Lung statics 2 Flashcards
What are the 3 determinants of lung volume?
- pulmonary compliance
- chest wall compliance
- respiratory muscles
Define the following:
Pao
Pbs
Palv
Ppl
Pab
Pao: Pressure at the airways opening (mouth)
Pbs: Pressure on the body surface
Palv: Alveolar pressure
Ppl: Intrapleural pressure
Pab: Abdominal pressure
Define:
trans-pulmonary pressure
trans-lung pressure
trans-chest wall pressure
trans-respiratory system pressure
- Trans-pulmonary pressure: Pao-Ppl
- Trans-lung pressure: Palv-Ppl
- Trans-chest wall pressure: Ppl - Pbs
- Trans-respiratory system pressure: Pao-Pbs
What is compliance?
A measure of stiffness. It is the slope of the pressure-volume curve.
What is the stiffness of highly compliant lungs?
What is the stiffness of non-compliant lungs?
Greater compliance = looser lung
Lower compliance = stiffer lung
What is the formula for compliance?
Compliance is the slope of the pressure-volume curve:
C = ΔV/ΔP
When there is no flow, the pressure at the airway opening should be the same as …
alveolar pressure
Compliance is obtained using the … pressure.
transpulmonary (Ptp) (Pao-Ppl)
Transpulmonary pressure (Ptp) is used as a measure of…
the elastic recoil pressure (Pel) of the lungs and airways
What instrument is used to measure compliance? How does it work?
An esophageal balloon (compliance) catheter is swallowed and measures the pressure inside the esophagus, since the esophageal pressure reflects the pleural pressure.
Which requires more pressure to exert a change in volume in the lungs?
a) inspiration
b) expiration
a) inspiration
(see graph in your slides!)
Why is the pressure-volume curve different between inspiration and exhalation? What is this difference called?
It takes significantly more pressure to exert a change in volume in the lungs during inspiration compared to expiration. This difference is called “hysteresis”.
How does emphysema/COPD affect elastic recoil of the lungs?
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.
How does pulmonary fibrosis affect elastic recoil of the lungs?
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.
What are the 3 determinants of compliance?
- Tissue forces (lung)
- Surface tension
- Compliance of the chest wall
What do tissue forces in the lung arise from?
From the elastin-collagen-proteoglycan network of the lung tissue and the elastic recoil of the lung tissue.
Lungs have a tendency to want to…
deflate
What can decrease tissue forces in the lung?
Destruction of lung parenchymal architecture by diseases like emphysema. Lung tissue loses its elastic recoil and becomes more compliant.
What can increase tissue forces in the lung?
Lung scarring (fibrosis). Lung parenchyma thickens and stiffens due to fibrosis, making it stiffer and less compliant.
Surface tension occurs at the …. of the alveoli.
air-liquid interface
Surface tension arises from…
the tendency of the liquid molecules to stick together rather than to associate with air molecules - will cause the alveoli to collapse.
What prevents the alveoli from collapsing under their surface tension?
Pulmonary surfactant, which modifies surface tension
What is surfactant composed of?
Small layer of phospholipids in the alveoli which reduces surface tension.
What happens when lungs are filled with saline?
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.
Why is surface tension important for adequate function of the lungs?
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.
For a given surface tension, which alveolus is more likely to collapse?
a) alveolus with smaller radius
b) alveolus with greater radius
a) alveolus with smaller radius
The smaller the radius of the alveoli, for the same surface tension, the … the pressure inside the alveoli
a) greater
b) smaller
a) greater
Smaller alveolus = greater intra-alveolar pressure
Describe Laplace’s law on surface tension and alveolar instability
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).
If you are at RV (maximum forceful expiration), how does the chest wall react?
Chest wall wants to expand
What are the two vital properties of pulmonary surfactant?
- 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)
- Surfactant promotes alveolar stability (reduces surface tension in smaller alveoli more than in larger alveoli, preventing them from collapsing into larger alveoli).
At functional residual capacity (FRC), the lungs and the chest wall are…
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).
As opposed to the lungs, the chest wall always has a tendency to …
expand outward
At residual volume (RV), the lungs and chest wall and lungs are no longer at equilibrium. Elaborate.
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.
At total lung capacity (TLC), the lungs are no longer at equilibrium. Elaborate.
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.
What is the resting position of the chest wall?
Around 60-80% of the TLC.
*Review, the blue, green and red graph in your slides.
What determines the residual volume in young individuals?
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.
What determines the residual volume in older adults?
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).
Other than age, what can influence the residual volume between individuals?
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.
How does gravity affect lung compliance? Why do basal (lower) zones have smaller resting volumes.
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.
Why is the lower zone of the lungs more easily inflated?
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.
At TLC, both the upper and lower part of the lung are maximally inflated….
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
At RV, both the upper and lower part of the lungs are maximally deflated…
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
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?
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.
