Composition, Structure and Biophysical Functions of Pulmonary Surfactants: Their Deficiency and Strategy for Remedy
Pulmonary surfactant (PS) is a complex mixture of lipids and proteins that forms a monomolecular film at the lung-air interface. PS is a barrier material of the lungs and serves dual purposes: i) it lowers the surface tension; and ii) it participates in innate immune defense of the lung. Surface tension lowering property of PS is achieved by forming a surface film (monolayer) which is highly enriched in dipalmitoylphosphatidylcholine. There are four surfactant proteins, (SP) designated as SP-A, SP-B, SP-C and SP-D. While SP-A and SP-D are hydrophilic, SP-B and SP-C are hydrophobic and are also biocompatible among all the mammalians. This article starts with a brief account of the history of research on PS, anatomy of lung, alveolar metabolism, composition and methodologies adopted for in vitro evaluation of the PS system. The possible molecular mechanism of film formation (adsorption), and of film adaptation to surface changes (phase transition) during the process or respiration have been described in detail. Major disorders of the surfactant system along with its clinical consequence, and the potentials of surfactant therapy in the treatment of some of these disorders have been discussed. A brief account of the evolutionary development of pulmonary surfactant has been also presented.