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Endocrine System
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Endocrine System
Exocrine glands- secrete their products into
ducts. The endocrine glands secrete their
products (hormones) into the extracellular space around the secretory cells
The human body contains two different
kinds of glands:
are the pituitary (hypophysis), thyroid, parathyroids, adrenals (suprarenals), pancreas, testes, ovaries, pineal (epiphysis cerebri), and thymus.
Their products, called hormones, are used to maintain homoestasis by changing the physiological activities of cells.
Target cells- cells that respond to the effects of a hormone. These cells contain receptors for the hormone and thus bind to it, leading to a response.
The Endocrine Glands
Earl W. Sutherland, Jr. who
received the Nobel Prize in 1971 proposed a hypothesis explaining the mechanism that controls hormonal activity.
Certain organs and tissues respond to a hormone, but others do not. Different organs may respond differently to the same hormone. For example a pituitary hormone, FSH stimulates the maturation of eggs.
Hormonal Action
The cell’s DNA determine the sequence of events
triggered by a hormone. The hormones produced by the endocrine glands
are either the steroids, which are lipid soluble, and the protein hormones.
The steroids can enter a cell through its membrane while protein hormones cannot. The steroid hormones include the following: testosterone, progesterone, estrogens, aldosterone and corticoids. Protein hormones are insulin, glucagoo, thyroxene and the pituitary hormones.
Hormonal Action
Steroid hormones work differently. These fat-
soluble hormones pass through the membrane and once inside the cell, they combine with receptors.
The hormone receptor complex moves into the nucleus and attaches at a particular location on a chromosome. The attachment activates one or more genes that can cause the synthesis of new proteins that are related to the cell’s specific function.
Hormonal Action
Hormonal ActionAccording to Sutherland, when the
protein hormone binds to its receptor in the cell membrane, an enzyme
called adenylate cyclase is activated in the cell.
Adenylate cyclase catalyzes the conversion of ATP to cyclic AMP
(adenosine monophosphate. Cyclic AMP is called a ‘second messenger’ which causes the
response inside the cell that the hormone (first messenger)
began. The presence of cyclic AMP sets in motion the response
programmed in the cell’s DNA.
EndocrineGlands
Also called the hypophysis, the pituitary gland is located directly under the hypothalamus portion of the brain. The pituitary is about the size of a bean and is connected by a stalk of nerve cells and blood vessels to the hypothalamus. This shows a direct link between the nervous system and the endocrine system.
Pituitary Gland
Pituitary Gland
The daily secretions of the pituitary are only
less than one millionth of a gram yet they control most of the other endocrine glands. That is the reason why the pituitary is often referred to as the master gland.
It is the anterior lobe of the pituitary which functions as the master control system; it is in turn, controlled by the hypothalamus (brain).
Pituitary Gland
The anterior lobe secretes the following hormones.1. The thyroid stimulating hormones (TSH) stimulates the thyroid to secrete thyroxine;2. The adrenocorticotropic hormone (ACTH) stimulates cortex of the adrenal glands; 3. The gonadotropic hormone or the follicle-stimulating hormone (FSH);4. The luteinizing hormone (LH) which controls the secretion of ovaries and testes;5. The somatotropic hormone (STA) or growth hormone; and6. The prolactin a hormone that stimulate milk production in the mammary glands.
Pituitary Gland
The posterior pituitary secretes the following hormones:
1. Oxytocin which stimulates uterine contractions during pregnancy, and milk released after childbirth; and
2. Antidiuretic hormone (ADH) or vasopressin which aids water reabsorption in the kidneys and stimulates the smooth muscles of the arteries.
Pituitary Gland
Pituitary GlandPituitary gland
Located at the base of the brain, the pituitary gland is protected
by a bony structure called the sella turcica of
the sphenoid bone.
Median sagittal through the hypophysis of an adult monkey.
Semidiagrammatic.
Location of Pituitary Gland
Pituitary Gland
A normal sized hand (left) and the
enlarged hand of someone with
acromegaly (right)
Diagram of pituitary and pineal glands in
the human brain
Oversecretion of the growth hormone during the period of skeletal development causes giantism. The body grows beyond the normal size and may reach more than 2 ½ meters (8 feet) in height, and 180 kilograms (400 pounds) in weight. Death usually occurs before the person reaches 30 years of age.
Acromegaly is a form of giantism that occurs in adults after skeletal development is complete. The skeleton does not lengthen any further, but some cartilage and bones thicken. This widens the face, hands and feet.
An insufficient secretion of the growth hormone produces the dwarf or midget. The height of the pituitary dwarf is about the size of a six-year-old child. A dwarf has a normal intelligence and normal body proportions. He usually dies before he reaches fifty. Some dwarfism is caused by the deficiencies of the thyroid hormone rather than the pituitary growth hormone.
Disorders of the Pituitary Gland
The thyroid gland is situated near the junction
of the larynx and the trachea. The two lobes of the thyroid are connected by a bridge of tissue called an isthmus. The thyroid secretes thyroxine, an amino acid containing iodine.
Thyroxine controls the metabolism, the rate at which food and oxygen are used to generate heat and energy. The synthesis of thyroxine and its release are controlled by a thyroid-stimulating hormone (TSH) of the pituitary gland.
Thyroid Gland
The overactivity of the thyroid gland results in
hyperthyroidism (Graves’ disease). An oversecretion of TSH of the pituitary or a thyroid tumor causes this condition. The symptoms includes nervousness, irritability, increased heart rate and blood pressure, weakness, weight loss, elevated use of oxygen at rest and bulging eyes. Drug therapy and radioactive iodine have replaced surgery as a treatment for hyperthyroidism.
Disorders of the Thyroid Glands
The underactivity of the thyroid is called
hypothyroidism. This condition causes a swelling in the neck due to an enlarged thryroid. Hypothyroidism, usually called goiter, is caused by insufficient iodine in the diet. Most cases of persons with goiter live in the areas far from the ocean where iodine content in the soil and water supply is low. Goiter practically disappeared when small amounts of iodine where added to the table salt and drinking water in these affected areas.
Disorders of the Thyroid Glands
The symptoms of hypothyroidism include
decreased heart rate, blood pressure, and body temperature; lowered basal metabolism rate; and underactivity of the nervous system. Hypothyroidism during infancy causes cretinism, which results in a mental retardation and irregular development of bones and muscles. If the condition occurs during adulthood, myxedema results. Swollen facial features and possible mental retardation are the effects of myxedema.
Disorders of the Thyroid Glands
parathyroid
gland
WHAT ARE THE PARATHYROID GLANDS?
Theparathyroidglandsarefourtinyglands,locatedintheneck,thatcontrolthebody'scalciumlevels.
Size-agrainofrice(weighsapproximately30milligramsandis3-4millimetersindiameter).
Theparathyroidsproduceahormonecalledparathyroidhormone(PTH).
WHAT IS PARATHYROID HORMONE?
PTHraisesthebloodcalciumlevelby: ■breakingdownthebone(wheremostofthebody's
calciumisstored)andcausingcalciumrelease ■increasingthebody'sabilitytoabsorbcalciumfrom
food Whenthebloodcalciumlevelistoolow,PTHis
releasedtobringthecalciumlevelbackuptonormal.Whenthecalciumlevelisnormalorgetsalittletoohigh,normalparathyroidswillstopreleasingPTH.
WHY DO WE NEED CALCIUM?
Weneedcalciumtobuildandmaintainstrongbonesandhealthycommunicationbetweenthebrainandvariouspartsofthebody.
itregulatesmusclecontractionincludingtheheartbeat.
italsoplaysakeyroleinnormalbloodcoagulation(clotting)
In mammals, the adrenal glands (also known as suprarenal glands) are endocrine glands that are located on the top of the kidneys. They are chiefly responsible for releasing hormones in response to stress through the synthesis of corticosteroids such as cortisol and catecholamines such as adrenaline (epinephrine) and noradrenaline. They also produce androgens in their innermost cortical layer.
The adrenal glands affect kidney function through the secretion of aldosterone, and recent data (1998) suggest that adrenocortical cells under pathological as well as under physiological conditions show neuroendocrine properties; within normal adrenal glands, this neuroendocrine differentiation seems to be restricted to cells of the zona glomerulosa and might be important for an autocrine regulation of adrenocortical function
The two-adrenal glands are located on top of the kidneys. The outer portion of each adrenal is the cortex and the inner part is the medulla. The adrenal cortex provides more than fifty identified cortical hormones. The most widely known are the mineralocorticoid group, the glucocorticoids and the cortical sex hormones (mostly male androgens).
The adrenal medulla secretes epinephrine or adrenalin when stimulated by the nervous system. This hormone permits the body to react quickly and strongly during emergencies (the “flight or fight” effect). The secretion of adrenalin is important for the body’s survival in times of stress.
Disorders of the
Underactivity of the cortex produces Addison’s disease. The symptoms of this condition include anemia, weakness, fatigue, increased blood potassium, and decrease blood sodium. Overactivity of the cortex produces Cushing’s disease and the adrenogenital syndrome. The symptoms of Cushing’s disease include fattening of the face, chest and the abdomen with striations on the abdomen. The adrenogenital syndrome makes a female acquire male characteristics while, in males, it causes an accelerated sexual development.
PANCREAS
PANCREAS
The pancreas is located near the stomach and duodenum of the small intestine. As secretory organ, the pancreas has the following functions:
1. To produce and release digestive enzymes into the small intestines,
2. To secrete several hormones into the bloodstream.
PANCREAS
The true endocrine portion of the pancreas is found in a cluster of cells called the islets of Langerhans. The alpha cells of the islets of Langerhans secrete glucagon and the beta cells secrete insulin. When the blood sugar rises insulin is secreted. Insulin facilitates glucose transport across the cellular membranes and helps convert glucose to glycogen in the liver. The liver acts as regulator of blood sugar by taking up excess glucose when insulin and blood sugar are high. Sugar is released into the blood by the liver when insulin and blood sugar are low. It is the hormone, glucagon that stimulates the liver to convert stored glycogen to glucose, thereby increasing the blood sugar concentration.
PANCREAS
When the beta cells of the islets of Langerhans do not produce enough insulin, the person develops diabetes mellitus, an inherited disease. Medicinal insulin is used to treat this disease. Low blood sugar or hypoglycemia is caused by excessive insulin. This condition can be controlled by a regulated diet, low in carbohydrates. Carbohydrates tend to stimulate large secretions of insulin, thereby removing the sugar from the bloodstream. Disorders of the
Gonadsarethemaleandfemaleprimaryreproductiveorgans.Themale
gonadsarethetestesandthefemalegonadsaretheovaries.These
reproductivesystemorgansarenecessaryforsexualreproductionasthey
areresponsiblefortheproductionofmaleandfemalegametes.Gonads
alsoproducesexhormonesneededforthegrowthanddevelopmentof
primaryandsecondaryreproductiveorgansandstructures.
Gonads
Asacomponentoftheendocrinesystem,bothmaleandfemalegonadsproducesex
hormones.Maleandfemalesexhormonesaresteroidhormonesandassuch,canpassthrough
thecellmembraneoftheirtargetcellstoinfluencegeneexpressionwithincells.Gonadal
hormoneproductionisregulatedbyhormonessecretedbytheanteriorpituitaryinthebrain.
Hormonesthatstimulatethegonadstoproducesexhormonesareknownasgonadotropins.
Thepituitarysecretesthegonadotropinsluteinizinghormone(LH)andfollicle-stimulating
hormone(FSH).Theseproteinhormonesinfluencereproductiveorgansinvariousways.LH
stimulatesthetestestosecretethesexhormonetestosteroneandtheovariestosecrete
progesteroneandestrogens.FSHaidsinthematurationofovarianfollicles(sacscontaining
ova)infemalesandspermproductioninmales.
Gonads
Female Gonad Hormones
Theprimaryhormonesoftheovariesareestrogensandprogesterone.
Estrogens-Groupoffemalesexhormonesimportantforreproductionandthedevelopmentoffemalesexcharacteristics.Estrogensareresponsibleforgrowthandmaturationoftheuterusandvagina;breastdevelopment;wideningofthepelvis;greaterfatdistributioninthehips,thighs,andbreast;uteruschangesduringthemenstrualcycle;andincreasedgrowthofbodyhair.
Progesterone-Hormonethatfunctionstopreparetheuterusforconception;regulatesuteruschangesduringthemenstrualcycle;increasessexualdesire;aidsinovulation;andstimulatesglanddevelopmentformilkproductionduringpregnancy.
Androstenedione-Androgenhormonethatservesasaprecursortotestosteroneandestrogens.
Activin-Hormonethatstimulatestheproductionandreleaseoffollicle-stimulatinghormone(FSH).Italsoassistsinmenstrualcycleregulation.
Inhibin-HormonethatinhibitstheproductionandreleaseofFSH.
Gonads
Male Gonad Hormones
Androgensarehormonesthatprimarilyinfluencethedevelopmentofthemalereproductivesystem.Althoughfoundinmuchhigherlevelsinmen,androgensarealsoproducedinwomen.
Testosterone-Sexhormoneimportantforthedevelopmentofmalesexorgansandsexcharacteristics.Testosteroneisresponsibleforincreasedmuscleandbonemass;increasedgrowthofbodyhair;developmentofbroadshoulders;deepeningofthevoice;andgrowthofthepenis.Testosteroneisthemainandrogensecretedbythetestes.
Androstenedione-Hormonethatservesasaprecursortotestosteroneandestrogens.
Inhibin-HormonethatinhibitsthereleaseofFSHandisthoughttobeinvolvedinspermcelldevelopmentandregulation.
Gonads
GonadsGonads: Hormonal Regulation
Sexhormonesmayberegulatedbyotherhormones,byglandsandorgans,andbyanegativefeedbackmechanism.Hormonesthatregulatethereleaseofotherhormonesarecalledtropichormones.Gonadotropinsaretropichormonesthatregulatethereleaseofsexhormonesbygonads.ThemajorityoftropichormonesandthegonadotropinsFSHandLHaresecretedbytheanteriorpituitary.Gonadotropinsecretionisitselfregulatedbythetropichormonegonadotropin-releasinghormone(GnRH),whichisproducedbythehypothalamus.GnRHreleasedfromthehypothalamusstimulatesthepituitarytoreleasethegonadotropinsFSHandLH.FSHandLHinturnstimulatethegonadstoproduceandsecretesexhormones.
Theregulationofsexhormoneproductionandsecretionisalsoanexampleofanegativefeedbackloop.Innegativefeedbackregulation,theinitialstimulusisreducedbytheresponseitprovokes.Theresponseeliminatestheinitialstimulusandthepathwayishalted.ThereleaseofGnRHstimulatesthepituitarytoreleaseLHandFSH.LHandFSHstimulatethegonadstoreleasetestosteroneorestrogenandprogesterone.Asthesesexhormonescirculateintheblood,theirrisingconcentrationsaredetectedbythehypothalamusandpituitary.ThesexhormoneshelptoinhibitthereleaseofGnRH,LH,andFSH,whichresultsindecreasedsexhormoneproductionandsecretion.
Gonads
Gonads: Gamete Production
Theproductionofspermcellsisknownasspermatogenesis.Thisprocessoccurscontinuouslyandtakesplacewithinthemaletestes.Themalegermcellorspermatocyteundergoesatwopartcelldivisionprocesscalledmeiosis.Meiosisproducessexcellswithonehalfthenumberofchromosomesastheparentcell.Haploidmaleandfemalesexcellsuniteduringfertilizationtobecomeonediploidcellcalledazygote.Hundredsofmillionsofspermmustbereleasedinorderforfertilizationtotakeplace.
Oogenesis(ovumdevelopment)occursinthefemaleovaries.AftermeiosisIiscomplete,theoocyte(eggcell)iscalledasecondaryoocyte.Thehaploidsecondaryoocytewillonlycompletethesecondmeioticstageifitencountersaspermcellandfertilizationbegins.Oncefertilizationisinitiated,thesecondaryoocytecompletesmeiosisIIandisthencalledanovum.Whenfertilizationiscomplete,theunitedspermandovumbecomeazygote.Azygoteisacellthatisattheearlieststageofembryonicdevelopment.
Gonads
Hypogonadism
Onefunctionofthetestesistosecretethehormonetestosterone.Thishormoneplaysanimportantroleinthedevelopmentandmaintenanceofmanymalephysicalcharacteristics.Theseincludemusclemassandstrength,fatdistribution,bonemass,spermproduction,andsexdrive.
Hypogonadisminmenisadisorderthatoccurswhenthetesticles(gonads)donotproduceenoughtestosterone.Primaryhypogonadismoccurswhenthereisaproblemorabnormalityinthetesticlesthemselves.Secondaryhypogonadismoccurswhenthereisaproblemwiththepituitaryglandinthebrain,whichsendschemicalmessagestothetesticlestoproducetestosterone.
Hypogonadismcanoccurduringfetaldevelopment,atpuberty,orinadultmen.Whenitoccursinadultmen,hypogonadismmaycausethefollowingproblems:
Erectiledysfunction(theinabilitytoachieveormaintainanerection)
Infertility
Decreasedsexdrive
Decreaseinbeardandgrowthofbodyhair
Decreaseinsizeorfirmnessofthetesticles
Decreaseinmusclemassandincreaseinbodyfat
Enlargedmalebreasttissue
Mentalandemotionalsymptomssimilartothoseofmenopauseinwomen(hotflashes,moodswings,irritability,depression,fatigue)
Therearevariouscausesofhypogonadism,including:
Klinefelter's syndrome—Thissyndromeinvolvesthepresenceofabnormalsexchromosomes.AmalenormallyhasoneXchromosomeandoneYchromosome.TheYchromosomecontainsthegeneticmaterialwiththecodesthatdeterminethemalegender,andrelatedmasculinecharacteristicsanddevelopment.MaleswithKlinefelter'ssyndromehaveanextraXchromosome,whichcausesabnormaldevelopmentofthetesticles.
Undescended testicles—Thetesticlesdevelopinsidetheabdomenandusuallymovedownintothescrotumbeforebirth.Sometimes,thisdoesnotoccur.However,inmostcases,thetesticlesdescendbythechild'sfirstbirthday.Anundescendedtesticlethatremainsoutsidethescrotumthroughoutchildhoodcanresultinabnormaltesticulardevelopment.
Hemochromatosis—Hemochromatosis,ortoomuchironintheblood,cancausethetesticlesorthepituitaryglandtomalfunction.
.
Testicular trauma —Damagetothetesticlescanaffecttheproductionoftestosterone.
Cancer treatment—Chemotherapyorradiationtherapy,commontreatmentsforcancer,caninterferewithtestosteroneandspermproductionbythetesticles.
Normal aging —Oldermengenerallyhavelowerlevelsoftestosterone,althoughthedeclineofthehormonevariesgreatlyamongmen.
Pituitary disorders—Problemsaffectingthepituitarygland—includingaheadinjuryorpituitarytumor—caninterferewiththegland'ssignalstothetesticlestoproducetestosterone.
Medications —Certaindrugscanaffecttestosteroneproduction.Commonlyusedpsychiatricdrugsandsomemedicinesusedtotreatgastroesophagealrefluxdisease(GERD)maycausehypogonadism.
This is a pea-sized gland located in the midbrain. Melatonin is one of the chemicals isolated from the pineal. Melatonin affects the skin pigmentation.
Pineal Gland
Diagram of pituitary and pineal glands in
the human brain Micrograph of a normal pineal gland –
very high magnification
Thymus Gland The thymus is found under the
breast bone and is active only during childhood. The thymers atrophies and, later on, ceases activity after fifty years of age. The thymus appears to form antibodies for immunity.
The true functions of the thymus and pineal glands are still under investigation.
Gastrin, secreted from the stomach,
stimulates the production of hydrochloric acid when the food enters the mouth. The secretion stimulates the release of pancreatic juice to neutralize the stomach acid. Cholecystokinin (CCK) stimulates the release of pancreatic enzymes and gall bladder contraction.
Digestive Hormones
The prostaglandins constitute a family of more than a dozen fatty acid compounds exerting a wide diversity of hormonal effects. These hormones are found in the intestines, liver, kidneys, pancreas, heart, lungs, thymus, brain and reproductive organs. The richest source of this hormone is human seminal fluid. It was earlier taught that the prostaglandins are came from prostate gland, hence the name.
The effects of prostaglandins are very specific and, often, opposite actions are produced by closely related compounds. In general, the effects of prostaglandins are found in a smooth muscle contraction, secretion, blood flow and metabolism.
Prostaglandins
Major Endocrine Glands Hormone Principal Function
Hypothalamus (via posterior
pituitary)
Antediuretichormone(ADH)
oxytocin
Constrictarterioles,promotesreabsorptionofwaterinkidneysand
sweatglandsInmales,causesspermejection:
infemales,stimulatescontractionofuterinemusclesduringchildbirth,ejectionandmaternalbehaviors
Hypothalamus
Releasingandinhibiting
hormones
Releasinghormonesstimulatereleaseofhormonesfromanteriorpituitary:inhibitinghormonesinhibitreleaseofhormonesinanterior
pituitaryThyroid Thyroxine
CalcitoninRegulatesgrowthand
development:increasesmetabolicrateofmostbodycells:increasesbody
temperatureInhibitsincreaseofcalciumfrom
bonesParathyroid Parathormone Promotesabsorptionofcalcium
byintestines:promotesreabsorptionofcalciumbykidneystimulatesreleaseof
calciumfrombone
Adrenal Medulla Adrenalinandnoradrenalin
Constrictsomebloodvessels:increaserateandforceofcontractionsoftheheart:increasesinstabolicrate.Increaseslevelsofsugarandacidsin
theblood
Adrenal Cortez Glucocorticoids
Aldosterone
Testosterone
Increasebloodsugar,regulatesugar,lipidandfatmetabolism,anti-
inflammatoryeffects
Increasereabsorptionofsaltinthekidney
Causesmasculinizationofbodyfeatures,growth
Pancreas Insulin
Glucagon
Decreasesbloodglucoselevelsbyincreasinguptakeofglucoseinto
cellsandconvertingglucosetoglycogen,especiallyinliver:regulates
fatmetabolism
Convertsglycogentoglucose,therebyraisingbloodglucoselevels
Ovaries Estrogen Causesdevelopmentoffemalesecondarysexualcharacteristicsandmaturationofeggs:promotegrowthofuterinelining:hasgeneraleffectson
metabolism
Progesterone
Stimulatesdevelopmentof
uterineliningandformationofplacenta
Testes Testosterone Stimulatesdevelopmentofgenitaliaandmalesecondarysexual
characteristics:stimulatespermatogenesisandgrowth:hasgeneraleffectstometabolism
OTHER SOURCES OF HORMONES
Thymus Thymosin Stimulatesmaturationofcellsofimmunesystem
Pineal Melatonin Regulatesbiologicalclock:mayregulateonsetofpuberty
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