Endocrine glands consist of clusters of secretory cells surrounded by supporting connective or connective tissue that provides them with blood vessels, lymphatic capillaries, and nerves. The secretory part of the gland is made up of specialized epithelium that has been modified to produce secretions and the secreted products (the hormones) pass into the extracellular space around the secretory cells.
The endocrine glands of the human body include: the pituitary gland, the thyroid gland, the parathyroid glands, the adrenal glands, and the pineal gland. In addition, several organs contain endocrine tissue which, although it does not constitute an endocrine gland by itself, is part of the structure of the organ in question. This is the case in the hypothalamus, thymus, heart, pancreas, stomach, liver, small intestine, kidneys, ovaries, testes, placenta, or in adipose tissue or blood cells such as lymphocytes. Endocrine glands and endocrine tissue constitute the Endocrine System. The science that deals with the structure and functions of the endocrine glands and the diagnosis and treatment of disorders of the endocrine system is called Endocrinology.
A hormone is a chemical substance secreted by a cell or group of cells, which exerts physiological effects on other cells of the organism. There are local hormones that act on target cells close to their site of release. They can be paracrine, such as histamine, which acts on neighboring cells, or autocrine, such as interleukin, which acts on the same cell that secreted it.
There are general or circulating hormones that diffuse from the extracellular space to the interior of the capillaries and are transported by the blood to all the tissues of the organism, acting only on those cells that have specific receptors for them and are therefore called target cells. Some of the general hormones affect all or almost all the cells of the organism, such as growth hormone or thyroid hormones.
Other general hormones affect only specific tissues. Hormone secretions are produced in very low concentrations and have very powerful effects. Circulating hormones can remain in the blood and have their effects within minutes or hours after their secretion. Over time, circulating hormones are inactivated by the liver and excreted by the kidneys. In case of liver or kidney failure the excessive number of hormones or their metabolic products in the blood can cause health problems.
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