Essay, Research Paper
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).
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.
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
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).
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
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
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
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. The symptoms of
mild AMS are headache, dizziness, fatigue, shortness of breath, loss of appetite,
nausea, disturbed sleep, and a general feeling of malaise (Princeton, 1995).
These symptoms tend to increase at night when respiration is slowed during sleep.
Mild AMS does not interfere with normal activity and symptoms generally subside
spontaneously as the body acclimatizes to the higher elevation.
Moderate AMS includes a severe headache that is not relieved by
medication, nausea and vomiting, increasing weakness and fatigue, shortness of
breath, and decreased coordination called ataxia (Princeton, 1995). Normal
activity becomes difficult at this stage of AMS, although the person may still
be able to walk on their own. A test for moderate AMS is to have the individual
attempt to walk a straight line heel to toe. The person with ataxia will be
unable to walk a straight line. If ataxia is indicated it is a clear sign that
immediate descent is required. In the case of hiking or climbing it is important
to get the affected individual to descend before the ataxia reaches the point
where they can no longer walk on their own.
Severe AMS presents all of the symptoms of mild and moderate AMS at an
increased level of severity. In addition there is a marked shortness of breath
at rest, the inability to walk, a decreasing mental clarity, and a potentially
dangerous fluid buildup in the lungs.
There is really no cure for Acute Mountain Sickness other than
acclimatization or descent to a lower altitude. Acclimatization is the process,
over time, where the body adapts to the decrease in partial pressure of oxygen
molecules at a higher altitude. The major cause of altitude illnesses is a rapid
increase in elevation without an appropriate acclimatization period. The process
of acclimatization generally takes 1-3 days at the new altitude. Acclimatization
involves several changes in the structure and function of the body. Some of
these changes happen immediately in response to reduced levels of oxygen while
others are a slower adaptation. Some of the most significant changes are:
Chemoreceptor mechanism increases the depth of alveolar ventilation.
This allows for an increase in ventilation of about 60% (Guyton, 1969). This is
an immediate response to oxygen debt. Over a period of several weeks the
capacity to increase alveolar ventilation may increase 600-700%.
Pressure in pulmonary arteries is increased, forcing blood into portions
of the lung which are normally not used during sea level breathing.
The body produces more red blood cells in the bone marrow to carry
oxygen. This process may take several weeks. Persons who live at high altitude
often have red blood cell counts 50% greater than normal.
The body produces more of the enzyme 2,3-biphosphoglycerate that
facilitates the release of oxygen from hemoglobin to the body tissues (Tortora,
The acclimatization process is slowed by dehydration, over-exertion, alcohol and
other depressant drug consumption. Longer term changes may include an increase
in the size of the alveoli, and decrease in the thickness of the alveoli
membranes. Both of these changes allow for more gas transfer.
TREATMENT FOR AMS
The symptoms of mild AMS can be treated with pain medications for
headache. Some physicians recommend the medication Diamox (Acetazolamide). Both
Diamox and headache medication appear to reduce the severity of symptoms, but do
not cure the underlying problem of oxygen debt. Diamox, however, may allow the
individual to metabolize more oxygen by breathing faster. This is especially
helpful at night when respiratory drive is decreased. Since it takes a while for
Diamox to have an effect, it is advisable to start taking it 24 hours before
going to altitude. The recommendation of the Himalayan Rescue Association
Medical Clinic is 125 mg. twice a day. The standard dose has been 250 mg., but
their research shows no difference with the lower dose (Princeton, 1995).
Possible side effects include tingling of the lips and finger tips, blurring of
vision, and alteration of taste. These side effects may be reduced with the 125
mg. dose. Side effects subside when the drug is stopped. Diamox is a sulfonamide
drug, so people who are allergic to sulfa drugs such as penicillin should not
take Diamox. Diamox has also been known to cause severe allergic reactions to
people with no previous history of Diamox or suffer allergies. A trial course of
the drug is usually conducted before going to a remote location where a severe
allergic reaction could prove difficult to treat. Some recent data suggests that
the medication Dexamethasone may have some effect in reducing the risk of
mountain sickness when used in combination with Diamox (University of Iowa,
Moderate AMS requires advanced medications or immediate descent to
reverse the problem. Descending even a few hundred feet may help and definite
improvement will be seen in descents of 1,000-2,000 feet. Twenty-four hours at
the lower altitude will result in significant improvements. The person should
remain at lower altitude until symptoms have subsided (up to 3 days). At this
point, the person has become acclimatized to that altitude and can begin
ascending again. Severe AMS requires immediate descent to lower altitudes (2,000
- 4,000 feet). Supplemental oxygen may be helpful in reducing the effects of
altitude sicknesses but does not overcome all the difficulties that may result
from the lowered barometric pressure.
This invention has revolutionized field treatment of high altitude
illnesses. The Gamow bag is basically a portable sealed chamber with a pump. The
principle of operation is identical to the hyperbaric chambers used in deep sea
diving. The person is placed inside the bag and it is inflated. Pumping the bag
full of air effectively increases the concentration of oxygen molecules and
therefore simulates a descent to lower altitude. In as little as 10 minutes the
bag creates an atmosphere that corresponds to that at 3,000 – 5,000 feet lower.
After 1-2 hours in the bag, the person’s body chemistry will have reset to the
lower altitude. This lasts for up to 12 hours outside of the bag which should be
enough time to travel to a lower altitude and allow for further acclimatization.
The bag and pump weigh about 14 pounds and are now carried on most major high
altitude expeditions. The gamow bag is particularly important where the
possibility of immediate descent is not feasible.
OTHER ALTITUDE-INDUCED ILLNESS
There are two other severe forms of altitude illness. Both of these
happen less frequently, especially to those who are properly acclimatized. When
they do occur, it is usually the result of an increase in elevation that is too
rapid for the body to adjust properly. For reasons not entirely understood, the
lack of oxygen and reduced pressure often results in leakage of fluid through
the capillary walls into either the lungs or the brain. Continuing to higher
altitudes without proper acclimatization can lead to potentially serious, even
HIGH ALTITUDE PULMONARY EDEMA (HAPE)
High altitude pulmonary edema results from fluid buildup in the lungs.
The fluid in the lungs interferes with effective oxygen exchange. As the
condition becomes more severe, the level of oxygen in the bloodstream decreases,
and this can lead to cyanosis, impaired cerebral function, and death. Symptoms
include shortness of breath even at rest, tightness in the chest, marked fatigue,
a feeling of impending suffocation at night, weakness, and a persistent
productive cough bringing up white, watery, or frothy fluid (University of Iowa,
1995.). Confusion, and irrational behavior are signs that insufficient oxygen is
reaching the brain. One of the methods for testing for HAPE is to check recovery
time after exertion. Recovery time refers to the time after exertion that it
takes for heart rate and respiration to return to near normal. An increase in
this time may mean fluid is building up in the lungs. If a case of HAPE is
suspected an immediate descent is a necessary life-saving measure (2,000 – 4,000
feet). Anyone suffering from HAPE must be evacuated to a medical facility for
proper follow-up treatment. Early data suggests that nifedipine may have a
protective effect against high altitude pulmonary edema (University of Iowa,
HIGH ALTITUDE CEREBRAL EDEMA (HACE)
High altitude cerebral edema results from the swelling of brain tissue
from fluid leakage. Symptoms can include headache, loss of coordination (ataxia),
weakness, and decreasing levels of consciousness including, disorientation, loss
of memory, hallucinations, psychotic behavior, and coma. It generally occurs
after a week or more at high altitude. Severe instances can lead to death if not
treated quickly. Immediate descent is a necessary life-saving measure (2,000 -
4,000 feet). Anyone suffering from HACE must be evacuated to a medical facility
for proper follow-up treatment.
The importance of oxygen to the functioning of the human body is
critical. Thus the effect of decreased partial pressure of oxygen at higher
altitudes can be pronounced. Each individual adapts at a different speed to
exposure to altitude and it is hard to know who may be affected by altitude
sickness. There are no specific factors such as age, sex, or physical condition
that correlate with susceptibility to altitude sickness. Most people can go up
to 8,000 feet with minimal effect. Acclimatization is often accompanied by fluid
loss, so the ingestion of large amounts of fluid to remain properly hydrated is
important (at least 3-4 quarts per day). Urine output should be copious and
From the available studies on the effect of altitude on the human body
it would appear apparent that it is important to recognize symptoms early and
take corrective measures. Light activity during the day is better than sleeping
because respiration decreases during sleep, exacerbating the symptoms. The
avoidance of tobacco, alcohol, and other depressant drugs including,
barbiturates, tranquilizers, and sleeping pills is important. These depressants
further decrease the respiratory drive during sleep resulting in a worsening of
the symptoms. A high carbohydrate diet (more than 70% of your calories from
carbohydrates) while at altitude also appears to facilitate recovery.
A little planning and awareness can greatly decrease the chances of
altitude sickness. Recognizing early symptoms can result in the avoidance of
more serious consequences of altitude sickness. The human body is a complex
biochemical organism that requires an adequate supply of oxygen to function. The
ability of this organism to adjust to a wide range of conditions is a testament
to its survivability. The decreased partial pressure of oxygen with increasing
altitude is one of these adaptations.
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