EVOLUTION OF SURFACE-ACTIVITY RELATED FUNCTIONS OF VERTEBRATE PULMONARY SURFACTANT

1. Pulmonary surfactant is a mixture of lipids and proteins that Lines the air-liquid interface of the lungs of all vertebrates. In mammals, it functions to reduce and vary surface tension, which helps to decre ase the work of breathing, provide alveolar stability and prevent alve olar oedema, The present review examines the evolution and relative im portance of these surface activity related functions in the lungs of v ertebrates. 2. The surface activity of surfactant from fish, amphibian s, birds and most reptiles is generally very low correlating with a lo w body temperature and a low disaturated phospholipid content of their surfactant, In contrast, the surfactant of those reptiles with a high er preferred body temperature, as well as that of birds and mammals, h as a much higher surface activity. 3, The two main functions of surfac tant in mammals are to provide alveolar stability and to increase comp liance of the relatively stiff bronchoalveolar lung. As the respirator y units of most non-mammalian vertebrates are up to 1000-fold larger a nd up to 100-fold more compliant, surfactant is not required for these functions. 4, In non-mammals, surfactant appears to act as an anti-gl ue preventing the adhesion of respiratory surfaces that may occur when the lungs collapse (e,g, during diving, swallowing of prey or on expi ration). Surfactant also controls lung fluid balance. These functions can be fulfilled by a surfactant with relatively low surface activity and may represent the primitive functions of surface active material i n vertebrate lungs.