The internal environment of our bodies is constantly kept within strict limits. Take for example temperature. It must be a challenge keeping our fleshy selves at 37°C while the outside fluctuates wildly! Or how about blood glucose concentration? We’ve all heard of diabetes – if it goes too high our organs sustain damage, if it goes too low a coma may be induced or even death.
Welcome to homeostasis – the maintenance of physiological parameters within optimal range.
The operation of an oven is an easy example of negative feedback acting both ways, which is how it usually acts. That means that there are deviations in two opposing directions. If an oven is set at 220°C, both a decrease and an increase in temperature is a deviation. So if the temperature drops or rises, a sensor picks that up and commands the heater to turn on or off.
This is negative feedback because it returns the system to its original state.
Sometimes completely separate sensors control a rise or fall in temperature. This separation gives a high level of control.
In contrast with negative feedback, positive feedback drives a system further away from its original state, often starting out with small disturbances which cause a factor which further stimulates the disturbance. For example, a panicking sheep will cause more sheep around to get panicked, potentially causing a stampede.
Unlike the sharp control available in negative control loops, positive feedback may end up spiralling itself out of control and self-destructing.