Refer to figure.

HYPOXIA

Hypoxia occurs when the tissues in the body do not receive enough oxygen. Hypoxia can be caused by several factors including an insufficient supply of oxygen, inadequate transportation of oxygen, or the inability of the body tissues to use oxygen. A healthy person is normally able to compensate for altitudes up to approximately 10000 - 12000 ft.

An insidious characteristic of hypoxia is that its early (and probably the most dangerous) symptom include euphoria (false sense of well being). This can prevent the pilot from recognizing a potentially hazardous situation because it puts him/her in such a state of mind that individual well being, as well as that of the passengers, is a low priority. You should remain alert for the other symptoms of hypoxia such as headache, increased response time, impaired judgement, drowsiness, dizziness, tingling fingers and toes, numbness, blue fingernails and lips (cyanosis), visual disturbances, lack of concentration, fatigue and limp muscles. The speed and order of appearance of signs, and the severity of symptoms, produced by breathing air at altitude depend on the individual.

Even if you learn the early symptoms of hypoxia, do not assume that you will be able to take corrective action whenever they occur. Because when the brain starts to feel the effects of hypoxia, the higher reasoning portion of the brain is the first affected. This means that judgment and cognitive skills diminish from the very start.