LUNG COMPLIANCE

Lung Compliance is defined as the magnitude of the change in lung volume produced by a given change in the trans pulmonary pressure .
        CL= change in VL/ change in Ptp
Thus, the greater the lung compliance, the easier it is to expand the lungs at any given change in trans pulmonary pressure. Compliance can be considered the inverse of stiffness. The less compliant the lung, the more energy is required to produce a given amount of expansion. i.e., when lung compliance is abnormally low, (increased stiffness), intrapleural pressure must be made more sub atmospheric than usual during inspiration to achieve lung expansion.

DETERMINANTS OF LUNG COMPLIANCE :

• One is the stretchability of the lung tissues, particularly their elastic connective tissues. Thus, the thickening of the lung tissues decreases lung compliance.
• Another equally important determinant is the surface tension at the air-water interfaces within the alveoli. The surface of the alveolar cells is moist, so the alveoli can be picturized as air-filled sacs lined with water. At an air-water interface, the attractive forces between the water molecules, known as surface tension, make the water lining like a stretched balloon that constantly tends to shrink and resists further stretching. Thus, expansion of the lung requires energy not only to stretch the connective tissue of the lung but also to overcome the surface tension of the water lining of the alveoli.
• Surfactants, markedly reduces the cohesive forces and thus, lowers the surface tension, which increases lung compliance and makes it easier to expand the lungs.