system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How does the air pressure in a airplane change depending on altitude if the plane is sealed shut?
gpt
The plane is not sealed completely shut.

Compressed air is tapped off of the the engines (bleed air), and this then goes through the air-conditioning packs and into the cabin. There are then outflow valves in the fuselage where the air exits, so there is a continuous circulation of air. The outflow valves are controllable to set the cabin pressure.

The reason that the plane isn't pressurised up to ground level pressure is to reduce stress on the airframe through the repeated pressurisation/depressurisation cycles.