When you face a perceived threat, a tiny region at the brain's base, called the hypothalamus, sets off an alarm system in the body. An example of a perceived threat is a large dog barking at you during your morning walk. Through nerve and hormonal signals, this system prompts the adrenal glands, found atop the kidneys, to release a surge of hormones, such as adrenaline and cortisol.
Adrenaline makes the heart beat faster, causes blood pressure to go up and gives you more energy. Cortisol, the primary stress hormone, increases sugar, also called glucose, in the bloodstream, enhances the brain's use of glucose and increases the availability of substances in the body that repair tissues.
Cortisol also slows functions that would be nonessential or harmful in a fight-or-flight situation. It changes immune system responses and suppresses the digestive system, the reproductive system and growth processes. This complex natural alarm system also communicates with the brain regions that control mood, motivation and fear.