Topic 2 Haemodynamics

Question 1

What are the three layers of an artery?

Answer 1

Tunica intima
Tunica media
Tunica adventitia

Question 2

Describe the tunica intima

Answer 2

The innermost layer of an artery. Consists of endothelial cells and a basement membrane. Function is to prevent clots and to respond to local changes in blood flow by controlling artery diameter

Question 3

Describe the tunica media

Answer 3

Predominantly composed of smooth muscle arranged in circular and longitudinal orientation. Controls vessel diameter and contributes to the elasticity of the blood vessel

Question 4

Describe the tunica adventitia

Answer 4

Outermost layer of the artery. Composed mostly of collagen and elastin. Gives support to blood vessels and contributes to elasticity

Question 5

Describe how the tunica media changes in arteries

Answer 5

The tunica media is relatively thick in proximal arteries while thinning in distal arteries and arterioles

Question 6

What is poiseuille’s equation?

Answer 6

P1 – P2 = 8ηlQ/ π r4

P1 – P2 : Tube Pressure difference
η: viscosity of the liquid
l: length of the tube
r: radius of the tube
Q: Flow in the tube

Question 7

How does radius affect flow and pressure?

Answer 7

Viscosity and length of an artery can be considered constant, so radius exerts a significant influence on flow and pressure (especially since the radius is raised to the fourth power). Doubling the radius will allow 16x the blood flow.

Question 8

What is hydraulic resistance?

Answer 8

Applying force to fluid through a tube will produce some resistance. Opposing forces to fluid motion is determined by the pressure applied to the tube and the flow rate which results

Question 9

What is total resistance?

Answer 9

Total resistance is the sum of all individual resistances over changing diameters

Question 10

What is the powerful influence on resistance?

Answer 10

Changing diameter

Question 11

Describe a monophasic waveform

Answer 11

• continuous flow through diastole
• associated with high level of continuous flow in an artery
• can occur in normal arteries where distal vessels are dilated in organs that require constant flow such as renal arteries
• can occur in post-exercise states
• can occur distal to an obstruction

Question 12

Describe a biphasic waveform

Answer 12

• Forward velocity in systole and brief reversal during the early part of diastole
• associated with increasing constriction of the distal vessels which reduces the continuous flow and enhances strength of the reflected wave
• seen in arteries where distal circulation is in moderate degree of constriction such as leg and arm arteries

Question 13

Describe a triphasic waveform

Answer 13

• created by an increasing distal resistance causing a stronger reflected wave and reducing the level of continuous flow
• triphasic waveform has the later arrival of the reflected pulse or pulse from multiple reflected sites

Question 14

Describe laminar flow

Answer 14

Constant, steady, velocity profile.
As blood flows through an artery, friction caused by the wall of the artery slows the outermost lamina, and this in turn slows the layer next to it

Question 15

Describe turbulent flow

Answer 15

Blood speed increases until the cohesive nature of the lamina is disrupted by the increasing difference between speed of the adjacent lamina. When laminar flow is disrupted, blood flow loses some of its energy as heat and sound

Question 16

How does turbulent flow appear on a spectral display?

Answer 16

Spectral broadening, due to increase in velocities

Question 17

What is Bernoulli’s principle?

Answer 17

Bernoulli’s principle shows that energy is conserved between any 2 points in a tube assuming that no energy is lost due to friction against the artery wall.

Question 18

What is a critical stenosis?

Answer 18

A critical narrowing of an artery that results in a significant reduction in maximal flow capacity in a distal vascular bed.