Final answer:
At functional residual capacity, the outward recoil of the chest wall is equal and opposite to the inward recoil of the lungs, facilitating passive expiration.
Step-by-step explanation:
At functional residual capacity (end of quiet expiration) the outward recoil of the chest wall is equal in magnitude but opposite in direction to the inward recoil of the lungs. This balance between the chest wall's expansion and the lungs' natural elasticity allows for the pressures to equilibrate, enabling passive expiration without requiring muscular contraction. The recoil property of the lung tissue drives it to collapse inward, while the elastic work conducted by the intercostal muscles, chest wall, and diaphragm during inhalation is relaxed, allowing the thoracic cavity to decrease in volume and air to be expelled.