examples of positive feedback homeostasis

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From Wikipedia

Feedback

Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e. same defined) event / phenomenon (or the continuation / development of the original phenomenon) in the present or future. When an event is part of a chain of cause-and-effect that forms a circuit or loop, then the event is said to "feed back" into itself.

Feedback is also a synonym for:

  • Feedback signal - the information about the initial event that is the basis for subsequent modification of the event
  • Feedback loop - the causal path that leads from the initial generation of the feedback signal to the subsequent modification of the event
  • Audio feedback- the special kind ofpositive feedback that occurs when a loop exists between an audio input and output.

Overview

Feedback is a mechanism, process or signal that is looped back to control a system within itself. Such a loop is called a feedback loop. In systems containing an input and output, feeding back part of the output so as to increase the input is positive feedback (regeneration); feeding back part of the output in such a way as to partially oppose the input is negative feedback (degeneration).

In more general terms, a control system has input from an external signal source and output to an external load; this defines a natural sense (or direction) or path of propagation of signal; the feedforward sense or path describes the signal propagation from input to output; feedback describes signal propagation in the reverse sense. When a sample of the output of the system is fed back, in the reverse sense, by a distinct feedback path into the interior of the system, to contribute to the input of one of its internal feedforward components, especially an active device or a substance that is consumed in an irreversible reaction, it is called the "feedback". The propagation of the signal around the feedback loop takes a finite time because it is causal.

The natural sense of feedforward is defined chemically by some irreversible reaction, or electronically by an active circuit element that has access to an auxiliary power supply, so as to be able to provide power gain to amplify the signal as it propagates from input to output. For example, an amplifier can use power from its controlled power reservoir, such as its battery, to provide power gain to amplify the signal; but the reverse is not possible: the signal cannot provide power to re-charge the battery of the amplifier.

Feedforward, feedback and regulation are self related. The feedforward carries the signal from source to load.

Negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. For example, in a population of foxes (predators) and rabbits (prey), an increase in the number of foxes will cause a reduction in the number of rabbits; the smaller rabbit population will sustain fewer foxes, and the fox population will fall back. In an electronic amplifier feeding back a negative copy of the output to the input will tend to cancel distortion, making the output a more accurate replica of the input signal

Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.

For example, in an organism, most positive feedback provides for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and are believed to play a key role in morphogenesis, growth, and development of organs, all processes that are, in essence, a rapid escape from the initial state. Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level. Chemical potential energy for irreversible reactions or electrical potential energy for irreversible cell-membrane current powers the feedforward sense of the process. However, in the case of morphogenesis, feedback may only be enough to explain the increase in momentum of the system, and may not be sufficient in itself to account for the movement or direction of its parts.

When a public-address system is used with a microphone to amplify speech, the output from a random sound at the microphone may produce sound at a loudspeaker that reaches the microphone such as to reinforce and amplify the original signal (positive feedback), building up to a howl (of frequency dependent upon the acoustics of the hall). A similar process is used deliberately to produce oscillating electrical signals.

Feedback is distinctly different from reinforcement that occurs in learning, or in conditioned reflexes. Feedback combines immediately with the immediate input signal to drive the responsive power gain element, without changing the basic responsiveness of the system to future signals. Reinforcement changes the basic responsiveness of the system to future signals, without combining with the immediate input signal. Reinforcement is a permanent change in the responsiveness of the system to all future signals. Feedback is only transient, being limited by the duration of the immediate signal.

Types of feedback

When feedback acts in response to an event/phenomenon, it can influence the input signal in one of two ways:

  1. An in-phase feedback signal, where a positive-going wave on the input leads to a positive-going change on the output, will amplify the input signal, leading to more modification. This is known as positive feedback.
  2. A feedback signal which is inverted, where a positive-going change on the input leads to a negative-going change on the output, will dampen the effect of the input signal, leading to less modification. This is known as negative feedback.
  • Note that an increase or decrease of the feedback signal here refers to themagnitude relative to the input signal's absolute value, without regard to the polarity or sign of the feedback signal. For example if the input signal changes by 100 then a change in feedback signal value from +5 to +10 is a positive feedback. If the input signal changes by -100 then a change in the feedback signal from -5 to -10 is also a positive feedback.

Positive feedback tends to increase the


From Yahoo Answers

Question:What is a positive and negative feedback mechanism in the human body and Describe it, please give examples also

Answers:A positive feedback mechanism in the body is relatively rare. The two best described are defecation and childbirth. It is defined as a mechanism whose output stimulates an increase in output (i.e. birth). Also, "A system's response to external stimuli leading to further changes that serve to reinforce the initial response, thereby creating and accelerating a cause and effect loop." This mechanism is potentially dangerous because it will cause the body to respond as strong as it can until there is a release of stimulation (birth or uterine rupture). A negative feedback mechanism is one where the output causes a decrease in subsequent output (a balancing loop). There are lots of negative feedback loops in the body as this is the way that most systems maintain homeostasis ("The ability or tendency of an organism or cell to maintain internal equilibrium by adjusting its physiological processes.") A negative loop could be hormone secretion. Hormone levels decrease so the body stimulates the organ to produce more. At a certain level, the body recognizes that there is too much and pulls back production (the negative feedback).

Question:

Answers:Negative Feedback will prevent something from becoming excessive, Positive feedback will encourage it. An example is oxytocin during uterine contractions for positive feedback and for negative feedback, consider blood vessels which send signals to the brain if blood pressure is getting to high. The brain sends signals back to the blood vessels and heart (the effectors) and the blood vessels vasodilate (increase in diameter) because the heart rate decreases.This causes blood pressure to decrease. So its negative feedback, it causes something to stop or decrease, it does not encourage a process but rather inhibits it (ex. it prevents blood pressure from increasing too much). I really hope that makes sense.

Question:Describe the concept of stoichiometric homeostasis. How does one test for its existence? Which types of organisms tend to exhibit stoichiometric homeostasis, and which do not. Finally, how the presence of lack of stoichiometric homeostasis in soil bacteria and plants affect the response of terrestrial nutrient cycles to additions of a limiting nutrient.

Answers:A dry topic. Not given to light replies - or in your case, any replies. So I'll tackle: Stoichiometric homeostasis: One of the most significant themes in ecology. This is the concept that compares the elemental makeup of the tissues in living organisms with their environment. Quoting biologist J. Persson at University of Oslo, and fellow scientists in their report, "To be or not to be what you eat: Regulation of stoichiometric homeostasis among autotrophs and heterotrophs": "Homeostasis is the resistance to change of consumer body composition in response to the chemical composition of consumer's food." Until recently, autotrophs were assumed to be flexible. In contrast, heterotrophs, which were "confined to a constant (strictly homeostatic) body composition," were not. Now there's evidence to challenge that. So these guys tested it. "We examined the degree to which autotrophs and heterotrophs regulate stoichiometric homeostasis (P:C, N:C, N:P, or % P and %N). ... There was a wide range of responses from strictly homeostatic to non-homeostatic. Even within heterotrophic organisms, varying levels of homeostasis were observed... [as well as] significant differences between groups. For example, aquatic macroinvertebrates were significantly more homeostatic in terms of P:C than terrestrial invertebrates." And "with regard to N:P, heterotrophs are significantly more homeostatic than autotrophs. ..." Studying stoichiometric homeostasis helps to clarify many soil food-web relationships, "commonly driven by elemental imbalances between consumers and their resources." In stoichiometrics, organisms become molecules and ecosystems are organisms. The link is referenced below. I highly recommend one of the greatest texts on this subject: "Ecological Stoichiometry: Biology of Elements from Molecules to the Biosphere" by Sterner and Elser. There is one chapter posted online, and some of it is worth posting here: "Redfield's congruence in nutrient ratios between plankton and their aquatic medium indicated a balanced flow of C, N, and P in and out of the biota. The 'Redfield ocean' is a biological circulatory system with constant C:N:P stoichiometry moving vast quantities of constant proportions of these three elements vertically over thousands of meters. A second congruence was that the line describing the N and P data had a zero intercept, indicating that these two elements would be depleted from ocean waters simultaneously. The same was not true for carbon: there was a surplus of carbonate when N and P were depleted." They continue: "Simultaneous depletion of N and P was surprising. There is no a priori reason to expect ocean water to contain N and P in proportions identical to biological demand. Why then should this measure of the chemistry of the ocean--such a vast proportion of the Earth's surface and subjected to major influences from geology, meteorology, and others--have an N:P ratio that matches biological demand? Redfield's (1958) answer was that the biota itself determined the relative concentrations of N and P in the deep sea. He suggested that it was P that ultimately determined the biological productivity of the world's oceans, and that biological feedbacks adjusted the level of N so that its availability matched the availability of P (Fal-kowski et al. 1998). Similar arguments were later applied to soils (Walker and Adams 1958, 1959). Redfield's findings were important in a very broad context: his work was instrumental in fostering a view that the ocean's biota has a major influence on the chemistry of even this vast volume of water. In their abstract, "Soil Nutrient Stoichiometry as Influenced by Fire Return Intervals in Ponderosa Pine Forests," researcher Joss Mckinnon and colleagues declared, "Nitrogen deficiency is the primary form of nutrient limitation experienced by vegitation in western Montana. However, an examination of the quantity of available N in soil will not provide a comprehensive view of nutrient limitation status due to the complex nutrient requirements of plant species. Rather an analysis of the ratio of plant available N to plant available phosphorus (P) provides a more precise characterization of the nutrient status of the soil. Limited research has examined the role of natural fire intervals on the stoichiometric relationship between these nutrients in this system. We identified seven clustered sites in wilderness areas that represent stands that have been exposed to fire 0, 1, 2, or 3 or more times in the last 120 years across three wilderness areas in the Inland Northwest. The sites with three or more fires represent a fire return interval similar to what is thought to be natural. Mineral soil samples were collected from each of the seven sites and analyzed for total C, N and P, potentially mineralizable N (PMN), NH4+, NO3- and PO43-. Forest litter and foliage samples were also collected and analyzed for total C, N and P. Discussion of the relati

Question:1. The part of the kidney that takes the filtrate from the nephron and filters it to the ureter is called: a) renal cortex b) renal medulla c) renal pelvis d) renal capsule 2. what is the difference between afferent and efferent arterioles? 3. The [a) Urethra b) Ureter c) uvula d) Uterus] allows urin stored in the bladed to be expelled?....(is bladed suppose to be bladder?) 4. when woman is in labour, postive feedback activated right? 5. Why do people become dehydrated after drinking too much alcohol? 6. How does kidney help maintain a constant blood pH?

Answers:1. renal pelvis 2. the afferent arterioles deliver blood to the glomerulus, the efferent arterioles take blood away. 3. urethra 4. yes, a positive feedback mechanism is activated during labor 5. alcohol inhibits the secretion of a hormone known as antidiuretic hormone (ADH). when secreted, it prevents water loss, so when it is inhibited by alcohol, water is lost through urine. 6. tubular cells in the kidney synthesize HCO3- ions to balance the excess of H+ ions in the blood (high blood levels of H+ ions cause pH to drop). by creating more HCO3- ions, excess H+ ions are excreted.

From Youtube

Morphing algebraic shapes that emits particles (particles.cpp HPMC sample) :This example demonstrates using the surface generated by HPMC as input to a geometry shader that emits particles randomly over the surface. The particles are pulled by gravity, and uses the scalar field passed to HPMC to determine when particles hit the surface, and in this case, they bounce. To test if a particle hits the surface is done by evaluating the sign of the scalar field at the position of the particle at the beginning of the timestep and at the end. This approach is a bit too simple for these shapes, as they usually have a great deal of regions with multiple zeros, and this leads to the artefact of particles falling through the surface at some places. The following render loop is used: * Use HPMC to determine the iso-surface of the current scalar field * Render the iso surface, but tap vertex position and normals into a transform feedback buffer. * Pass this buffer into a geometry shader that emits particles (points) at some of the triangles, output stored in another transform feedback buffer. * Pass the particles from the previous frame into a geometry shader that does a series of Euler-steps to integrate velocity and position, checking for collisions in-between. The output of this pass is concatenated at the end of the newly created particles using transform feedback. * Render the particles using a geometry shader that expands the point positions into quadrilateral screen-aligned billboards.

PPV Enjoyment/How Environment affects your enjoyment/BIG DVD Update :I talk about how when you enjoy the flow of the show, it adds to the PPV. Sorry if I made the assumption the skypetalkers are sometimes negative in skype calls during PPVs. I made my statement off of one experience so it wasn't right for me to accuse them on all. I did say they are positive, like when I referenced Turning Point as an example. Feedback on the DVDs I got from ROH and ECW is much appreciated.