18. Lung Mechanics Flashcards
What happens to the breathing values in an obstructive disease and give a chronic and acute cause?
- TLC increases (overall)
- Residual volume increases (closed off airways)
- All other proportions decrease
- Chronic - COPD (narrow airways, break down of parenchyma, less recoil of alveoli)
- Acute - Asthma
What happens to TLC and residual volume in a restrictive disease and give an example of a pulmonary and extra-pulmonary cause?
- TLC decreases (overall)
- Residual volume decreases
- Decrease in all volumes - proportions remain the same
- Pulmonary - Lung fibrosis
- Extra-pulmonary - obesity
Describe the transrespiratory pressure (graph) in a restrictive and obstructive diseased lung
- Restrictive - shallower, lower default point (large change in pressure for small change in volume) - unwilling to distort
- Obstructive - steeper, higher default point (large change in volume with small change in pressure) - lost elasticity
- Transrespiratory pressure is always 0 at FRC
- FRC itself is different in both
(always zero at FRC)
In the transrespiratory pressure graph, why is the end of inspiration negative and where is the end of expiration?
• End of inspiration - decrease in pleural pressure
• End of expiration - FRC (passive)
- forced expiration required to make the pressure more positive
Explain the interaction between alveolar pressure and flow rate
- Change in alveolar pressure causes a change in flow rate
- Expand chest wall => increased thoracic cavity capacity => negative pressure inside => air flows in
- When alveoli fill with gas - equilibrium (no pressure difference)
- Tension compresses gas molecules out of alveoli at start of expiration - positive pressure
Compare a graph of compliance to elastance?
• Compliance - volume (y-axis) vs pressure (x-axis)
(willingness to change shape after pressure applied)
• Elastance - pressure (y-axis) vs volume (y-axis)
(tendency to recoil to original volume)
Compare the intrapleural pressure throughout the lungs
• On average: -5 cmH2O
• More negative at the apex due to gravity (-8 cmH2O)
- greater transmural pressure required to change volume
- alveoli less compliant as they are fully inflated
• Smaller transmural pressure at the base due to gravity
- less effort required to inflate these alveoli
- smaller and more compliant
How does breathing change if the lungs are fluid-filled?
- More compliant - smaller pressure needed to increases volume
- Usually surfactant creates air-water surface tension
- Fluid-water interface does not exhibit surface tension
Why does a air-water interface create surface tension?
- In the water, all the water molecules interact
- At one layer, air is on that side
- No matching force on one side
- Tension created across the top
Describe the distribution of water in the alveoli?
- More dense around the outside due to the shape
- Water molecules on the inside attracted to other water molecules across the alveolar space
- If the alveolus is too small, this can cause it to collapse
What produces pulmonary surfactant, what is it made up and how does it prevent collapse of small airways?
• Type II Pneumocytes
• Surfactant is made up of:
- 80% polar phospholipids
- 10% non-polar lipids
- 10% protein
• Splits water molecules, reducing surface tension between them
• Prevents collapse and increases compliance (can work less hard to breathe)
Where does peak resistance occur?
- Flow rapidly decreases at bifurcations
* Peak resistance around the 4th generation
What is conductance and how does conductance and resistance change with increasing lung volume?
- Conductance - how well the airways conduct and allow air to pass through
- Conductance increases
- Resistance decreases
What does homogenous expansion describe?
- Alveoli share walls
- When one alveolus moves, the others around it also move
- No focal high stress concentration
- Expansion at the same rate and similar volume
When does non-homogenous expansion occur?
- Restrictive lung disease e.g. fibrosis - one part is resistant to expansion and another part is fine - stress point created between
- Obstructive lung disease e.g. emphysema - one part wants to expand more due to broken tissue - connection point becomes a focal point for damage (collagen can be subject to permanent damage)
