Closing volume is the volume of lung
High-Yield Explanation
The closing capacity (CC) is the volume in the lungs at which its smallest airways, the respiratory bronchioles, collapse. It is defined mathematically as the sum of the closing volume and the residual volume. The alveoli lack suppoing cailage and so depend on other factors to keep them open Ref: guyton and hall textbook of medical physiology 12 edition page number:325,326,327