When you encounter a threat, whether physical danger or a looming deadline, your body launches a coordinated response across nearly every organ system within seconds. Your brain triggers a hormonal cascade that raises your heart rate, floods your bloodstream with glucose, sharpens your focus, and temporarily shuts down functions like digestion that aren’t needed for immediate survival. This response evolved to keep you alive in emergencies. Problems arise when it stays switched on.
The Hormonal Cascade Behind Stress
The stress response runs through a communication chain linking three structures: the hypothalamus (a small region at the base of your brain), the pituitary gland (just below it), and the adrenal glands (which sit on top of your kidneys). This chain fires in sequence like a relay.
First, the hypothalamus releases a signaling hormone called CRH into a network of blood vessels connecting it to the pituitary gland. CRH tells the pituitary to produce ACTH, which travels through your bloodstream to the adrenal glands. The adrenal glands then produce cortisol, the hormone most people associate with stress. Cortisol is the final product, and it’s responsible for many of the body-wide changes you feel during a stressful event. Under normal conditions, cortisol levels peak in the early morning (roughly 10 to 20 mcg/dL around 6 to 8 a.m.) and drop by more than half in the evening (3 to 10 mcg/dL around 4 p.m.). Stress overrides that rhythm and pushes levels higher regardless of the time of day.
Cortisol also regulates itself through a feedback loop. When levels get high enough, the hypothalamus and pituitary detect it and dial back their signaling. This is supposed to act like a thermostat, turning off the response once the threat passes. Chronic stress can blunt that feedback mechanism, leaving cortisol elevated for longer than it should be.
What Happens in the First Few Seconds
Before the hormonal relay even finishes, your sympathetic nervous system fires directly. This is the “fight or flight” branch of your nervous system, and it works through nerve signals rather than hormones, so the effects are almost instant. Your heart rate climbs, your heart contracts more forcefully, and blood flow redirects away from your skin and digestive organs toward your large muscles. Blood pressure rises. Your airways widen so you can take in more oxygen, though paradoxically this can also trigger rapid, shallow breathing that feels like shortness of breath.
Your liver begins breaking down stored glycogen into glucose and producing new glucose through a process that converts amino acids and other molecules into usable fuel. Blood sugar spikes to supply your muscles and brain with energy. Blood clotting factors increase, preparing you for potential injury. Even your pupils dilate to let in more light. All of this happens in the span of a few heartbeats.
A Newly Discovered Fast Track for Blood Sugar
Scientists recently identified a surprising shortcut in this process. A 2025 study published in Nature found that a region of the brain’s amygdala (the area that processes fear and threat) can signal the liver to produce glucose through a direct nerve pathway, bypassing the slower hormonal system entirely. In experiments, activating this circuit raised blood sugar levels rapidly without any involvement of cortisol or insulin. This amygdala-to-liver connection helps explain why blood sugar can spike almost immediately during acute fear or panic, faster than hormones alone could account for.
How Stress Affects Digestion
Your gut is one of the first casualties of the stress response. When your sympathetic nervous system activates, it suppresses the movement of food through your stomach, a process called gastric emptying. In practical terms, this means your stomach holds onto its contents longer than usual, which is why stress often produces nausea, bloating, or that heavy “pit in your stomach” feeling. At the same time, stress tends to speed up activity in the colon. This combination of a sluggish stomach and an overactive colon is the most common digestive pattern during stress, and it explains why anxiety can cause both loss of appetite and urgent trips to the bathroom.
Anger, fear, pain, and even intense exercise all slow gastric emptying in healthy people. If you’ve ever noticed you can’t eat before a big presentation or felt your stomach churn during an argument, you were experiencing this shift in real time.
What Chronic Stress Does to Your Immune System
In the short term, stress actually primes certain immune defenses. But when cortisol stays elevated for weeks or months, it begins suppressing immune function in meaningful ways. Cortisol enters immune cells passively and binds to receptors inside them, triggering a chain of events that dials down inflammation. This sounds beneficial, and in small doses it is. But sustained suppression reduces your body’s ability to fight infections and heal wounds.
One specific mechanism involves T cells, a critical part of your adaptive immune system. Cortisol shifts T cells away from inflammatory “attack mode” and toward a less aggressive anti-inflammatory state. It also interferes with the energy supply T cells need to function by reducing their ability to use glucose. The result is an immune system that responds more slowly and less effectively. This is why people under prolonged stress tend to catch colds more easily and recover from injuries more slowly.
How Chronic Stress Reshapes the Brain
Sustained stress doesn’t just change how the brain functions. It changes its physical structure. The hippocampus, a region essential for memory and learning, is particularly vulnerable to prolonged cortisol exposure. Studies in both animals and humans show that chronic stress or long-term glucocorticoid exposure leads to volume loss in the hippocampus. The neurons in one section of the hippocampus (called CA3) show dendritic atrophy, meaning the branching extensions that neurons use to communicate with each other physically shrink. This has been linked to depressive behavior in animal models.
The amygdala tells a different story. While the hippocampus shrinks, the amygdala actually grows with chronic stress. Its neurons develop more connections, which may heighten fear and emotional reactivity. In other words, the part of your brain responsible for threat detection gets stronger, while the part responsible for calm reasoning and memory gets weaker. This creates a feedback loop where chronic stress makes you more sensitive to future stress.
Cardiovascular Damage Over Time
The cardiovascular system is built to handle short bursts of stress. Temporary spikes in heart rate and blood pressure are normal and harmless. The damage comes from repetition. When the sympathetic nervous system stays chronically activated, levels of epinephrine and norepinephrine remain elevated. These hormones cause small arteries and veins to contract persistently, raising both systolic and diastolic blood pressure. Over time, this contributes to hypertension.
Chronic stress also injures the inner lining of blood vessels, called the endothelium. Healthy endothelial cells produce nitric oxide, which keeps vessels relaxed and flexible. Stress reduces nitric oxide production, making vessels stiffer and more prone to damage. Once the endothelium is compromised, immune cells called macrophages move in and absorb fats from the bloodstream, forming foam cells. These foam cells accumulate and develop into atherosclerotic plaque, the fatty buildup that narrows arteries and raises the risk of heart attack and stroke.
Cortisol and catecholamines also disrupt lipid metabolism, tilting the balance of fats in your blood in unfavorable directions. Combined with the direct vascular damage and elevated blood pressure, this creates a triple threat for long-term cardiovascular health. Research has established that chronic stress is a critical independent risk factor for atherosclerosis, separate from diet, exercise, or genetics.
Why the Response Becomes Harmful
The stress response is not inherently destructive. It evolved as a survival tool, and for brief, intense threats it works exactly as designed. The mismatch is between the biology and modern life. Your body can’t distinguish between a predator and a difficult boss. Financial anxiety, relationship conflict, and work pressure activate the same hormonal and nervous system pathways that prepare you to run from physical danger. When those pathways fire daily for months or years, the protective response becomes the source of damage: weakened immunity, arterial plaque, elevated blood sugar, a shrinking hippocampus, and a hyperactive fear center.
The body does have a built-in off switch. The parasympathetic nervous system, sometimes called “rest and digest,” counterbalances the sympathetic system once a threat passes. Chronic stress creates an imbalance where the sympathetic side dominates and the parasympathetic side can’t fully restore equilibrium. This autonomic imbalance is a consistent finding in people experiencing long-term psychological stress and underlies many of the physical health consequences that follow.