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Assist prof. of Medical Physiology
• Body functions are regulated by 2 systems;
Nervous
system
Endocrine
system
Rapid onset
Slow onset
Short duration
Long duration
nervous system
Endocrine
gland
Blood vessels
Receptor
Biological effect
Target
cell
• Endocrine glands are groups of cells that
produce specific chemicals, called hormones,
having well defined effects on body functions.
• Also, called ductless glands since their
secretion is not conveyed along ducts but pass
directly into blood and lymphatic vessels.
General features of hormones (def.)
1) A specific chemical substance
2) Secreted by ductless gland
3) In a catalytic amount (very small amounts),
4) Transported by the blood (directly or through
lymphatics), To a specific target cells (which have a
specific hormone receptors),
5) Where it produces:
» physiologic,
» morphologic and
» biochemical responses
• Both nervous and endocrine systems form together a biological
communication network for integration of the body response to
a changing environment.
• Examples of this link:
• 1) Hypothalamic neurosecretory cells,
• A) Some neurons delivered substances through
hypothalamo-hypophyseal portal vessels to the ant.
pituitary to regulate its secretions.
•
• B) Other hypothalamic neurons send their axons to
the post pituitary, where they release neurosecretory
products directly into the blood stream.
Hypothalamic neurosecretory cells
2- Innervation of the endocrine glands: Most, endocrine
glands, receive nerves that appear to control their
blood supply and secretory activity.
3- Also, thyroid, gonadal and adrenocortical hormones
act on the CNS to inhibit or stimulate the secretory
activity of the hypothalamic neurons.
Secretion of adrenaline and noradrenaline from adrenal medulla
by symp. N.F.
Feedback of Thyroid Hormones on Pituitary Gland
• Virtually all organs of the body exhibit endocrine function
I-Endocrine glands
II- Organs with endocrine functions
1- Hypothalamus
2- Pituitary gland
3- Thyroid gland.
4- Parathyroid glands
5- Suprarenal glands
6- Endocrine portion of the pancreas
7- Primary sex organs: testes and
ovaries
8- Thymus gland
9- Pineal gland
1- Heart
2- Kidney
3- Liver
4- Skin
5-GIT
6- Placenta
Some hormones act only locally, e.g.:
1- Paracrine hormones, which diffuse for a short distance through the
interstitial space to affect neighbouring cells.
2- Autocrine hormones, which act on the same cells.
3- Juxtacrine, whereby one cell interact with specific receptor on juxtaposed cells.
Example: PGs, histamine, serotonin, bradykinin, epinephrine,
acetylcholine, endorphins, encephalins, GIT hormones.
Biological
effect
Chemical nature of hormones
Protein H.
P.P
-Pituitary gland
-Hypothalamus
-Calcitonin.
-Parathyroid H. Pancreatic H.
-GIT H.
Steroid H.
A.A
-Thyroid h.
From cholesterol:
-Catechol.
-Melatonin
A) Adrenal cortical H.
B) Sex H.
C) D3.
Synthesis:
• Amino acid and steroid hormones are synthesized
through series of enzymatic reactions
• Peptide hormones are synthesized as proteins in the
ribosomes.
Storage :
• Catecholamines and polypeptide hormones are stored in
secretory granules
• thyroxin and steroid hormones not stored in granules &
present in the free form within the cytoplasm .
• 2 groups of hormones derived from the amino acid
tyrosine
• Thyroid hormones and Catecholamines
Most endocrine glands produce their hormones
continually at levels determined by:
a) Body requirements.
b) Rate of hormone inactivation.
c) Rate of hormone clearance from the body.
1. Catecholamine and polypeptide hormones are stored
in secretory granules, are released by the process of
exocytosis.
2. Thyroxin and steroid hormones are present in the free
form within the cytoplasm and leave the cell by simple
transfer through the plasma membrane.
The released hormones enter the blood, where they
may circulate in 2 forms:
– 1. Free (unbound) part: the active part which binds to
receptor.
– 2. Bound part: carried by specific albumins and globulins
which are synthesized in the liver.
In general, steroid and thyroid hormones are bound to transport
proteins,
whereas polypeptide and other amine hormones circulate in a
free form.
The plasma half-life of a hormone (time needed for the
concentration of the hormone to decrease to its half ) is correlated
with the % of protein binding.
For example,
– Thyroxin is 99.98% protein bound and has a
plasma half-life of 6 days,
– Whereas aldosterone, a steroid hormone, is only
15% bound and its plasma half-life of 25 minutes.