The Endocrine System
• Contributes to:
– control of growth,
development,
reproduction,
behaviour, energy
metabolism, and
water balance
• By:
– Secreting hormones
• To control
– Organ and tissue
functions
Endocrine System
• A system of ductless
secretory organs (glands)
located in various parts of
the body
• Include
–
–
–
–
–
–
–
–
–
Pineal
Anterior/posterior pituitary
Thyroid
Parathyroid
Thymus
Adrenal
Islets of Langerhans,
Ovaries
Testes
• Main function
– Secrete hormones directly
into the blood or
extracellular fluid
Hypothalamus
• Is not a gland but a
region of the brain
• Part of the nervous
system
• Very important for
function of endocrine
system
• Produce neurohormones
that stimulate or inhibit
production of other
hormones in the
pituitary gland
Hormones of The Hypothalamus
• Thyrotropin-releasing hormone (TRH)
– Stimulates release of thyroid-stimulating hormone (TSH)
• Gonadotropin-releasing hormone (GnRH)
– Stimulates release of follicle-stimulating hormone (FSH) and
luteinizing hormone (LH)
• Growth hormone-releasing hormone (GHRH)
– Stimulates release of growth hormone (GH)
• Corticotropin-releasing hormone (CRH)
– Release of adrenocorticotropic hormone (ACTH)
• Somatosin
– Inhibits the release of growth hormone (GH)
• Dopamine
– Inhibits the release of thyroid-stimulating hormone (TSH)
Hormones: Maintaining Homeostasis
• Chemical management
system for the body
• Chemicals produced by
cells in one part of the
body that regulate the
processes of cells in
another part of the body
• Chemical messengers
– act on cells from another
part of the body
• Local regulators
(paracrine)
– act on nearby cells
• Self regulators (autocrine)
– cells that produce
chemicals to stimulate
their own cellular
processes
Tropic vs Non-Tropic Hormones
• Tropic
Hormones
– Target
endocrine
glands
• Nontropic
Hormones
– Target
cells,
tissues,
and organs
Hormones
• Produced and secreted
by cells, tissues and
organs that compose the
endocrine system
(glands) directly into the
blood or extracellular
fluid
• Hormones are circulated
throughout the body
• Only target cells will
respond to specific
hormones
• Hormones are broken
down by enzymes in
target cell, liver or
kidneys where they are
reused or excreted
Hormones
• Secreted in an inactive form – prohormones
• Prohormones are converted by target cells or by
enzymes in the blood to an active form
• Angiotensinogen → angiotensin
Hormones
• Protein hormones
– Consist of AA (3 to 200 in
length)
– Usually hydrophilic (water
soluble)
– Diffuse well through blood
– Cannot pass through lipid
bilayer
• Steroid hormones
– Derived from cholesterol
– Not water soluble
– Usually encased with protein
(protein carrier) to travel
through blood
– Pass easily through lipid
bilayer
Hormone Mechanisms
• Water-Soluble
– Cannot pass membrane
– Bind to receptor molecules in
the cell membrane
– Signal is activated
– Secondary messenger is
activated (cAMP – cyclic
adenosine monophosphate)
– Change is caused inside cell
– Acts in the cytosol or the
nucleus
– Regulate protein production, ion
channels
– Activation of protein kinases
• Glucagon
– Breakdown of glycogen into
glucose
Controlling Blood Glucose Levels
Hormone Mechanisms
• Lipid-Soluble
– Can pass membrane (lipid)
– Bind to receptors inside a
cell (cytosol or nucleus)
– Turn on or off an action of a
specific gene
– Changes amount of protein
that is synthesized by cell
• Aldosterone
– Increase sodium absorption
→ increases water retention
→ increase blood pressure
Major Features of Hormone Mechanisms
• Only the cells that contain surface or internal receptors for
the hormones respond to the hormones
• Once bound to their receptors, hormones produce a response
by turning cellular processes on or off. They do this by
altering the proteins that are functioning in or produced by
the cell
• Hormones are effective in very small concentrations because
of the amplification that occurs in both the surface and
internal receptor mechanisms
• The response to a hormone differs among target organs and
among species
Hormones: Negative Feedback Mechanisms
• Secretion of hormones are regulated by negative
feedback mechanisms
• Hormones inhibit other hormones
• Multiple hormones can be secreted at a time
The Pituitary Gland
• The Master Gland
– Produces hormones that
control most of the other
glands in the endocrine system
• Made up of anterior lobe and
posterior lobe
• Links endocrine system to
nervous system via portal
vein (hypothalamus)
• Influenced by hypothalamus
– Releasing hormones/inhibiting
hormones
2. portal vein
4. anterior pituitary
gland
5. hypophyseal vein
6. posterior pituitary
gland
8. pituitary stalk
9. capillary network
10. neurons
11. neurosecretory
cells
12. hypothalamus
Anterior Pituitary Gland
• Major hormones
secreted into the
bloodstream
• Tropic
– Growth hormone
(both)
– thyroid-stimulating
hormone
– adrenocorticotropic
hormone
– follicle-stimulating
hormone
– luteinizing hormone
• Nontropic
– Prolactin
– Melanocytestimulating hormone
Prolactin
• Promote milk
production in
mammary glands
• Suckling on nipple
causes stimulus
• Regulated by
dopamine
• Too much
– Hyperprolactinaemia
• Too little
– Hypoprolactinaemia
Growth hormone
• Cell division, protein
synthesis, bone growth
• Release of growth
hormone stimulates
release of IGF in liver
(insulin growth factor)
that stimulates these
functions
• Conversion of glycogen
to glucose, fats to fatty
acids – regulates levels in
blood
• Stimulates cells to take
up FA, AA and limits
muscle cells to take up
glucose
Growth hormone
• Underproduction
– Dwarfism
– Heart disease, increased fat
• Cause
– Genetic, benign tumour on pituitary
gland
• Overproduction
– Acromegaly – body tissues get
larger over time
– Gigantism – excessive production of
growth hormone
• Cause
– Benign tumour on pituitary gland
Thyroid-stimulating hormone
• Controls production of the
thyroid hormones
• Underproduction
– Hyperthyroidism
• Cause
– Thyroid is producing too much
thyroid hormone
• Overproduction
– Hypothyroidism
• Cause
– Thyroid if producing too much
thyroid hormone
Adrenocorticotropic hormone
• Controls production of cortisol,
adrenaline, noradrenaline
• Underproduction
– Cushing’s syndrome
• Cause
– Steroid medication, tumour of
pituitary gland
• Overproduction
– Cushing’s disease
– Addison’s disease (loss of function of
cortex of adrenal gland)
• Cause
– Adenoma (non-cancerous tumour) in
the pituitary gland
– Autoimmunity
Follicle-stimulating hormone/luteinising hormone
• Puberty development/function of
the gonads (testes/ovaries)
• Sex steroid production
• Germ cell production (sperm/eggs)
• Underproduction
– Incomplete development at puberty
– Infertility
• Overproduction
– Turner syndrome (female is missing
entire/parts of x chromosome)
– Kallmann’s syndrome (failure to
start/complete puberty)
• Cause
– Testicular/ovarian failure,
Melanocyte-stimulating hormone
• Causes darkening in humans by
releasing melanin in the skin and
hair and eyes
• Specialized cells called
melanocytes release melanin
• Protects from UV rays
• Overproduction
– Increase production of melanin
• Cause
– Prolonged exposure to sun or skin
tanning
• Underproduction
– Lack of skin pigmentation
– Loss of natural protection from UV
rays and sun
• Cause
– Damage to pituitary glands
Posterior pituitary gland
• Stores and releases
2 major hormones
into the
bloodstream
– Antidiuretic
hormone
(vasopressin)
– oxytocin
• These hormones are
produced by the
hypothalamus and
stored here
Antidiuretic Hormone (vasopressin)
• Causes distal
convoluted
tubule to
become
permeable to
water
• Helps maintain
water balance
• Overproduction
– Kidneys retain
too much water
• Underproduction
– Kidneys excrete
too much water
Oxytocin
• Contraction of
the womb
• Lactation
• Overproduction
– Not clear
• Underproduction
– Linked to autism
Thyroid gland
• Located in the
front of the
throat and
shaped like a
bow tie
• Secretes
– Thyroxine (T₄)
– calcitonin
Thyroxine (T₄)
• Prohormone (inactive)
– Active form is triiodothyronine
• Contains 4 iodine atoms
• Regulates body’s metabolic rate,
heart and digestive function,
muscle control, brain
development, maintenance of
bones
• Overproduction
– Thyrotoxicosis – too much
thyroxine in bloodstream
recognized by goitre
• Causes
– Hyperthyroidism
– Graves Disease
• Underproduction
– Hypothyroidism
• Causes
– Autoimmune diseases, poor
iodine diet
Calcitonin
• Reduces levels of
Calcium (Ca²⁺) in
the blood stream
• Opposes the action
of parathyroid
hormone
• Overproduction
– No apparent effect
on body
• Underproduction
– No apparent effect
on body
Parathyroid gland
• 4 spherical glands (size of a pea) located on each
side of the posterior surface of the thyroid gland
• Secretes
– Parathyroid hormone
Parathyroid hormone
• Stimulates enzymes
in kidneys to convert
vitamin D into
calcitrol increasing
absorption of Ca²⁺
and phosphates from
food
• Underproduction
– Muscle cramps
– Osteoporosis
• Overproduction
– Kidney stones
Pineal gland
• Located near the centre of the
brain
• Secretes
– melatonin
• Secretion of melatonin is
controlled by circadian rhythm
(biological processes that
fluctuate on a 24 hour
timetable)
• Helps to synchronize biological
clock (jet lag, sleep disorders)
• Melatonin also produced in
retina of the eye
• Overproduction
– Reduced core body temperature
• Underproduction
– No apparent effect on the body
Adrenal glands
• Consist of two regions
1. Adrenal medulla
– contains highly
modified neurosecretory neurons
2. Adrenal cortex
– contains non-neural
endocrine cells
Adrenal medulla
• Secretes
– Epinephrine, norepinephrine
• These chemicals
can act as
hormones or
neurotransmitters
(transmit nerve
signals to brain)
• Part of the “fight or
flight” response
Epinephrine
• Released when body
encounters stresses
• Increase heart rate
• glycogen and fat
breakdown
• Major blood vessels
dilate (increase blood
flow)
• Blood vessels in skin
constrict (chills and
sweating)
• Increase in blood
pressure
• Reduces water loss
• Digestive system slows
• Used to counter
anaphylaxis
Adrenal cortex
• Secretes
– Aldosterone, cortisol
Glucocorticoids
• Cortisol – Helps raise
blood glucose levels
using three mechanisms
– Stimulate synthesis of
glucose from fats and
proteins
– Reduce glucose uptake
by the body cells except
in the central nervous
system
– Promote breakdown of
fats and proteins into
fatty acids and amino
acids as alternative fuels
Mineralocorticoids
• Aldosterone
– Increase amount of
sodium /water
reabsorption in
bloodstream
– Increase amount of
potassium removal in
urine
– Increase blood pressure
• Overproduction
– High blood pressure,
low potassium, alkaline
blood
• Causes
– Adrenal tumour
• Underproduction
– Addison’s disease, low
blood pressure
Regulating blood sugar
• Occurs automatically in
our body
• Pancreas – contain
both exocrine and
endocrine glands
• Exocrine
– secretes digestive
enzymes into the small
intestine
• Endocrine
– Islets of Langerhans Secretes insulin (beta
cells) and glucagon
(alpha cells)
Insulin/Glucagon
• Regulate the
ability of most
tissues in the body
to metabolize fuel
substances
(glucose, fats,
proteins)
Insulin
• Secreted by beta cells
• Lower blood glucose levels
by
– Acts on skeletal muscles,
liver cells, adipose tissue
(fat) to uptake glucose
• In the Liver
– Lowers fatty acid levels
– promotes fatty acid uptake
and storage in adipose
tissue
– Inhibits breakdown of fats
into fatty acids
– Lowers amino acid levels
– Promotes protein synthesis
– Inhibits breakdown of
proteins
Glucagon
• Secreted by alpha cells
• Increase blood glucose
levels by
– Stimulating breakdown of
glycogen into glucose
– Stimulates breakdown of
fats into fatty acids
– Stimulates breakdown of
proteins into amino acids
– Stimulate cells to use
amino acids and noncarbohydrates to
synthesize glucose
Glucose levels throughout the day
Unstable levels of glucose
• Hyperglycemia (above 200mg/dL of blood)
– Blood glucose levels are too high
– (norm 115-200mg/dL)
• Symptoms
– Frequent urination, sugar in the urine, vision problems,
fatigue, weight loss
• Hypoglycemia (below 70mg/dL of blood)
– Blood glucose levels are too low
– (norm 70-115mg/dL)
• Symptoms
– Nervousness, cold sweats, hunger, headaches, weakness
Diabetes
• High glucose levels in the blood • Classified into 3 different types
caused by problems with insulin – Type 1
production
– Type 2
• Symptoms
– Gestational
– Frequent urination, increased
thirst/appetite
Type 1
• Also called juvenile
diabetes or insulindependant
• Beta cells do not
produce any insulin
• Daily administration
of insulin is required
usually by injection
or pump
Type 2
• Reduced insulin
production or the
inability of insulin to bind
to its receptors properly
• Developed in adulthood
and is associated with
obesity
• 90% of diabetics have this
type
• Controlling diet and
exercise helps restore
normal levels of insulin
production
Gestational
• Occurs in about 2 to
10% of pregnant
women
• High blood glucose
levels develop
during pregnancy
• Usually a temporary
condition but does
increase the risk of
both mother and
child developing
later in life
Reproductive Hormones
• Gonads (sex glands)
– Males – testes
– Females – ovaries
• Sex hormones
– Androgens,
estrogens, progestins
• Regulate
development of
– male and female
reproductive systems,
sexual characteristics,
mating behaviour
Female Reproductive System
• Pair of ovaries
– Located in
abdominal cavity
– Produce female
gametes (ova, eggs)
– Produce estrogen
and progesterone
• FSH and LH from
the pituitary gland
stimulate the
maturation of the
follicles in the
ovary and trigger
ovulation
Estrogen
• Estradiol
– Stimulates
maturation of
the sex organs at
puberty
– Release of egg
during ovulation
– Development of
secondary
sexual
characteristics
(breast
development,
body hair,
widening of
pelvis)
– Sex drive
Progestins
• Progesterone
– Secreted by
the corpus
luteum in the
ovary
– Maintains
uterus for
implantation
of a fertilized
egg
– Growth and
development
of an embryo
Oogenesis
• Production and
release of eggs
(ova) by the ovaries
• Releases oocytes immature eggs that
have undergone 1
meiotic division
• Polar body is
associated with it
– Disintegrates
quickly
• Females produce
up to 1 million
• Only ~380 are
ovulated before
menopause
Ovulation
• Monthly release of one or
a few developing oocytes
into the oviduct
• Burst of LH causes follicle
to rupture
• Ova becomes ovum
• Moves through the oviduct
(fallopian tubes) via cilia
that line these tubes
• Fertilization occurs here in
the oviduct
• undergoes second meiotic
division only if penetrated
by sperm cell producing a
zygote
• If not fertilized egg will
degenerate
Ovarian cycle
• Occurs from
puberty to
menopause
• Involves release of
a mature egg
approx every 28
days
• Coordinated with
the menstrual
cycle (month)
– Prepares the
uterus to implant
the egg if
fertilization occurs
Corpus luteum
• LH causes ruptured follicle
to grow into an enlarged
yellowish structure
• Initiates luteal phase –
prepares uterus to receive
an egg
• If egg is fertilized:
• Acts as an endocrine
gland- secretes estrogens,
progesterone and inhibin
• Progesterone – inhibits
GnRH – FSH LH
• Inhibin prevents secretion
of FSH
• If egg is not fertilized
• Corpus luteum shrinks
Menstrual cycle
• Begins at day 0
• Results from the
breakdown of the
endometrium
• Releases blood and
tissue breakdown
products from the
uterus to the outside
through the vagina
• Day 4 or 5 – flow
ceases and
endometrium begins
to grow again
• Same hormones that
control ovarian cycle
control this cycle
Menopause
• High levels of sex
hormones stops
• Late 40’s or early
50’s
• Menstrual/Ovarian
cycle stops
• Side effects
– Hot flashes,
headaches, mood
swings
• Treated with HRT
(hormone
replacement
therapy)
Male reproductive system
• Testes
– Affect the development
of male secondary
characteristics
– Secrete androgens
(testosterone)
– Stimulates puberty,
facial hair, vocal cords,
sex drive
– Spermatogenesis –
production of sperm
• Release of
testosterone in the
body is controlled by
LH which is controlled
by GnRH
Spermatogenesis
• Sperm development from spermatogonia
• Takes about 9 to 10 weeks - spermatogonium to
sperm
• Testes produce about 130 million fertile sperm each
day
Spermatogenesis
• Leydig cells - secrete
testosterone
• Sertoli cells – supply nutrients to
spermatocytes and seal them off
from body’s blood supply
• Coiled seminiferous tubules
located in epididymis store
mature sperm
• Vas deferens – transport sperm
upon ejaculation