What substance covers the alveoli and reduces surface tension?

The substance that covers the alveoli and reduces surface tension is specifically known as surfactant. Surfactant is a complex mixture of lipids and proteins produced by the type II alveolar cells in the lungs. Its primary function is to lower the surface tension at the air-liquid interface within the alveoli, which prevents the collapse of these tiny air sacs during exhalation and facilitates gas exchange. By reducing surface tension, surfactant increases lung compliance, making it easier for the lungs to expand and contract during breathing. This is crucial for maintaining proper respiratory function and ensuring that alveoli can remain open and available for gas exchange efficiently.