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Flashcards in Flow Through Tubes Deck (25)
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1
Q

Define Pressure (P)

A

Force exerted per unit area (surface area)

2
Q

Define Pressure Gradient (∆P)

A

Difference in forces exerted (per unit area) at either end/side of an object

3
Q

Define Flow (Q)

A

The volume of liquid passing a given level of the circulation/airways per unit time

4
Q

Define Resistance

A

A force that tends to oppose the flow of a substance

5
Q

Define Velocity

A

The rate of movement of fluid particles along a vessel/airway

6
Q

Define viscosity

A

The property of resistance to flow in a fluid or semifluid

7
Q

How is a pressure gradient calculated? How can this be applied clinically?

A

∆P=QR (BP= CO x SVR)

8
Q

What is the relationship between flow and pressure gradient?

A

Flow is directly proportional to pressure gradient assuming all other things are equal

9
Q

Describe the relationship between resistance and radius if the flow is constant in laminar flow.

What is the clinical application of this?

A

Laminar flow means that the resistance is greatly affected by the radius of the tube (if flow is constant)

Narrowing of vessels/valves or airways means that the pressure gradient (i.e. blood pressure) has to be greatly increased to maintain constant flow. The velocity will also be increased, causing a wheeze in airways or bruits in blood vessels.

10
Q

What affects resistance? Give a clinical example

A

Resistance depends on the radius of the vessel/airway and the viscosity of the fluid

E.g. Bronchoconstriction in asthma, reduced flow of air entry.

11
Q

How is flow (Q) calculated? (Laminar flow)

What is the result of this?

A

Flow = pressure gradient/ resistance

(resistance= 8nl/πr4)

Q=∆Pπr4

8nl

Flow is directly proportional to r4

  • Radius doubled: flow increases 16 times
  • Radius halved: flow decreases 16 times
12
Q

Describe the relationship between flow, velocity and cross sectional area of the circulation/airways if the flow is constant.

A

If the flow is constant, velocity is inversely proportional to the cross-sectional area (V=Q/A) (A=𝜋𝑟2) (decrease in area= increase in velocity)

13
Q

What is laminar flow?

A

Fluid moves in organised layers through tube. Velocity will be highest in the centre of the tube and lowest at the outer edges.

14
Q

Describe the relationship between flow, velocity and cross sectional area if the pressure gradient (not the flow) is constant (in laminar flow)

A

If the pressure gradient is constant, a reduction in cross sectional area results in decreased velocity in laminar flow and a decrease in flow due to increased resistance from smaller vessel.

15
Q

What is Pouiselle’s Law?

A

The resistance of the vessel depends on its radius, its length and the viscosity of the fluid:

Resistance = 8nl/πr4

n=viscosity

l=length

r=radius

16
Q

What would happen to the velocity and pressure gradient if the radius of a tube was halved and the flow is kept constant?

A

Velocity would increase x4

Pressure gradient would increase x16

17
Q

What would happen to the velocity and flow if the radius of a tube was halved and the pressure gradient was kept constant?

A

Velocity would reduce x4

Flow would reduce x16

18
Q

What is turbulent flow?

What are the consequences of this clinically?

A

Breaking up of layers of laminar flow causing flow to become disordered.

Turbulent flow means greater resistance, can result in damage to the endothelium of blood vessels

19
Q

What makes flow more likely to be turbulent?

A

Increased velocity (from narrowed vessel)

Low viscosity (less adhesion between layers)

High blood vessel diameter (difficult for layers to maintain ordered flow)

20
Q

What is the effect of branching of the circulation on resistance?

A

Resistances combine in vessels that are in parallel rather than in series; the overall resistance is reduced.

The flow through each vessel/airway will be inversely proportional to its resistance.

(Arterioles have the highest resistance as they are arranged more in series)

21
Q

What happens if the resistance of resistance vessels (small arteries and arterioles) increases?

A

The heart must increase the pressure gradient by increasing stroke work to maintain cardiac output

22
Q

What is transmural pressure?

A

Transmural pressure = intravascular pressure - extravascular pressure.

Transmural pressure has to be >0 in order to keep blood vessels open

23
Q

What is the effect of resistance from increasing pressure in blood vessels?

A

Increased pressure causes decrease in resistance in blood vessels as they are distensible rather than rigid. Therefore increased pressure causes an increase in radius which reduces resistance.

24
Q

What is the advantage of distensibility of blood vessels?

A

Distensibility of blood vessels gives them capacitance, allowing them to store blood.

25
Q
A