The Endocrine System and Negative Feedback

The endocrine system is the body's system of chemical control. It is made of glands that secrete chemicals called hormones directly into the bloodstream.

A hormone is a chemical messenger. Once a gland releases a hormone, the blood carries it around the body to a target organ, where it produces an effect. Only the target organ responds, because only its cells have the right receptors for that hormone — the blood reaches every organ, but most ignore the hormone.

Term	Meaning
Gland	An organ that secretes (releases) a hormone
Hormone	A chemical messenger that travels in the blood
Target organ	The organ a particular hormone acts on

Hormones travel in the blood, not along nerves. This is the single most important fact about the endocrine system: glands secrete hormones directly into the bloodstream, and the blood carries them to their target organ.

Because hormones move at the speed of the bloodstream rather than as an electrical impulse, the endocrine system is slower to act than the nervous system — but its effects last much longer.

For AQA 8461 you must be able to identify the position of six glands on a diagram of the human body. Learn their positions and what each releases.

Gland	Position in the body
Pituitary gland	At the base of the brain
Thyroid	In the neck
Pancreas	Behind the stomach
Adrenal glands	One on top of each kidney
Ovaries (female)	Lower abdomen
Testes (male)	In the scrotum

AQA asks you to point to these six glands on a body diagram. A frequent slip is placing the adrenal glands in the neck — they sit on top of the kidneys.

Each gland releases one or more hormones with a specific job. The table below links each gland to its main hormone(s) and the effect they produce — a common exam format is to give you the gland and ask for the hormone, or the other way round.

Gland	Hormone(s)	Main effect
Pituitary	Several (e.g. FSH, LH, ADH, growth hormone)	Acts on other glands to release further hormones
Thyroid	Thyroxine	Controls metabolic rate; growth and development
Pancreas	Insulin (and glucagon)	Controls blood glucose concentration
Adrenal glands	Adrenaline	Prepares the body for 'fight or flight'
Ovaries	Oestrogen	Controls the menstrual cycle; female development
Testes	Testosterone	Controls sperm production; male development

The pancreas is unusual: it acts as both a gland (secreting hormones) and a digestive organ. Its role in controlling blood glucose with insulin is covered in detail in the diabetes topic.

(Extra context — not required by AQA 8461: the specific hormones FSH, LH and ADH are named here only to show the pituitary controls many processes. The detail of FSH and LH belongs to the reproduction/menstrual-cycle topic, and ADH to water balance.)

The pituitary gland sits at the base of the brain and is described as the 'master gland'.

It earns that name because it secretes several hormones into the blood in response to body conditions, and many of those hormones act on other glands, stimulating them to release their own hormones to bring about effects. So the pituitary does not just act directly — it controls a chain of other glands.

A worked example of this chain — controlling the thyroid:

1. The pituitary detects that the body needs more activity from the thyroid.
2. The pituitary secretes a hormone (TSH) into the blood.
3. The blood carries it to the thyroid gland.
4. The thyroid responds by releasing thyroxine.
5. Thyroxine then acts on body cells to raise the metabolic rate.

The pituitary is the master gland because the hormones it releases control other glands. It responds to body conditions and triggers a cascade — one gland switching on another.

Both the nervous system and the endocrine system are control systems, but they work in completely different ways. Knowing exactly how they differ is a guaranteed exam comparison.

Feature	Nervous system	Endocrine system
Signal type	Electrical impulses (and chemicals at synapses)	Chemical hormones
Travels via	Neurones (nerve cells)	The bloodstream
Speed	Very fast	Slower
Duration of effect	Short-lived	Long-lasting
How targeted	Precise — acts on a specific part	More general — affects target organs around the body

Worked comparison — the same situation, two systems. Imagine you see a dog running towards you.

• Nervous response: light from the dog hits receptors in your eye → an electrical impulse travels along neurones to your brain → your brain sends impulses to leg muscles → you step back. This happens in a fraction of a second and stops as soon as the impulses stop.
• Hormonal response: your adrenal glands release adrenaline into the blood → the blood carries it to the heart, muscles and other organs → your heart races and you feel "on edge". This builds up over a few seconds and the feeling lasts for minutes after the dog has gone.

Nervous = fast, brief, precise. Hormonal = slower, long-lasting, more general. The same trigger can set off both at once.

(Higher Tier only — the role of thyroxine, and its control by negative feedback, is HT content in AQA 8461.)

Thyroxine is a hormone released by the thyroid gland. It stimulates the basal metabolic rate — the rate at which your body uses energy at rest — and plays an important role in growth and development.

The level of thyroxine in the blood is kept steady by negative feedback: when a level rises too high, the body acts to bring it back down; when it falls too low, the body acts to raise it. The response always opposes the change.

Worked example — trace the negative-feedback loop:

So a fall in thyroxine triggers a response that raises it, and a rise triggers a response that lowers it. The two opposing actions hold the level around a steady set point.

In negative feedback the response always opposes the change that triggered it. Low thyroxine → more is made; high thyroxine → less is made. That is what keeps the level stable.

(Higher Tier only — the role of adrenaline is HT content in AQA 8461.)

Adrenaline is produced by the adrenal glands (on top of the kidneys) in times of fear or stress.

Its job is to prepare the body for 'fight or flight' — to either confront a threat or run from it. To do this, adrenaline:

• increases the heart rate, and
• boosts the delivery of oxygen and glucose to the brain and muscles.

More oxygen and glucose reaching the muscles means more respiration, releasing the energy needed for sudden action. More reaching the brain sharpens alertness.

Crucial distinction — adrenaline is NOT controlled by negative feedback. Adrenaline is a response to a stimulus (fear or stress): the adrenal glands release it when a threat is detected, and the level drops once the threat passes. Thyroxine is held at a steady set point by feedback; adrenaline is switched on and off by the situation.

Hormone	Released by	Trigger	Controlled by
Thyroxine	Thyroid	Body's metabolic needs	Negative feedback (steady level)
Adrenaline	Adrenal glands	Fear or stress	Response to a stimulus (not feedback)

Thyroxine is controlled by negative feedback. Adrenaline is not — it is released in response to fear or stress and is not held at a set point. Mixing these up is a classic Higher Tier error.

1. Saying hormones travel along nerves

Hormones travel in the blood, not along neurones. Glands secrete them directly into the bloodstream, which carries them to the target organ. Saying a hormone "travels along a nerve" describes the nervous system instead.

2. Getting the nervous-vs-hormonal comparison backwards

The endocrine system is slower but longer-lasting and more general. The nervous system is faster but shorter and more precise. A common slip is writing that hormonal effects are fast or brief.

3. Thinking the master gland acts on the body directly only

The pituitary is the master gland because many of its hormones act on other glands, making them release further hormones. It controls a chain of glands, not just direct effects.

4. Placing the adrenal glands or pancreas in the wrong spot

The adrenal glands sit on top of the kidneys; the pancreas is behind the stomach; the thyroid is in the neck. Mislabelling positions loses marks on diagram questions.

5. Saying adrenaline is controlled by negative feedback (HT)

Only thyroxine is controlled by negative feedback. Adrenaline is a response to a stimulus — fear or stress — and is not held at a steady set point. Do not describe a feedback loop for adrenaline.

6. Confusing what thyroxine and adrenaline each do (HT)

Thyroxine controls the basal metabolic rate (and growth/development). Adrenaline increases heart rate and boosts oxygen and glucose delivery for 'fight or flight'. Keep their roles separate.