What are hormones and endocrine glands?
Hormones are chemical messengers made by endocrine glands (ductless glands). They are released directly into the blood, reach every part of the body, and act only on the specific cells or organs (target cells) that can respond to them.
Why do we need hormones as well as nerves?
Nerve impulses are fast but reach only cells joined to nerves, and a nerve cell needs time to reset. Many body cells, and slow long-term changes such as growth, are better controlled by hormones, which reach every cell through blood.
Main endocrine glands and their hormones
- Hypothalamus (brain): makes releasing hormones that tell the pituitary what to do.
- Pituitary: growth hormone controls growth and development. Too little in childhood causes dwarfism; too much causes gigantism. It also controls other glands, so it is often called the master gland.
- Thyroid: thyroxin controls carbohydrate, protein and fat metabolism. Iodine is needed to make it; lack of iodine causes goitre.
- Adrenal: adrenaline prepares the body for emergencies.
- Pancreas: insulin lowers blood sugar. Too little insulin causes diabetes.
- Testes (males): testosterone brings changes at puberty.
- Ovaries (females): oestrogen brings changes at puberty.
Thymus and pineal gland are also endocrine glands you may meet in higher classes.
Adrenaline: the emergency hormone
When we are scared or excited, adrenaline goes into the blood. The heart beats faster so more oxygen reaches muscles, blood supply to the gut and skin is reduced (muscles of small arteries contract), breathing becomes faster and the body is ready to fight or run away. This whole response happens in seconds and helps in danger.
Feedback mechanism: controlling hormone amounts
The timing and amount of a hormone must be just right. The body uses feedback: the result of a hormone's action controls how much more is released. Example: when blood sugar rises after a meal, cells in the pancreas detect it and release more insulin. Insulin helps cells take in sugar, so the level falls. When it falls, insulin release slows down. This keeps blood sugar steady.
Board exam tip: gland-hormone-function tables, iodised salt and goitre, dwarfism/gigantism, diabetes and the feedback mechanism are common 1 to 3 mark questions.
Nervous control vs hormonal control
| Nervous | Hormonal |
|---|---|
| Electric impulses along nerves | Chemicals carried in blood |
| Very fast, short-lived | Slower, long-lasting |
| Reaches only cells connected to nerves | Reaches all cells; acts on target cells |
Key formulas and definitions
- Hypothalamus → releasing hormones → pituitary
- Pituitary → growth hormone (dwarfism / gigantism)
- Thyroid → thyroxin (needs iodine; lack → goitre)
- Adrenal → adrenaline (fight or flight)
- Pancreas → insulin (lowers blood sugar; lack → diabetes)
- Testes → testosterone; Ovaries → oestrogen (puberty)
- Feedback: high sugar → more insulin → sugar falls → less insulin
Worked examples
1. Why is iodised salt recommended in our diet?
The thyroid needs iodine to make thyroxin. Without enough iodine, thyroxin is not made properly and the thyroid swells (goitre). Iodised salt supplies this iodine.
2. A child grows much taller than normal for his age. Which gland and hormone may be over-active?
The pituitary gland, releasing too much growth hormone (gigantism).
3. Why does your heart race when a dog suddenly chases you?
Fear makes the adrenal glands release adrenaline into the blood. It reaches the heart and makes it beat faster, sending more oxygen to muscles so you can run.
4. Explain with an example how feedback controls hormone release.
After a sweet snack, blood sugar rises. The pancreas detects the rise and releases insulin. Insulin lowers blood sugar. As it falls, the pancreas releases less insulin. The effect of the hormone itself controls its release.
5. Hormones travel to every part of the body in blood. Why doesn't every organ respond to every hormone?
Only target cells have the right receptors for a given hormone, so only they respond; other cells ignore it.
Common mistakes
- Calling endocrine glands 'duct glands'. They are ductless; hormones go straight into blood.
- Saying insulin increases blood sugar. It lowers it.
- Linking goitre to lack of vitamin. It is lack of iodine.
- Writing that the pituitary is controlled by nothing. The hypothalamus controls it through releasing hormones.