Final answer:
Surfactant is a lipoprotein film that reduces surface tension and keeps the alveoli open during inspiration.
Step-by-step explanation:
The lipoprotein film that enables the air sacs (alveoli) to stay open and receive air during inspiration is called surfactant. Surfactant is a mixture of phospholipids and lipoproteins that works to reduce the surface tension between the alveoli tissue and the air within the alveoli. It prevents small alveoli from collapsing and prevents large alveoli from expanding too much. By lowering the surface tension, surfactant helps maintain the elasticity and functionality of the alveoli.