![]() Negative feedback loops, in conjunction with the various stimuli that can affect a variable, typically produce a condition in which the variable oscillates around the set point. ![]() Negative feedback loops are inherently stable systems. Although some may consider this a positive feedback loop, such terminology is not universally accepted. But if we just consider the effects of thrombin on itself, it is considered a positive feedback cycle. It should be noted that there are other aspects of blood clotting that keep the overall process in check, such that thrombin levels don’t rise without limit. This latter step leads to a positive feedback cycle, where an increase in thrombin leads to further increases in thrombin. One of the enzymes in the pathway, called thrombin, not only acts on the next protein in the pathway but also has an ability to activate a protein that preceded it in the cascade. For example, during blood clotting, a cascade of enzymatic proteins activates each other, leading to the formation of a fibrin clot that prevents blood loss. In most cases, positive feedback is harmful, but there are a few instances where positive feedback, when used in limited fashion, contributes to normal function. The term positive feedback is typically used as long as a variable has an ability to amplify itself, even if the components of a loop (receptor, control center and effector) are not easily identifiable. Because a change in an input causes responses that produce continued changes in the same direction, positive feedback loops can lead to runaway conditions. Positive feedback loops are inherently unstable systems. For example, an increase in the concentration of a substance causes feedback that ultimately causes the concentration of the substance to decrease.
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