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Stress is one of the most heavily researched topics in psychology because it sits at the intersection of mind and body: a purely psychological event — a deadline, an argument, an exam — sets in motion a cascade of measurable physiological changes that affect almost every organ system. To understand stress at A-Level you must be able to describe, in mechanistic detail, how the perception of a threat is translated into a bodily response. This lesson lays the physiological foundation for the entire Stress option. It explains the two great pathways of the stress response — the fast-acting sympathomedullary (SAM) pathway and the slower hypothalamic-pituitary-adrenal (HPA) axis — and shows how Hans Selye's General Adaptation Syndrome ties them together into an account of why prolonged stress damages health.
Key Definition: Stress is the physiological and psychological state that arises when an individual perceives that the demands placed on them exceed their ability to cope. The stress response is the coordinated set of bodily changes — mediated chiefly by the autonomic nervous system and the endocrine system — that prepare the organism to deal with a perceived threat.
The crucial idea to grasp from the outset is that the stress response is fundamentally a survival mechanism. The same physiology that prepares a gazelle to flee a lion prepares a student to sit an exam — which is both the elegance and the problem of the system, because a response evolved for brief physical emergencies is repeatedly triggered by the chronic, psychological stressors of modern life.
This lesson addresses the following points in AQA A-Level Psychology (7182), Paper 3, Section C (Stress):
Assessment objectives engaged: AO1 (knowledge of the SAM pathway, the HPA axis, the role of cortisol, and Selye's General Adaptation Syndrome) and AO3 (evaluation of the physiological account of stress — the GAS model, gender bias in the fight-or-flight literature, the dual role of cortisol, and the methodological basis of the evidence). This topic also draws directly on Paper 2 Biopsychology (the nervous and endocrine systems, the fight-or-flight response), so it is strongly synoptic.
When a threat is sudden and short-lived — a near-miss in traffic, a loud bang, an unexpected confrontation — the body responds within seconds through the sympathomedullary pathway, often abbreviated to the SAM pathway. This is the physiological basis of the fight-or-flight response first described by Walter Cannon (1932).
The sequence begins in the brain. A potential threat is detected by the sensory systems and appraised; the amygdala flags it as threatening and alerts the hypothalamus, which acts as the command centre linking the nervous system to the body's physiology. The hypothalamus activates the sympathetic branch of the autonomic nervous system (ANS). Sympathetic neurons travel down to the adrenal medulla — the inner core of the two adrenal glands that sit on top of the kidneys — and stimulate it to release the hormones adrenaline (epinephrine) and noradrenaline (norepinephrine) directly into the bloodstream.
These hormones circulate rapidly and act on multiple target organs to produce the characteristic fight-or-flight changes:
| Physiological change | Adaptive purpose |
|---|---|
| Increased heart rate and blood pressure | Pumps more oxygenated blood to skeletal muscles for action |
| Increased breathing rate | Raises oxygen intake and expels carbon dioxide faster |
| Pupil dilation | Improves visual acuity and peripheral vision to detect threat |
| Glycogenolysis in the liver | Converts stored glycogen to glucose, raising blood sugar for energy |
| Blood diverted from digestive system to muscles | Prioritises energy where it is needed for fight or flight |
| Increased sweating | Cools the body in anticipation of physical exertion |
| Suppression of immune and reproductive functions | Diverts resources from non-urgent maintenance to immediate survival |
The defining feature of the SAM pathway is speed. Because it is driven by direct sympathetic neural stimulation of the adrenal medulla (a neural trigger producing a hormonal output), adrenaline can be detected in the blood within seconds of a threat being perceived. The pathway is summarised in the diagram below.
graph TD
A[Acute stressor perceived] --> B[Hypothalamus]
B --> C[Sympathetic branch of the ANS]
C --> D[Adrenal medulla]
D --> E[Adrenaline and noradrenaline released into bloodstream]
E --> F[Fight-or-flight response<br/>raised heart rate, breathing, glucose, alertness]
F --> G[Threat passes - parasympathetic rebound restores resting state]
Once the threat has passed, the parasympathetic branch of the ANS — the antagonistic "rest and digest" system — gradually restores the body to its baseline state, lowering heart rate, slowing breathing, and reactivating digestion. This return to homeostasis is sometimes called the parasympathetic rebound. The SAM pathway is therefore self-limiting: it fires hard, fast, and briefly, then switches off.
Key Definition: The sympathomedullary (SAM) pathway is the route by which acute stress produces the fight-or-flight response: the hypothalamus activates the sympathetic nervous system, which stimulates the adrenal medulla to secrete adrenaline and noradrenaline.
The SAM pathway is well suited to brief emergencies, but many stressors are not brief. A demanding job, a long illness, financial worry, or caring for a relative can persist for weeks, months, or years. For these chronic stressors a second, slower, longer-lasting system dominates: the hypothalamic-pituitary-adrenal axis, or HPA axis. Where the SAM pathway is a neural sprint, the HPA axis is an endocrine marathon.
The HPA axis works as a hormonal relay through three "stations":
This three-step endocrine cascade is shown below.
graph TD
A[Chronic stressor perceived] --> B[Hypothalamus]
B --> C[Releases CRH<br/>corticotrophin-releasing hormone]
C --> D[Anterior pituitary gland]
D --> E[Releases ACTH<br/>adrenocorticotrophic hormone]
E --> F[Adrenal cortex]
F --> G[Releases cortisol into bloodstream]
G --> H[Negative feedback to hypothalamus and pituitary<br/>reduces CRH and ACTH]
H --> B
A key regulatory feature of the HPA axis is its negative-feedback loop. When cortisol levels in the blood rise, this is detected by receptors in the hypothalamus and the pituitary, which respond by reducing their output of CRH and ACTH. Less ACTH means less cortisol, so cortisol is brought back down toward its set point. This is the same homeostatic principle of negative feedback that controls body temperature and blood glucose, studied in A-Level Biology. Under healthy conditions the loop keeps cortisol within a normal daily rhythm (higher in the morning, lower at night). Under chronic stress, however, the system can become dysregulated — cortisol stays persistently elevated, and over time the feedback receptors may become less sensitive, which is one route by which long-term stress damages the body.
Exam Tip: A common way to lose marks is to muddle the two pathways. Anchor them to two contrasts: (1) SAM = acute / fast / adrenaline / adrenal medulla; HPA = chronic / slow / cortisol / adrenal cortex; and (2) SAM is triggered neurally (sympathetic nerves) whereas the HPA axis is an endocrine relay (CRH → ACTH → cortisol). Getting the medulla/cortex distinction the right way round is a frequent discriminator.
Cortisol is the central hormone of the chronic stress response, and the specification names it explicitly, so it deserves close attention. In the short term, cortisol is genuinely helpful: it is a "double-edged" hormone whose benefits and costs depend entirely on duration.
The beneficial, short-to-medium-term actions of cortisol include:
The harmful, long-term consequences of chronically elevated cortisol are precisely what make chronic stress dangerous:
This dual nature of cortisol — protective in the short term, corrosive in the long term — is the single most important idea in the physiology of stress, and it is the mechanism that links the biological pathways to the health consequences (immune suppression, cardiovascular disease) studied in the rest of the option.
The endocrinologist Hans Selye carried out the foundational research that united acute and chronic stress into a single model. While studying the effects of injecting rats with various substances, Selye noticed that the rats developed the same pattern of physiological changes — enlarged adrenal glands, shrunken immune (lymphatic) tissue, and stomach ulcers — almost regardless of what he injected, and even when he exposed them to cold, heat, or physical restraint. He concluded that the body responds to any prolonged stressor with a non-specific physiological reaction, which he named the General Adaptation Syndrome (GAS).
The three stages are summarised below.
| Stage | What happens | Pathway involved |
|---|---|---|
| Alarm reaction | The threat is perceived; the SAM pathway fires, adrenaline is released, and the fight-or-flight response is mobilised. | SAM (acute) |
| Resistance | If the stressor persists, the body adapts and appears to cope; the HPA axis sustains the response and cortisol maintains energy supply. Outward functioning seems normal, but resources are being depleted. | HPA (chronic) |
| Exhaustion | Prolonged demand depletes the body's resources; the systems that maintained resistance can no longer cope. Cortisol remains high, the immune system is compromised, and stress-related illness (hypertension, ulcers, infection) becomes likely. | HPA exhaustion |
Selye's model is important precisely because it bridges the two pathways: the alarm stage corresponds to the acute SAM response, the resistance stage corresponds to the sustained HPA response, and the exhaustion stage explains why chronic stress is so damaging — the body simply cannot maintain the resistance response indefinitely.
Key Definition: The General Adaptation Syndrome (GAS) is Selye's (1936) model proposing that the body responds to any prolonged stressor through three stages — alarm, resistance, and exhaustion — with continued stress eventually depleting resources and producing illness.
A theme that runs through the whole Stress option is the contrast between acute and chronic stress, and the physiology explains exactly why the two have such different consequences. An acute stressor is intense but brief: the SAM pathway fires, adrenaline surges, the body acts, and the parasympathetic rebound rapidly restores balance. In evolutionary terms this is an efficient, self-correcting design — the gazelle that survives the lion's charge returns to grazing within minutes, its cortisol and adrenaline already falling. Acute stress is, in moderation, not only harmless but sometimes beneficial: a short burst of arousal sharpens attention and performance, and the brief immunosuppression of an acute stressor may even be followed by a temporary enhancement of certain immune defences.
A chronic stressor, by contrast, keeps the HPA axis activated for days, weeks, or months. Cortisol remains persistently elevated rather than returning to baseline, and it is this sustained exposure — not the stress hormones themselves — that produces harm. The table below summarises the contrast and explains why chronic stress is the form most strongly linked to illness in the rest of the option.
| Feature | Acute stress | Chronic stress |
|---|---|---|
| Duration | Seconds to minutes | Days, weeks or months |
| Dominant pathway | SAM (adrenaline) | HPA axis (cortisol) |
| Resolution | Rapid parasympathetic rebound | Little or no resolution; cortisol stays high |
| Effect on immunity | Brief suppression, sometimes followed by enhancement | Sustained immunosuppression |
| Health consequence | Usually harmless, can aid performance | Hypertension, cardiovascular disease, infection (Selye's exhaustion) |
This distinction also helps explain the phenomenon of individual differences in vulnerability to stress. People differ in the reactivity of their HPA axis — how strongly and for how long cortisol is released in response to a given stressor — and in the efficiency of the negative-feedback loop that switches it off. Those with a more reactive or poorly regulated stress system experience higher and more prolonged cortisol exposure to the same objective stressor, which may partly explain why some individuals develop stress-related illness while others, exposed to similar demands, do not. The physiology is therefore not a fixed, identical response in everyone; it is modulated by genetics, early experience, and the cognitive appraisal of the stressor — a point that recurs when individual differences in personality and hardiness are studied later in the option.
The physiology of stress is one of the most strongly synoptic topics on the entire A-Level specification, with direct links to Biology and to Paper 2 Biopsychology.
A major strength of the physiological account is that it rests on objective, replicable biological evidence, which gives it scientific credibility. The roles of adrenaline, the sympathetic nervous system, ACTH, and cortisol have been established using direct physiological measures — adrenaline and cortisol can be assayed in blood and saliva, and the activity of the HPA axis can be tracked hormonally. Because these are quantifiable, observable measures rather than subjective self-report, the account is grounded in falsifiable evidence. The implication is that the SAM and HPA pathways are not speculative constructs but mechanisms that can be tested and replicated across laboratories and species, lending the physiological explanation of stress strong internal validity and a firm scientific footing.
However, much of the foundational evidence — including Selye's work — derives from non-human animals, which raises questions about generalisability to humans. Selye's (1936) General Adaptation Syndrome was developed almost entirely from research on rats, and much early work on the HPA axis used animal models. Although the broad physiology of the stress response is highly conserved across mammals, there are species differences, and — critically — humans differ from rats in that we appraise stressors cognitively: a deadline is only stressful if it is interpreted as threatening. The implication is that a model built on animals risks neglecting the cognitive appraisal that determines whether the stress response is triggered at all in humans, so the purely physiological account is best combined with cognitive models such as Lazarus's transactional theory of stress, which emphasises appraisal.
The General Adaptation Syndrome has been criticised for being overly general and for over-emphasising the non-specific nature of the response. Selye argued that the body responds identically to any stressor, but later research (e.g., Mason, 1971) suggested that different stressors can produce different patterns of hormonal response, depending partly on the emotional reaction they provoke — fear, anger, and uncertainty may activate the system differently. The implication is that the GAS may oversimplify by treating all stress as equivalent; the type of stressor and the individual's emotional and cognitive reaction to it appear to matter, which the original "non-specific" model does not adequately capture. This refines rather than refutes Selye, but it shows the model is incomplete.
A further important limitation is that the early fight-or-flight and stress literature is gender-biased (a beta-bias), which limits its applicability to women. Taylor et al. (2000) argued that stress research was conducted predominantly on males, partly because female hormonal cycles were treated as a confounding variable. Taylor proposed that women are more likely to show a "tend-and-befriend" response — protecting offspring (tend) and forming protective social alliances (befriend) — mediated by oxytocin and oestrogen rather than by adrenaline alone. The implication is that a model presented as a universal human response may in fact describe a predominantly male pattern, meaning psychologists risk over-generalising from one sex to the whole population and overlooking adaptive female responses to stress.
The physiological model is also reductionist, which is both a strength and a limitation. By reducing stress to the activity of the SAM pathway, the HPA axis, and the hormone cortisol, the account achieves precision and testability — it is parsimonious and scientific. However, this reductionism risks neglecting the higher-level psychological and social factors that determine whether a situation becomes stressful in the first place: the same physiological hardware is triggered by an exam, a bereavement, or a job interview, and what differs is the meaning the person assigns to the event. The implication is that a complete account of stress requires an interactionist approach that combines the physiological mechanisms with cognitive appraisal and social context, rather than treating stress as nothing more than cortisol and adrenaline.
A practical strength of understanding the physiology of stress is its considerable real-world application, which adds to the model's value. Knowing that the parasympathetic branch antagonises sympathetic arousal, and that chronic HPA activation raises cortisol, directly informs stress-management techniques: relaxation, controlled breathing, and biofeedback recruit the parasympathetic "rest and digest" system, while drug therapies such as beta-blockers target the cardiovascular effects of adrenaline and benzodiazepines dampen arousal. The implication is that the physiological account is not merely descriptive but has produced evidence-based interventions that protect long-term health, demonstrating that this is a model with genuine applied utility in health psychology and clinical practice.
Exam Tip: When evaluating the physiology of stress, the strongest answers move beyond "it's supported by evidence." Develop the cortisol point both ways (adaptive short-term and damaging long-term), use the Taylor et al. (2000) tend-and-befriend critique with the correct term (beta-bias), and finish with the interactionist conclusion that physiology alone cannot explain why a situation is appraised as stressful.
Describe and evaluate the physiology of the stress response, including the SAM pathway and the HPA axis. (16 marks)
This is a standard 16-mark extended-response essay: 6 marks AO1 (description of the SAM pathway, the HPA axis, and the role of cortisol) and 10 marks AO3 (evaluation). There is no AO2 in this question because it contains no applied scenario or stem — AO2 marks are awarded only when a question asks you to apply your knowledge to a specific context, and this question does not.
The stress response has two pathways. The first is the SAM pathway, which deals with sudden stress. When a person sees a threat, the hypothalamus activates the sympathetic nervous system. This makes the adrenal medulla release adrenaline into the blood. Adrenaline increases heart rate and breathing and releases glucose, which gets the body ready to fight or run away. When the threat is over, the parasympathetic nervous system calms the body down.
The second pathway is the HPA axis, which deals with long-term stress. The hypothalamus releases CRH, which makes the pituitary gland release ACTH. ACTH then makes the adrenal cortex release cortisol. Cortisol gives the body energy but if there is too much of it for a long time it can suppress the immune system. Selye called the three stages of stress alarm, resistance and exhaustion.
One strength is that the explanation is supported by biological evidence because cortisol can be measured. One weakness is that a lot of the research, like Selye's, was done on rats, so it might not apply to humans. Another weakness is that Taylor said the response is based on men and that women show tend-and-befriend instead, so it could be biased.
The physiology of stress involves two pathways that operate over different timescales. The sympathomedullary (SAM) pathway responds to acute stressors. A threat is perceived and relayed to the hypothalamus, which activates the sympathetic branch of the autonomic nervous system. This stimulates the adrenal medulla to secrete adrenaline and noradrenaline into the bloodstream. These hormones increase heart rate and blood pressure, raise breathing rate, dilate the pupils, and trigger glycogenolysis in the liver to release glucose, preparing the body for fight or flight. Once the threat passes, the parasympathetic division restores the resting state.
The hypothalamic-pituitary-adrenal (HPA) axis handles chronic stress. The hypothalamus releases CRH, which stimulates the anterior pituitary to secrete ACTH; ACTH acts on the adrenal cortex to release cortisol. Cortisol maintains a steady supply of glucose and helps sustain blood pressure, but when chronically elevated it suppresses the immune system and contributes to cardiovascular problems. A negative-feedback loop normally keeps cortisol in check. Selye's (1936) General Adaptation Syndrome describes three stages — alarm (SAM), resistance (HPA), and exhaustion — explaining why prolonged stress leads to illness.
A strength is the strong, objective evidence base: adrenaline and cortisol can be measured directly, giving the account scientific credibility. However, much of the evidence comes from animals — Selye used rats — so generalisation to humans is uncertain, especially as humans appraise stressors cognitively. Furthermore, Taylor et al. (2000) argued the fight-or-flight model is gender-biased, proposing that women show a tend-and-befriend response mediated by oxytocin, which suggests a beta-bias in the traditional account.
The physiology of the stress response comprises two complementary pathways operating over different timescales, together with Selye's overarching model of how stress unfolds. The sympathomedullary (SAM) pathway governs the acute response. On perceiving a threat, sensory information reaches the hypothalamus, which activates the sympathetic branch of the autonomic nervous system; sympathetic neurons stimulate the adrenal medulla to secrete adrenaline and noradrenaline directly into the bloodstream. These hormones act on multiple target organs to produce the fight-or-flight response — raised heart rate and blood pressure to perfuse the muscles, increased breathing, pupil dilation, hepatic glycogenolysis to raise blood glucose, and the suppression of non-urgent functions such as digestion. Because the trigger is neural, the response is almost immediate, and the antagonistic parasympathetic division subsequently restores homeostasis (the parasympathetic rebound).
Chronic stressors recruit the slower hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus secretes corticotrophin-releasing hormone (CRH), which stimulates the anterior pituitary to release adrenocorticotrophic hormone (ACTH); ACTH acts on the adrenal cortex to release cortisol. Cortisol is a double-edged hormone: in the short term it mobilises energy through gluconeogenesis, sustains blood pressure, and reduces inflammation, but chronic elevation suppresses immune function and damages the cardiovascular system, while desensitising the negative-feedback receptors that should switch the axis off. Selye's (1936) General Adaptation Syndrome integrates the two pathways into three stages — alarm (the acute SAM response), resistance (sustained HPA activation in which the body appears to cope while depleting resources), and exhaustion (resource depletion and stress-related illness).
The principal strength of this account is its objective, replicable evidence base: adrenaline and cortisol are measured directly via blood and salivary assays, making the model falsifiable and scientifically credible. Nevertheless, the evidence is heavily reliant on animals — Selye's GAS was derived from rats — which limits generalisability, since humans uniquely appraise stressors cognitively, so the same physiology may or may not be triggered depending on interpretation. The model is also gender-biased: Taylor et al. (2000) argued that fight-or-flight research was conducted predominantly on males and that women may show a tend-and-befriend response mediated by oxytocin, constituting a beta-bias. Finally, the account is reductionist; by reducing stress to cortisol and adrenaline it gains precision but risks neglecting the cognitive appraisal and social context that determine whether an event becomes stressful at all. Taken together, the physiological model offers a precise, well-evidenced, and clinically useful account of how the body responds to stress, but a complete explanation requires an interactionist approach that combines these mechanisms with cognitive and social factors that determine why a stressor is experienced as stressful in the first place.
The Mid-band response correctly identifies both pathways and the GAS stages, and offers three relevant evaluation points, but the description lacks precise mechanism (for example, it does not mention noradrenaline, glycogenolysis, or negative feedback) and the evaluation is asserted in single sentences rather than explained, so it remains at the level of identification. The Stronger response describes both pathways accurately with appropriate terminology, develops each evaluation point with an explanation, and correctly distinguishes the medulla from the cortex; its shortfall is that it stops before drawing out implications and offers no overall conclusion. The Top-band response is detailed and accurate throughout — it captures the dual role of cortisol, the negative-feedback loop, the neural-versus-endocrine contrast, and the way GAS integrates the pathways — uses precise evaluative terminology ("beta-bias," "reductionist," "cognitive appraisal"), develops each point into a point–evidence–explanation–implication chain, and ends with a synoptic interactionist conclusion. Note that none of the answers awards AO2, because the question contains no applied scenario.
This content is aligned with the AQA A-Level Psychology (7182) specification.