The previous lesson delivered hot, high-pressure air to the wing root; this lesson spends it on the people. It must arrive as fresh air moving through the cabin, at a liveable temperature, at a pressure the human body can work with, seven miles above the weather. The machinery divides along those jobs: the packs make the air cold enough, the trim air makes each zone warm enough, and the pressurisation system decides how much of it is allowed to leave.
Three jobs
The pressurisation and air-conditioning system of an aeroplane controls ventilation, temperature and pressure, and each is a separate control loop:
ventilation — flow-control valves set how much air moves through the cabin;
temperature — the packs and trim air set how warm it is;
pressure — the outflow valve sets how much of it stays.
A fourth quantity rides along uncontrolled by default, and the syllabus asks about it separately: humidity.
Humidity is controlled mostly by removal: the packs' water separator wrings the condensed moisture out of the cold conditioned air before it reaches the cabin, and the cabin is kept deliberately dry, because moisture condensing on cold structure is the corrosion feed-stock of the first lesson. Recirculated cabin air returns some moisture to the mix, and some aircraft add humidifiers for crew rest or premium zones; the dry-eyed long-haul cabin is a design choice, not an oversight.
The pressurisation principle
The core idea fits in one sentence: the packs blow a nearly constant flow of air into the hull, and the cabin's pressure is controlled by throttling how fast that air escapes through the outflow valve. The fuselage is the pressure vessel of the structures module, inflated and relieved continuously: close the outflow valve and the cabin descends (pressure rises); open it and the cabin climbs. The components of the pressurisation system:
The pneumatic system as the power source — the packs' steady inflow is what the whole scheme throttles; no inflow, no pressurisation.
Outflow valve — the controlled exit, usually a large valve at the rear of the hull, modulating between nearly closed (maximum differential) and wide open (unpressurised).
Outflow valve actuator — the motors that drive it: typically an AC motor for automatic control and an independent DC motor for the manual mode, so the valve can always be moved somehow.
Pressure controller — the brain, scheduling cabin altitude against the aircraft's altitude and driving the actuator to achieve it.
Excessive differential pressure-relief valve — a mechanical safety valve that opens if the differential exceeds the structural limit, whatever the controller thinks.
Negative differential pressure-relief valve — the mirror-image guard: if outside pressure exceeds cabin pressure (a fast descent with the cabin lagging), it opens inward, because the hull is built to be inflated, not crushed.