What is a negative/postive feedback system in homeostasis?
Negative/positive feedback system in homeostasis are "systems" (mechanisms, processes) that respond to a stimulus based on a reference values.
In control engineering, we have two kinds of controls:
1) feedback; 2) feed-forward. In the former, the system just respond, without any possibility to change its behavior, these systems does not seem to exist in biological systems, they tend to be "stupid." Whereas in the latter we have changes due to desired/needed workings.
Further, in the feedback, we have: 1) positive; 2) negative. The majority of the systems in biological systems are negative; positives tend to be unstable; but lesser than feed-forward.
In a positive feedback, you have an increase of the output in response to an input, whereas in negative you have an decrease in response to an input.
For instance, as glucose increases in bloodstream, insulin is produced, after a while, the glucose levels decrease; insulin-glucose is a negative feedback system. Another example, now positive, as glucose levels decrease, glycogen is released and broken down as a response to glucagon, after a while you have an increase in glucose; glucagon-glucose is a positive feedback system. Thus, glucose control is a homeostatic process; problems could signal medical conditions such as diabetes.
You may assume that a homeostatic system is composed of negative and positive feedback, activated exclusively, not at once. Further, there is a neutral domain, where the system is at rest, no feedback.
Some homeostatic systems are mainly negative, e.g. CO2.
Homeostasis. https://en.wikipedia.org/wiki/Homeostasis. Accessed on 26 02 2016.