During a 'fight or flight' response, adrenaline redirects blood from your digestive system to your muscles, preparing your body for immediate action.

The 'fight or flight' response was first formally described by Harvard physiologist Walter Cannon in 1915. When the brain perceives a threat, the amygdala signals the hypothalamus, which triggers the adrenal glands to release epinephrine (adrenaline) into the bloodstream. This hormone binds to alpha-adrenergic receptors on blood vessel walls throughout the gastrointestinal tract, causing vasoconstriction—a narrowing that dramatically reduces blood flow to the stomach and intestines. Research shows that during intense stress, digestive blood flow can drop by as much as 80 percent.The redirected blood flows to skeletal muscles and the lungs, maximizing physical performance and oxygen delivery. The sudden reduction in blood and oxygen supply to the stomach creates the characteristic sensation of "butterflies" or sharp cramping. At the same time, the body suppresses digestive enzyme production and slows peristalsis—the rhythmic muscle contractions that move food through the digestive tract. This metabolic shift is an evolutionary adaptation that conserves energy for immediate physical action.A 2014 study published in Autonomic Neuroscience demonstrated that this autonomic nervous system response occurs within seconds of a perceived threat. Once the danger passes, the parasympathetic nervous system activates to restore normal digestive function.