The innate intelligence of the body uses the human brain to preside over every function of your body including those of the heart and blood vessels by designating authority to the autonomic nervous system, two groups that oppose and balance each other.
The sympathetic nerves send the heart rate soaring: Danger, stress and exertion signal this system to speed up the flow of blood. The parasympathetic system counters by slowing the heart down, conserving energy for the demands of everyday life.
The sympathetic nervous system is part of the flight response that primes us for action. You step off a curb; subsequently a speeding taxicab skids around the corner, heading straight for you. The innate intelligence of the body responds to such dangers by signaling the sympathetic nerves to pour out adrenaline and noradrenaline (now called epinephrine and norepinephrine).
Both chemicals constrict blood vessels, raise blood pressure, and speed the heart. As the cab bears down, your heart pounds faster and faster, your blood pressure rises, breathing deepens, and muscle tense. You leap for safety and collapse on the sidewalk, sweating and gasping for breath.
In extreme emergencies the sympathetic nerves can send the heart rate soaring as high as 200 beats per minute, preparing us for unusual feats of strength or action.
Amazing isn’t it?