Session 5

Question 1

Define flow and velocity

Answer 1

Flow - the volume of fluid passing a given point per unit time
Velocity - the rate of movement of fluid particles along the tube

Question 2

Describe laminar flow

Answer 2

There is a gradient of velocity from the middle to the edge of the vessel. The fluid moves in concentric layers.

Question 3

Describe turbulent flow

Answer 3

Follows a high mean velocity, low viscosity and/or irregular vessel lumen. The velocity gradient breaks down, fluid tumbles over and flow resistance is greatly increased.
It can be heard upon auscultation as a bruit if cardiac valves become sclerotic or when atherosclerotic blockages obstruct an artery.

Question 4

For a vessel with a constant pressure driving flow, what does the mean velocity depend on?

Answer 4

The viscosity of the fluid and radius of the tube

Question 5

Define viscosity

Answer 5

The extent to which fluid layers resist sliding over each other. It determines the slope of the gradient of velocity in the vessel.

Question 6

Describe the effects of changes in tube diameter on flow rate

Answer 6

At a constant viscosity (slope gradient), the wider the tube the faster the middle layers move. Hence mean velocity is proportional to cross sectional area.

Question 8

Why are children’s airways more prone to compromised flow than adults?

Answer 8

A 2mm reduction in a child’s trachea, caused by an inserted tube, reduces flow more than a 2mm reduction in the diameter of an adults trachea.

Question 9

What can cause hyperviscosity syndrome?

Answer 9

Abnormally high plasma protein levels or rbc/wbc count

Question 10

Why can functional cardiac murmurs be heard in severe anaemia?

Answer 10

As a result of high blood velocities and/or reduced viscosity of the blood due to low rbc count

Question 11

Give the equation for pressure difference

Answer 11

Pressure difference = flow x resistance

Resistance increases as viscosity and length increase and as r^4 decreases

Question 12

Why do capillaries offer little collective resistance despite being narrow?

Answer 12

Due to their parallel arrangement their individual resistances do not add up

Question 13

Describe the pattern of flow resistance and pressure over the systemic circulation

Answer 13

Over the whole resistance, flow is the same at all points.
Arteries are low resistance, so their pressure drop is small.
Arterioles are high resistance so their pressure drop is high.
There is a small pressure drop in capillaries, venules and veins.

Question 14

Describe the relationship between arteriole resistance and arterial pressure

Answer 14

The higher the resistance of the arterioles at a constant flow, the higher the arterial pressure.
The higher the cardiac output at a constant arteriole resistance, the higher the arterial pressure.

Question 15

How does distensibility of the blood vessels affect the relationship between flow and pressure?

Answer 15

As the vessel stretches, the diameter of the lumen increases so resistance falls and flow increases.
The higher the pressure in a vessel, the easier it is for blood to flow through it.

Question 16

How does distensibility of blood vessels produce the property of capacitance?

Answer 16

As vessels widen, more blood transiently flows in than out and vice versa. This allows distensibility vessels to store blood (mostly veins).

Question 17

What is Poiseuille’s law?

Answer 17

When blood flow is steady and laminar in blood vessels larger than arterioles, the blood flow is:
Proportional to the pressure difference
Proportional to the fourth power of the radius (both velocity and flow are proportional to area)
Inversely proportional to the length of the vessel
Inversely proportional to the viscosity of the blood

Question 18

What is the compliance of a vessel?

Answer 18

The ability to distend and increase volume due to pressure increase

Question 19

Define total peripheral resistance

Answer 19

The sum of the resistance of all the peripheral vasculature in the systemic circulation

Question 20

What is the advantage in having distensible vessel walls in terms of pressure waves?

Answer 20

Compliance acts to store mechanical energy during systole and dissipates energy more gradually over diastole. It dampens the pulsatile nature of the systolic pressure wave.

Question 21

What is the pulse pressure?

Answer 21

The difference between diastolic and systolic pressures. At rest ~40mmHg

Question 22

How is the average pressure calculated?

Answer 22

Diastolic pressure plus 1/3 pulse pressure

Question 23

What is the pulse wave?

Answer 23

Contraction of the ventricles generates a pulse wave, which propagates along the arteries faster than blood. This is felt as a pulse where arteries come close to the surface and are pushed against a reasonably hard surface.

Question 24

Describe flow autoregulation

Answer 24

Vasomotor tone can be decreased due to metabolic activity and release of local vasodilator metabolites (H+, K+, adenosine). Increased blood supply removes metabolic factors with a gradual return to vasomotor tone by SNS.

Question 25

What is reactive hyperaemia?

Answer 25

Occurs when circulation is cut off for ~minute, then restored. The local arterioles dilate maximally due to a build up of local metabolites.

Question 26

What is central venous pressure and what factors affect it?

Answer 26

The pressure in the great veins supplying the heart.

Depends on return of blood from the body, pumping of the heart, gravity and muscle pumping.

Question 27

Give the equation that relates flow to velocity

Answer 27

Flow = velocity x area