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Endocrine System
Dr. Michael P. Gillespie
Mediator Molecules in Nervous
& Endocrine Systems
 The nervous system utilizes
neurotransmitters to control body functions.
 The endocrine system utilizes hormones to
control body functions.
Site Of Mediator Action In
Nervous & Endocrine Systems
 The neurotransmitters perform their action
close to the site of release.
 The hormones usually perform their action
far from their site of release.
Types Of Target Cells In
Nervous & Endocrine Systems
 The nervous system acts upon muscle cells
(smooth, cardiac, and skeletal), glands, and
other neurons.
 The endocrine system acts upon virtually all
cells of the body.
Time To Onset Of Action In
Nervous & Endocrine Systems
 In the nervous system, action typically
occurs within milliseconds of
neurotransmitter release.
 In the endocrine system, action can take
seconds to days to occur after release of the
hormone.
Duration Of Action In Nervous
& Endocrine Systems
 The actions tend to be briefer in duration in
the nervous system and longer in duration in
the endocrine system.
Comparison Of Control By The
Nervous & Endocrine Systems
 Refer to table 18.1 on page 587
Hormones
 A hormone is a mediator molecule that is released
in one part of the body but regulates activity of cells
in other parts of the body.
 Most hormones enter the interstitial fluid and then
the bloodstream.
 Hormones travel through the bloodstream to cells
throughout the body.
 Several neurotransmitters are also hormones (i.E.
norepinephrine).
Functions Of Hormones
 Help regulate:
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Chemical composition and volume of the
internal environment (interstitial fluid).
Metabolism and energy balance.
Contraction of smooth and cardiac muscle fibers.
Glandular secretions.
Some immune system activities.
 Control growth and development.
Functions Of Hormones
 Regulate operation of reproductive systems.
 Help establish circadian rhythms.
“Supersystem”
 The nervous and endocrine systems function
together.
 Parts of the nervous system stimulate or
inhibit the release of hormones.
 Hormones can promote or inhibit the release
of nerve impulses.
Exocrine Glands Versus
Endocrine Glands
 Exocrine glands (Exo = outside) – secrete
their products into ducts that carry secretions
into body cavities, into the lumen of an
organ, or to the outer surface of the body.
 Endocrine glands – secrete their hormones
into the interstitial fluid surrounding the
secretory cells.
Exocrine Glands Versus
Endocrine Glands
 Exocrine glands.
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Sudoriferous (sweat).
Sebaceous (oil).
Mucous.
Digestive.
Exocrine Glands Versus
Endocrine Glands
 Endocrine glands.
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Pituitary.
Thyroid.
Parathyroid.
Adrenal.
Pineal.
Other organs that secrete hormones:

Hypothalamus, thymus, pancreas, ovaries, testes, kidneys,
stomach, liver, SI, skin, heart, adipose tissue, & placenta.
Endocrine System
 All endocrine glands and hormone secreting
cells comprise the endocrine system.
 Endocrinology (-logy = study of) is the study
of the science of endocrine glands, function
of endocrine glands, diagnosis of endocrine
disorders and treatment of endocrine
disorders.
Hormone Receptors
 Hormones bind with specific receptors.
 Only target cells for a given hormone have specific
receptors that bind and recognize that hormone.
 A target cell can have anywhere between 2000 and
100,000 receptors for a particular hormone.
 Receptors are constantly being synthesized and
broken down to meet the needs of the body.
Down-regulation
 If a hormone is present in excess, the number
of target cell receptors may decrease.
 Down-regulation decreases the
responsiveness of the target cell to the
hormone.
Up-regulation
 When a hormone (or neurotransmitter) is
deficient, the number of receptors may
increase.
 Up-regulation makes a target cell more
receptive to a specific hormone.
Synthetic Hormones
 Synthetic hormones in the form of drugs can
block the receptors from naturally occurring
hormones.
Circulating & Local Hormones
 Circulating hormones – pass from secretory
cells that make them into the interstitial fluid
and then into the blood.

Most hormones are of this type.
 Local hormones – act locally on neighboring
cells or on the same cells that secreted them
without first entering the bloodstream.
Local Hormones
 Paracrines – (para = beside or near) act on
neighboring cells.
 Autocrines – (auto – self) act on the same
cell that secreted them.
Example Of A Local Hormone
 Interleukin 2 (IL-2) is an example of a local
hormone.
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It is released by helper T cells during immune
responses.
It acts on nearby immune cells (paracrine) and
on itself (autocrine).
This generates more helper T cells and boosts the
immune response.
Duration of Local Versus
Circulating Hormones
 Local hormones usually are inactivated
quickly.
 Circulating hormones linger longer.
 The liver eventually deactivates circulating
hormones and the kidneys excrete them.
Chemical Classes Of Hormones
 Lipid-soluble hormones.
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Steroid hormones.
Thyroid hormones.
Nitric oxide (NO).
Chemical Classes Of Hormones
 Water-soluble hormones.
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Amine hormones.
Peptide hormones and protein hormones.
Eicosanoid hormones.
Prostaglandins.
 Leukotrienes.

Hormone Transport In Blood
 Most water-soluble hormones are transported
in their “free” form (not attached to plasma
proteins).
 Most lipid-soluble hormones are bound to
transport proteins.
Hormone Receptors
 Lipid-soluble hormones – the receptors are
located inside the target cells.
 Water-soluble hormones – the receptors are
located within the plasma membrane of the
target cells.
Action Of Lipid-soluble
Hormones
 Lipid soluble hormones turn specific genes
of the nuclear DNA on or off.
 This directs the synthesis of a new protein
(often an enzyme).
 These new proteins alter the cells activity.
Action Of Water-soluble
Hormones
 Water soluble hormones are the first
messenger. They activate the second
messenger i.e. cyclic AMP (cAMP).
 This initiates a cascade of events within the
cell that produces millions of enzymes to
catalyze reactions.
 Phosphodiesterase inactivates cAMP.
Responsiveness Of The Target
Cell
 The responsiveness of the target cell depends
upon the following:
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The hormone’s concentration.
The number of the hormone receptors on the
target cell.
Influences exerted by other hormones.
Influences Of Other Hormones
 Permissive effect – the action of a 2nd
hormone is required for the 1st hormone to
take effect.
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Thyroid hormones (2nd) allow epinephrine to
stimulate lipolysis.
Influences Of Other Hormones
 Synergistic effect – the sum of the actions of the 2
hormones is greater than either hormone
individually.
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Estrogens and FSH promote development of oocytes.
 Antagonistic effect – one hormone opposes the
actions of another.
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Insulin promotes synthesis of glycogen and glucagon
stimulates breakdown of glycogen.
Control Of Hormonal Secretion
 Hormone secretion is regulated by:
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Signals from the nervous system.
Chemical changes in the blood.
Other hormones.
Hypothalamus
 Serves as a major integrating link between
the nervous system and the endocrine
system.
 Painful, stressful, and emotional experiences
cause changes in hypothalamic activity.
 Synthesizes at least 9 different hormones.
 Regulates the pituitary gland.
Pituitary Gland (hypophysis)
 Synthesizes at least 7 different hormones.
 Release of anterior pituitary hormones is
stimulated by releasing hormones and
suppressed by inhibiting hormones from the
hypothalamus.
Types Of Anterior Pituitary
Cells & Their Hormones
 Somatotrophs – secretes human growth
hormone (hGH) or somatotropin, which
stimulates tissues to secrete insulinlike
growth factors (IGFs).
 Thyrotrophs – secrete thyroid-stimulating
hormone (TSH) or thyroptropin.
Types Of Anterior Pituitary
Cells & Their Hormones
 Gonadotrophs – secrete follicle-stimulating
hormone (FSH) and luteinizing hormone
(LH) which act on the gonads.
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They stimulate the secretion of estrogen and
progesterone and the maturation of oocytes in
the ovaries.
They stimulate the secretion of testosterone and
sperm production in the testes.
Types Of Anterior Pituitary
Cells & Their Hormones
 Lactotrophs – secrete prolactin (PRL), which
initiates milk production.
 Corticotrophs – secrete adrenocorticotropic
hormone (ACTH) or corticotropin, which
stimulates the adrenal cortex to secrete
glucocorticoids.
Tropic Hormones (tropins)
 Hormones that influence another gland are
called tropic hormones or tropins.
Control Of Secretion By The
Anterior Pituitary
(adenohypophysis)
 The hypothalamus secretes five releasing
hormones and two inhibiting hormones.
 Negative feedback loops from hormones
released from target glands decrease the
release from the anterior pituitary gland.
Human Growth Hormone &
Insulinlike Growth Factors
 The main function of hGH is to promote synthesis
of IGFs.
 IGFs cause cells to grow and multiply.
 They help to maintain the mass of muscles and
bones.
 They promote healing of injuries and tissue repair.
 They enhance lypolysis in adipose tissue.
Release Of hGH
 Two hypothalamic hormones control the
release of hGH:
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Growth hormone releasing hormone (GHRH).
Stimulated by hypoglycemia.
 Inhibited by hyperglycemia.
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Growth hormone inhibiting hormone (GHIH).
Stimulated by hyperglycemia.
 Inhibited by hypoglycemia.
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Thyroid-stimulating Hormone
 Thyroid-stimulating hormone (TSH) stimulates the
synthesis and secretion of two thyroid hormones:
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Triiodothyronine (T3).
Thyroxine (T4).
 Thyrotropin-releasing hormone (TRH) from the
hypothalamus controls TSH secretion.
 Negative feedback from T3 and T4 inhibits the
release of TRH.
Follicle-stimulating Hormone
 FSH initiates the development of ovarian
follicles and stimulates follicular cells to
secrete estrogens in females.
 FSH stimulates sperm production in the
testes in males.
Follicle-stimulating Hormone
 Gonadotropin-releasing hormone (GnRH)
from the hypothalamus stimulates FSH
release.
 Estrogens in females and testosterone in
males suppresses release of GnRH and FSH
through negative feedback systems.
Luteinizing Hormone (LH)
 In females LH triggers ovulation.
 FSH and LH work together to stimulate the
release of estrogen.
 In males, LH stimulates the release of
testosterone from the testes.
Prolactin (PRL)
 Initiates and maintains secretion of milk by
the mammary glands.
 By itself, prolactin has only a weak effect.
Prolactin (PRL)
 Only with the effects of estrogens, progesterone,
glucocorticoids, hGH, thyroxine and insulin does
PRL bring about milk.
 The hypothalamus secretes both inhibitory and
excitatory hormones that regulate PRL secretion:
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Prolactin-inhibiting hormone (PIH).
Prolactin-releasing hormone (PRH).
Hypersecretion Of Prolactin
 In males – erectile dysfunction.
 In females – galactorrhea (inappropriate
lactation) and amenorrhea (absence of
menstrual cycles).
Adrenocorticotropic Hormone
(ACTH)
 ACTH controls secretion of cortisol and
other glucocorticoids by the cortex of the
adrenal gland.
 Corticotropin-releasing hormone (CRH)
from the hypothalamus stimulates secretion
of ACTH.
 Glucocorticoids cause inhibition of CRH and
ACTH through negative feedback systems.
Principle Actions Of Anterior
Pituitary Hormones
 Table 18.4 page 600.
Posterior Pituitary
(neurohypophysis)
 The posteror pituitary does not synthesize
any hormones; however, it does store and
release two hormones from the
hypothalamus:
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Oxytocin (OT).
Antidiuretic hormone (ADH) a.k.a. vasopressin.
Oxytocin
 During delivery, oxytocin enhances
contraction of smooth muscle cells in the
wall of the uterus.
 After delivery, oxytocin stimulates milk
ejection (“letdown”) from the mammary
glands in response to the suckling infant.
Antidiuretic Hormone (ADH)
 ADH is a substance that decreases urine
production.
 ADH causes the kidneys to return more
water to the blood.
Summary Of Posterior Pituitary
Hormones
 Table 18.5 page 602.
Thyroid Gland
 Located inferior to the larynx (voice box).
 Right and left lateral lobes connected by an
isthmus.
 Anterior to the trachea.
 Highly vascular.
 Consists of thyroid follicles (spherical sacs).
 The walls of each follicle contain follicular cells,
which extend into the lumen of the follicle.
Follicular Cells
 Produce two thyroid hormones (both are lipid
soluble).
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Thyroxine (tetraiodothyronine or T4).
Triiodothyrronine (T3).
Parafollicular Cells
 Also known as C cells.
 Produce the hormone calcitonin (CT).
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Regulates calcium homeostasis.
CT inhibits the action of osteoclasts.
CT accelerates the uptake of calcium and
phosphates into the bone matrix.
Actions Of Thyroid Hormones
 Increase basal metabolic rate (BMR).
 Calorigenic effect.
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Helps to regulate body temperature.
 Regulate metabolism.
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Protein synthesis.
Increase the use of glucose and fatty acids for ATP.
Increase lypolysis.
 Accelerate body growth, especially of the nervous
system.
Parathyroid Glands
 Embedded into the posterior surface of the
lateral lobes of the thyroid gland.
 Superior and inferior parathyroid glands.
 Two kinds of epithelial cells.
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Chief (principal) cells – produce parathyroid
hormone (PTH) or parathormone.
Oxyphil cell – function unknown.
Parathyroid Hormone
 Regulates levels of calcium, magnesium, and
phosphate ions in the blood.
 PTH increases the activity and # of
osteoclasts.
 PTH stimulates the kidneys to synthesize
calcitrol (active form of vitamin D).

Calcitrol stimulates increased absorption of Ca2+
from the GI tract.
Adrenal Glands
 The paired adrenal (suprarenal) glands lie
superior to each kidney.
 Consists of the adrenal cortex (80-90% of the
gland) and the adrenal medulla.
 Highly vascularized.
Adrenal Gland Hormones
 Adrenal Cortex – steroid hormones.
 Adrenal Medulla – 3 catecholamine
hormones.
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Norepinephrine.
Epinephrine.
Dopamine.
Aldosterone (Cortex)
 Regulates homeostasis of sodium ions and
postassium ions.
 Promotes the secretion of H+ in the urine to
regulate acid-base balance. Prevents acidosis
(pH below 7.35).
 Renin-angiotensin-aldosterone (RAA)
pathway controls the secretion of aldosterone
(Controls blood pressure).
Glucocorticoids (Cortex)
 Regulate metabolism and resistance to stress.
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Cortisol (hydrocortisone).
Corticostrerone.
Cortisone.
 Low levels of glucocorticoids, mainly cortisol,
stimulate the hypothalamus to secrete corticotropinreleasing hormone (CRH), which promotes the
release of ACTH from the anterior pituitary, which
in turn stimulates glucocorticoid secretion.
Effects Of Glucocorticoids
 Increase the rate of protein breakdown.
 Stimulates glucose formation by breaking
down glycogen stores and through
gluconeogenesis.
 Stimulates lypolysis.
Effects Of Glucocorticoids
 Resistance to stress by supplying ATP and
raising BP in cases of severe blood loss.
 Anti-inflammatory effects – inhibit WBCs
(also slows wound healing).
 Depression of immune responses (utilized
with organ transplant recipients).
Androgens (Cortex)
 The adrenal cortex secretes small amounts of
androgens (primarily dehydroepiandrosterone
DHEA).
 Assists in early growth of axillary and pubic hair in
both sexes.
 Contributes to libido and provides a source of
estrogens after menopause in females.
 ACTH stimulates its secretion.
Adrenal Medulla Hormones
 Epinephrine (adrenaline).
 Norepinephrine (noradrenaline).
 These two hormones augment the fight or flight
response.
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Increase the heart rate and force of contraction.
Dilates the airways in the lungs.
Shunts blood to heart, liver, skeletal muscles, and
adipose tissue.
Increases blood levels of glucose and fatty acids.
Pancreatic Islets
 Both and endocrine and exocrine gland.
 Pancreatic islets (a.k.a. islets of Langerhans).
 4 major cell types:
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Alpha (A) cells – secrete glucagon.
Beta (B) cells – secrete insulin.
Delta (D) cells – secretes somatostatin (identical
to growth hormone inhibiting hormone).
F cells – secrete pancreatic polypeptide.
Pancreatic Hormones
 Glucagon raises blood glucose levels.
 Insulin lowers blood glucose levels.
 Somatostatin inhibits both glocagon and insulin
release.
 Pancreatic polypeptide inhibits somatostatin
secretion, gallbladder contraction and secretion of
digestive enzymes by the pancreas.
Regulation Of Glucagon &
Insulin Secretion
 Hypoglycemia stimulates release of
glucagon.
 Glucagon causes hepatocytes to convert
glucagon to glucose (gkycogenolysis).
 Hyperglycemia inhibit release of glucagon.
Regulation Of Glucagon &
Insulin Secretion
 Insulin allows glucose to diffuse into cells,
increases amino acid uptake by cells, and
increaes fatty acid uptake by cells.
 This facilitates glucose conversion into
glycogen (glycogenesis), synthesis of
proteins, and synthesis of fatty acids
(lipogenesis).
Gonads
 Ovaries (female gonads).

Produce steroid hormones.
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Estrogens.
Progesterone.
Produce inhibin.
Produce relaxin.
 Testes (male gonads).
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Produce testosterone (an androgen).
Produce inhibin.
Females Sex Hormones
 Estrogen and progesterone along with FSH
and LH (from the anterior pituitary), regulate
the menstrual cycle, maintain pregnancy, and
prepare the mammary glands for lactation.
 Maintain the feminine secondary sex
characteristics (larger breasts and hips).
Inhibin & Relaxin
 Inhibin inhibits secretion of FSH.
 Relaxin increases the flexibility of the pubic
symphisis during pregnancy and helps dilate
the cervix during labor and delivery.
Male Sex Hormones
(Androgens)
 Testosterone regulates the production of
sperm.
 Stimulates the production of male secondary
sex characteristics (beard growth and
deepening of the voice).
Pineal Gland
 A small endocrine gland attached to the roof
of the third ventrical of the brain.
 Secretes melatonin.
 More melatonin is released in darkness.
 Melatonin contributes to the body’s
biological clock.
Seasonal Affective Disorder
(SAD)
 Thought to be due to overproduction of
melatonin during the winter months.
 Full spectrum bright-light therapy can assist
with SAD and jet lag.
Thymus
 Located behind the sternum between the
lungs.
 Hormones produced – thymosin, thymic
humoral factor (THF), thymic factor (TF),
amd thymopoeietin.
 Promotes the maturation of T cells and may
retard the aging process.