LUNG-TISSUE RHEOLOGY AND 1 F NOISE

The mechanical properties of lung tissue are important contributors to both the elastic and dissipative properties of the entire organ at no rmal breathing frequencies. A number of detailed studies have shown th at the stress adaptation in the tissue of the lung following a step ch ange in volume is very accurately described by the function t(-k), for some small positive constant k. We applied step increases in length t o lung parenchymal strips and found the ensuing stress recovery to be extremely accurately described by t(-k) over almost 3 decades of time, despite the quasi-static stress-length characteristics of the strips being highly nonlinear. The corresponding complex impedance of lung ti ssue was found to have a magnitude that varied inversely with frequenc y. We note that this is highly reminiscent of a phenomenon known as 1/ f noise, which has been shown to occur ubiquitously throughout the nat ural world. 1/f noise has been postulated to be a reflection of the co mplexity of the system that produces it, something like a central limi t theorem for dynamic systems. We have therefore developed the hypothe sis that the t(-k) nature of lung tissue stress adaptation follows fro m the fact that lung tissue itself is composed of innumerable componen ts that interact in an extremely rich and varied manner. Thus, althoug h the constant k is no doubt determined by the particular constituents of the tissue, we postulate that the actual functional form of the st ress adaptation is not.