General Principles of Physiology Practice Test – Study Guide & Prep Resources

The hormone primarily responsible for the 'fight or flight' response is adrenaline, also known as epinephrine. This hormone is released from the adrenal medulla in response to stress and is a key player in preparing the body to react to perceived threats. When adrenaline is released into the bloodstream, it triggers a series of physiological changes: it increases heart rate, elevates blood pressure, and enhances the energy supply by increasing blood glucose levels. These changes facilitate a rapid response to danger, enabling the body to either confront the threat or flee from it.

In comparison, cortisol, while also involved in stress responses, is a steroid hormone that has more prolonged effects on metabolism and immune function rather than the immediate fight or flight reaction. Insulin plays a critical role in glucose metabolism and does not have a direct role in acute stress responses. Thyroxine, a thyroid hormone, regulates metabolism but is not specifically tied to immediate stress responses like adrenaline is. Therefore, adrenaline's rapid action and effects on the cardiovascular and metabolic systems make it the dominant hormone during the fight or flight response.