The passive nature of expiration refers to which characteristic?

The passive nature of expiration relates directly to the concept that it does not require active contraction of respiratory muscles. During normal breathing, expiration typically occurs as a result of the relaxation of the diaphragm and intercostal muscles following inhalation. This relaxation allows the elastic recoil of the lung tissue and the thoracic cavity to reduce the volume of the chest and push air out of the lungs without the need for muscular effort.

When considering the other options, the idea that expiration only occurs during sleep is inaccurate, as breathing happens continuously, whether awake or asleep. Similarly, saying it happens only during exercise contradicts the understanding that expiration occurs at rest as well. Finally, even though expiration does occur at all times, the statement about requiring no muscle effort highlights the key passive characteristic more effectively than merely stating that it occurs consistently.