A 58-year-old man with COPD presents with bluish discoloration of his lips, tongue, and oral mucosa. On examination, the tongue is distinctly blue. Arterial blood gas shows PaO2 = 55 mmHg and SaO2 = 82% on room air. The clinical sign marked **B** in the diagram indicates the threshold at which cyanosis becomes clinically visible. Which of the following best describes the pathophysiological basis for the appearance of cyanosis in this patient?