Hormones in animals and feedback control
Endocrine glands pour hormones straight into the blood to control growth, metabolism, sugar levels and emergencies. A feedback mechanism keeps each hormone at the right level.
Why animals need hormones as well as nerves
Nerve impulses are fast, but they have limits. They reach only cells that are connected to nerves, and a cell needs time to reset before it can send the next impulse. So animals also use hormones: chemical messengers made by endocrine glands and released directly into the blood. Blood carries hormones to every cell, and only the target cells respond.
Endocrine glands and their hormones
| Gland | Where | Hormone | Job |
|---|---|---|---|
| Hypothalamus | Brain | Releasing hormones | Make the pituitary release its hormones |
| Pituitary | Brain, below the hypothalamus | Growth hormone | Controls growth and development; too little in childhood causes dwarfism |
| Thyroid | Neck | Thyroxin | Regulates carbohydrate, protein and fat metabolism; needs iodine |
| Pancreas | Below the stomach | Insulin | Lowers the blood sugar level |
| Adrenal glands | On top of the kidneys | Adrenaline | Prepares the body for an emergency |
| Testes | Scrotum (males) | Testosterone | Brings the male changes of puberty |
| Ovaries | Lower abdomen (females) | Oestrogen | Brings the female changes of puberty; regulates the menstrual cycle |
Adrenaline in an emergency
- When you are frightened or must act fast, the adrenal glands release adrenaline into the blood.
- The heart beats faster, sending more oxygen to the muscles.
- Small arteries to the digestive system and skin narrow, so less blood goes there and more goes to the muscles.
- The breathing rate increases, as the diaphragm and rib muscles contract faster.
- Together, these changes prepare the body to face the danger or run from it.
Hormone problems examiners ask about
- Goitre: a swollen neck, caused by too little iodine in the diet, so the thyroid cannot make enough thyroxin. Iodised salt prevents it.
- Diabetes: blood sugar stays high because the pancreas makes too little insulin. Many patients are treated with insulin injections.
- Dwarfism: too little growth hormone during childhood.
Draw it: feedback control of blood sugar
- back to Blood sugar rises after a meal
The level of a substance controls how much hormone is released. Another example: when growth hormone is low, the hypothalamus releases a releasing hormone that makes the pituitary release more growth hormone.
Salt packets in Indian kitchens say iodised because iodine lets the thyroid make thyroxin and prevents goitre. Many families know someone with diabetes who checks blood sugar regularly and may take insulin.
2 marks: “Why is the use of iodised salt advised?” The thyroid needs iodine to make thyroxin, which regulates carbohydrate, protein and fat metabolism. Too little iodine causes goitre.
3 marks: “How does adrenaline prepare the body for an emergency?” Released from the adrenal glands into the blood; heart beats faster, more oxygen to muscles; less blood to the digestive system and skin; faster breathing.
3 marks: “What is a feedback mechanism? Explain it with the example of insulin.” Control of hormone release by the level of a substance. Blood sugar rises → pancreas releases more insulin → sugar falls → less insulin is released.
Iodine → thyroid → thyroxin: no iodine, the neck swells (goitre). Insulin is the sugar police of the pancreas.