Positive and Negative Feedback Systems

The body can regulate its internal environment through feedback systems. A feedback system is a cycle of events in which the status of a body condition is monitored, evaluated, changed, remonitored, reevaluated and so on. Each monitored variable, such as body temperature, blood pressure, or blood glucose level, is called a controlled condition. Any disruption that changes a controlled condition is called a stimulus

A feedback system has three basic parts: a receptor, a control center, and an effector. A receptor is a body structure such as a nerve ending or sensory receptor that monitors changes in a controlled condition and sends input, usually in the form of a nerve impulse or chemical signal, to a control center, which in the body is typically the brain. A control center sets the range of values within which a controlled condition should be maintained, evaluates the input it receives, and generates output commands, usually in the form of nerve impulses, hormones, or other chemical signals.

An effector is a body structure, such as a muscle or gland that receives output from the control center and produces a response or effect that changes the controlled condition. Almost every organ or tissue in the body can behave as an effector. Specific examples include muscle and glands.

Negative feedback mechanisms

Almost all homeostatic control mechanisms are negative feedback mechanisms. These mechanisms change the variable back to its original state or “ideal value”.

A good example of a negative feedback mechanism is a home thermostat (heating system). The thermostat contains the receptor (thermometer) and control center. If the heating system is set at 70 degrees Fahrenheit, the heat (effector) is turned on if the temperature drops below 70 degrees Fahrenheit. After the heater heats the house to 70 degrees Fahrenheit, it shuts off effectively maintaining the ideal temperature.

The control of blood sugar (glucose) by insulin is another good example of a negative feedback mechanism. When blood sugar rises, receptors in the body sense a change . In turn, the control center (pancreas) secretes insulin into the blood effectively lowering blood sugar levels. Once blood sugar levels reach homeostasis, the pancreas stops releasing insulin.

Positive feedback mechanisms
A positive feedback mechanism is the exact opposite of a negative feedback mechanism. With negative feedback, the output reduces the original effect of the stimulus. In a positive feedback system, the output enhances the original stimulus. A good example of a positive feedback system is child birth. During labor, a hormone called oxytocin is released that intensifies and speeds up contractions. The increase in contractions causes more oxytocin to be released and the cycle goes on until the baby is born. The birth ends the release of oxytocin and ends the positive feedback mechanism.

Another good example of a positive feedback mechanism is blood clotting. Once a vessel is damaged, platelets start to cling to the injured site and release chemicals that attract more platelets. The platelets continue to pile up and release chemicals until a clot is formed.

Just remember that positive feedback mechanisms enhance the original stimulus and negative feedback mechanisms inhibit it.