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The Endocrine System
Life Science
FLSS
All images used are taken from copyright-free sources e.g. Wikicommons Media or produced by UWS staff.
Homeostasis
• Maintenance of a relatively constant
internal environment
• Maintained by nervous system and
endocrine system working together
Internal communication
NERVOUS
ENDOCRINE
fast
slower, but more
sustained
mostly electrical
chemical
“hard wired”
more diffuse
Glands
• Exocrine glands – secretions discharged
onto epithelial surface of an organ directly,
or via a duct
• Endocrine glands (ductless glands) –
secretions (hormones) discharged into
blood (or lymph) directly
•United States Governemnt (2005) The endocrine system
•http://commons.wikimedia.org/wiki/Endocrine_system?uselang=engb#mediaviewer/File:Illu_endocrine_system.jpg
Hormones
•
•
•
•
•
•
activities of entire organs
growth and development
reproduction
sexual characteristics
usage and storage of energy
levels of fluid, salt and sugar in the blood
Hormones
Carried in blood to target organs, influence
cellular activity by binding to receptors
UWS Staff (2015)
Hormones
Chemically, 2 categories:
• Proteins – synthesised from amino acid
chains
• Lipid hormones – based on cholesterol
(steroids)
Control of endocrine function
Control systems
sensor
control centre
effector
UWS Staff (2015)
Negative feedback
Effector response decreases effect
of original stimulus
eg. If body temperature increases, effector
responds so that temperature decreases
Negative feedback
Direct (short) feedback control
• good example is control of blood sugar
by insulin and glucagon
– level of blood glucose tightly regulated
– set point 90mg/100ml
Positive feedback control
• Rare in physiological systems
• Stimulus increases progressively,
disrupting homeostasis
• Under normal conditions has a defined
end point
– Sneeze
– Formation of blood clot
– Birth of a baby
variable
Positive
feedback
increases
change
sensor
Negative
feedback
reverses
changes
control centre
effector
UWS Staff (2015)
Hormone release can be controlled
• Directly – by levels of chemicals in the
blood acting directly on an endocrine
gland
• Indirectly – by release of other
hormones secreted by the hypothalamus
and anterior pituitary
How is glucose regulated?
The problem:
• cells need a continuous supply and
exercise requires sudden increase in
supply
• but variable input (infrequent feeding)
leads to large swings in available
glucose
Control of blood sugar
2 antagonistic hormones
– Insulin
– Glucagon
• Produced in islets of Langerhans in
pancreas
• Secreted in direct response to blood
glucose levels
• Hypothalamus and pituitary not involved
Control of blood sugar
• Insulin decreases blood glucose
• Glucagon increases blood glucose
Pancreas
OpenStax College (2013)
https://commons.wikimedia.org/wiki/File:1820_The_Pancreas.jpg
Alpha cells
secrete glucagon
Beta cells secrete
insulin
http://en.wikiversity.org/wiki/Blausen_gallery_2014#mediaviewer
/File:Blausen_0701_PancreaticTissue.png
Blausen.com staff. “Blausen Gallery 2014”
Pancreatic Islet
Actions of insulin
 Increases uptake of glucose into cells
 Enhances storage of glycogen in liver
 Inhibits conversion of amino acids and
fatty acids into glucose
Actions of glucagon
 Stimulates breakdown of glycogen in
liver
 enhances conversion of amino acids
and fatty acids into glucose
Diabetes mellitus
• Which hormone is deficient in this
condition?
– Insulin
• How can this deficit be rectified?
– Insulin injection
Diabetes mellitus
• Failure of insulin system
hyperglycaemia
• Type I
(insulin dependent)
(juvenile onset)
• Little or no insulin produced
• No rise in insulin in response to
• Type II
(non-insulin dependent)
(maturity onset)
• Insulin usually produced
• Insensitivity to insulin
blood glucose
Indirect feedback control
Involves release of hormones from the
hypothalamus and anterior pituitary gland
OpenStax College (2013)
https://commons.wikimedia.org/wiki/File:1806_The_Hypothalamus-Pituitary_Complex.jpg
Indirect feedback control
• Involves release of hormones from the
hypothalamus and the anterior pituitary
gland
• These glands act together to regulate
activity of many other endocrine glands
Anterior pituitary
• Hormones from hypothalamus control
secretion of 6 anterior pituitary
hormones
• Some of these stimulate or inhibit
secretion of other endocrine glands
(target glands)
• Others have a direct effect on target
tissues
HYPOTHALAMUS
releasing hormones
ANTERIOR PITUITARY
trophic hormones
ENDOCRINE GLANDS
hormones
TARGET ORGAN
UWS Staff (2015)
Hypothalamus linked to
•Posterior lobe of pituitary gland by nerve fibres
•Anterior lobe of pituitary by blood vessels
And controls output of hormones from both lobes
OpenStax College (2013)
https://commons.wikimedia.org/wiki/File:1808_The_Anterior_Pituitary_Complex.jpg
https://commons.wikimedia.org/wiki/File:1807_The_Posterior_Pituitary_Complex.jpg
Posterior pituitary
• Secretes Oxytocin (acts on smooth muscle
of uterus and milk ducts) and ADH (acts on
kidney to reduce urine volume)
• These are synthesised in the
hypothalamus and passed along nerve
fibres to posterior pituitary where they are
stored
• Release triggered by nerve impulses
from the hypothalamus
Anterior pituitary
hormone
Function
Growth hormone (GH)
Stimulates growth and division of most body
cells
Thyroid stimulatng
hormone (TSH)
Stimulates growth and activity of thyroid
gland
Adenocorticotrophic
hormone (ACTH)
Increases output of adrenal steroids
(especially cortisol)
Prolactin (PRL)
Stimulates lactation
Follicle stimulating
hormone (FSH)
In female- stimulates production of Graafian
follicle
In male – stimulates production of sperm
Luteinising hormone (LH)
In female- triggers ovulation and maintains
corpus luteum in ovary. In male – stimulates
interstitial cells in testes to produce
testosterone
Makes 2 related thyroid hormones
• Tri-iodothyronine (T3)
• Thyroxine (T4 )
OpenStax College (2013)
https://commons.wikimedia.org/wiki/File:1811_The_Thyroid_Gland.jpg
CFCF (2014)
https://commons.wikimedia.org/wiki/File:Illu_thyroi
d_parathyroid.jpg
Thyroid gland
Mikael Häggström (2009)
https://commons.wikimedia.org/wiki/File:Thyroid_system.png
Hypothalamus
TRH (TSH releasing hormone)
Anterior pituitary
TSH (thyroid stimulating hormone)
Thyroid
T3 & T4
UWS Staff (2015)
Regulation of thyroid hormone
HYPOTHALAMUS
TRH
Negative
feedback
ANTERIOR PITUITARY
Negative
feedback
TSH
Thyroid
T3
T4
UWS Staff (2015)
Comparison of Nervous &
Endocrine Systems
Characteristic
Nervous system
Endocrine system
Mediator molecules
Neurotransmitters released
in response to nerve
impulses
Hormones delivered to
tissues throughout the body
by the blood
Cells affected
Muscle cells, gland cells,
other neurons
Virtually all body cells (but
often only some types in
response to any individual
hormone)
Time to onset of
action
Typically within
milliseconds
Seconds - hours or days
Duration of action
Generally briefer
Generally longer
GLAND
HORMONE(S)
FUNCTION
Anterior pituitary various trophic
hormones
Affects other
endocrine glands
Posterior
pituitary
oxytocin, ADH
Smooth muscle
contraction, water
balance
Pineal
melatonin
biorhythms
Thyroid
T3, T4, calcitonin
Control of
metabolic rate and
blood Ca levels
Parathyroid
parathyroid
hormone
Control blood Ca
levels
GLAND
HORMONE(S) FUNCTION
Thymus
thymosin
Adrenals
Medulla
Cortex
adrenaline,
noradrenaline
corticosteroids
Development of
immune system
Response to stress,
control of water and
electrolyte balance
Testes
testosterone
Maintenance of male
sexual function,
secondary
characteristics
Ovaries
oestrogen
Development and
maintenance of female
reproductive structures
GLAND
HORMONE(S) FUNCTION
Ovaries
progesterone
Pancreas
Placenta
Digestive
system
Prepares uterine lining
and mammary glands
for pregnancy
Insulin, glucagon Control of blood
glucose levels
Human chorionic Maintenance of
gonadotrophin
pregnancy, preparation
and others
of the breasts for
lactation
e.g. gastrin,
Influence secretion of
secretin,
digestive juices
cholecystokinin