Stress is the body's response to any physical, mental or emotional changes in a person's life. This response includes the release of a hormone - Adrenaline, in the body. It causes an increase in the heart rate, breathing, blood pressure and blood sugar levels. It also diverts the blood flow from your digestive system to your muscles. This is called the Flight or Fight response. All stressors are not equal. There is no single level of stress that is optimal for all people.