The Physiological Effects of High Altitudes

It is important for people traveling to high altitude to understand the risks of decreased oxygen at high altitudes. We all love the experience of hiking a great trail to get to a beautiful view of the valley below.

You flew earlier that day from Los Angeles to Denver, CO and that afternoon you rent a car and drive to a trailhead at 8000 ft and hike to a peak that is over 10,000 ft. You were feeling great, but now you have a massive headache and feel nauseous. Unless you head down the mountain, you are in for more serious problems.

What is High Altitude?

High altitude is considered to be between 5,000 and 11,500 feet (1,524 and 3,505.2 m) above sea level. Very high altitude is any altitude between 11,500 and 18,000 feet (5,486.4 m), and extreme altitude is anything above 18,000 feet.

What causes Altitude Sickness?

Altitude sickness is the name given to the human body’s physiological reactions that occur as a result of low oxygen pressure that exists at high altitudes. At higher elevations oxygen levels decrease. The problem of Acute Mountain Sickness starts when acclimatization does not keep pace with your ascent to high altitude. This happens when you ascend to quickly or go from sea level to high altitudes in a day and your body cannot get as much oxygen as it needs or is accustomed too. Even the top endurance athletes know to get to a high altitude race location a few days early to acclimate to the altitude.

Symptoms of Altitude Sickness include:

  • Headache (headache)
  • Fatigue (physical exhaustion)
  • Sleep Disorder
  • Nausea and Vomiting
  • Digestive Disorders
  • Agitation
  • Vertigo

Who can get altitude sickness?

Anyone who goes to high altitude can get Acute Mountain Sickness. It is primarily related to individual genetics and how fast you go up. There is no difference in age, gender, physical fitness, or previous visits. Some people acclimatize quickly, and have no problems; others acclimatize slowly and feel bad for a few days and others progress to the more serious forms.  There is no way to predict who is likely to get sick at high altitudes

What is Acclimatisation?

The main cause of altitude sickness is going too high too quickly. Flying to a high altitude destination does not allow the body to acclimate quickly. With time however, your body will adapt to the decrease in oxygen at a specific altitude. This process is known as acclimatisation and generally takes one to three days at any given altitude, e.g. If you go from sea level to 5,000 feet and spend several days at that altitude, your body will acclimatise to 5,000 feet. If you then climb to 10,000 feet your body has to acclimatise once again.

Your body undergoes several changes to deal with the decreased oxygen levels at higher altitudes:

  • You breath faster and deeper
  • You experience shortness of breath with exertion
  • You will exhibit a different breathing pattern as you sleep
  • You will awaken more frequently at night
  • Your body’s hematocrit will increase as more red blood cells are produced.
  • You will experience increased urination
  • The body increases productions of an enzyme that causes the release of oxygen from haemoglobin to the body tissues.
  • All of these changes are a normal response to decreased oxygen level.

See Tips on how to prevent altitude sickness

Medications used to treat Acute Mountain Sickness



  •  Diamox
  •  Dexamethasone
  •  Nifedipine
  •  Furosemide
  •  Ibuprofen

Altitude Sickness Medications