The Physiology of Stress and Stress-Related Illness

Stress is the body's physiological response to a demand or threat that is perceived to exceed a person's ability to cope. When you meet a stressor, your body does not sit still: a chain of nervous and hormonal signals prepares you to act.

The physiology of stress splits into two questions, and AQA assesses both:

• How does the body respond over time? This is described by Hans Selye's general adaptation syndrome (GAS).
• Which biological pathways carry the response? There are two: a fast one for short bursts (the sympathomedullary pathway) and a slow one for ongoing pressure (the hypothalamic-pituitary-adrenal system).

Feature	Acute (short-term) stress	Chronic (ongoing) stress
Pathway	Sympathomedullary (SAM)	Hypothalamic-pituitary-adrenal (HPA)
Speed	Fast, seconds	Slower, sustained
Key hormone	Adrenaline, noradrenaline	Cortisol
Gland	Adrenal medulla	Adrenal cortex

Stress is not simply "feeling worried". For this topic it is a measurable cascade of hormones and nervous activity — get the biology right and the marks follow.

Hans Selye noticed that animals exposed to very different stressors (cold, injury, toxins) showed the same underlying bodily changes. He concluded that the body has a general, non-specific response to any prolonged stressor, which he called the general adaptation syndrome. It unfolds in three stages.

Stage	What happens in the body
1. Alarm	A stressor triggers sympathetic arousal (fight-or-flight); adrenaline is released.
2. Resistance	The body adapts and appears to cope, but is draining resources; cortisol stays elevated.
3. Exhaustion	Resources are depleted, the stress response can no longer be maintained, and stress-related illnesses appear.

The word non-specific is the important one: the same three-stage pattern appears no matter what the stressor is. During resistance a person may look and feel fine, which is deceptive, because their reserves are quietly being used up.

"Non-specific" means the physiological response is broadly the same regardless of the type of stressor. This is precisely the claim later researchers challenged (see the Evaluation slide).

For a sudden threat you need an instant response, and the sympathomedullary pathway (SAM) delivers it. It runs through the nervous system, so it is fast-acting.

The sequence is: the hypothalamus activates the sympathetic branch of the autonomic nervous system (ANS), which stimulates the adrenal medulla (the inner part of the adrenal gland). The adrenal medulla releases adrenaline and noradrenaline into the bloodstream, producing rapid fight-or-flight changes.

These hormones raise heart rate, redirect blood to the muscles, dilate the pupils and release stored glucose. Because the signal travels along nerves, the whole response fires within seconds. Once the threat passes, the parasympathetic branch of the ANS returns the body to rest.

Adrenaline (adrenal medulla) is the signature of the fast acute pathway. If a question stresses speed and fight-or-flight, this is the pathway to describe.

When a stressor persists for hours, days or weeks, a slower hormonal system takes over: the hypothalamic-pituitary-adrenal (HPA) system. It relies on hormones travelling through the blood rather than fast nerve impulses, so it is slower but sustained.

The sequence runs through three structures, each releasing a hormone that triggers the next:

The hypothalamus releases CRH (corticotropin-releasing hormone), which tells the pituitary gland to release ACTH (adrenocorticotropic hormone). ACTH travels to the adrenal cortex (the outer part of the adrenal gland), which releases cortisol. Cortisol mobilises energy by raising blood glucose, keeping the body fuelled for a prolonged demand. But when cortisol stays high for a long time, it begins to cause harm, and that is where illness comes in.

Cortisol (adrenal cortex) is the signature of the slow, chronic HPA system. The name gives you the order: Hypothalamus, Pituitary, Adrenal.

The reason stress physiology matters clinically is that chronic activation of these systems damages health. AQA requires two named consequences: immunosuppression and cardiovascular disorders.

Immunosuppression. Chronically high cortisol reduces the activity of the immune system, a process called immunosuppression. With fewer active immune cells the body is more vulnerable to infection and heals more slowly.

• Kiecolt-Glaser et al. (1984) measured medical students' immune function before and during exams. Natural killer (NK) cell activity was lower during the exam period, when stress was highest.
• Kiecolt-Glaser et al. (1995) found that stressed participants (carers of relatives with dementia) took longer to heal from a small standardised wound than matched controls.

Cardiovascular disorders. Chronic stress repeatedly raises heart rate and blood pressure. Over time this damages blood vessels and increases the risk of hypertension (high blood pressure) and coronary heart disease (CHD). The link can be:

Route	How stress harms the heart
Direct	Physiological wear: raised blood pressure damages vessel walls over time.
Indirect	Unhealthy coping: stressed people may smoke more, drink more or eat poorly.

Note the direction of the cortisol effect. In the short term stress can prime the body, but chronic cortisol suppresses immunity. Writing that stress "boosts the immune system" loses marks.

A strong answer weighs the physiological account rather than just describing it. Develop three or four points fully.

GAS is pioneering but based on animals. Selye's research used rats, so generalising to human psychological stress is uncertain, because humans appraise stressors cognitively in a way rats do not. His claim that the response is non-specific has also been challenged: Mason (1975) found that different stressors produced different patterns of hormone release, suggesting the response is more specific than Selye proposed.

The stress-illness link has strong supporting evidence, but limitations. Kiecolt-Glaser's controlled studies give solid support for immunosuppression. However, much of the wider evidence is correlational, so other factors (diet, exercise, genetics) may explain the illness rather than stress itself. There are also individual differences: the same stressor affects people's health very differently, which a purely physiological model struggles to explain.

Immunosuppression is not the whole story. Brief, acute stress can actually enhance immune function, preparing the body to deal with injury. The relationship is therefore more complex than "stress equals suppression" and depends on whether the stress is short-lived or chronic.

The physiological account is useful but reductionist: it downplays the psychological appraisal that decides whether an event is experienced as stressful in the first place. Two people meeting the same event may mount very different stress responses.

1. Confusing the SAM pathway with the HPA system

The sympathomedullary pathway is fast, uses the sympathetic nervous system, and releases adrenaline from the adrenal medulla. The HPA system is slow, works through CRH and ACTH, and releases cortisol from the adrenal cortex. Swapping medulla and cortex, or adrenaline and cortisol, is the most common error here.

2. Getting the three GAS stages out of order

The order is always alarm → resistance → exhaustion. Illness appears in the exhaustion stage, once resources are depleted, not during alarm or resistance.

3. Saying cortisol boosts immunity

Chronically high cortisol suppresses the immune system. It is short-term acute stress that may briefly enhance immune function; sustained cortisol does the opposite.

4. Treating the stress-illness correlation as proven cause

Much of the evidence linking stress to illness is correlational. Say that stress is associated with raised illness risk, and note that lifestyle and other factors may contribute, rather than claiming stress definitively causes the illness.

5. Naming the wrong hormone chain in the HPA system

The order is hypothalamus → CRH → pituitary → ACTH → adrenal cortex → cortisol. Students often muddle CRH and ACTH or attach cortisol to the pituitary. Use the name as a memory aid: Hypothalamus, Pituitary, Adrenal.