Positive Feedback
When a change occurs in the body, there are two general ways that the body can respond. In negative feedback, the body responds in such a way as to reverse the direction of change. This is the type of response that is involved in maintaining constant conditions inside the body (see Homeostasis.) The second type of response is positive feedback. This means that if a change occurs in some variable, the response is to change that variable even more in the same direction. The result is a continuing spiral of change. (Eventually, negative feedback may take over to put a limit on things.)An example of positive feedback is the phenomenon of autacatalysis, which occurs in some digestive enzymes such as pepsin. Pepsin is a protein-digesting enzyme that works in the stomach. However, the stomach does not secrete pepsin; it secretes an inactive form, called pepsinogen. When one pepsinogen molecule becomes activated, it helps to activate other pepsinogens nearby, which in turn can activate others. In this way, the number of active pepsin molecules can increase rapidly, by using positive feedback.
The pepsinogen-pepsin system is thought to be one of the ways that the stomach avoids digesting itself. Pepsin is a powerful enzyme for breaking down proteins, and could damage the very cells that produce it. By secreting it in an inactive form, the cells in the stomach's lining play it safe. Other cells, which produce mucus, help protect the stomach lining once pepsin is activated.