There are two
major kinds of environmental stresses at high altitude for humans. First, there are
the alternating daily extremes of climate that often range from hot,
sunburning
days to freezing nights. In addition, winds are often strong and humidity
low, resulting in rapid dehydration. Second, the air pressure is lower.
This is usually the most significant limiting factor
in high mountain regions.
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| Air pressure decreases as altitude increases | |
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Click
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The percentage of oxygen in the air at two miles (3.2 km.) is the same as at sea level (21%). However, the air pressure is 30% lower at the higher altitude due to the fact that the atmosphere is less dense--that is, the air molecules are farther apart.
When we
breathe in air at sea level, the atmospheric pressure of about 14.7 pounds per square inch (1.04 kg. per cm.2)
causes oxygen to easily pass through selectively
permeable
lung membranes into the blood. At high altitudes, the lower air
pressure makes it more difficult for oxygen to enter our vascular systems. The
result is hypoxia
, or oxygen
deprivation. Hypoxia usually begins with the inability to do normal physical
activities, such as climbing a short flight of stairs without fatigue. Other early
symptoms include a lack of appetite, distorted vision, and difficulty with memorizing and thinking clearly.
In serious cases, pneumonia-like symptoms (pulmonary edema
)
and an abnormal accumulation of fluid around the brain (cerebral edema
)
develop, leading to death within a few
days if there is not a return to normal air pressure levels.
There is also an increased risk of heart failure due to the added stress
placed on the lungs, heart, and arteries at high altitudes.
When we travel to high altitudes, our bodies initially develop inefficient physiological responses. There is an increase in breathing and heart rate to as much as double even while resting. Pulse rate and blood pressure go up sharply as our hearts pump harder to get more oxygen to the cells. These are stressful changes, especially for people with weak hearts.
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| Initial inefficient response to low oxygen pressure |
Later, a more efficient response normally develops as acclimatization takes place. More red blood
cells and capillaries
are produced to carry more oxygen.
The lungs increase in size to facilitate the osmosis
of oxygen and carbon dioxide. There
is also an increase in the vascular network of muscles which enhances the transfer of
gases.
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Beginning of successful acclimatization to low oxygen pressure |
However, successful acclimatization rarely results in the same level of physical and mental fitness that was typical of altitudes close to sea level. Strenuous exercise and memorization tasks still remain more difficult. In addition, the rate of miscarriages is usually higher at altitudes above two miles.
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| Increased fitness level after successful acclimatization to low oxygen pressure |
On returning to sea level after successful acclimatization to high altitude, the body usually has more red blood cells and greater lung expansion capability than needed. Since this provides athletes in endurance sports with a competitive advantage, the U.S. maintains an Olympic training center in the mountains of Colorado. Several other nations also train their athletes at high altitude for this reason. However, the physiological changes that result in increased fitness are short term at low altitude. In a matter of weeks, the body returns to a normal fitness level.
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| Enhanced fitness level for a short period of time after returning to low altitude |
Who Is Most
Likely to Have Hypoxia?
Most lowland people begin to develop hypoxia symptoms at 1-2 miles altitude. However, there are some permanent settlements in the Andes Mountains in South America and the Himalaya Mountains in Asia that are at altitudes of 3 miles. Mountain climbers have reached peaks that are over 5 miles high, but only by using tanks of oxygen to assist in breathing. The highest peaks are too high for any human to acclimatize naturally.
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Climbers at the
top of Mt.
Logan, Yukon |
There is considerable variability between individuals and between populations in their ability to adjust to the environmental stresses of high mountain regions. Usually, the populations that are most successful are those whose ancestors have lived at high altitudes for thousands of years. This is the case with some of the indigenous peoples living in the Andes Mountains of Peru and Bolivia as well as the Tibetans and Nepalese in the Himalaya Mountains. The ancestors of many people in each of these populations have lived above 13,000 feet (ca. 4000 meters) for 5,000-10,000 years.
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| Peruvian Indian woman and Tibetan man |
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(Her cheeks are
red primarily due to increased blood
flow near the skin surface. |
The implication is that natural selection
over thousands of years results in some populations being genetically more suited to the
stresses at high altitude. However, different
populations respond physiologically to low oxygen pressure in somewhat
different ways. The primary solution of Indians from the high mountain
valleys in Peru and Bolivia has been to produce more hemoglobin
in
their blood and to increase their lung expansion
capability. In contrast, the common solution of Tibetans and Nepalese who live at
high attitudes generally has been to breathe
faster in order to take in more oxygen and to have
broader arteries and capillaries, thereby allowing much higher rates of
blood flow and subsequently greater amounts of oxygen delivered to their
muscles.