3. Physiology
        3.16. SAQs
            3.16.12. Respiratory
       Mechanics of breathing Lung compliance

Lung compliance

Describe the factors that affect lung compliance. (03A14, 1990)

Definition of compliance


Specific compliance

Dynamic compliance vs static compliance

Static compliance is the compliance measured when there is no gas flow into or out of the lung.

Lung compliance and chest wall compliance

Compliance is made up of lung compliance and chest compliance

=> 1/Ct = 1/Cl + 1/Ccw

(Ct = total compliance, Cl = lung compliance, Ccw = chest wall compliance)


Factors affecting compliance

  1. Lung elastic recoil
  2. Lung volume
  3. Pulmonary blood flow
  4. History of recent ventilatory
  5. Bronchial smooth muscle tone
  6. Disease

Lung's elastic recoil

Due to:

Surface tension accounts for 70% of the elastic recoil


Lung volume

The slope of the P-V curve is not constant across different lung volumes.

At high lung volumes, compliance is reduced because more pressure is required to stretch the already stretched elastic tissues further.

At very low volumes, compliance is reduced because of closed airway and collapsed alveoli

=> increased surface tension

=> increased pressure is needed to re-open the airway/alveoli

=> reduced compliance

At the base of the lung, at very low volumes, compliance is even more reduced because of positive intrapleural pressure


Posture affects compliance by affecting the lung volume.

Restriction of chest expansion also affect lung volmen and chest wall compliance.

Pulmonary blood flow

Contributes to stiffness of the lung, especially in the case of pulmonary congestion

History of recent ventilation

Prolonged periods of hypoventilation without periodic deep breath may lead to reduced compliance.

Maybe related to atelectasis.

Bronchial smooth muscle tone

In animal studies, increased bronchocontriction can lower time constant and reduced dynamic compliance.

Static compliance is probably not affected.


In diseased lungs, where time constant for the alveolis are different, units with higher time constants are slow to fill and empty.

With higher respiratory rate, the problem worsens:

=> units with high time constant hypoventilates

=> less lung units participate in volume changes

=> dynamic compliance reduced.

With collapsed alveoli

=> greatly increased surface tension

=> very high pressure is required to re-open airway/alveoli

Diseases that REDUCE compliance:

Diseases that INCREASE compliance:

(see West p97)


Additional notes

NB: 1/compliance = elastance


Recoil pressure of the lung (transpulmonary) = alveolar pressure - intrapleural pressure

Recoil pressure of the chest = intrapleural pressure - atmospheric pressure

Examiner's comments


To be added later

Measurement of compliance see Nunn p54

Elastic forces - lung vs chest wall (20050121-10)


? is compliance highest at FRC

? diagram of pressure volume loop


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