How is closing capacity defined in terms of lung volumes?

Closing capacity is defined as the volume of air in the lungs at which small airways become closed during expiration. This concept primarily includes the closing volume, which is the volume of air in the lungs when the lungs start to close off the small airways, and the residual volume, which is the volume of air remaining in the lungs after a forced expiration. When we add both the closing volume and the residual volume, we accurately describe the total volume at which airway closure begins to occur, thereby defining the concept of closing capacity.

This understanding is critical in respiratory physiology because it helps in assessing lung function, particularly in individuals with conditions that affect lung compliance or airway resistance. By combining these two measurements, we gain insight into the aeration and ventilation status of the lungs, especially in clinical settings when evaluating diseases such as chronic obstructive pulmonary disease (COPD) or restrictive lung diseases.