What determines the lung functional residual capacity: does the chest wall exert inward elastic recoil, lungs exert outward elastic recoil, alveolar pressure equal pleural pressure, or do lungs and chest wall exert equal and opposing recoil?

From the Research

The lung functional residual capacity is defined by the point at which the lungs and chest wall exert equal and opposing recoil forces. At this point, the inward elastic recoil of the lungs is balanced by the outward elastic recoil of the chest wall, resulting in a stable volume of air remaining in the lungs after a normal passive exhalation. This balance is crucial for maintaining a healthy respiratory system, and alterations in either lung or chest wall mechanics can disrupt this balance and change the functional residual capacity (FRC). For example, conditions like emphysema can reduce lung elastic recoil, resulting in increased FRC and hyperinflation, while restrictive lung diseases or obesity may decrease FRC by altering chest wall mechanics or reducing lung compliance 1. Key factors that influence the FRC include:

• The elastic properties of the lungs and chest wall
• The surface tension of the alveoli, regulated by pulmonary surfactant
• The position of the body and the use of inhaled and intravenous anesthetics, which can affect chest wall forces
• The presence of lung diseases, such as emphysema or restrictive lung diseases, which can alter lung mechanics and compliance. Understanding the equilibrium that defines the FRC is essential for managing patients with respiratory diseases and for optimizing respiratory function in various clinical settings 1.