The importance of surface tension The importance of surface tension. If two connected alveoli have the same surface tension, then the smaller the radius, the greater the pressure tending to collapse the sphere. This could lead to alveolar instability, with smaller units emptying into larger ones. Alveoli typically do not have the same surface tension, however, because surface forces vary according to surface area as a result of the presence of surfactant: the relative concentration of surfactant in the surface layer of the sphere increases as the radius of the sphere falls, augmenting the effect of surfactant at low lung volumes. This tends to counterbalance the increase in pressure needed to keep alveoli open at diminished lung volume and adds stability to alveoli, which might otherwise tend to collapse into one another. Surfactant thus protects against regional collapse of lung units, a condition known as atelectasis, in addition to its other functions. (r, radius of alveolus; T, surface tension; P, gas pressure.) Source: Pulmonary Disease, Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e Citation: Hammer GD, McPhee SJ. Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e; 2013 Available at: https://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/Books/961/Ham007_Fig_09-07.gif&sec=53629584&BookID=961&ChapterSecID=53555690&imagename= Accessed: October 15, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved