Outline the non-respiratory functions of the lung.

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College Answer

In general, this question was not well answered. In particular candidates did not demonstrate sufficient depth of information. Nunn’s Applied Respiratory Physiology devotes an entire chapter to this topic (Ch.12, in the 6th edition).

Discussion

With these sorts of college remarks, one is hardly able to guess what the marking criteria had contained. One can only assume that the examiners wanted something like the contents of Nunn's, except condensed into a ten-minute answer.

Thus, the nonrespiratory functions of the lungs are:

  • Trap for airborne particles: 
    • Physical defence mechanisms
      • Upper airway filtering systems (nasal turbinates)
      • Mucociliary escalator
      • Reflexes
        • Sneezing
        • Coughing
    • Cellular defence mechanisms
      • Alveolar macrophages
      • Lung neutrophils
      • Mast cells
    • Immunologic defence mechanisms
      • Lymphatic system of the lung
      • Immunoglobulin in mucus and cell surfaces
      • Direct antibacterial action of surfactant (Wu et al, 2003)
  • Reservoir of blood: the lungs contain about 10% of the circulating blood volume
  • Route of drug administration (eg. nebulised steroids and bronchodilators)
  • Route of drug elimination  (eg. volatile anaesthetics, paraldehyde)
  • Metabolism (eg. conversion of of angiotensin-I, and degradation of neutrophil elastase by α1-antitrypsin)
  • Modulator of acid-base balance by virtue of CO2 elimination
  • Modulator of the clotting cascade: the lungs contain thromboplastin, heparin and tissue plasminogen activator
  • Filter for the bloodstream: particles larger than an RBC are trapped (~8  μm size barrier), which includes clots, tumour cells and other emboli
  • Modulation of body temperature: heat loss can occur by respiration

References

References

Heinemann, H. O., and A. P. Fishman. "Nonrespiratory functions of mammalian lung." Physiological Reviews 49.1 (1969): 1-47.

Harada, Ruth N., and John E. Repine. "Pulmonary host defense mechanisms." Chest 87.2 (1985): 247-252.

Chitkara, Rajinder K., and Faroque A. Khan. "Endocrine Functions of the Lung." Hormonal Actions in Non-endocrine Systems. Springer, Dordrecht, 1983. 147-199.