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The Effects of Altitude in a Nutshell

by Globetrooper Todd | 2 Responses
Acute Mountain Sickness

I wrote about the effects of altitude in a previous post, The science behind mountain sickness. However, just after writing that note, I bumped into a paramedic who also has an interest in the effects of altitude. He’s worked for the State Emergency Service and is an avid snowboarder.

It was quite an interesting chat and it certainly gave me a better appreciation of how the human body works at altitude.

Rather than delving even deeper into jargon, I came away from the discussion with three main points that explain the effects of altitude on the human body:

  • Less oxygen in each breath - the percentage of oxygen in the air does NOT actually decrease at altitude. But the atmospheric (or barometric) pressure decreases, meaning the air is less dense, and hence, each breath contains less oxygen. So each molecule of air still contains about 21% oxygen, but there are fewer air molecules in each breath. In scientific-speak, the partial pressure of oxygen in the lungs is reduced. And this reduction means our bodies operate less efficiently.
  • Less room in the lungs for oxygen – In addition to there being less oxygen in each breath, there’s also less room in the lungs for inspired oxygen. This is because our bodies saturate incoming air with water and saturated water vapour has a constant partial pressure. It’s difficult to explain this more simply, but basically if something (partial pressure of oxygen in inhaled air) decreases at altitude, and we remove a constant (partial pressure of saturated water vapour), then the result (gases, including oxygen for delivery to our blood) decreases even further.
  • Less pressure to absorb oxygen – with each breath, carbon dioxide is exchanged for oxygen (see gas exchange). The primary force that feeds this process is atmospheric pressure. As atmospheric pressure drops (when we climb to higher altitudes), the gas exchange process becomes less efficient. So in addition to less oxygen per breath, and less room in the lungs for oxygen, there’s also less pressure to aid the process of oxygen absorption.

These three points mean that our bodies are much less efficient at altitude. Everything deteriorates, not just our climbing speed. I’ll talk about the effects of less oxygen in an upcoming post. So stay tuned.

Posted in Featured, Mountaineering | May 10th, 2010

2 Responses to The Effects of Altitude in a Nutshell

  1. Both of your articles distill the essence of this subject so it is easily understood.

    I had assumed the opposite in relation to the levels of water vapour at different altitudes.

    Thanks for sharing this with us.

  2. Hi EnT,

    I agree, it doesn’t sound intuitive. Colder air (which is typical at higher altitudes) would hold less water, not more. And heavier breathing should expel more water, which would explain how easy it is to become dehydrated at altitude.

    But (and I’ve edited the post for clarity), the water vapour issue refers directly to the atmospheric pressure. Why? Because when we breathe in, our nose/mouth/etc saturates the air with water. So while the levels of water in the outside air fluctuate with temperature, the level of saturation inside our bodies is mostly constant.

    Now, the issue is that the partial pressure of saturated water vapour is also constant at altitude, while the partial pressure of oxygen in our lungs decreases. And this is where I wan’t very clear in the post. If air pressure in our lungs is made up of the pressures of water and gases, and water vapour stays constant, then the gas pressures drop even further at altitude.

    So, the partial pressure of oxygen in the outside air reduces due to the lower pressure, BUT furthermore, as it enters our bodies and the air is saturated with water in our mouths/airways/etc, the partial pressure of oxygen drops even further (because water vapour is constant).

    At sea level, the partial pressure of oxygen inside us is 6% less than outside us (due to the water saturation). At altitude, say the summit of Everest, it is 19% less.

    I hope that makes sense. If not, try this (http://www.dr-amy.com/rich/oxygen/) explanation. I had to read it 5 times before I was convinced.

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