Chapter 4 of 8

All 8 chapters
  1. 1The nervous system, readunderstood
  2. 2Reflexes, readunderstood
  3. 3The eye, readunderstood
  4. 4Hormones, readunderstood
  5. 5Blood sugar and diabetes, readunderstood
  6. 6Temperature and water balance, readunderstood
  7. 7Hormones and the menstrual cycle, readunderstood
  8. 8Plant responses, readunderstood

S4.5.4 Chapter 4

Hormones

Your heart pounds when you get a fright, and you still feel shaky a minute later. What is carrying that message?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Compare nerves and hormones by speed and how long effects last
  • Name the main glands and the hormones they make

Nerves are fast, but their messages are short. Many jobs in the body need a slower message that lasts longer. Growing, changing in puberty and keeping the blood sugar steady are examples. The body sends these messages with hormones.

A hormone is a chemical messenger. It is made by a gland, an organ that makes chemicals and releases them. The hormone goes into the blood and is carried all round the body. It acts on a target organ, an organ that has receptor sites for it.

Four boxes joined by arrows that point down. The boxes are labelled gland makes a hormone, hormone enters the blood, blood carries it round the body and target organ responds.gland makes a hormonehormone enters the bloodblood carries it round thebodytarget organ responds
The path of a hormone. Unlike a nerve impulse, it travels in the blood.

The blood reaches every organ, but a hormone acts only on its target organs. Only the cells of a target organ have receptor sites that fit that hormone, as a key fits one lock. These are tiny parts of the cell's surface. The other cells do not react to it.

Nerves and hormones do different jobs. An impulse goes along a neurone to one place. It acts in a fraction of a second and soon stops. A hormone must be made, released and carried in the blood. It takes longer to arrive, but its effect can last for minutes, hours or years.

A table with the columns Feature, Nerves and Hormones. Signal: impulse for nerves, chemical for hormones. Route: neurones for nerves, blood for hormones. Speed: fast for nerves, slower for hormones. Effect: short for nerves, long for hormones.FeatureNervesHormonessignalimpulsechemicalrouteneuronesbloodspeedfastslowereffectshortlong
Nerves and hormones compared. The body needs both.

A gland that sends its hormone into the blood is called an endocrine gland. The body has several. Look at where they lie. The first picture is a female body and the second is a male body. The glands in the neck and the trunk are the same in both. Only the last gland differs.

The head and body of a female from the front. The labels are pituitary, thyroid, adrenal glands, pancreas, kidneys and ovaries. The pituitary is under the brain, the thyroid is in the neck, one adrenal gland sits on top of each kidney, the pancreas lies across the middle of the trunk and the ovaries are low in the trunk. The pancreas is drawn in front of the right-hand kidney and adrenal gland, so their outlines overlap.thyroidadrenalglandskidneysovariespituitarypancreas
The glands in a female body. The ovaries are low in the trunk. The pancreas lies in front of the kidney and adrenal gland on one side, so their outlines overlap in the drawing.
The head and body of a male from the front. The labels are pituitary, thyroid, adrenal glands, pancreas, kidneys and testes. The pituitary is under the brain, the thyroid is in the neck, one adrenal gland sits on top of each kidney, the pancreas lies across the middle of the trunk and the testes hang below the trunk. The pancreas is drawn in front of the right-hand kidney and adrenal gland, so their outlines overlap.thyroidadrenalglandskidneystestespituitarypancreas
The glands in a male body. The testes hang outside the trunk. The other glands are in the same places. The pancreas again overlaps the kidney and adrenal gland on one side.

The pituitary gland is a small gland under the brain. It releases several hormones. Growth hormone makes children grow. ADH controls the water in the body. FSH and LH act on the ovaries or the testes. Some of its hormones make other glands work, so it controls other glands.

The thyroid gland is in the neck. It makes thyroxine, which sets the rate of the chemical reactions in the body. The thyroid needs iodine to make it. Too little iodine in food can make the thyroid swell.

The two adrenal glands lie on top of the kidneys. They make adrenaline. The pancreas lies behind the stomach and makes insulin and glucagon, which control blood sugar. You will study them in the next chapter.

The ovaries make oestrogen and progesterone. The testes make testosterone. These hormones bring about the changes of puberty and control the making of eggs and sperm. Puberty starts when the pituitary releases more FSH and LH.

A table with the columns Gland and Hormone. Pituitary: growth hormone, ADH, and FSH and LH. Thyroid: thyroxine. Adrenal glands: adrenaline. Pancreas: insulin and glucagon. Ovaries: oestrogen and progesterone. Testes: testosterone.GlandHormonepituitarygrowth hormonepituitaryADHpituitaryFSH and LHthyroidthyroxineadrenaladrenalinepancreasinsulinpancreasglucagonovariesoestrogenovariesprogesteronetestestestosterone
The main glands and some of the hormones they release. The adrenal row stands for the adrenal glands.

Adrenaline shows nerves and hormones working together. You see a danger. Nerves quickly tell the adrenal glands to release adrenaline. The blood carries it round the body. Your heart beats faster, your breathing quickens, and your liver releases glucose, so that your muscles get more energy and oxygen. Your pupils widen.

Four boxes joined by arrows that point down. The boxes are labelled danger is seen, nerves tell the adrenal glands, adrenaline enters the blood and heart and breathing speed up.danger is seennerves tell the adrenalglandsadrenaline enters the bloodheart and breathing speed up
A fright. Nerves start the response, and a hormone keeps it going. You may feel shaky for a few minutes after the danger has passed. Adrenaline is still in the blood.

How much slower is a hormone? Here are two round numbers. A nerve impulse takes about 20 ms to cross the body. Blood takes about 60 s to make a full trip round it. A hormone must make that trip, so it needs about that time to reach every part.

Worked example

Suppose an impulse takes 20 ms to cross the body, and a hormone takes 60 s. How many times longer does the hormone take?

  1. Put both times in the same unit. 60 s = 60 × 1 000 = 60 000 ms.
  2. Divide the longer time by the shorter time: 60 000 ÷ 20 = 3 000.

Answer: 3 000 times

Try it

Two ways to send a message

You need: at least five people, standing in a line, a clock or phone timer showing seconds, a small piece of paper, paper and a pencil to record times.

Be safe: Walk, never run. Keep the floor clear of bags and shoes.

  1. Stand in a line, with a gap of about one arm between people. The first person is the receptor. The last person is the target organ.
  2. Nerve message. The first person squeezes the hand of the next person. Each person squeezes the next as soon as they feel a squeeze. Time how long the squeeze takes to reach the last person.
  3. Hormone message. Write a message on the piece of paper. The first person walks it along the line and hands it to the last person. Time how long it takes.
  4. Repeat each way three times and write down the times. Compare them.

What you will see: The squeeze message is fast, but it is passed from one person to the next, like an impulse along neurones. The walking message is slower. It is carried by one person and goes to the target, like a hormone in the blood. The times in the picture are an example. Yours will differ.

If you cannot try it, this is what happens.
A table with the columns Message and Time. Squeeze: 2 s. Walk with paper: 7 s. These are example times.MessageTimesqueeze2 swalk with paper7 s

A wrong idea many people have

Hormones are only about puberty and having babies.

Why it feels right: Hormones are often talked about with puberty, so it seems that is all they do.

The test that breaks it: Look at the table of glands. Adrenaline prepares you for danger. Insulin and glucagon control blood sugar. ADH controls water in the body. Thyroxine sets the rate of chemical reactions. None of these is about puberty.

What is true: Hormones control many things in every body, at every stage of life. The hormones of puberty are only some of them.

Key words

hormone
a chemical messenger that is made by a gland and carried in the blood.
gland
an organ that makes a chemical and releases it.
target organ
an organ with receptor sites for a hormone, which responds to it.
pituitary gland
a small gland under the brain that releases hormones, some of which control other glands.
thyroid gland
a gland in the neck that makes thyroxine.
adrenal glands
two glands on top of the kidneys that make adrenaline.
adrenaline
a hormone that gets the body ready for danger.
pancreas
a gland behind the stomach that makes insulin and glucagon.

Back to your guess

The answer is A: A chemical in the blood, which reaches the heart and other organs.

Nerves start the response very quickly. But a gland also releases adrenaline into the blood. The blood carries it to the heart and to other organs, and it keeps working for a few minutes.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. How does a hormone get from a gland to its target organ?

    Tap your answer.

    1. Yes, that is right. A gland releases the hormone into the blood. The blood carries it all round the body to the target organs.

    2. Not this one. Nerve impulses run along neurones. A hormone is a chemical, and the blood carries it. Go to S4.5.1. Try another.

    3. Not this one. A chemical crosses a synapse between neurones. A hormone goes in the blood, far beyond a synapse. Try another.

    See the answer

    It is carried in the blood.

    A gland releases the hormone into the blood. The blood carries it all round the body to the target organs.

    • If you chose B: Nerve impulses run along neurones. A hormone is a chemical, and the blood carries it. Go to S4.5.1.
    • If you chose C: A chemical crosses a synapse between neurones. A hormone goes in the blood, far beyond a synapse.

  2. Which is true of hormones, compared with nerve impulses?

    Tap your answer.

    1. Not this one. It is the other way round. A hormone has to be carried in the blood, so it is slower. Try another.

    2. Not this one. Nerve impulses act in a fraction of a second. Hormones are slower, and last far longer. Try another.

    3. Yes, that is right. A hormone has to be made, released and carried in the blood, so it arrives later. Its effect can last for minutes, hours or years.

    See the answer

    They are slower, and their effects last longer.

    A hormone has to be made, released and carried in the blood, so it arrives later. Its effect can last for minutes, hours or years.

    • If you chose A: It is the other way round. A hormone has to be carried in the blood, so it is slower.
    • If you chose B: Nerve impulses act in a fraction of a second. Hormones are slower, and last far longer.

  3. Blood carries a hormone to every organ, yet only some respond. Why?

    Tap your answer.

    1. Not this one. Blood reaches every organ. The target organs are the ones that respond. Try another.

    2. Yes, that is right. A hormone fits only the receptor sites of its target cells, as a key fits one lock. The cells of other organs have no such sites.

    3. Not this one. The hormone is not destroyed in other organs. They simply have no receptor sites for it, so they do not respond. Try another.

    See the answer

    Only the target organs have receptor sites that fit the hormone.

    A hormone fits only the receptor sites of its target cells, as a key fits one lock. The cells of other organs have no such sites.

    • If you chose A: Blood reaches every organ. The target organs are the ones that respond.
    • If you chose C: The hormone is not destroyed in other organs. They simply have no receptor sites for it, so they do not respond.

  4. Which gland releases adrenaline?

    Tap your answer.

    1. Not this one. The thyroid makes thyroxine. It sets the rate of the chemical reactions in the body. Try another.

    2. Not this one. The pituitary releases growth hormone, ADH, FSH and LH and others. It does not release adrenaline. Try another.

    3. Not this one. The pancreas makes insulin and glucagon. They control blood sugar. Try another.

    4. Yes, that is right. The two adrenal glands lie on top of the kidneys. They release adrenaline, which gets the body ready for danger.

    See the answer

    The adrenal glands

    The two adrenal glands lie on top of the kidneys. They release adrenaline, which gets the body ready for danger.

    • If you chose A: The thyroid makes thyroxine. It sets the rate of the chemical reactions in the body.
    • If you chose B: The pituitary releases growth hormone, ADH, FSH and LH and others. It does not release adrenaline.
    • If you chose C: The pancreas makes insulin and glucagon. They control blood sugar.

  5. Which gland releases hormones that control other glands?

    Tap your answer.

    1. Yes, that is right. The pituitary lies under the brain. Some of its hormones, such as FSH and LH, make other glands release their own hormones.

    2. Not this one. The thyroid makes thyroxine. It is controlled by a hormone from the pituitary. Try another.

    3. Not this one. The pancreas makes insulin and glucagon. Insulin and glucagon act mainly on the liver and muscles. They do not make other glands release hormones. Try another.

    See the answer

    The pituitary gland

    The pituitary lies under the brain. Some of its hormones, such as FSH and LH, make other glands release their own hormones.

    • If you chose B: The thyroid makes thyroxine. It is controlled by a hormone from the pituitary.
    • If you chose C: The pancreas makes insulin and glucagon. Insulin and glucagon act mainly on the liver and muscles. They do not make other glands release hormones.

  6. What does the hormone thyroxine control?

    Tap your answer.

    1. Not this one. Insulin and glucagon from the pancreas control blood sugar. Try another.

    2. Yes, that is right. Thyroxine from the thyroid sets how fast the body's chemical reactions go.

    3. Not this one. ADH from the pituitary controls the water in urine. Try another.

    See the answer

    The rate of the chemical reactions in the body

    Thyroxine from the thyroid sets how fast the body's chemical reactions go.

    • If you chose A: Insulin and glucagon from the pancreas control blood sugar.
    • If you chose C: ADH from the pituitary controls the water in urine.

  7. From an earlier chapter

    Which kind of neurone carries impulses from the brain or spinal cord to a muscle or a gland?

    Tap your answer.

    1. Not this one. A sensory neurone carries impulses in, from a receptor. Go to S4.5.1. Try another.

    2. Not this one. A relay neurone is short and stays inside the brain or spinal cord. Try another.

    3. Yes, that is right. A motor neurone carries impulses out from the central nervous system to an effector, which is a muscle or a gland.

    See the answer

    A motor neurone

    A motor neurone carries impulses out from the central nervous system to an effector, which is a muscle or a gland.

    • If you chose A: A sensory neurone carries impulses in, from a receptor. Go to S4.5.1.
    • If you chose B: A relay neurone is short and stays inside the brain or spinal cord.

  8. Suppose a nerve impulse takes 20 ms and a hormone takes 60 s. How many times longer does the hormone take?

    See the answer

    3 000 times

    60 s is 60 000 ms. 60 000 ÷ 20 = 3 000.

  9. A nerve impulse takes 40 ms to reach a muscle. How many seconds is that?

    See the answer

    0.04 s

    One second is 1 000 ms. 40 ÷ 1 000 = 0.04 s.

Keep it

Copy this into your exercise book.

Hormones are chemicals made by glands, carried in the blood to target organs. Slower than nerves, but they last longer. Pituitary controls other glands. Thyroid: thyroxine. Adrenal glands: adrenaline. Pancreas: insulin and glucagon.

Draw: A body outline with the pituitary, thyroid, adrenal glands and pancreas marked and named.

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