What is lung compliance? How is it that surfactants increase lung complianc

Lung compliance is basically how easy it is for the lungs and thorax to expand. Surfactant (phospholipids) are produced by type II alveolar cells and they help lung compliance by reducing surface tension and keeping the small alveoli open.

The ability of the lungs to reversibly change shape can be quantified using two parameters. One of these parameters is compliance, which is a measure of how easy it is to stretch the lung (during inhalation). The other measure is elastance, which is a measure of how readily the lung returns to its original shape (during exhalation). A highly compliant lung stretches more in response to a pressure change than does a less compliant lung. Compliance is a function of the change in lung volume divided by the change in transpulmonary pressure (\({\Delta}\)V/\({\Delta}\)P). Therefore, for a highly compliant lung, generating the same transpulmonary pressure upon inhalation will result in greater filling of the lung, which is desirable. A force that resists lung inflation (and thus reduces compliance) is surface tension of the thin layer of liquid that exists in the small airways and alveoli. Surface tension is generated mostly through hydrogen bonding, and causes two wet surfaces to stick together. This is detrimental to lung filling. For example, in premature human babies, surfactants are not present in the lungs in sufficient amounts to contribute to lung compliance, which makes breathing difficult. Surfactants, however, such as lipoprotein surfactants, reduce the surface tension of the fluid layer lining the lungs by disrupting the hydrogen bonds, increasing lung compliance and permitting lung inflation.