Not every message travels along a nerve. Endocrine glands release hormones into the blood, which carries them all round the body, but only their target organs respond. Adrenaline gets you ready to fight or run in seconds; insulin and the sex hormones work over hours to years. Glucagon and adrenaline's effect on blood glucose are Extended content.
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. The adrenal glands secrete adrenaline, the pancreas insulin, the testes testosterone and the ovaries oestrogen. Adrenaline is secreted in fight or flight situations and increases breathing rate, heart rate and pupil diameter. Nervous control is faster but shorter-lasting than hormonal control.
Choose male or female, then click each gland. The panel names the hormone it secretes and its target organs.
| Gland | Where | Hormone |
|---|---|---|
| Adrenal glands | on top of each kidney | adrenaline |
| Pancreas | behind the stomach | insulin (and, Extended, glucagon) |
| Testes | in males | testosterone |
| Ovaries | in females | oestrogen |
A hormone is carried in the blood plasma, not by red blood cells, and it is made by a gland, not by the target organ. Do not confuse the adrenal glands with the kidneys they sit on.
Press A sudden fright and watch the readings change, then press Calm down. Note how slowly the readings return to normal.
| Reading in the sim | Calm | After a fright | Why it helps |
|---|---|---|---|
| Heart rate | about 70 per min | up to about 160 per min | more blood, oxygen and glucose to the muscles |
| Breathing rate | about 12 per min | up to about 30 per min | more oxygen in, more carbon dioxide out |
| Pupil diameter | about 3 mm | up to about 7 mm | more light enters the eye |
| Blood glucose (Extended) | about 5 mmol/dm3 | up to about 7.5 mmol/dm3 | more glucose for respiration in muscles |
Adrenaline increases pupil diameter. Do not mix this up with the pupil reflex, where bright light makes the pupil smaller.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the glands tab, which hormone do the adrenal glands secrete?
Which gland is shown only when the female body is selected?
In the adrenaline tab, what happens to the pupil diameter after a sudden fright?
In the race tab, which response starts first and fades fastest?
Press Send both: a nerve impulse and a hormone set off at the same moment. Compare the two lines on the response graph: when each response starts and how long it lasts.
| Nervous control | Hormonal control | |
|---|---|---|
| Message | electrical impulses along neurones | chemicals carried in the blood |
| Speed of action | very fast (milliseconds) | slower (seconds to hours) |
| Duration of effect | short-lived | longer-lasting |
| Where it acts | one precise place | target organs anywhere in the body |
The syllabus comparison is about speed of action and duration of effect. Write both halves: nerves are faster and their effect is shorter; hormones are slower and their effect lasts longer.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which is the best definition of a hormone?
Which effect is caused by adrenaline?
Extended: which gland secretes glucagon?
Compared with nervous control, hormonal control is:
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