HormonesCambridge IGCSE Biology: Revision notes
Section 1
What is a hormone?
A hormone is a chemical substance, produced by a gland, and carried by the blood, which alters the activity of one or more specific target organs.
Hormones are part of the body's chemical (endocrine) control system, working alongside — and differently from — the nervous system.
Section 2
Key endocrine glands and their hormones
Several glands secrete specific hormones:
| Gland | Hormone |
|---|---|
| Adrenal glands | Adrenaline |
| Pancreas | Insulin (and glucagon) |
| Testes | Testosterone |
| Ovaries | Oestrogen |
Each hormone travels in the blood to its target organ(s), where it produces a specific effect.
Don't confuse the pancreas's hormonal role (insulin/glucagon) with its separate role producing digestive enzymes — both happen in the same organ but are different functions.
Section 3
Adrenaline: the 'fight or flight' hormone
Adrenaline is secreted by the adrenal glands in stressful or dangerous ('fight or flight') situations. It prepares the body for rapid action by causing:
- Increased breathing rate
- Increased heart rate
- Increased pupil diameter
- Increased blood glucose concentration (metabolic activity), giving muscles more available energy
Think of adrenaline as the body's own emergency alarm system — it primes every system needed to run or fight, all at once, within seconds.
Section 4
Insulin and glucagon: controlling blood glucose
The pancreas monitors and controls blood glucose concentration using two antagonistic hormones:
- Insulin — secreted when blood glucose is too high; decreases blood glucose concentration by promoting uptake and storage
- Glucagon — secreted when blood glucose is too low; increases blood glucose concentration
The liver is the main organ acting on these hormone signals to raise or lower glucose levels, forming a negative feedback loop that keeps blood glucose close to a set point.
In Type 1 diabetes, the pancreas does not produce enough insulin. It is treated by injecting insulin (matched to diet and activity) to keep blood glucose within a safe range.
Remember the direction each way: Insulin In-creases storage and so decreases blood glucose; glucagon raises it back up when it is low.
Section 5
Comparing nervous and hormonal control
Hormones and nerves both coordinate responses in the body, but they differ importantly:
| Feature | Nervous control | Hormonal control |
|---|---|---|
| Speed | Very fast | Slower |
| Duration of effect | Short-lived | Longer-lasting |
| How it travels | Electrical impulses along neurones | Chemicals in the blood |
This is why a reflex (nervous) happens almost instantly, whereas a hormonal effect like the response to stress builds up and fades more gradually.
Must Know
- A hormone is a chemical made by a gland, carried in the blood, that acts on a target organ
- Adrenal glands → adrenaline; pancreas → insulin and glucagon; testes → testosterone; ovaries → oestrogen
- Adrenaline causes 'fight or flight' effects: increased breathing rate, heart rate, pupil diameter and blood glucose
- Insulin decreases blood glucose; glucagon increases it; the liver acts on both signals
- Type 1 diabetes is a lack of insulin, treated with insulin injections
- Hormonal control is slower and longer-lasting than fast, short-lived nervous control
That's the notes covered.
Carry on to the next subtopic.