Altitude Symptoms

As you climb to higher elevations, the altitude begins to change the way your body functions. As the air gets thinner ,the amount of available oxygen in each breath decreases. As well, your body's tissues have a harder time getting the oxygen they need for metabolism, and you enter the state of reduced oxygen called hypoxia. Your body attempts to adapt to this environmental change but adaption takes time. There is a great variation how each individual adaps.

Signs of Acute Mountain sickness

Lack of appetite, nausea, vomiting
Fatigue or weakness
Pins and needles
Shortness of breath
Persistent rapid pulse
Peripheral edema
Loss of coordination
Fullness or tightness of chest
Reduced urine output
Heavy feeling in the legs

Severe symptoms

Symptoms that may indicate life-threatening altitude sickness include:

High-Altitude Pulmonary Edema (fluid in the lungs)- HAPE
HAPE occurs in about 2% of those adjusting to altitudes above 10,000 feet. It can be fatal.
Symptoms similar to bronchitis
Persistent dry cough
Shortness of breath even when resting

High-Altitude Cerebral edema (swelling of the brain)- HACE
Usually develops in unacclimatized climbers above 10,000 feet although it can occur as low as 8500 feet. It can lead to death.
Headache that does not respond to analgesics
Unsteady gait
Gradual loss of consciousness
Increased nausea
Retinal haemorrhage

Also see

A few ways to cope with Altitude