<unk>  is a surface-active lipoprotein complex formed by type II alveolar cells. The proteins and lipids that comprise<unk>  have both a hydrophilic region and a hydrophobic region. By adsorbing to the air-water interface of alveoli with the hydrophilic headgroups in the water and the hydrophobic tails facing towards the air, the main lipid component of<unk> , dipalmitoylphosphatidylcholine, reduces surface tension.
Pulmonary surfactant
Alveolar-capillary barrier
Extracellular matrix
Earth's atmosphere
Alveolar duct
Mucous membrane of nose
Lumen 
Base of the heart