How do stimulants increase heart rate?

Release dopamine: Dopamine has pretty much the same kind of activity on the heart and vascular system as adrenaline and noradrenaline do. These are the bodies own neurotransmitters that are part of the "fight or flight" response, when we are under stress. They increase blood flow to our brain and muscles so that we can respond to danger or stress. Stimulants all work by releasing dopamine.