What substance is responsible for reducing surface tension in the lungs?

Surfactant is the substance that reduces surface tension in the lungs. It is a complex mixture of lipids and proteins, primarily produced by the type II alveolar cells in the lungs. Surfactant plays a critical role in maintaining the stability of the alveoli, which are the small air sacs where gas exchange occurs. By reducing surface tension, surfactant prevents the alveoli from collapsing, especially during exhalation, when the volume of air in the alveoli decreases. This action allows for easier inflation of the lungs during breathing and helps optimize respiratory efficiency.

Phospholipids are indeed components of surfactant, but they alone do not provide the function of reducing surface tension without the other proteins and components that make up the surfactant complex. Cilia are hair-like structures that help to move mucus and debris out of the respiratory tract but do not directly influence surface tension in the alveoli. Alveolar fluid is the fluid found within the alveoli that can be affected by surfactant, yet it is the surfactant that is specifically responsible for decreasing surface tension to allow for proper lung function.