Concept preview

What you will be able to do

1. 01Explain how negative feedback keeps a variable near its set point and name the parts of a control loop.
2. 02Describe the control of blood glucose by insulin and glucagon as a negative-feedback system.
3. 03Explain thermoregulation and the control of thyroxine as further examples of negative feedback.
4. 04Contrast positive feedback (as in oxytocin during childbirth) with negative feedback and explain why positive feedback is rarer.

The cells of your body work well only within a narrow range of conditions: the temperature, the amount of sugar in the blood, the balance of water and salts, and the acidity must all be held close to fixed values. Keeping these internal conditions steady, even while the outside world changes, is called homeostasis, and it is one of the deepest ideas in all of biology. The endocrine system is a master of homeostasis, because hormones are perfectly suited to slow, whole-body adjustment. The tool it uses, again and again, is feedback: the body measures a condition, compares it with a target value, and then acts to correct any difference. This topic is about how those feedback loops work.