Endocrine System
Dr Khamis Al Hashmi
Department of physiology
Room # 006
Ext. 3435
Email: kh@squ.edu.om
Nervous Vs Endocrine system
 Nervous
and endocrine systems acts
together to coordinate functions of body
system.
 Nervous system releases;
Neurotransmitors
 Endocrine
system releases; Hormones
Nervous Vs Endocrine system (cont.)
Characteristic
Nervous system
Endocrine system
Mediators molecules Neurotransmitters Hormones
Site of mediator
action
Close to site of
release
Far from site of
release
Types of target cells Muscle cells,
Cells throughout
the body
Time to onset of
action
Seconds to hours
Duration of action
gland cells, other
neurons
Within
milliseconds
Briefer
(milliseconds)
Longer (seconds
to days)
Endocrine Vs Exocrine glands
Exocrine gland: secrete their products into
ducts that carry the secretion into body
cavities, into lumen of organs, or to the outer
surface of the body.
 E.g. Sweat, sebaceous, mucus, digestive
glands.


Endocrine glands: secrete their products into
the interstitial fluid surrounding the secretory
cells
diffused into blood capillaries
carried to target cells.
Definitions
Hormone:
is a mediator molecule that is released in
one part of the body but regulates activity of cells in
other parts of the body.
Target cell:
A cells whose activity is affected by a
particular hormone.
Receptor: A specific molecule or cluster of molecules
that recognizes and binds a particular ligand (hormone)
Circulating hormones Vs Local hormones
A) Circulating hormones
B) Local hormones:
1) Paracrines
E.g. Nitric oxide.
IL2
2) Autocrines
E.g. IL2
Local hormones usually are inactivated quickly
Chemical classes of hormones
a)
Water-soluble hormones:
-
Amine hormones
(Retain an amino group
(NH3+)
-
-
Peptide and protein hormones
Eicosanoid hormones (derived from
arachidonic acid. Eg. Prostaglandins, leukotrienes)
b)
Lipid-soluble hormones:
-
Steroid hormones
Thyroid hormones
Nitric oxide
a) Water soluble hormone
b) Lipid soluble hormone
Hormone transport in the blood
 Water
soluble hormones circulate in a
free form (Unbound).
 Lipid soluble hormones bound to
transport proteins. (>90%).
Importance:
- Increase the solubility in blood.
- Slow the rate of excretion by kidney or
degradation by liver.
- Provide a ready reserve of hormone.
Responsiveness of a target cell to a hormone
depend on:
1) Concentration of the hormone
2) Number of receptors
3) Influence by other hormone (hormones interaction)
1) Concentration of the hormone
–
The hormone’s rate of secretion (all hormones)
–
Rate of metabolic inactivation and excretion (all hormones)
–
Rate of metabolic activation (few hormones)
–
Extent of binding to plasma protein (lipophilic hormones)
Control of hormone secretion

Important to prevent overproduction or
underproduction
Three types of signals:
1. Signals from the nervous system
Eg. Nerve impulse to adrenal medulla
2.
Chemical changes in the blood.
Eg. Blood Ca2+ level
3.
Epinephrine
parathyroid hormone
Other hormone
Eg. Ant. Pituitary hormones
glands
hormones of other
Negative-feedback control
Short loop feedback
long loop feedback
Positive-feedback
control
Diurnal (circadian) rhythm

The secretion rate of many hormones rhythmically
fluctuate up and down as function of time.
– Diurnal (or circadian), i.e. “day-night”. characterised
by repetitive oscillations in hormone levels that are
very regular and have a frequency of one cycle every
24 hours. Eg. Cortisol secretion.
– Other. Monthly menstrual period.

Negative feedback control mechanisms operate to
maintain whatever set point is established for that
time.
Clearance of hormones from the blood
– Metabolic destruction by the tissues
– Binding with the tissues.
– Excretion by the liver into the bile.
– Excretion by the kidneys into urine.
2) Number of receptors
Down regulation:
A hormone present in excess
–
–
–
–
–
Receptors
Destruction of the receptor by lysosomes
Decrease production of receptors
Inactivation of some of the receptor molecules
Inactivation of some of the intracellular signaling molecules
Temporary sequestration of the receptor to the inside of the
cell.
Up regulation:
A hormone is deficient
Receptors
3) Hormones interaction:
Permissiveness (permissive effect): powerful action of a
hormone on target cells require a simultaneous or recent
exposure to a second hormone.
Eg. Epinephrine and thyroid hormones in lipolysis.
Synergism (synergistic effect): the effect of two hormones
acting together is greater or more extensive than the effect of
each hormone acting alone.
Eg. FSH and estrogen for normal development of oocytes in
ovaries.
Antagonism (antagonistic effect): one hormone opposes the
action of another hormone.
Eg. Insulin and glucagon.
Endocrine dysfunction
a)
Decrease in hormone activity



Decrease in hormone secretion by the
endocrine gland (hyposecretion).
Increase removal of the hormone from the
blood
Abnormal tissue responsiveness to the
hormone:

Lack of target cell receptors.
Lack of an enzyme essential to the target cell
response
b)
Increase in hormone activity



Increase in hormone secretion by the
endocrine gland (hypersecretion)
Reduced plasma protein binding of the
hormone (too much free, biologically active
hormone
Decreased removal of the hormone from
the blood:

Decreased inactivation.
Decreased excretion
Location of endocrine glands
Organ
Hormones secreted
Hypothalamus
Releasing and inhibitory hormones (e.g.
thyrotropin releasing hormones,
growth
hormone
inhibitory
hormone), ADH, oxytocin.
Anterior pituitary gland
Tropic
hormones:
ACTH,
LH,FSH), GH, prolactin.
Posterior pituitary gland
Oxytocin, ADH (vasopressin)
Thyroid gland
Thyroxine, Tri- iodothyronine.
Adrenal gland
Mineralocorticoids (e.g. aldosterone),
glucocorticoids
(e.g.
cortisol),
catecholamines (e.g. epinephrine,
nor-epinephrine).
Parathyroid glands
Parathyroid hormone.
Pancreatic Islets
Insulin, glucagon.
Gonads
Testosterone, estradiol.
TSH
Hypothalamus and pituitary gland
Anterior pituitary
Hypophyseal
portal system
Posterior Pituitary
Hypothalamohypophyseal
tract