Figure 17-19 Relationship between functional residual capacity (FRC) and closing capacity (CC). FRC is the amount of gas in the lungs at end-exhalation during normal tidal breathing, shown by the level of each trough of the sine wave tidal volume. CC is the amount of gas that must be in the lungs to keep the small conducting airways open. This figure shows three different CCs, as indicated by the three different straight lines. See the text for an explanation of why the three different FRC-CC relationships depicted result in normal or low ventilation-perfusion (V̇A/) relationships or atelectasis. The abscissa is time. (Redrawn from Benumof JL: Anesthesia for Thoracic Surgery, 2nd ed. Philadelphia, WB Saunders, 1995, Chapter 8.)