Fear the the speed of your heart?

Why does your heart beat faster when your scared?Fear the the speed of your heart?
It's due to adrenaline aka epinephrine. Epinephrine is a "fight or flight" hormone which is released from the adrenal glands whenever danger threatens. When secreted it floods out the endocrine gland into the bloodstream to instantly prepare the body for action in emergency situations. This action by your body leads to an increase in heart rate.
Adrenaline is released into the bloodstream by the adrenal glands when the brain says its time to 'get out of Dodge'.