Adrenaline
The release of adrenaline causes rapid changes to your blood flow and increases your breathing and heart rate, to get you ready to defend yourself (fight) or to run away (flight). You become pale, sweat more and your mouth becomes dry.
Your body responds in this way to all types of stress as if it were a physical threat. You may merely be having an argument with someone, but your body may react as though you were facing a wolf. If the threat is physical, you use the effects of the adrenaline appropriately – to fight or to run, and when the danger is passed your body recovers. But if the stress is emotional, the effects of adrenaline subside more slowly, and you may go on feeling agitated for a long time. If the causes of stress are long-term, you may always be tensed up to deal with them and never relaxed. This is very bad for both your physical and your mental health.