A 48-year-old Indian man with cirrhosis presents with progressive dyspnea and orthodeoxia (shortness of breath when upright, relieved when lying flat). Arterial blood gas on room air shows PaO2 58 mmHg with an elevated A-a gradient. When placed on 100% inspired oxygen, his PaO2 rises only to 85 mmHg. The mechanism marked **C** in the diagram (right-to-left shunt) is responsible for this refractory hypoxemia. Which of the following best explains why supplemental oxygen fails to correct the hypoxemia in this patient?