The primary function of surfactant is

Surfactant is a mixture of dipalmitoylphosphatidylcholine, other lipids, and proteins. The phospholipids line up in the alveoli with their hydrophobic fatty acid tails facing the alveolar lumen. Surface tension is inversely propoional to their concentration per unit area. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse I accordance with Laplace law. Surfactant molecules move apa as alveoli enlarge during inspiration and surface tension increases, whereas it decreases when they move closer together during expiration.Ref: Ganong's Review of Medical Physiology Kim E Barrett, Susan M Barman, Scott Boitano, Heddwen L Brooks 23rd edition, page no -597