Hypoxia

Humans require a continuous supply of oxygen for the body to function normally.

Oxygen passes from the lungs into the bloodstream and is then pumped by the heart to the body's tissues and organs.

The brain has a particularly high requirement for oxygen.

If the body's cells do not receive enough oxygen, normal function begins to deteriorate.

This condition of insufficient oxygen is known as hypoxia.

In flying, the main cause of hypoxia is the reduction in atmospheric pressure with increasing altitude.

Oxygen and Altitude

The atmosphere is made up mainly of:

Nitrogen — 78%

Oxygen — 21%

Other gases — 1%

As altitude increases, atmospheric pressure decreases.

At approximately 18,000 ft, atmospheric pressure is about half that at sea level.

Although pressure decreases with altitude, the percentage of oxygen remains approximately 21%.

However, the partial pressure of oxygen decreases, so less oxygen is able to pass from the lungs into the bloodstream.

As a result, less oxygen is available to the body's tissues and, in particular, the brain.