All real-world systems are composed of many such interacting feedback loops — animals, machines, businesses, and ecosystems, to name a few. There are two types of feedback loops: positive and negative. Positive feedback amplifies system output, resulting in growth or decline. Negative feedback dampers output, stabilizes the system around an equilibrium point.
Positive feedback loops are effective for creating change, but generally result in negative consequences if not moderated by negative feedback loops. For example, in response to head and neck injuries in football in the late s, designers created plastic football helmets with internal padding to replace leather helmets. The helmets provided more protection, but induced players to take increasingly greater risks when tackling.
More head and neck injuries occurred after the introduction of plastic helmets than before. By concentrating on the problem in isolation e. This resulted in more injuries which resulted in additional redesigns that made the helmet shells harder and more padded and so on.
Negative feedback loops are effective for resisting change. For example, the Segway Human Transporter uses negative feedback loops to maintain equilibrium. As a rider leans forward or backward, the Segway accelerates or decelerates to keep the system in equilibrium. To achieve this smoothly, the Segway makes hundreds of adjustments every second.
Given the high adjustment rate, the oscillations around the point of equilibrium are so small as to not be detectable. However, if fewer adjustments were made per second, the oscillations would increase in size and the ride would become increasingly jerky. A key lesson of feedback loops is that things are connected —changing one variable in a system will affect other variables in that system and other systems.
This is important because it means that designers must not only consider particular elements of a design, but also their relation to the design as a whole and to the greater environment. From Graph Algebra by Courtney Brown:. Feedback loops are typically used to accomplish regulation and control. A feedback loop is like an input, but its origin is from within the system itself, not from outside the system.
In many systems, the output reenters the system as another input. This is exactly what happens with a microphone and speakers when the sound from the speakers feed back into the microphone, often causing a loud squeal. Sanjay Bakshi, a visiting professor at MDI wrote an email to one of his students on positive feedback:. In my view, its not correct to always view positive feedback loops in business as destructive, though they well might be. For example a run on a bank can bring it down on its knees in a very short time period and it can spread systemic risk to other banks.
Similarly stock market bubbles can be thought of positive feedback loops — high prices feed optimism which feeds high prices — it does not last for ever, but it can last for a long long time. Production of human red blood cells erythropoiesis - A decrease in oxygen is detected by the kidneys and they secrete erythropoietin.
This hormone stimulates the production of red blood cells. Negative Feedback in Nature Dive into different negative feedback loops that you can find in nature. The carbon cycle - The equilibrium of this cycle will change in accordance with carbon dioxide emissions.
Plant photosynthesis - The photosynthesis in plants speeds up in response to increased levels of carbon dioxide. Carbonation - Rain and carbon dioxide combine with limestone to make calcium bicarbonate. This increases when the temperature lowers and is a factor in glacial weathering.
Population of predators and prey - If the numbers of prey decrease, then some predators will starve, and their numbers will decrease. Greenhouse effect - More radiation escapes into space from the upper atmosphere than from the lower portion. Global warming will reduce the rate of this escaping radiation, and this will lessen the greenhouse effect. Mechanical Negative Feedback Mechanics is also full of different physical negative feedback loops.
Here are a few examples. The extreme muscular work of labor and delivery are the result of a positive feedback system Figure 1. The first contractions of labor the stimulus push the baby toward the cervix the lowest part of the uterus. The cervix contains stretch-sensitive nerve cells that monitor the degree of stretching the sensors. These nerve cells send messages to the brain, which in turn causes the pituitary gland at the base of the brain to release the hormone oxytocin into the bloodstream.
Oxytocin causes stronger contractions of the smooth muscles in of the uterus the effectors , pushing the baby further down the birth canal. This causes even greater stretching of the cervix. The cycle of stretching, oxytocin release, and increasingly more forceful contractions stops only when the baby is born. At this point, the stretching of the cervix halts, stopping the release of oxytocin.
A second example of positive feedback centers on reversing extreme damage to the body. Following a penetrating wound, the most immediate threat is excessive blood loss. Less blood circulating means reduced blood pressure and reduced perfusion penetration of blood to the brain and other vital organs. If perfusion is severely reduced, vital organs will shut down and the person will die.
The body responds to this potential catastrophe by releasing substances in the injured blood vessel wall that begin the process of blood clotting. As each step of clotting occurs, it stimulates the release of more clotting substances. This accelerates the processes of clotting and sealing off the damaged area. Clotting is contained in a local area based on the tightly controlled availability of clotting proteins. This is an adaptive, life-saving cascade of events.
When body temperature rises, the hypothalamus initiates several physiological responses to decrease heat production and lose heat:. These effects cause body temperature to decrease. Many homeostatic mechanisms, like temperature, have different responses if the variable is above or below the set point. When temperature increases, we sweat, when it decreases, we shiver. These responses use different effectors to adjust the variable.
In other cases, a feedback loop will use the same effector to adjust the variable back toward the set point, whether the initial change of the variable was either above or below the set point. For example, pupillary diameter is adjusted to make sure an appropriate amount of light is entering the eye. If the amount of light is too low, the pupil dilates, if it is too high, the pupil constricts. This might be compared to driving. If your speed is above the set point the value you want it to be , you can either just decrease the level of the accelerator i.
Blood pressure is created initially by the contraction of the heart. Changes in the strength and rate of contraction will be directly related to changes in blood pressure. Changes in the volume of blood would also be directly related to changes in blood pressure. Changes in the diameter of the vessels that blood travels through will change resistance and have an opposite change on blood pressure. Blood pressure homeostasis involves receptors monitoring blood pressure and control centers initiating changes in the effectors to keep it within a normal range.
Due to synchronization of insulin release among the beta cells, basal insulin concentration oscillates in the blood following a meal. The oscillations are clinically important, since they are believed to help maintain sensitivity of insulin receptors in target cells. This loss of sensitivity is the basis for insulin resistance. Thus, failure of the negative feedback mechanism can result in high blood glucose levels, which have a variety of negative health effects.
In particular, we will discuss diabetes type 1 and type 2. Diabetes can be caused by too little insulin, resistance to insulin, or both. Type 1 Diabetes occurs when the pancreatic beta cells are destroyed by an immune-mediated process. Because the pancreatic beta cells sense plasma glucose levels and respond by releasing insulin, individuals with type 1 diabetes have a complete lack of insulin. In this disease, daily injections of insulin are needed.
Also affected are those who lose their pancreas. Once the pancreas has been removed because of cancer, for example , diabetes type 1 is always present. Type 2 Diabetes is far more common than type 1. It makes up most of diabetes cases. It usually occurs in adulthood, but young people are increasingly being diagnosed with this disease. In type 2 diabetes, the pancreas still makes insulin, but the tissues do not respond effectively to normal levels of insulin, a condition termed insulin resistance.
Over many years the pancreas will decrease the levels of insulin it secretes, but that is not the main problem when the disease initiates.
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