Negative feedback
A control loop that counteracts change, pushing a system back towards its set point; it keeps body temperature and blood sugar steady.
In a negative feedback loop, a change triggers a response that works against it. If a level rises too high, the system brings it down; if it falls too low, the system raises it. The result is stability around a target value, the set point. A thermostat is the everyday example: when a room gets too cold the heating switches on, and when it is warm enough it switches off.
Every such loop has the same parts: a sensor that measures the quantity, a control centre that compares it with the set point, and an effector that makes the correction. The body runs many of these loops to keep its internal conditions steady, a state called homeostasis.
Blood sugar is a good example. An adult keeps only about 4 grams of glucose dissolved in the blood at any moment, and holds the fasting level in a narrow range, about 4.4 to 6.1 millimoles per litre. After a meal, rising glucose is detected by beta cells in the pancreas, which release insulin; insulin makes muscle, fat and liver cells take up glucose, so the level falls. When glucose drops, insulin release stops and alpha cells release glucagon, which makes the liver release stored glucose. In type 1 diabetes the insulin side of the loop fails, which is why insulin injections save lives.
Engineers use the same principle in feedback control, constantly measuring an error and steering to reduce it.
The opposite, positive feedback, amplifies a change instead of resisting it.
See it in action
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