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The Endocrine SystemThe Endocrine SystemThe Endocrine SystemThe Endocrine System

Chapter 13

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EndocrineEndocrineEndocrineEndocrine

• to interstitial fluid circulation• exocrine- secreted to ducts lumen or

outside the body• Endocrine glands:

Pituitary, thyroid, parathyroid, adrenal & pineal

• Hormone secretion + other functions:Hypothalamus, thymus, pancreas, ovaries,

testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue & placenta

Figure 13.1Figure 13.1

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Hormone OperationHormone Operation Hormone OperationHormone Operation

• General chemical signal in circulation• Slower than nerve responses• Target cells must have a specific

receptor Response determined by responding

cell, i.e. different cells may respond differently to the same hormone

• Cell may respond to more than one hormone, i.e. has more than one type receptor

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Hormone ChemistryHormone ChemistryHormone ChemistryHormone Chemistry

• Soluble in lipids = Hydrophobicsteroids, e.g. testosterone, estrogens, etc.thyroid hormones, e.g. T3, T4 Nitric oxide (NO)

• Water soluble= HydrophillicAmino acid derivatives, e.g. epinephrine,

norepinephrinePeptides, e.g. antidiuretic Hormone (ADH),

oxytocinProteins, e.g. insulin & growth hormone

• General Action depends on chemistry

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Lipid Soluble ActionLipid Soluble ActionLipid Soluble ActionLipid Soluble Action

• Hormone detaches from carrier in blood stream

• Diffusion through interstitial fluid & cell membrane into cell

• Binds to & activates receptor• Receptor-hormone complex alters

gene expression• If new mRNA protein synthesis• New proteins alter cell activity

Figure 13.2Figure 13.2

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Water-Soluble ActionWater-Soluble ActionWater-Soluble ActionWater-Soluble Action

• Diffuses from blood and binds to receptor in plasma membrane

• Starts reaction inside cell forming second messenger Cyclic AMP is a common one

• Second messenger causes activation of several proteins (enzymes)

• Activated proteins produce physiological responses

• Second messenger is inactivated

Figure 13.3Figure 13.3

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Control of SecretionsControl of SecretionsControl of SecretionsControl of Secretions

• Release occurs in short bursts• Regulated by:

Signals from nervous system, e.g. adrenal medulla release of epinephrine

Chemical changes in blood, e.g Blood Ca2+ affects parathyroid hormone

Other hormones, e.g. ACTH from pituitary stimulates cortisol release from adrenal cortex

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Hypothalamus & Hypothalamus & PituitaryPituitary

Hypothalamus & Hypothalamus & PituitaryPituitary

• Major link between nervous & endocrine systems

• Hypothalamic Cells synthesize at least 9 hormones

• Pituitary synthesizes 7• Regulate growth, development,

metabolism & homeostasis

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PituitaryPituitaryPituitaryPituitary

• Two lobes; anterior & posterior•Hypophyseal portal veins

Connect capillaries in hypothalamus to capillaries in anterior pituitary

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Hypothalamus Hypothalamus Pituitary PituitaryHypothalamus Hypothalamus Pituitary Pituitary

• Axons of hypothalamic neurons (neurosecretory cells) end near capillaries of hypothalamus

• Secrete Releasing hormones or Inhibiting hormones portal veins

Regulate release of anterior pituitary hormones

Figure 13.4Figure 13.4

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Human Growth Hormone Human Growth Hormone (hGH)(hGH)

Human Growth Hormone Human Growth Hormone (hGH)(hGH)

• Promotes synthesis of IGFs = somatomedins in liver, muscle, cartilage & bone

• Released in bursts (~2 hour intervals)

• Hypothalamus Growth Hormone Releasing Hormone (GHRH) & Growth Hormone Inhibiting Hormone (GHIH )Regulated by blood glucose levels

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Thyroid Stimulating Thyroid Stimulating HormoneHormone

Thyroid Stimulating Thyroid Stimulating HormoneHormone

• Stimulates the formation & secretion of Thyroid hormones from thyroid gland

• Hypothalamus Thyrotropin Releasing Hormone (TRH)- no TIHRegulated by circulating thyroid hormone

levels

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Follicle Stimulating Hormone Follicle Stimulating Hormone (FSH) & Luteinizing Hormone (FSH) & Luteinizing Hormone

(LH)(LH)

Follicle Stimulating Hormone Follicle Stimulating Hormone (FSH) & Luteinizing Hormone (FSH) & Luteinizing Hormone

(LH)(LH)

• In females:FSH starts follicle developmentLH stimulates formation of corpus luteum &

secretion of progesterone• In males:

FSH stimulates sperm production in testesLH stimulates release of testosterone

• Gonadotrophin releasing Hormone (GnRH) from hypothalamus is suppressed by high levels of estrogen in females and testosterone in males

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Prolactin (PRL)Prolactin (PRL)Prolactin (PRL)Prolactin (PRL)

• Initiates & maintains milk production by mammary glands

• Ejection of milk depends on oxytocin• Prolactin inhibiting hormone (PIH)

suppresses prolactin release• High levels of Estrogens PRH

prolactin release• Unknown function in males

Hypersecretion impotence

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Adrenocortcotrophic Adrenocortcotrophic Hormone (ACTH)Hormone (ACTH)

Adrenocortcotrophic Adrenocortcotrophic Hormone (ACTH)Hormone (ACTH)

• Controls production & secretion of glucocorticoids from adrenal cortex

• Corticotrophin Releasing Hormone (CRH) from hypothalamus stimulates secretion of ACTH

• Stress related stimuli can also stimulate ACTH release

• Glucocorticoids inhibit CRH & ACTH release

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Melanocyte Stimulating Melanocyte Stimulating Hormone (MSH)Hormone (MSH)

Melanocyte Stimulating Melanocyte Stimulating Hormone (MSH)Hormone (MSH)

• Small circulating amounts• Excess causes skin darkening

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Posterior PituitaryPosterior PituitaryPosterior PituitaryPosterior Pituitary

• axon terminals from hypothalamus-• Release hormones • Oxytocin- enhance smooth muscle

contraction during birth & milk ejectionmay play role in emotional bonding

• Antidiuretic Hormone (ADH) = vasopressinCauses kidney to retain more water

Vasoconstriction increase in blood pressure

high blood osmotic pressure increase secretion

Figure 13.5Figure 13.5

Figure 13.6Figure 13.6

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Thyroid GlandThyroid GlandThyroid GlandThyroid Gland

• Below larynx- two lobesfollicular cells surround follicles thyroxin (T4) & triiodothyronine

(T3)

Stored in follicle

• Parafollicular cells (C-cells)calcitonin

Figure 13.7aFigure 13.7a

Figure 13.7bFigure 13.7b

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Thyroid HormonesThyroid HormonesThyroid HormonesThyroid Hormones

• T4 & T3 increase basal metabolic rate, protein synthesis & growth

• Blood level is controlled via feedback through hypothalamusIncreased body ATP demand can also

raise blood levels

• Calcitonin inhibits osteoclasts decrease in blood Ca2+

Feedback control on blood levels

Figure 13.8Figure 13.8

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Parathyroid GlandsParathyroid GlandsParathyroid GlandsParathyroid Glands

• Small round masses in posterior of thyroid gland

• Chief cells release parathyroid hormone (PTH)

• Regulator of Ca2+, Mg2+ & HPO42-

Increases number & activity of osteoblastsSlows loss of Ca2+ & Mg2+ in urinePromotes production of calcitriol increases

rate of Ca2+, Mg2+ & HPO42- absorption in GI

tract

Figure 13.9Figure 13.9

Figure 13.10Figure 13.10

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• Fattened organ in curve of duodenum

• Mostly an exocrine organ for digestion

• Endocrine cells in pancreatic islets• Several cell types:

• alpha cells glucagon• beta cells insulin

PancreasPancreasPancreasPancreas

Figure 13.11aFigure 13.11a

Figure 13.11bFigure 13.11b

Figure 13.11cFigure 13.11c

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Actions of Insulin & Actions of Insulin & GlucagonGlucagon

Actions of Insulin & Actions of Insulin & GlucagonGlucagon

• Low blood glucose stimulates glucagon release

• Glucagon stimulates liver glucose release increased blood glucose

• High glucose levels stimulate insulin release• Insulin increase glucose transport into

skeletal muscle and adipose cells decreased blood glucose

• Insulin promotes Amino Acid uptake, protein synthesis & lipid storage

• ANS also modulates hormone release

Figure 13.12Figure 13.12

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Adrenal GlandAdrenal GlandAdrenal GlandAdrenal Gland

• Near kidneys• Two separate gland structures- • Adrenal cortex and adrenal

medulla• 3 zones in Cortex-3 steroid

hormonesOuter zone mineralocorticoidsMiddle zone glucocorticoidsInner Zone androgens

Figure 13.13aFigure 13.13a

Figure 13.13bFigure 13.13b

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MineralocorticoidsMineralocorticoidsMineralocorticoidsMineralocorticoids

• Aldosterone is the major form• Stimulates Na+ reabsorption from

urine to blood• Stimulates excretion of K + into urine• Part of renin-angiotensin-aldosterone

pathwayDecreased BP release of renin from kidneyRenin causes angiotensinogen angiotensin IIn lungs Angiotensin converting enzyme (ACE)

causes Angiotensin I angiotensin IIAngiotensin II causes Aldosterone release

Figure 13.14Figure 13.14

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Glucocorticoid actionGlucocorticoid actionGlucocorticoid actionGlucocorticoid action

• Increase rate of protein breakdown• Stimulate liver formation of glucose• Breakdown of triglycerides in

adipose• Anti-inflammatory effects-

Inhibit white blood cells

• Depresses immune system• Regulated by negative feedback

through hypothalamus

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AndrogensAndrogensAndrogensAndrogens

• Small amount secreted from adrenal cortex

• Contribute to libido in females• Converted to estrogens by other

body tissues• Stimulate axillary hair growth in

both boys & girls• Contribute to adolescent growth

spurt

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Adrenal MedullaAdrenal MedullaAdrenal MedullaAdrenal Medulla

• Consists of sympathetic post ganglionic cells

• stimulated by preganglionic sympathetic neurons

• Releases Epinephrine and norepinephrine

• gives systemic sympathetic effects• occurs during strong physiological

stress

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GonadsGonadsGonadsGonads

• Produce gametes• Release sex steroids (testosterone

or estrogen & progesterone) • Also hormone inhibin

Inhibits FSH release

• hormones from pituitary (FSH & LH)• Ovaries also produce a hormone

relaxin during pregnancy• details later in course

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PinealPinealPinealPineal

• Small gland attached to roof of third ventricle of brain

• Produces melatonin• Sets bodies biological clock

More released in darkness

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Other hormonesOther hormonesOther hormonesOther hormones

• Prostaglandins (PG) & leukotrienes (LT)• Derived from fatty acids• Act locally in most tissues & released

from most body cells• LTs stimulate white blood cells &

mediate inflammation• PGs affect many visceral functions &

also modulate inflammation, promote fever & intensify pain

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Stress ResponsesStress ResponsesStress ResponsesStress Responses

• Part of homeostatic responses• When successful leads to extra

physiological capacity and long term adaptation

• Initial “fight-or-flight” responseNerve mediated response-sympathetic

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Stress- Resistance ReactionStress- Resistance ReactionStress- Resistance ReactionStress- Resistance Reaction

Slower & longer Than initial responseHypothalamus Increased CRH, GHRH, TRH

• CRHACTHCortisol mobilize metabolites (amino acids, glucose & fat)

• GHRHhGH mobilize fats & glucose for energy and promote tissue growth & repair

• TRHTSHthyroid hormones increased Metabolic capacity

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AgingAgingAgingAging

• Some decrease in function with aging• Loss of negative feedback sensitivity, e.g.

decline in circulating thyroid hormones• PTH levels rise loss of bone mass• Less glucocorticoid production• Slower release of insulin• Thymus declines after puberty• Ovary response to gonadotrophins stops• Slow decline in testosterone production