Effects Of Altitude On Human Physiology

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

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 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.


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