Composition of the EndocrineSystemThe endocrine system is composed oforgans that produce and secrete hormones.Because these organs perform mainly asecretory function, they are also referred toas glandsTwo types of glands:– EXOCRINE– ENDOCRINE
EXOCRINE glands secrete their productsinto ducts.The ducts transport the products into bodycavities, into the spaces within organs, orinto the body surface.EXOCRINE glands include oil glands,sweat glands, mucous glands, and salivaryglands.
ENDOCRINE GLANDSENDOCRINE glands secrete theirproducts into the extracellular spacesurrounding the secretory cells.Endocrine glands are sometimes calledductless glands.Their products are the HORMONES,which diffuse from here to enter thebloodstream.
The primary endocrine glands of the bodyare the pituitary gland, the thyroid gland,the parathyroid glands, the adrenal gland,the pancreas, and the sex glands, or gonads(the testes in the male and ovaries in thefemale).Endocrine glands that provide a minor rolein body maintenance include the pinealgland and the thymus gland.
The stomach, kidneys, small intestine, andthe placenta, all have a secondary role asendocrine glands
HORMONESHormones are the chemical units producedby endocrine glands.Hormones are the means by whichendocrine glands provide control of bodyactivities to maintain homeostasis.
HORMONAL ACTIONHormones are released in very smallquantities, because they are extremelypotent compounds.Once released by secretory cells into theextracellular space, they find their way bydiffusion into the bloodstream.A given hormone will have an effect onlyon a particular type of cell. This is calledthe target cell.
The effect is limited to the target cellsbecause only they contain special proteinmolecules in their plasma membrane thatserve as receptors, which recognize andbind to specific hormones while rejectingothers.Cells other than target cells are not affectedby a hormone, because they lack theappropriate receptors.
Once a hormone has united with thereceptor on a target cell, it begins to exertits effect.Its effect is to alter the cell’s metabolicprocesses.Examples: a hormone may change the ratesof enzyme activities, the rate of proteinsynthesis, the rates of secretion, or the ratesat which materials are transported across theplasma membrane.
Although there are many types of hormonesthat differ chemically, hormones may begrouped into two broad categories on thebasis of their solubility: those that dissolvein water, or are water-soluble; and thosethat dissolve in lipids, or are lipid-soluble.
WATER-SOLUBLEHORMONESHormones that are soluble in waterinclude molecules that are composed ofamino acids.Because these hormones are solubleonly in water, they cannot pass throughthe lipid plasma membrane.
Question: how can they produce an effecton the cell if they cannot penetrate themembrane?This is done by passing the signal to asecond-messenger system located within thecell. One that uses a compound calledcyclic AMP (adenosine monophasphate).
The enzymatic cascade activated bythe second-messenger system has anenormous amplification effect withinthe cell.A single hormone molecule triggers asingle enzyme, which catalyzesliterally hundreds of reactions.
Water-soluble hormones that serve as firstmessengers in this system includeepinephrine, norepinephrine (NE),antidiuretic hormone (ADH), oxytocin(OT), calcitonin (CT), and parathyroidhormone (PTH).
LIPID-SOLUBLE HORMONESHormones that dissolve in lipids includemainly steroid hormones.Because the plasma membrane is composedof a bilayer of lipid molecules, steroidhormones can pass directly through it bydiffusion to enter the target cell quite easily.(recall that steroids are a type of lipid also;lipids dissolve in other lipids).
Lipid-soluble hormones activate genes tosynthesize new proteins and enzymes.The protein products that are newly formedinclude enzymes that promote the metabolicactivities specified by the hormone.Lipid-soluble hormones that stimulateprotein synthesis include aldosterone,cortisol, testosterone, estrogen, andthyroxine.
PROSTAGLANDINSProstaglandins are a group of chemicals thatalso have regulating effects on cells.They are lipids that are produced by manydifferent parts of the body.Like hormones, they are extremely potentcompounds and are released in very smallquantities.
Specifically, prostaglandins stimulate orinhibit the formation of cyclic AMP,thereby modulating the effect of hormonesthat use cyclic AMP as a second messenger.Because they do not induce their own effectbut instead modify the effect of a hormone,prostaglandins as a group are notconsidered true hormones.
Some prostaglandins reduce blood pressureand open airways by causing smoothmuscles to relax, others have the oppositeeffect.Other types inhibit the secretion of HCLfrom the stomach wall, increase intestinalcontractions, stimulate contraction of theuterus, regulate metabolism, causeinflammation, and even cause fever.
HORMONAL CONTROLFEEDBACK CONTROL--how does anendocrine gland “know” how muchhormone to produce and release?This information, or feedback, is providedby way of chemical signals that are sent tothe endocrine gland.There are two systems that operate in thismanner: negative feedback systems andpositive feedback systems.
NEGATIVE FEEDBACKNegative feedback systems control theamount of hormone released by providing aresponse in the opposite direction to that ofthe stimulus.In these systems, the secretion of a hormonethat accelerates a body activity is inhibitedby the negative feedback signal, and thesecretion of a hormone that slows the samebody activity is stimulated yet further.
Negative feedback systems are the mostcommon method of hormone regulation inthe body.
POSITIVE FEEDBACKSYSTEMSPositive feedback systems regulatehormone secretion by providing a responsein the same direction as the stimulus.When the desired response stimulated byhormone action occurs, a chemical feedbacksignal causes the endocrine gland toincrease its rate of hormone release andmore responses are stimulated.
Positive feedback systems tend to causeextreme changes in conditions in the bodyand are therefore quite unstable anduncommon.
Example: the production of oxytocin by thepituitary gland during childbirth. Itstimulates contractions of the uterus. Itsrising levels in the blood cause theformation of products that stimulate furtheroxytocin production, and uterinecontractions respond by graduallyincreasing in strength until birth isaccomplished.
NERVOUS CONTROLA second way of controlling hormonerelease is by the nervous system.Nervous control is responsible forregulating only some endocrine glands,such as the adrenal medulla and secretorycells in the hypothalamus of the brain.These glands secrete hormones when theyreceive nerve impulses.
THE ENDOCRINEGLANDSSome are in the head,some are in the neck, andsome in the abdominalcavity.
PITUITARY GLANDThe pituitary gland, or hypophysis, islocated at the base of the brain.It is about the size of a pea and weighs only0.5 gram (0.02 ounce).It is attached to the hypothalamus by anarrow stalk, called the infundibulum, andlies within a bony cavity formed by thesella turcica of the sphenoid bone.
The pituitary gland produces manyhormones, some of which control theactivities of several other endocrine glands.It thereby influences a wide range of bodyfunctions.The pituitary gland consists of two portions:an anterior lobe and a posterior lobe.
ANTERIOR LOBEWithin its epithelium are five differenttypes of secretory cells that releaseseven types of hormones.The release of these hormones iscontrolled by chemical secretions fromthe hypothalamus, called regulatingfactors.
GROWTH HORMONEGrowth hormone (GH) stimulates bodycells to grow and divide.On a more short-term basis, the nutritionalstatus of your body affects the release ofGH in order to maintain a relativelyconstant blood sugar level.When sugar levels are low, a conditioncalled hypoglycemia exists and thehypothalamus is stimulated to releaseregulating factors.
Once these factors reach the anteriorlobe, GH is released into thebloodstream. As a result, blood sugarlevels rise. (convert glycogen intoglucose)High levels of sugar in the blood, orhyperglycemia, cause the oppositeeffect (GH is inhibited).
Thus, your blood sugarlevels are kept relativelyconstant by negativefeedback mechanisminvolving Gh
MELANOCYTE-STIMULATING HORMONEMSH--stimulates theproduction of melanin inthe skin, causing the skinto increase inpigmentation.
PROLACTINIn combination with other hormones,prolactin (PRL) stimulates and maintainsmilk secretion by the mammary glands infemales.The actual ejection of milk is controlled bya hormone released by the posterior lobe,called oxytocin.
The combined secretion andejection of milk from themammary glands is anactivity referred to aslactation.
THYROID-STIMULATINGHORMONEThe production and secretion of hormonesby the thyroid gland are stimulated by TSH.It is influenced by the body’s metabolicrate, levels in the blood of a thyroidhormone called thyroxine, and otherfactors.
ADRENOCORTICOTROPICHORMONEThe production and secretion ofhormones released by the outerregion or, cortex, of the adrenalgland are controlled by the ACTH.Its release is also influenced byvarious forms of stress.
FOLLICLE-STIMULATINGHORMONEFSH has a different effect upon the twosexes.In females, FSH stimulates the developmentof eggs, or ova, each month within theovaries.It also stimulates the cells in the ovaries tosecrete estrogens, the female sex hormone.
In the male, FSH stimulates the productionof sperm by the testes.FSH production is controlled by regulatingfactors released from the hypothalamus inresponse to estrogens in the female and totesterone in the male, in the manner of anegative feedback system.
LUTEINIZING HORMONELH also plays a different role in each of thetwo sexes.In females, it works together with estrogensto stimulate the ovary to release an ovum (aprocess called ovulation). and prepare theuterus for implantation of the fertilizedovum.
In males, LH stimulates cellswithin the testes to produce andsecrete testosterone.LH secretion is controlled by thehypothalamus by way of negativefeedback.
OXYTOCINOXYTOCIN (OT) stimulatecontraction of smooth muscle inthe wall of the uterus.It also stimulates cells aroundmammary ducts to contract,thereby causing milk to eject.
ANTIDIURETIC HORMONEADH regulates fluid balancein the body.ADH causes a decrease inurine output and an increasein body fluid volume.
THYROID GLANDThe THYROID GLAND is the prominentorgan in the neck.Located slightly below the larynx in front ofthe trachea.Its follicles contain a clear liquid calledcolloid.The three primary hormones are: thyroxine.also known as T4; triiodothyronine (T3),and calcitonin.
Thyroxine andTriiodothyronine playimportant roles inmetabolism and growth.
CALCITONINCalcitonin reduces the calcium andphosphate levels in the blood.Calcium concentrations must be kept withinnarrow limits for normal nerve and musclefunction, and both ions are essential mineralcomponents of bone.
PARATHYROID GLANDSThe parathyroid glands are fouror five pea-shaped masses ofglandular epithelium.They secrete one hormone,called parathyroid hormone(PTH).
Parathyroid Hormone (PTH)Parathyroid hormone(PTH) plays an importantrole in maintaining thecalcium and phosphate levelsin the blood.
ADRENAL GLANDSThe Adrenal Glands are paired,triangular masses that lie atop eachkidney.Like the kidneys, they are locatedbehind the membrane that encloses theabdominal cavity, which is called theperitoneum
Adrenal MedullaThe adrenal medulla iscomposed of modified nervetissue.It secretes two hormones ,epinephrine andnorepinephrine
Adrenal CortexIt occupies the larger portion of the adrenalgland.The secretory cells in each one of theadrenal cortex secrete steroid hormones.These hormones are synthesized fromcolesterol. They include three classes ofcompounds: mineralocorticoids,glucocorticoids, and sex hormones.
MineralocorticoidsThe primarymineralocorticoid isaldosterone.This steroid hormonemaintains body fluid balance.
Sex HormonesThe two classes of sex hormonesreleased by the adrenal cortex areandrogens, which have amasculinizing effect, andestrogens, which have feminizingeffects.
PancreasThe pancreas is a soft, oblong organlocated in the abdominal cavity behind thestomach.It is actually two body systems, since itperforms two distinct functions.It is an endocrine gland, since it secretestwo important hormones into the bloodstream.
It is also a digestive organ, because of itssecretion of digestive enzymes into ductsthat empty into the small intestine.The endocrine cells of the pancreas formclusters called the islets of Langerhans.The hormones play important roles inproviding body cells with sufficientamounts of energy. They do this byregulating the amount of sugar in the blood.
GLUCAGON--stimulates theconversion of glycogen into the simplesugar glucose.INSULIN--has the opposite effect tothat of glucagon on liver cells: itstimulates the formation of glycogenfrom glucose.
GonadsThe gonads are the sex organs; that is, theyare the organs that produce the sex cells andsecrete the primary sex hormones.In females, the ovaries , they secreteestrogens, which are the primary femalesex hormoneIn males, the testes secrete testosterone.
Pineal GlandThe pinal gland is a small structure withinthe cranial cavity associated with the brain.It is sometimes called the epithalamusbecause it is attached to the upper margin ofthe thalamus.It secretes one hormone, melatoninthe pineal gland in reptiles and birds, hasbeen shown to regulate reproduction cycles,hibernation cycles, and migration patterns
Thymus GlandThe thymus gland is a prominent structurein infants and young children butdiminishes in size with advancing age.It is a soft, irregularly shaped structure thatlies in the mediastinum on top of the heart.It secretes a hormone known as thymosin,which stimulates the production of certainwhite blood cells called T lymphocytes
The thymus glandplays an important rolein immunity.