Chaos theory: the transforming role of functional systems

The review of applications of chaos theory in physiology shows, that this theory makes possible to get quantitative measure of degree of the order for such processes as neuronal activity, heart rate, electroencephalogram etc. The basic concepts of chaos theory--fractal dimension. Liapunov exponents, entropy, some algorithms for obtaining of quantitative characteristics of degree of the order for different processes--have been considered in this paper. The main areas for application chaos theory in physiology and medicine have been revealed and some examples of practical and theoretical achievements in this new region of investigation are given, as well as existing limitations and problems with results interpretation. Entirely new possibilities for understanding of the order in physiological processes are shown when estimation is made from the point of view of the functional system theory.