Figure 49-6 Effect of hypoxic pulmonary vasoconstriction (HPV) on PaO₂ . As the amount of lung that is made hypoxic is increased (x axis), arterial oxygen tension (PaO₂ ) decreases (y axis). In the range of 30% to 70% hypoxic lung, the normal expected amount of HPV increases PaO₂ from arrhythmogenic levels to much higher and safer levels. Normal cardiac output, hemoglobin concentration, and mixed venous oxygen tension (Pv̄O₂ ) are assumed. (Data from Marshall BE, Marshall C: Continuity of response to hypoxic pulmonary vasoconstriction. J Appl Physiol 59:189, 1980.)