tissue generates is less.
THE WALL BALLOONS OUT and this further reduces the effectiveness of the wall to withstand the pressure.
Eventually an aneurysm may occur.
There is often a local INFLAMMATION reaction in an atheroma which causes destruction of elastin fibres in the artery wall, weakening it and making it more prone to stretch and form an aneurysm when there is a blood pressure spike.

Question 50

Why and where do aneurysms most commonly occur?

Answer 50

Aneurysms often occur just before branch points in arteries.
Due to the pulsatile flow of blood, pressure waves can reflect from the branch point, setting up eddies or vortices in the blood.
These eddies can damage the endothelial wall, leading to local inflammation.
This causes destruction of elastin fibres in the artery wall, weakening it and making it more prone to stretch if there is a spike in blood pressure
Hypertension and artery disease are major risks factors for aneurysm formation.

Question 51

In which arteries are aneurysms most common and why?

Answer 51

Aneurysms occur commonly in the cerebral arteries, with an estimated frequency of 1-8% in the general population.
Cerebral arteries are much more convoluted than arteries in other organs. This “twistiness” contributes to extra stress on the walls and the formation of ‘berry’ aneurysms at branch points.

Question 52

Which are the 3 ways in which arteries can present?

Answer 52

They can be discovered incidentally during MRI or angiography, they can rupture causing subarachnoid brain haemorrhage, or they can present with symptoms of mass effect on neural structures.

Question 53

How can cerebral aneurysms be treated?

Answer 53

Cerebral aneurysms can nowadays be treated by clipping or coiling.
In coiling a fine wire is pushed into the swollen artery to form a coil.
Blood clots around the coil; this takes the pressure off the wall and forms a stable structure