Final answer:
Surfactant keeps the alveoli open at the end of expiration by reducing surface tension and preventing their collapse, thus facilitating efficient gas exchange in the lungs.
Step-by-step explanation:
Surfactant is responsible for keeping alveoli open at the end of expiration. During expiration, the diaphragm and intercostal muscles relax, and the air is passively expelled from the lungs. However, the alveoli are kept from collapsing by surfactant, which lowers the surface tension at the air-lung interface. This substance aids in preventing alveoli collapse and maintains open airways, contributing to efficient gas exchange.
Without surfactant, alveoli would be at risk of collapse due to the high surface tension of the water that lines the alveoli. Surfactant molecules disrupt these cohesive forces, ensuring that the alveoli remain inflated and that breathing can proceed with less effort. Its presence is critical, especially at the end of expiration when the lungs are at their smallest volume and the pressure could potentially collapse the alveoli.