The pressurisation system, keeps the internal pressure some psi above outside pressure ( for example 5 Psi above outside) at 38000ft, the pressure outside would be nearing 0psi (maybe 1 or 2), so internal pressure would be around 6 or 7 psi . As the aircraft descends,the outside preure will increase, so internal will increase also, until it reaches about 14 psi (average ground level pressure)
This is a very simplified explantion... but close enough