Fig. 6. Effect of chronic hypoxia of 9 to 24 hours on HIF1 hydroxylation. Model results show the amount of hydroxylated [HIF1] per [PHD2] for four different ratios of PHD2 synthesis rate to HIF1 synthesis rate relative to the maximum [HIF1]_hydroxylated[PHD2]. HIF1 synthesis is a function of [O₂] and duration of hypoxia, whereas PHD2 synthesis is a function of [HIF1] and duration of hypoxia. HIF1 accumulation begins at 4 hours, and measurable PHD2 synthesis follows at 8 hours.