Surfactant is synthesised by type II alveolar epithelial cells. It is comprised of about 90% lipids, predominantly the phospholipid dipalmitoylphosphatidylcholine and cholesterol, along with 5%-10% proteins. It is spread as a monolayer at the J
air-fluid interface. r l
It reduces the surface tension at the alveolar air-fluid interface. This stabilises f the alveoli, preventing alveolar collapse during expiration, i.e. at low lung n volumes. It changes surface tension characteristics as the surface is stretched. a
Surface tension increases as the alveoli distend and reduces as the alveoli con- o
tract. By Laplace's law, with a fall in alveolar pressure during expiration and reduction in alveolar radius, the pressure across the wall rises and would lead to alveolar collapse if surface tension were to remain constant. Reduction of surface tension increases lung compliance and reduces the work of breathing. Surfactant also reduces pulmonary capillary filtration and the transudation of fluid into the alveoli.
It is responsible for hysteresis in the lung, a phenomenon wherein the inflation and deflation volume-pressure curves of the lung are different. Surfactant may contribute to the lung defence mechanism.
Surfactant deficiency is a primary cause of neonatal respiratory distress syndrome, and exogenous surfactant therapy improves gas exchange and increases lung compliance in this condition.
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