The heart rate is controlled by chronotropic influences on the spontaneous electrical activity of SA nodal cells.
An increase in sympathetic activity increases the heart rate, whereas an increase in parasympathetic activity decreases the heart rate.
These neural inputs have immediate effects (within 1 beat) and therefore can cause very rapid adjustments in cardiac output.
Generally, the faster the heart beats, the more blood can be pumped over a particular period of time.
If the heart rate is too slow, , or is too fast, then cardiac output can be impaired.