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Homeostatic Feedback Mechanisms

Positive and negative feedback systems are used by the body to maintain homeostasis and keep internal body fluctuations close to the set point.

The hormones produced by the glands of the endocrine system play an important role in homeostasis. An effector gland is a structure of the endocrine system that releases hormones in the body. Effector glands are one of the three structural elements required to regulate homeostasis. When the endocrine system uses the effector glands to release hormones or chemical messengers in the body, these hormones help regulate cellular activity. However, this hormonal release is controlled by a stimulus. The stimulus is influenced by internal and external changes occurring in the body. The body controls this connection between a stimulus and hormonal response through the action of feedback regulation. There are two types of homeostatic feedback regulatory mechanisms—negative feedback and positive feedback.

Negative feedback is a homeostatic feedback system that counteracts a stimulus by inhibiting the direction of the stimulus. A negative feedback system counteracts the original change made by the body in response to a stimulus.

An example of negative feedback is the regulation of blood sugar, primarily glucose, levels. After an animal has eaten, blood glucose levels rise. Cells in the pancreas, a digestive accessory organ, sense the increase and release the hormone insulin in response. Insulin causes blood glucose levels to decrease. The decrease in blood glucose then shuts off insulin production, completing the negative feedback loop. However, when an animal has not eaten, blood glucose levels decrease. The decrease in blood glucose level is sensed by another type of cell in the pancreas. These cells release the hormone glucagon, which causes the breakdown of glycogen, a storage carbohydrate in animals, to glucose. Glucose is released and blood levels increase. The rise in blood glucose inhibits the further production of glycogen. These are both negative feedback loops as the response is to reverse the original stimulus (changing glucose levels) that originally initiated a homeostatic response. There are many other negative feedback mechanisms used to maintain homeostasis of a variety of internal conditions, including blood pressure, electrolyte balance, fluid volume, and oxygen levels.
A negative feedback loop is a regulatory mechanism that helps the body restore a body value (whether upper or lower) to the ideal body value. For example, this loop can be used in an animal's body to raise its internal temperature in response to a cold environmental stimulus.
Positive feedback is a homeostatic feedback system that maintains a stimulus, often increasing the action of a stimulus. For example, when an animal experiences an injury, the blood will clot in response to the injured site. This clotting occurs because the blood vessel is damaged. Thus, in response to this stimulus, the body will recruit platelets (cell fragments in the blood) to the site. As platelets aggregate to the site, positive feedback occurs when the platelets release chemicals, stimulating the attraction of more platelets to the injured site. This positive feedback helps speed up the rate of wound healing. Positive feedback is less common in biological systems. However, another example is the production of milk by mammals as the baby suckles. This suckling stimulates the mammary glands to produce more milk for the baby to drink.
When an animal injures itself and a blood vessel wall is damaged, the positive feedback system will function to recruit platelets (a component of blood that helps form a clot) to the site of injury as a way to facilitate wound healing. Positive feedback systems are implemented to maintain the direction of the original stimulus or by increasing it in certain cases.