Explain the process of ventilation in human lungs.

Ventilation is movement of air into and out of lungs;
volume of thorax/lungs/chest increased/decreased;
pressure in thorax/lungs/chest decreased/increased;

air flows from higher to lower pressure / air flows until the pressures are equal;

During inspiration/inhalation:

external intercostal muscles contract so ribcage moved up/out; diaphragm contracts so moves down/becomes flatter;
internal intercostal/abdomen (wall) muscles relax;

During expiration/exhalation:

external intercostal muscles relax so ribcage moved down/in; diaphragm relaxes;
recoil of elastic fibres that stretched during inspiration;
internal intercostal muscles contract (during forced ventilation); abdomen (wall) muscles contract (during forced ventilation);