Quiet Expiration is primarily a passive process.

Quiet expiration is principally due to the elastic recoil of the costal cartilage and the relaxation of the muscles of inspiration. This also aided by the recoil of the abdominal muscles which increases the intrabdominal pressure pushing the diaphragm up. All these actions result in a decrease in the chest volume with a resultant increase in intralveolar pressure.

In Forced Expiration, the process is an active one with contraction of the abdominal muscles resulting in pressure against the diaphragm and depression of the rib cage. The internal intercostals contract and help in the depression of the rib cage. All these actions decrease the volume of the chest and increasing the intralveolar pressure.