Two Coordination Systems
The body is coordinated by two systems that work together. The nervous system sends fast electrical messages that act for a short time. The endocrine system sends slower chemical messages called hormones, which travel in the blood and often act for a longer time. Together they keep the body working smoothly.
Key idea
A hormone is a chemical made by an endocrine gland, released directly into the blood, and carried to a target organ where it produces a specific effect.
Main Glands and Their Hormones
| Gland | Hormone | Main effect |
|---|---|---|
| Pituitary | Growth hormone | Controls body growth; also controls other glands |
| Thyroid | Thyroxine | Controls the rate of metabolism |
| Adrenal | Adrenaline | Prepares the body for action (fight or flight) |
| Pancreas | Insulin / glucagon | Controls blood sugar level |
| Ovaries / testes | Oestrogen / testosterone | Control sexual development |
Controlling Blood Sugar
The pancreas keeps blood glucose steady, an example of homeostasis. After a meal, blood sugar rises; the pancreas releases insulin, which helps cells absorb glucose and lowers the level. When blood sugar falls too low, it releases glucagon, which raises the level again. This balancing keeps a healthy, near-constant supply of energy to cells.
The endocrine system usually works by negative feedback: when a hormone brings a value back towards normal, the gland is signalled to slow its release again. This self-adjusting control is the main reason why levels such as blood sugar and body temperature do not swing wildly in a healthy person, but instead stay within a narrow, safe range.
Example
Before an exam a student feels their heart race and hands sweat. Adrenaline released by the adrenal gland has prepared the body for 'fight or flight'.
Remember
Nervous = fast and short; endocrine = slower and longer. Insulin lowers blood sugar, glucagon raises it.