Answer
Both negative and positive feedback mechanisms are used to maintain homeostasis in the body, keeping the body's internal conditions within a narrow range. However, negative feedback mechanisms are much more commonly used, while positive feedback mechanisms are used much more rarely. In a negative feedback mechanism a stimulus (such as rising body temperature or low blood glucose levels) is detected by a receptor, which then sends signals to a control center. The control center determines the response to the stimulus, and this response is carried out by an effector. Two examples of variable controlled by negative feedback mechanisms are body temperature and blood volume levels. In a positive feedback mechanism, the body also reacts to a stimulus, but in this case the body's response increases the original stimulus, which in turn increases the body's response. Therefore, the stimulus and response build off of each other, creating a "cascade" effect. The process of blood clotting is controlled by a positive feedback mechanism.
Work Step by Step
Negative and positive feedback mechanisms are discussed on pages 9-11. Each mechanism is first triggered by a stimulus (such as rising body temperature), which is detected by a receptor. The receptor sends information called "input" to a control center, which analyzes the input and determines the body's response. The control center then sends "output" to an effector, which carries out the body's response. A negative feedback mechanism reduces the effects of the stimulus (think of sweating in response to the stimulus of a rising body temperature). A positive feedback mechanism increases the effects of the stimulus.
