Biomedical Engineering Reference
In-Depth Information
1.
INTRODUCTION
An Anacreontick
Busy, curious, thirsty fly,
Gently drink, and drink as I;
Freely welcome to my cup,
Could'st thou sip, and sip it up;
Make the most of Life you may,
Life is short and wears away.
Just alike, both mine and thine,
Hasten quick to their decline;
Thine's a Summer, mine's no more,
Though repeated to threescore;
Threescore Summers when they're gone,
Will appear as short as one.
—
William Oldys
When Oldys wrote this verse in the eighteenth century, poetically express-
ing the similarity of life histories of different organisms and their corresponding
rates of life processes, he surely did not appreciate that the difference in perspec-
tive between a fly and a human is not merely psychological, but has, in fact, a
physiological basis. For example, it is now known that the total number of life-
time heartbeats is approximately the same for all mammals, even though larger
mammals generally live longer than smaller ones. Indeed, lifespan varies in a
systematic way with the body size and body temperature of an organism (1), and
it is believed that this is a consequence of the rate at which organisms live and
process energy. Almost all physiological variables and all biological rates and
times, including heart and respiratory rates (2,3), gestation periods (2,4), devel-
opmental times (5), sleep times (6,7), and even cancer growth rates (8-11), scale
in a systematic and interrelated way with body size and temperature. In addition,
many structural properties, including the radius of the aorta (12), the density of
mitochondria (13), and genome length scale in a similar fashion (14,15). All of
these phenomena scale with mass as a power law with exponents that are in-
variably simple multiples of one quarter. In the past few years we and our col-
leagues have attempted to discover the mathematical form of these scaling
relationships, and perhaps most importantly, to develop mechanistic, dynamical
theories for their origins, based on an underlying set of general principles
(1,12,16). Through the development of these theories and the gathering of em-
pirical data, we have discovered new scaling relationships for the rates and times
of many cellular (13), physiological (5), and ecological processes (17-20).
Our starting point is the assumption that at every organizational level there
exist average idealized organisms, or biological systems, whose properties are
