In an emergency the sympathetic nervous system the body by

The answer to this question would be fight or flight mechanism, which will increase heart rate and judgment time.

The sympathetic nervous system will be activated when something threatening and helping the organism to make a fast decision. In this condition, the heart rate of the individual will be increased and the heart will pump more blood so you can do much more work than normal condition. 
In this state, the individual will fight the threat or simply running away from it.