Lung compliance

From WikiLectures

Pulmonary compliance - is the ratio of the change in volume to the change in interpleural pressure that caused the change.

Interpleural pressure[edit | edit source]

Is the pressure between the pleurae - visceral and parietal.

  • It is always negative. On inspiration: -0.8 kPa, on expiration: -0.33 kPa.
  • The IPT value at resting expiration is not uniform.
  • Standing examinee has IPT more negative in upper parts of lungs than in base of lungs - probably due to weight of lungs. The consequence is different ventilation of the basal and apical parts of the lungs.
  • Importance of IPT negativity - keeps lungs expanded - allows chest volume changes to be monitored and thus lung ventilation is secured.

Pneumothorax[edit | edit source]

Pneumothorax is a disturbance of the pleural cavity - air is present - the lung shrinks, breathing is impaired, and hypoxia is imminent.

Pneumothorax
  1. Enclosed - air enters the pleural cavity from the alveolar space spontaneously or after lung injury.
  2. Open - when the chest wall is injured - stab wound.
  3. Valvular (tension) - air enters the pleural cavity with each respiratory movement but cannot escape.

Factors determining lung compliance[edit | edit source]

Elasticity of lung tissue[edit | edit source]

The lung is an elastic organ. The elasticity is due to the reticular arrangement of connective tissue. During exspiration, the fibers contract and bend. The elasticity is 1/3 of the total elasticity of the lung.

Surface tension of alveoli at the interface between alveolar air and alveolar lining[edit | edit source]

Compliance depends on the surface tension between the gas and fluid - that is, the internal surface area of the alveoli and the exchange of respiratory gases. For example, we have a bubble that is surrounded by a fluid - its surface tension will create an overpressure inside the bubble relative to the external pressure, its value determined by Laplace's law:

  • If the mouth of the cylinder (ductus alveolaris) is covered by a flat soap bubble, then r is high and P is small.

When we want to increase the volume of the bubble (alveolus) we have to decrease r and thus increase P - a large opening pressure is required. Further inflation increases r and decreases P. Alveoli behave similarly. In interconnected alveoli, the smaller alveolus may shrink in favour of the larger one, but in normal lungs this is prevented by surfactant.

Surfactant[edit | edit source]

  • It reduces surface tension (more in smaller alveoli than in larger alveoli). It also prevents lung collapse.
  • In premature infants, the lungs have not had time to develop a functional surfactant.
  • Surface tension is therefore high and atelectasis occurs → alveolar collapse → Respiratory Distress Syndrome (RDS). Lung damage also occurs with oxygen poisoning. This is partly due to oxidative destruction of surfactant.
  • Compliance decreases, alveoli collapse and pulmonary edema develops.


For more information go to: Surfactant.


References[edit | edit source]

Related articles[edit | edit source]

Literature used[edit | edit source]

  • SILBERNAGL, Stefan – DESPOPOULOS, Agamemnon. Atlas fyziologie člověka. 3. edition. Praha : Grada, 2004. 435 pp. ISBN 80-247-0630-X.
  • GANONG, William, F. Přehled lékařské fyziologie. 1. edition. Jinočany : H & H, 1995. 681 pp. ISBN 80-85787-36-9.
  • BROŽEK, Gustav – HERGET, Jan – VÍZEK, Martin. Poznámky k přednáškám z fysiologie : První díl, Dýchání, cirkulace, svaly, neurofysiologie. 1. edition. Jinočany : H & H, 1999. 229 pp. ISBN 80-86022-48-X.