THE ENDOCRINE SYSTEM
Like the Nervous System, the Endocrine System work through
a group of chemical messengers. The endocrine messengers
are called hormones.
Hormones: Substances secreted by one group of cells that
travel through the blood stream and regulate the metabolic
functions of other cells.
Hormones can affect only cells that have receptors that can
bind the hormone
Target cells: Those cell types that can be affected by a
given hormone
FUNCTIONS OF THE ENDOCRINE SYSTEM
The Endocrine System helps to control and integrate:
1- Reproduction
2- Growth
6- Energy balance
7- Cellular metabolism
3- Development
4- Defense against stress
5- Water, electrolyte and nutrient balance
The Endocrine system helps to maintain homeostasis by
coordinating and regulating the activities of cells, tissues,
organs, and systems throughout the body
Homeostasis
It is the ability of the human body to maintain
relatively stable internal conditions (temperature,
PH, etc) even though the outside world changes
continuously.
A good homeostasis is indispensable to maintain the
organism in good health. Most diseases can be
regarded as a result of disturbance of homeostasis.
o
Example: The body temperature must be near to 37 C
o
(98.6 F). When temperature increases only three
o
degrees (to 40 C), the organism is in danger.
Temperature: - 20 o C
Temperature: + 50 o C
Homeostatic Control Mechanisms maintain the temperature
of the bodies near to 37 oC, and both men are in good health
FEEDBACK SYSTEMS
1- Negative feedback mechanisms:
Causes the variable to change in a direction opposite to that
of the initial change.
or
2- Positive feedback mechanisms:
Causes the variable to accelerate the change in the same
direction that the initial disturbance.
or
1- ENDOCRINE SYSTEM IN NEGATIVE
FEEDBACK MECHANISMS
First Hormonal Mechanism
[Calcium]
in blood
Parathyroid
hormone is
released by
parathyroid
gland
Stimulates the
osteoclasts to
resorb bone,
releasing calcium
to the blood
[Calcium]
in blood
Second Hormonal Mechanism
[Calcium]
in blood
Calcitonin
hormone is
released by
thyroid gland
Inhibits osteoclasts
to resorb bone, and
encourages
calcium salt deposit
in bone matrix
[Calcium]
in blood
2- ENDOCRINE SYSTEM IN POSITIVE
FEEDBACK MECHANISMS
Positive feedback mechanisms are used to amplify or perpetuate
events. They are referred as “cascades”.
Example:
HYPOTALAMUS
SENSOR
OXYTOXIN
DELIVERY! Uterine
contractions
CONTROL
CENTER
EFFECTOR
PITUITARY
GLAND
Control of Hormone Secretion
Most of hormone secretions are controlled by the
Hypothalamus and the Pituitary gland
Hypothalamus
Pituitary gland
Corpus callosum
Septum pellucidum
Fornix
Intermediate
mass
Thalamus
Epithalamus
Pineal
gland
Anterior
commissure
Optic chiasm
Pituitary gland
Hypothalamus
Mamillary body
Three mechanisms of Hypothalamic Control over Endocrine Function
Posterior lobe
Anterior lobe
TSH, FSH, LH,
ACTH, GH, PRL,
MSH
Oxytoxin and Antidiuretic
Hormone (ADH)
melanocyte
stimulating
hormone
Hormones produced by the Anterior Lobe or
Adenohypophysis (1)
2
1
5
Hormones produced by the Anterior Lobe or
Adenohypophysis (2)
4
GONADOTROPINS
3
3
6
melanocyte stimulating
hormone
Hormones produced by the Hypothalamus and released
by the posterior pituitary
Hormonal Control of Water Retention in the Body
The body controls how much water is either sent to the
bladder (in urine) or retained in circulation through a
hormone called “antidiuretic hormone” (ADH)
When the body needs water, receptors in the hypothalamus
sense the increase of solute concentration in the blood,
and the hypothalamus orders the release of ADH from the
pituitary gland.
ADH works on the distal nephron tubule and the collecting
duct increasing their permeability to water. Consequently,
more water moves out of the nephron tubule and collecting
duct and thus back into circulation
Hormonal Control of Water Retention in the Body
HYPOTALAMUS
SENSOR
OXYTOXIN
DELIVERY! Uterine
contractions
CONTROL
CENTER
EFFECTOR
PITUITARY
GLAND
The Thyroid Gland
Isthmus
Right
lobe
Left
lobe
Calcitonin
T4 or Thyroxine
T3 or Triiodothyronine
Thyroid Hormones
1- T4 or thyroxine
2- T3 or triiodothyronine
3- Calcitonin
1- T4 or thyroxine
2- T3 or triioodothyronine
Follicular hormones (they are
produced by the follicular cells)
1- They increase the rate of oxygen uptake and the basal metabolic rate
2- They enhance the effects of sympathetic stimulation (increasing heart
rate, blood pressure, nervousness, and sweating)
3- They help to maintain body temperature
4- They help to regulate growth, development, and intracellular metabolism
by increasing protein synthesis and lipid breakdown
5- They stimulate RBC formation, and speed up bone turnover
3- Calcitonin
It is released in response to high levels of calcium in the blood
[Calcium]
in blood
Calcitonin
hormone is
released by
thyroid gland
Inhibits osteoclasts
to resorb bone, and
encourages
calcium salt deposit
in bone matrix
[Calcium]
in blood
The Parathyroid Glands
Thyroid gland, posterior view
Parathyroid
glands
Parathyroid
glands
The Parathyroid Hormone
It increases calcium blood levels by
targeting bone and kidneys
1- In bones
[Calcium]
in blood
Parathyroid
hormone is
released by
parathyroid
gland
Stimulates the
osteoclasts to
resorb bone,
releasing calcium
to the blood
[Calcium]
in blood
2- In kidneys
- It increases calcium reabsortion
- It increases formation and
Increases absorption of calcium and
secretion of calcitriol
phosphate from the intestines
Adrenal or Suprarenal Glands
Medulla (It is considered
part of Sympathetic
Autonomic nervous System)
Catecholamines
(Epinephrine and
Norepinephrine)
Sex hormones
( androgens)
Glucocorticoids
(Cortisol, Cortisone and
Corticosterone)
Mineralocorticoids
(Aldosterone)
Hormones of the Adrenal Glands
1- Mineralocorticoides (Aldosterone)
-It increases sodium ion and water reabsortion in kidneys
-It increases potassium secretion in kidneys
Aldosterone secretion is regulated by the renin-angiotencin mechanism
Blood pressure or
plasma solute
concentration
Renin-angiotencin
mechanism
Aldosterone
secretion
2- Glucocorticoids (Corticosterone and Cortisol)
-They help the body to resist stressors
-They depress the immune system and the inflammatory response
3- Gonadocorticoids or Sex Hormones (Androgens)
-They are masculinizing hormones.
- In females they stimulate sexual drive. After menopause they are
converted to estrogens.
4- Catecholamines (Epinephrine and Norepinephrine)
They increase:
-Cardiac activity -Blood pressure
-Glycogen breakdown -Blood glucose levels
-Triglyceride breakdown
Three mechanisms of Hypothalamic Control over Endocrine Function
The Pancreas
Endocrine pancreas
Islets of Langerhans
1- Beta cells: Insulin
2- Alpha cells: Glucagon
3- Delta cells: Somatostatin
4- F cells: Pancreatic polypeptide
Exocrine pancreas
Pancreatic acini
They secrete large quantities of
an alkaline, enzyme rich fluid
1- Glucagon (Released by alpha cells)
-It stimulates the liver to produce glucose
-It stimulates to breakdown glycogen
Glucose in
blood
Pancreas
Glucagon
Liver
Glycogen
(Alpha cells)
Glucose
2- Insulin (Released by beta cells)
- High blood glucose
- Parasympathetic stimulation
- High level of some aminoacids
Pancreas
(Beta cells)
Insulin
- It increases the rate of glucose uptake by cells
- It increases cellular metabolism
- It increases the storage of lipid and glycogen
- Increases aminoacids absorption and protein synthesis
The Gonads
Spermatogonia
Seminiferous
tube
Primary
spermatocyte
Secondary
spermatocyte
Spermatids
Sperm (immature)
Sustentacular or Sertoli
cells: Produce Inhibin
Interstitial or Leydig cells:
Produce androgens
(testosterone)
The Development of the Egg
Corpus
luteum : Progesterone
Primary
oocytes
Secondary Primary
follicles follicles
Tertiary
follicles
Follicles: Estrogens and Inhibin
The Pineal Gland
Pineal gland
Melatonin:
1- Inhibiting reproductive
functions
2- Protecting against
damage for free radicals
3- Setting circadian
rhythms
Hormones of the Kidneys
1- Calcitriol:
It stimulates uptake of calcium and phosphate ions from the gut
2- Erythropoietin:
It stimulates the production of red blood cells by red bone marrow
3- Renin:
It converts angiotensinogen into angiotensin-I and starts the cascade of
enzymatic changes known as the renin-angiotensin system
Atrial Natriuretic Peptide (ANP)
It is produced by the atrial cells of the heart
It reduces the blood volume and blood pressure