Exhalation, in contrast to inhalation, is described as which of the following?

• A. Requires muscular effort to effectively expel air.
• B. Is a passive process caused by increased intrathoracic pressure.
• C. Occurs when the diaphragm lowers and expels air from the lungs.
• D. Is an active process caused by decreased intrathoracic pressure.

Answer: (B)

Exhalation in normal breathing is a passive process driven by the elastic recoil of the lungs and chest wall after the inspiratory muscles relax. As these structures return to their resting lengths, intrathoracic pressure rises above atmospheric pressure, pushing air out of the lungs. That’s why exhalation is described as passive. The diaphragm’s downward movement is part of inhalation, when it contracts to enlarge the thoracic cavity; during exhalation it relaxes and moves upward. Increased intrathoracic pressure during exhalation explains why air exits the lungs, whereas decreased pressure would draw air in. Remember, if you’re exercising or forced to breathe out more forcefully, exhalation can involve active muscle use, but in quiet breathing it remains a passive process.