Surface tension is a force generated in the surface of a liquid at a gas-liquid interface. This occurs in the lungs between the alveolar gas and the fluid lining the alveoli, which is necessary to keep these delicate structures moist. Surface tension has the units of force per unit length in the surface of the interface (e.g., dynes per centimeter) because it is the force holding the liquid molecules together at the interface. It is the force that forms a skin on water when a glass is filled slightly above the rim. Surface tension tends to shrink the surface area of the interface, so the tendency for the alveolar surface area to shrink tends to decrease alveolar volume. Therefore, surface tension acts like a pressure outside the alveoli and tends to collapse them. The law of LaPlace relates the surface tension (T) and pressure (P) in a gas-filled, liquid-lined sphere with radius r:
Figure 4 shows why surface tension and the law of LaPlace are important in the lung. Pressures inside the alveoli are inversely proportional to their radius.
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