What primarily causes expiration during normal breathing?

• A. Active muscle contraction
• B. Elastic recoil
• C. Increased intrapulmonary pressure
• D. Diaphragm relaxation

Answer: (B)

During normal breathing, expiration is primarily caused by elastic recoil of the lungs. The lungs and chest wall have elastic properties that allow them to stretch during inhalation when air is drawn in, but they also return to their original shape when the muscles that were contracting during inhalation relax. This elastic recoil generates a pressure difference that helps exhale air out of the lungs without requiring active muscle contraction.

The other choices may involve different aspects of the respiratory process but do not accurately describe the primary mechanism of normal expiration. For instance, while diaphragm relaxation does occur during expiration, it is not the direct cause but rather part of the process that allows the elastic recoil to take effect. Increased intrapulmonary pressure can occur but is a result of the lung volume decreasing rather than being the main factor. Active muscle contraction is mainly associated with inspiration rather than normal expiration, as the primary action of expiration is passive during quiet breathing.