2. ENDOCRINE SYSTEM
• The word Endocrine derives from the Greek words
"endo" meaning inside , within, and "crinis"
meaning secrete.
• The endocrine system is a ductless system referring to
the collection of cells, glands, and tissues of an organism
that secrete hormones directly into the bloodstream to
control the organisms' physiological and behavioral
activities.
• The endocrine system is in contrast to the exocrine
system, which secretes its chemicals using ducts.
3. Other modes of secretion
• Exocrine Secretion
Exocrine glands release chemical substances through
ducts to outside the body or onto another surface within
the body for example the salivary glands, sweat glands
and glands within the gastrointestinal tract.
• Paracrines are secreted by the cells into
extracellular fluid and affect neighboring target cells
of a different type
• Autocrines secreted by cells into the extracellular fluid
and affect the function of the same cells that
produced them
4. HORMONE
• A hormone is a chemical substance
produced in particular parts of organisms by
specific cells, glands, and/or tissues and
then transported to other parts of the body,
with the intent of influencing a variety of
physiological and behavioral activities, such
as the processes of
digestion, metabolism, growth, reproduction
and mood control.
5. • Neurotransmitters
Released by axon terminals of the
neurons into the synaptic junctions and
act locally to control nerve cell functions
• Neuroendocrine
Are the hormones secreted by the
neurons into the circulating blood and
influence the function of target cells at
another location in the body
6. •Cytokines
Peptides secreted by cells into extracellular fluid and
can function as autocrine , paracrine or endocrine
hormones .eg interleukins and other lymphocykes that
are secreted by helper cells and act on other cells of
immune system
8. Classification of
hormones based on the
chemical nature
1- Protein and peptides including hormones
secreted by anterior and posterior pituitary, the
pancreas , parathyroid gland
2- Steroids secreted by the adrenal cortex, the
ovaries, testes
3- Derivative of the amino acid tyrosine,
secreted by thyroid gland , adrenal medullae.
9. Classification of
hormones based on the
location of receptors
• In or on the surface of cell
membrane
for protein , peptide and catecholamine hormones
• In the cell cytoplasm
for steroid hormones
• In the cell nucleus
for thyroid hormones
10. HORMONE PRODUCTION AND
STORAGE
• Polypeptide and protein hormone are stored
in secretory vesicles until needed.
Most of the hormones in the body are polypeptides and
protein
Range in size from small peptides with as few as 3 amino
acid to protein with almost 200 amino acid
polypeptides with 100 or more amino acids are called
proteins and those with fewer than 100 amino acid are
peptides
Synthesized on the rough endoplasmic reticulum of
different endocrine cells
11. Usually synthesized first as large protein that are not biologically
active
Preprohormones are cleaved to form smaller prohormone in
the endoplasmic reticulum
Prohormones are transferred to Golgi apparatus for packing
into secretory vesicles
enzymes in the vesicles cleave the prohormone to produce
smaller and logically active hormone and inactive fragments
Vesicles are stored within the cytoplasm and many are bound
to the cell membrane until their secretions is needed
Secretion of the hormones occurs when the secretory vesicles
fuse with the cell membrane and the granular contents are
extruded into the interstitial fluid or directly into the blood
stream by exocytosis
12. • Steroid hormones are usually synthesized
from cholesterol and not stored.
•
13. chemical structure of steroid hormones is similar to that
of cholesterol
They are lipid soluble and consists of three cyclohexyl
rings and one cyclopentyl ring combined into a single
structure .
Usually very little hormone storage in steroid producing
endocrine cells
Large stores of cholesterol esters in cytoplasm vacuoles can
be rapidly mobilized for steriod synthesis after a stimulus
14. •Amine hormones are derived from
tyrosine
two groups of hormones derived from tyrosine
The thyroid and the adrenal medullary hormones
thyroid hormones are synthesized and stored in the
thyroid gland and incorporated into macromolecules
of the protein THYROGLOBULIN which is stored in
large follicles within the thyroid gland
Hormone secretion occurs when the amines are split
from thyroglobulin and free hormones are then
released into blood stream
15.
16. Epinephrine and norepinephrine are formed in the
adrenal medulla which normally secretes about 3 to 4
times more epinephrine than norepinephrine
Catecholamine are taken up into preformed vesicles and
stored until secreted
17. Hormone secretion ,
transport and clearance
from the blood
• HORMONE SECRETION AFTER A STIMULUS AND DURATION
OF DIFFERENT HORMONES
• Each of the different hormones has its own
characteristic onset and duration of action
norepinephrine and epinephrine are secreted with in
seconds after the gland is stimulated and may develop full
action within another few seconds to minutes
The actions of thyroid hormones and growth hormones
require months for full effects
18. • Concentration of hormones in the circulating
blood and hormonal secretion rates
the concentration of hormones required to control metabolic
and endocrine functions are incredibly SMALL
Their concentration in the blood ranges from as little as 1
picogram /ml of blood to few microgram /ml of blood
Rate of secretion of various hormones are extremely small ,
usually measured in microgram or milligram per day
19. FEED BACK CONTROL OF
HORMONE SECRETION
• Negative feedback mechanism
Is most commonly applied principle for regulating hormone
secretion
It prevents over activity of the hormone system
Is self -limiting
A hormone has biologic actions that, directly or indirectly ,
inhibit further secretion of the hormone
20. FEED BACK CONTROL OF
HORMONE SECRETION
• Positive feedback mechanism
is rare
Is explosive and self – reinforcing
Surges of hormone can occur with positive feed back
A hormone has biologic actions that, directly or indirectly,
cause more secretion of the hormone
21. • Cyclical variations occur in hormone
releases
there are periodic variations in hormone release
that are influenced by seasonal changes , various
stages of development and aging , the diurnal (daily)
cycle and sleep
Growth hormone is markedly increased during the
early period of sleep but is reduced during the early
period of sleep
The secretary rates of Glucocorticoid secretion like
Cortisol are high in the early morning but low in the
late evening
22. Transport of hormones in the
blood
• Water soluble hormones ( protein and peptides ) are
dissolved in the plasma and transported from their site
of synthesis to target tissues
• Steroid and thyroid hormones circulate in the blood
while being mainly bound to plasma proteins
• 10 % of steroid or thyroid hormones in the plasma exist
free in the solution
23. CLEARANCE OF HORMONES
FROM THE BLOOD
2 factors can increase or decrease the concentration of a
hormone in the blood
1. Rate of secretion of hormone in the blood
2. Rate of removal of hormone from the blood which is
called METABOLIC CLEARANCE RATE
METABOLIC CLEARANCE RATE = rate of disappearance from
the plasma / concentration of hormone
