Stress is a natural reaction to excess demand, pressure or excitement. Hormonal and chemical defence mechanisms are triggered in your body. This is often called the 'fight or flight' reaction. It evolved to help us deal with dangerous or life-threatening situations.
You may begin to sweat. The blood vessels to your skin constrict, muscle blood vessels swell, and the stress hormones adrenaline and cortisol are released. Adrenaline accelerates your heart rate, cortisol raises your blood sugar levels.
With these changes, your body adapts itself to cope with short-term stress. It can even be enjoyable to deliberately trigger stress reactions in situations of your own choosing, e.g. playing sports, on theme-park rides, or taking part in the performing arts.