TL;DR:
Homeostasis is keeping internal conditions within a narrow range around a set point.
Negative feedback reverses a change using a sensor, a control centre and an effector. It is the main way the body stabilises things.
Positive feedback amplifies a change until something ends it, as in childbirth contractions and blood clotting.
Run up a flight of stairs and your face goes red, you sweat, and you breathe hard. Ten minutes later you are back to normal. Your body kept its temperature close to 37 °C without any decision on your part. That is homeostasis, and it works the same way a thermostat does.
The idea in plain words
Homeostasis is the maintenance of a roughly constant internal environment, for conditions such as temperature, blood glucose, water balance and pH. The target value is the set point. The body does not hold it perfectly. It swings a little above and below, and corrections bring it back.
The parts of a feedback loop
Stimulus: a change in the condition.
Sensor (receptor): detects the change.
Control centre: compares it with the set point and decides on a response. For temperature, this is in the brain.
Effector: a muscle or gland that carries out the response.
Response: the change that pushes the condition back toward the set point.
In negative feedback, the response opposes the stimulus. Too high leads to a response that lowers it, and too low leads to one that raises it. The result is stability. A household thermostat works the same way: the temperature sensor tells the controller, the controller switches the heater, and the heater changes the temperature.
Worked example: body temperature control
Say you start exercising and your core temperature rises from 37.0 °C to 37.6 °C. The numbers are only an illustration.
Stimulus: Muscles release heat, and the blood warms. The condition has moved
+0.6 °Cabove the set point.Sensors: Temperature receptors in the skin and brain detect the rise.
Control centre: The brain compares the temperature with the set point and finds it too high.
Effectors: Sweat glands release sweat, which cools the skin as it evaporates. Blood vessels near the skin widen, so more heat is lost to the air.
Result: Temperature falls back toward 37.0 °C. Once it does, the signal weakens, and the effectors ease off.
Now reverse it. In the cold, the temperature falls and the control centre triggers shivering, where muscles release heat, and narrows the blood vessels near the skin to keep warm blood in the core. The temperature rises back toward the set point. Two opposite responses, one goal.
A second loop: blood glucose
The same pattern controls blood sugar. After a meal, glucose in the blood rises. Cells in the pancreas sense it and release the hormone insulin. Insulin tells liver and muscle cells to take up glucose and store it, so blood glucose falls back toward its set point.
If glucose falls too low, a different hormone, glucagon, signals the liver to release stored glucose. Two hormones with opposite effects act as a pair, like heater and air conditioner, so the system can push in both directions. When this system does not work properly it is a medical matter, so questions about your own health belong with a doctor.
Why the temperature wobbles
Every loop has a delay: sensing, signalling and responding take time. During that delay the condition keeps moving, so it overshoots a little before the response catches up. That is why body temperature, and a room with a thermostat, drift slightly above and below the set point instead of sitting exactly on it.
Positive feedback: the exception
In positive feedback, the response amplifies the stimulus, pushing the condition further away. It does not stabilise anything, so it appears only in processes that have a clear end point.
Childbirth: contractions push the baby against the cervix, which triggers the release of a hormone that causes stronger contractions. The loop ends with the birth.
Blood clotting: platelets at a wound release chemicals that attract more platelets, and the clot builds until the wound is sealed.
Fruit ripening: a ripening fruit gives off ethylene gas, which speeds ripening in nearby fruit.
How to check your answer
Does the response push the variable back toward the set point? If yes, it is negative feedback.
Have you named the sensor, the control centre and the effector in order?
Did you say what the effector actually does, such as 'sweat glands release sweat', not just 'the body cools down'?
For positive feedback, did you say what stops it?
Common mistakes
Thinking negative means bad. It means the response opposes the change.
Thinking positive means good. It means the response adds to the change.
Saying the body keeps temperature exactly constant. It varies within a range.
Naming a tissue instead of an effector. The effector is the thing doing the work: a muscle or a gland.
Where you will use it next
Blood sugar control by insulin, water balance in the kidneys and the control of breathing rate all use the same loop. Plants also regulate water: see how plants move water from roots to leaves. If you are ill or concerned about your own temperature, speak to a doctor. This article is general science.
Frequently asked questions
Is homeostasis only about temperature?
No. It includes blood glucose, water content, pH, blood pressure and more.
Can a negative feedback loop stop working?
Yes. If a sensor, control centre or effector is damaged, the loop fails and the condition can drift. That is the basis of many diseases.
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