Maintenance of the essential composition of lung surfactant phospholipid is critical for the survival of all mammalian species. Lung surfactant is secreted from specialized type 2 epithelial cells in the lung alveolus and forms a continuous lining layer at the air-liquid interface throughout the lungs. To provide adequate gas exchange surface area in the lungs to support respiration, alveolar diameters must be very small, giving a large surface area:volume ratio. One consequence of the small dimensions of the alveolus is that surface tension forces contribute significantly to the dynamics of lung function. Surfactant opposes surface tension in the lungs. It is the absence of adequate surfactant that leads directly to lung collapse and the high incidence of morbidity and mortality associated with neonatal respiratory distress syndrome.
Lung surfactant has a unique phospholipid composition, containing PC16:0/16:0 as 40-60% of total PC and monounsaturated phosphatidylglycerol (PG) species as 10-15% of total phospholipid. Phos-phatidylglycerol is not found at such high concentration in any other membrane of the body. PC16:0/ 16:0 is the principal surface-active component of lung surfactant, has a gel:liquid crystalline transition temperature of 41 °C, and consequently is, in effect, solid at a body temperature of 37 °C. It has been suggested that the compressed PC16:0/16:0 mono-layer at the air-liquid interface survives the high surface pressures within the lungs by forming a solid monomolecular sheet, and it thus prevents any surface tension effects. At the same time, PC16:0/16:0 is metabolically inert, and one proposed specialized role for PG is to fluidise PC16:0/16:0 and facilitate its metabolic processing, secretion, and adsorption to the air-liquid interface.
This composition of lung surfactant is restricted to air-breathing animals. Comparative studies with reptiles, amphibia, and lower vertebrates have shown that concentration of PC16:0116:0 in surfactant correlates with the ratio of lung:body surface area as a measure of an animal's reliance on lung-mediated respiration. Lung surfactant from amphibia, by comparison, also contains phospholi-pid, but this is largely cholesterol and unsaturated PC, which is thought to serve an antiglue function. By analogy with lung surfactant, phospholipid-rich surfactants have been described for other epithelial surfaces, including the stomach, eustachian tube, and synovial surfaces, where they are thought to create a protective hydrophobic lining layer. The comparison with lung surfactant is somewhat misleading, however, because the PC fraction of these other epithelial secretions contains minimal PC16:0/ 16:0 and high contents of mono- and diunsaturated species.
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