In contrast to inhalation, exhalation:

In contrast to inhalation, exhalation:

• A. Is an active process requiring muscle contraction.
• B. Is a passive process caused by decreased intrathoracic pressure.
• C. Is a passive process caused by increased intrathoracic pressure.
• D. Requires chest wall contraction.

Answer: (C)

The mechanism being tested is that breathing at rest uses muscle action to inhale, but relies on passive recoil for exhale. Inhalation happens when the diaphragm and intercostal muscles contract, expanding the chest and lowering intrathoracic pressure to draw air in. Exhalation, in contrast, is normally a passive process: as the chest and lungs recoil, the volume inside the thorax decreases and intrathoracic pressure rises above atmospheric, pushing air out. Only with forced breathing do expiratory muscles contract to actively raise pressure further. So exhalation is a passive process caused by increased intrathoracic pressure, which matches the correct choice.

The mechanism being tested is that breathing at rest uses muscle action to inhale, but relies on passive recoil for exhale. Inhalation happens when the diaphragm and intercostal muscles contract, expanding the chest and lowering intrathoracic pressure to draw air in. Exhalation, in contrast, is normally a passive process: as the chest and lungs recoil, the volume inside the thorax decreases and intrathoracic pressure rises above atmospheric, pushing air out. Only with forced breathing do expiratory muscles contract to actively raise pressure further. So exhalation is a passive process caused by increased intrathoracic pressure, which matches the correct choice.