Homeostasis

The body keeps each variable stable by using a negative feedback loop. The idea is simple: any change away from the normal level triggers a response that reverses the change, bringing the variable back to normal.

A negative feedback loop has four components:

1. A norm (the desired set point, e.g. 37 °C or 5 mmol/L glucose)
2. A receptor that detects when the variable has drifted away from the norm
3. A coordinator that decides what corrective action to take
4. An effector that carries out the correction

The crucial feature is that the correction is opposite to the change. If the variable goes up, the response brings it back down; if it goes down, the response brings it back up.

A simple analogy

A central-heating thermostat works the same way:

• The thermostat is set to (say) 20 °C.
• A sensor measures the room temperature.
• If the room cools below 20 °C, the thermostat switches the heater ON and the temperature rises.
• If the room warms above 20 °C, the thermostat switches the heater OFF and the room cools down.

The body works exactly like this for many of its variables. Most of the body's "thermostats" sit in the hypothalamus, a small region at the base of the brain that monitors the blood as it flows past.

The opposite: positive feedback

There is also positive feedback, where a change triggers a response that amplifies the change. This is rare in the body and is usually unhelpful for maintaining stability. Childbirth (where stretching of the cervix triggers more contractions, which stretch the cervix further) is one of the few useful examples. For homeostasis, the body relies almost entirely on negative feedback.