A quick note on pressurization
Cabin pressurization is controlled by regulating the discharge of conditioned cabin air through the outflow valves.
Two outflow valves are installed at the rear of the cabin. The valves normally operate in parallel.
Cabin altitude and full ventilation rates can be maintained by either valve.
There are two cabin altitude controllers, A and B. Each controller controls both outflow valves.
Positive and negative pressure relief valves protect the fuselage against excessive pressure differential.
The pressurization system has automatic and manual operating modes.
and following on from what has already been mentioned,
For takeoff, the system provides a small positive pressurization prior to rotation to cause a smooth transition to the cabin altitude climb schedule. During climb, cabin altitude increases on a schedule related to airplane climb rate and flight plan cruise altitude. When the FMC climb path has a planned level segment, it is included in the total time required for the airplane to reach the top of climb and cabin altitude continues to increase during the level segment. If the airplane climb flight path is above the FMC climb path and maximum cabin pressure differential is reached during the climb, cabin rate then becomes a function of airplane climb rate so maximum cabin differential pressure is not exceeded.
Hope this helps,
Jason.