Chapter 4 | 193An important and exciting application area of thermodynamics is biological
systems, which are the sites of rather complex and intriguing energy transfer
and transformation processes. Biological systems are not in thermodynamic
equilibrium, and thus they are not easy to analyze. Despite their complexity,
biological systems are primarily made up of four simple elements: hydrogen,
oxygen, carbon, and nitrogen. In the human body, hydrogen accounts for
63 percent, oxygen 25.5 percent, carbon 9.5 percent, and nitrogen 1.4 percent of
all the atoms. The remaining 0.6 percent of the atoms comes from 20 other ele-
ments essential for life. By mass, about 72 percent of the human body is water.
The building blocks of living organisms are cells,which resemble miniature
factories performing functions that are vital for the survival of organisms. A
biological system can be as simple as a single cell. The human body contains
about 100 trillion cells with an average diameter of 0.01 mm. The membrane
of the cell is a semipermeable wall that allows some substances to pass
through it while excluding others.
In a typical cell, thousands of chemical reactions occur every second during
which some molecules are broken down and energy is released and some new
molecules are formed. This high level of chemical activity in the cells, which
maintains the human body at a temperature of 37°C while performing the
necessary bodily tasks, is called metabolism.In simple terms, metabolism
refers to the burning of foods such as carbohydrates, fat, and protein. The rate
of metabolism in the resting state is called the basal metabolic rate,which is
the rate of metabolism required to keep a body performing the necessary
functions (such as breathing and blood circulation) at zero external activity
level. The metabolic rate can also be interpreted as the energy consumption
rate for a body. For an average male (30 years old, 70 kg, 1.8-m^2 body surface
area), the basal metabolic rate is 84 W. That is, the body dissipates energy to
the environment at a rate of 84 W, which means that the body is converting
chemical energy of the food (or of the body fat if the person has not eaten)
into thermal energy at a rate of 84 W (Fig. 4 –37). The metabolic rate
increases with the level of activity, and it may exceed 10 times the basal
metabolic rate when a body is doing strenuous exercise. That is, two people
doing heavy exercising in a room may be supplying more energy to the room
than a 1-kW electrical resistance heater (Fig. 4 –38). The fraction of sensible
heat varies from about 40 percent in the case of heavy work to about 70 per-
cent in the case of light work. The rest of the energy is rejected from the body
by perspiration in the form of latent heat.
The basal metabolic rate varies with sex, body size, general health conditions,
and so forth, and decreases considerably with age. This is one of the reasons
people tend to put on weight in their late twenties and thirties even though they
do not increase their food intake. The brain and the liver are the major sites of
metabolic activity. These two organs are responsible for almost 50 percent of
the basal metabolic rate of an adult human body although they constitute only
about 4 percent of the body mass. In small children, it is remarkable that about
half of the basal metabolic activity occurs in the brain alone.
TOPIC OF SPECIAL INTEREST* Thermodynamic Aspects of Biological Systems
*This section can be skipped without a loss in continuity.
FIGURE 4 –37
An average person dissipates energy to
the surroundings at a rate of 84 W
when resting.
© Vol. 124/PhotoDisc1.2 kJ/s1 kJ/sFIGURE 4 –38
Two fast-dancing people supply more
energy to a room than a 1-kW electric
resistance heater.