Figure 17-21 Two-compartment model of the lung in which the anatomic and alveolar dead space compartments have been combined into the total (physiologic) dead space (VD). FACO₂ = alveolar CO₂ fraction; FECO₂ = mixed expired CO₂ fraction; FICO₂ = inspired CO₂ fraction; V̇A = alveolar ventilation; V̇A/ = 1 means ventilation and perfusion are equal in liters per minute; V̇CO₂ = carbon dioxide production; V̇E = expired minute ventilation. Normally, the amount of CO₂ eliminated at the airway (V̇E × FECO₂ ) equals the amount of CO₂ removed by alveolar ventilation (V̇A × FACO₂ ) because there is no CO₂ elimination from alveolar dead space (FICO₂ = 0).