The air pressure there is 1/3 of that at sea level -- I thought I remembered back from the days of the Apollo and Soyuz docking, that the Apollo carried a pure O2 atmosphere at 1/5 sea level pressure (vs. the Soyuz which carried an Earth atmosphere mixture at full sea level pressure). Soyuz cosmonauts would have to go through decompression in order to visit the Apollo craft, as would Apollo astronauts upon returning from a visit to the Soyuz. But there was no special preparation required other than the normal training of the spacemen.
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Human beings also depend on the pressure of the atmosphere for breathing. Atmospheric pressure is a expressed in millimeters of mercury (mm Hg), or (in the older units used by the Space Shuttle Program) in pounds per square inch (PSI). This represents the amount of force (pounds) acting over a given area (square inch). At sea level, the atmosphere presses in with a force of 14.7 pounds on every square inch of surface. This is enough force to raise a column of mercury up to a height of 760 mm. This atmospheric pressure helps us to inhale and exhale and also makes it possible to keep certain gases dissolved within the bloodstream. If air pressure becomes too low, some gases can come out of solution, and body fluids could even begin to boil at normal body temperatures. The correct mixture of gases in the atmosphere and the pressure of the atmosphere must both be maintained in order for us to breathe.
On Earth, air is a mixture of nitrogen (79%), oxygen (20%), carbon dioxide (about 0.04%), and trace amounts of other gases at a pressure of 760 mm Hg. This gives a partial pressure of oxygen of about 152 mm Hg (20% of 760) (3.06 PSI). On Mercury, Gemini and Apollo missions, the atmospheric pressure in U.S. space vehicles decreased from 14.7 PSI to 5 PSI during liftoff, then maintained 5 PSI of pure oxygen for the duration of the flight. Since the percentage of oxygen was 100%, the partial pressure of oxygen was 5 PSI -- more than what is needed for human survival. On the Space Shuttle, the crew compartment life support system normally maintains the atmosphere at 14.7 PSI, with 79% nitrogen and 21% oxygen. The life support system in the Space Shuttle spacesuits maintains 4.3 PSI of pure oxygen. If air pressure is reduced too rapidly, several physiologic problems can result, including "the bends."