The Evolution of the Vertebrate Pulmonary Surfactant System

Lung structure and function vary widely among vertebrates. Despite their diversity, all lungs are internal, fluid-lined structures that change volume and hence face similar biophysical problems. For example, if the surface tension of the fluid lining is high, this may lead to collapse or flooding of the lung In mammals, these problems are largely overcome by the presence of a mixture of surface-active lipids and proteins (pulmonary surfactant), which lowers the surface tension of the fluid lining, particularly at very low lung volumes. This action is due primarily to a disaturated phospholipid (DSP), predominantly dipalmitoylphosphatidylcholine (DPPC), which exists in the ordered, gel state below 41°C Cholesterol (CHOL) and unsaturated phospholipids (USPs) promote respreading upon inflation by converting DPPC to the disordered, liquid-crystalline state. It appeared to us that a DSP-rich surfactant, with its high phase transition temperature, is likely to be of only limited use in the lungs of ectothermic ...