Surfactant is composed mostly of lipid molecules and is secreted by type II pneumocytes from around 33 weeks gestation. Its key effect is to reduce surface tension at the air water interface inside alveoli, significantly increasing lung compliance and thereby decreasing the work of breathing. Its absence in premature neonates explains infant respiratory distress syndrome.
Changes in surfactant distribution during the respiratory cycle may explain hysteresis in pressure volume relationships.
By decreasing surface tension surfactant decreases the tendency for small alveoli to collapse into large alveoli, allowing alveoli of different sizes to coexist (by La Place's law pressure is inversely proportional to alveolar radius).
See A Thinking Approach to Physiology, pages 76 - 79.
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