Resp 17 - Lung mechanics

Question 1

How does TLC change in obstructive and restrictive disease?

Answer 1

Obstructive - increases because RV increases a lot (because alveoli have less recoil). Other volumes decrease.
Restrictive - decreases (every volume, especially RV)

Question 2

How does the change in volume per unit pressure change as you move further from FRC?

Answer 2

Change in volume per unit pressure DECREASES as you move further from FRC - basically gradient of curve decreases. This is because it is easier to breather around FRC and when you inspire/expire a lot it’s hard to change volume significantly. (sigmoid shaped curve)

Question 3

What is transrespiratory pressure?

Answer 3

The pressure between the inside of the airways and the outside atmosphere

Question 4

How does the pleural pressure change from FRC to a) end of tidal inspiration and b) end of tidal expiration?

Answer 4

End of tidal inspiration - more negative than FRC
End of tidal expiration - FRC (lungs passively recoil inwards to return the pleural pressure to FRC)
You can only get a more positive intrapleural pressure than FRC if you do FORCED expiration.

Question 5

What is the significance of the shape of the curve with regards to ease of tidal breathing?

Answer 5

Close to FRC, you get a large change in volume per unit pressure (gradient) so we can relatively easily inspire and expire in tidal breathing. The further we deviate from FRC, the more difficult it becomes to increase the volume.

Question 6

How does the volume-pressure curve change in obstructive and restrictive disease?

Answer 6

Obstructive - moves up and squished (FRC higher) - gradient is higher - larger volume for smaller pressure because tissue looses elasticity so takes up lots of volume (then unable to get it out)

Restrictive - moves down and spreads (FRC lower) - gradient lower, have to generate more pressure

Question 7

Describe negative pressure breathing referring to the alveolar pressure and volume.

Answer 7

The respiratory muscles work to decrease intrathoracic pressure and so the alveoli are pulled open thus increasing their volume and decreasing their pressure (generating negative pressure). This creates a pressure gradient between the alveoli and the atmosphere so air is drawn into the alveoli to re-establish the initial pressure. Once pressure is re established, lungs recoil creating positive alveolar pressure, thus air flows out.

Question 8

Define compliance.

Answer 8

The willingness of a material to distend under pressure.

Question 9

Define elastance.

Answer 9

The tendency of something to recoil to its original volume.

Question 10

In terms of change in volume per unit pressure, how is the ease of inflation of the lung different to deflation?

Answer 10

Inflation is more difficult that deflation (requires more pressure per unit change in volume) because the lungs have a natural tendency to recoil

Question 11

How is the ease of inflating and deflating a fluid-filled lung different to an air-filled lung?

Answer 11

It is much easier to inflate a fluid-filled lung - it is more compliant. The air-filled lung has a surfactant-air interface which has surface tension and hence decreases compliance. The fluid-filled lung has a fluid-water interface, which doesn’t have surface tension so the lung is more compliant.
(surface tension = when you have surface of water not exposed to opposing force like air)

Question 12

What type of cell produces surfactant?

Answer 12

Type II pneumocyte

Question 13

What is the role of surfactant?

Answer 13

It breaks up the water molecules, reduces surface tension and hence prevents alveolar collapse.

Question 14

What are the effects of surfactant on surface tension and compliance?

Answer 14

It reduces the surface tension and hence increases the compliance of the lungs

Question 15

What is conductance?

Answer 15

How well the airways will conduct and allow air to pass through.

Question 16

Why doesn’t the resistance of the airways increase as you go down the generations?

Answer 16

Because flow also decreases - you must consider flow not only the size of airways. Until 4th generation, airways get smaller so more resistance. However, when air can go through many many branches, resistance will decrease as flow is divided.

Question 17

When does peak resistance occur?

Answer 17

Around the 4th generation

Question 18

How do conductance and resistance change with increasing lung volume? Why does this happen?

Answer 18

Conductance increases and resistance decreases with increasing volume because when the lungs inflate, the airways dilate (i.e. the radius increases).

Question 19

Describe the phenomenon of interdependence with regards to alveoli.

Answer 19

The function of any single alveolus is dependent on the function of the alveoli around it.

Question 20

Describe what is considered a ‘stress point’ in obstructive and restrictive disease.

Answer 20

In a lung with obstructive and restrictive disease, there will be a border between normal lung tissue and lung tissue that is either resistance to stretch (restrictive) or wants to stretch too much (obstructive)

Question 21

Explain why forced expiration can lead to the collapsing of airways.

Answer 21

Forced expiration leads to a dramatic increase in pleural pressure. There is a gradient of pressure between the alveoli and the atmosphere. The sudden increase in pleural pressure could mean that at a point along the collapsible tubes, the pleural pressure exceed the internal pressure of the airways and hence the airways collapse.