Strong emotional events, like intimidation, trigger a physical response in the body. A mixture of stress and the 'flight or flight response' triggers the hypothalamus to increase production of adrenaline in the adrenal glands. The glands also produce cortisol, a chemical which 'turns off' essential functions, but it is the adrenaline which increases the heart rate.
Your heart has adrenergic receptors that register the increased amount of adrenaline in the bloodstream. They then, in turn, trigger the next set of alarms that increase heart rate.
From an evolutionary point of view, acute stress response prepares the body for either a fight, or flight. Increased heart and breathing rates ensures that oxygen is transported to the muscles for physical exertion. In addition, shutting off non-essential 'services' makes you focussed and alert.
Triggering an increased heart rate allows you to 'act and react'. Different chemicals then need to be triggered in order to 'repair'.