Stress generally refers to two things: the psychological perception of pressure, on the one hand, and the body's response to it, on the other, which involves multiple systems, from metabolism to muscles to memory. Through hormonal signaling, the perception of danger sets off an automatic response system, known as the fight-or-flight response, that prepares all animals to meet a challenge or flee from it. A stressful event—whether an external phenomenon like the sudden appearance of a snake on your path or an internal event like fear of losing your job when the boss yells at you—triggers a cascade of hormones, including adrenaline and cortisol, that surge through the body, speeding your heartbeat and the circulation of blood, mobilizing fat and sugar for fast energy, focusing attention, preparing muscles for action, and more. It generally takes some time for the body to calm down after the stress response has been triggered.
Lifesaving as the stress response is, it was meant to solve short-term, life-threatening problems, not extended difficulties such as daily traffic jams or marital problems. Prolonged or repeated arousal of the stress response, a characteristic of modern life, can have harmful physical and psychological effects, including heart disease and depression.