Hormonal coordination: the endocrine system
The endocrine system is the second great communication network of the body. Glands secrete chemical messengers called hormones directly into the bloodstream, which carries them to target organs that have specific receptors for them. Hormonal effects are slower than nervous responses but tend to be longer-lasting and more widespread.
Key glands and their hormones (you must know these)
| Gland | Where | Main hormone(s) | Effect |
|---|---|---|---|
| Pituitary | Base of the brain | Many — including TSH, FSH, LH, ADH, growth hormone | "Master gland" — controls other glands |
| Thyroid | Neck | Thyroxine | Regulates metabolic rate; growth and development |
| Adrenal | On top of kidneys | Adrenaline | Prepares body for "fight or flight" |
| Pancreas | Behind stomach | Insulin and glucagon | Control blood glucose (B5.7) |
| Ovaries (in females) | Pelvis | Oestrogen | Menstrual cycle, female secondary characteristics |
| Testes (in males) | Scrotum | Testosterone | Sperm production, male secondary characteristics |
The pituitary is the master gland because it secretes hormones that target other endocrine glands, telling them when to release their hormones.
Hormones vs nerves — the comparison
| Feature | Nervous | Hormonal |
|---|---|---|
| Signal | Electrical impulse | Chemical (hormone) |
| Pathway | Along neurones | In the bloodstream |
| Speed | Very fast (milliseconds) | Slower (seconds → days) |
| Duration | Short-lived | Long-lasting |
| Specificity | Very precise target | Affects all cells with the right receptor |
The two systems work together — adrenaline, for example, is released by nerves stimulating the adrenal glands.
Adrenaline ("fight or flight")
Released by the adrenal glands in response to fear or stress. Effects:
- Increases heart rate (more blood pumped)
- Increases breathing rate (more O₂ taken in)
- Diverts blood from skin and gut to muscles (so muscles can respire faster)
- Dilates pupils (better vision)
- Raises blood glucose (more fuel for respiration)
Adrenaline doesn't go through negative feedback — it acts quickly and is broken down once the threat passes.
Thyroxine and negative feedback (HT)
Thyroxine is released continuously by the thyroid gland and sets the body's basal metabolic rate. It is controlled by negative feedback:
- The hypothalamus releases TRH when thyroxine is low.
- TRH stimulates the pituitary to release TSH.
- TSH stimulates the thyroid to release more thyroxine.
- Rising thyroxine inhibits the hypothalamus and pituitary — TRH and TSH fall.
- Thyroxine secretion drops back to the set point.
This loop keeps thyroxine concentration in the blood within a narrow range — too much and you lose weight, get a fast heart rate and overheat (hyperthyroidism); too little and you become tired, gain weight and feel cold (hypothyroidism).
⚠Common mistakes
- Saying nerves are faster and longer-lasting. They're faster but shorter-lasting.
- Calling all hormones "released by glands". Yes — but secreted into the blood, which is what makes them hormones rather than local signals.
- Confusing oestrogen and testosterone with FSH/LH. Oestrogen and testosterone come from the ovaries/testes; FSH and LH come from the pituitary and act on them.
- Saying the pituitary controls the body directly. It mostly works through other glands.
Links
Foundation for B5.7 (insulin and glucagon), B5.8 (ADH and water balance), B5.9 (reproductive hormones, FSH/LH/oestrogen).
AI-generated · claude-opus-4-7 · v3-deep-biology