There are two types of feedback systems. Negative feedback systems and positive feedback systems.In negative feedback systems a small change in one direction is detected by sensory receptors and effectors work to reverse the change caused by the initial stimulus. Negative feedback systems are important in maintaining optimal pH, temperature, blood sugar levels and water balance.Positive feedback systems reinforce the change. There are relatively few in the body. The blood clotting cascade is under positive feedback control as is uterine contraction during childbirth. There are also situations where positive feedback control is harmful such as hypothermia.