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Positive and Negative Feedback

Negative and Positive Feedback Mechanisms


The endocrine system helps regulate and maintain various body functions by synthesizing and releasing hormones.  It is composed of glands located through out the body that secrete chemicals called hormones directly into the blood.  Hormones stimulate various body tissues. <_o3a_p>

            The hormone levels in the blood are regulated by a highly specialized homeostatic mechanism called feedback.  Information regarding the hormone level or its effect is fed back to the gland that the hormone secreted from.  Due to positive and negative feedback, our body will be in homeostasis.<_o3a_p>


Negative Feedback

In negative feedback, the response will reverse or cause the opposite effect of the original stimulus.  Negative feedback can be explained with the process of insulin production and release.  After a meal the blood sugar level will be elevated due to the absorption of sugars from the digestive tract.  This triggers the release of insulin from pancreas.  Insulin converts sugar into cells and hence the blood sugar level drops.  This low blood sugar level will result in the cessation of insulin release.  This insulin response is a negative feedback.  Most endocrine glands are under the control of negative feedback mechanisms <_o3a_p>


Another example of negative feedback is the regulation of the blood calcium level. The parathyroid glands secrete parathyroid hormone, which regulates the level of calcium in the blood. If calcium decreases, the parathyroid glands sense the decrease and secrete more parathyroid hormone. The parathyroid hormone stimulates calcium release from the bones and increases the calcium uptake into the bloodstream from the collecting tubules in the kidneys. Conversely, if blood calcium increases too much, the parathyroid glands reduce parathyroid hormone production. Both responses are examples of negative feedback because in both cases the effects are negative (opposite) to the stimulus.


Negative feedback mechanisms act like a thermostat in the home. As the temperature rises (deviation from the ideal normal value), the thermostat detects the change and triggers the air-conditioning to turn on and cool the house. Once the temperature reaches its thermostat setting (ideal normal value), the air conditioning turns off. <_o3a_p>


Positive Feedback

Positive feedback mechanisms are rare.  It amplifies changes rather than reversing them.  The release of oxytocin from the posterior pituitary gland during labor is an example of positive feedback mechanism.  Oxytocin stimulates the muscle contractions that push the baby through the birth canal.  The release of oxytocin result in stronger or augmented contractions during labor. The contractions intensify and increase until the baby is outside the birth canal. When the stimulus to the pressure receptors ends, oxytocin production stops and labor contractions cease.

Positive feedback mechanisms control self-perpetuating events that can be out of control and do not require continuous adjustment. In positive feedback mechanisms, the original stimulus is promoted rather than negated. Positive feedback increases the deviation from an ideal normal value.

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