Positive Vs. Negative Feedback Mechanisms

Positive and negative feedback mechanisms refer to any process that regulates the value of one variable in the face of another variable, thus increasing or decreasing the change in the value of the initial variable. In biology, feedback mechanisms are related to an organism maintaining homeostasis.

Homeostasis refers to the tendency of an organism to regulate its internal processes to achieve a certain equilibrium state. Homeostasis is the result of many parts of the body working together to regulate the value of some biological variable, be it oxygen content, insulin levels, or dopamine lives.  The difference between positive and negative feedback mechanisms is related to the direction of regulation of a variable.

Positive feedback loops magnify the change in the variable while negative feedback loops minimize the change in the variable.

An example of a positive feedback mechanism in humans is the production of oxytocin during labor. Contractions during labor result in the production of oxytocin. The presence of elevated levels of oxytocin stimulates more contractions, which creates more oxytocin, which causes more contractions, and so on.  An example of a negative feedback mechanism is the regulation of blood sugar. High levels of blood glucose are sensed by receptors in the body. In response, the pancreas produces more insulin until glucose levels fall back to normal. Positive and negative feedback mechanisms in biology thus constitute the precise balancing act required for living organisms to achieve homeostasis.

Feedback Mechanism Sketch

Any feedback process, positive or negative, can be represented as having 5 main elements: stimulus, receptor, input, output, and response. The stimulus is the thing that produces a change in the variable to be regulated. The receptor refers to whatever entity detects the initial change in the variable. Input refers to the information that travels from the sensor to the control center (e.g. the brain) which determines the appropriate response. The output is the signal that the control center sends back to the system. Lastly, the response is the action of the system that regulates the original variable.

Any feedback mechanisms can be represented as consisting of these 5 main elements. The difference between a positive and negative feedback loop is in the effect the response has on the variable to be regulated. Positive feedback loops result in an increase in the change of the variable while negative loops result in a decrease in the change of the variable.

Here is a very simple example of a positive feedback loop: Holding a live microphone up to a speaker. The high pitched wailing sound is the result of the microphone picking up ambient noise in the environment, transmitting that sound to be played out of the speaker, which the microphone picks up, which generates more sound out of the speaker, and so on. The loop amplifies the original audio signal to produce the loud wailing sound anyone who has used a microphone is familiar with. For a negative feedback loop, a simple example is your house thermostat. Thermostats detect the ambient air and will turn on or off to keep the inside of the house at a constant temperature. This feedback mechanism attempt to minimize the change in the regulated variable and so is a negative feedback mechanism.

Biological Feedback Mechanisms

Every living organism relies on keeping certain biological variables at certain values. The tendency of an organism to regulate its biological variables to reach an equilibrium state is called homeostasis. Organisms use feedback mechanisms to maintain homeostasis. Given that most biological processes rely on minimizing the change of a variable to keep it within a certain range, most feedback mechanisms found in living creatures are negative.

There are some examples of positive feedback loops in organisms though, One example is in the phenomena of fruit ripening in trees. Some fruit producing trees have a tendency to ripen all their fruit at once, usually without much visual display. Apple trees can go from unripe to completely ripe in seemingly a single night. Once the first fruit ripens, it secretes a chemical called ethylene through its skin which causes surrounding fruit to ripen. Those fruits then produce more ethylene, making the ripening spread through the tree almost like a wave rippling from the original ripened fruit. This positive feedback mechanism is taken advantage of in fruit production, where crops are treated with ethylene to ripen fruit more quickly.

Another example of a positive feedback mechanism is in human blood clotting. Injury to tissue releases a chemical which stimulates platelet production. The platelets release more of the chemical, which produces more platelets and so on until the blood is clotted. Some varieties of blood clotting disorder arise from a malfunction in this positive feedback loop.

Negative feedback loops, in contrast, are more common in living organisms. An example of negative feedback in humans is temperature regulation. Humans are endotherms, meaning that they regulate their body temperature through internal processes. Normal human body temperature sits at about 98.6°F. When the body gets too hot, sweat is produced and blood vessels will expand to radiate away excess heat. Conversely, when the body gets too cold, blood vessels will contract to prevent heat from radiating away.

One more example of negative feedback in living organisms is in the regulation of blood pressure. Blood pressure has to be high enough to pump blood not so hight it damages the blood vessels. Baroreceptors (cells that detect pressure) monitor blood pressure levels to keep them in check. If blood pressure is too high, chemicals sent to the brain which in turn slows down heart rate and expand blood vessels. if blood pressure is too low, the brain will increase heart rate and contract blood vessels. The interplay of these mechanisms is what allows humans to maintain a constant blood pressure.

In summation, positive and negative feedback mechanisms are mechanisms that regulate the change in the value of a variable in the face of another variable. Positive feedback mechanisms result in an increase in the change of the variable and negative feedback mechanisms result in a decrease in the change of a variable. In living organisms, positive and negative feedback mechanisms allow an organism to regulate their internal environment and achieve homeostasis.

About Alex Bolano

When Alex isn't nerdily stalking the internet for science news, he enjoys tabletop RPGs and making really obscure TV references. Alex has a Masters's degree from the University of Missouri-St. Louis.

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