Lecture 27 : Compliance and Elastic Properties Flashcards
(36 cards)
What is required for lung expansion?
Distending pressure - transpulmonary pressure, Ptp
How is transpulmonary pressure calculated?
Ptp = Palv - Piv
Describe the relationship between Ptp and Pip:
Ptp increases with more negative Pip (greater suction)
- The greater the elastic recoil of the lungs, the greater change in Ptp required to fill the lungs
How is lung compliance measured?
Measured as the change in volume (Vd) for a given change in pressure (Ptp)
* C = Vd/Ptp
What is lung compliance?
The degree that the lung will distend for a given change in the Ptp
- expandability/stretchiness
What accounts for 25% of elastic recoil of the lungs?
Resistance to stretching by the elastic fibres
What does lung compliance determine?
How effective the lung expands at given Ptp
What is emphysema?
Destruction of alveoli → alveoli with large air spaces
* High compliance
* Only small pressure change is required to inflate the compliant lung
* Little elastic recoil and the lungs tend to remain inflated
* Expiratory muscle activity is required to deflate the lungs
What are the disadvantages of high compliance?
- Elastic recoil is reduced
- Passive recoil of lungs during expiration is not very efficient
- Expiratory muscle activity may be required even during tidal breathing
How does compliance affect volume changes when pressure changes?
In a lung with high compliance, a small pressure change would results in a large volume change
What is the advantage of high compliance?
Easy to inflate the lungs - little respiratory muscle activity required
What 2 factors do lung compliance and elastic recoil of the lungs depend on?
- The elastic elements in the alveolar interstitium
- The surface tension at the interface between air and a layer of fluid on the wall of the alveolus
What are the advantages of low compliance?
- Elastic recoil is high
- Passive recoil of lungs during expiration is not a problem
What is the disadvantage of low compliance?
Difficult to inflate the lungs - strong inspiratory muscle activity required
What is pulmonary fibrosis?
Restrictive lung disease caused by deposition of fibrotic material
* Low compliance
* Inspiration is difficult - inspiratory muscles have
to work harder
* High elastic recoil and higher Ptp is needed for inflation
* Lung volumes are decreased, frequency increased
* Breath shallowly and rapidly
Describe the relationship between number of elastic fibres and resistance:
The more elastic fibres, the greater resistance to stretching
Does stretching of the lung tissue (expansion) require energy?
Yes
What does surface tension do to alveoli?
In spherical structures this creates a tendency for the alveoli to collapse
What creates surface tension within lungs?
- The walls of alveoli are coated with a thin film of water
- Water molecules are attracted to each other = surface tension
Describe the attraction of water molecules to water and air, and how this affects surface area:
- Water molecules are more attracted to each other than to air
- Causes the tendency aiming to decrease surface area
What accounts for 75% of elastic recoil of the lungs?
Surface tension
What produces pulmonary surfactant?
Type II alveolar cells
What causes airway resistance?
Friction in airways
What is respiratory distress syndrome (RDS)?
Premature babies can’t synthesize pulmonary surfactant - Type II
alveolar cells too immature
* Work required to overcome the surface tension in the alveoli is too exhausting
* Lungs collapse
