Which statement about exhalation is true?

Which statement about exhalation is true?

• A. Inhalation is active
• B. Exhalation is passive
• C. Exhalation is active
• D. Both phases require muscle effort

Answer: (B)

During normal, quiet breathing, exhalation happens passively. After the diaphragm and external intercostals contract to fill the lungs, they relax and the lungs and chest wall recoil elastically. This recoil increases intrapulmonary pressure above ambient, pushing air out without active muscle effort. Inhalation, by contrast, is active because it requires muscle contraction to enlarge the thoracic cavity. Active expiration only occurs during forced or heavy breathing when expiratory muscles contract. So the true statement is that exhalation is passive.

During normal, quiet breathing, exhalation happens passively. After the diaphragm and external intercostals contract to fill the lungs, they relax and the lungs and chest wall recoil elastically. This recoil increases intrapulmonary pressure above ambient, pushing air out without active muscle effort. Inhalation, by contrast, is active because it requires muscle contraction to enlarge the thoracic cavity. Active expiration only occurs during forced or heavy breathing when expiratory muscles contract. So the true statement is that exhalation is passive.