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 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 leve,l 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 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 mmHg so the partial pressure of O2 would be
approximately 152 mmHg.
 
760 mmHg x 0.20 = 152 mmHg
 
A similar computation can be made for CO2 if we know that
the concentration is approximately 4%. The partial pressure
of CO2 would then be about 0.304 mmHg at sea level.
 
Gas transfer at the alveoli follows the rule of simple
diffusion. Diffusion is movement of molecules along a
concentration gradient from an area of high concentration
to an area of lower concentration. Diffusion is the result
of collisions between molecules. In areas of higher
concentration there are more collisions. The net effect of
this greater number of collisions is a movement toward an
area of lower concentration. In Table 1 it is apparent that
the concentration gradient favors the diffusion of oxygen
into and carbon dioxide out of the blood (Gerking, 1969).
Table 2 shows the decrease in partial pressure of oxygen at
increasing altitudes (Guyton, 1979).
 
Table 1
 
ATMOSPHERIC AIR ALVEOLUS VENOUS BLOOD
 
OXYGEN 152 mmHg (20%) 104 mmHg (13.6%) 40 mmHg
 
CARBON DIOXIDE 0.304 mmHg (0.04%) 40 mmHg (5.3%) 45 mmHg
 
Table 2
 
ALTITUDE (ft.) BAROMETRIC PRESSURE (mmHg) Po2 IN AIR (mmHg)
Po2 IN ALVEOLI (mmHg) ARTERIAL OXYGEN SATURATION (%)
 
0 760 159* 104 97
 
10,000 523 110 67 90
 
20,000 349 73 40 70
 
30,000 226 47 21 20
 
40,000 141 29 8 5
 
50,000 87 18 1 1
 
*this value differs from table 1 because the author used
the value for the concentration of O2 as 21%.
 
The author of table 1 chose to use the value as 20%.
 
In a normal, non-stressed state, the respiratory system
transports oxygen from the lungs to the cells of the body
where it is used in the process of cellular respiration.
Under normal conditions this transport of oxygen is
sufficient for the needs of cellular respiration. Cellular
respiration converts the energy in chemical bonds into
energy that can be used to power body processes. Glucose is
the molecule most often used to fuel this process although
the body is capable of using other organic molecules for
energy.
 
The transfer of oxygen to the body tissues is often called
internal respiration (Grollman, 1978). The process of
cellular respiration is a complex series of chemical steps
that ultimately allow for the breakdown of glucose into
usable energy in the form of ATP (adenosine triphosphate).
The three main steps in the process are: 1) glycolysis, 2)
Krebs cycle, and 3) electron transport system. Oxygen is
required for these processes to function at an efficient
level. Without the presence of oxygen the pathway for
energy production must proceed anaerobically. Anaerobic
respiration sometimes called lactic acid fermentation
produces significantly less ATP (2 instead of 36/38) and
due to this great inefficiency will quickly exhaust the
available supply of glucose. Thus the anaerobic pathway is
not a permanent solution for the provision of energy to the
body in the absence of sufficient oxygen.
 
The supply of oxygen to the tissues is dependent on: 1) the
efficiency with which blood is oxygenated in the lungs, 2)
the efficiency of the blood in delivering oxygen to the
tissues, 3) the efficiency of the respiratory enzymes
within the cells to transfer hydrogen to molecular oxygen
(Grollman, 1978). A deficiency in any of these areas can
result in the body cells not having an adequate supply of
oxygen. It is this inadequate supply of oxygen that results
in difficulties for the body at higher elevations.
 
A lack of sufficient oxygen in the cells is called anoxia.
Sometimes the term hypoxia, meaning less oxygen, is used to
indicate an oxygen debt. While anoxia literally means "no
oxygen" it is often used interchangeably with hypoxia.
There are different types of anoxia based on the cause of
the oxygen deficiency. Anoxic anoxia refers to defective
oxygenation of the blood in the lungs. This is the type of
oxygen deficiency that is of concern when ascending to
greater altitudes with a subsequent decreased partial
pressure of O2. Other types of oxygen deficiencies include:
anemic anoxia (failure of the blood to transport adequate
quantities of oxygen), stagnant anoxia (the slowing of the
circulatory system), and histotoxic anoxia (the failure of
respiratory enzymes to adequately function).
 
Anoxia can occur temporarily during normal respiratory
system regulation of changing cellular needs. An example of
this would be climbing a flight of stairs. The increased
oxygen demand of the cells in providing the mechanical
energy required to climb ultimately produces a local
hypoxia in the muscle cell. The first noticeable response
to this external stress is usually an increase in breathing
rate. This is called increased alveolar ventilation. The
rate of our breathing is determined by the need for O2 in
the cells and is the first response to hypoxic conditions.