ENDOCRINE
SYSTEM
By
Dr. Khaled Ibrahim
AT THE END OF THIS SESSION, STUDENT SHOULD BE ABLE TO:
 Define hormones and list the major functions of hormones in the body.
 Classify hormones according to their chemical nature.
 Outline the endocrine glands and exemplify some of their hormones.
 Describe briefly the major mechanisms for hormone actions.
GUYTON & HALL Textbook of Medical Physiology, 12th ed. page: 881-882 &
886-891.
A gland = is a group of cells that produce a secretion with a specific
chemical composition
This secretion
May be:
Conveyed along a duct
Exocrine gland
Poured directly into blood and lymphatics
without a duct
Endocrine gland
Duct
Sweat gland
An example of exocrine gland
Digestive glands
Mammary gland
Definition of endocrine glands
They are ductless glands (have no ducts)
-----> pouring their secretion directly into the
blood and lymphatics, producing specific
chemical substances called HORMONES.
General features of hormones
Hormone is:
 A specific chemical substance (with a specific composition),
 Secreted by ductless gland,
 In a catalytic amount (very small amounts),
 Transported by the blood stream
lymphatics),
 To a specific target cells
receptors),
(direct or indirect through
(which have a specific hormone
 Where it produce physiologic,
biochemical responses.
morphologic
and
General functions of Hormones
 Maintenance of chemical homeostasis (= maintain the
internal chemical environment of the body constant; i.e.,
the body water, osmolarity, blood glucose, plasma lipids,
appetite, nitrogen balance, etc……..)
 Regulation of body growth and maturity.
 Maintenance of reproduction.
 Affect the general human behaviour.
Hormone secreting tissues
1) Endocrine glands.
2) Organs with endocrine functions
Endocrine
Glands
Organs with endocrine functions
1 - Heart -----> atrial natriuretic peptide
2 - Kidney ------> erythropoitic factor, renin, active vitamin
D3
3 - Liver -----> somatomedins & 25-hydroxycholecalciferol
4 - Skin -----> calciferol
5 - GIT -----> gastrin, pancreozymin, secretin, VIP
6 - Placenta ------> estrogen, progesterone, HCG, HCS,
LHRH & relaxin
Local
Hormones
In
which
a
chemical
substance secreted by one
cell acts on the same cell.
Local hormones
as:
Prostaglandins
Histamine
In which a chemical substance released by one cell
Serotonin
In which a chemical substance
Bradykinin
diffuses for a short distance in the interstitial space
released by one cell acts on a
GIT hormones
& actson
onjuxtaposed
the Neighbouring
cells
specific receptor
cell
Endorphines
Types of Chemical Regulation
Chemical nature of hormones
Polypeptide
Hormones
Amino acid
Hormones
Steroid
Hormones
Chemical nature of hormones
Protein hormones
Small M.W.
Polypeptide
Steroid hormones
Derived from cholesterol
Derived from a.a. as:
- Thyroid hormones
The remaining
hormones
- Adrenal cortical h.
- Sex hormones
- Catecholamines
Glands of endodermal origin
- Active vitamin D3
- Melatonin
Glands of mesodermal origin
Glands of ectodermal origin
Mechanism of hormone action
 To exert its action, the hormone must bind to specific, high-affinity cellular
receptors present in the target cell.
 These receptors may be present:
1) On the cell membrane of the target cell (surface receptor).
2) Inside the target cell (cytoplasmic or nuclear receptor).
1- Surface receptors:
For water-soluble hormones as: peptide hormones & catecholamines.
1
2
3
Activation of membrane-bound enzyme
system
4
These second messenger may be:
1) Cyclic nucleotide as: cAMP or cGMP
2) Rise of intracellular Ca++ concentration.
3) Increase in the cell membrane permeability to Na+, K+ and Cl - .
Hormone-sensitive adenyl cyclase has 3 components:
GDP
Catalytic
component
= Inactive
adenyl
cyclase
α
GDP
Inactive
adenyl
cyclase
α
α
2- Inside the target cells:
For steroid and thyroid hormones, also for vitamin D.