Hormones and homeostasis
Chemical coordination, blood glucose control and negative feedback.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 14.2.1Define a hormone and compare nervous and hormonal control.
- 14.2.2Explain homeostasis, including the control of blood glucose by insulin.Supplement
6 minute read
Hormones and homeostasis
A hormone is a chemical substance produced by a gland, carried by the blood, which alters the activity of one or more specific target organs.
Nervous versus hormonal control
| Nervous | Hormonal | |
|---|---|---|
| Transmitted by | electrical impulses along neurones | chemicals in the blood |
| Speed | very fast | slower |
| Duration | short-lived | longer-lasting |
| Target | precise, localised | widespread target organs |
Adrenaline illustrates the hormonal route: released from the adrenal glands when frightened, it increases heart rate and breathing rate and diverts blood to the muscles — the "fight or flight" response.
Homeostasis
Homeostasis is the maintenance of a constant internal environment. Body temperature, blood glucose and water content are all held within narrow limits, because enzymes only work properly within them.
Negative feedback
The control mechanism always follows the same loop: a change away from the norm is detected, a response is triggered that counteracts the change, and conditions return to normal — at which point the response stops.
Blood glucose control
- Blood glucose too high (after a meal) → the pancreas releases insulin → the liver converts excess glucose into glycogen for storage → blood glucose falls.
- Blood glucose too low → the pancreas releases glucagon → the liver converts glycogen back into glucose → blood glucose rises.
In type 1 diabetes the pancreas produces insufficient insulin, so blood glucose can rise dangerously high. It is treated with insulin injections and by managing diet.
Think of it like this
Negative feedback is a thermostat. It does not aim to make the room a particular temperature and stop caring — it constantly notices deviation and pushes back the other way, which is why the temperature stays steady rather than drifting.
Worked examples
Method, step by step
Explain how blood glucose concentration is brought back to normal after a meal high in carbohydrate.
- 1Digestion produces glucose, which is absorbed into the blood, raising blood glucose above normal.
- 2The pancreas detects the rise and secretes insulin into the blood.
- 3Insulin travels to the liver, which converts excess glucose into glycogen for storage.
- 4Blood glucose concentration falls back to normal, and insulin secretion decreases.
The pancreas detects the rise and releases insulin, which causes the liver to convert excess glucose into glycogen, lowering blood glucose back to normal — an example of negative feedback.
Common misconceptions
- Thinking homeostasis means nothing changes. Conditions fluctuate constantly; homeostasis keeps them oscillating within narrow limits.
- Confusing glycogen (animal storage carbohydrate in the liver) with glucagon (the hormone) — the names differ by one letter and cost marks every year.
- Believing insulin "removes" glucose. It causes the liver to convert glucose to glycogen for storage.
In the exam
- Blood glucose answers should name three things: the stimulus (high or low glucose), the hormone, and the effect on the liver.
- When comparing nervous and hormonal control, give speed and duration — they are the two contrasts marked most often.