Effects Of Altitude On Human Physiology Essay
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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 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 choose to use the value as 20%.
CELLULAR RESPIRATION
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.
ANOXIA
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 oxygendemand 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.
BODY RESPONSE TO ANOXIA
If increases in the rate of alveolar respiration are insufficient to supply the oxygen needs of the
cells the respiratory system responds by general vasodilation. This allows a greater flow of blood
in the circulatory system. The sympathetic nervous system also acts to stimulate vasodilation
within the skeletal muscle. At the level of the capillaries the normally closed precapillary sphincters
open allowing a large flow of blood through the muscles. In turn the cardiac output increases both
in terms of heart rate and stroke volume. The stroke volume, however, does not substantially
increase in the non-athlete (Langley, et.al., 1980). This demonstrates an obvious benefit of regular
exercise and physical conditioning particularly for an individual who will be exposed to high
altitudes. The heart rate is increased by the action of the adrenal medulla which releases
catecholamines. These catecholamines work directly on the myocardium to strengthen
contraction. Another compensation mechanism is the release of renin by the kidneys. Renin leads
to the production of angiotensin which serves to increase blood pressure (Langley, Telford, and
Christensen, 1980). This helps to force more blood into capillaries. All of these changes are a
regular and normal response of the body to external stressors. The question involved with altitude
changes becomes what happens when the normal responses can no longer meet the oxygen
demand from the cells?
ACUTE MOUNTAIN SICKNESS
One possibility is that Acute Mountain Sickness (AMS) may occur. AMS is common at high
altitudes. At elevations over 10,000 feet, 75% of people will have mild symptoms (Princeton,
1995). The occurrence of AMS is dependent upon the elevation, the rate of ascent to that
elevation, and individual susceptibility.
Acute Mountain Sickness is labeled as mild, moderate, or severe dependent on the presenting
symptoms. Many people will experience mild AMS during the process of acclimatization to a
higher altitude. In this case symptoms of AMS would usually start 12-24 hours after arrival at a
higher altitude and begin to decrease in severity about the third day....
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