UKC

Stephen Taylor explores the challenges that living and moving in high-altitude environments present to the human body and how to manage them...

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A lot of interesting info there. I've tried to make headway with the "there's just as much oxygen, but in percentage terms there isn't", but I'm struggling. If oxygen takes up 21% of a given volume of air at sea level but is around half of that at 5000m, does this mean there is a higher percentage of nitrogen accounting for the rest?

I'm not a scientist, but I believe it's to do with density, i.e. the air composition doesn't change but just becomes less dense as you go higher, meaning less availability of o2. Someone will correct me no doubt if that's not right. 

But if the percentage of oxygen per volume of air decreases, this suggests that the air composition does change? 

As the mountaineer ascends to higher altitudes, so air pressure decreases, and this has the effect of spreading out the essential oxygen molecules we depend on to survive and perform. To counter this decrease in oxygen intake, the body adapts by gradually increasing the red blood cells over a number of days. Other mechanisms come into play. We breathe harder and more frequently, and air-sacs in the extremities of the lungs begin to function. However, if the body is gradually worked hard over a couple of weeks such as 6-10 hours of strenuous activity daily, then the body will naturally increase the red blood cell count. It does not need to be at high altitudes for this to occur. Consequently, climbers and mountaineers had learned to ‘acclimatize’ to this ‘thin air’ as they ascend. The higher they ascend, the longer this period of acclimatization. Like all things however, it is a law of diminishing returns, and above a certain height no matter how long the climber acclimatizes, there is little or no improvement. It could be argued that the reverse is true. There comes a point when the body starts to deteriorate.

I should perhaps add to all this the importance of hydration, as this goes hand in hand with oxygen intake. The water molecules are also more spread out. So with the loss of pressure there is also less water vapour in the air we breathe, so more intake of H2O is required to avoid such dehydration.

Another interesting issue is the boiling point of water, which decreases with the higher altitude gain. So there comes a point (around 60,000ft,) when the water content in our bodies starts to boil!

Hope this helps.

An interesting article,  with a lot of good and useful information,  but the early use of % of oxygen is very confusing....  to be devil's advocate,  what completes the 100% as oxygen decreases? 

According to the link below and a quote from it the air composition is unchanged from sea level to above all mountains. 

"The composition of air is unchanged until elevation of approximately 10.000 m" 

https://www.engineeringtoolbox.com/air-composition-d_212.html

I think a lot of that was covered in the article, but I was trying to understand the science of how oxygen seems to decrease, but in reality doesn't? (or something like that)

Lower air pressure at altitude = less oxygen forced into lungs. Composition doesn't change but atmospheric pressure does. I think that's what you're asking?

So it probably means 10,000m rather than 10m?

Yes I don't really get it. Maybe the article writer can explain.

The air becomes less dense; there is less of everything in it per unit volume. So less oxygen per lung full. Unless I am completely misunderstanding things!

https://www.ukclimbing.com/articles/skills/series/altitude_and_acclimatisat...

The acclimatisation process for lowlanders

High altitude poses multiple challenges to human survival, but it's the lack of oxygen (hypoxia) that we will focus on here. The percentage of oxygen in the air at 4,000m is essentially the same as at sea level (21%). The main difference at altitude is that air pressure drops the higher we go, reducing the driving force to 'push' oxygen from the air in our lungs into our blood. At 4,000m the air pressure is 37% lower than it is at sea level. 

That is what I would've assumed beforehand, much like JayW said above. But you'd think this would be expressed in terms of total number of oxygen molecules per unit volume, not as a percentage. 

Ah OK, so there is just as much oxygen but as air pressure decreases we are unable to take in as much of it? Both the total amount of oxygen in the air and the percentage of oxygen per unit volume stay the same? 

Yes, I think it is poorly expressed.

A quick Google suggests that is not the case. Our diaphragm movement increases the volume of the lungs and air flows in. I can't find anything to suggest that the volume of air inhaled increases with atmospheric pressure; our lungs are inflated by our muscles, not air pressure (this just makes the air flow in to fill the available volume). Happy to be proved wrong though!

Yer, I think most other countries use a the comer and full stop the opposite way to the UK for numbers. 

No.

The proportion of oxygen : nitrogen  is approximately the same . But the pressure is reduced, and thus the number of molecules per cubic meter reduced.  Thus there is less oxygen available.

Your lungs expand/contract a volume.  IF there  are less molecules in that volume, less oxygen

The pressure of an ideal gas is proportional to the density (i.e. number of molecules per cubic metre) multiplied by the temperature. So a drop in pressure must have a corresponding drop in temperature or density but not necessarily both. I don't know what happens in practice at high altitudes, but the temperature certainly drops, so it's possible the density remains about the same as a sea level.

Either way, the drop in pressure would make it harder for our lungs to function efficiently, regardless of whether the density has changed.

Looked it up. Going from sea level to 3000m, pressure drops by about 20% and temperature by about 7%.

Nothing in what Natalie has quoted suggests the volume of air taken in increases with pressure, the mass of that air is what will change. The higher mass of air will contain a greater mass of oxygen.

Your muscles increase the volume of your lungs, but air pressure is what fills them. At altitude they will fill with the same volume of lower pressure air, hence less oxygen by mass.

Sorry, yes, I misread it. It was about pressure forcing oxygen from the lungs into the bloodstream, not forcing air into the lungs. It seems to be saying that this is a more important factor than the lower mass of oxygen entering the lungs.

Edit: Or are these the same thing (assuming same temperature); the lower pressure is simply due to fewer molecules moving at the same average speed, so with the same chance of making it through to the bloodstream?

This does seem unlikely, especially since your muscles can produce the force needed for this. The actual amount of oxygen is surely far more important.

It’s sort of one and the same - fewer molecules per unit volume = lower pressures. Gas transfer between alveoli and capillaries in the lungs relies mostly on concentration gradients amongst other things (look up Fick’s law and Henry’s law). Less O2 per unit volume in lungs at higher altitudes means a less steep alveolar-blood capillary gradient of O2 so reduced partial pressure of O2 across those membranes. Change in mechanical ventilation is therefore a compensatory response to try increase the partial pressure of O2 and recruit more alveoli to aid gas exchange. Of course there are lots of other compensatory mechanisms and disease processes that influence this!

Thanks, it's an interesting subject.

Another factor is that a base level of oxygen in our blood is needed for our metabolism with no physical movement. That baseline will increase with cold and lower oxygen levels. Less oxygen in the blood minus that baseline number means less available to physically work than the proportional drop in pressure. The body reacts to such situations and cold adding additional problems (especially frostbite risk). 

Higher than average meteorological pressure makes a big difference to Everest success rates  (better weather and higher oxygen intake).