The Effects of Altitude On Human Physiology


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.

HIGH ALTITUDE

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).

HUMAN RESPIRATORY SYSTEM

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 place.

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 alveoli.

GAS TRANSFER

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