High altitudes can make you sick.
High altitude has a number of effects on your system. This is largely because the air pressure is lower at high altitude, which means there is not as much oxygen. This in turn means that your body does not get as much oxygen with each breath, which has some very real biological effects.
Causes
Every biological effect of high altitude is caused by the same thing -- less oxygen in every breath of air. This has a variety of effects, depending on how quickly you ascended to high altitude and how high the altitude is. However, this is the common denominator for all biological effects of high altitude.
Acclimating
A slow ascent to high altitude causes your body to adjust to the fact that there is less oxygen in the air. Your body produces more red blood cells in order to carry more oxygen from each breath. This raises your blood pressure in order to move oxygen faster, and you start breathing deeper without thinking about it. These are all examples of how your body starts to make changes in order to compensate for the reduced oxygen around you. Prior to acclimating, your body will not be receiving enough oxygen, which can cause dehydration, headaches and generally feeling hungover. These symptoms are thought to be caused by mild brain swelling due to lack of oxygen.
Breathing Changes
At high altitudes, people will start to breathe differently. When they are awake, they will breathe faster, as they need to breathe more in order to take in the same amount of oxygen as they would at sea level. However, this results in less carbon dioxide than usual in their system. Since your brain uses high carbon dioxide levels to tell you to breathe again, this results in a "holding, letting out" pattern of breathing when you are asleep at high altitude. This usually stops once your body acclimates and you no longer breathe as quickly.
Pulmonary Edema
When you ascend to very high altitudes very quickly, you run the risk of pulmonary edema. When you are in areas with adequate oxygen, the pressure keeps fluid out of your lungs and brain. However, when you ascend too quickly, you lose this pressure and fluid can leak into either of these two organs. This is an extremely serious issue with high altitude, and it can result in death.
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