Glands and hormones
The endocrine system is made of endocrine glands that release chemical messengers called hormones directly into the blood. The blood carries each hormone all over the body, but it acts only on its target organ, which can recognise it. Endocrine glands have no ducts, so hormones enter the blood, not a tube.
Because hormones travel in blood, their effects begin more slowly than nervous responses but last much longer and can reach many organs at once.
Some important hormones
| Gland | Hormone | Main effect |
| Pancreas | Insulin | Lowers blood glucose |
| Pancreas | Glucagon | Raises blood glucose |
| Adrenal gland | Adrenaline | Prepares body for action |
| Thyroid gland | Thyroxine | Controls metabolic rate |
Key idea
Insulin and glucagon work as opposites to keep blood glucose steady: insulin lowers it, glucagon raises it.
Adrenaline and thyroxine
Adrenaline is released when a person is frightened, angry or excited. It speeds up the heartbeat and breathing, and releases glucose for energy — the "fight or flight" preparation. Because it is carried in the blood, its effects can last for several minutes after the alarming event has passed. Thyroxine from the thyroid gland controls the rate of metabolism, affecting growth and how fast energy is used. The endocrine glands are spread throughout the body — for example the pancreas lies below the stomach, the two adrenal glands sit above the kidneys, and the thyroid gland is in the neck — yet the blood links them all into one coordinating system that works alongside the nervous system.
Example
After a large meal, blood glucose rises. The pancreas releases insulin, which helps body cells take in glucose and the liver store it, so the blood glucose level falls back to normal. Between meals, glucagon does the reverse.
Remember
Endocrine glands are ductless and secrete hormones into the blood. Effects are slow to start but widespread and long-lasting — the opposite balance to the nervous system.