The closing volume is:

A. The volume remaining in the lung at the end of expiration below which alveolar collapse begins to occur, resulting in physiologic shunting

C. Not changed during surgery

DISCUSSION: The closing volume is conceptually the remaining lung volume at the end of expiration below which alveolar collapse begins to occur, causing intrapulmonary right-to-left shunting and thus desaturation of blood in the left atrium. In a normal young person this closing volume is well below the functional residual capacity (FRC); thus, such physiologic shunting does not occur until there is a decrease in the elastic properties of the lung. Although FRC gradually increases with age, so does the effective closing volume. Eventually some alveoli are being underventilated (at endexpiration), allowing physiologic right-to-left shunting to occur. Closing volume is unchanged, but FRC decreases during surgery (i.e., shunting occurs). Closing volume has no direct relationship to the oxygen content of the mixed venous blood.