High Altitude Physiology – Explained

As riders, mountaineers, and out doors people, we know that some of the best of what the outdoors has to offer is often found above tree line. Here in Colorado, for example, many of the 10th Mountain Division huts are at or above 12,000 feet. This kind of altitude is a strain for even the fittest amongst us. We’re all familiar with that feeling of being out of breath, trying desperately to keep up with our buddies. Even if you’re not suffering severe symptoms of acute mountain sickness (AMS), you’ll probably still have a lousy night’s sleep on a hut trip at that elevation.

Oh, the Os

The oxygen percentage in air at sea level (~21%) is actually the same as when you are summiting a 14er in Colorado. What changes is the air pressure. Because the air pressure gets lower and lower as you go higher and higher, the partial pressure of oxygen decreases, and you take in less oxygen per breath. When you summit at 14,000 ft, you are breathing 40% less oxygen compared to sea level.  There’s your problem!  This condition is known as hypoxia, which occurs any time the body is deprived of adequate oxygen supply.

Effects of Altitude on Your Physiology

Your vital signs are the first indicators that your body is reacting to an increase in altitude:

  • Respiration rate, or the number of breaths one takes per minute, increases. An average, resting adult in a non-hypoxic environment takes between 12-18 breaths/min. When at altitude, you will notice an increase in the number of breaths you are taking.
  • Basal metabolic rate increases. Your basal metabolic rate is the number of calories your body needs to function at rest. The number of calories you will need to function at altitude goes up. This is a contributory reason as to why Colorado has the lowest obesity rate in the country.
  • Oxygen saturation levels in your blood decrease. (It’s interesting to bring along a fingertip pulse oximeter when you go to altitude to monitor the percentage and compare that to how you’re feeling. Be sure to know your baseline before you climb.)
  • Pulse rate increases. Normally, the average adult’s heart will beat somewhere around 60-70 beats/min. At altitude, the pulse rate will go up as the heart needs to work harder to carry oxygen through the body.
  • The kidneys kick in and start producing Erythropoietin (EPO) which results, eventually, in an increased red blood cell mass.

These responses to hypoxia (low oxygen) are universal.  However, as you have likely observed, altitude affects everyone differently and fitness level is not necessarily a good predictor for altitude tolerance.  I’ve observed elite cyclists suffering terribly at even moderate altitudes and have also seen people who struggle with even moderate exercise adjust to altitude quickly with no problem at all.  What we do know is that the slower you go up, the less chance there is of feeling lousy or experiencing symptoms of AMS.  Interestingly, the summiting success rate of older climbers on Mt. Kilimanjaro is actually higher than that of younger, presumably fitter climbers.  It is widely thought that this is because older climbers ascend more slowly which gives their body more time to adapt than their speedier, younger counterparts.

If you go too high too fast, you could start developing any of the well-known symptoms of AMS to varying degrees:

  • Loss of appetite
  • Headache
  • Nausea
  • Difficulty sleeping
  • Lethargy

Some people’s condition may worsen into more serious situations like high altitude cerebral edema (HACE), where they typically demonstrate the “umbles” – mumbles, stumbles, and bumbles (similar symptoms to being drunk).  High altitude pulmonary edema (HAPE) is another serious situation where you would observe shortness of breath and crackling lung sounds. If anyone in your party starts experiencing any of these symptoms, it’s time to call emergency services and get to a lower altitude immediately.


The easiest and quickest way to alleviate AMS symptoms is to go to a lower elevation. Relief is usually instantaneous but that’s not always practical or convenient. It’s always good to travel with a basic first aid kit, and I would suggest adding the following if you’re going high:

  • Advil (Ibuprofen)
  • Diamox (acetazolamide)
  • Dexamethasone

Advil is available over-the-counter, but the others require a prescription from your family physician, along with instructions on how/when to administer and applicable contraindications.


Low and slow.  The slower you can acclimatize the better.  As an example, if some members of your party are flying from sea level to Colorado, be sure that they spend as many days as possible in the Denver/front-range area at 5-7k’ before heading up to the mountains.  Again, the longer and slower the better.  Being fit and staying off the booze certainly can help, but each person responds to altitude differently so it’s difficult to predict. If you’ll excuse the shameless plug, through my business, I speak to many athletes and outdoorspeople who use simulated altitude to slowly ramp up their elevation before a trip so there’s no time lost acclimatizing on arrival. My wife and I sleep in an altitude tent for the entirety of the back-country ski season so that I don’t have to worry about AMS. We increase our altitude gradually (500’/night) and take as long as two weeks to work our way up to 10-12k’.

Altitude has real and vital effects on our bodies. Understanding what those effects are, what they look like, and how to avoid negative consequences increase our odds of successfully navigating complications when they arise. As always, preparation and knowledge are the keys to maximizing safety and enjoyment of some of the greatest playgrounds in the world.

~by Jon Jonis, CEO of Mountain Air Health, maker of automatically controlled altitude simulation systems for cardio improvement, pre-acclimatization, and weight loss