What Stress Is and How the Body Responds¶
Stress is a state of disrupted homeostasis — the body's attempt to maintain a stable internal environment thrown off balance 1. When the brain perceives a threat, it triggers the sympathetic nervous system, setting off a cascade of physical and hormonal changes that prepare the body for immediate action 2. Heart rate rises. Blood pressure elevates. Respiration quickens 2. Blood flow shifts toward essential organs, while non-critical functions — digestion, growth, reproductive hormone maintenance — are downregulated 1.
This mobilization, the fight-or-flight response, is fundamentally protective. When confronting genuine danger, these physiological changes promote survival by directing available resources toward the immediate demand 1. The response involves a release of epinephrine and norepinephrine from the sympathetic nervous system, alongside cortisol secreted from the adrenal glands 3.
Short-term stressors are generally beneficial for immediate survival 4. The trouble begins when the perceived threat comes not from a single, resolvable danger but from constant daily pressures. In that context, the same protective response can start to damage the very systems it was built to defend 1.
The HPA Axis and the Physiology of the Stress Response¶
The stress response runs through several interconnected biological pathways. When a person experiences stress, physiological responses are activated through a combination of molecular regulation, hormone secretion, and the activation of the hypothalamic-pituitary-adrenal (HPA) axis 5. These responses raise blood pressure and heart rate, and they also shift how a person perceives and reacts to the stressful situation itself 5.
The HPA axis works by sending a signal from the hypothalamus to the pituitary gland, which then prompts the adrenal gland to release stress hormones 3. The effects of this hormonal cascade can take hours to weeks to fully materialize from the initial stress response 3. Response strength depends on the intensity and duration of the stressor, yet the effects can persist far longer than the initial exposure or interpretation of that stressor 3.
Hans Selye organized the body's response to stress into a three-stage model called the General Adaptation Syndrome (GAS) 6. In the alarm stage, the body's initial reaction involves a temporary drop in physiological resistance, activation of the sympathetic nervous system, and release of stress hormones like adrenaline and cortisol — which simultaneously suppress the immune system, increasing vulnerability to illness 6. In the resistance stage, the body adapts and builds defenses against the ongoing stressor 6. If stress remains unresolved, the body enters the exhaustion stage: energy reserves are depleted, physical and mental fatigue set in, and serious health consequences including cardiovascular disease and immune dysfunction become possible 6.
Selye's original framework has since been refined. The HPA axis and the noradrenergic and adrenergic nerves show different patterns of response depending on the type of stressor, rather than a single stereotyped reaction 7. There are also sex differences worth noting: the fight-or-flight characterization most accurately describes the response of male animals under threat, while the female response to non-life-threatening stress has been characterized as "tend-and-befriend" 7.
Cortisol plays a particularly central role. It helps break down stored fuel and inhibit unnecessary systems during the stress response 1. Cortisol typically spikes in the latter parts of the stress response and remains elevated during recovery, increasing hunger and motivation to eat as a way of replenishing calories that may have been burned while responding to a stressor 1. Another hormone, dehydroepiandrosterone (DHEA), also rises during a stress response and promotes brain growth and recovery, affecting emotions, immune reactions, mood, and behavior 1.
Acute vs. Chronic Stress and Allostatic Load¶
The distinction between acute and chronic stress is not merely a matter of duration. It reflects fundamentally different physiological consequences.
Short-term stressors are generally beneficial for immediate survival 4. Chronic exposure tells a different story. Chronic stress can lead to cardiovascular disease, gastrointestinal disorders, and weakened immune function 2. Under chronic stress, many regulatory mechanisms in the body lose their ability to maintain physiological homeostasis 3. Hormones like vasopressin, aldosterone, epinephrine, and norepinephrine — whose purpose is to increase blood pressure so active muscles receive oxygenated blood — become harmful in a resting state, contributing to hypertension and ultimately coronary heart disease 3.
To describe this cumulative wear and tear, researchers developed the concepts of allostasis and allostatic load. Allostasis refers to the adaptive processes that maintain homeostasis through the production of mediators such as adrenalin, cortisol, and other chemical messengers 7. These mediators promote adaptation in the aftermath of acute stress, but they also contribute to allostatic overload — the wear and tear on the body and brain that results from being chronically stressed 7. Allostatic load leads to impaired immunity, atherosclerosis, obesity, bone demineralization, and atrophy of nerve cells in the brain 8.
The damage from chronic stress does not come simply from an exhaustion of the body's defenses. The stress mediators themselves turn on the body and cause damage when they are mismanaged or overused 7. Recognizing this — that it is the overactivity of the same systems that protect us, not merely their depletion, that drives harm — is central to understanding why chronic stress is so destructive 7.
The brain is not exempt. The hippocampal formation, which expresses high levels of adrenal steroid receptors and plays an important role in learning and memory, is vulnerable to the effects of stress and trauma. In animal models, neurons in the hippocampus and prefrontal cortex show atrophy under repeated stress, while neurons in the amygdala show a growth response 8.
Eustress, Recovery, and Stress Inoculation¶
Not all stress is harmful. Short-term stressors are generally beneficial for immediate survival 4, and this productive dimension of stress is sometimes called eustress: stress that motivates and enhances performance rather than depleting the organism.
Mindset shapes the physiological stress response in measurable ways. In one experiment involving a mock interview, researchers measured cortisol and DHEA in participants randomly assigned to watch either a "stress is enhancing" or a "stress is debilitating" video beforehand. Those who watched the "stress is enhancing" video had a dramatically greater rise in DHEA compared to the other group — a shift linked to improved health outcomes in response to stress 1. How a person interprets a stressor can alter the hormonal profile of their response, not just their subjective experience of it.
Recovery matters just as much as the response itself. Continuous activation of the stress response without recovery can cause irreversible damage to vital organs and systems 6. The goal, then, is not to eliminate stress but to improve the efficiency of the adaptive response while minimizing the overactivity of those same systems — since that overactivity underlies many of the common diseases of modern life 7.
Stress management techniques including relaxation strategies, exercise, and social support can help mitigate physiological effects and promote overall well-being 2. Individual differences in stress responses, influenced by genetic and environmental factors, mean that no single approach works for everyone 2. Future research into the role of endogenous opiates in the stress response may open new strategies for enhancing individual coping, as may further investigation of how stressors influence immune function 2.