In a normal adult, which phase of respiration is longer?

In a typical adult, the phase of expiration is longer than inspiration during normal, quiet breathing. This is due to the mechanics of the respiratory process. When air is inhaled (inspired), it is quickly drawn into the lungs, primarily driven by the contraction of the diaphragm and external intercostal muscles. This phase is relatively short-lived.

Conversely, expiration, or exhalation, involves the passive recoil of the diaphragm and the elastic recoil of lung tissue. Once the air has been brought into the lungs, the body allows for a longer period of time for the air to be expelled. This passive process can take longer than the active process of inhalation, resulting in a longer duration for expiration in normal breathing patterns.

Furthermore, normal anatomical and physiological factors, such as airway resistance and lung compliance, also play a role in the duration of these phases, contributing to the perception that expiration is longer than inspiration under resting conditions.