Final answer:
Pulmonary surfactants are a complex mixture of phospholipids and lipoproteins that reduce the surface tension in the alveoli of the lungs, preventing their collapse during exhalation.
Step-by-step explanation:
In a mature normal lung, special alveolar cells secrete pulmonary surfactants. These surfactants, which are a complex mixture of phospholipids and lipoproteins, work to reduce the surface tension between the alveoli tissue and the air in the alveoli. By lowering the surface tension, the surfactants prevent the collapse of alveoli during exhalation.