The physical properties of normal lungs removed after death.

It has been known for over a century that the lungs retain some of their elastic properties after death. Carson (1820) was the first to investigate the elastic recoil of the lungs by measuring the force required to inflate them, and this type of experiment has been repeated by numerous investigators with confusing results. The only conclusion that can be drawn from the literature is that the elastic properties of normal lungs after death are unpredictable. In the experiments to be described the lungs have been inflated by negative pressure in a plethysmograph, and some of the factors responsible for changes in elastic behaviour after death have been analysed in an attempt to explain why previous investigations have not given consistent results. Pulmonary elasticity has been defined as the ability of the lungs to recover their original size by elastic recoil when a force deforming them is removed. Until recently the lungs have been thought to obey Hooke's law, which states that the deformation of an elastic body is directly proportional to the force applied. This is a theoretical law to which no known material conforms. Recent work on the elastic properties of biological materials suggests that the lungs are more probably visco-elastic. A visco-elastic system is a combination of a viscous system in which the response to a force. depends on the time the force is applied, and an elastic system in which the response depends on the magnitude of the force applied.