Course Material
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How Does This Relate to You?

It means that your body has to function with less oxygen:

  • Your body needs the same amount of oxygen at altitude as it does at sea level.
  • Your body has to adjust, or acclimatize, to the altitude.

Acclimatization is the process by which your body adjusts and adapts to the decrease in oxygen available to the tissues.

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How the Body Acclimatizes

Increased Respiration

During the first week of adaptation, a variety of changes take place. The depth and rate of breathing increases in response to lower concentrations of oxygen in the blood, causing more carbon dioxide to be lost and more oxygen to be delivered to the alveoli (tiny air sacs in the lungs).

The increased respiratory rate begins within the first few hours of arriving at altitudes as low as 5,000 feet. The lost carbon dioxide causes the body to become more alkaline.

To compensate for the body's increasing alkalinity, the kidneys excrete bicarbonate (an alkaline substance) in the urine. This adaptation occurs within 24 to 48 hours after hyperventilation starts.


Urinary output increases to rid the body of bicarbonate.

Increased Heart Rate

Cells require a constant supply of oxygen; therefore, heart rate increases so the heart pumps more blood and oxygen to the body. Except at extreme altitudes, heart rate returns to near normal after acclimatization.

Pulmonary Artery Pressure

In the lungs, the pulmonary capillaries (tiny blood vessels) constrict, increasing resistance to flow through the lungs and raising pulmonary blood pressure to help “push” oxygen from the air sacs into the blood stream. Dangerously high blood pressure in the pulmonary artery may cause fluid to escape from the capillaries and leak into the lungs (pulmonary edema).

Red Cell Production

As acclimatization continues, the bone marrow contributes by increasing red blood cell production. New red blood cells become available in the blood within four to five days, increasing the blood's oxygen-carrying capacity. An acclimatized person may have 30 to 50 percent more red blood cells than his counterpart at sea level.

Enzyme and Tissue Changes

Within the blood cells, 2,3-Diphosphoglycerate (DPG) increases. This is an organic phosphate that helps oxygen to combine with red blood cells. Production of myoglobin, the intramuscular oxygen-carrying protein in red blood cells, also increases.

The body develops more capillaries in response to altitude. This improves the transport and spread of oxygen by shortening the distance between the cell and capillary.

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The Acclimatization Process is Different for Everyone
  • Time varies for the different bodily reactions discussed in the How the Body Acclimatizes section.
  • Individuals vary widely in the ability to acclimatize, both in degree and time.
  • The average time for acclimatization is three to five days.
  • Prior experience at altitude does not exclude you from altitude related illnesses.
  • Some individuals are unable to acclimatize at all.
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Normal Body Responses to Altitude

These responses are your body’s way of adapting to altitude:

  • Hyperventilation
  • Shortness of breath on exertion
  • Increased urination
  • Cheyne-Stokes respiration (also known as periodic breathing), which is an abnormal breathing pattern during sleep
  • Sleep disturbances
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How to Achieve Acclimatization
  • Use graded ascents—use an intermediary camp to acclimatize (you may not have the opportunity to do this because participants are normally flown from sea level directly to altitude).
  • If you climb high, sleep low.
  • Stay properly hydrated.
  • Do not over-exert.
  • Rest upon arrival.
  • Eat a high carbohydrate diet.
  • Avoid alcohol, tobacco, depressant drugs (e.g., sleeping pills and narcotics).