Surfactant

Surfactant is a lipoprotein molecule which is produced by alveolar type 2 cells in the lungs. It is required in the lungs to reduce 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  Because the alveoli are so small, the water molecules are close enough for the attraction of hydrogen bonds to occur, applying a force making it harder for the alveoli to expand, reducing the internal pressure and therefore creating a gradient for air to move from the higher atmospheric pressure into the alveoli.