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| Endocrine glands | |
|---|---|
 The major endocrine glands: | |
| Details | |
| System | Endocrine system |
| Identifiers | |
| Latin | glandulae endocrine |
| MeSH | D004702 |
| TA98 | A11.0.00.000 |
| TA2 | 3852 |
| TH | H2.00.02.0.03072 |
| FMA | 9602 |
| Anatomical terminology | |
The endocrine system is a network of glands and organs located throughout the body. Along with thenervous system, it makes theneuroendocrine system, which controls and regulates many of the body's functions.Endocrine glands are ductlessglands of theendocrine system that secrete their products,hormones, directly into theblood. The major glands of the endocrine system include thepineal gland,pituitary gland,pancreas,ovaries,testicles,thyroid gland,parathyroid gland,hypothalamus andadrenal glands. The hypothalamus and pituitary glands areneuroendocrineorgans.[1]
Thepituitary gland hangs from the base of thebrain by thepituitary stalk, and is enclosed by bone. It consists of a hormone-producing glandular portion of theanterior pituitary and a neural portion of theposterior pituitary, which is an extension of thehypothalamus. The hypothalamus regulates the hormonal output of the anterior pituitary and creates two hormones that it exports to the posterior pituitary for storage and later release.
Four of the six anterior pituitary hormones aretropic hormones that regulate the function of other endocrine organs. Most anterior pituitary hormones exhibit a diurnal rhythm of release, which is subject to modification by stimuli influencing the hypothalamus.
Somatotropic hormone orgrowth hormone (GH) is ananabolic hormone that stimulates the growth of all body tissues especially skeletal muscle and bone. It may act directly, or indirectly viainsulin-like growth factors (IGFs). GH mobilizes fats, stimulates protein synthesis, and inhibits glucose uptake and metabolism. Secretion is regulated bygrowth hormone-releasing hormone (GHRH) andgrowth hormone-inhibiting hormone (GHIH), or somatostatin. Hypersecretion causesgigantism in children andacromegaly in adults;hyposecretion in children causespituitary dwarfism.
Thyroid-stimulating hormone promotes normal development and activity of thethyroid gland.Thyrotropin-releasing hormone stimulates its release; negative feedback of thyroid hormone inhibits it.
Adrenocorticotropic hormone stimulates theadrenal cortex to releasecorticosteroids. Adrenocorticotropic hormone release is triggered bycorticotropin-releasing hormone and inhibited by risingglucocorticoid levels.
Thegonadotropins—follicle-stimulating hormone andluteinizing hormone regulate the functions of the gonads in both sexes. Follicle-stimulating hormone stimulates sex cell production; luteinizing hormone stimulatesgonadal hormone production. Gonadotropin levels rise in response togonadotropin-releasing hormone. Negative feedback of gonadal hormones inhibits gonadotropin release.
Prolactin promotes milk production in human females. Its secretion is prompted byprolactin-releasing hormone and inhibited byprolactin-inhibiting hormone.
The intermediate lobe of the pituitary gland secretes only one enzyme that ismelanocyte stimulating hormone. It is linked with the formation of the black pigment in our skin called melanin.
Theneurohypophysis stores and releases two hypothalamic hormones:
The thyroid gland is located in the front of the neck, in front of thethyroid cartilage, and is shaped like a butterfly, with two wings connected by a centralisthmus. Thyroid tissue consists of follicles with a stored protein called colloid, containingthyroglobulin, a precursor to other thyroid hormones, which are manufactured within the colloid.
Thethyroid hormones increase the rate ofcellular metabolism, and includethyroxine (T4) andtriiodothyronine (T3). Secretion is stimulated by the thyroid-stimulating hormone, secreted by the anterior pituitary. When thyroid levels are high, there is negative feedback that decreases the amount of Thyroid-stimulating hormone secreted. Most T4 is converted to T3 (a more active form) in the target tissues.
Calcitonin, produced by theparafollicular cells (C cells) of the thyroid gland in response to rising blood calcium levels, depresses blood calcium levels by inhibiting bone matrix resorption and enhancing calcium deposit in bones.Excessive secretion cause hyperthyroidism and deficiency cause hypothyroidism.
The parathyroid glands, of which there are 4–6, are found on the back of the thyroid glands, and secreteparathyroid hormone.[2] This causes an increase in blood calcium levels by targeting bone, the intestine, and the kidneys. The parathyroid hormone is the antagonist ofcalcitonin. Parathyroid hormone release is triggered by falling blood calcium levels and is inhibited by rising blood calcium levels.
The adrenal glands are located above the kidneys in humans andin front of the kidneys in other animals. The adrenal glands produce a variety of hormones includingadrenaline and the steroidsaldosteronecortisol andDehydroepiandrosterone sulfate (DHEA).[3] Adrenaline increases blood pressure, heart rate, and metabolism in reaction to stress, the aldosterone controls the body's salt and water balance, the cortisol plays a role in stress response and the dehydroepiandrosterone sulfate (DHEA) produces aids in production of body odor and growth of body hair during puberty.
The pancreas, located in the abdomen, below and behind the stomach, is both anexocrine and an endocrine gland. Thealpha andbeta cells are the endocrine cells in thepancreatic islets that releaseinsulin andglucagon and smaller amounts of other hormones into the blood. Insulin and glucagon influenceblood sugar levels. Glucagon is released when the blood glucose level is low and stimulates the liver to releaseglucose into the blood. Insulin increases the rate of glucose uptake and metabolism by most body cells.
Somatostatin is released bydelta cells and acts as an inhibitor of GH, insulin, and glucagon.
The ovaries of the female, located in the pelvic cavity, release two main hormones. Secretion ofestrogens by the ovarian follicles begins atpuberty under the influence of follicle-stimulating hormone. Estrogens stimulate the maturation of the female reproductive system and the development of secondary sexual characteristics.Progesterone is released in response to high blood levels ofluteinizing hormone. It works with estrogens in establishing themenstrual cycle.
The testes of the male begin to producetestosterone at puberty in response to luteinizing hormone. Testosterone promotes maturation of the male reproductive organs, development of secondary sex characteristics such as increased muscle and bone mass, and the growth of body hair.
The pineal gland is located in thediencephalon of the brain. It primarily releasesmelatonin, which influences daily rhythms and may have anantigonadotropic effect in humans.[citation needed] It may also influence the melanotropes and melanocytes located in the skin.[citation needed]
Many body organs not normally considered endocrine organs contain isolated cell clusters that secrete hormones. Examples include theheart (atrial natriuretic peptide);gastrointestinal tract organs (gastrin,secretin, and others); theplacenta (hormones of pregnancy—estrogen,progesterone, and others); thekidneys (erythropoietin andrenin); thethymus;skin (cholecalciferol); andadipose tissue (leptin andresistin).
Endocrine glands derive from all three germ layers.[citation needed]
The natural decrease in function of the female's ovaries during late middle age results inmenopause. The efficiency of all endocrine glands seems to decrease gradually as ageing occurs. This leads to a generalized increase in the incidence ofdiabetes mellitus and a lowermetabolic rate.
Local chemical messengers, not generally considered part of the endocrine system, includeautocrines, which act on the cells that secrete them, andparacrines, which act on a different cell type nearby.
The ability of a target cell to respond to a hormone depends on the presence of receptors, within the cell or on its plasma membrane, to which the hormone can bind.
Hormone receptors are dynamic structures. Changes in the number and sensitivity of hormone receptors may occur in response to high or low levels of stimulating hormones.
Blood levels of hormones reflect a balance betweensecretion and degradation/excretion. Theliver andkidneys are the major organs that degrade hormones; breakdown products are excreted in urine and faeces.
Hormone half-life and duration of activity are limited and vary from hormone to hormone.
Interaction of hormones at target cellsPermissiveness is the situation in which a hormone cannot exert its full effects without the presence of another hormone.
Synergism occurs when two or more hormones produce the same effects in a target cell and their results are amplified.
Antagonism occurs when a hormone opposes or reverses the effect of another hormone.
The endocrine glands belong to the body's control system. The hormones which they produce help to regulate the functions of cells and tissues throughout the body. Endocrine organs are activated to release their hormones by humoral, neural, or hormonal stimuli. Negative feedback is important in regulatinghormone levels in the blood.
Thenervous system, acting through hypothalamic controls, can in certain cases override or modulate hormonal effects.
Diseases of the endocrine glands are common,[5] including conditions such asdiabetes mellitus,thyroid disease, andobesity.
Endocrine disease is characterized by irregulated hormone release (a productivepituitary adenoma), inappropriate response to signalling (hypothyroidism), lack of a gland (diabetes mellitus type 1, diminishederythropoiesis inchronic kidney failure), or structural enlargement in a critical site such as the thyroid (toxic multinodular goitre). Hypofunction of endocrine glands can occur as a result of the loss of reserve, hyposecretion,agenesis, atrophy, or active destruction. Hyperfunction can occur as a result of hypersecretion, loss of suppression,hyperplastic, orneoplastic change, or hyperstimulation.
Endocrinopathies are classified as primary, secondary, or tertiary. Primary endocrine disease inhibits the action of 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.[citation needed]
As thethyroid, and hormones have been implicated in signaling distant tissues to proliferate, for example, theestrogen receptor has been shown to be involved in certainbreast cancers. Endocrine, paracrine, and autocrine signaling have all been implicated in proliferation, one of the required steps ofoncogenesis.[6]
Other common diseases that result from endocrine dysfunction includeAddison's disease,Cushing's disease andGrave's disease. Cushing's disease and Addison's disease are pathologies involving the dysfunction of the adrenal gland. Dysfunction in the adrenal gland could be due to primary or secondary factors and can result in hypercortisolism or hypocortisolism. Cushing's disease is characterized by the hypersecretion of the adrenocorticotropic hormone due to a pituitary adenoma that ultimately causes endogenous hypercortisolism by stimulating the adrenal glands.[7] Some clinical signs of Cushing's disease include obesity, moon face, and hirsutism.[8] Addison's disease is an endocrine disease that results from hypocortisolism caused by adrenal gland insufficiency. Adrenal insufficiency is significant because it is correlated with decreased ability to maintain blood pressure and blood sugar, a defect that can prove to be fatal.[9]
Graves' disease involves the hyperactivity of the thyroid gland which produces the T3 and T4 hormones.[8]Graves' disease effects range from excess sweating, fatigue, heat intolerance and high blood pressure to swelling of the eyes that causes redness, puffiness and in rare cases reduced or double vision.[citation needed]
Graves' disease is the most common cause ofhyperthyroidism; hyposecretion causescretinism in infants andmyxoedema in adults.
Hyperparathyroidism results inhypercalcemia and its effects and in extreme bone wasting.Hypoparathyroidism leads tohypocalcemia, evidenced by tetany seizure and respiratory paralysis.Hyposecretion of insulin results in diabetes mellitus; cardinal signs are polyuria, polydipsia, and polyphagia.
