Scaling - Rivers, blood and transportation networks - Reply

The idea behind our theorem1 is simple. It can be illustrated by using airline travel as an example. Consider a stream of people (blood) leaving London (heart) at a steady rate and fanning out to all parts of the world (body). The number of people leaving London each day and arriving elsewhere at their final destinations (metabolic rate) is denoted by B. Assuming that the people travel along a locally connected network and that the transit time for each local hop is the same (say, 1 day), the number of people in transit at any given time (blood volume) is proportional to B, but with a proportionality constant that is given by the mean number of hops from all the destination cities to London.