Odds are you either love or hate the jittery, stomach-drop feeling of a good scare. And whether you’re one to fight or flee, your body is ready to give you the energy you’ll need.

Vishal Gohil, Ph.D., professor in the Texas A&M College of Agriculture and Life Sciences Department of Biochemistry and Biophysics, said fear triggers the adrenal gland to release adrenaline, a hormone that targets specific organs to help you react quickly.

“When you’re in a scary situation — say it’s Halloween and you just saw a ghost — it incites fear and you might want to run away,” Gohil said. “To do that, your muscles suddenly need extra energy, and your heart must beat faster. That burst of energy you need comes from the effect of adrenaline on the body.”

Adrenaline is sometimes called the “fight or flight” hormone, but it’s also responsible for the remarkable feats of strength that have been reported in moments of extreme fear or danger, like giving a person the ability to lift a car enough to save someone, as it provides a temporary surge of energy often followed by a period of fatigue.

Adrenaline does all this by initiating a cascade of reactions in the body to switch our energy production into overdrive and directing energy to where it’s needed most, and it all begins in a matter of seconds.

The body’s sugar vault

You can feel the effects of adrenaline in tense muscles and an increased heart rate, but you might not know that it also targets the liver.

The liver stores a form of the sugar glucose, a key energy source for our body’s cells. It’s stored in the liver in long chains called glycogen specifically so that the body has a source of quick energy available when needed.

Under normal circumstances, Gohil said the body uses both carbohydrates and fats for most energy needs, but during moments of stress, the fast energy from glucose is more useful.

After units of glucose are broken off the glycogen chains, Gohil said they’re released into the bloodstream for cells throughout the body to absorb, spiking blood sugar levels. At the same time, adrenaline signals the heart and muscles to ramp up a process called glycolysis, the breakdown of glucose within our cells, to extract the energy from this simple sugar.

Fast energy over efficiency

Typically, most of the body’s energy needs are supplied by the mitochondria, which is why it’s earned the nickname as the powerhouse of the cell. But the processes mitochondria use are too slow for the immediate energy demands of a fight-or-flight situation.

So, instead of relying on the mitochondria’s efficiency, adrenaline directs cells to rely on glycolysis. Cells generate a molecule called adenosine triphosphate, or ATP, quicker this way, though at a lower yield per glucose molecule.

“ATP is the body’s energy currency,” Gohil said. “Mitochondrial ATP production is very efficient at extracting energy, but it’s a slower process. During flight or fight, when you need a rapid burst of energy, it’s glycolysis — the faster biochemical pathway — that is critical.”

By increasing the reliance on glycolysis, Gohil said energy production goes from a slow drip to a downpour.

Triaging energy needs

Adrenaline also ensures energy is used where it’s most needed. Normal maintenance processes in the body are slowed to direct ATP toward muscles and heart function.

“The body wants to divert all your energy to your muscles and heart,” Gohil said. “You don’t want to use ATP for storing glucose, because that consumes energy that could be used for running or fighting.”

In moments of fear, this fine-tuned system ensures that available energy is directed appropriately.

So, the next time you visit a haunted house, take a moment between those pounding heartbeats to appreciate the biochemical processes keeping you ready to either face or flee those zombies and werewolves. Or maybe just wait for the inevitable crash that will come a few minutes later.