L26. Compliance and Elastic Properties Flashcards
(20 cards)
Compliance?
Compliance is determined by the elasticity of the materials
- The greater the elastic recoil of the lungs the greater the change in Ptp required to fill the lungs. Lung compliance determines how effective the lung expands at a given Ptp/force to inflate
E.g. a hot water bottle has low compliance –> has high Ptp/force to inflate –> high recoil
E.g. a balloon has high compliance –> has low Ptp/force to inflate –> low recoil
Lung compliance?
- The degree to which the lung will distend for a given change in the Ptp is known as the compliance, C (expandability/stretchiness)
- Measured as the change in volume (Vd) for a given change in pressure (Ptp)
- C = Vd / Ptp
- In a lung with high compliance (easily distensible), a small pressure change would result in a large volume change
High compliance?
Advantages:
- Easy to inflate the lungs (little respiratory muscle activity required)
Disadvantages:
- Elastic recoil is reduced
- Passive recoil of lungs during expiration isn’t very efficient
- Expiratory muscle activity may be required even during tidal breathing
Emphysema?
Example of high compliance
Emphysema causes destruction of alveoli –> alveoli with large air spaces
- Characterised by high compliance
- Only a small pressure change is required to inflate the compliant lung
- The lungs in patients with emphysema have little elastic recoil and the lungs tend to remain inflated
- Expiratory muscle activity is required to deflate the lungs
Low compliance?
Advantages:
- Elastic recoil is high
- Passive recoil of lungs during expiration is not a problem
Disadvantages:
- Difficult to inflate the lungs (strong inspiratory muscle activity required)
Pulmonary fibrosis?
Example of low compliance
- PF is a restrictive lung disease caused by deposition of fibrotic material
- Characterised by low compliance
- Inspiration is difficult (inspiratory muscles have to work harder)
- PF patients have high elastic recoil and higher Ptp is needed for inflation; lung volumes are decreased, frequency increased
- A patient with restrictive lung disease breathe shallowly and rapidly
Determinants of lung compliance and elastic recoil?
- 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
Elastic elements?
- Resistance to stretching by the elastic fibres accounts for 25% of the elastic recoil
- The more elastic fibres, the greater resistance to stretching
- Stretching of the lung tissue (expansion of the lung) requires energy
Surface tension?
- The walls of alveoli are coated with a thin film of water
- Water molecules are attracted to each other
- This attraction creates a force called surface tension
- Water molecules are more attracted to each other than to air
- Causes the tendency aiming to decrease surface area
- In spherical structures (e.g. alveoli) this creates a tendency for the alveoli to collapse
Surface tension of alveolar fluid?
- Surface tension accounts for 75% of the elastic recoil of the lungs
Fortunately, our alveoli do not collapse, and inhalation is relatively easy because the lungs produce a substance called surfactant that reduces surface tension - Fortunately, our alveoli do not collapse, and inhalation is relatively easy because the lungs produce a substance called surfactant that reduces surface tension
Surfactant?
- Type II alveolar cells produce pulmonary surfactant
- Surfactant forms a layer between the water and air, reduces the interaction between the water molecules
–> Reduces surface tension; breaks up the attracting forces between water molecules at the surface
–> Surfactant increases lung compliance
Respiratory distress syndrome?
Premature babies before the 35th week of gestation cannot synthesise pulmonary surfactant - Type II alveolar cells too immature
- The work required to overcome the surface tension in the alveoli is too exhausting
- The lungs collapse and death follows if no therapy initiated :(
- Therapy is to administer exogenous surfactant via the trachea
Airway resistance?
Mainly due to friction in the airways
- Under normal circumstances, during quiet breathing, friction is very low so airway resistance is also low
Air flow = Patm - Palv / Airway resistance
Determinants of airway resistance?
- Viscosity of air (low, because low density (gas))
- Length of airways (fixed)
- Diameter/radius (large - but variable)
- The most important contributor to changes in airways resistance is the radii of the airways
* Increased radius = reduced airways resistance
* Reduced radius = increased airways resistance
- The major site of airways resistance in the lungs is in the medium-sized bronchi-bronchioles
What can change the radius of the airways?
- Physical factors –> lateral/radial traction
- Chemical factors –> allergens, pollutions (e.g. asthma)
- Neural factors –> autonomous nervous system
Physical factors - lateral/radial traction?
Lung tissues are connected to each other
- As the lungs expand, the increasing Ptp exerts a force on the airways, pulling them open (increase the diameter)
- Increased diameter –> reduced resistance
- Change is proportional to volume change
Chemical factors - asthma?
- Blockage by mucus; reduces the radius of the airways –> increases the airways resistance
- Local inflammatory mediators, (e.g. histamine) cause smooth muscle to contract (bronchoconstriction)
- Both events increase airway resistance
- Leading to reduced ventilation of alveoli
- Reduced gas exchange
Neural factors?
Stimulation of parasympathetic nerves to airways smooth muscle causes bronchoconstriction
- Target for treatment in cases where there is too much bronchoconstriction
- Muscarinic ACh receptor blockers used for treatment of chronic bronchitis
Intrapleural pressure (Pip) becomes more negative during expiration BECAUSE during expiration the volume of the thorax increases
A. If both statements are true, and the second causes the first
B. If both statements are true, but the second does not cause the first
C. If the first is true and the second is false
D. If the first is false and the second is true
E. Both statements are false
E
With the physics/ideas behind airways, what also demonstrates similar laws?
Blood flow –> pressure and resistance
