4.3.7 - The physiology of stress

What stress physiology explains

In Psychology, stress can mean the pressure placed on a person, the person's appraisal of that pressure, or the body's biological response to it. This lesson is about the third meaning: the physiological response that prepares the body to deal with a perceived stressor.

AQA wants you to know four linked ideas:

• The sympathomedullary pathway (SAM): the fast, short-term stress pathway for acute threat.
• The hypothalamic-pituitary-adrenal system (HPA): the slower, longer-lasting stress pathway for continuing stress.
• The role of cortisol: the hormone that helps sustain the stress response but prolonged or poorly regulated exposure can contribute to health risks.
• General adaptation syndrome (GAS): Selye's model of how the body adapts to stress across alarm, resistance and exhaustion.

AO2: Acute and chronic stressors

Imagine Noah sees a cyclist heading straight towards him. His heart pounds, his breathing quickens and he jumps out of the way. That is an acute stressor: sudden, immediate and short-lived. Now imagine Priya has been caring for a seriously ill relative for two years while also working full time. That is a chronic stressor: long-lasting and difficult to escape.

The two cases involve different time courses. Noah's body needs a rapid burst of arousal. Priya's body may need a sustained response, which is why cortisol and the HPA system become especially important.

Exam focus

Strong answers do not just list symptoms such as "sweating" or "a fast heart rate". They explain the biological chain that causes those symptoms. A useful exam habit is to write in sequences: stressor -> hypothalamus -> pathway -> hormone -> body effect.

The sympathomedullary pathway

The sympathomedullary pathway, often shortened to SAM, is the body's rapid response to a perceived immediate threat. It is closely linked to the fight or flight response.

The sequence is:

1. A stressor is perceived as threatening.
2. The hypothalamus coordinates the immediate stress response.
3. The sympathetic branch of the autonomic nervous system is activated.
4. Sympathetic nerves stimulate the adrenal medulla, the inner part of the adrenal gland.
5. The adrenal medulla releases adrenaline and noradrenaline into the bloodstream.
6. These hormones produce physiological arousal that prepares the body for rapid action.

AO1: What adrenaline and noradrenaline do

The sympathetic nervous system and adrenal hormones act together to coordinate a whole-body response. Some effects are produced directly by sympathetic nerves as well as by hormones in the blood:

Sweating illustrates why this distinction matters: sympathetic nerves stimulate sweat glands directly. Do not describe every sympathetic effect as being caused only by adrenaline released into the blood.

Once the threat has passed, the parasympathetic branch helps return the body towards baseline. Heart rate and breathing slow, digestion resumes and arousal decreases. This recovery matters because the SAM pathway is designed for short-term mobilisation, not constant activation.

AO2: Applying SAM to a scenario

If Leila's hands shake before a driving test, you could apply SAM like this: Leila appraises the test as threatening, so her hypothalamus activates the sympathetic branch of the autonomic nervous system. This stimulates the adrenal medulla to release adrenaline and noradrenaline. Adrenaline increases muscle tension and arousal, which helps explain her shaking hands even though she does not actually need to fight or flee.

The HPA system and cortisol

The hypothalamic-pituitary-adrenal system, or HPA system, is slower than the SAM pathway but its effects last longer. It is especially important when a stressor continues beyond the immediate moment.

SAM and HPA are not mutually exclusive switches: an acute stressor can activate both, but neural sympathetic effects appear faster than the hormonal cortisol response. Chronic stress involves repeated activation or altered regulation, not necessarily continuously high cortisol in every person.

The HPA sequence is:

1. The hypothalamus responds to a perceived stressor.
2. The hypothalamus releases corticotropin-releasing hormone, often called CRH or CRF in Psychology resources.
3. CRH/CRF stimulates the anterior pituitary gland.
4. The pituitary gland releases ACTH into the bloodstream.
5. ACTH stimulates the adrenal cortex, the outer part of the adrenal gland.
6. The adrenal cortex releases cortisol.

AO1: The role of cortisol

Cortisol helps the body cope with continuing demand by making energy available. It increases glucose availability and supports alertness, so in the short term it is adaptive. A student revising under pressure or a nurse handling a long emergency shift may need sustained energy and concentration.

The problem is duration. If cortisol remains high for a long time, it can suppress immune activity and place strain on body systems. This is why chronic stress is linked to later lessons on illness, immunosuppression and cardiovascular problems.

AO1: Negative feedback

The HPA system is partly controlled by a negative feedback loop. When cortisol levels rise, the hypothalamus and pituitary gland reduce their release of CRH/CRF and ACTH. This reduces further cortisol secretion. Negative feedback is useful because it prevents the stress response from continuing unnecessarily once the body has enough cortisol.

AO2: Applying HPA to a scenario

Maya has experienced months of workplace pressure. She is not reacting to one sudden danger; she is dealing with a chronic stressor. Her HPA system may be repeatedly activated or its regulation may change. Cortisol helps make energy available, but prolonged exposure can affect immune activity. This gives one possible explanation for an association with infections; the scenario does not establish Maya's cortisol level or show that stress caused any particular illness.

Keeping the two adrenal regions separate

Both adrenal glands sit above the kidneys, but each has two functionally different regions. The medulla receives sympathetic nerve signals and releases adrenaline/noradrenaline. The cortex responds to ACTH from the pituitary and releases cortisol. Thus, “the adrenal gland releases a stress hormone” is too vague to identify a pathway.

For example, if ACTH rises and is followed by cortisol, the link is pituitary → adrenal cortex. It would be incorrect to insert the adrenal medulla between ACTH and cortisol. Negative feedback is a return signal that reduces further CRH and ACTH release; it does not reverse the forward sequence.