Alarm strikes your body within seconds of recognizing a threat. Here’s what’s going down.

1. Amygdala

Before you consciously process a problem, this almond-shaped brain region activates and begins to amp up the sympathetic nervous system’s fight-or-flight response.

2. Hippocampus and frontal cortex

The rational centers of our brain kick in, analyzing whether the perceived visual or auditory input shows a true danger.

3. Cardiovascular system

Stress hormones increase your heartbeat and breathing rate while dilating the tiny airways in your lungs. This allows more oxygen to reach your muscles.

4. Endocrine glands

Signals from the amygdala hit the hypothalamus, which starts a cascade of activity throughout the endocrine system. The result: a surge of adrenaline and cortisol.

5. Gastrointestinal system

Fear really can make you poop your pants. That and retching may be side effects of shrinking blood vessels in the GI tract—diverting resources to give you strength.

6. Musculoskeletal system

Endocrine signals push glucose and other energy-storing molecules out of reserve and into the blood, which rushes in to fuel muscles should you need to escape or fend off a threat.

This article was originally published in the Winter 2018 Danger issue of Popular Science.