When frightened or even in stressfull situation, the adrenal gland releases adrenalin into the blood stream to circulate in the body to influence many organs including the heart. This cause the rate of heart beat to increase several times in order to suppy the muscles of the body with blood glucose and oxygen for action i.e to help the muscle to react quickly and vigorously by running away or putting up a fight or struggle

when you are frighten

The hypothalamus sends an electrical impulse down your nerves to your adrenal glands, which sit on top of your kidneys. These produce adrenaline and send it into your bloodstream. Adrenaline can cause: your breathing to speed up; your pupils to dilate; saliva production to slow down or stop; blood to drain away from non-critical organs like skin and stomach; decreased sensitivity to pain; and your heart rate to speed up. This is an evolved process to help us escape or neutralise dangerous situations