When the diaphragm relaxes during quiet breathing expiration, what contributes to lung volume reduction?

• A. Elastic recoil
• B. Muscle contraction
• C. Intrapleural pressure increase
• D. Transpulmonary pressure decrease

Answer: (A)

When the diaphragm relaxes during quiet breathing expiration, elastic recoil plays a crucial role in lung volume reduction. The lungs have intrinsic elastic properties, which means they tend to return to their original size after being stretched during inhalation. This elastic recoil is due to the elastic fibers in lung tissue and the surface tension within the alveoli.

As the diaphragm and intercostal muscles relax, the thoracic cavity becomes smaller, resulting in an increase in intra-thoracic pressure. This pressure change facilitates the passive process of expiration, allowing air to flow out of the lungs. The elastic recoil effectively reduces lung volume, making space for the outgoing air.

While increases in intrapleural and intra-alveolar pressures also occur during expiration, it is the elastic properties of the lung tissue itself that primarily drive the reduction in lung volume during this phase of breathing.