Because a change in an input causes responses that produce continued changes in the same direction, positive feedback loops can lead to runaway conditions. 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.
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.
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. 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. This latter step leads to a positive feedback cycle, where an increase in thrombin leads to further increases in thrombin.
But if we just consider the effects of thrombin on itself, it is considered a positive feedback cycle. Although some may consider this a positive feedback loop, such terminology is not universally accepted.
Negative feedback loops are inherently stable systems. 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.
For example, negative feedback loops involving insulin and glucagon help to keep blood glucose levels within a narrow concentration range. If glucose levels get too high, the body releases insulin into the bloodstream. In a positive feedback mechanism, the output of the system stimulates the system in such a way as to further increase the output.
As noted, there are some physiologic processes that are commonly considered to be positive feedback, although they may not all have identifiable components of a feedback loop. In these cases, the positive feedback loop always ends with counter-signaling that suppresses the original stimulus.
A good example of positive feedback involves the amplification of labor contractions. The contractions are initiated as the baby moves into position, stretching the cervix beyond its normal position.
The feedback increases the strength and frequency of the contractions until the baby is born. After birth, the stretching stops and the loop is interrupted. Another example of positive feedback occurs in lactation, during which a mother produces milk for her infant.
During pregnancy, levels of the hormone prolactin increase. Prolactin normally stimulates milk production, but during pregnancy, progesterone inhibits milk production.
At birth, when the placenta is released from the uterus, progesterone levels drop. As a result, milk production surges. As the baby feeds, its suckling stimulates the breast, promoting further release of prolactin, resulting in yet more milk production. This positive feedback ensures the baby has sufficient milk during feeding.
The above provide examples of beneficial positive feedback mechanisms. However, in many instances, positive feedback can be potentially damaging to life processes. For example, blood pressure can fall significantly if a person loses a lot of blood due to trauma.
Blood pressure is a regulated variable that leads to the heart increasing its rate i. These changes to the heart cause it to need more oxygen and nutrients, but if the blood volume in the body is too low, the heart tissue itself will not receive enough blood flow to meet these increased needs. In the elderly and others, however, that loop sometimes weakens, putting their health dangerously at risk.
By unraveling the complexity of the thirst mechanism, scientists are developing better treatments for people who lose their sense of thirst and are gaining greater knowledge about many other basic human behaviors. The positron emission tomography PET image at top, taken after subjects received an infusion of a concentrated saline solution into the blood to stimulate thirst, shows regions of activity in the left side of the brain in thirsty subjects.
This activity changed dramatically after their thirst was quenched. In particular, the yellow and orange areas above indicate activity along the cingulate cortex that was extinguished later. Water is the most abundant molecule in the human body, making up about 70 percent of our body weight.
It performs a host of important internal functions, from maintaining body temperature; to transporting vitamins, minerals, hormones, and other substances; to lubricating joints, eyes, and intestines. We can survive for only a matter of days without water. Thirst serves as an automatic reminder of that fact—and, thus, plays a crucial role in keeping us alive. Yet people sometimes lose their sense of thirst.
The elderly in particular are prone to not feeling thirsty, even as they become dehydrated. Certain brain injuries also can prevent people from recognizing when they need to drink. Recently, reports of people, particularly marathon runners, who drink too much water in an overzealous attempt to avoid dehydration have hit the news.
Is thirst negative or positive feedback? Do feedbacks reflect positive or negative? What are the disadvantages of feedback in communication? What is the importance of feedback in education? What are the four types of feedback? However, emerging evidence suggests a clear role for thirst as a feedforward adaptive anticipatory response that precedes physiological challenges.
These anticipatory responses are promoted by rises in core body temperature, food intake prandial and signals from the circadian clock.
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