The fight or flight response is an automatic physiological reaction to a perceived stressful or frightening event. It is also known as the acute stress response and is a survival mechanism that enables people and other mammals to react quickly to life-threatening situations. The fight or flight response is triggered by a release of hormones that prompt the body to stay and fight or run away and flee. The perception of threat activates the sympathetic nervous system, which triggers the fight-or-flight response, providing the body with a burst of energy so that it can respond to perceived dangers. The chain reaction brought about by the fight or flight response can include increased blood pressure, heart rate, blood sugars, and fats in order to supply the body with extra energy. In addition to physiological reactions, there is also a psychological component to the fight or flight response. Automatic reactions include a quickening of thought and an attentional focus on salient targets such as the source of the threat and potential avenues for escape. Secondary psychological responses can include appraisals about the meaning of the body reactions. The fight or flight response was originally described by American physiologist Walter Bradford Cannon in the book "Bodily changes in pain, hunger, fear and rage". The fight or flight response is one of the three most basic stress responses, along with freeze and fawn. The goal of the fight, flight, freeze, and fawn response is to decrease, end, or evade danger and return to a calm, relaxed state.