Endocrine system



The endocrine system is a chemical messenger system comprising feedback loops of the hormones released by internal glands regulating distant target organs. 



The major endocrine glands are the thyroid gland and the adrenal glands. 



The hypothalamus is the neural control center for all endocrine systems. 



The glands that signal each other in sequence are referred to as an axis: such as the hypothalamic-pituitary-adrenal axis. 



Many other organs are part body systems have secondary endocrine functions, including bone, kidneys, liver, heart and gonads: kidney secretes the endocrine hormone erythropoietin. 



Hormones can be amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins.



The glands that secrete hormones comprise the endocrine system. 



The endocrine system is contrasted to exocrine glands, which secrete hormones to the outside of the body.



Paracrine signalling occurs between cells over a relatively short distance. 



Endocrine glands have no ducts.



Endocrine glands are vascular, and commonly have intracellular vacuoles or granules that store their hormones. 



Exocrine glands, such as salivary glands, sweat glands, and glands within the gastrointestinal tract, tend to be much less vascular and have ducts or a hollow lumen.



The endocrine system consists of several systems that operate via feedback loops: Several important feedback systems are mediated via the hypothalamus and pituitary.



TRH – TSH – T3/T4



GnRH – LH/FSH – sex hormones



CRH – ACTH – cortisol



Renin – angiotensin – aldosterone



leptin vs. insulin



Endocrine glands secrete their products, hormones, directly into interstitial spaces and then absorbed into blood rather than through a duct. 



The major glands of the endocrine system include: pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. 



The hypothalamus and pituitary gland are neuroendocrine organs.



There are many types of cells that make up the endocrine system.






Anterior pituitary gland



Pineal gland



Posterior pituitary gland



Thyroid gland



Thyroid follicular cells of the thyroid gland produce and secrete T3 and T4 in response to elevated levels of TRH.



TRH is produced by the hypothalamus, and subsequent elevated levels of TSH, produced by the anterior pituitary gland, which further regulates the metabolic activity and rate of all cells, including cell growth and tissue differentiation.



The parathyroid gland epithelial cells  are supplied with blood from the inferior and superior thyroid arteries and secrete ((parathyroid hormone)) (PTH). 



PTH acts on bone, the kidneys, and the GI tract to increase calcium reabsorption and phosphate excretion, stimulates the conversion of Vitamin D to its most active variant, 1,25-dihydroxyvitamin D3, which further stimulates calcium absorption in the GI tract.



A ((hormone)) is any of a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behavior. 



Hormones function to communicate between organs and tissues for physiological regulation and behavioral activities.



Hormones aid in digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress, growth and development, movement, reproduction, and mood.



Hormones bind to specific receptor proteins in the target cell resulting in a change in cell function: changes to the activity of existing proteins, or slower changes in the expression of target genes. 



The amino acid–based hormones, amines and peptide or protein hormones, are water-soluble and act on the surface of target cells via signal transduction pathways.



Steroid hormones are lipid-soluble, and move through the plasma membranes of target cells to act within their nuclei.



Typically endocrine system uses  the circulatory system to reach distant target organs. 



Other modes, include: paracrine, autocrine, and neuroendocrine signaling. 



Autocrine signaling: a cell secretes a hormone or chemical messenger that binds to autocrine receptors on the same cell, leading to changes in the cells.



Paracrine signaling refers to slower acting, targeting cells in the same tissue or organ.



A juxtacrine signal refers to a type of intercellular communication that is transmitted by oligosaccharides, lipids, or protein components of a cell membrane.



A juxtacrine signal may affect either the emitting cell or the immediately adjacent cells.



A juxtacrine signal occurs between adjacent cells that possess closely opposed plasma membranes, linked by transmembrane channels (connexons).



The gap between the cells is usually be between only 2 and 4 nm.



Diseases of the endocrine system are common: diabetes mellitus, thyroid disease, and obesity. 



Endocrine diseases are characterized by abnormal hormone release, inappropriate response to signaling, lack of a gland, or structural enlargement in a gland.



Reasons for an endocrine gland hypo function: loss of reserve, hyposecretion, agenesis, atrophy, and active destruction. 



Endocrine gland hyperfunction can occur as a result of hypersecretion, loss of suppression feedback, hyperplastic or neoplastic change, or hyperstimulation.



Endocrinopathies are categorized as primary, secondary, or tertiary. 



Primary endocrine disease inhibits its action on downstream glands. 



Secondary endocrine disease is indicative of a problem with the pituitary gland. 



Tertiary endocrine disease is associated with dysfunction of the hypothalamus and its releasing hormones.



Endocrine, paracrine, and autocrine signaling have all been implicated in proliferative steps of oncogenesis.



Hormone dysfunctional related diseases: include Addison’s disease, Cushing’s disease and Graves’ disease. 



Cushing’s disease and Addison’s disease are pathologies involving the dysfunction of the adrenal gland. 



Addison’s disease is an endocrine disease that results from hypocortisolism caused by adrenal gland insufficiency, and it is correlated with decreased ability to maintain blood pressure and blood sugar, a defect that can prove to be fatal.



Graves’ disease involves the hyperactivity of the thyroid gland which produces the T3 and T4 hormones:


excess sweating, fatigue, heat intolerance and high blood pressure.




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