A system approach modelling of the three-stage non-linear kinetics in biological ageing

There is a widely observed non-linear kinetics in the ageing of biological systems, which is characterised by three successive stages, (1) the ageing rate is firstly high, but decreases quickly to a minimum, from which (2) it remains nearly constant during the major part of the process until (3) it starts increasing again up to the final collapse of the system. Such kinetics are also encountered in the ageing of inert systems. It is shown that a model useful for the follow-up of operating inert systems allows to find back typical curves and laws related to the ageing of biological systems (mortality rate curves, survival curves, growth curves, Gompertz law, ...). In this model, ageing is seen as a multifactorial process. The classical concepts of lifespan, longevity and life expectancy are given new light using the model, which also gives clues to explain both the discrepancy in the age of death of individuals in a given population and the wide range of lifespans of species encountered in nature. Finally, the model shows in which directions accelerated senescence testing protocols should be orientated for a better understanding of the underlying phenomena and for life prediction purposes. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.