The plateau portion of the O2−Hb curve________
Both (a) and (b) above are correct.
The plateau region is where O2 is being loaded onto Hb. In this region PPO2 is 100 mm Hg. Looking at the O2−Hb curve, note that at a blood PPO2 of 100 mmHg, Hb is 97.5% saturated. Therefore Hb in the systemic arterial blood normally is almost fully saturated. If the alveolar and consequently the arterial , fall below normal. There is little reduction in the total amount of O2, transported by the blood until the, falls below 60 mmHg, because of the plateau region of the curve. Even if the arterial PPO2 falls from 100 to 60 mmHg, The % Hb saturation is still remarkably high at 90% .