In 1929, von Neergaard wrote “It may be possible that the surface tension of the alveoli is diminished by concentration of surface-active substances against other physiologic solutions” (14). He was referring to the existence of a surface film in the alveoli and his assumption was based on the following observations. Neergaard measured pressure-volume diagrams from human and animal lungs, first filling them with air and then with liquid. The surprising result was that the pressure necessary for filling the lung with liquid was only half the pressure necessary for filling the lung with air (Fig. 1). His explanation of this remarkable difference was based on the assumption that in each alveolus there must be a barrier between air and fluid (such as in the wall of a soap-bubble) with a tendency to diminish its size according to the law of Laplace. The amount of retraction pressure for the lung is larger than the retraction force of the elastic fibers. By filling the alveoli with liquid, the air-liquid barrier is replaced by a liquid-to-liquid barrier without any surface tension. The retraction pressure, measurable in the fluid-filled lung, is therefore equal to the retraction pressure of the elastic fibres. The same procedure for determining the influence of surface tension on the overall retraction of the lung was later reported by other scientists, and is now a well-established method.
KeywordsSurface Tension Platelet Activate Factor Surfactant System Pulmonary Surfactant Lung Edema
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