What primarily influences functional residual capacity?

• A. Airway resistance
• B. Elastic recoil of lungs and chest wall
• C. Surfactant production
• D. Alveolar ventilation

Answer: (B)

Functional residual capacity (FRC) is the volume of air present in the lungs at the end of a normal expiration. It is primarily influenced by the elastic recoil of the lungs and the chest wall. The elastic recoil is the tendency of the lung tissue to return to its original size and shape after being stretched, and this force is balanced by the outward pull of the chest wall. When these forces are in equilibrium, FRC is established.

At FRC, the forces of lung collapse due to elastic recoil and the forces trying to expand the thoracic cavity reach a balance. If the elastic recoil increases (such as with conditions like pulmonary fibrosis), FRC may decrease, whereas conditions that decrease recoil (like emphysema) can lead to an increased FRC.

While other factors like airway resistance and surfactant production can influence lung mechanics and efficiency of gas exchange, they do not have as direct an effect on the measurement of FRC as the elastic recoil does. Airway resistance primarily affects airflow and dynamic lung volumes rather than static volumes like FRC. Surfactant production plays a crucial role in preventing alveolar collapse and improving compliance, but its impact on FRC is indirect compared to the direct influence of elastic recoil. Alveolar ventilation