Coaching Centre
Hormones and the endocrine system
What hormones are and how they reach target cells, the main endocrine glands and their hormones, the fight-or-flight response, how the nervous and endocrine systems compare and work together, and what happens when glands go wrong, including diabetes and goitre.
You step on a stick in the bush and for a split second it looks like a snake. You jump back before you know why. That first jolt is your nervous system. But your heart keeps pounding and your hands keep shaking for minutes after you realise it was only a stick. That lingering part is a hormone, and it is still circulating in your blood.
What hormones are
The endocrine system is made of glands that release chemical messengers called hormones straight into the blood. The blood carries a hormone all around the body, so in a sense every cell gets the message. But only target cells respond, because only they have the right receptor proteins that the hormone fits into, like a key into a lock.
Hormones control things that need to happen over minutes, days or years: growth, puberty, the menstrual cycle, how fast cells use energy, and the amount of sugar and water in the blood. A few, like adrenaline, act within seconds.
Not all glands are endocrine glands. Sweat glands and salivary glands release their products through tubes called ducts onto a surface, and they are called exocrine glands. Endocrine glands have no ducts; their hormones go into the blood.
The main endocrine glands
| Gland | Hormone(s) | Main effects |
|---|---|---|
| Hypothalamus (in the brain) | Releasing hormones; makes ADH | Controls the pituitary gland. This is where the nervous and endocrine systems meet. |
| Pituitary gland | Growth hormone; ADH; hormones that control other glands | Growth of bones and muscles. ADH makes the kidneys reabsorb more water, so you make less, darker urine. Often called the "master gland" because it controls the thyroid, adrenal glands, ovaries and testes. |
| Pineal gland | Melatonin | Released when it gets dark; makes you sleepy and keeps your daily body clock in time |
| Thyroid gland (in the neck) | Thyroxine | Controls metabolism, the rate cells use energy; needed for normal growth and brain development |
| Adrenal glands (on the kidneys) | Adrenaline; cortisol | Adrenaline prepares the body for fight or flight. Cortisol helps the body cope with longer-term stress. |
| Pancreas | Insulin; glucagon | Insulin lowers blood glucose; glucagon raises it |
| Ovaries | Oestrogen; progesterone | Female puberty, the menstrual cycle and pregnancy |
| Testes | Testosterone | Male puberty and sperm production |
Fight or flight
When you get a fright, your nervous and endocrine systems act as a team.
- Your senses detect the danger and the brain, including the hypothalamus, recognises it.
- Within a fraction of a second, sympathetic nerves (4A) speed up the heart and breathing.
- Sympathetic nerves also signal the inner part of the adrenal glands to release adrenaline into the blood. It reaches target cells all over the body within seconds.
- Adrenaline keeps the heart beating hard and fast, makes the liver release glucose into the blood, widens the airways and pupils, sends more blood to the muscles and less to the gut and skin.
The result is more glucose and oxygen reaching your muscles, so they can release more energy to fight or run. Your mouth goes dry and you may feel "butterflies" because digestion has been put on hold. Once the danger is gone, the parasympathetic nerves slow things down, but the adrenaline takes a few minutes to be broken down, so the effects fade gradually.
- With nerves and hormone working, press the snake button. Describe the shape of the heart rate graph over the next 15 minutes.
- Switch to "Nerves only" and try again. Which part of the response is missing? What does that tell you about adrenaline?
- A person with a transplanted heart has no nerves connected to it. Try "Hormone only". How does their heart rate respond, and why is it slower to rise?
- Look at the blood glucose line. Why is extra glucose useful when you're in danger?
Nerves and hormones compared
| Nervous system | Endocrine system | |
|---|---|---|
| Type of message | Electrical impulse (chemical at synapses) | Chemical (hormone) |
| Carried by | Neurons | Blood |
| Speed | Very fast, milliseconds | Slower, seconds to hours |
| How long the effect lasts | Short, stops when impulses stop | Longer, until the hormone is broken down |
| Where it acts | Exact place: one muscle or gland | Spread out: any target cells with receptors |
| Examples | Reflexes, walking, speaking | Growth, puberty, blood glucose, water balance |
The two systems are linked through the hypothalamus. It is part of the brain, with neurons receiving messages from all over the body, but it also controls the pituitary gland, which controls many of the other glands.
When glands go wrong
Diabetes. In Year 8 you saw how insulin and glucagon keep blood glucose steady and how diabetes is treated. In type 1 diabetes, the immune system destroys the insulin-making cells in the pancreas, usually in childhood, so the person must inject insulin or wear a pump. In type 2 diabetes, the body still makes insulin but cells stop responding to it properly. Type 2 makes up most cases and is linked to genes, being overweight and inactivity, so it can often be delayed or prevented by diet and exercise.
Goitre. The thyroid needs iodine to make thyroxine. Without enough, the gland swells into a lump in the neck called a goitre, and people feel tired and cold. Iodine deficiency during pregnancy can affect a baby's brain development. Many Australian soils are low in iodine, and in the early 2000s surveys found that mild iodine deficiency had come back in parts of Australia. Since 2009, bakers in Australia have had to use iodised salt in most bread.
Too much or too little growth hormone in childhood can make a person unusually tall or short. Children with too little can now be treated with growth hormone injections.
Test yourself
Why does adrenaline affect the heart but not the bones, even though it reaches both?
Only target cells with matching receptors respond.
Which gland is often called the "master gland"?
The pituitary releases hormones that control several other glands.
Compared with a nerve impulse, a hormone's effect is usually
Hormones take time to travel in blood and to be broken down.
On a hot day without a drink, more ADH is released. What happens?
ADH helps the body save water.
A goitre is caused by a lack of
The thyroid needs iodine to make thyroxine.
Label the endocrine glands.
Which system does each description fit?
Hormones are made by glands. The pancreas releases to lower blood glucose. The thyroid makes , which controls metabolism. The pineal gland releases to help you sleep. The adrenal glands release in an emergency.
Explain how the nervous and endocrine systems work together when a person is frightened, and why their heart keeps racing after the danger has passed.
The eyes detect the danger and sensory neurons carry the message to the brain, where the hypothalamus triggers the sympathetic nervous system. Within a fraction of a second, sympathetic nerves speed up the heart and breathing. They also stimulate the adrenal glands to release adrenaline into the blood. Adrenaline reaches target cells all over the body: it keeps the heart beating fast and strongly, makes the liver release glucose, widens the airways and diverts blood from the gut to the muscles. Together this delivers more glucose and oxygen to the muscles so they can release energy to fight or run. When the danger passes, nerve impulses stop almost at once, but the adrenaline already in the blood takes several minutes to be broken down, so the heart keeps racing for a while.