Modern Engineering Thermodynamics

716 CHAPTER 17: Thermodynamics of Biological Systems
EXAMPLE 17.9
Determine the locomotion transport number of a 60.0 kg person traveling at 15.0 miles per hour on a 15.0 kg bicycle while
expending 400. W of power pedaling.
Solution
Using Eq. (17.22) with P = 400: W,w = ð60:0 + 15:0Þð9:81Þ = 735 N, and V = 15:0mph= ð15:0 miles/hÞð1:609 km/mileÞ =
24:14 km/h = 24,140 m/h gives
T =
P
wV = ð400:N .m/sÞð3600:s/hÞ
= 0:0812
ð735 NÞð24,140 m/hÞ
Exercises
25. Determine the locomotion transport number of a 0.100 kg fish using 0.150 J/s to swim at a velocity of 0.500 m/s.
Answer: T fish = 0.30.
26. An aircraft weighing 22.0 × 10 3 lbf uses 3.00 × 10 3 horsepower to fly at 200. mph. Determine its locomotion transport
number. Answer: T airplane = 0.256.
27. Determine the most efficient velocity for the bicyclist in Example 17.9 if his or her basal metabolic rate is 73.1 J/s,
frontal area is 0.750 m 2 , aerodynamic drag coefficient is 1.50, and the local density of air is 1.21 kg/m 3 .
Answer: V most efficient = 3.77 m/s = 13.6 km/h.
Figure 17.13 presents data on the dimensionless locomotion
transport number T vs. body mass for a large
variety of birds, fish, land animals, and machines. The
value of the locomotion transport number for an animal
of a given mass clearly depends directly on the percentage
of its body mass dedicated to locomotion
muscles. This percentage is greatest in fish, next largest
in birds, and smallest in two- and four-legged runners.
Note that fish have the lowest T values and are therefore
the most efficient mobile animals. Figure 17.13 also has
points for various machines, and the machines that are
the most efficient at transport are trains and ships.
The locomotion efficiency for a given animal becomes
much lower when it is forced to travel in a different
medium. A human consumes 30 times more energy in
swimming than does a fish of equivalent mass. Penguins
are highly adapted toswimming,butonland
they waddle around with a locomotion transport number
twice as high as any land animal of equivalent mass.
Log (transport number)
1.5
1.0
0.5
0.0
−0.5
−1.0
−1.5
Birds and insects
Fish
Land animals
Humans
Motorcycles
Automobiles
Airplanes
Trains
Bicycles and
ships
−7 −6 −5 −4 −3 −2 −1 0 1 2 3 4 5 6 7
Log (body mass in kg)
FIGURE 17.13
The average locomotion transport number vs. mass for a variety
of animals and machines.
17.9 THERMODYNAMICS OF AGING AND DEATH
There are several important theories of biological aging, but perhaps the most popular is that of molecular error
propagation. This theory states that molecular reproduction by enzymes is not perfect. The entropy production
of molecular synthesis over a significant period of time cannot be insignificant with regard to the information
content (or structure) of the molecule being synthesized. Thus, both evolution and aging depend on how the living
system responds to error accumulation at the molecular level. Ultimately, the errors build up such that the
system can no longer function properly and a catastrophic event leading to death occurs. Equation (17.11) is
the energy rate balance applied to the living system of Figure 17.5. It accounts for all the energy flows into and
out of the system and the state of the energy within the system at any time. It reveals nothing about the aging
process or life span of the system. However, the life span of mammals in captivity has been accurately correlated
with body mass as
Life span of mamals ðin yearsÞ = 11:8 m 0:2 (17.26)
where the body mass m is in kilograms. Table 17.6 lists the pulse rates, breathing rates, and life spans of various
mammals calculated using Eqs. (17.18), (17.19), and (17.26). Except for human life span, the results of these
calculations are reasonably accurate.