Response and Coordination
Senior 3 · Biology
Response and Coordination
Lesson 4 of 4
25 min
You are walking home after dark when a dog barks right behind a fence. In a fraction of a second you jump: that was your nervous system. But a minute later, long after you have seen that the dog is tied up, your heart is still pounding, you are breathing fast and your hands are shaking. Something is still acting on your body, and it is not travelling along neurones. It is a hormone, adrenaline, carried in your blood.
Definition
Hormone: a chemical messenger made by an endocrine gland, carried in the blood, that changes the activity of one or more target organs. Example: insulin, made in the pancreas, acts on the liver and muscles.
Endocrine gland: a gland with no duct, which releases its hormones directly into the blood flowing through it. Example: the thyroid gland.
Exocrine gland: a gland that releases its secretion through a duct. Example: the salivary glands. The pancreas is both: it sends digestive enzymes along the pancreatic duct (Unit 6) and releases hormones into the blood.
Target organ: an organ whose cells have receptors for a particular hormone, so that they respond to it.
The endocrine glands together make up the endocrine system. Because hormones travel in the blood, they reach every part of the body, but only cells with the right receptors respond. Figure 1 shows where the main glands are.
Figure 1: positions only; not to scale. The kidneys are drawn dashed only to show where the adrenal glands sit.
| Gland | Hormone | Main target | Main effect |
|---|---|---|---|
| Pituitary | Antidiuretic hormone (ADH) | Kidneys (collecting ducts) | More water is reabsorbed into the blood, so less, more concentrated urine is made |
| Pituitary | Growth hormone | Bones and muscles | Growth in childhood and adolescence |
| Pituitary | FSH and LH | Ovaries and testes | Control the making of eggs, sperm and sex hormones (Unit 12) |
| Thyroid | Thyroxine | Most body cells | Controls the metabolic rate and normal growth and development; it contains iodine |
| Adrenal glands | Adrenaline | Heart, liver, lungs, blood vessels, eyes | Prepares the body for "fight or flight" |
| Pancreas | Insulin | Liver and muscles | Lowers blood glucose |
| Pancreas | Glucagon | Liver | Raises blood glucose |
| Ovaries | Oestrogen and progesterone | Uterus and other organs | Female secondary sexual characteristics; the menstrual cycle (Unit 12) |
| Testes | Testosterone | Many organs | Male secondary sexual characteristics; sperm production (Unit 12) |
The pituitary is often called the "master gland" because several of its hormones control other glands. It is itself controlled by the hypothalamus just above it. ADH is in fact made in the hypothalamus and stored in and released from the pituitary.
In fear, anger or excitement, nerve impulses make the adrenal glands release adrenaline. Within seconds it:
All of these supply the muscles with more glucose and oxygen, ready for action.
After a meal of ubugari or rice, glucose is absorbed and blood glucose rises. Cells in the islets of Langerhans in the pancreas detect this and release insulin, which makes the liver and muscles take up glucose and store it as glycogen. When blood glucose falls, for example between meals or during a long run, other islet cells release glucagon, which makes the liver break glycogen down into glucose. Two hormones with opposite effects hold blood glucose within a narrow range. ADH does the same job for the water content of the blood. Unit 10 shows how each of these works as a feedback loop, and what goes wrong in diabetes.
Worked example 1: nerves or hormone?
Look again at the barking dog. Sort each response into nervous or hormonal, and explain the timing.
| Response | Mainly | Reason |
|---|---|---|
| Jumping within half a second | Nervous | Impulses along neurones reach the leg muscles in milliseconds |
| Heart still racing a minute later | Hormonal (adrenaline) | Adrenaline stays in the blood and keeps acting until the liver breaks it down |
| Fast breathing for several minutes | Hormonal (adrenaline) | Same reason: a long-lasting chemical message |
Conclusion: the nervous system gives the fast first response; adrenaline keeps the body ready for action for longer. The two systems work together.
Worked example 2: drinking a litre of water
A student empties her bladder, drinks 1 litre (1000 cm³) of water, and collects her urine every 30 minutes (illustrative data):
| Time after drinking (min) | 0–30 | 30–60 | 60–90 | 90–120 | 120–150 | 150–180 |
|---|---|---|---|---|---|---|
| Urine volume (cm³) | 60 | 240 | 320 | 180 | 80 | 60 |
| Feature | Nervous system | Endocrine (hormonal) system |
|---|---|---|
| Form of the message | Electrical impulses along neurones (chemical only across synapses) | Chemical: hormones |
| Carried by | Neurones | The blood |
| Speed | Very fast: milliseconds | Slower: seconds to hours |
| How long the effect lasts | Usually short; stops when the impulses stop | Usually longer; until the hormone is broken down |
| Where it acts | Precisely, on particular muscles or glands | Widely, on every cell that has receptors for the hormone |
| Examples | Reflexes, walking, speaking | Growth, puberty, blood glucose, water balance |
Warning
Common mistakes. (1) Writing that hormones are carried "in the nerves" or travel to one organ only: they are carried in the blood to the whole body, and only target cells respond. (2) Mixing up glucagon (a hormone) and glycogen (the stored carbohydrate): glucagon makes the liver break down glycogen. (3) Saying insulin "turns glucose into energy": it makes cells take up glucose and makes the liver and muscles store it as glycogen. (4) Writing that ADH makes more urine: ADH makes the kidneys keep water, so less urine is made.
What Are Hormones, FuseSchool. Watch for the idea of a target organ and for the examples of glands; compare them with Figure 1 and the table.
Tip
Memory hook: "glucaGONe". Glucagon is released when glucose is gone (low), and it raises it again. ADH: "Anti-Diuretic" means "against making lots of urine".
Practice: hormones
Key points
Further reading
Sources and credits
Written by Ganzaa: the text, Figure 1, the tables, the worked examples (Worked example 2 uses illustrative data) and the practice questions. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
