Modern Engineering Thermodynamics

702 CHAPTER 17: Thermodynamics of Biological Systems
CRITICAL THINKING
The energy conversion efficiencies for plants and animals defined in Eqs. (17.14) and (17.15) are what we define in Chapter
10 as a first law efficiency. Can you use the general definition of a second law efficiency given in Chapter 10 to formulate a
second law efficiency for plants and animals? What difficulties are encountered in evaluating this new efficiency?
Our conceptual understanding of physiological work is often quite different from our earlier (Chapter 4)
definition of thermodynamic work. For example, when an animal walks along a horizontal surface it does no
net thermodynamic work. There is no net change in kinetic or potential energy, and there is no appreciable sliding
friction between the animal’s feet and the ground. Only when the animal moves against an external force
(such as hydrodynamic drag or inertia forces) is any classical thermodynamic work done. Walking does involve
what we culturally call work, but in thermodynamic jargon the energy associated with constant velocity motion
along a horizontal plane (in the absence of hydrodynamic drag) merely involves a net conversion of internal
energy into heat. Thus, the thermodynamic efficiency of this type of motion in animals (or machines) is zero. If,
instead, an animal walks on a horizontal treadmill, then it does do thermodynamic work. This work appears as
friction or electricity, depending in the treadmill design. Part of the friction in this case is external to the animal
and is measurable as work in the classical thermodynamic sense. The remaining part of the energy expenditure
is internal losses within the animal and appears as metabolic heat. Note, however, that the thermodynamic
work efficiency of walking on a treadmill returns to zero if the entire treadmill apparatus is included in the system
with the animal.
A key element in understanding the thermodynamics of biological systems is comprehending the role of the heat
transfer term in the energy rate balance equation of these systems. Since this equation by itself is useful only if
you have just one unknown term, since it is not usually satisfactory to simply ignore or set equal to zero those
terms for which we do not have values, and since ðdU/dtÞ system
is perhaps the most difficult term of all to measure
accurately, then it becomes absolutely necessary that a means be found to give accurate measurements of _Q:
17.5 METABOLISM
The metabolic energy in the resting state is called the basal metabolic rate (BMR). The BMR is essentially the
energy required to keep the molecular machinery of life operating at a zero activity level. Similar measurements
at a higher activity level produce intermediary metabolic rate results. The basal metabolic rate for humans
depends on age, sex, height, general health conditions, and the like. Figure 17.6 shows the variation in the
average BMR per unit body surface area for human malesandfemalesasafunctionofage.Itisnotuncommon
to have BMR variations around these normal (or average) values of ±15% for any one individual.
Table 17.3 shows the breakdown in energy consumption comprising the BMR in the adult human body. The
large energy consumption of the brain is surprising; the brain of a five-year-old child may account for up to
50% of its BMR.
300
Basal metabolic rate
per unit surface area (kJ/m 2 h)
280
260 Male
240
220
200
Human metabolic rate
180
160
140
Female
120
100
0 10 20 30 40 50 60 70 80
Age (years)
FIGURE 17.6
Average BMR per unit area for humans vs. age.