An increase in heart rate is one component of the "fight or flight" response. When we are startled, frightened, or angry, our bodies automatically ready themselves to stand up and fight or to run away. In order to achieve this response, the sympathetic nervous system rapidly releases epinephrine from the adrenal glands into the blood stream. Epinephrine is also known as adrenaline and is responsible for the ‘adrenaline rush’ that we experience during arousing situations. Epinephrine released at nerve terminals surrounding the heart causes an increase in the strength and frequency of the heart beats so that more blood is pumped faster throughout the body to provide oxygen and fuel to muscles in preparation of our impending decision to either fight or fly.

Activation of the sympathetic nervous system and the resultant release of epinephrine is responsible for physiological reactions in addition to increased heart rate. These other responses also help the body prepare to react and include dilation of the pupils to increase visual acuity, the breakdown of liver glycogen so that glucose is readily available to fuel muscles, enhanced lung function, hypervigilance, and the redirection of blood flow from central organs to the muscles. During this time, the parasympathetic nervous system, the part of the central nervous system that is responsible for digestion, reproduction, and energy storage, is shut down. The body is focused on survival and the activities of the sympathetic nervous system.

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