Adapting to High Altitude

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.


Air pressure
as altitude

  drawing of the earth's surface and the atmosphere above it showing that with increasing distance from the earth, the gas molecules in the atmosphere are farther apart and the air pressure is lower

Click here for more information about the earth's atmosphere 

The percentage of oxygen in the air at two miles (3.2 km.) is essentially 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 click this icon to hear the preceding term pronounced 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 click this icon to hear the preceding term pronounced, 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 of "high altitude sickness" include a lack of appetite, vomiting, headache, distorted vision, fatigue, and difficulty with memorizing and thinking clearly.  In serious cases, pneumonia-like symptoms (pulmonary edema click this icon to hear the preceding term pronounced) due to hemorrhaging in the lungs and an abnormal accumulation of fluid around the brain (cerebral edema click this icon to hear the preceding term pronounced) develop.  Pulmonary and cerebral edema usually results in 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 mountain areas, 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.

Initial inefficient
response to low
oxygen pressure

  graph illustrating initial inefficient physiological response to low oxygen pressure

Later, a more efficient response normally develops as acclimatization takes place.  Additional red blood cells and capillaries click this icon to hear the preceding term pronounced are produced to carry more oxygen.  The lungs increase in size to facilitate the osmosis click this icon to hear the preceding term pronounced of oxygen and carbon dioxide.  There is also an increase in the vascular network of muscles which enhances the transfer of gases.

Beginning of
to low oxygen

  graph illustrating beginning of successful acclimatization to low oxygen pressure after initial decline in fitness

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 because fetuses receive less oxygen from their mothers.

Increased fitness
level after successful
acclimatization to
low oxygen pressure

  graph illustrating fitness level after successful acclimatization to low oxygen pressure--it is somewhat lower than it was at sea level

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. 

Enhanced fitness
level for a short
period of time
after returning to
low altitude

graph illustrating enhanced fitness level after returning to sea level

Who Is Most Likely to Have High Altitude Sickness?

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 rarely without using tanks of oxygen to assist in breathing.  The highest peaks are too high for any human to acclimatize to the point that they could stay there for prolonged periods.

Climbers at the top
of Mt. Logan, Yukon
Territory, Canada
(altitude 19,850 feet)

  photo of climbers at the peak of the snow covered Mt. Logan, Yukon Territory, Canada (19,850 feet altitude)

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 at least 2,700 years.

Peruvian Indian
woman and
(both from
high altitude

  photos of an Andean woman and a Himalayan man

(Her cheeks are red primarily due to increased
blood flow near the skin surface.  More red
blood cells help her get oxygen to the tissues
of her body.)

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 click this icon to hear the preceding term pronounced in their blood and to increase their lung expansion capability.  Both result in an increase of oxygen carried by the blood.  In contrast, the common solution of Tibetans and Nepalese who live at high altitudes 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, despite the fact that they have relatively normal hemoglobin levels.  A recent study of Tibetan villagers who live their lives at around 15,000 feet has shown that they have 10 oxygen-processing genes not commonly found in lowland populations.  The EPAS1 gene is particularly important in adapting to environments with consistently low oxygen pressure.

Whether you personally will experience high altitude sickness in the future may be at least partly a consequence of your genetic inheritance.  Those individuals who have low expression levels of the PDP2 gene generally have more severe symptoms.  This gene codes for a protein that assists in the conversion of food into fuel for our bodies.  In some way, it apparently also helps in acclimatization to low oxygen pressure.

NEWS:  In the March 15, 2011 issue of the Journal of Epidemiology & Community Health, researchers from the University of Colorado School of Medicine and the Harvard School of Global Health reported that people generally live longer at high altitudes and have a lower risk of dying from coronary artery disease.  This positive effect occurs unless people have chronic breathing problems.  The researchers speculated that mild hypoxia improves the way the heart functions and produces new blood vessels that increase blood flow for the heart.  An alternative explanation presented by the authors is that increased exposure to ultraviolet radiation from the sun at higher altitudes increases the body's ability to produce vitamin D, which has beneficial effects on the heart.

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Copyright 1998-2012 by Dennis O'Neil. All rights reserved.
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