Download Body Temperature

adapted from Chemistry: The Central Science, 2003.
For most of us, the question, “Are you running a fever?” was one of our first
introductions to medical diagnosis. Indeed, a deviation in body temperature of
only a few degrees indicates that something is amiss. In the laboratory you may
have tried to maintain a solution or a water bath at a constant temperature, only
to find how difficult it can be to keep the solution within a very narrow
temperature range. Yet our bodies manage to maintain a near-constant
temperature in spite of widely varying weather, levels of physical activity, and
periods of high metabolic activity (such as after a meal). How does the human
body manage this task, and how does it relate to the topics we have discussed
this chapter?
Maintaining a near-constant temperature is one of the primary physiological
functions of the human body. Normal body temperature generally ranges from
35.8--37.2°C (96.5--99°F). This very narrow temperature range is essential to
proper muscle function and to the control of the rates of biochemical reactions in
the body. This temperature is regulated by a portion of the human brain stem
called the hypothalamus. The hypothalamus acts as a thermostat for body
temperature. When body temperature rises above the high end of the normal
range, the hypothalamus triggers mechanisms to lower the temperature. It
likewise triggers mechanisms to increase the temperature if the body
temperature drops too low.
To qualitatively understand how the body’s heating and cooling mechanisms
operate, you need to view the body as a thermodynamic system. The body
increases its internal energy content by ingesting foods from the surroundings.
The foods, such as glucose (C6H12O6), are metabolized – a process that is
essentially controlled oxidation to CO2 and H2O:
C6H12O6(s) + 6 O2(g)
6 CO2(g) + 6 H2O(l)
ΔH = -2803 kJ
Roughly 40% of the energy produced is ultimately used to do work in the form of
muscle and nerve contractions. The remainder of the energy is released as heat,
part of which is used to maintain body temperature. When the body produces
too much heat, as in the times of heavy physical exertion, it dissipates the excess
to the surroundings.
Heat is transferred from the body to the surroundings primarily by radiation,
convection, and evaporation. Radiation is the direct loss of heat from the body
to cooler surroundings, much as a hot stovetop radiates heat to its surroundings.
Convection is heat loss by virtue of heating air in contact with the body. The
heated air rises and is replaced with cooler air, and the process continues.
Warm clothing, which usually consists of insulating layers of material with “dead
air” in between, decreases convective heat loss in cold weather. Evaporative
cooling occurs when perspiration is generated at the skin surface by sweat
glands. Heat is removed from the body as the perspiration evaporates into the
surroundings. Perspiration is predominately water, so the process involved is the
endothermic conversion of liquid water into water vapor:
H2O(l)
H2O(g)
ΔH = +44.0 kJ
The speed with which evaporative cooling occurs decreases as the
atmospheric humidity increases, which is why people seem to be
more sweaty and uncomfortable on hot and humid days.
When the hypothalamus senses that the body temperature has risen too
high, it increases heat loss from the body in two principal ways. First, it
increases the flow of blood near the surface of the skin, which allows for
increased radiational and convective cooling. The reddish “flushed”
appearance of a hot individual is the result of this increased subsurface blood
flow. Second, the hypothalamus stimulates the secretion of perspiration from
the sweat glands, which increase evaporative cooling. During periods of extreme
activity, the amount of liquid secreted as perspiration can be as high as 2–4 liters
per hour. As a result, water must be replenished to the body during these
periods. If the body loses too much fluid through perspiration, it will no longer be
able to cool itself and blood volume decreases, which can lead to heat
exhaustion or the more serious and potentially fatal heat stroke, during which
the body temperature can rise to as high as 41-45°C (106-113°F).
When body temperature drops too low, the hypothalamus
decreases the blood flow to the surface of the skin,
thereby decreasing heat loss. It also triggers small
involuntary contractions of muscles; the biochemical
reactions that generate the energy to do this work also
generate more heat for the body.
When these
contractions get large enough—as when the body feels a
chill—a shiver results. If the body is unable to maintain a
temperature above 35°C (95°F), the very dangerous
condition called hypothermia can result.
The ability of the human body to maintain its temperature by “tuning” the amount
of heat it generates and transfers to its surroundings is truly remarkable. If you
take courses in human anatomy and physiology, you will see many other
applications of thermochemistry and thermodynamics to the ways in which the
human body works.