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Feed back loops in the human body

Whats an example of a positive OR negative feedback cycle in the human body. Explain why it is positive or negative feedback, describing the entire cycle from beginning to end.

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Let's start with definitions:

Negative Feedback Cycle: "Feedback that reduces the output of a system, such as the action of heat on a thermostat to limit the output of a furnace" - answers.com. In biology there are many examples of negative feedback, most are related to homeostasis, the processes of keeping an animal or human body in balance. Namely, negative feedback is most often used to end a disturbance in the body's normal functioning; for example, when a condition is abnormal, say your blood pressure is high, the high pressure will trigger processes in the body to decrease blood pressure. See below for a more thorough example.

Positive Feedback Cycle: This is just the opposite of negative feedback and is used in nature to enhance a disturbance/response. Positive feedback leads to amplification of the disturbance. A good example from chemistry follows: "Indeed, chemical and nuclear fission based explosives offer an excellent physical demonstration of positive feedback. Bombarding fissile material with neutrons ...

Solution Summary

In this solution I define negative and positive feed back loops in relation to biology and the human body. I then site examples of a number of these loops in operation in the human body.