As you climb, the environment is constantly changing. You can observe this in part, such as the changes in fauna and flora and the fall in air temperature. Other changes are apparently unnoticed. One is the gradually decreasing air pressure and the associated decrease in the oxygen pressure in the blood, which means that less oxygen can bind to the hemaglobin. This is the red pigment in the (therefore) red blood cells. These red blood cells transport the oxygen to the organs, where it is used as fuel to generate the energy the body needs. The body's oxygen requirement does not decrease at altitude for the same performance. The decrease in supply means that the higher you climb, the more oxygen your body needs for basic functions such as breathing and the functioning of all organs. In percentage and in absolute terms, less oxygen is available for the muscles, while the heart and lungs have to work harder and harder. Walking, and even more so climbing, requires more and more effort as you ascend and is therefore increasingly difficult to sustain over a long period. Above 6,000 metres, steady and continuous climbing is no longer possible and frequent stops to "catch your breath" are necessary. Above 8000 metres, several breaths are required per step. At the summit of Mount Everest (8850 metres) the amount of oxygen bound to haemaglobin is only half of the normal amount. More than a third of all oxygen is then needed just to keep the breathing muscles going, compared to about 7% at sea level. The rest is almost entirely needed to maintain necessary body functions. This means that the amount of oxygen needed for climbing and descending is reduced to almost zero. Therefore, a person cannot survive for long at that altitude without extra oxygen. However, there are big differences between people in the capacity to adapt physically to altitude; this adaptive capacity is largely based on aptitude and also influences the risk of getting altitude sickness.
While climbing, your body tries to adjust in different ways, but it gets less and less successful the higher you climb. This follows the most important changes that occur without you having any influence.
The physical reactions at height due to oxygen deficiency:
1. Your heart starts beating (at rest) about 10 to 15 beats per minute faster, so that more blood is pumped around per time unit. This reaction starts almost immediately with the rise. After about ten days, the heart rate drops to (almost) normal values again through adjustment, unless you are extremely high (above 5000 meters). Before you climb it is useful to determine what your resting heart rate is. Then you have a comparison value, if you want to check later if your wrist has risen too strongly, which may indicate altitude sickness. Incidentally, the maximum heart rate (which is age and gender-related) decreases with increasing. This reduces the maximum oxygen transport and therefore the maximum oxygen uptake capacity, despite all attempts to adjust your body. You notice that especially with a considerable effort.
2. The blood supply to your brain increases to provide the brain tissue, which is very sensitive to oxygen deficiency, with more oxygen. The blood supply to some other tissues and organs, such as the intestines, decreases to save oxygen.
3. You start to breathe deeper, which means that air also enters parts of the lung that do not normally participate in the exchange of oxygen in the blood. This too is a reaction of the body that becomes stronger in parallel with the rise.
4. You breathe faster. Normally we breathe in and out about 12 times per minute at rest. At altitude this frequency during rest gradually rises to 15 to 20, and during physical exertion itself to above 50. A breathing speed at rest above 20 is always a reason to think of starting pulmonary edema. By breathing deeper more quickly, more oxygen is absorbed into the blood per unit of time and more carbon dioxide (carbon dioxide) is exhaled. At extreme heights, the frequency at which a person can still breathe becomes the limiting factor. The combination of deeper and faster breathing leads to liters of extra fluid loss per day through the lungs, further enhanced by the fact that air at height is very dry and the body tries to moisten that dry air. Dehydration at height is therefore a major threat.
5. Carbon dioxide (CO2) is formed in our body as a waste product of the metabolism. The greatly increased breathing due to the lack of oxygen leads to a much too large exhalation of CO2. This results in an increase in the acidity of the blood: the pH rises, the blood becomes less 'acidic'. This situation would lead to all kinds of health problems if our body could not do anything about it. However, the body tries to adjust to this situation. The kidneys respond to high acidity by washing out bicarbonate, but this compensation mechanism lasts a few days and is not complete. In the meantime, due to the high acidity, you are going to breathe less frequently, which increases the oxygen deficiency. That sometimes leads to sleeping problems at night.
6. The blood pressure in your arterial lung vessels and capillaries increases considerably, also almost in proportion to the increase. This is due to a reaction to the decreasing oxygen pressure in the inhaled air, causing the blood vessels to contract in many places in the lungs. This phenomenon is amplified when cold air is inhaled. To compensate, in other lung parts, more smaller pulmonary vessels that normally have little or no blood flow through which are further open. As a result, per breath, more oxygen can be absorbed from the inhaled air in those places. However, the net effect is that the average blood flow over the entire lung decreases, so that less oxygen can be absorbed per breath. Due to the higher blood pressure in the lung vessels, blood pressure in the right part of the heart can also rise. Heart abnormalities in this part of the heart can therefore cause problems faster at height. The high pressure in the lungs sometimes causes leakage of the vessels, which can lead to serious problems.
7. The amount of red colouring in your blood increases due to an increase in the enzyme erythropoetin (EPO), which stimulates the production of red blood cells. This allows more oxygen to be transported to the cells. A person fully adapted to altitude may end up with 30 to 50% more red blood cells. This EPO reaction starts within hours, but an effective increase in haemoglobin only occurs after many weeks of staying at altitude, so a holiday of, say, three days is of no benefit. However, the amount of a certain enzyme (2,3-diphosphoglycerate) in the red blood cells increases slightly at altitude. This stimulates the release of oxygen to the cells somewhat. Prolonged stays at altitude also lead to adaptations in the tissues (e.g. better blood circulation in the muscles, more small blood vessels, more red blood cells), which then allow for more efficient transport and use of oxygen.
8. You temporarily pee more water above 2500 meters. You notice this, for example, because you have to get out of bed (sleeping bag) once or twice at night. Due to the loss of moisture that results, you dry out a little and your blood becomes slightly more concentrated. This is all part of a normal acclimatization process. Some people, on the other hand, retain moisture and gain weight in a few days. It is not clear whether this increases the risk for the development of altitude sickness, although it does indicate a less good acclimatization.
9. For reasons that are not yet understood in detail, the body reacts to oxygen deficiency, in combination with height, in another, undesirable way: the walls of the small blood vessels in the body start to 'leak' a bit. The high pressure in the vessels plays a role in this, but certain substances are also released in the body that increase the permeability of blood vessels. As a result, fluid comes out of the blood vessels, which ends up in the surrounding tissues where it is withdrawn from the blood circulation. For this reason you also dry out a little (further), even though the moisture is present elsewhere in your tissues. That is probably the case with many people who climb to altitudes above 2500 meters, but not everyone notices that. On average, it concerns a total of 5 to 10% of the total blood volume at sea level, ie approximately 250 to 500 ml. The most serious forms of fluid leakage due to oxygen deficiency are found in the lungs and in the brain.
10. During the first few days, most of the acclimatization takes place at the height at which you are; any altitude sickness complaints usually disappear within that period. After an average of a week to 10 days, the acclimatization process is largely completed, with the exception of the aforementioned adjustment reactions in the long term. After that, it is very unlikely that you will become highly ill at that altitude. If you climb further, you run another risk: Your body must adjust to a new height every time. Your body no longer fully acclimatises above about 5,500 meters and hardly more than 7,000 meters. The acclimatization of your body is lost again after descent at about the same pace as the adaptation when climbing. If you go down at a height after two or two weeks, and then go up again after two weeks, your body will have to start practically adjusting to the new height again. Even after a much longer stay at a higher altitude and a subsequent stay of more than a week at sea level, your body must partially adjust (depending on the duration of the low stay). Then you run the same risks as people who climb to that height for the first time. This problem occurs, for example, with people who live high up and who periodically stay at a much lower level for work or vacation. In such situations, the body does have more red blood cells, because the lifespan of these cells is approximately 120 days, so that the oxygen supply is somewhat better than with 'lowlanders'.
Most of the reactions mentioned by your body contribute to a less sharp decrease in the concentration of oxygen in your blood than would be the case without these reactions. However, this adjustment is still insufficient compared to the initial situation: The moment you departed. The health problems that result from this lack of oxygen are all summarized under the concept of 'altitude sickness', although it actually concerns a few different types of illness due to lack of oxygen. The severity of the problems varies: From 'little means' to 'life-threatening'.
Author: Han Willems - More info about altitude sickness
