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The Endocrine System
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Unit 1
Chapter 13
Endocrine
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• to interstitial fluid circulation
• exocrine- secreted to ducts  lumen or
outside the body
• Endocrine glands:
• Hormone secretion + other functions:
Hypothalamus, thymus, pancreas, ovaries,
testes, kidneys, stomach, liver, small intestine,
skin, heart, adipose tissue & placenta
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Pituitary, thyroid, parathyroid, adrenal & pineal
Figure 13.1
Hormone Operation
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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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• General chemical signal in circulation
• Slower than nerve responses
• Target cells must have a specific
receptor
Hormone Chemistry
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• Soluble in lipids = Hydrophobic
steroids, e.g. testosterone, estrogens, etc.
thyroid hormones, e.g. T3, T4
Nitric oxide (NO)
Amino acid derivatives, e.g. epinephrine,
norepinephrine
Peptides, e.g. antidiuretic Hormone (ADH), oxytocin
Proteins, e.g. insulin & growth hormone
• General Action depends on chemistry
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• Water soluble= Hydrophillic
• 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
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Lipid Soluble Action
Figure 13.2
Water-Soluble Action
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• Diffuses from blood and binds to receptor
in plasma membrane
• Starts reaction inside cell forming second
messenger
• Second messenger causes activation of
several proteins (enzymes)
• Activated proteins produce physiological
responses
• Second messenger is inactivated
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Cyclic AMP is a common one
Figure 13.3
Control of Secretions
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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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• Release occurs in short bursts
• Regulated by:
• 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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Hypothalamus & Pituitary
Pituitary
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• Two lobes; anterior & posterior
• Hypophyseal portal veins
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Connect capillaries in hypothalamus to
capillaries in anterior 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
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Hypothalamus  Pituitary
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Figure 13.4
Human Growth Hormone (hGH)
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• Promotes synthesis of IGFs =
somatomedins
• Released in bursts (~2 hour intervals)
• Hypothalamus Growth Hormone
Releasing Hormone (GHRH) & Growth
Hormone Inhibiting Hormone (GHIH )
Regulated by blood glucose levels
Unit 1
in liver, muscle, cartilage & bone
Thyroid Stimulating Hormone
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Regulated by circulating thyroid hormone
levels
Unit 1
• Stimulates the formation & secretion
of Thyroid hormones from thyroid
gland
• Hypothalamus Thyrotropin
Releasing Hormone (TRH)- no TIH
Follicle Stimulating Hormone (FSH)
& Luteinizing Hormone (LH)
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• In females:
FSH starts follicle development
LH stimulates formation of corpus luteum &
secretion of progesterone
• In males:
• Gonadotrophin releasing Hormone
(GnRH) from hypothalamus is suppressed
by high levels of estrogen in females and
testosterone in males
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FSH stimulates sperm production in testes
LH stimulates release of testosterone
• 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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Prolactin (PRL)
• 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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Adrenocortcotrophic Hormone
(ACTH)
Melanocyte Stimulating
Hormone (MSH)
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• Small circulating amounts
• Excess causes skin darkening
Posterior Pituitary
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• axon terminals from hypothalamus• Release hormones
• Oxytocin- enhance smooth muscle contraction
during birth & milk ejection
may play role in emotional bonding
Causes kidney to retain more water
Vasoconstriction increase in blood pressure
high blood osmotic pressure increase secretion
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• Antidiuretic Hormone (ADH) = vasopressin
Figure 13.5
Figure 13.6
Thyroid Gland
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• Below larynx- two lobes
follicular cells surround follicles
 thyroxin (T4) & triiodothyronine (T3)
Stored in follicle
calcitonin
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• Parafollicular cells (C-cells)
Figure 13.7a
Figure 13.7b
Thyroid Hormones
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• T4 & T3 increase basal metabolic
rate, protein synthesis & growth
• Blood level is controlled via feedback
through hypothalamus
• Calcitonin inhibits osteoclasts 
decrease in blood Ca2+
Feedback control on blood levels
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Increased body ATP demand can also
raise blood levels
Figure 13.8
Parathyroid Glands
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Increases number & activity of osteoblasts
Slows loss of Ca2+ & Mg2+ in urine
Promotes production of calcitriol  increases
rate of Ca2+, Mg2+ & HPO42- absorption in GI
tract
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• Small round masses in posterior of thyroid
gland
• Chief cells release parathyroid hormone
(PTH)
• Regulator of Ca2+, Mg2+ & HPO42-
Figure 13.9
Figure 13.10
Pancreas
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• Fattened organ in curve of duodenum
• Mostly an exocrine organ for digestion
• Endocrine cells in pancreatic islets
• alpha cells glucagon
• beta cells insulin
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• Several cell types:
Figure 13.11a
Figure 13.11b
Figure 13.11c
• 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
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Actions of Insulin & Glucagon
Figure 13.12
Adrenal Gland
Near kidneys
Two separate gland structuresAdrenal cortex and adrenal medulla
3 zones in Cortex-3 steroid hormones
Outer zone  mineralocorticoids
Middle zone  glucocorticoids
Inner Zone  androgens
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•
•
•
•
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Figure 13.13a
Figure 13.13b
Mineralocorticoids
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Decreased BP  release of renin from kidney
Renin causes angiotensinogen angiotensin I
In lungs Angiotensin converting enzyme (ACE)
causes Angiotensin I  angiotensin II
Angiotensin II causes Aldosterone release
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• Aldosterone is the major form
• Stimulates Na+ reabsorption from urine
to blood
• Stimulates excretion of K + into urine
• Part of renin-angiotensin-aldosterone
pathway
Figure 13.14
Glucocorticoid action
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•
•
•
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Increase rate of protein breakdown
Stimulate liver formation of glucose
Breakdown of triglycerides in adipose
Anti-inflammatory effects-
• Depresses immune system
• Regulated by negative feedback through
hypothalamus
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Inhibit white blood cells
• 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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Androgens
• 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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Adrenal Medulla
Gonads
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• Produce gametes
• Release sex steroids (testosterone or
estrogen & progesterone)
• Also hormone inhibin
• hormones from pituitary (FSH & LH)
• Ovaries also produce a hormone relaxin
during pregnancy
• details later in course
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Inhibits FSH release
Pineal
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• Small gland attached to roof of third
ventricle of brain
• Produces melatonin
• Sets bodies biological clock
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More released in darkness
• 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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Other hormones
Stress Responses
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Nerve mediated response-sympathetic
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• Part of homeostatic responses
• When successful leads to extra
physiological capacity and long term
adaptation
• Initial “fight-or-flight” response
Stress- Resistance Reaction
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Slower & longer Than initial response
• CRHACTHCortisol  mobilize
metabolites (amino acids, glucose & fat)
• GHRHhGH mobilize fats & glucose for
energy and promote tissue growth & repair
• TRHTSHthyroid hormones increased
Metabolic capacity
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Hypothalamus  Increased CRH, GHRH,
TRH
• 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
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Aging