Mathematically, the cardiac output can be calculated by multiplying the heart rate times the
stroke volume. In anemia, the
cardiac output increases, and that allows more hemoglobin to be exposed to the peripheral tissues, making up for the decreased hemoglobin concentration. Accordingly, the heart rate increases, which gives us one of the cardinal clinical manifestations of anemia, tachycardia, or fast heart rate.
The heart does not act alone to increase the cardiac output.