V/Q Ratio "Normal" V/Q = 0.8 V = alveolar ventilation Q = pulmonary blood flow (perfusion) "Normal" V/Q depends on "normal" respiratory rate, tidal volume, and cardiac output PaO2 = 100 mm Hg PaCO2 = 40 mm Hg Ventilation/perfusion matching is essential for ideal gas exchange of O2 and CO2 "ideal" V/Q = 1.0 exercise → ↑ cardiac output → vasodilation of apical arteries V/Q approaches 1.0 Distribution of V/Q in Lung Ventilation and perfusion are nonuniformly distributed in normal, upright lung Zone 1 apex of lung decreased ventilation and decreased perfusion V/Q is highest (= 3.0) because of relatively greater decrease in perfusion wasted ventilation PaO2 is highest and PaCO2 is lowest organisms that thrive in high O2 (e.g., TB) flourish in apex of lung Zone 3 base of lung V/Q is lowest (= 0.6) wasted perfusion PaO2 is lowest and PaCO2 is highest V/Q Defects Dead Space (V/Q → ∞) e.g., pulmonary embolism (blood flow obstruction) ventilation of lung regions that are not perfused wasted ventilation physiologic dead space no gas exchange occurs PAO2 has same composition as humidified, inspired air (= 150 mm Hg) PACO2 = 0 mm Hg 100% O2 improves PaO2 Shunt (V/Q = 0) e.g., airway obstruction, dependent portion of lung in ARDS can act as "shunt" perfusion of lung regions that are not ventilated wasted perfusion no gas exchange pulmonary capillary blood has same composition as venous blood 100% O2 does not improve PaO2