Effects of high altitude on humans

# Overview
There are several effects of high altitude on humans:
The percentage saturation of hemoglobin with oxygen determines the content of oxygen in our blood. After the body reaches around 7000 feet (2100 m) above sea level, the saturation of oxyhemoglobin begins to plummet.

# Effects
Altitude acclimatization, the physiological adaptions to altitude, can have immediate and long term effects.

## Immediate effects
- Hyperventilation
- Fluid loss (due to a decreased thirst drive and decrease in ADH)
- Increase in heart rate (HR)
- Slightly lowered stroke volume

## Longer term effects
- Lower lactate production (because reduced glucose breakdown decreases the amount of lactate formed).
- Compensatory alkali loss in urine
- Decrease in plasma volume
- Increased Hematocrit (polycythemia)
- Increase in RBC mass
- Higher concentration of capillaries in skeletal muscle tissue
- Increase in myoglobin
- Increase in mitochondria
- Increase in aerobic enzyme concentration
- Increase in 2,3-BPG
- Hypoxic pulmonary vasoconstriction
- Right ventricular hypertrophy

# Altitude and athletic performance
In the athletic arena, it is thought that acclimatization from living and training at high altitudes enhances performance compared to living and training at sea level.  However, this may not always be the case. Any positive acclimatization effects may be negated by a de-training effect as the athletes are usually not able to exercise with as much intensity at high altitudes compared to sea level.
This conundrum led to the development of the altitude training modality known as "Live-High, Train-Low" whereby the athlete spends many hours a day resting and sleeping at one (high) altitude, but performs a significant portion of their training, possibly all of it, at another (lower) altitude. A series of studies conducted in Utah in the late '90s by researchers Ben Levine, Jim Stray-Gundersen, and others, showed significant performance gains in athletes who followed such a protocol for several weeks.  
Other studies have shown performance gains from merely performing some exercising sessions at altitude, yet living at sea-level. 
For those who wish to adjust to high altitudes, or to obtain the associated athletic performance, but without being at high altitudes, state-of-the-art altitude acclimatization devices exist.  Chambers that reduce barometric pressure, or hypoxic systems (altitude tents or altitude rooms) with increased nitrogen concentration (which reduces oxygen), are used by athletes to acclimatize to high altitudes.
To achieve the full potential athletic gains from at-rest altitude acclimatization, one must maintain altitude exposure for a significant period of time and the effects are only transitory.  A study  using simulated altitude exposure for 18 days, yet training closer to sea-level, showed performance gains were still evident 15 days later.
The physiological adaptation that is mainly responsible for the performance gains achieved from altitude training, is a subject of discussion among researchers.
Some, including American researchers Ben Levine and Jim Stray-Gundersen, claim it is primarily the increased Red Blood Cell Volume .
Others, including Australian researcher Chris Gore, and New Zealand researcher Will Hopkins, dispute this and instead claim the gains are primarily a result of other adaptions such as a switch to a more economic mode of oxygen utilization