Haemodynamics I

Question 1

Recall Darcy’s law.

Answer 1

Flow = pressure difference/ resistance to flow

Flow = Pa - CVP/ TPR

Question 2

Recall Bernoulli’s law.

Answer 2

Flow = pressure (PV) + ρV^2/2 + ρgh

Question 3

Define blood flow, perfusion and velocity of blood flow.

Answer 3

Blood flow - volume of blood flowing per unit time (ml/min).

Perfusion - blood flow per given mass of tissue (ml/min/g).

Velocity of blood flow - blood flow divided by cross-sectional area through which the blood flows (cm/s).

Question 4

Describe and explain how blood velocity differs throughout the circulation.

Answer 4

Velocity highest in aorta because lowest total cross-sectional area of any vascular bed. Capillaries have highest total cross-sectional area:

Aorta > arteries > arterioles > capillaries
Vena cave > veins > venules > capillaries

Question 5

Describe laminar blood flow and where it occurs.

Answer 5

Occurs in most arteries, arterioles, veins and venules.

“Concentric shells”:
Zero velocity at walls due to molecular interactions.
Maximum velocity at centre.

RBCs driven into centre of narrow vessels, increasing velocity of flow.

Question 6

Describe bolus flow in capillaries.

Answer 6

RBCs have larger diameter than capillaries - RBCs move in single file.

Plasma columns are trapped between RBCs.

Uniform velocity.

Little internal friction - very low resistance.

Question 7

Describe turbulent flow of blood and where it occurs.

Answer 7

Occurs in ventricles, aorta, and other vessels if atheroma is present.

Non-linear flow with local currents - due to increased pressure and velocity.

Question 8

Explain the significance of Reynold’s number and describe the equation from which it is derived.

Answer 8

Reynold’s number (Re) denotes the likelihood of turbulent flow occurring in a blood vessel. Turbulent flow occurs when Re > 2000. A high Re value is caused by bruits, ejection murmurs.

Re = ρVD / μ.

Re = (density x velocity x diameter) / viscosity

Question 9

Name 4 factors that affect arterial blood pressure.

Answer 9

1. Cardiac output.
2. Properties of arteries - aorta.
3. Peripheral resistance - arterioles.
4. Blood viscosity - haematocrit.

Question 10

Arterial blood pressure involves interactions between which 4 key relationships?

Answer 10

1. Systolic pressure.
2. Diastolic pressure.
3. Pulse pressure.
4. Mean blood pressure.

Question 11

Explain the role of aortic elasticity during left ventricular ejection and diastole.

Answer 11

During LV ejection:
60-80% of SV is stored in aorta and arteries as they expand.
Energy is stored in stretched elastin.

During LV diastole:
Energy is returned to the blood as the walls of the aorta and arteries contract (recoil).
This sustains diastolic blood pressure and flow when the heart is relaxed.

Question 12

What is pulse pressure? Give two equations.

Answer 12

Pulse pressure = systolic pressure - diastolic pressure.

Pulse pressure = stroke volume / compliance.

Question 13

Changes in pulse pressure indicate changes in stroke volume. Explain why.

Answer 13

The greater the stroke volume, the greater the stretch of arteries, meaning increased systolic pressure. Increased systolic pressure > increased pulse pressure.

Question 14

How does decreased compliance affect systolic and pulse pressure?

Answer 14

Increases in stroke volume increase systolic and pulse pressure disproportionally.

Question 15

What is the main cause of stiff arteries (decreased compliance)?

Answer 15

Arteriosclerosis due to old age.

Question 16

Explain how pulse pressure changes as arteries become more distal (further away from the aorta). How does this relationship change with age?

Answer 16

In young people pulse pressure increases in distal arteries due to increased stiffness.

Age increases the stiffness of vessels, particularly the aorta. This means that a high pulse pressure is present throughout the arterial tree.

Question 17

How is pulse pressure altered in aortic stenosis and aortic regurgitation?

Answer 17

Aortic stenosis - narrowing of aortic valve, poor ejection. Slower upstroke, smaller peak in pulse pressure.

Aortic regurgitation - leaky aortic valve. Fast upstroke, larger peak, poor diastolic run off - due to blood entering aorta/ventricles during diastole.

Question 18

How is mean blood pressure calculated?

Answer 18

Diastolic blood pressure + 1/3 pulse pressure.