Increasing rates of work will increase the energy demand of the body. To fulfill the energy needs, the heart needs to pump more blood.
The rate of blood pumped by heart is known as cardiac output while stroke volume is the amount of blood pumped each contraction. It is calculated by: heart rate x stroke volume
The easiest variable to changed to increase the cardiac output would be heart rate. When a person increases his rates of work, the heart rate will be increased to increases the cardiac output so the heart can supply the energy needed.