A 52-year-old man with chronic obstructive pulmonary disease (COPD) is admitted with an acute exacerbation. Arterial blood gas shows: pH 7.32, PaCO₂ 62 mmHg, HCO₃⁻ 32 mEq/L, PaO₂ 58 mmHg. His hemoglobin is 16 g/dL (elevated due to chronic hypoxemia). Despite supplemental oxygen therapy, his tissue oxygen delivery remains suboptimal. Which factor is primarily responsible for facilitating oxygen unloading to tissues in this patient's setting?