Type-2 cells in the alveolar wall contain detergent-like chemicals. These chemicals are called surfactants. Detergents are chemicals like phospholipids. It reduces the surfactant tension of the alveolar wall. Prevents alveolus from collapsing. Contracts and expands the lungs with less effort. Facilitates rapid exchange of CO2 and O2. Destroys germs (bacteria). Surfactant secretion begins in the 23-week-old human fetus.