Human body is extremely adaptable at high altitude. The atmospheric pressure at high altitude is less and thinner compared to ground level. This may result to altitude sickness that can be extremely fatal and even experienced people who are used at high altitude have died from its effects.
Mountain People
People living at the mountain have adjusted to life at high altitude. Such people have more production of red blood cells compared to normal people and therefore can efficiently carry more oxygen.
Pilots flying at high altitude
Pilots flying planes at high altitude cannot adapt to the atmospheric pressure. They have to wear a mask with tanks of pressurized air that supplies air for them. Without the mask, they would become unconscious and lose control of the plane.
Astronauts in Space
Since there is no atmosphere in space, therefore, there is no supply of oxygen. Astronauts breath air in the space capsule. However, when leaving the spacecrafts, they take air tanks with pressurized air. These generally contain 100
percent oxygen that is lower than normal pressure.
With so much less oxygen, your body has to breathe more to get the same amount of the essential molecule. The oxygen drop combined with the decreased air pressure packs a one-two punch to your cardiovascular system. In order for your lungs to breathe air in without duress, the pressure has to be higher outside your body. But at high altitudes, the outside air pressure is lower than it is inside your lungs, making it more difficult to pull in the thinner air and for your veins to pump oxygen throughout the body.
If altitude sickness remains untreated, it could result to headache, dizziness, swelling of the hands, feet, and face and gradual loss of consciousness. So when you are trying to climb mountains atleast 8,000 ft above ground level, make sure to stop at base camps so that your body can get used to the conditions.
References:
http://scienceline.org/2015/02/why-do-you-feel-so-awful-at-high-altitudes/
https://www.medicalnewstoday.com/articles/179819.php
