The endocrine system | AQA A-Level Psychology Revision
- Revision Notes
- Aug 3
- 12 min read
Updated: 6 days ago
For 7182 specification, first teach in September 2025
AQA A-Level Psychology | Free Revision Notes
Estimated study time: 40 minutes
These The endocrine system A-Level Psychology revision notes explain how glands release hormones to communicate with target parts of the body. You will examine how endocrine communication differs from the rapid neural communication covered in the process of synaptic communication. This lesson also prepares you to apply both communication systems to the body’s response to a threat. The AQA specification requires knowledge of the endocrine system’s function, glands and hormones.
Learning Objectives 🎯
By the end of this revision page, you should be able to:
Define the endocrine system.
Explain the function of the endocrine system.
Explain the role of glands.
Explain the role of hormones.
Describe how hormones reach and affect target parts of the body.
Compare endocrine and nervous communication.
Apply endocrine communication to unfamiliar situations.
Revision Notes 📚
The endocrine system A-Level Psychology revision overview
The endocrine system is a biological communication system consisting of glands that release chemical messengers called hormones.
Hormones are carried in the bloodstream and affect particular target cells, tissues or organs.
The basic sequence is:
Gland → hormone released → bloodstream → target cell or organ → biological effect
The endocrine system contributes to the regulation and coordination of bodily processes.
Unlike the nervous system, which transmits information through neurons, the endocrine system communicates chemically through the bloodstream.
What is the endocrine system?
The endocrine system is a network of glands located throughout the body.
Its function is to:
Produce hormones.
Release hormones into the bloodstream.
Carry chemical messages around the body.
Influence the activity of target cells or organs.
Coordinate bodily processes and responses.
The endocrine system allows one part of the body to communicate with another without sending a signal directly along a neuron.
What is a gland?
A gland is an organ or group of specialised cells that produces and releases a chemical substance.
Within the endocrine system, glands produce and release hormones.
The role of an endocrine gland can be summarised as:
Producing a particular hormone.
Releasing that hormone when required.
Releasing it into the bloodstream.
Allowing the hormone to travel to its target.
A gland is therefore the source of the hormonal message.
Glands release hormones
When an endocrine gland is activated, it releases a hormone.
The hormone enters the bloodstream and circulates through the body.
The gland does not need to be physically next to the part of the body that the hormone affects. The bloodstream provides the communication route between the gland and its target.
For example:
A gland releases a hormone.
The hormone enters the blood.
The blood transports it around the body.
The hormone reaches target cells or organs.
The target responds.
What is a hormone?
A hormone is a chemical messenger produced and released by an endocrine gland.
Hormones travel through the bloodstream and influence the activity of target cells, tissues or organs.
Their role is to carry information chemically from the gland that releases them to the part of the body that responds.
A complete definition should include:
That a hormone is a chemical messenger.
That it is released by a gland.
That it travels through the bloodstream.
That it affects a target part of the body.
Hormones and target cells
Although hormones circulate through the bloodstream, they do not affect every cell in the same way.
A target cell is a cell that can respond to a particular hormone.
Target cells contain receptors that are suitable for that hormone.
The process can be represented as:
Hormone reaches target cell → binds to a suitable receptor → changes the cell’s activity
Cells without the appropriate receptor do not respond to the hormone in the same way.
This allows a hormone to travel widely through the body while producing a particular effect at its target.
Hormones as chemical messages
A hormone carries biological information rather than behaving as a physical instruction.
When it reaches an appropriate target, it changes the activity of that target.
Hormonal effects may include changes in:
The activity of an organ.
Bodily functioning.
A physiological response.
Behaviour associated with the biological change.
The exact effect depends on:
Which hormone was released.
Which target receptors are present.
Which part of the body responds.
The route of endocrine communication
Endocrine communication follows a clear pathway:
Stage 1: A gland is activated
A change within or outside the body leads to activity in an endocrine gland.
Stage 2: A hormone is released
The gland secretes a chemical messenger.
Stage 3: The hormone enters the bloodstream
The blood carries the hormone around the body.
Stage 4: The hormone reaches target cells
Only cells with appropriate receptors respond to it.
Stage 5: A biological change occurs
The hormone alters the activity of the target cells or organ.
The complete pathway is:
Change detected → gland activated → hormone released → bloodstream → target receptor → response
An example involving adrenaline
The AQA specification names adrenaline as part of the following lesson on the fight or flight response.
Adrenaline illustrates the difference between a gland and a hormone:
The adrenal glands are glands within the endocrine system.
Adrenaline is a hormone released by these glands.
The hormone enters the bloodstream.
It travels to target parts of the body.
It contributes to biological changes associated with responding to a threat.
The complete role of adrenaline is covered in adrenaline and responses to danger.
Glands and hormones are not the same
Students sometimes use the terms gland and hormone interchangeably.
They describe different parts of the endocrine system.
Gland | Hormone |
Produces and releases a chemical messenger | Is the chemical messenger |
Is a biological structure | Is a chemical substance |
Remains in a particular location | Travels through the bloodstream |
Acts as the source of communication | Carries the message to a target |
Example: adrenal gland | Example: adrenaline |
A gland releases a hormone. A hormone does not release a gland.
How the endocrine system coordinates the body
The endocrine system allows bodily activity to be coordinated over time.
A hormone released in one location can reach target cells elsewhere in the body.
This makes it possible for the body to produce a coordinated response involving more than one target.
For example:
A situation creates a biological demand.
An endocrine gland releases a hormone.
The bloodstream distributes the hormone.
Relevant target organs respond.
The body’s activity changes to meet the demand.
The endocrine system therefore forms part of the biological basis of behaviour described in biological structures and neurochemistry.
The nervous system as a communication system
The nervous system communicates using neurons.
Information travels:
Electrically along neurons.
Chemically between neurons at synapses.
The nervous system includes:
The central nervous system.
The peripheral nervous system.
Sensory, relay and motor neurons.
Synaptic transmission.
You can review its organisation through central, peripheral, somatic and autonomic systems.
Nervous communication pathway
A simplified nervous pathway is:
Stimulus → receptor → sensory neuron → central nervous system → motor neuron → effector
Signals travel rapidly along specific neural pathways.
At a synapse:
An electrical signal reaches the end of a neuron.
Neurotransmitters are released.
They cross the synaptic gap.
They affect the postsynaptic neuron.
This should not be confused with endocrine communication, where hormones travel through the bloodstream.
Comparing endocrine and nervous communication
The endocrine and nervous systems both transmit information, but they use different methods.
Feature | Endocrine communication | Nervous communication |
Main messenger | Hormones | Electrical signals and neurotransmitters |
Route | Bloodstream | Neurons and synapses |
Source | Endocrine glands | Neurons |
Speed | Generally slower | Generally faster |
Duration | Effects may last longer | Effects are often shorter-lived |
Targeting | Hormones circulate widely but act on cells with suitable receptors | Signals travel along particular neural pathways |
Main role | Coordinates bodily activity through hormonal messages | Rapid transmission and coordination of information |
Difference in speed
Nervous communication is generally faster than endocrine communication.
A nervous signal travels rapidly along a neural pathway. Hormones must:
Be released by a gland.
Enter the bloodstream.
Travel through the circulation.
Reach their target cells.
Produce an effect.
This means endocrine responses usually take longer to begin.
A comparison answer should use the word generally because biological communication is complex and exact timings vary.
Difference in duration
Hormonal effects may continue after the hormone has been released.
Endocrine communication is therefore generally associated with longer-lasting changes.
Neural communication is often more immediate and shorter-lived.
Nervous response | Endocrine response |
Rapid onset | Slower onset |
Often brief | May continue for longer |
Suitable for immediate communication | Suitable for maintaining bodily changes |
A correct comparison should distinguish how quickly the response begins from how long it lasts.
Difference in communication route
The systems use different physical routes.
Endocrine system
Hormones travel in the bloodstream.
Nervous system
Signals travel along neurons and cross synapses using neurotransmitters.
This distinction can be summarised as:
Endocrine: gland to blood to target.
Nervous: neuron to synapse to neuron or effector.
Difference in targeting
Nervous signals travel along particular neural pathways.
This allows rapid communication with a relatively specific destination.
Hormones circulate through the bloodstream more widely. However, they affect only target cells with appropriate receptors.
It is therefore inaccurate to say that hormones affect every part of the body equally.
The bloodstream distributes the hormone widely, but receptor matching makes the effect selective.
Hormones and neurotransmitters
Hormones and neurotransmitters are both chemical messengers, but they operate differently.
Hormone | Neurotransmitter |
Released by an endocrine gland | Released by a presynaptic neuron |
Travels through the bloodstream | Crosses a synaptic gap |
May travel a considerable distance | Travels across a very small gap |
Acts on target cells with appropriate receptors | Binds to receptors on a postsynaptic cell |
Part of endocrine communication | Part of nervous communication |
The similarities and differences become clearer by reviewing neurotransmitters, excitation and inhibition.
Similarities between endocrine and nervous communication
Despite their differences, both systems:
Communicate information within the body.
Use chemical messengers.
Depend on receptors.
Affect the activity of target cells.
Help coordinate bodily responses.
Can contribute to behaviour.
For example:
Neurotransmitters bind to receptors on postsynaptic cells.
Hormones bind to receptors on target cells.
The route and timing differ, but both rely on a chemical fitting an appropriate receptor.
How the nervous and endocrine systems work together
The nervous and endocrine systems are not completely separate.
They can operate together to coordinate a response.
A general sequence might be:
The nervous system detects and processes a change.
A neural signal activates an endocrine gland.
The gland releases a hormone.
The hormone travels through the bloodstream.
Target organs respond.
The bodily response is maintained or coordinated.
This combination allows the body to produce:
A rapid initial response through the nervous system.
A wider or longer-lasting response through the endocrine system.
This interaction is central to the nervous and hormonal response to stress.
Applying endocrine communication
Consider the following scenario:
A gland releases a chemical messenger after a person encounters a demanding situation. The chemical travels through the blood and changes the activity of several target organs.
This describes endocrine communication because:
A gland produces the messenger.
The messenger is a hormone.
It travels through the bloodstream.
It reaches target organs.
It produces biological changes.
It would not be described as purely nervous communication because the messenger is not travelling along a neuron.
Applying nervous communication
Consider another situation:
A receptor detects a sudden sound. An electrical signal travels rapidly towards the central nervous system along a sensory neuron.
This describes nervous communication because:
Information travels along a neuron.
The signal is directed towards the central nervous system.
The pathway produces rapid communication.
The three neuron types involved are explained in sensory, relay and motor pathways.
Applying both systems to one situation
Consider this example:
Priya sees an object moving rapidly towards her. She immediately steps backwards. Chemical messengers are also released into her bloodstream, producing changes in internal bodily activity.
The systems can be applied as follows:
Sensory neurons carry visual information towards the central nervous system.
The central nervous system processes the information.
Motor neurons carry instructions to Priya’s skeletal muscles.
Her immediate movement involves nervous communication.
An endocrine gland releases hormones.
The hormones travel through her bloodstream.
They affect target organs and contribute to internal bodily changes.
The two systems contribute different but connected parts of the response.
A method for identifying the communication system
Ask the following questions.
Is the message travelling along a neuron?
This indicates nervous communication.
Is a chemical crossing a synaptic gap?
This is a neurotransmitter involved in nervous communication.
Is a gland releasing a chemical into the bloodstream?
This indicates endocrine communication.
Is the effect rapid and directed along a particular pathway?
This is more characteristic of nervous communication.
Is the effect slower to begin and longer-lasting?
This is more characteristic of endocrine communication.
A method for answering comparison questions
A good comparison should identify one feature and explain both systems in relation to it.
For example:
Endocrine communication is generally slower because hormones must travel through the bloodstream from a gland to target cells. In contrast, nervous communication is generally faster because electrical signals travel rapidly along neurons.
This is stronger than writing:
The endocrine system is slow. The nervous system is fast.
The developed answer explains why the systems differ.
Comparison structure
Use the following structure:
State the comparison point.
Explain the endocrine system.
Use a comparison word.
Explain the nervous system.
State why the difference matters where appropriate.
Useful comparison words include:
Whereas
In contrast
Both
Similarly
Unlike
However
Writing an effective explanation of the endocrine system
A strong answer might state:
The endocrine system is a biological communication system consisting of glands that release hormones. Hormones are chemical messengers carried through the bloodstream. They bind to appropriate receptors on target cells or organs and alter their activity. Endocrine communication is generally slower than nervous communication, but its effects may last longer.
This answer:
Defines the endocrine system.
Explains the role of glands.
Explains the role of hormones.
Identifies the bloodstream as the communication route.
Explains target receptors.
Makes a developed comparison with nervous communication.
What a complete comparison needs
For a question comparing nervous and endocrine communication, consider:
Type of messenger.
Route travelled.
Speed.
Duration.
Targeting.
One relevant similarity.
A complete response does not need every possible comparison unless the mark allocation requires it.
For a two-mark question, one developed distinction may be enough. For a longer question, use several direct comparisons.
Key Words 🔑
Key word | Student-friendly definition | How it may be used in an exam |
Endocrine system | A communication system of glands that release hormones into the bloodstream. | Explain how chemical messages coordinate bodily activity. |
Gland | An organ or group of specialised cells that produces and releases a chemical substance. | Identify the source of a hormone. |
Hormone | A chemical messenger released by a gland and carried through the bloodstream. | Explain endocrine communication. |
Bloodstream | The route through which hormones travel around the body. | Compare hormonal and neural communication routes. |
Target cell | A cell that can respond to a particular hormone. | Explain why hormones do not affect every cell equally. |
Target organ | An organ containing cells that respond to a particular hormone. | Apply endocrine communication to a bodily response. |
Receptor | A structure to which a particular chemical messenger can bind. | Explain how a target responds to a hormone. |
Adrenal gland | An endocrine gland that releases adrenaline. | Connect this lesson with the fight or flight response. |
Adrenaline | A hormone involved in the fight or flight response. | Apply endocrine communication to a threatening situation. |
Nervous system | The communication system using neurons and synapses. | Compare neural and hormonal communication. |
Neuron | A specialised cell that transmits information through the nervous system. | Identify the route used in nervous communication. |
Neurotransmitter | A chemical messenger released by a neuron that crosses a synaptic gap. | Distinguish chemical communication at a synapse from hormonal communication. |
Synapse | The junction between two neurons. | Identify where neurotransmitters act. |
Communication system | A system that allows information to pass between different parts of the body. | Explain a similarity between the nervous and endocrine systems. |
Common Mistakes ⚠️
Mistake: Using gland and hormone as though they mean the same thing.
Why this is incorrect:A gland is the structure that produces and releases the chemical messenger. The hormone is the chemical messenger itself.
How to improve:Use the sequence: gland produces hormone.
Mistake: Saying that hormones travel along neurons.
Why this is incorrect:Hormones travel through the bloodstream.
How to improve:Distinguish endocrine communication through blood from nervous communication along neural pathways.
Mistake: Saying that neurotransmitters and hormones travel through the same route.
Why this is incorrect:Neurotransmitters cross a synaptic gap, whereas hormones travel through the bloodstream.
How to improve:Identify both the messenger and its route.
Mistake: Saying that hormones affect every cell in the body.
Why this is incorrect:Hormones affect target cells that contain appropriate receptors.
How to improve:Explain that hormones circulate widely but act selectively.
Mistake: Describing endocrine communication as electrical.
Why this is incorrect:The endocrine system communicates using chemical hormones.
How to improve:Reserve electrical transmission for signals travelling along neurons.
Mistake: Saying endocrine communication is always immediate.
Why this is incorrect:Hormones must be released and transported through the bloodstream before affecting their targets.
How to improve:Describe endocrine communication as generally slower than nervous communication.
Mistake: Saying that slower communication means the endocrine system is ineffective.
Why this is incorrect:Endocrine effects may last longer and help coordinate wider bodily changes.
How to improve:Treat speed and duration as different characteristics rather than value judgements.
Mistake: Claiming that nervous communication is entirely chemical.
Why this is incorrect:Communication is electrical along neurons and chemical across synapses.
How to improve:Use the sequence electrical, chemical, electrical when describing neural transmission.
Mistake: Describing adrenaline as a gland.
Why this is incorrect:Adrenaline is a hormone. It is released by the adrenal glands.
How to improve:Separate the biological structure from the chemical it releases.
Mistake: Writing two separate descriptions instead of making a comparison.
Why this is incorrect:A comparison question requires a direct similarity or difference.
How to improve:Use phrases such as “whereas”, “in contrast” and “both systems”.
Mistake: Saying that the nervous and endocrine systems operate independently in every situation.
Why this is incorrect:The systems can work together to coordinate a response.
How to improve:Explain how neural activity may activate a gland and lead to hormonal release.
Exam-Style Questions ✍️
Question 1
Which one of the following is the best definition of a hormone?
A. An electrical signal travelling along a neuron
B. A chemical messenger released by a gland
C. A cell that receives sensory information
D. A gap between two neurons
[1 mark]
Question 2
Define the endocrine system.
[2 marks]
Question 3
Explain the role of glands in endocrine communication.
[3 marks]
Question 4
Explain how a hormone can affect a target organ located away from the gland that released it.
[4 marks]
Question 5
Distinguish between a gland and a hormone.
[4 marks]
Question 6
Explain one difference between a hormone and a neurotransmitter.
[4 marks]
Question 7
Compare endocrine communication with nervous communication.
Refer to both speed and the route of communication in your answer.
[4 marks]
Question 8
A gland releases a chemical messenger into the blood. The chemical reaches several organs, but only some cells respond.
Explain this process using your knowledge of hormones and target cells.
[4 marks]
Question 9
During a demanding situation, Freya immediately moves away from danger. A hormone is then released and travels through her bloodstream.
Using your knowledge of biological communication, explain how nervous and endocrine communication are involved in Freya’s response.
[6 marks]
Question 10
Explain the function of the endocrine system.
Refer to glands, hormones, the bloodstream and target cells in your answer.
[6 marks]
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