Hormones
Last reviewed: 23.04.2024
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Hormones are a group of compounds of various chemical structures characterized by the ability, after isolation from the cells in which they form, to reach the target cells (most often with blood) and by binding to specific protein molecules of target cells (receptors) to cause in the latter more or less specific changes in metabolism. About 100 hormones are described in humans. After inactivation, hormones are excreted from the body in an inactive form. The rate of formation and destruction of hormones depends on the needs of the body.
The main sites for the synthesis of hormones are the hypothalamus, anterior and posterior pituitary glands, thyroid and parathyroid glands, pancreatic islets, the cortex and adrenal medulla, sex glands, placenta, certain cells of the gastrointestinal tract, brain, myocardium, adipose tissue. Hormones can also form tumors of non-endocrine tissues (the so-called ectopic production of hormones).
Transport of hormones is carried out by blood. Most hormones (especially protein and peptide nature) are highly soluble in water, hence, in blood plasma. The exception is T 4 and steroid hormones. They are transported by blood with the help of special carrier proteins. Solubility and interaction with the carrier affect the half-life of hormones in the blood. Most peptide hormones have a very short half-life of 20 minutes or less. Hydrophobic steroid hormones have a significantly longer half-life (cortisol about 1 h, T 4 - 7 days).
Hormones circulate in the blood at very low concentrations (usually about 10 -6 -10 -9 moles / L), but the number of molecules corresponding to this concentration is huge (10 17 -10 14 molecules / liter) - virtually trillions of molecules per 1 liter of blood . This huge amount of hormone molecules makes possible their influence on each individual cell of the body and the regulation of its specific metabolic processes. Circulating hormones do not act on all cells in the same way. The selectivity of the action of hormones is provided by specific receptor proteins localized on the cell membrane or in the cytoplasm of target cells. The number of receptors on cell membranes can be thousands or even tens of thousands. The number of receptors on the target cell is variable and is regulated, as a rule, by the action of the corresponding hormones. Usually, with a constantly elevated concentration of the hormone in the blood, the number of its receptors decreases. Specificity of receptors in many cases is low, therefore they can bind not only hormones, but also compounds similar to them in structure. The latter circumstance can cause a violation of hormonal regulation, manifested by tissue resistance to the action of hormones.
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