Non-respiratory functions of the lung

Which one of the following options is a function performed by the lung?

a) Conversion of angiotensinogen to angiotensin I

b) Secretion of immunoglobulin E into bronchial mucus

c) Uptake and metabolism of histamine

d) Deactivation of prostaglandin E2

e) Manufacture of phosphatidylinositol biphosphate, the phospholipid component of

surfactant

Answer: d

Explanation

The non-respiratory functions of the lung are numerous and should not be overlooked in

favour of the more commonly questioned functions of gas exchange. Angiotensinogen is

converted to angiotensin I in the plasma by renin produced by the juxtaglomerular

apparatus of the kidney. Angiotensin I is virtually devoid of physiological action except

as a precursor to angiotensin II. This conversion occurs in the lungs catalysed by

angiotensin converting enzyme (ACE) located on pulmonary capillary endothelial

cells. As angiotensin II is a significant vasoconstrictor (more potent than noradrenaline)

it follows that ACE inhibitor drugs are effective antihypertensives. Angiotensin converting

enzyme also inactivates almost all of the bradykinin in the lung. The accumulation of

bradykinin in patients on ACE inhibitor therapy is responsible for the side effect of

cough, as bradykinin is an irritant. The lung secretes immunoglobulin A into bronchial

mucus as a component of defence against infection (cf. IgE component in asthma

pathophysiology). 5-hydroxytryptamine (serotonin) is almost completely eliminated

by passage through the lungs – it is taken up intact rather than metabolised.

Noradrenaline is partially deactivated, whereas histamine and dopamine traverse the

pulmonary circulation without decrement. Prostaglandin E1, E2 and F2α are deactivated

in the lung whereas prostaglandin A2 and I2 (prostacyclin) are not. Phosphatidylinositol

biphosphate (PIP2) is a component of G-protein second messenger systems. The active

phospholipid in surfactant is dipalmitoyl phosphatidylcholine. Some other nonrespiratory

lung functions are: as a reservoir of blood; a filter of debris, platelet clumps

and bubbles; storage of heparin in mast cells; arachidonic acid metabolism; and maintenance of the connective tissue architecture of the lung.