The HPA system | AQA A-Level Psychology Revision

For 7182 specification, first teach in September 2025

AQA A-Level Psychology | Free Revision Notes

Estimated study time: 55 minutes

These HPA system A-Level Psychology revision notes explain how the hypothalamic-pituitary-adrenal system produces a sustained physiological response to continuing stress. You will follow the sequence from the hypothalamus to the pituitary gland and adrenal cortex, before examining how cortisol maintains the stress response. The HPA system develops your understanding of adaptation to continuing stress [General adaptation syndrome] and prepares you to compare prolonged activation with the rapid fight-or-flight response [The sympathomedullary pathway].

Learning Objectives 🎯

By the end of this revision page, you should be able to:

• Define the hypothalamic-pituitary-adrenal system.

• Describe the sequence of physiological activity within the HPA system.

• Explain the roles of the hypothalamus, pituitary gland and adrenal cortex.

• Explain how cortisol helps the body respond to a continuing stressor.

• Apply the HPA system to unfamiliar examples of prolonged stress.

• Compare the HPA system with the sympathomedullary pathway.

Revision Notes 📚

What does the AQA specification require?

For the Stress option, AQA requires knowledge of the physiology of stress, including:

• General adaptation syndrome.

• The hypothalamic-pituitary-adrenal system.

• The sympathomedullary pathway.

• The role of cortisol.

The course also examines how stress may contribute to immunosuppression and cardiovascular disorders.

For this lesson, you should know:

1. What activates the HPA system.

2. The order in which its structures and hormones act.

3. Where each hormone is released.

4. The role of the adrenal cortex.

5. How cortisol supports a prolonged stress response.

6. Why sustained cortisol production may eventually be harmful.

What is the HPA system?

The hypothalamic-pituitary-adrenal system, usually abbreviated to the HPA system, is the physiological pathway associated with the body’s response to a continuing stressor.

Its name identifies its three main components:

• Hypothalamic refers to the hypothalamus.

• Pituitary refers to the pituitary gland.

• Adrenal refers to the adrenal glands.

The pathway produces cortisol, a corticosteroid hormone which helps the body maintain its response when stress continues.

The essential sequence is:

Stressor → hypothalamus → pituitary gland → adrenal cortex → cortisol

Learning that order is vital. AQA has previously assessed whether students can distinguish the adrenal cortex, which forms part of the HPA system, from the adrenal medulla, which is involved in the sympathomedullary pathway.

Why does the body need a prolonged stress pathway?

An immediate threat requires a rapid response. However, some stressors continue for hours, days, weeks or longer.

Examples include:

• Continuing examination pressure.

• A sustained high workload.

• Long-term caring responsibilities.

• Repeated daily demands.

• Ongoing financial or relationship difficulties.

A brief burst of physiological activation would not be sufficient to meet a demand that continues over time. The HPA system therefore helps maintain the body’s response.

This connects with the resistance stage of the body’s three-stage stress response [General adaptation syndrome]. During resistance, the person may continue functioning, but physiological activity remains elevated.

The HPA system step by step

Step 1: A continuing stressor is detected

The process begins when a situation is perceived as stressful.

The stressor activates the hypothalamus, a brain structure involved in coordinating physiological responses.

The hypothalamus acts as the starting point for both:

• The HPA system.

• The sympathomedullary pathway.

The pathways then follow different routes and have different roles.

Step 2: The hypothalamus releases CRH

The hypothalamus releases corticotrophin-releasing hormone, abbreviated to CRH.

CRH acts on the pituitary gland.

Its role is to communicate that the body requires a sustained stress response.

A clear exam sentence would be:

In response to a continuing stressor, the hypothalamus releases CRH, which stimulates the pituitary gland.

Step 3: The pituitary gland releases ACTH

The pituitary gland responds by releasing adrenocorticotrophin hormone, abbreviated to ACTH, into the bloodstream.

ACTH travels to the adrenal glands.

Its target is the adrenal cortex, which is the outer part of the adrenal gland.

The pituitary gland does not release cortisol. It releases ACTH, which then stimulates the adrenal cortex.

Step 4: ACTH stimulates the adrenal cortex

ACTH reaches and stimulates the adrenal cortex.

The adrenal cortex responds by releasing corticosteroid hormones, including cortisol.

This distinction has been assessed directly by AQA:

• HPA system: adrenal cortex and corticosteroids.

• Sympathomedullary pathway: adrenal medulla and adrenaline.

Do not use cortex and medulla interchangeably. They are different parts of the adrenal gland with different roles.

Step 5: The adrenal cortex releases cortisol

The adrenal cortex releases cortisol into the bloodstream.

Cortisol helps the body continue responding to the stressor.

Its effects include:

• Maintaining the supply of energy available to the body.

• Increasing the availability of glucose.

• Supporting continued physiological activity.

• Temporarily reducing processes that are not immediately necessary for coping with the stressor.

This makes cortisol adaptive in the short term. The body has more energy available to meet the continuing demand.

The complete sequence

Stage	Structure	Chemical released	Target or result
1	Hypothalamus	CRH	Stimulates the pituitary gland
2	Pituitary gland	ACTH	Travels through the blood to the adrenal cortex
3	Adrenal cortex	Cortisol	Maintains the prolonged physiological stress response

An effective memory chain is:

Hypothalamus releases CRH → Pituitary releases ACTH → Adrenal cortex releases Cortisol

Understanding the three hormones

The HPA system includes several chemical signals. Each has a distinct source and role.

Hormone	Full name	Released by	Main role
CRH	Corticotrophin-releasing hormone	Hypothalamus	Stimulates the pituitary gland
ACTH	Adrenocorticotrophin hormone	Pituitary gland	Stimulates the adrenal cortex
Cortisol	A corticosteroid hormone	Adrenal cortex	Maintains the response to prolonged stress

A common revision error is remembering the hormone names but not their sources.

Try learning each as a complete statement:

• The hypothalamus releases CRH.

• The pituitary gland releases ACTH.

• The adrenal cortex releases cortisol.

The role of cortisol

Cortisol is the main hormone associated with the HPA stress response.

Its role is to help the person continue dealing with a stressor after the immediate response has begun.

Cortisol helps by increasing the availability of energy. This is useful when a person must remain alert and active over an extended period.

For example, someone dealing with several weeks of demanding work may need to:

• Maintain attention.

• Continue completing tasks.

• Mobilise energy repeatedly.

• Remain physiologically prepared for further demands.

Cortisol supports this continuing response.

Cortisol and glucose

One effect of cortisol is to increase the availability of glucose in the bloodstream.

Glucose provides energy for bodily activity.

During prolonged stress:

1. The stressor remains.

2. Cortisol continues to be released.

3. Glucose remains available.

4. The person has energy to continue responding to the demand.

This is adaptive while the response is needed.

However, maintaining physiological activity for too long may create health costs.

Adaptive and harmful effects of cortisol

Cortisol should not be described as simply harmful.

Short-term or controlled cortisol release	Sustained cortisol release
Helps make energy available	Keeps the body physiologically activated
Supports continued coping	May interfere with normal bodily functioning
Helps the person respond to a demand	May contribute to immunosuppression
Is an adaptive part of the stress response	May increase vulnerability to illness

The key issue is duration.

Cortisol is useful when it helps the body meet a demand, but sustained production during chronic stress can become harmful.

Cortisol and immune functioning

During stress, energy and resources are directed towards coping with the immediate demand.

Sustained cortisol production can reduce immune functioning. This may interfere with the activity of white blood cells and reduce the body’s ability to fight infection.

The sequence is:

1. A stressor continues.

2. The HPA system remains active.

3. Cortisol production is sustained.

4. Immune activity is suppressed.

5. Vulnerability to infection may increase.

This link is developed fully in the relationship between stress and health [Stress and illness].

Cortisol does not mean that illness is inevitable. It may increase vulnerability when stress is severe or prolonged.

Returning towards normal functioning

When the stressor ends, continued activation is no longer needed.

The HPA system is regulated through negative feedback. As cortisol levels rise, signals are sent which reduce further activity of the hypothalamus and pituitary gland.

This means:

• Less CRH is released.

• Less ACTH is released.

• The adrenal cortex produces less cortisol.

• The body can return towards its usual level of activity.

Negative feedback prevents the stress response from continuing indefinitely when the demand has passed.

What happens during prolonged stress?

During prolonged stress, the stressor repeatedly or continuously activates the HPA system.

The sequence may continue for an extended period:

1. The hypothalamus remains activated.

2. CRH continues to stimulate the pituitary gland.

3. The pituitary continues releasing ACTH.

4. ACTH continues stimulating the adrenal cortex.

5. Cortisol remains elevated.

6. Energy remains available for coping.

7. Sustained activation may eventually impair immune functioning.

This is why the HPA system is associated more closely with chronic stress than with an immediate emergency.

Applying the HPA system to workplace stress

Consider this example:

Theo has worked under intense pressure for several months. He regularly works late and worries about missing his targets. Although he continues meeting most deadlines, he feels constantly tense and has recently experienced repeated infections.

A developed application might explain:

• Theo’s continuing workload is a prolonged stressor.

• His hypothalamus may release CRH.

• CRH stimulates the pituitary gland to release ACTH.

• ACTH stimulates the adrenal cortex.

• The adrenal cortex releases cortisol.

• Cortisol helps maintain energy so Theo can continue working.

• Sustained cortisol production may suppress immune functioning, which could help explain his increased vulnerability to infection.

The source of Theo’s stress could be developed further through workload and occupational control [Workplace stress].

Application structure

For an application question, use:

Scenario detail → HPA component → physiological process → consequence

For example:

Theo has experienced pressure for several months, so the stressor is chronic. This would repeatedly activate the HPA system, causing the adrenal cortex to release cortisol. Sustained cortisol could reduce immune functioning, increasing Theo’s vulnerability to infection.

Avoid writing:

Theo is stressed, so his HPA system releases cortisol.

This skips the hypothalamus, CRH, pituitary gland, ACTH and adrenal cortex.

Acute and prolonged stress compared

Immediate stress	Prolonged stress
Requires rapid mobilisation	Requires a sustained response
Closely associated with the sympathomedullary pathway	Closely associated with the HPA system
Involves the adrenal medulla	Involves the adrenal cortex
Releases adrenaline	Releases cortisol
Produces rapid fight-or-flight arousal	Maintains energy and physiological activity

The two systems are not completely isolated. A stressor may activate both, particularly when an immediate threat develops into an ongoing demand.

The next lesson examines adrenaline and rapid physiological arousal [The sympathomedullary pathway].

HPA system and general adaptation syndrome

General adaptation syndrome and the HPA system describe stress at different levels.

General adaptation syndrome	HPA system
Describes the stress response across stages	Explains a specific physiological pathway
Includes alarm, resistance and exhaustion	Includes the hypothalamus, pituitary and adrenal cortex
Explains adaptation over time	Explains how cortisol is released
Links prolonged stress to depleted resources	Links sustained cortisol with prolonged physiological activity

The HPA system is especially relevant to the resistance stage, when the body attempts to maintain its response to a continuing stressor.

If activation continues for too long, sustained cortisol may contribute to the health problems associated with exhaustion.

HPA system and the endocrine system

The HPA system is an endocrine pathway because its glands release hormones into the bloodstream.

It contains a chain of communication:

• One structure releases a hormone.

• That hormone stimulates another gland.

• The second gland releases another hormone.

• The final hormone produces physiological effects.

This differs from the rapid transmission of electrical impulses through the nervous system.

The endocrine response is generally slower but longer lasting, which makes it suitable for prolonged stress.

Comparing cortisol and adrenaline

Cortisol	Adrenaline
Released by the adrenal cortex	Released by the adrenal medulla
Part of the HPA system	Part of the sympathomedullary pathway
Associated with prolonged stress	Associated with rapid fight or flight
Helps maintain energy availability	Rapidly increases physiological arousal
Can contribute to immunosuppression when sustained	Prepares the body for immediate action

📌 Exam tip: Before writing an answer, check whether the question asks about cortisol or adrenaline. That will usually tell you which pathway is required.

Measuring the stress response

Cortisol provides a physiological indication that the stress response has been activated.

Physiological measures are useful because they do not rely entirely on a person accurately reporting how stressed they feel.

However:

• A biological measure does not reveal the precise source of the stress.

• People may differ in their physiological responses.

• The same person’s response may vary across situations.

The course considers objective indications of arousal more broadly in biological indicators of stress [Physiological measures of stress].

Limits of an HPA-only explanation

The HPA system explains the physiological sequence involved in prolonged stress. It does not, by itself, fully explain:

• Why one person perceives a situation as stressful and another does not.

• Why individuals differ in their ability to cope.

• How control over the situation changes the response.

• How personality or social support affects stress.

For example, commitment, challenge and control [Hardiness] may influence whether a person experiences a demand as threatening and how successfully they cope.

The HPA system is therefore a physiological mechanism within a wider explanation of stress, rather than a complete account of every individual response.

Key Words 🔑

Key word	Student-friendly definition	How it may be used in an exam
HPA system	The hypothalamic-pituitary-adrenal pathway which produces a sustained response to stress	Name and explain the pathway involved in prolonged stress
Hypothalamus	A brain structure which begins the HPA response by releasing CRH	Identify the starting point of the pathway
CRH	Corticotrophin-releasing hormone, released by the hypothalamus	Explain how the pituitary gland is stimulated
Pituitary gland	An endocrine gland which releases ACTH during the HPA response	Identify the second structure in the pathway
ACTH	Adrenocorticotrophin hormone, released by the pituitary gland	Explain how the adrenal cortex is stimulated
Adrenal gland	A gland containing the cortex and medulla	Distinguish which part belongs to each stress pathway
Adrenal cortex	The outer part of the adrenal gland which releases cortisol	Use it in explanations of the HPA system
Adrenal medulla	The inner part of the adrenal gland which releases adrenaline	Contrast it with the adrenal cortex
Cortisol	A corticosteroid hormone which maintains the response to continuing stress	Explain energy availability and the effects of prolonged stress
Corticosteroid	A type of hormone produced by the adrenal cortex	Classify cortisol accurately
Glucose	A source of energy made more available during the stress response	Explain how cortisol helps maintain coping
Chronic stress	Stress which continues over a prolonged period	Identify when the HPA system is especially important
Negative feedback	A regulatory process which reduces HPA activity when cortisol is sufficiently high	Explain how the response returns towards normal
Immunosuppression	Reduced immune functioning	Explain a possible consequence of sustained cortisol
Endocrine system	A communication system in which glands release hormones into the bloodstream	Explain why the HPA response is slower and longer lasting than a neural response

Hints from the Examiner Reports 💡

No lesson-specific examiner guidance was identified in the provided reports.

However, past AQA assessment materials highlight several precise distinctions that are important for this topic:

Examiner hint: Learn the sequence as connected cause and effect. Do not present the hypothalamus, pituitary gland and adrenal cortex as an unconnected list.

Examiner hint: State that the pituitary gland releases ACTH. ACTH is not released by the adrenal gland.

Examiner hint: Link the HPA system to the adrenal cortex, not the adrenal medulla. AQA’s 2022 mark scheme identified stimulation of the adrenal cortex to produce corticosteroids as the correct description.

Examiner hint: Distinguish cortisol from adrenaline. Cortisol belongs to the HPA system, whereas adrenaline belongs to the sympathomedullary pathway.

Examiner hint: Application should use the duration of the stressor. Words such as “for several months”, “continuing” or “long term” are clues that the HPA system is relevant.

Examiner hint: Develop the link between cortisol and illness. Do not stop at “cortisol is released”. Explain that sustained cortisol may suppress immune functioning.

Examiner hint: In an extended response, use the whole scenario. A continuing demand, maintained performance, poor health and repeated infection may each support a different stage of the explanation.

Common Mistakes ⚠️

Mistake: Putting the stages of the HPA system in the wrong order.

Why this is incorrect:Each hormone stimulates the next structure in a fixed sequence.

How to improve:Learn the chain: hypothalamus, pituitary gland, adrenal cortex.

Mistake: Saying that the hypothalamus releases ACTH.

Why this is incorrect:The hypothalamus releases CRH. The pituitary gland releases ACTH.

How to improve:Pair each structure with its hormone while revising.

Mistake: Saying that the pituitary gland releases cortisol.

Why this is incorrect:The pituitary gland releases ACTH, which stimulates the adrenal cortex to release cortisol.

How to improve:Write the complete causal sentence rather than jumping directly from the pituitary to cortisol.

Mistake: Linking the HPA system to the adrenal medulla.

Why this is incorrect:The HPA system stimulates the adrenal cortex. The medulla belongs to the sympathomedullary pathway.

How to improve:Remember: cortex equals cortisol.

Mistake: Saying that cortisol is adrenaline.

Why this is incorrect:They are different hormones released from different parts of the adrenal gland.

How to improve:Create a comparison table showing pathway, adrenal structure, hormone and duration.

Mistake: Describing the HPA response as instantaneous.

Why this is incorrect:The HPA system is associated with maintaining a response to prolonged stress.

How to improve:Link rapid immediate stress with the sympathomedullary pathway and prolonged stress with the HPA system.

Mistake: Saying that cortisol is always harmful.

Why this is incorrect:Cortisol is adaptive because it makes energy available to deal with a continuing demand.

How to improve:Distinguish useful short-term release from harmful sustained production.

Mistake: Saying that cortisol directly guarantees illness.

Why this is incorrect:Sustained cortisol may suppress immune functioning and increase vulnerability. Illness is not inevitable.

How to improve:Use careful terms such as “may contribute to” or “increases vulnerability”.

Mistake: Naming the structures without explaining what each does.

Why this is incorrect:An outline question requires a connected physiological process.

How to improve:Use active verbs: releases, stimulates, travels and maintains.

Mistake: Applying the HPA system to a brief surprise without considering duration.

Why this is incorrect:A sudden immediate threat is more directly associated with the sympathomedullary pathway.

How to improve:Look for evidence that the stressor continues over time.

Mistake: Saying the person consciously activates the HPA system.

Why this is incorrect:The HPA response is a physiological process rather than a deliberate decision.

How to improve:Describe the structures and hormones involved rather than conscious choice.

Exam-Style Questions ✍️

Question 1

Which of the following structures releases cortisol during the HPA response?

A. HypothalamusB. Pituitary glandC. Adrenal cortexD. Adrenal medulla

[1 mark]

Question 2

What is meant by the hypothalamic-pituitary-adrenal system?

[2 marks]

Question 3

Outline the sequence of activity in the HPA system.

[5 marks]

Question 4

Explain the role of cortisol in the physiological response to stress.

[4 marks]

Question 5

Explain two differences between the HPA system and the sympathomedullary pathway.

[4 marks]

Question 6

Priya has been caring for a family member for several months while continuing to work full time. She says that she remains alert throughout the day even when she has slept poorly. Recently, she has experienced several minor infections.

Use your knowledge of the HPA system to explain Priya’s physiological response.

[6 marks]

Question 7

A researcher takes cortisol samples from a participant at three points during a demanding six-week placement.

Point in placement	Relative cortisol level
Before the placement	Low
End of week 1	High
End of week 6	High

Using the HPA system, explain what the pattern might indicate.

[4 marks]

Question 8

Arun works in a busy office. A temporary emergency causes his heart rate to increase rapidly. The workload then remains high for several months. Arun continues working but feels constantly tense and has begun experiencing frequent colds.

Explain the physiological response to stress. Refer to the HPA system and Arun in your answer.

[8 marks]

Answers and mark scheme

Question 1

Answer: C

Cortisol is released by the adrenal cortex.

[1 mark]

Question 2

Award up to two marks:

• The HPA system is a physiological or endocrine pathway involved in responding to stress.

• It connects the hypothalamus, pituitary gland and adrenal cortex and results in cortisol release.

[2 marks]

Question 3

Award one mark for each relevant stage:

• A continuing stressor activates the hypothalamus.

• The hypothalamus releases CRH.

• CRH stimulates the pituitary gland.

• The pituitary gland releases ACTH.

• ACTH stimulates the adrenal cortex, which releases cortisol.

[5 marks]

Question 4

Award up to four marks:

• Cortisol is released by the adrenal cortex.

• It helps maintain the response to a continuing stressor.

• It increases the availability of glucose or energy.

• If cortisol remains elevated, it may suppress immune functioning and increase vulnerability to illness.

[4 marks]

Question 5

Award up to two marks for each developed difference:

• The HPA system involves the adrenal cortex, whereas the sympathomedullary pathway involves the adrenal medulla.

• The HPA system releases cortisol, whereas the sympathomedullary pathway releases adrenaline.

• The HPA system is associated with prolonged stress, whereas the sympathomedullary pathway produces a rapid immediate response.

• The HPA response is slower and longer lasting, whereas the sympathomedullary response is faster and shorter lived.

Maximum [4 marks].

Question 6

Award one mark for each clear application, up to six marks:

• Caring and working for several months represents prolonged stress.

• Priya’s hypothalamus may release CRH.

• CRH stimulates the pituitary gland to release ACTH.

• ACTH stimulates the adrenal cortex.

• Cortisol helps maintain alertness or energy availability while the demand continues.

• Sustained cortisol may suppress immune functioning, which could increase her vulnerability to infections.

Credit other accurate applications of the pathway.

[6 marks]

Question 7

Award up to four marks:

• The low initial level indicates that the HPA system was not strongly activated before the placement.

• The high level at week 1 suggests activation of the HPA response after the stressor began.

• The cortisol level remaining high at week 6 suggests that the stressor is continuing.

• Sustained cortisol indicates prolonged physiological activation and could increase the risk of immunosuppression if maintained.

The results indicate an association with the placement but do not independently prove that the placement caused the cortisol changes.

[4 marks]

Question 8

Indicative content may include:

Knowledge and understanding

• A prolonged stressor activates the hypothalamus.

• The hypothalamus releases CRH.

• CRH stimulates the pituitary gland.

• The pituitary releases ACTH.

• ACTH stimulates the adrenal cortex.

• The adrenal cortex releases cortisol.

• Cortisol maintains energy availability.

• Sustained cortisol may reduce immune functioning.

Application to Arun

• The temporary emergency may initially trigger a rapid physiological response.

• The workload lasting for months is a prolonged stressor.

• Arun’s HPA system may therefore remain active.

• Cortisol could help him continue working despite the demand.

• Constant tension is consistent with sustained physiological activation.

• Frequent colds may indicate increased vulnerability resulting from reduced immune functioning.

A high-level answer will give the sequence accurately, distinguish prolonged from immediate stress and link several details from Arun’s situation to the physiological processes.

[8 marks]