![]() When the level of blood glucose falls sufficiently, the stimulus for insulin release disappears and insulin is no longer secreted.Insulin has the major effect of facilitating entry of glucose into many cells of the body - as a result, blood glucose levels fall.Elevation of blood glucose concentration stimulates endocrine cells in the pancreas to release insulin.Glucose from the ingested lactose or sucrose is absorbed in the intestine and the level of glucose in blood rises.Drink a glass of milk or eat a candy bar and the following (simplified) series of events will occur: As thyroid hormone levels decay below the threshold, negative feedback is relieved, TRH secretion starts again, leading to TSH secretion.Īnother type of feedback is seen in endocrine systems that regulate concentrations of blood components such as glucose. Inhibition of TRH secretion leads to shut-off of TSH secretion, which leads to shut-off of thyroid hormone secretion. This is an example of "negative feedback". When blood concentrations of thyroid hormones increase above a certain threshold, TRH-secreting neurons in the hypothalamus are inhibited and stop secreting TRH.TSH binds to receptors on epithelial cells in the thyroid gland, stimulating synthesis and secretion of thyroid hormones, which affect probably all cells in the body.Neurons in the hypothalamus secrete thyroid releasing hormone (TRH), which stimulates cells in the anterior pituitary to secrete thyroid-stimulating hormone (TSH).The basic mechanisms for control in this system (illustrated to the right) are: The thyroid hormones thyroxine and triiodothyronine ("T4 and T3") are synthesized and secreted by thyroid glands and affect metabolism throughout the body. An important example of a negative feedback loop is seen in control of thyroid hormone secretion. When temperature drops back below the set point, negative feedback is gone, and the furnace comes back on.įeedback loops are used extensively to regulate secretion of hormones in the hypothalamic-pituitary axis. When the furnace produces enough heat to elevate temperature above the set point of the thermostat, the thermostat is triggered and shuts off the furnace (heat is feeding back negatively on the source of heat). The heating system in your home is a simple negative feedback circuit. Negative feedback is seen when the output of a pathway inhibits inputs to the pathway. Instances of positive feedback certainly occur, but negative feedback is much more common. Shutting off secretion of a hormone that has a very short halflife causes circulating hormone concentration to plummet, but if a hormone's biological halflife is long, effective concentrations persist for some time after secretion ceases.įeedback circuits are at the root of most control mechanisms in physiology, and are particularly prominent in the endocrine system. Rate of degradation and elimination: Hormones, like all biomolecules, have characteristic rates of decay, and are metabolized and excreted from the body through several routes.Rate of delivery: An example of this effect is blood flow to a target organ or group of target cells - high blood flow delivers more hormone than low blood flow.Such control is mediated by positive and negative feedback circuits, as described below in more detail. Rate of production: Synthesis and secretion of hormones are the most highly regulated aspect of endocrine control.The concentration of hormone as seen by target cells is determined by three factors: Almost inevitably, disease results when hormone concentrations are either too high or too low, and precise control over circulating concentrations of hormones is therefore crucial. The physiologic effects of hormones depend largely on their concentration in blood and extracellular fluid. Endocrine System > Hormones, Receptors and Control Systems Control of Endocrine Activity
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