A positive feedback loop is a system where one variable increases the quality of another variable which in turn increases the quantity/occurrence of the first variable. Positive feedback loops are processes that occur within feedback loops in general, and their conceptual opposite is a negative feedback loop.
The mathematical definition of a positive feedback loop is defined as a phenomenon where a positive gain exists within a loop of cause and effect. Positive feedback systems like these are inherently unstable systems that will usually end with some sort of divergence from the system’s normal equilibrium, possibly with chaotic behavior or exponential growth.
Positive Feedback Loops Vs Negative Feedback Loops
As a practical example of a positive feedback loop, consider the process of blood clotting, where the arrival of platelets at a site releases clotting factors that cause more platelets to arrive at the injury site. Also consider the process of childbirth, where the stretching of the walls of the uterus leads to contractions, and the contractions further stretch the uterine walls in a cycle that continues until the birthing process is over. Notice that in both instances, the positive feedback involves the response of one variable (release of clotting factors, stretching of the uterus) that reinforces or amplifies the detected change (platelet arrival, contractions).
Contrast this with negative feedback loops, which are responses that reverse the detected change. The function of negative feedback loops is often to reduce undesirable effects. An example of a negative feedback loop is the thermoregulation of the body, like sweating when you are too hot. The sweat reduces body temperature and will continue until you are back to acceptable body temperature.
The Relationship Between Homeostasis And Feedback Loops
In biology, the concept of feedback loops is applied alongside the concept of homeostasis, which is the use of dynamic physiological processes by the cells of an organism to maintain an internal environment suitable for normal function and the continued existence of that organism. The goal of homeostasis is to keep certain important internal variables like blood pressure and body temperature within a range of acceptable values.
Regarding feedback loops, as they are related to homeostasis, the function of a feedback loop is to help the body maintain homeostasis. Feedback loops are defined as systems that adjust levels of the specific variable whenever identifiable receptors, effectors, methods of communication, and control centers are present. It would be helpful to define these terms:
- Variables are any number of parameters or constraints that have levels monitored or affected by the body’s feedback system.
- Receptors are responsible for detecting changes in the level of the different variables.
- Control centers, also known as integrators, are systems in the body that are responsible for comparing the detected levels of the variable to the target levels of the variable and generating a response if the measurements and targets are different.
- Effectors are devices that carry out the changes necessary to adjust levels of the variable and restore homeostasis. Some method of communicating between the various systems and subsystems involved in the feedback loop is also necessary so that the system knows when to begin and end the various parts of the loop.
An example of an organism’s attempts at maintaining homeostasis is, as described above, thermoregulation. When it is too cold out, your body shivers and mammals get goosebumps which increases the distance of blood to the exterior of the skin, decreasing heat loss. Another example of the body’s attempt to maintain homeostasis is during physical activity, where when the body is stressed for long periods of time it begins to sweat, which causes it to lose water and salt. The low level of water in the body creates the release of hormones responsible for the feeling of thirst. Many medical conditions result from a failure of the body to maintain some type of homeostasis.
A Closer Look At Positive Feedback Loops
Positive feedback loops are systems where a change in one aspect of the system will cause more changes in that aspect of the system. Because a change in the level of a variable causes responses that further increase the level of that variable, a positive feedback loop can grow quickly and unstable-y, leading to a runaway condition. Positive feedback loops can often be harmful because of this tendency to become unstably, though in some cases, such as the aforementioned clotting of blood, they can be beneficial. In the case of blood clotting, an enzyme called thrombin acts activates proteins that cause the cascading effect, but there are also other aspects of the blood clotting system that keep it in check and don’t allow levels of thrombin to increase beyond acceptable levels.
“Childbirth is more admirable than conquest, more amazing than self-defense, and as courageous as either one.” — Gloria Steinem
Let’s look at one more example of positive feedback loops found within the body. After a child is born, the mother produces milk for her offspring, a process known as lactation. During the course of the pregnancy levels of the hormone known as prolactin increase, and prolactin is typically responsible for stimulating the production of milk. Yet during the course of the pregnancy, levels of progesterone inhibit the production of milk. After the birth of the child levels of progesterone drop and as the child drinks milk the release of further prolactin is promoted, which thereby leads to the production of more milk and more prolactin. This positive feedback loop is necessary to make sure the child has enough milk during the time it nurses. When the baby begins to wean off of milk, the levels of prolactin in the mother’s body return to their normal levels.
Examples Of Positive Feedback Loops Outside Of The Body
An example of a positive feedback loop in nature outside of the body is drought. Droughts become more intense due to a positive feedback loop. When there isn’t enough rain, plants die off, which limits how much water vapor is added to the air from the planet’s surface. Dry dust is also added to the atmosphere which ends up absorbing water and making the air even dryer. The lower amount of water vapor in the air means that days get hotter and drier, which decreases the chance that clouds will form, which decreases the chance that it will rain – exacerbating the drought.