A 42-year-old woman with severe anemia (hemoglobin 6.5 g/dL) secondary to chronic GI bleeding presents with palpitations and dyspnea on exertion. Her heart rate is 110 bpm, blood pressure 118/72 mmHg, and cardiac output (measured by thermodilution) is 6.8 L/min (normal 4–5 L/min). After transfusion to Hb 10 g/dL, her heart rate falls to 92 bpm and cardiac output decreases to 5.2 L/min. Which of the following BEST explains the physiologic basis for the elevated cardiac output in severe anemia?