A THEORY OF MIXED CHAINS APPLIED TO SAFETY FACTORS IN BIOLOGICAL-SYSTEMS

The skeleton of a leg is a chain of bones, breakage of any one of whic h makes the whole leg useless. The oxygen-transport system of mammals is a chain of processes, any one of which may limit the rate of use of oxygen. The simple argument that a chain is as strong as its weakest link suggests that chains both of structures and of processes would be best designed to have the strengths or capacities of all their links matched. However, this paper shows that if the performance of a link b uilt to a given design is to some extent unpredictable, it may be bett er to design links for unequal performance. In chains of structures, l inks made from less expensive or more variable materials should be des igned to be somewhat stronger than others. In chains of processes, the plant for less expensive or more predictable processes should be desi gned for higher maximum rates of working. These conclusions offer poss ible explanations for some observations regarding frequencies of fract ure in horse leg bones and the diffusing capacities of mammalian lungs . (C) 1997 Academic Press Limited.