Changes in altitude have a profound effect on the human body. The body
attempts to maintain a state of homeostasis or balance to ensure the optimal
operating environment for its complex chemical systems. Any change from this
homeostasis is a change away from the optimal operating environment. The body
attempts to correct this imbalance. One such imbalance is the effect of
increasing altitude on the body\'s ability to provide adequate oxygen to be
utilized in cellular respiration. With an increase in elevation, a typical
occurrence when climbing mountains, the body is forced to respond in various
ways to the changes in external
environment. Foremost of these changes is the diminished ability to obtain
oxygen from the atmosphere. If the adaptive responses to this stressor are
inadequate the performance of body systems may decline dramatically. If
prolonged the results can be serious or even fatal. In looking at the effect
of altitude on body functioning we first must understand what occurs in the
external environment at higher elevations and then observe the important
changes that occur in the internal environment of the body in response.

In discussing altitude change and its effect on the body mountaineers
generally define altitude according to the scale of high (8,000 - 12,000
feet), very high (12,000 - 18,000 feet), and extremely high (18,000+ feet),
(Hubble, 1995). A common misperception of the change in external environment
with increased altitude is that there is decreased oxygen. This is not
correct as the concentration of oxygen at sea level is about 21% and stays
relatively unchanged until over 50,000 feet (Johnson, 1988).
What is really happening is that the atmospheric pressure is decreasing and
subsequently the amount of oxygen available in a single breath of air is
significantly less. At sea level the barometric pressure averages 760 mmHg
while at 12,000 feet it is only 483 mmHg. This decrease in total atmospheric
pressure means that there are 40% fewer oxygen molecules per breath at this
altitude compared to sea level (Princeton, 1995).

The human respiratory system is responsible for bringing oxygen into the
body and transferring it to the cells where it can be utilized for cellular
activities. It also removes carbon dioxide from the body. The respiratory
system draws air initially either through the mouth or nasal passages. Both
of these passages join behind the hard palate to form the pharynx. At the
base of the pharynx are two openings. One, the esophagus, leads to the
digestive system while the other, the glottis, leads to the lungs. The
epiglottis covers the glottis when swallowing so that food does not enter the
lungs. When the epiglottis is not covering the opening to the lungs air may
pass freely into and out of the trachea.
The trachea sometimes called the "windpipe" branches into two bronchi which
in turn lead to a lung. Once in the lung the bronchi branch many times into
smaller bronchioles which eventually terminate in small sacs called alveoli.
It is in the alveoli that the actual transfer of oxygen to the blood takes
The alveoli are shaped like inflated sacs and exchange gas through a
membrane. The passage of oxygen into the blood and carbon dioxide out of the
blood is dependent on three major factors: 1) the partial pressure of the
gases, 2) the area of the pulmonary surface, and 3) the thickness of the
membrane (Gerking, 1969). The membranes in the alveoli provide a large
surface area for the free exchange of gases. The typical thickness of the
pulmonary membrane is less than the thickness of a red blood cell. The
pulmonary surface and the thickness of the alveolar membranes are not
directly affected by a change in altitude. The partial pressure of oxygen,
however, is directly related to altitude and affects gas transfer in the

To understand gas transfer it is important to first understand something
about the
behavior of gases. Each gas in our atmosphere exerts its own pressure and
acts independently of the others. Hence the term partial pressure refers to
the contribution of each gas to the entire pressure of the atmosphere. The
average pressure of the atmosphere at sea level is approximately 760 mmHg.
This means that the pressure is great enough to support a column of mercury
(Hg) 760 mm high. To figure the partial pressure of oxygen you start with the
percentage of oxygen present in the atmosphere which is about 20%. Thus
oxygen will constitute 20% of the total atmospheric pressure at any given
level. At sea level the total atmospheric pressure is 760