A hormone (frae Greek ὁρμή, "impetus") is ony member o a cless o signalin molecules produced bi glands in multicellular organisms that are transportit bi the circulatory system tae target distant organs tae regulate physiology and behaviour. Hormones hae diverse chemical structurs, mainly o 3 clesses: eicosanoids, steroids, an amino acid/protein derivatives (amines, peptides, an proteins). The glands that secrete hormones comprise the endocrine seegnalin system. The term hormone is whiles extendit tae include chemicals produced bi cells that affect the same cell (autocrine or intracrine seegnallin) or nearbi cells (paracrine seegnallin).
Hormones are uised tae communicate atween organs an tishies for physiological regulation an behavioural activities, sic as digeestion, metabolism, respiration, tishie function, sensory perception, sleep, excretion, lactation, stress, growthe an development, muivement, reproduction, an muid.[1][2] Hormones affect distant cells bi bindin tae speceefic receptor proteins in the target cell resultin in a chynge in cell function. Whan a hormone binds tae the receptor, it results in the activation o a seegnal transduction pathwey that teepically activates gene transcription resultin in increased expression o target proteins; non-genomic effects are mair rapid, an can be synergistic wi genomic effects.[3] Amino acid–based hormones (amines an peptide or protein hormones) are watter-soluble an act on the surface o target cells via seicont messengers; steroid hormones, bein lipid-soluble, muive throu the plasma membranes o target cells (baith cytoplasmic an nuclear) tae act within thair nuclei.
Hormone secretion mey occur in mony tishies. Endocrine glands are the cardinal example, but speicialised cells in various ither organs an aw secrete hormones. Hormone secretion occurs in response tae speceefic biochemical seegnals frae a wide range o regulatory systems. For instance, serum calcium concentration affects parathyroid hormone synthesis; bluid succar (serum glucose concentration) affects insulin synthesis; an acause the ootputs o the painch an exocrine pancreas (the amoonts o gastric juice an pancreatic juice) acome the input o the smaw thairm, the smaw thairm secretes hormones tae stimulate or inhibit the painch an pancreas based on hou busy it is. Regulation o hormone synthesis o gonadal hormones, adrenocortical hormones, an thyroid hormones is eften dependent on complex sets o direct influence an feedback interactions involvin the hypothalamic-pituitary-adrenal (HPA), -gonadal (HPG), an -thyroid (HPT) aixes.
Upon secretion, certaint hormones, includin protein hormones an catecholamines, are watter-soluble an are sicweys readily transportit throu the circulatory system. Ither hormones, includin steroid an thyroid hormones, are lipid-soluble; tae allou for their widespread distribution, thir hormones maun bond tae carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) tae form ligand-protein complexes. Some hormones are completely active whan released intae the bluidstream (as is the case for insulin an growthe hormones), while ithers are prohormones that maun be activatit in speceefic cells throu a series o activation steps that are commonly heichly regulatit. The endocrine system secretes hormones directly intae the bluidstream teepically intae fenestratit capillaries, whauras the exocrine system secretes its hormones indirectly uisin ducts. Hormones wi paracrine function diffuise throu the interstitial spaces tae nearbi target tishie.
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