Topic 14.3 · Core + Supplement
Hormones and Endocrine Glands
Alongside the nervous system, the body has a second coordination system built on chemistry rather than electricity. Instead of an impulse travelling along a neurone, a hormone travels dissolved in the blood — and that single difference in transport is what explains almost every other difference between the two systems.
What a hormone is
A hormone is a chemical substance, produced by a gland, carried by the blood, that alters the activity of one or more specific target organs.Every part of that definition earns its place: chemical substance, produced by a gland, carried by the blood, acting on specific target organs. A hormone is not an electrical impulse and does not travel along a neurone, however quickly its effects might appear — the fight-or-flight response below can feel fast, but the underlying transport is still a chemical travelling in the bloodstream, not a nerve signal. It’s also worth resisting the idea that a gland aims a hormone at one single named organ — every target organ with the matching receptors can respond, wherever the blood happens to carry the hormone.
Endocrine glands and their hormones
For this syllabus, four gland-hormone pairings are required, and Cambridge expects them matched precisely:
| Gland | Hormone |
|---|---|
| Adrenal glands | adrenaline |
| Pancreas | insulin |
| Testes | testosterone |
| Ovaries | oestrogen |
The pancreas is worth a second look on this list, because it secretes two different hormones: insulin, and — at Supplement level — glucagon, which raises rather than lowers blood glucose concentration. A single gland producing more than one hormone is the exception here, not evidence that glands are interchangeable; don’t assume every gland on this list secretes only one hormone just because most of them do. The detailed reproductive roles of testosterone and oestrogen belong to human reproduction rather than this chapter — here, what matters is being able to pair each gland with its hormone without hesitation.
Adrenaline and the fight-or-flight response
Adrenaline is secreted by the adrenal glands in situations associated with fight or flight, and it prepares the body for rapid activity. At Core level, its required effects are increased breathing rate, increased heart rate, and increased pupil diameter. Increased breathing rate provides the oxygen needed for a rise in aerobic respiration; increased heart rate transports substances such as oxygen and glucose more rapidly; and increased pupil diameter lets more light enter the eye. At Supplement level, adrenaline also increases blood glucose concentration, giving the body more glucose available for respiration during increased activity — and it’s worth being explicit that this is an increase, the opposite of what insulin does, since the two hormones are easy to mix up if you’re thinking about blood glucose in general rather than which hormone is doing which job.
Nervous control versus hormonal control
Cambridge requires this comparison in specifically two dimensions: speed of action, and duration of effect.
| Feature | Nervous control | Hormonal control |
|---|---|---|
| Speed | Faster | Slower |
| Duration | Usually shorter-lasting | Usually longer-lasting |
Nervous control suits responses that must happen immediately; hormonal control produces changes that persist for longer, which matters for processes — growth, or the ongoing regulation of blood glucose covered on the blood glucose controlpage — that need a sustained effect rather than a single instant one. “Slower” doesn’t mean “weaker” or “less important,” and the two systems aren’t alternatives where the body picks only one — they’re different mechanisms of coordination that can, and often do, contribute to the same overall response.