In 1953 Sir John Hunt led a British expedition to climb Mount Everest for the first time. Sitting at 8848m, many believed this to be impossible. However, the expedition used supplementary oxygen for the last part of the climb (often called the “Death Zone”) and was successful.
While a very impressive feat, this suggested that humans could not survive without additional oxygen at barometric pressures this low. However, in 1978 Peter Habeler and Reinhold Messner managed to climb Everest without oxygen. Our understanding of how survival is possible at such high altitudes has developed hugely over the last century, particularly since the advent of aircraft.
The problem with high altitudes
At sea level, the atmospheric pressure is about 760Torr – it is made up of 21% O2, 0.04% CO2 and the rest is mostly N2. The pressure produced by O2 is called the partial pressure of O2 – this is 21% of 760Torr = 159Torr. At high altitudes, such as the peak of Everest the atmospheric pressure is only 250Torr. This reduces the partial pressure of O2 to only 52.5.
This means that significantly less oxygen is inhaled with each breath. What’s more, there is a roughly fixed amount of water vapour in your lungs. The volume of this increases hugely as you ascend, leaving less space available for oxygen. Water vapour has a partial pressure of 47Torr which means that at an atmospheric pressure of 47Torr (which occurs at 19,200m) the lungs will be completely filled with water vapour, with no more room for O2.
Even while breathing pure oxygen, the water vapour in our lungs limits the heights at which humans survive. It is estimated that the highest possible altitude that humans could survive at while breathing pure O2 is about 10,400m – roughly the cruising height of most commercial aircraft.
Acute mountain sickness
Altitude/ mountain sickness begins within 48 hours of arriving at arriving at altitude. Initially, you feel light headed and even euphoric. After a few more hours, this wears off leaving you feeling exhausted. Everything seems to take much more effort than normal and a feeling of dizziness doesn’t help. Many people wake repeatedly in the night, feeling like they are suffocating.
The only way to cure mountain sickness is to descend to a lower altitude. Some tourists make the mistake of paying someone to carry them up the mountain when they can no longer walk. This can be fatal as pulmonary and cerebral oedemas can occur.
The only way to survive at altitudes as high as those of the peak of Everest is to breath faster. Hyperventilation increases the amount of O2 drawn into the lungs.
There is a problem with this, however. Your body is wired so that it is the partial pressure of CO2 in the blood that controls the rate of breathing. The pH of the blood (and therefore the concentration of CO2 as it is acidic) is measured by central chemoreceptors in the brain. CO2 is normally a good proxy for the O2 concentration in the blood since they are linked – O2 is used in respiration to produce CO2.
At sea level, the oxygen concentration in the lungs is far higher than is needed – even if breathing is substantially reduced. On the other hand, the rate of breathing has a large effect on the concentration of CO2 in the lungs and tissues. Measuring the pH and therefore the CO2 concentration helps to match the respiratory rate to the concentration of the gas in your body.
Hyperventilation is necessary at altitude to get enough O2, but hyperventilation decreases the CO2 in the blood which suppresses further hyperventilation.
This can result in a very abnormal pattern of breathing when you first arrive at altitude which is particularly noticeable when someone is asleep – alternate periods of breathing and breath holding – sometimes this enough to wake the person up with a feeling of being suffocated.
Acclimatisation is essentially the process of removing the brake on breathing initially imposed by the reduced level of CO2 in the blood. The mechanism of this is not entirely understood, but certainly includes long-term compensation of pH regulation by the kidneys.
Not everyone can acclimatise sufficiently to maintain such a low CO2 concentration to allow the increase in breathing rate and so these people will not be able to live at high altitudes.
Haemoglobin levels vary based on altitude
Another difference between people who live and are acclimatised to altitudes compared to those living at sea level is their haematocrit. Haematocrit describes how much haemoglobin per unit of blood there is. The higher the level of haemoglobin, the higher the oxygen-carrying capacity. People at altitude, therefore, have more red blood cells than low-landers.
Some athletes try to exploit this and will do training at high altitudes to increase their haematocrit to give than an advantage over other competitors when they return to sea level. The downside of this is that it increases the viscosity of the blood and consequently the heart has to work harder to pump it around the body. This means that people who have lived all their life at altitude have a very large muscular heart as well as proportionately larger lungs.
Depressurisation of commercial aircraft
The huge increase in air travel in the past few decades means that almost everyone is familiar with the phrase “in the event of a sudden loss of cabin pressure, oxygen masks will fall from the overhead lockers”. We are also reminded to fit our own masks before helping other people.
Commercial aircraft are usually pressurised to an altitude of 1500-2400 metres. The reason they are not pressurised to the atmospheric pressure at sea level is that it would be too expensive to maintain such a large pressure difference across the walls of the plane as well as the fact that it is not really necessary.
At these pressures, there is plenty of oxygen in the air to sufficiently oxygenate the blood of all passengers – plus everyone is sitting down and not being very active. Passengers with heart or lung disease can struggle with this however and may need supplementary oxygen.
De-pressurisation of aircraft at high altitude can result in loss of consciousness
If a window blows out at cruising altitude (10,400m), the rapid release of pressure will cause anything not strapped down to be sucked out of the cabin. The cabin would fill with a fine mist as the water vapour in the air condenses.
It is essential to put your oxygen mask on quickly as at this height you will lose consciousness in under thirty seconds. “Useful” time – where the pilot is capable of taking corrective action is even smaller – there was once a case that upon depressurisation, the pilot dropped his glasses – by the time he had reached down to pick them up it was too late – he lost consciousness.
Other problems at high altitudes
While the decrease in oxygen partial pressure is not the only problem faced by mountaineers, pilots, balloonists and people who live at high altitudes, it is the main limiting factor to our survival at such heights. Other factors such as the cold, dehydration and intense solar radiation also make life in these places very difficult and potentially inhabitable.
Our knowledge and understanding of how our bodies adapt to altitude is revolutionising our exploration of the world above 5000m. Approximately 4000 people have now climbed Everest since 1953 and this number is growing each year. Rest stops to acclimatise to the altitude are now used as part of all mountaineering expeditions and indeed it is common to use a strategy where you return to lower altitudes to sleep and rest before climbing higher.