The Endocrine System

The Endocrine SystemSecond-messenger system of the bodyUses chemical messengers (hormones) that

are released into the bloodHormones control several major processes

ReproductionGrowth and developmentMobilization of body defensesMaintenance of much of homeostasisRegulation of metabolism

Hormone OverviewHormones are produced by specialized cellsCells secrete hormones into extracellular

fluidsBlood transfers hormones to target sitesThese hormones regulate the activity of

other cells

The Chemistry of HormonesHormones are classified chemically as

Amino acid–based, which includesProteinsPeptidesAmines

Steroids—made from cholesterolProstaglandins—made from highly active

lipids

Mechanisms of Hormone ActionHormones affect only certain tissues or

organs (target cells or target organs)Target cells must have specific protein

receptorsHormone-binding alters cellular activity

Effects Caused by HormonesChanges in plasma membrane permeability

or electrical stateSynthesis of proteins, such as enzymesActivation or inactivation of enzymesStimulation of mitosisPromotion of secretory activity

The Chemistry of HormonesTwo mechanisms in which hormones act

Direct gene activationSecond-messenger system

Direct Gene Activation (Steroid Hormone Action)Diffuse through the plasma membrane of

target cellsEnter the nucleusBind to a specific protein within the nucleusBind to specific sites on the cell’s DNAActivate genes that result in synthesis of

new proteins

Figure 9.1a

NucleusCytoplasmSteroidhormone Receptor

protein

Hormone-receptorcomplex

DNA

mRNANewprotein

Plasmamembraneof targetcell

Second-Messenger System (Nonsteroid Hormone Action)Hormone binds to a membrane receptorHormone does not enter the cellSets off a series of reactions that activates

an enzymeCatalyzes a reaction that produces a

second-messenger moleculeOversees additional intracellular changes

to promote a specific response

Figure 9.1b

CytoplasmNonsteroidhormone (firstmessenger)

Enzyme

Receptorprotein

Secondmessenger

Effect on cellular function,such as glycogenbreakdown Plasma

membraneof target cell

ATP

cAMP

Control of Hormone ReleaseHormone levels in the blood are mostly

maintained by negative feedbackA stimulus or low hormone levels in the

blood triggers the release of more hormoneHormone release stops once an appropriate

level in the blood is reached

Hormonal Stimuli of Endocrine GlandsMost common stimuliEndocrine glands are activated by other

hormonesExamples:

Anterior pituitary hormones

Hormonal Stimuli of Endocrine Glands

Figure 9.2a

Humoral Stimuli of Endocrine GlandsChanging blood levels of certain ions

stimulate hormone releaseHumoral indicates various body fluids such

as blood and bileExamples:

Parathyroid hormoneCalcitoninInsulin

Humoral Stimuli of Endocrine Glands

Figure 9.2b

Neural Stimuli of Endocrine GlandsNerve impulses stimulate hormone releaseMost are under the control of the

sympathetic nervous systemExamples include the release of

norepinephrine and epinephrine by the adrenal medulla

Figure 9.2c

Neural Stimuli of Endocrine Glands

Major Endocrine OrgansPituitary glandThyroid glandParathyroid glandsAdrenal glandsPineal glandThymus glandPancreasGonads (Ovaries and Testes)Hypothalamus

Location of Major Endrocrine Organs

Figure 9.3

Pituitary GlandSize of a peaHangs by a stalk from the hypothalamus in

the brainProtected by the sphenoid boneHas two functional lobes

Anterior pituitary—glandular tissuePosterior pituitary—nervous tissue

Often called the “master endocrine gland”

Hormones of the Anterior PituitarySix anterior pituitary hormones

Two affect non-endocrine targetsGrowth hormoneProlactin

Four stimulate other endocrine glands (tropic hormones)Thyroid-stimulating hormone (thyrotropic hormone)Adrenocorticotropic hormoneTwo gonadotropic hormones

Hormones of the Anterior PituitaryCharacteristics of all anterior pituitary

hormonesProteins (or peptides)Act through second-messenger systemsRegulated by hormonal stimuli, mostly

negative feedback

Hormones of the Anterior Pituitary

Hormones of the Anterior PituitaryGrowth hormone

General metabolic hormoneMajor effects are directed to growth of

skeletal muscles and long bonesPlays a role in determining final body sizeCauses amino acids to be built into proteinsCauses fats to be broken down for a source

of energy

Hormones of the Anterior PituitaryGrowth hormone (GH) disorders

Pituitary dwarfism results from hyposecretion of GH during childhood

Gigantism results from hypersecretion of GH during childhood

Acromegaly results from hypersecretion of GH during adulthood

Hormones of the Anterior PituitaryGigantism

Figure 9.5a

Hormones of the Anterior PituitaryDwarfism

Figure 9.5b

Hormones of the Anterior PituitaryProlactin (PRL)

Stimulates and maintains milk production following childbirth

Function in males is unknownAdrenocorticotropic hormone (ACTH)

Regulates endocrine activity of the adrenal cortex

Thyroid-stimulating hormone (TSH)Influences growth and activity of the thyroid

gland

Hormones of the Anterior PituitaryGonadotropic hormones

Regulate hormonal activity of the gonadsFollicle-stimulating hormone (FSH)

Stimulates follicle development in ovariesStimulates sperm development in testes

Luteinizing hormone (LH)Triggers ovulation of an egg in femalesStimulates testosterone production in males

Pituitary–Hypothalamus RelationshipHormonal release is regulated by releasing

and inhibiting hormones produced by the hypothalamus

Hypothalamus produces two hormonesThese hormones are transported to

neurosecretory cells of the posterior pituitaryOxytocinAntidiuretic hormone

The posterior pituitary is not strictly an endocrine gland, but does release hormones

Hormones of the Posterior PituitaryOxytocin

Stimulates contractions of the uterus during labor, sexual relations, and breastfeeding

Causes milk ejection in a nursing woman

Hormones of the Posterior PituitaryAntidiuretic hormone (ADH)

Inhibits urine production by promoting water reabsorption by the kidneys

In large amounts, causes vasoconstriction leading to increased blood pressure

Also known as vasopressin

Hormones of the Posterior Pituitary

Figure 9.6