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Our body relies on many complex equilibria (the effect of long term weightlessness on bone density is one example). Our blood's chemistry is a particularly good example of how gases and pressure change in an equilibrium.
The pressure
of the earth's atmosphere decreases at higher elevations. At the altitude of
Mount Everest (8,850 meters), the pressure of the atmosphere is only about 30% of the
pressure at sea level. We breathe in atmospheric oxygen, and our blood transports it
for cellular respiration. The oxygen is carried through the bloodstream attached to
hemoglobin molecules (which are found in our red blood cells).
Hemoglobin is a complex protein molecule that contains four iron atoms in its centre. We will represent it by the symbol Hem. When hemoglobin is exposed to oxygen it binds to the oxygen to form oxyhemoglobin, which we will represent by the symbol HemO2. This is a gas pressure equilibrium, and can be represented by the equation:
O2 (g) + Hem (aq) 
HemO2 (aq)
le Châtelier's principle predicts that the reaction will respond to a decrease in gas pressure by shifting to the side with more gas molecules. This is shown in the following table.
| Applied Stress | Le Châtelier's Principle Prediction of Response to Stress |
|||
| O2 (g) | + Hem (aq) | |
HemO2 (aq) | |
| Decrease P |  Increase |
Increase |
Decrease |
|
The net result is that there is a lot less HemO2 to carry oxygen to the cells. If a person were quickly transported from sea level to the height of Mount Everest they would probably die, and would most definitely become unconscious. This condition is known as hypoxia, and is what would happen if an airplane were suddenly depressurized. The cells, particularly in the brain, would not get enough oxygen to function.
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So how can mountain climbers get to the top of Mount Everest? Most carry oxygen with them, but some have actually climbed it without using any oxygen support. In order to do this, the climbers become acclimatized in stages to increasing elevation. They do this by setting up different camps at different altitudes. They climb up to a higher camp, usually sleep overnight, then return to a lower camp. During a recovery period at lower altitude the body responds by producing more hemoglobin molecules. This increases the concentration of Hem in the blood. Using the effect of concentration and le Châtelier's principle we reach the conclusion that:
| Applied Stress | Le Châtelier's Principle Prediction of Response to Stress |
|||
| O2 (g) | + Hem (aq) | |
HemO2 (aq) | |
| Increase [Hem] |  Decrease |
Decrease |
 Increase |
|
The reaction will respond by shifting to the right, so there is more HemO2. Even though there is less oxygen pressure, the result is that the body can carry enough oxygen to (barely) supply its needs.
Climbing at high altitudes is incredibly dangerous. The lack of oxygen makes the brain function poorly. Climbers often make bad judgments that they would not make if they were thinking clearly. The ratio of persons who have made it to the summit of Everest compared to those who have died is almost 1 in 4.
