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The Effects of Altitude on Human Physiology

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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

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