Lung statics 2

Question 1

What are the 3 determinants of lung volume?

Answer 1

1. pulmonary compliance
2. chest wall compliance
3. respiratory muscles

Question 2

Define the following:
Pao
Pbs
Palv
Ppl
Pab

Answer 2

Pao: Pressure at the airways opening (mouth)
Pbs: Pressure on the body surface
Palv: Alveolar pressure
Ppl: Intrapleural pressure
Pab: Abdominal pressure

Question 3

Define:
trans-pulmonary pressure
trans-lung pressure
trans-chest wall pressure
trans-respiratory system pressure

Answer 3

1. Trans-pulmonary pressure: Pao-Ppl
2. Trans-lung pressure: Palv-Ppl
3. Trans-chest wall pressure: Ppl - Pbs
4. Trans-respiratory system pressure: Pao-Pbs

Question 4

What is compliance?

Answer 4

A measure of stiffness. It is the slope of the pressure-volume curve.

Question 5

What is the stiffness of highly compliant lungs?
What is the stiffness of non-compliant lungs?

Answer 5

Greater compliance = looser lung
Lower compliance = stiffer lung

Question 6

What is the formula for compliance?

Answer 6

Compliance is the slope of the pressure-volume curve:
C = ΔV/ΔP

Question 7

When there is no flow, the pressure at the airway opening should be the same as …

Answer 7

alveolar pressure

Question 8

Compliance is obtained using the … pressure.

Answer 8

transpulmonary (Ptp) (Pao-Ppl)

Question 9

Transpulmonary pressure (Ptp) is used as a measure of…

Answer 9

the elastic recoil pressure (Pel) of the lungs and airways

Question 10

What instrument is used to measure compliance? How does it work?

Answer 10

An esophageal balloon (compliance) catheter is swallowed and measures the pressure inside the esophagus, since the esophageal pressure reflects the pleural pressure.

Question 11

Which requires more pressure to exert a change in volume in the lungs?
a) inspiration
b) expiration

Answer 11

a) inspiration
(see graph in your slides!)

Question 12

Why is the pressure-volume curve different between inspiration and exhalation? What is this difference called?

Answer 12

It takes significantly more pressure to exert a change in volume in the lungs during inspiration compared to expiration. This difference is called “hysteresis”.

Question 13

How does emphysema/COPD affect elastic recoil of the lungs?

Answer 13

Emphysema/COPD are obstructive diseases that lead to a reduction in the elastic recoil of the lungs (i.e. increased compliance). Therefore, it takes less pressure to exert a change in volume of the lungs

Make sure you can draw the emphysema PV curve compared to normal.

Question 14

How does pulmonary fibrosis affect elastic recoil of the lungs?

Answer 14

Pulmonary fibrosis leads to the thickening and stiffening of lung tissue, resulting in increased lung stiffness (i.e. decreased compliance). Therefore, it takes more pressure to exert a change in volume of the lungs.

Make sure you can draw the pulmonary fibrosis PV curve compared to normal.

Question 15

What are the 3 determinants of compliance?

Answer 15

1. Tissue forces (lung)
2. Surface tension
3. Compliance of the chest wall

Question 16

What do tissue forces in the lung arise from?

Answer 16

From the elastin-collagen-proteoglycan network of the lung tissue and the elastic recoil of the lung tissue.

Question 17

Lungs have a tendency to want to…

Answer 17

deflate

Question 18

What can decrease tissue forces in the lung?

Answer 18

Destruction of lung parenchymal architecture by diseases like emphysema. Lung tissue loses its elastic recoil and becomes more compliant.

Question 19

What can increase tissue forces in the lung?

Answer 19

Lung scarring (fibrosis). Lung parenchyma thickens and stiffens due to fibrosis, making it stiffer and less compliant.

Question 20

Surface tension occurs at the …. of the alveoli.

Answer 20

air-liquid interface

Question 21

Surface tension arises from…

Answer 21

the tendency of the liquid molecules to stick together rather than to associate with air molecules - will cause the alveoli to collapse.

Question 22

What prevents the alveoli from collapsing under their surface tension?

Answer 22

Pulmonary surfactant, which modifies surface tension

Question 23

What is surfactant composed of?

Answer 23

Small layer of phospholipids in the alveoli which reduces surface tension.

Question 24

What happens when lungs are filled with saline?

Answer 24

When lungs are filled with saline rather than air, the air-liquid interface in the alveoli is absent, so surface tension does not develop. The collapsing pressure in the alveoli is therefore reduced, making it easier to inflate the lungs.
In other words, elastic recoil decreases, making the lung more compliant.

Question 25

Why is surface tension important for adequate function of the lungs?

Answer 25

Surface tension creates a collapsing pressure in the alveoli that contributes to the elastic recoil of the lungs during inflation. Without surface tension, the lungs would be much more compliant.

Question 26

For a given surface tension, which alveolus is more likely to collapse?
a) alveolus with smaller radius
b) alveolus with greater radius

Answer 26

a) alveolus with smaller radius

Question 27

The smaller the radius of the alveoli, for the same surface tension, the … the pressure inside the alveoli
a) greater
b) smaller

Answer 27

a) greater

Smaller alveolus = greater intra-alveolar pressure

Question 28

Describe Laplace’s law on surface tension and alveolar instability

Answer 28

P=2T/r

For a given surface tension…
1. Smaller alveoli (smaller r) will generate a higher pressure
2. Larger alveoli (greater r) will generate a lower pressure

Therefore, smaller alveoli have a higher pressure than larger alveoli, so they have a tendency to collapse into larger alveoli (alveolar instability).

Question 29

If you are at RV (maximum forceful expiration), how does the chest wall react?

Answer 29

Chest wall wants to expand

Question 30

What are the two vital properties of pulmonary surfactant?

Answer 30

1. Surfactant lowers the surface tension across all alveoli, making it easier to inflate and deflate the lungs with each breath (increases compliance of the lung)
2. Surfactant promotes alveolar stability (reduces surface tension in smaller alveoli more than in larger alveoli, preventing them from collapsing into larger alveoli).

Question 31

At functional residual capacity (FRC), the lungs and the chest wall are…

Answer 31

At FRC, the lungs and chest wall are in a state of mechanical equilibrium.

The chest wall wants to expand outward, The lungs want to collapse inward. At FRC, these two opposing forces are equal and opposite (at equilibrium).

Question 32

As opposed to the lungs, the chest wall always has a tendency to …

Answer 32

expand outward

Question 33

At residual volume (RV), the lungs and chest wall and lungs are no longer at equilibrium. Elaborate.

Answer 33

At RV, the outward elastic recoil of the chest wall is greater than the inward elastic recoil of the pulmonary tissue.
The respiratory system wants to reinflate.

Question 34

At total lung capacity (TLC), the lungs are no longer at equilibrium. Elaborate.

Answer 34

At around 60-80% of TLC, both the chest wall and the lungs want to collapse inward (inward elastic recoil).
The respiratory system wants to deflate.

Question 35

What is the resting position of the chest wall?

Answer 35

Around 60-80% of the TLC.

*Review, the blue, green and red graph in your slides.

Question 36

What determines the residual volume in young individuals?

Answer 36

The limitations of their chest wall - while lungs recoil inward, the chest wants to recoil outward, so there is a limit as to how much air you can expire until you can no longer overcome the force of the chest recoil.

Question 37

What determines the residual volume in older adults?

Answer 37

Premature airway closure - with age, we lose elastic recoil in the lungs, which causes smaller airways to collapse. It therefore takes longer to empty the lungs (increased RV).

Question 38

Other than age, what can influence the residual volume between individuals?

Answer 38

Limit of expiratory force (i.e. respiratory muscle strength).

If some condition is causing muscle weakness, you will be too weak to breathe out correctly, leading to an increased RV.

Question 39

How does gravity affect lung compliance? Why do basal (lower) zones have smaller resting volumes.

Answer 39

Gravity causes the basal zones of the lungs to be compressed by the lung tissue above. These compressed zones therefore have smaller lung volumes at rest.

Question 40

Why is the lower zone of the lungs more easily inflated?

Answer 40

Pleural pressure in those areas is less negative, so these zones lie on the steeper part of the PV curve (higher compliance). Therefore, small increases in pressure result in large increases in volume.

More compressed alveoli resist expansion less than more expanded alveoli.

Question 41

At TLC, both the upper and lower part of the lung are maximally inflated….

Answer 41

Both regions are located at the flatter part of the PV curve (the lungs resist expansion more, it takes more pressure to create changes in volume).

Larger volumes
Low compliance
Not easily ventilated

Question 42

At RV, both the upper and lower part of the lungs are maximally deflated…

Answer 42

Both regions are located at the steeper part of the curve (both regions are more compliant to expansion, it takes less pressure to create significant changes in volume).

Smaller volumes
Greater compliance
Easily ventilated

Question 43

At FRC, the upper part of the lungs is more inflated than the lower part of the lungs. How does this affect ventilation of the upper vs lower part of the lungs?

Answer 43

Lower zones of the lungs are ventilated more easily than upper part of the lungs, because the lower zones are located on the steeper part of the curve, whereas the upper zones are located on the flatter part of the curve.