Chapter 4 Homeostatic Mechanisms • MHR 109Human body temperature is also an example
of dynamic equilibrium. However, homeostasis is
much more complex than the temperature control
mechanisms in a house. The human body features
a series of control mechanisms to keep its
temperature at approximately 37°C. For example,
when your skin is cold, “goose bumps” appear.
Goose bumps are an attempt by the body to fluff up
non-existent fur. The fluffed-up fur of other mammals
traps air that acts as an extra layer of insulation.
This reaction is similar to birds fluffing up their
feathers in winter. In humans the reaction is not
very effective because we have minimal body hair.
A second control mechanism is shivering. When
we shiver, much of the energy processed by the
muscle action is given off as heat, which is
transferred throughout the body. Shivering is not
something we consciously control. Rather, it is the
involuntary contraction of muscles to generate heat
when the body detects a drop in temperature (see
Figure 4.3).
A third control mechanism employed to maintain
internal temperature is the transfer of heat energy
to the body’s most critical parts. When this
mechanism, known as vasoconstriction, occurs, the
diameter of blood vessels near the skin is reduced
and blood circulation is concentrated in the core of
the body to keep the major organs functioning. If
the body’s temperature continues to drop,
hypothermiaresults. At this point, the body no
longer has the energy to shiver; extremities such
as fingers, toes, and ears begin to freeze; and blood
flow to the brain decreases, resulting in impaired
judgement, sleepiness, and eventual loss of
consciousness. Ultimately, death may occur.
To maintain homeostasis, the body also
frequently needs to rid itself of excess heat. It
accomplishes this in two ways. First, as shown in
Figure 4.4, sweat is released onto the surface of the
skin. Energy in the form of heat is then given off as
the sweat evaporates. Second, the body reverses the
mechanism of vasoconstriction. In a process called
vasodilation, blood vessels near the skin dilate to
transport more blood (and thus heat) to the skin’s
surface, where the heat is released. If excess heat
cannot be eliminated fast enough, heat exhaustion
and hyperthermia, or an unusually high body
temperature, may result. As with hypothermia,
hyperthermia can cause death.Occasionally hypothermia can save lives. Some people who
have fallen into very cold water have been revived up to one
hour later. In most cases, drowning victims suffer brain
damage after five minutes due to oxygen deprivation.
However, hypothermia slows down bodily processes,
reducing the demand for oxygen by the brain. Victims
of near-drowning in cold water may thereby escape
brain damage.BIO FACT
Figure 4.3The muscle action of shivering generates heat,
which is then distributed throughout the body.
Figure 4.4To rid itself of excess heat, the body releases
sweat onto the skin. Cooling occurs as the sweat evaporates.