Your traffic light just turned green, and you’re about to pull out into an intersection when you notice another car is barreling through and will hit you. Quick, hit the brakes! The car passes in front of you; the crash is averted. Your heart is racing. You’re holding your breath. Exhale. Your hands are gripping the steering wheel tightly, and your entire body has tensed in anticipation of the collision. Relax, you’re safe.
In that split-second, near-miss accident, several systems in your body kicked into gear. One of those is a part of your brain called the hypothalamic-pituitary-adrenal (HPA) system, which released a cascade of chemicals—such as adrenaline, steroid hormones, and cortisol—that kicked up your heart rate, helped your brain with that split-second decision, and increased glucose in the bloodstream to give you a burst of energy to react. You didn’t even have to tell it what to do. This is your body’s natural reaction to stress.