Hypoxia 

Humans require oxygen which is transferred to the blood through the lungs. 

The heart pumps the blood through the body to every cell, but the brain has the highest oxygen requirement. 

For normal body operation sufficient oxygen is required and if for any reason the amount of oxygen reduces the human body will start to degrade.  

If the body cells do not receive enough oxygen the body organs - in particular the brain - will not function properly.

This condition of insufficient oxygen is known as hypoxia.  

In flying, the main reason for hypoxia is due to reduced atmospheric pressure at altitude.

The atmosphere is made up of 3 main gasses:

Nitrogen                    - 78%

Oxygen                      - 21%

Trace Gases              - 1%

As we climb into the atmosphere, the pressure around us reduces.

For example, at 18'000ft, the atmosphere has approximately half the weight as at the surface.  

Even though the pressure is reducing, the percentages of each gas remain the same.