Concepts of Lifetime Fitness

September 1, 1997

Homeostasis is the state of equilibrium in which the internal environment of the
human body remains relatively constant.  Two excellent examples of homeostasis
are how the body maintains a constant temperature and blood pressure during
strenuous physical activity or exercise.  Although there are many other
activities in the body that display homeostasis, I will only discuss these two.  
Temperature in the human body is usually kept at approximately 37 degrees
Celsius.  To maintain such a strict temperature, the body has a few functions to
combat the outside elements.  People cannot make themselves cold as readily as
make themselves hot, however I will mention both homeostasis functions.  When
the external temperature decreases, a portion of the brain called the
hypothalamus detects the drop by means of the blood.  To compensate, the brain
sends chemical and electrical impulses to the muscles.  These impulses tell the
muscles to begin to contract and relax at very high intervals.  This is commonly
known as shivering.  The production of Adenosine Triphosphate or ATP in the
mitochondria of the muscles produces heat.  If the body temperature does not
rise immediately after this, then a second function begins. The brain will
signal the blood vessels near the skin to constrict or narrow in diameter.  This
occurs so the heat deep in the muscles is conserved.  Since the vessels are now
smaller in diameter, less blood is needed to fill them.  Since less blood is
needed through the vessels, the heart begins to slow.  If the body remains in
this slowed state, hypothermia could result.  Hypothermia is the condition in
which metabolic processes are inhibited.  The medical world has taken advantage
of this by inducing hypothermia in patients that are undergoing organ

To fight temperatures higher than normal, as in exercise or on hot days, the
body reacts in the opposite way than with cold.  Again, the hypothalamus detects
the change of temperature in the blood.  The brain signals blood vessels not to
constrict, but to dilate.  This increases the diameter of the vessels, and
results in the need for more blood. Since more blood is needed to fill the
vessels, the heart pumps faster and that causes respiration to increase.  The
increased respiration will make the body exhale some of the internal heat, like
placing a fan in a window to cool a room.  The blood vessels are dilated so the
heat deep in the muscles is easily released.  Another commonly known mechanism
to fight heat is sweating.  Sweat glands found throughout the body are
stimulated by the hypothalamus to excrete sweat and when the sweat evaporates,
the skin is cooled.  If the body is not cooled by the time all of the internal
water supply is used, it could go into hypothermia. This is when the body
becomes dehydrated and proteins begin to denature.  Hypothermia can result in
certain death if the water supply is not immediately replenished.  Some
advantages to these mechanisms are the cleansing effect of sweating and weight
loss.  Sweat, when excreted, removes waste materials such as bacteria and water. 
Fat material, during exercise, is actually "eaten" by the body thus reducing
overall weight.

The second example of homeostasis is blood pressure regulation.  When the
hydrostatic pressure of blood is above normal, pressure sensors in the blood
vessels tell the brain through chemical means.  The brain will then stimulate
the heart to contract or beat in slower intervals.  This will cause less blood
to enter the blood vessels and that will lower the hydrostatic pressure.  If the
pressure is lower than normal, the exact opposite happens.  The sensors in the
vessels tell the brain and the brain will then make the heart beat faster so
more blood enters the vessels and the pressure is raised.

The body uses many mechanisms to regulate temperature and blood pressure.  Be it
stimuli to the heart from the brain or messages from the blood, the body
maintains its internal environment through a process called homeostasis.

Category: Miscellaneous