The exponential increase in mortality rate with age attributed to wearing-out of biological components

A mortality rate which increases exponentially with advancing age is a curious phenomenon seen in several higher organisms including man. It is explained here through an analogy between death of the organism and failure of a complex physical system consisting of many components. According to reliability theory the risk of failure in such a system increases exponentially whenever the components are connected in series and are all subject to failure behavior of a certain general type. Existing theory is however restricted to systems of identical components and in order to enhance the model's biological relevance the theory is extended to include non-identical components. It is shown that the components are all required to undergo a “wearing-out” process in which the risk of failure increases progressively. Conceptually, biological “components” may be identified by subdividing the organism into minimal subsets of cells such that each subset is vital for survival, e.g. a critical mass of heart muscle, or a critical branch of the arterial tree. According to this schema the wearing-out of each component would stem from the wearing-out of its constituent cells. Another implication of the theory is that the lifespan of subjects who have managed to reach age 30 is distributed according to an extreme value distribution. The theory could be useful in the analysis of the mortality patterns in long-term clinical trials to detect the costs and benefits of therapeutic intervention.