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Surfactant plays a crucial role in the alveoli primarily by lowering surface tension. The alveoli are small air sacs in the lungs where gas exchange occurs, and their inner surfaces are lined with a thin layer of liquid. Without surfactant, the surface tension created by this liquid would be significantly high, resulting in difficulty in lung expansion during inhalation and an increased risk of alveolar collapse (atelectasis).
Surfactant is a complex mixture of lipids and proteins secreted by type II alveolar cells that decreases surface tension, allowing the alveoli to remain open and stable at different lung volumes. This reduction in surface tension is vital for normal respiratory function because it enables easier inflation of the lungs, enhances lung compliance, and improves overall gas exchange efficiency by maintaining the surface area of the alveoli.
In contrast to the other options, surfactant does not increase surface tension, nor does it directly facilitate gas exchange or provide structural support to the alveolar walls. Its primary and most vital function is centered around lowering surface tension to prevent collapse and maintain alveolar stability.