Pulmonary surfactant composed of myelin and lecithin is mainly secreted continuously by Type II pneumocyte beginning from 20th week of gestation. Also Know: Surfactant has both lipid (90%) and protein (10%) component. About half of the lipids are dipalmitoylphosphatidylcholine and the remaining are phosphatidylglycerol, cholesterol and other lipids. Half of the proteins are apoproteins and other half is composed of proteins normally found in blood plasma. Function of surfactant: The surfactant greatly reduces the surface tension allowing easier expansion and collapse of alveoli during during respiration and the resulting changes in pressure. It also helps the alveoli to expand and shrink at the same rate thereby reducing the chance for isolated overexpansion and total collapse of alveolar sacs. Ref: The Big Picture: Medical Biochemistry by Lee W. Janson, chapter 17