Refer to figure.
PRESSURIZATION SYSTEMS
The aim of a cabin pressurization system is to maintain the cabin altitude at an optimum pressure, irrespective of the height at which the aircraft is flying. For aircraft with passenger cabins, the maximum cabin altitude of an aircraft must be limited to between 6000 and 8000 ft. At this altitude, the air will provide sufficient oxygen for normal use.

Control. Achieved by feeding air into the cabin (through air conditioning packs) and restricting its flow out - by using electric motor-operated outflow valves. The valves are automatically controlled by the aircraft’s pressurization system when in automatic control.

• If a higher pressure is needed inside the cabin, the outflow is reduced by closing the outflow valves (which increases the pressure);
• To reduce cabin pressure, the outflow valve slowly opens, allowing more air to escape.