Check Airman, Thanks for the vote, but it was a Cut 'N' Paste from Nasa:
http://spaceflight.nasa.gov/shuttle/...ref/index.html
There is a fundamental difference between Aircraft & Space craft when one considers Pressurization.
Basically: Aircraft fly in the reservoir of air that is used for pressurization, whereas space craft (currently) need to generate (albeit a by-product of fuels) or carry reservoirs of pure gasses to create the same.
In aircraft: air is drawn from its’ surroundings by using engine driven “Blowers or Compressors” to provide a flow of air through the pressure vessel, outflow valves control the pressure and since the reservoir is effectively endless, the blowers/compressors can supply more air than is necessary just to pressurize, so leaks here and there are of no real consequence. (Obviously a bad door seals etc. have an effect)
However in a spacecraft since the source is in short supply, the pressure vessel it’s self has to be of a higher standard, NO or Very Low losses.
Hope that helps?