Your enzymes work best in a narrow range of temperature and pH (see Temperature, pH and Enzymes). So your body works hard to keep its inside steady, whatever happens outside. That steadiness is homeostasis.
Key idea
Homeostasis keeps conditions inside the body within a narrow range around a set point. Most of it runs on negative feedback: a sensor detects a change, a control center compares it with the set point, and effectors act to reverse the change. Positive feedback does the opposite: it amplifies a change until some event ends it.
Why it works
A home thermostat is the classic model. When the room cools below the setting, the sensor tells the control box, which switches the furnace on; the room warms, and the furnace switches off. The response undoes the change that triggered it, which is why it is called negative. Because the fix starts only after a change is sensed, the value wobbles a little around the set point rather than holding perfectly still.
| Part | Job | In temperature control |
|---|---|---|
| Stimulus | a change from the set point | body temperature rises |
| Sensor | detects the change | temperature sensors in the skin and brain |
| Control center | compares with the set point and sends signals | the hypothalamus in the brain |
| Effector | carries out the response | sweat glands, skin blood vessels |
| Response | reverses the change | sweating; temperature falls |
Worked example: Negative or positive?
As a fruit ripens it gives off ethylene gas, which makes nearby fruit ripen and give off more ethylene. Which kind of feedback is this?
- The response (more ethylene) makes the change (ripening) bigger, not smaller.
- It stops only when the fruit are ripe.
Answer: Positive feedback.
Worked example: Naming the parts
Blood sugar rises after a meal. Cells in the pancreas detect it and release insulin, and body cells take up glucose. Which is the sensor, and which the effector?
- The pancreas cells detect the change: sensor (and here also the control center).
- Body cells and the liver, acting on insulin, carry out the response: effectors.
Answer: Sensor: pancreas cells. Effectors: body cells and the liver.
Common mistake
Two misreadings. "Negative" does not mean bad, and "positive" does not mean good: negative feedback reverses a change, positive feedback amplifies one. And homeostasis does not hold values perfectly fixed; they wobble within a range around the set point.