Cabin pressure

From: (Wayne Dockery)
Organization: The Pipeline
Date:         22 May 95 12:50:58 
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If the cabin in a commercial passenger aircraft is
pressurized, how is it possible to exhange fresh
air from the outside since I would assume the aircraft
is somewhat airtight?  Also, is the pressure managed
by a air-compressor or is it simply "scooped" from the
outside via some sort of duct-work into the cabin thereby
generating the pressure?