Circulatory

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Hormones have diverse chemical structures, mainly of 3 classes: The glands that secrete hormones comprise the endocrine signaling system. The term hormone is sometimes extended to include chemicals produced by cells that affect the same cell autocrine or intracrine signalling or nearby cells paracrine signalling.

Hormones are used to communicate between organs and tissues for physiological regulation and behavioral activities, such as digestion, metabolism , respiration , tissue function, sensory perception , sleep , excretion , lactation , stress , growth and development , movement , reproduction , and mood. When a hormone binds to the receptor, it results in the activation of a signal transduction pathway that typically activates gene transcription resulting in increased expression of target proteins; non-genomic effects are more rapid, and can be synergistic with genomic effects.

Hormone secretion may occur in many tissues. Endocrine glands are the cardinal example, but specialized cells in various other organs also secrete hormones. Hormone secretion occurs in response to specific biochemical signals from a wide range of regulatory systems. For instance, serum calcium concentration affects parathyroid hormone synthesis; blood sugar serum glucose concentration affects insulin synthesis; and because the outputs of the stomach and exocrine pancreas the amounts of gastric juice and pancreatic juice become the input of the small intestine , the small intestine secretes hormones to stimulate or inhibit the stomach and pancreas based on how busy it is.

Regulation of hormone synthesis of gonadal hormones , adrenocortical hormones , and thyroid hormones is often dependent on complex sets of direct influence and feedback interactions involving the hypothalamic-pituitary-adrenal HPA , -gonadal HPG , and -thyroid HPT axes.

Upon secretion, certain hormones, including protein hormones and catecholamines, are water-soluble and are thus readily transported through the circulatory system.

Other hormones, including steroid and thyroid hormones, are lipid-soluble; to allow for their widespread distribution, these hormones must bond to carrier plasma glycoproteins e. Some hormones are completely active when released into the bloodstream as is the case for insulin and growth hormones , while others are prohormones that must be activated in specific cells through a series of activation steps that are commonly highly regulated.

The endocrine system secretes hormones directly into the bloodstream , typically via fenestrated capillaries , whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissue. Hormonal signaling involves the following steps: Hormone producing cells are typically of a specialized cell type, residing within a particular endocrine gland , such as the thyroid gland , ovaries , and testes.

Hormones exit their cell of origin via exocytosis or another means of membrane transport. The hierarchical model is an oversimplification of the hormonal signaling process. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin , which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal.

The rate of hormone biosynthesis and secretion is often regulated by a homeostatic negative feedback control mechanism. Such a mechanism depends on factors that influence the metabolism and excretion of hormones. Thus, higher hormone concentration alone cannot trigger the negative feedback mechanism. Negative feedback must be triggered by overproduction of an "effect" of the hormone. One special group of hormones is the tropic hormones that stimulate the hormone production of other endocrine glands.

For example, thyroid-stimulating hormone TSH causes growth and increased activity of another endocrine gland, the thyroid , which increases output of thyroid hormones. To release active hormones quickly into the circulation , hormone biosynthetic cells may produce and store biologically inactive hormones in the form of pre- or prohormones. These can then be quickly converted into their active hormone form in response to a particular stimulus. Eicosanoids are considered to act as local hormones.

They are considered to be "local" because they possess specific effects on target cells close to their site of formation. They also have a rapid degradation cycle, making sure they do not reach distant sites within the body. Most hormones initiate a cellular response by initially binding to either cell membrane associated or intracellular receptors. A cell may have several different receptor types that recognize the same hormone but activate different signal transduction pathways, or a cell may have several different receptors that recognize different hormones and activate the same biochemical pathway.

Receptors for most peptide as well as many eicosanoid hormones are embedded in the plasma membrane at the surface of the cell and the majority of these receptors belong to the G protein-coupled receptor GPCR class of seven alpha helix transmembrane proteins.

The interaction of hormone and receptor typically triggers a cascade of secondary effects within the cytoplasm of the cell, often involving phosphorylation or dephosphorylation of various other cytoplasmic proteins, changes in ion channel permeability, or increased concentrations of intracellular molecules that may act as secondary messengers e.

Some protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism. For steroid or thyroid hormones, their receptors are located inside the cell within the cytoplasm of the target cell. These receptors belong to the nuclear receptor family of ligand-activated transcription factors. To bind their receptors, these hormones must first cross the cell membrane.

They can do so because they are lipid-soluble. The combined hormone-receptor complex then moves across the nuclear membrane into the nucleus of the cell, where it binds to specific DNA sequences , regulating the expression of certain genes , and thereby increasing the levels of the proteins encoded by these genes. Some are associated with the plasma membrane.

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