Figure 17-41 Changes in the minute ventilation and the circulation of healthy awake humans during progressive isocapnic hypoxia and hyperoxic hypercapnia. PETCO₂ , end-tidal PCO₂ ; PETO₂ , end-tidal PO₂ ; , cardiac output; S₁ , slope during the first phase of slowly increasing ventilation and/or circulation; S₂ , slope during the second phase of sharply increasing ventilation and/or circulation. V̇E, expired minute ventilation. (Redrawn from Serebrovskaya TV: Comparison of respiratory and circulatory human responses to progressive hypoxia and hypercapnia. Respiration 59:35, 1992.)