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2 out of 6

The mechanism of action of surfactant is: (AIIMS NOV 2007)

A Breaks the structure of water in the alveoli

B Lubricate the flow of Co2 diffusion

C Makes the capillary surface hydrophilic

D All of the above

Ans. B Lubricate the flow of Co2 diffusion


I. The surface tension in alveoli is produced due to air-fluid interphase.

II. The low surface tension when the alveoli are small is due to the presence in the fluid lining the alveoli of surfactant, a lipid surface –tension-lowering agent. Surfactant is a mixture of DPPC, other lipids, and proteins.

III. It does this by not dissolving uniformly in the fluid lining the alveolar surface. Instead, part of the molecule dissolves, while the remainder spreads over the surface of the water in the alveoli,thereby breaking the structure of water present inside the alveoli.

IV. This surface has from one twelfth to one half the surface tension of a pure water surface.

V. If the surface tension is not kept low when the alveoli become smaller during expiration, they collapse in accordance with the law of Laplace.

VI. In spherical structures like the alveoli, the distending pressure equals 2 times the tension divided by the radius (P=2T/r) ; if T is not reduced as r is reduced , the tension overcomes the distending pressure. Surfactant also helps tp prevent pulmonary edema.

VII. It has been calculated that if were not presents, the unopposed surface tension in the alveoli would produce a 20mm Hg force favoring transudation fluid from the blood into the alveoli.