What is the main characteristic of passive expiration?

The main characteristic of passive expiration is that it involves the relaxation of muscles. During passive expiration, the diaphragm and intercostal muscles, which were previously contracted during inhalation, relax. This relaxation allows the elastic recoil of the lungs and thoracic cage to naturally push air out of the lungs without the need for additional muscular effort.

This process primarily occurs during normal, resting breathing when the body does not require forced or additional ventilation, which would involve active muscle contractions. Therefore, the relaxation of the respiratory muscles is key to enabling this passive process. Other options suggest active involvement or limitation of the process that do not accurately represent how passive expiration functions.