In which of the following a reduction in aerial oxygen tension occurs?

A gas exes its pressure when in a free state. pO2 is exeed by the oxygen that is dissolved in plasma, and not by the oxygen that has combined with Hb. In anaemia and CO poisoning, the oxygen that has combined with Hb is decreased; PO2 is not affected. In the moderate exercise, O2 delivery by Hb is facilitated by the increase of H+, CO2, and temperature. Thus, O2 is liberated from Hb, but PO2 will be unaffected. Alveolar and aerial PO2 depend on alveolar ventilation, alveolar PCO2, and the respiratory quotient; the relationship is given by alveolar air equation. Alveolar O2 is determined by a balance between the rate of O2 removal and the rate of O2 replenishment by ventilation. Metabolic demands of the tissues will determine the rate of O2 removal. If the demands are normal, but O2 replenishment is reduced (as in hypoventilation), alveolar O2 will decrease. And subsequently, aerial PO2 will decrease. In all other cases paial pressure of oxygen is unaffected