Modern Engineering Thermodynamics

718 CHAPTER 17: Thermodynamics of Biological Systems
A POSSIBLE DEFINITION OF A LIVING SYSTEM (I.E., LIFE)
Living systems are uniquely characterized by a continuously decreasing entropy over their life spans.
dS
dt living system
= m ds
dt + s dm
< 0
dt living system
becomes more complex at the molecular level. Note that this does not happen with the aging of machines,
whose entropy generally increases monotonically with age. Consequently, we can postulate that living systems
are defined by the following unique characteristic:
_Q
T b
+∑
in
_ms −∑
out
_ms
> _S P (17.28)
and that death occurs when this inequality is violated. What does Eq. (17.28) tell us about the system as it ages?
Based on our experiences with the machinery of the Industrial Age, we intuitively feel that old age corresponds
to degeneration. In humans, the skin becomes wrinkled, teeth and hair are permanently lost, hearing and sight
diminish, joints stiffen—itseemsasthoughpeople“wear out” as they become older. Actually, what are normally
described as degenerative signs of aging are really the result of continued growth, that is, continued systemic
molecular organization. Skin becomes wrinkled because the collagen molecules of the skin crosslink to
form a more rigid (less elastic) and complex structure. The same thing happens in the lens of the eye, where the
macromolecular cross-linking makes the lens so rigid that the eye muscles can no longer change its shape to
make it focus properly. Molecular cross-linking also causes loss of hearing sensitivity, and cross-linking within
the lubricating fluid of the joints causes this fluid to thicken, which makes the joints arthritic and painful to
move. We also see cross-linking and thickening in other biofluids such as blood. It appears that growth in molecular
complexity continues long after physical maturity is reached and is the cause of many of the common
symptoms of aging. If a biological system were to continue to grow (but not add mass), then its ultimate state
would be one of complete rigidity with a very low entropy but with little mobility potential and very low predator
survivability. Thus, a living system becomes more delicate as it ages beyond physical maturity and consequently
is more prone to death resulting from failure of one of its major subsystems, such as the circulatory or
the respiratory system. Cancer is curious in that it represents a reversion to cellular growth and appears to function
as a mechanism for preventing the entropy of a living system from becoming too low.
According to the inequality of Eq. (17.28), the life span of a living system could be extended by decreasing
the system (or body) temperature. Thus, even though _Q is decreasing with age, the ratio _Q/T b could be made
as large as desired by selectively lowering the body temperature. Figure 17.14 presents survival curves for common
houseflies raised from birth in environments of different (but constant) temperatures. The longest life
spans occur at the lowest environmental temperature
(16°C). These insects have also been shown to exhibit
increased life spans when they were raised for part of
their lives at one temperature and then spent the
remainder of their lives at a lower temperature. Similarly,
their life spans have been shortened by raising
the environmental temperature slightly.
Using survival curves such as those of Figure 17.14,
researchers developed survival equations similar
to those used in describing the kinetics of first-order
chemical reactions,
dN
dt
= −k d N (17.29)
where N is the number of survivors at time t, and k d is a
death rate constant. The constant k d is often found to be
independent of t but dependent on the environmental
Percent surviving
100
90
80
70
60
50
40
30
20
10
0
30°C 27°C 21°C 18°C
25 50 75 100 125 150 175
Age (days)
FIGURE 17.14
Survival curves for houseflies raised at different constant
environmental temperatures.