GCSE · Biology · AQA · Spec 8461

Human endocrine system

Right now, chemical messages in your blood are reaching organ after organ. Most organs ignore them completely. So how does the right organ know a message is meant for it?

The endocrine glands

Where your hormones come from

View: Female body. Showing 3 layers: Body, Glands in every body, Ovaries

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tap a gland to see its hormone

Female body: pituitary, thyroid, adrenal glands, pancreas — and two ovaries. Tap any gland to see what it releases.

Switch between the female and male body, then tap each gland to find out which hormone it releases and what that hormone controls.

The big puzzle

How does a hormone find its target?

A hormone is a chemical made and released by a gland. It goes straight into the bloodstream and is carried in the blood plasma to a particular organ — its target organ — where it has its effect. But the blood flows all round the body. So how does the hormone end up affecting only its target?

Which idea is closest to what you think right now?

Each gland is linked to its target, so the hormone is sent straight there and nowhere else.The hormone reaches every organ, so every organ responds — the target just responds the most.The blood carries it everywhere, but only cells with the right receptor respond to it.

How sure are you?

I'm sureI think soI'm guessing

Each gland is linked to its target, so the hormone is sent straight there and nowhere else.

This is a common misconception

It's a sensible guess — but there's no private delivery route. The gland releases its hormone into the bloodstream, and the blood carries it all round the body, past organ after organ. So the targeting can't happen on the journey. It happens when the hormone arrives.

The hormone reaches every organ, so every organ responds — the target just responds the most.

This is a common misconception

You've got the first half right: the blood really does carry the hormone all round the body. But reaching an organ isn't the same as affecting it. An organ responds only if its cells carry the matching receptor. No matching receptor means no effect at all — however much hormone flows past.

The blood carries it everywhere, but only cells with the right receptor respond to it.

This matches the evidence

Spot on. The cells of the target organ carry specific receptors on their surface, and the hormone binds to them. Organs whose cells don't have that matching receptor simply aren't affected. The route isn't targeted — the receptor is.

One message, sent to everyone — only the cells with the matching receptor can read it.

The master gland

Why the pituitary is the boss

Start at the pituitary gland. Draw arrows to show which gland releases which hormone, and what each hormone acts on. Then compare your web with ours.

Pituitary gland
TSH (thyroid-stimulating hormone)
Thyroid gland
Thyroxine
FSH (follicle-stimulating hormone)
Ovaries
Oestrogen

Add a link

This factor……made this happen or worse

No links yet.

Predict, then check

Puberty and the menopause both bring big changes to the body. Both are linked to two hormones: oestrogen and progesterone.

What do you think happens to the levels of oestrogen and progesterone at puberty, and then at the menopause?

Two control systems

Nerves or hormones?

Your body has two systems for controlling what goes on inside it. Tick where each feature belongs: the nervous system, the endocrine system, both — or neither.

A Nervous system
B Endocrine system

Sends electrical impulses along nerves
Glands secrete hormones into the blood
Acts quickly
Slower to act
Response is short-lived
Effects last much longer
Helps regulate conditions in the body
Works together with the other system
Carries hormones along nerves

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

The endocrine system is made of glands that secrete hormones directly into the bloodstream to control and regulate changes and conditions in the body.
A hormone is a chemical produced and secreted by a gland, carried in the blood plasma to a target organ, where it brings about an effect.
Where the six glands are, and the hormone each one releases.
Why a hormone only affects its target organ — the receptor, not the route.
Why the pituitary is the master gland, and how the endocrine system compares with the nervous system.

The big picture

The endocrine system is a set of glands that secrete hormones straight into the bloodstream to control and regulate changes and conditions in the body. A hormone is carried in the blood all round the body, but it only affects a target organ whose cells carry the matching receptor. The pituitary is the master gland because its hormones act on other glands. Compared with nerves, hormones are slower to act but their effects last much longer — and hormone levels change throughout life.

Key points

1Pituitary (brain): its hormones act on other glands. Thyroid (neck): thyroxine, which controls metabolic rate. Adrenal glands (above the kidneys): adrenaline, for fight or flight.

2Pancreas: insulin and glucagon, which regulate blood glucose concentration — and it is an organ with other functions too.

3Ovaries: oestrogen, which controls female secondary sex characteristics and has a role in the menstrual cycle. Testes: testosterone; they control male secondary sex characteristics and sperm production.

4Hormones reach every organ in the blood, but only target cells with specific receptors that the hormone binds to respond.

5The pituitary is the master gland: TSH stimulates the thyroid to secrete thyroxine, and FSH acts on the ovaries, indirectly raising oestrogen.

6Nervous system: fast, electrical impulses, short-lived. Endocrine system: slower, hormones in the blood, much longer lasting. They work together.

7Hormone levels change through life: oestrogen and progesterone rise at puberty and fall at the menopause.

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Worked example

Problem

TSH is carried all round the body in the blood, yet it affects the thyroid gland. Explain how TSH brings about its effect, and what happens as a result.

⚠ Watch out

Mixing up where the 'stimulating' hormones come from. TSH and FSH are both made by the pituitary gland — TSH is not made by the thyroid, and FSH is not made by the ovaries. They are named after the gland they act on.

🧠

Memory hook

Think of a hormone as a message broadcast to the whole body in the blood — only cells with the matching receptor can read it. And the pituitary is the head teacher, telling the other glands when to send their messages.

✓

Check yourself

Cover the diagram. From head to hips, name each gland, where it is and the hormone it releases. Then explain in one sentence why a hormone only affects its target organ.

Flashcards

(14)

What is the endocrine system?

A system of glands that secrete hormones directly into the bloodstream, to control and regulate changes and conditions in the body.

What is a hormone?

A chemical produced and secreted by a gland, carried in the blood plasma to a target organ, where it brings about an effect.

What is a gland?

An organ or tissue that produces and secretes substances such as hormones.

Why does a hormone only affect its target organ?

Only cells of the target organ carry specific receptors on their surface for that hormone to bind to. Other organs lack them, so they are not affected.

Thyroid gland: where, which hormone, and what does it control?

In the neck. It secretes thyroxine, which controls the body's metabolic rate.

Adrenal glands: where, which hormone, and what does it do?

Above the kidneys. They secrete adrenaline, which prepares the body for 'fight or flight' in stressful situations.

Which hormones does the pancreas secrete, and what do they regulate?

Insulin and glucagon, which regulate blood glucose concentration. The pancreas is also an organ with other functions.

Ovaries: which hormone, and what does it do?

Oestrogen. It controls the development of female secondary sex characteristics and has a role in the menstrual cycle.

Testes: which hormone, and what do they control?

Testosterone. The testes control the development of male secondary sex characteristics and the production of sperm.

Where is the pituitary gland, and why is it called the 'master gland'?

In the brain. It secretes several hormones that act on other glands, stimulating them to secrete further hormones.

What does TSH do, and where is it made?

Thyroid-stimulating hormone is made by the pituitary gland. It stimulates the thyroid gland to secrete thyroxine.

What does FSH do, and where is it made?

Follicle-stimulating hormone is made by the pituitary gland. It acts on the ovaries and indirectly causes oestrogen levels to rise.

Nervous system vs endocrine system: speed and duration?

Nervous: electrical impulses along nerves — fast, but short-lived. Endocrine: hormones in the blood — slower, but much longer lasting.

How do oestrogen and progesterone levels change through life?

They rise at puberty and fall at the menopause — so body changes can come from a hormone level rising or falling.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How does a hormone find its target?

Why the pituitary is the boss

Nerves or hormones?

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