Modern Engineering Thermodynamics

17.9 Thermodynamics of Aging and Death 717
Table 17.6 Metabolic Characteristics of Typical Mammals
Mammal
Body Mass (kg)
Pulse Rate
(beats/min)
Breathing Rate
(breaths/min)
Life Span (years)
Shrew 0.003 1030 230 3.7
Mouse 0.03 580 130 5.9
Rat 0.2 360 80 8.6
Cat 2.8 190 42 14
Dog 15.9 120 27 21
Horse 700 47 10 44
Elephant 4000 30 6.7 62
Even though we know that the metabolic heat generation rate _Q decreases with increasing age (e.g., see
Figure 17.6), this effect must be offset by increasing size (i.e., growth) and eating less or exercising less as age
increases. Experiments in which test animals were fed a very low (starvation level) daily diet showed that
they generally had a lower metabolic rate and lived longer than did their counterparts who were fed a normal
or excessive diet. These particular results, however, occur only when the starvation diet was begun before the
animal reached sexual maturity. If it was begun later in life, it had no significant effect on metabolic rate or on
life span.
An entropy rate balance on a living system is
_Q
+∑ _ms −∑ _ms + _S P = dS
T b in out
dt
(17.27)
The first term is the entropy transport due to the metabolic heat transfer, and since _Q < 0, it is negative. The
combination of the second and third terms is the net entropy transport into the system via the mass flow
of food, respiration, and wastes. Since the entropy of the incoming food is lower than the entropy of the
outgoing wastes (both are at the same temperature, but the molecular order of the food is more complex
than that of the wastes) and the input and output mass flow rates averaged over a long period of time are
essentially the same, these two terms taken together also are negative. The last term on the left side of
Eq. (17.27) is the rate of entropy production, which by the second law of thermodynamics must always be
positive. The term on the right side is the time rate of changeoftheentropyoftheentirebiologicalsystem,
and it can be either positive or negative depending on the net sign of the left side. Thus, we find that, for
any living system,
_S P > 0
_Q
T b
< 0
∑
in
_ms −∑
out
_ms < 0
But, these conditions alone are not sufficient to define a living system. The one characteristic that seems to make
a living system unique is its peculiar affinity for self-organization, and this characteristic corresponds to a continual
decrease in the system’s entropy over its life span. As the system “lives,” it grows and ages and generally
CRITICAL THINKING
Is life unique in your opinion? That is, are living systems scientifically distinguishable from nonliving systems? Given a
sealed box containing an object, how can you tell whether or not the object is living without opening the box? What physical
tests could you perform to determine the life state of the object in the box?