What is a hormone?
A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more target organs. The organs that make hormones form the endocrine system.
Remember
- Nervous control is fast and short-lived; hormonal control is usually slower but longer-lasting and can affect the whole body.
Some important hormones
| Hormone | Gland | Effect |
| Adrenaline | Adrenal gland | Prepares body for "fight or flight" |
| Insulin | Pancreas | Lowers blood glucose |
| Glucagon | Pancreas | Raises blood glucose |
Homeostasis and blood glucose
Homeostasis is the maintenance of a constant internal environment. It works by negative feedback: when a factor changes from its normal level, the body acts to bring it back.
Key idea
Negative feedback: a rise in a factor triggers a response that lowers it; a fall triggers a response that raises it — keeping the factor close to a set point.
After a meal, blood glucose rises. The pancreas releases insulin, which makes the liver take up glucose and store it as glycogen, so the level falls. If blood glucose falls too low, for example during exercise, the pancreas releases glucagon, which makes the liver convert glycogen back to glucose. The two hormones work as opposites, keeping blood glucose within narrow limits — a clear example of homeostasis by negative feedback.
Worked example
In type 1 diabetes the pancreas cannot make enough insulin. Blood glucose stays high after eating. Treatment is often injecting insulin, which lowers blood glucose by helping the liver and cells take glucose in.