An adrenaline rush triggers your liver to release stored glucose into your bloodstream, providing instant energy for your muscles.

The physiological process of releasing energy during stress is called glycogenolysis. When the amygdala detects a threat, it signals the hypothalamus, which triggers the adrenal glands to release epinephrine (adrenaline) into the bloodstream. This hormone travels to the liver and binds to specific adrenergic receptors on liver cell surfaces.Once adrenaline attaches to these receptors, it activates an enzyme called glycogen phosphorylase, which rapidly breaks down glycogen—a large glucose polymer stored in the liver and skeletal muscles. The liver typically stores 100 to 120 grams of glycogen, representing about 8% of its total weight. This stored energy is converted into glucose-1-phosphate and then into free glucose.The glucose is immediately released into the bloodstream, raising blood sugar levels within seconds. This mechanism ensures that the brain and muscles have a continuous supply of ATP—the primary energy currency of cells—to sustain intense physical exertion. Research published in Nature Education highlights that this process is critical for survival in high-stress environments.This response was first described in detail by Walter Cannon in 1915 in his book Bodily Changes in Pain, Hunger, Fear and Rage. Cannon's work laid the foundation for understanding how the sympathetic nervous system coordinates with the endocrine system. Modern sports science now uses this knowledge to explain how athletes access "second winds" during high-intensity competition.