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Assist prof. of Medical Physiology
• Body functions are regulated by 2 systems;
nervous system
Blood vessels
Endocrine gland
Biological effect
Receptor
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 substance2) Secreted by ductless gland3) 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.
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 functions1- Hypothalamus2- Pituitary gland3- Thyroid gland. 4- Parathyroid glands 5- Suprarenal glands6- Endocrine portion of the pancreas7- Primary sex organs: testes and ovaries 8- Thymus gland9- Pineal gland
1- Heart 2- Kidney 3- Liver 4- Skin5-GIT6- 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 juxta-posed cells.
Example: PGs, histamine, serotonin, bradykinin,
epinephrine, acetylcholine, endorphins,
encephalins, GIT hormones.
Biological effect
Biological effect
Chemical nature of hormones Chemical nature of hormones
Protein H. Steroid H.
From cholesterol:
A) Adrenal cortical H.
B) Sex H.
C) D3.
From cholesterol:
A) Adrenal cortical H.
B) Sex H.
C) D3.
P.PP.P A.AA.A
-Thyroid h.-Catechol.-Melatonin
-Thyroid h.-Catechol.-Melatonin
-Pituitary gland
-Hypothalamus
-Calcitonin.
-Parathyroid H. -
Pancreatic H.
-GIT H.
-Pituitary gland
-Hypothalamus
-Calcitonin.
-Parathyroid H. -
Pancreatic H.
-GIT H.
Synthesis:Synthesis:• Amino acid and steroid hormones are
synthesized through series of enzymatic reactions • Peptide hormones are synthesized as proteins in
the ribosomes.
Storage :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.
