Positive feedback is a feedback mechanism within the body. It is always referred to as positive feedback when the output variable in a control loop has a reinforcing effect on itself. While negative feedback tends to try to keep the changes in the variables involved as small as possible, positive feedback ensures that the changes are as large as possible.
What is positive feedback?
An example of a desired positive feedback mechanism is the buildup of potential in excitation conduction. The higher the incoming potential, that is, the stimulus that hits a neuron, the more sodium ion channels are opened. Positive feedback is also known as the positive feedback mechanism. With the help of positive feedback, certain variables within a control loop are continuously corrected. Whereas with negative feedback the end product in a control loop has an inhibiting effect on the output variable, with positive feedback there is an inherently continuous amplification. Processes in which positive feedback plays a role start rather slowly and then quickly become more intense. In physiological positive feedback, there is a stop signal that brings the process to a halt. In pathological positive feedback mechanisms, this is not the case, resulting in a vicious circle, the circulus vitiosus.
Function and task
Compared to negative feedback, the positive version is found rather rarely. An example of a desired positive feedback mechanism is the buildup of potential during excitation conduction. The higher the incoming potential, i.e., the stimulus that hits a neuron, the more sodium ion channels are opened. In turn, the more ion channels are opened for sodium, the higher the action potential becomes. Another example of positive feedback is the stomach‘s response to toxins. Normally, the muscles of the stomach contract evenly at regular intervals and then relax again. This gently mixes the food. When a toxin is registered, a positive feedback loop causes the stomach to contract forcefully. This forces the stomach contents backward through the esophagus toward the mouth and then vomits. One hormone whose release is controlled by positive feedback, among other things, is oxytocin. Oxytocin plays an important role in milk production (lactation). The infant’s sucking movements at the breast stimulate the release and production of oxytocin. The oxytocin in turn stimulates milk production. If the infant sucks more at the breast, more oxytocin is again produced and the milk flow is further stimulated. The stop mechanism here is also the infant. If he does not drink at the breast for a longer period of time, the oxytocin level drops and milk production is reduced.
Diseases and ailments
When the stop signal is absent from a positive feedback loop, a vicious circle is created. Such a vicious circle is found, for example, in heart failure. We speak of heart failure when the performance of the heart is no longer sufficient to supply the body with sufficient blood. To ensure that the organs and peripheral regions of the body still receive sufficient blood, oxygen and nutrients, the body activates various adaptation mechanisms. For example, it constricts the vessels so that blood pressure rises and blood reaches more distant areas. In addition, the so-called renin-angiotensin-aldosterone system is activated. This causes more water to be recovered in the kidney. This also results in an increase in blood pressure. Activation of the sympathetic nerve increases the heart rate and the contractile force of the heart as much as possible. As a result, the body is actually better supplied initially, but the increased blood pressure and cardiac activity put a strain on the already weakened heart and sooner or later lead to a worsening of the situation. As a result, the compensatory mechanisms are strengthened, and these in turn damage the heart in the long run. In the worst case, this vicious circle makes a heart transplant inevitable. Another example of a dangerous positive feedback loop is shock. Once the shock spiral is set in motion, it can be fatal. The causes of shock can vary.However, a good example of positive feedback is volume deficiency shock. Volume deficiency shock occurs when the body does not have sufficient blood volume available. This can be the case, for example, after an accident or when there is a lack of fluids. So that the vital organs are nevertheless supplied, a so-called centralization takes place. This means that the vessels in the arms and legs are made narrow. As a result, more blood is available to the internal organs and especially to the brain. So at first glance, the mechanism of centralization makes sense. However, due to the low blood flow, acidic metabolic end products form in the periphery. This leads to what is known as acidosis. This leads to increased vascular permeability and, at the same time, to vasodilatation. As a result, exactly what should actually be avoided by centralization happens. The blood pools in the periphery and is absent from the systemic circulation. The shock can lead to acute kidney failure, acute lung failure or acute liver failure. Insulin resistance in diabetes mellitus is also based on a vicious circle. When food is consumed, the pancreas secretes more insulin. The insulin ensures that the sugar from the food is transported from the blood into the cells. Due to various factors, insulin resistance can develop in the body’s cells. To ensure that the sugar is nevertheless absorbed by the cells, significantly more insulin must be secreted. Higher insulin levels lead to weight gain, but this increases insulin resistance. This in turn leads to increased insulin secretion.
