In physiology, the endocrine system is a system of glands, each of which secretes a type of hormone directly into the bloodstream to regulate the body. The endocrine system is in contrast to the exocrine system, which secretes its chemicals using ducts. It derives from the Greek words endo meaning inside, within, and crinis for secrete. 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 for hours to weeks. The nervous system sends information very quickly, and responses are generally short lived. Hormones are substances (chemical mediators) released from endocrine tissue into the bloodstream where they travel to target tissue and generate a response. Hormones regulate various human functions, including Metabolism, growth and development, tissue function, and mood. The field of study dealing with the endocrine system and its disorders is endocrinology, a branch of internal medicine.
Features of endocrine glands are, in general, their ductless nature, their vascularity, and usually the presence of intracellular vacuoles or granules storing 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.
The endocrine system is made up of a series of glands that produce chemicals called hormones. A number of glands that signal each other in sequence is usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis.
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Endocrine organs and secreted hormones
Central nervous system
Hypothalamus
Pineal body (epiphysis)
Secreted hormone | From cells | Effect |
---|---|---|
Melatonin | Pinealocytes | Antioxidant Monitors the circadian rhythm including inducement of drowsiness and lowering of the middle body temperature sleep cycle |
Pituitary Gland (hypophysis)
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 | Uterine contraction Lactation (letdown reflex) |
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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 |
Oxytocin and anti-diuretic hormone are not secreted in the posterior lobe, merely stored.
Thyroid
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Triiodothyronine | T3 | Thyroid epithelial cell | (More potent form of thyroid hormone) Stimulates body oxygen and energy consumption, thereby increasing the basal metabolic rate Stimulates RNA polymerase I and II, thereby promoting protein synthesis |
Thyroxine (tetraiodothyronine) |
T4 | Thyroid epithelial cells | (Less active form of thyroid hormone) (Acts as a prohormone to triiodothyronine) Stimulates body oxygen and energy consumption, thereby increasing the basal metabolic rate Stimulates RNA polymerase I and II, thereby promoting protein synthesis |
Calcitonin | Parafollicular cells | Stimulates osteoblasts and thus bone construction Inhibits Ca2+ release from bone, thereby reducing blood Ca2+ |
Alimentary system
Stomach
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Gastrin (Primarily) | G cells | Secretion of gastric acid by parietal cells | |
Ghrelin | P/D1 cells | Stimulate appetite,
secretion of growth hormone from anterior pituitary gland |
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Neuropeptide Y | NPY | increased food intake and decreased physical activity | |
Somatostatin | D cells | Suppress release of gastrin, cholecystokinin (CCK), secretin, motilin, vasoactive intestinal peptide (VIP), gastric inhibitory polypeptide (GIP), enteroglucagon
Lowers rate of gastric emptying Reduces smooth muscle contractions and blood flow within the intestine.[1] |
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Histamine | ECL cells | stimulate gastric acid secretion | |
Endothelin | X cells | Smooth muscle contraction of stomach[2] |
Duodenum
Secreted hormone | From cells | Effect |
---|---|---|
Secretin | S cells | Secretion of bicarbonate from liver, pancreas and duodenal Brunner's glands
Enhances effects of cholecystokinin Stops production of gastric juice |
Cholecystokinin | I cells | Release of digestive enzymes from pancreas
Release of bile from gallbladder hunger suppressant |
Liver
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Insulin-like growth factor (or somatomedin) (Primarily) | IGF | Hepatocytes | insulin-like effects
regulate cell growth and development |
Angiotensinogen and angiotensin | Hepatocytes | vasoconstriction
release of aldosterone from adrenal cortex dipsogen. |
|
Thrombopoietin | Hepatocytes | stimulates megakaryocytes to produce platelets[3] |
Pancreas
Secreted hormone | From cells | Effect |
---|---|---|
Insulin (Primarily) | β Islet cells | Intake of glucose, glycogenesis and glycolysis in liver and muscle from blood
intake of lipids and synthesis of triglycerides in adipocytes Other anabolic effects |
Glucagon (Also Primarily) | α Islet cells | glycogenolysis and gluconeogenesis in liver
increases blood glucose level |
Somatostatin | δ Islet cells | Inhibit release of insulin[4]
Inhibit release of glucagon[4] Suppress the exocrine secretory action of pancreas. |
Pancreatic polypeptide | PP cells | Self regulate the pancreas secretion activities and effect the hepatic glycogen levels. |
Kidney
Secreted hormone | From cells | Effect |
---|---|---|
Renin (Primarily) | Juxtaglomerular cells | Activates the renin-angiotensin system by producing angiotensin I of angiotensinogen |
Erythropoietin (EPO) | Extraglomerular mesangial cells | Stimulate erythrocyte production |
Calcitriol (1,25-dihydroxyvitamin D3) | Active form of vitamin D3
Increase absorption of calcium and phosphate from gastrointestinal tract and kidneys inhibit release of PTH |
|
Thrombopoietin | stimulates megakaryocytes to produce platelets[3] |
Adrenal glands
Adrenal cortex
Secreted hormone | From cells | Effect |
---|---|---|
Glucocorticoids (chiefly cortisol) | zona fasciculata and zona reticularis cells | Stimulates gluconeogenesis Stimulates fat breakdown in adipose tissue Inhibits protein synthesis Inhibits glucose uptake in muscle and adipose tissue Inhibits immunological responses (immunosuppressive) Inhibits inflammatory responses (anti-inflammatory) |
Mineralocorticoids (chiefly aldosterone) | Zona glomerulosa cells | Stimulates active sodium reabsorption in kidneys Stimulates passive water reabsorption in kidneys, thus increasing blood volume and blood pressure Stimulates potassium and H+ secretion into nephron of kidney and subsequent excretion |
Androgens (including DHEA and testosterone) | Zona fasciculata and Zona reticularis cells | In males: Relatively small effect compared to androgens from testes In females: masculinizing effects (i.e. excessive facial hair) |
Adrenal medulla
Secreted hormone | From cells | Effect |
---|---|---|
Adrenaline (epinephrine) (Primarily) | Chromaffin cells | Fight-or-flight response:
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Noradrenaline (norepinephrine) | Chromaffin cells | Fight-or-flight response:
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Dopamine | Chromaffin cells | Increase heart rate and blood pressure |
Enkephalin | Chromaffin cells | Regulate pain |
Reproductive
Testes
Secreted hormone | From cells | Effect |
---|---|---|
Androgens (chiefly testosterone) | Leydig cells | Anabolic: growth of muscle mass and strength, increased bone density, growth and strength,
Virilizing: maturation of sex organs, formation of scrotum, deepening of voice, growth of beard and axillary hair. |
Estradiol | Sertoli cells | Prevent apoptosis of germ cells[5] |
Inhibin | Sertoli cells | Inhibit production of FSH |
Ovarian follicle / Corpus luteum
Secreted hormone | From cells | Effect |
---|---|---|
Progesterone | Granulosa cells, theca cells | Support pregnancy[6]:
Other:
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Androstenedione | Theca cells | Substrate for estrogen |
Estrogens (mainly estradiol) | Granulosa cells | Structural:
Protein synthesis:
Fluid balance:
Gastrointestinal tract:
Melanin:
Cancer:
Lung function: |
Inhibin | Granulosa cells | Inhibit production of FSH from anterior pituitary |
Placenta (when pregnant)
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Progesterone (Primarily) | Support pregnancy[6]:
Other effects on mother similar to ovarian follicle-progesterone |
||
Estrogens (mainly Estriol) (Also Primarily) | Effects on mother similar to ovarian follicle estrogen | ||
Human chorionic gonadotropin | HCG | Syncytiotrophoblast | promote maintenance of corpus luteum during beginning of pregnancy
Inhibit immune response, towards the human embryo. |
Human placental lactogen | HPL | Syncytiotrophoblast | increase production of insulin and IGF-1
increase insulin resistance and carbohydrate intolerance |
Inhibin | Fetal Trophoblasts | suppress FSH |
Uterus (when pregnant)
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Prolactin | PRL | Decidual cells | milk production in mammary glands |
Relaxin | Decidual cells | Unclear in humans and animals |
Calcium regulation
Parathyroid
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Parathyroid hormone | PTH | Parathyroid chief cell | Calcium:
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Skin
Secreted hormone | From cells | Effect |
---|---|---|
Calcidiol (25-hydroxyvitamin D3) | Inactive form of vitamin D3 |
Miscellaneous
Heart
Secreted hormone | Abbreviation | From cells | Effect |
---|---|---|---|
Atrial-natriuretic peptide | ANP | Cardiac myocytes | Reduce blood pressure by:
reducing systemic vascular resistance, reducing blood water, sodium and fats |
Brain natriuretic peptide | BNP | Cardiac myocytes | (To a lesser degree than ANP) reduce blood pressure by:
reducing systemic vascular resistance, reducing blood water, sodium and fats |
Bone Marrow
Secreted hormone | From cells | Effect |
---|---|---|
Thrombopoietin | liver and kidney cells | stimulates megakaryocytes to produce platelets[3] |
Adipose tissue
Secreted hormone | From cells | Effect |
---|---|---|
Leptin (Primarily) | Adipocytes | decrease of appetite and increase of metabolism. |
Estrogens[11] (mainly Estrone) | Adipocytes |
Major endocrine systems
The human endocrine system consists of several systems that operate via feedback loops. Several important feedback systems are mediated via the hypothalamus and pituitary.[12]
- TRH - TSH - T3/T4
- GnRH - LH/FSH - sex hormones
- CRH - ACTH - cortisol
- Renin - angiotensin - aldosterone
Diseases
![](https://web.archive.org/web/20110803135400im_/http://upload.wikimedia.org/wikipedia/commons/thumb/9/98/Endocrine_disorders_world_map_-_DALY_-_WHO2002.svg/220px-Endocrine_disorders_world_map_-_DALY_-_WHO2002.svg.png)
Diseases of the endocrine system are common,[14] including conditions such as diabetes mellitus, thyroid disease, and obesity. Endocrine disease is characterized by disregulated hormone release (a productive pituitary adenoma), inappropriate response to signaling (hypothyroidism), lack of a gland (diabetes mellitus type 1, diminished erythropoiesis in chronic renal 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 loss of reserve, hyposecretion, agenesis, atrophy, or active destruction. Hyperfunction can occur as a result of hypersecretion, loss of suppression, hyperplastic or neoplastic 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 the thyroid, and hormones have been implicated in signaling distant tissues to proliferate, for example, the estrogen receptor has been shown to be involved in certain breast cancers. Endocrine, paracrine, and autocrine signaling have all been implicated in proliferation, one of the required steps of oncogenesis.[15]
Other types of signaling
The typical mode of cell signaling in the endocrine system is endocrine signaling. However, there are also other modes, i.e., paracrine, autocrine, and neuroendocrine signaling.[16] Purely neurocrine signaling between neurons, on the other hand, belongs completely to the nervous system.
Autocrine
Autocrine signaling is a form of signaling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on the same cell, leading to changes in the cells.
Paracrine
Paracrine signaling is a form of cell signaling in which the target cell is near the signal-releasing cell.
Juxtacrine
juxtacrine signaling is a type of intercellular communication that is transmitted via oligosaccharide, lipid, or protein components of a cell membrane, and may affect either the emitting cell or the immediately adjacent cells.
It occurs between adjacent cells that possess broad patches of closely opposed plasma membrane linked by transmembrane channels known as connexons. The gap between the cells can usually be between only 2 and 4 nm.
Unlike other types of cell signaling (such as paracrine and endocrine), juxtacrine signaling requires physical contact between the two cells involved.
Juxtacrine signaling has been observed for some growth factors, cytokine and chemokine cellular signals.
See also
- Releasing hormones
- Neuroendocrinology
- Nervous system
- Endocrine disruptor
- Human anatomy#Major organ systems
- Endocrine disease
- Endocrinology
References
- ^ Colorado State University - Biomedical Hypertextbooks - Somatostatin
- ^ Endo K, Matsumoto T, Kobayashi T, Kasuya Y, Kamata K (February 2005). "Diabetes-related changes in contractile responses of stomach fundus to endothelin-1 in streptozotocin-induced diabetic rats". J Smooth Muscle Res 41 (1): 35–47. doi:10.1540/jsmr.41.35. PMID 15855738. http://www.jstage.jst.go.jp/article/jsmr/41/1/35/_pdf.
- ^ a b c Kaushansky K (May 2006). "Lineage-specific hematopoietic growth factors". N Engl J Med. 354 (19): 2034–45. doi:10.1056/NEJMra052706. PMID 16687716.
- ^ a b Physiology at MCG 5/5ch4/s5ch4_17
- ^ Pentikäinen V, Erkkilä K, Suomalainen L, Parvinen M, Dunkel L (May 2000). "Estradiol acts as a germinal cell survival factor in the human testis in vitro". J Clin Endocrinol Metab. 85 (5): 2057–67. doi:10.1210/jc.85.5.2057. PMID 10843196. http://jcem.endojournals.org/cgi/pmidlookup?view=long&pmid=10843196.
- ^ a b c d Placental Hormones
- ^ Physiology at MCG 5/5ch9/s5ch9_13
- ^ Hould F, Fried G, Fazekas A, Tremblay S, Mersereau W (1988). "Progesterone receptors regulate gallbladder motility". J Surg Res 45 (6): 505–12. doi:10.1016/0022-4804(88)90137-0. PMID 3184927.
- ^ Hormonal Therapy
- ^ Massaro D, Massaro GD (2004). "Estrogen regulates pulmonary alveolar formation, loss, and regeneration in mice". American Journal of Physiology. Lung Cellular and Molecular Physiology 287 (6): L1154–9. doi:10.1152/ajplung.00228.2004. PMID 15298854.
- ^ Frühbeck G (July 2004). "The adipose tissue as a source of vasoactive factors". Curr Med Chem Cardiovasc Hematol Agents 2 (3): 197–208. doi:10.2174/1568016043356255. PMID 15320786. http://openurl.ingenta.com/content/nlm?genre=article&issn=1568-0169&volume=2&issue=3&spage=197&aulast=Frühbeck.
- ^ Sherwood, L. (1997). Human Physiology: From Cells to Systems. Wadsworth Pub Co
- ^ "Mortality and Burden of Disease Estimates for WHO Member States in 2002" (xls). World Health Organization. 2002. http://www.who.int/entity/healthinfo/statistics/bodgbddeathdalyestimates.xls.
- ^ Kasper et al. (2005). Harrison's Principles of Internal Medicine. McGraw Hill. pp. 2074. ISBN 0-07-139140-1.
- ^ Bhowmick NA, Chytil A, Plieth D, et al. (February 2004). "TGF-beta signaling in fibroblasts modulates the oncogenic potential of adjacent epithelia". Science 303 (5659): 848–51. doi:10.1126/science.1090922. PMID 14764882.
- ^ University of Virginia - HISTOLOGY OF THE ENDOCRINE GLANDS
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