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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