Principles of Haemodynamics

Question 1

Define haemodynamics

Answer 1

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

Question 2

Which factors affect haemodynamics

Answer 2

1) Force- controlled by cardiac contraction
2) Work- isovolumetric contraction then ejection
3) Pressure- difference in pressure between aorta and veins
4) Compliance- ability of arteries able to stretch
5) Resistance- arterioles
6) Flow Velocity- how blood slows down when it reaches the capillaries

Question 3

Central venous pressure

Answer 3

Blood pressure in the vena cavae, near the right atrium

CVP reflects the amount of blood returning to the heart

CVP also reflects ability of heart to pump blood back into arteries

Question 4

How can you increase cardiac output in terms of the venous system

Answer 4

Blood reservoirs in veins are mobilized through innervation from ANS

Veins contract

Pulmonary vein contracts》more blood in left ventricle》stretch》Starling’s law 》increases stroke volume so more blood ejected per heart beat

Question 5

Darcy’s law

Answer 5

Role of pressure in blood flow

Darcy’s law is flawed; blood flow is not just determined by pressure as blood can flow against a pressure gradient, as well as flow when there is no pressure at all

Question 6

Equation to calculate blood flow

Answer 6

Pressure difference
Blood flow= ——————————‐———
Resistance

Q = (P1-P2) / R

If resistance goes up, amount of blood flow goes down

Question 7

Bernoulli’s law

Answer 7

Role of pressure, kinetic energy, potential energy in blood flow…not just pressure!

Note: kinetic energy= momentum of blood

Note: potential energy= effect of gravity

Question 8

Define blood flow

Answer 8

Volume of blood flowing in a given time

ml/min

Question 9

Define perfusion

Answer 9

Blood flow per given mass of tissue

ml/min/g

Question 10

How does cross sectional area affect the velocity of blood flow

Answer 10

High CSA means slow velocity (speed)
Capillaries have a high CSA

Arterioles lead up to capillaries, their CSA begins to gradually increase thereby slowing down blood flow/velocity

In contrast, venules unite to form a vein post capillary, CSA begins to decrease again, this increases blood flow and velocity

Question 11

Equation for volume flow

Answer 11

Volume flow = Velocity x Area

Q = V x A

Question 12

Laminar blood flow pattern

Answer 12

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• laminar blood flow is mainly found in arteries, arterioles, venules and veins
• blood flow is fastest at the centre (max velocity)
• RBC move through centre
• blood flow slow at walls (zero velocity) due to friction

Question 13

Turbulent blood flow pattern

Answer 13

🌊🌊🌀🌀🌊🌊🥏

• turbulent blood flow is too fast, generates whirlpools and vortexes due to increased pressure and velocity
• can hear it using a stethoscope
• turbulent blood flow seen in ventricles, aorta, and sites of atheroma where extra pressure is needed

Question 14

Bolus blood flow pattern

Answer 14

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🔴➡️ 🔴➡️ 🔴➡️
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• RBC have a larger diameter than the diameter of capillaries so therefore move in single file and deform slightly
• uniform velocity, little internal friction, low resistance
• bolus blood flow found in capillaries

Question 15

Reynolds number

Answer 15

The number laminar flow needs to exceed to become turbulent flow (2000)

Re = (fluid mass density x velocity x diameter) / viscosity

Question 16

Arterial blood flow

Answer 16

• blood exerts pressure onto vessel walls
• left ventricle contracting also exerts pressure on vessel
• arterial pressure decreases in systemic circulation with distance from left ventricle

Question 17

Name 4 factors that affect arterial blood pressure

Answer 17

1) cardiac output
2) peripheral resistance
3) properties of arteries
4) blood viscosity

Question 18

Name 4 interactions in arterial blood pressure

Answer 18

1) systolic pressure during ejection
2) diastolic pressure during relaxation
3) pulse pressure (radial artery in forearm)
4) mean blood pressure

Question 19

How does the aorta minimize pressure difference

Answer 19

• it has elastin which is able to expand and recoil
• 60 to 80% of stroke volume is stored in aorta during LV ejection (contraction)
• energy stored in stretched elastin then released into blood during LV relaxation

Question 20

How to calculate pulse pressure

Answer 20

stroke volume / compliance

Question 21

what is dicrotic notch

Answer 21

aortic valve closes

Question 22

How to calculate compliance

Answer 22

change in volume / change in pressure

Question 23

Atherosclerosis

Answer 23

• disease where plaque builds up inside arteries
• aorta becomes less stretchy, loses its compliance
• low compliance during exercise would result in pressure being too high (systolic pressure and pulse pressure is increased disproportionally)

Question 24

why are elderly people prone to hemorrhagic strokes

Answer 24

• as we get older, our compliance decreases
• any modest increase in stroke volume can lead to a huge increase in arterial pressure
• artery wall becomes weakened leading to formation of an aneurysm
• aneurysm bulge ruptures, blood supply to brain is cut off

Question 25

why is pulse pressure at the aorta smaller than the pulse pressure further down the arterial tree?

Answer 25

aorta has a high compliance, as the distance from the aorta increases, the compliance decreases

areas which have a low compliance, such as further down the arterial tree, have a high pulse pressure

Question 26

name 2 ways to detect pulse pressure

Answer 26

radial artery in wrist

carotid artery in side of neck

Question 27

what is the mean blood pressure?

Answer 27

93.3 mmHg, area under curve

diastolic pressure (80) + 1/3 of pulse pressure (13.3)

influenced by various factors e.g. age, disease, exercise, emotions like anger and fear, and even sleep