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Stability of alveoli maintain by (AIIMS NOV 2009)

A Residual air

B Increase surface area by surfactant

C Negative intra pleural pressure

D Lung compliance

Ans. B Increase surface area by surfactant

I. Stability of alveoli of alveoli is mainly the function of surfactant which prevents their collapse under Surface tension.

II. Factors like Residual air, Negative intra pleural pressure, Compliance mainly affect the inflation and deflation of lung during respiration.

III. They do have some effect on stability on alveoli but the single most important factor remains the Surfactant. (Maximum work of breathing is done to oppose Surface tension).

IV. The surface tension in alveoli is produced due to air-fluid interphase. Surfactant is made up of PHOSPHOLIPID- DI-PALMITOIL-PHOPHATIDYL-CHOLINE (DPPC) Q + two major proteins having molecular weights of 32,000 and 10,000.

V. It is secreted by TYPE II ALVEOLAR EPITHELIAL CELLS (type II pneumocytes) Q.It reduces surface tension in alveoli by not dissolving uniformly in the fluid lining the alveolar surface.

VI. 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.

VII. Main functions:

a. It increases the Compliance Q

b. Reduces work of breathingQ

c. Prevents collapse of alveoli at end of expiration : law of Laplace(P=2T/r) Q

d. Prevents pulmonary edema by keeping the alveoli dryQ

e. Alveolar size regulation: As the alveoli increase in size, the surfactant becomes more spread out over the surface of the liquid. This increases surface tension effectively slowing the rate of increase of the alveoli. This also helps all alveoli in the lungs expand at the same rate.

It has high concentration in the fetal lungs at 20 weeks of gestation Q. However, it does not reach the surface of the lung until 28-38 weeks when it is present in amniotic fluid. Q .Maximum secretion occurs at 34 weeksQ