Positive Feedback

3 Key excerpts on "Positive Feedback"

• 

  eBook - ePub

  Introduction to Modeling in Physiology and Medicine

  • Claudio Cobelli, Ewart Carson(Authors)
  • 2019(Publication Date)
  • Academic Press

    (Publisher)

  Feedback is a fundamental feature of all physiological systems. It is vital in terms of ensuring physiological regulation and control. It is an ingredient of the complexity that characterizes physiological systems.

  At its simplest, feedback can be regarded as a mutual causality, whereby variable X has an effect on variable Y, and in turn variable Y has an effect on variable X. If an increase in variable X brings about an increase in variable Y, and that increase in variable Y brings about a decrease in variable X, the process is referred to as negative feedback.

  2.4.1 Negative feedback

  Glucose metabolism provides us with examples of such negative feedback. For instance, in a normal individual, an increase in blood glucose concentration (variable X), brought about by the ingestion of the carbohydrate component of a meal, causes an increase in the secretion of insulin (variable Y). The effect of this increased insulin level is to bring about a reduction in the blood glucose concentration toward a normal value. This negative feedback process is inherently regulatory, seeking to enhance the control of blood glucose concentration. Physiologically the effects of insulin on glucose are achieved by processes that include the chemical conversion of glucose into glycogen that is stored in the liver.

  2.4.2 Positive Feedback

  On the other hand, Positive Feedback corresponds to the situation in which variable X causes an increase in variable Y, which in turn, brings about a further increase in X. An example outside the physiological domain is the wage-price spiral. An increase in wages causes price increases that in turn act as a catalyst for further wage increases. This is clearly a destabilizing phenomenon.

  2.4.3 Inherent feedback

  The examples of feedback considered above relate to control systems in which the feedback link takes the form of a flow of material or information. There is, however, a further form of feedback that needs to be considered, namely inherent feedback.

  Consider the case of a metabolic system. Suppose that in a simple chemical reaction, it can be assumed that the rate of concentration decrease of chemical A, taking part in the reaction, is directly proportional to its concentration. Mathematically this can be expressed as:

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  eBook - ePub

  Human Ecology

  Basic Concepts for Sustainable Development

  • Gerald G Marten(Author)
  • 2010(Publication Date)
  • Routledge

    (Publisher)

  Every ecosystem and human social system has numerous positive and negative feedback loops. Both kinds of feedback are essential for survival. Negative feedback provides stability; it keeps important parts of the system within the limits required for proper functioning. Positive Feedback provides the capacity to change drastically when necessary. The development and growth of all biological systems – from cells and individual organisms to ecosystems and social systems – is based on the interplay of positive and negative feedback. Ecosystems and social systems can stay more or less the same for long periods, but sometimes they change dramatically and rapidly. They function best when they have an appropriate balance between the forces that promote change and the forces that provide stability.

  People constantly interact with these forces of change and stability. People depend upon negative feedback to ‘take care of things’ and keep everything functioning smoothly most of the time. When people try to improve their situation (‘development’ or ‘solving problems’), they use Positive Feedback to help make the changes they want. However, in addition to working for people, positive and negative feedback can also work against them. Sometimes people try to improve things or solve a problem, and no matter what they do, there is no improvement because they are working against negative feedback that prevents the changes that they want. Other times, people prefer things to stay the way they are, but Positive Feedback amplifies seemingly harmless actions into changes that they do not want. If we pay attention to the positive and negative feedbacks in our social systems and ecosystems, we can use the feedbacks to our advantage instead of struggling against them. In the case of ecosystems, this means fitting our activities with ecosystems to do things ‘nature’s way’, so nature does most of the work and keeps things going. The concrete meaning of ‘doing things nature’s way’ will become more apparent in subsequent chapters.

  Things to Think About                                              

  1. Think of examples of Positive Feedback at different levels of social organization in your social system: family and friends, neighbourhood, city, national, and international. Draw diagrams to show circular chains of effects (ie, feedback loops). Do some of the feedback loops generate sudden changes?
  2. Think of examples of the replacement of one thing by another in your social system or ecosystem during recent years. Draw a diagram to show the chain of effects and feedback loops that generated the replacement.

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  eBook - ePub

  Fundamentals of Applied Pathophysiology

  An Essential Guide for Nursing and Healthcare Students

  • Ian Peate, Ian Peate(Authors)
  • 2021(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Nearly all physiological variables are controlled by negative feedback mechanisms. When a response reverses the original stimuli, then the system is operating by negative feedback.

  In Figure 3.3 , the regulation of blood pressure is considered. Watson (2018 ) provides an example of negative feedback and the maintenance of homeostasis in the regulation of blood pressure. When the heart beats faster or harder, the blood pressure increases. If a stimulus causes the blood pressure to rise, the baroreceptors (these are pressure‐sensitive nerve cells) located in walls of particular blood vessels send impulses to the brain (the control centre) having sensed higher pressure. The brain responds by sending out impulses to the heart and blood vessels (the effectors). Usually, as a result the heart rate responds and decreases, and blood vessels dilate, with the blood pressure responding by returning to normal. Blood pressure is discussed further in Chapter 9 of this text.

  Figure 3.2 Regulation through feedback, a feedback system

  (Source: Tortora and Derrickson (2017). Reproduced with permission of John Wiley and Sons, Inc.).

  There are many more examples of negative feedback mechanisms that occur in the body. Thermoregulation, control of blood glucose and osmoregulation are but three examples.

  Positive Feedback mechanisms

  The use of Positive Feedback mechanisms is less common in the control of physiological variables. When a response improves the original stimuli, then the system is operating by Positive Feedback. Figure 3.4 uses the Positive Feedback control of labour contractions during birth of a baby as an example of homeostatic regulation by a Positive Feedback mechanism.

  When labour commences, the cervix and uterus stretch (the stimulus), stretch‐sensitive nerve cells located in the cervix (the receptors) will transmit nerve impulses (the input) to the brain (this is the control centre). The brain responds, causing the pituitary gland to release the hormone oxytocin (the output), which then stimulates the walls of the uterus (the effector) to powerfully contract (the response) and to contract at a faster rate. When the fetus moves as a result of the contractions, this will also result in further stretching of the cervix and more oxytocin is released, resulting in even more forceful contractions. This cycle continues until the baby is born, the birth ends and the stretching of the cervix ceases, the release of oxytocin ends as well as the Positive Feedback mechanism.

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