What substance is secreted by alveolar epithelium to reduce the surface tension between alveoli of the lungs?

The substance secreted by the alveolar epithelium to reduce surface tension between the alveoli is surfactant. Surfactant is a complex mixture of lipids and proteins produced primarily by type II alveolar cells in the lungs. Its primary role is to lower the surface tension at the air-liquid interface within the alveoli, which helps to prevent the alveoli from collapsing during exhalation and facilitates their expansion during inhalation. This is crucial for efficient gas exchange and overall respiratory function.

Surfactant ensures that the lungs can function effectively by maintaining stability in the alveoli, especially given their small size and the amount of pressure that can be generated during breathing. Without surfactant, the high surface tension could lead to atelectasis, or the collapse of the alveoli, which would severely impair oxygen uptake and carbon dioxide elimination.

Other substances mentioned, such as angiotensin, intrapleural fluid, and carbon dioxide, have different roles in the body and do not directly affect the surface tension within the alveoli. Angiotensin is primarily involved in blood pressure regulation, intrapleural fluid acts as a lubricant within the pleural cavity, and carbon dioxide is a waste product of metabolism that is exchanged in the