Exhalation is the passive process expelling air.

• A. Passive Process Drawing Air Into Lungs
• B. Active Process Drawing Air Into Lungs
• C. Passive Process Expelling Air
• D. Active Process Expelling Air

Answer: (C)

Exhalation during quiet breathing relies on the elastic recoil of the lungs and chest wall. When inspiratory muscles relax, the thoracic cavity decreases in volume and the lungs snap back to their resting shape. This rebound increases the pressure inside the airways above atmospheric pressure, so air is pushed out passively without needing additional muscle energy. That’s why exhalation is described as a passive process that expels air.

Inhalation, by contrast, is driven by muscle contraction (the diaphragm and external intercostal muscles) and is therefore an active process. Only during forceful exhalation—such as coughing or intense exercise—do expiratory muscles actively contract to push air out more quickly. For normal breathing, the passive expiration fits the statement exactly: it’s a passive process expelling air.