Surfactant

 Surfactant is a lipoprotein molecule which is produced by alveolar type 2 cells in the lungs. It is required in the lungs to maintain surface tension in alveolar type 1 cells. This maintenance of surface tension is key to allow the rapid diffusion of gases such as oxygen and carbon dioxide across the alveoli membrane. [1]

Reference

1. Griese, M. (1999). Pulmonary surfactant in health and human lung diseases: state of the art.. Eur Rispar J. (9), 1455-1476.