This physical reaction is related to powerful feelings such as fear, anger and frustration. Adrenaline causes the heart to beat faster and increases respiration rate. Another hormone is released that causes the liver to release sugar for extra energy. The mouth becomes dry, pupils of the eyes dilate, and the lungs take in more air. Blood moves from the stomach into the skeletal muscles.
Hearing becomes more acute and muscles contract.
This fight or flight response provides animals and people the extra energy and strength they may need to protect themselves in times of crisis. It allows the body to perform at maximum capacity for a short period of time. After the crisis has passed, the body systems return to normal, and the body is able to relax. However, sometimes people have problems or concerns that keep bothering them, and their bodies stay in a constant state of alert. We say that these people are feeling stress.