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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 AIRALVEOLUSVENOUS BLOOD
OXYGEN152 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 760159*104 97
10,000523 110 67 90
20,000349 73 40 70
30,000226 47 21 20
40,000141 29 85
50,00087 18 11
*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. 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.


ACCLIMATIZATION
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, 1993).


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 sulfa
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, 1995).

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.


GAMOW BAG
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 life-threatening illnesses.


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, 1995).


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.


CONCLUSION
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 clear.

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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