The law of LaPlace
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The law of LaPlace explains the a relationship between pressure, surface tension, and radius. It is used in the context of alveoli in the lung, whereby a single alveolus is modeled as being a perfect sphere.[1] For such an alveoli, the magnitude of inward-directed pressure (P) is equal to twice the surface tension (T) divided by the radius (r).
This equation implies that a larger surface tension involves a larger force pushing inwards into the object. Also, because the relationship of P and r is inverse, a smaller radius would result in a larger inward force. When this is applied to the context of pulmonary systems, if two alveoli are connected to the same airway, it would be expected that the smaller alveoli would experience a greater pressure, driving its collapse, and pushing all of its gas contents into other, larger, alveoli. This however does not happen, the reason being that pulmonary surfactant, a mixture secreted in the alvoeli that decreases surface tension, has a greater effect on smaller alveoli than larger ones. Therefore while an alveoli's small radius would on its own increase pressure, this increase is offset by the extra reduction in surface tension that is the result of pulmonary surfactant function.[1]
The LaPlace pressure also causes the process of emulsification to be more thermodynamically inefficient. To form the small, highly curved droplets of an emulsion extra energy is required to overcome the large pressure that results.
[edit] References
- ^ a b Sherwood, Lauralee. “Ch13”, Peter Adams: Human physiology from cells to systems, 6th, Thomson Brooks/Cole. ISBN 0-495-0185-0.
Tadros T.F. "Surfactants in Agrochemicals" - Surfactant Science series vol. 54, published by Dekker, 1995
