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Endocrine System

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  The endocrine system  refers to the collection of  glands of an organism that secrete hormones  directly into the circulatory system to be carried towards a distant target organ. The major endocrine glands include the  pineal gland,  pituitary gland,  pancreas, ovaries, testes, thyroid gland,  parathyroid gland, hypothalamus, gastrointestinal tract and adrenal glands. The endocrine system is in contrast to the exocrine system, which secretes its hormones using ducts. Examples of exocrine glands include the sweat glands, salivary glands, mammary glands, and liver . The endocrine system is an information signal system like the nervous system, yet its effects and mechanism are classifiably different. The endocrine system's effects are slow to initiate, and  prolonged in their response, lasting from a few hours up to weeks. The nervous system sends information very quickly, and responses are generally short lived. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. The field of study dealing with the endocrine system and its disorders is endocrinology, a branch of  internal medicine.  Special features of  endocrine glands are, in general, their ductless nature, their vascularity, and commonly the presence of intracellular vacuoles or granules that store their hormones. In contrast, 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.  In addition to the specialised endocrine organs mentioned above, many other organs that are part of other body systems, such as the kidney, liver , heart and gonads, have secondary endocrine functions. For example the kidney secretes endocrine hormones such as erythropoietin and renin.  A number of glands that signal each other in sequence are usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis.  As opposed to endocrine factors that travel considerably longer distances via the circulatory system, other signaling molecules, such as  paracrine factors involved in  paracrine signalling  diffuse over a relatively short distance. The word endocrine derives from the Greek words ἐνδο - endo-   inside, within, and κρίνειν krinein   to separate, distinguish .  Endocrine glands in the human head and neck and their hormones Hypothalamus  Secreted hormone Abbreviation Produced by Effect Thyrotropin-releasing hormone   TRH Parvocellular neurosecretory neurons   Stimulate thyroid-stimulating hormone (TSH) release from anterior pituitary  (primarily) Dopamine (Prolactin-inhibiting hormone) DA or PIH Dopamine neurons of the arcuate nucleus   Inhibit  prolactin released from anterior  pituitary  Growth hormone-releasing hormone   GHRH  Neuroendocrine neurons of the Arcuate nucleus Stimulate Growth hormone (GH) release from anterior pituitary  Somatostatin (growth hormone-inhibiting hormone) SS, GHIH, or SRIF  Neuroendocrine cells of the Periventricular nucleus Inhibit Growth hormone (GH) release from anterior pituitary Inhibit thyroid-stimulating hormone (TSH)  release from anterior pituitary  Gonadotropin-releasing hormone   GnRH or LHRH  Neuroendocrine cells of the Preoptic area Stimulate follicle-stimulating hormone (FSH) release from anterior pituitary  Stimulate luteinizing hormone (LH) release from anterior pituitary  Corticotropin-releasing hormone   CRH or CRF Parvocellular neurosecretory neurons of the Paraventricular  Nucleus Stimulate adrenocorticotropic hormone (ACTH) release from anterior pituitary  Vasopressin (antidiuretic hormone) ADH or AVP or VP Parvocellular neurosecretory neurons, Magnocellular neurosecretory neurons of the Paraventricular nucleus and Supraoptic nucleus Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume Pineal body (epiphysis) Secreted hormone From cells Effect Melatonin   Pinealocytes   Antioxidant   Monitors the circadian rhythm including induction of  drowsiness and lowering of the core body temperature  Pituitary gland (hypophysis) The  pituitary gland (or hypophysis) is an endocrine gland about the size of a pea and weighing 5 grams (0.18 oz) in humans. It is a protrusion off the bottom of the hypothalamus at the base of the  brain, and rests in a small, bony cavity (sella turcica) covered by a dural fold (diaphragma sellae). The pituitary is functionally connected to the hypothalamus  by the median eminence via a small tube called the infundibular stem or   pituitary stalk . The pituitary fossa, in which the  pituitary gland sits, is situated in the sphenoid  bone in the middle cranial fossa at the base of the  brain. The pituitary gland secretes nine hormones that regulate homeostasis and the secretion of other hormones. [ citation needed  ]   Anterior pituitary lobe (adenohypophysis) Secreted hormone Abbreviation From cells Effect Growth hormone (somatotropin) GH Somatotrophs   Stimulates growth and cell reproduction Stimulates Insulin-like growth factor 1  release from liver   Thyroid-stimulating hormone (thyrotropin) TSH Thyrotrophs   Stimulates thyroxine (T4) and triiodothyronine (T3) synthesis and release from thyroid gland  Stimulates iodine absorption by thyroid gland Adrenocorticotropic hormone (corticotropin) ACTH Corticotrophs   Stimulates corticosteroid (glucocorticoid  and mineralcorticoid) and androgen  synthesis and release from adrenocortical cells  Beta-endorphin  –   Corticotrophs   Inhibits perception of pain Follicle-stimulating hormone   FSH Gonadotrophs   In females: Stimulates maturation of ovarian follicles in ovary  In males: Stimulates maturation of seminiferous tubules In males: Stimulates spermatogenesis  In males: Stimulates production of androgen-binding protein from Sertoli cells of the testes  Luteinizing hormone   LH Gonadotrophs   In females: Stimulates ovulation  In females: Stimulates formation of  corpus luteum In males: Stimulates testosterone synthesis from Leydig cells (interstitial cells)   Prolactin   PRL Lactotrophs   Stimulates milk synthesis and release from mammary glands Mediates sexual gratification  Melanocyte-stimulating hormone   MSH Melanotropes in the Pars intermedia of the Anterior Pituitary Stimulates melanin synthesis and release from skin/hair  melanocytes  Posterior pituitary lobe (neurohypophysis) Secreted hormone Abbreviation From cells Effect Oxytocin   Magnocellular neurosecretory cells   In females: uterine contraction during  birthing, lactation (letdown reflex) when nursing Vasopressin (antidiuretic hormone) ADH or AVP Parvocellular neurosecretory neurons   Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume Endocrine system The endocrine system includes all of the glands of the body and the hormones produced by those glands. The glands are controlled directly by stimulation from the nervous system as well as by chemical receptors in the blood and hormones produced by other glands. By regulating the functions of organs in the body, these glands help to maintain the body’s homeostasis. Cellular metabolism, reproduction, sexual development, sugar and mineral homeostasis, heart rate, and digestion are among the many processes regulated by the actions of hormones.   Anatomy of the Endocrine System Hypothalamus  The  hypothalamus  is a part of the  brain  located superior and anterior to the brain stem and inferior to the  thalamus . It serves many different functions in the  nervous system , and is also responsible for the direct control of the endocrine system through the pituitary gland. The hypothalamus contains special cells called neurosecretory cells  —  neurons that secrete hormones:    Thyrotropin-releasing hormone (TRH)    Growth hormone-releasing hormone (GHRH)    Growth hormone-inhibiting hormone (GHIH)    Gonadotropin-releasing hormone (GnRH)    Corticotropin-releasing hormone (CRH)    Oxytocin    Antidiuretic hormone (ADH)
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