What is a feedback loop?

A feedback loop is created when a process is affected by its own outcomes. A feedback loop might cause a process to stop when it’s had enough effect, or cause it to have a bigger and bigger effect as it continues.

Feedback loops are very important in biology. They are used to keep our body environment stable (homeostasis), and are also used during extraordinary events like birth. 

This article explains what feedback loops are and how they work. It explains the difference between positive and negative feedback loops, and gives examples of how both are used in the body. 

Vocabulary: input and outputs

All processes have inputs and outputs.

Inputs are things that affect if and how the process goes ahead. Outputs are the results of the process. 

For example:

• Doing the sum “1 + 2 = 3”: the inputs are 1 and 2, and the output is 3
• Making bread: the inputs are flour, yeast, salt and heat, and the output is bread
• Using a fan heater: the inputs are the heater settings and electricity, and the output is heat

A feedback loop happens when the output of a process is used as (or affects) an input. 

Example of a feedback loop

One of the most familiar examples of a feedback loop is found in heating systems with thermostats. These are used to keep a room at a comfortable temperature when it’s cold outside. 

The thermostat’s inputs are (i) the current temperature of the room, and (ii) the desired temperature. It compares these two numbers, and if the room is colder than desired then it turns the heater on.

The output of the heater being turned on is heat. The room warms up, which changes the current temperature of the room. And because the current temperature of the room is also an input, this creates a feedback loop.

This feedback loop means that once the room is warm enough, the heater automatically switches itself off. This stops the room from getting uncomfortably hot.

Using a heater controlled by a feedback loop is much better than, for example, just setting the heater to come on for ten minutes every hour. The feedback loop means it can automatically respond to changing weather. For example, the heater will automatically stay on longer if it’s an especially cold day (and may not come on at all if it’s an especially warm day).  

Feedback loops are really useful for controlling systems in dynamic, changing environments. Like the environment found inside the human body. 

Negative feedback loops

Most of the feedback loops found in biological systems are negative feedback loops. In negative feedback loops, the outcome of the process leads to the process being slowed down or stopped. The example of a heating system above is an example of a negative feedback loop. The outcome of heating the room leads to the heater turning itself off.

Negative feedback loops are especially helpful to balance systems, because they stop processes once they’ve had enough of an effect.

In biology, it’s common to find two processes that work together to help balance systems. One will push the balance in one direction, and one the other. Each will be controlled by its own negative feedback loop.

For example, the amount of sugar in the blood is controlled by an organ called the pancreas. Its release of the two main sugar-control hormones is controlled by negative feedback loops.

• Insulin is released when blood sugar is high. Its action leads to a fall in blood sugar.
• Glucagon is released when blood sugar is low. Its action leads to a rise in blood sugar.

Can you see how feedback loops are used here? In each case, the output (the change in blood sugar) affects the input (whether the hormone is released). Because the action of the hormones leads to their release being slowed/stopped, these are both negative feedback loops.

By switching between the release of these two hormones, the body is constantly rebalancing blood sugar levels to keep them healthy. If you’re interested to learn more about this example, see our article Sugar Control in the Body.   

Positive feedback loops

These are used to make sure a process continues, or to build up to a greater and greater effect.

An excellent example of a positive feedback loop is that of pushing while giving birth. During labour, the foetus has to move from the uterus (womb) through the cervix into the vagina. The cervix is the main control point and it takes a big push for the foetus to get through.

During labour the foetus (all being well) is head-down in the uterus, with the top of its head against the cervix. Large amounts of a hormone called oxytocin are released. Oxytocin causes the mother’s muscles to strongly contract, pushing the foetus in the right direction. At first, this just means the foetus’ head pushes against the closed cervix. But there is a positive feedback loop in the system.

The pressure of the foetus’ head against the cervix (the output) feeds back to cause more oxytocin to be released.

Can you see how this is a positive feedback loop? More oxytocin means the mother pushes harder, so the foetus pushes harder against the cervix, so more oxytocin is released, so the mother pushes harder …

Positive feedback loops are only used when there is a way for the body to escape from them. Otherwise they would go on for ever. In this case, eventually the push is hard enough (and the cervix is dilated enough) that the baby gets through. This breaks the loop as it is no longer pushing against the cervix.

A positive feedback loop also controls milk production when breastfeeding. The action of the baby suckling causes the release of a hormone, prolactin, that causes more milk production. The more the baby suckles, the more prolactin is released, and the more milk will be produced. This positive feedback loop makes sure that milk is produced for as long as required, and is broken when the baby is no longer breastfed.

If you would like to learn more about these topics, see our article about Hormones in Pregnancy and Labour.