Bhj

1. Dr noura H:Abdirahman SH:omar
Endocrine system

2. Endocrine System
2
 The endocrine system is all the organs of the
body that are endocrine glands.
 An endocrine gland secretes hormones.
 Hormones are molecules that are secreted into
the blood.
 Hormones are substances that are secreted by
one group of cells that affects the physiology of
another group of cells (organs).
 The endocrine system is controlled by the
pituitary gland and the hypothalamus.
 Compared to most other organs in the body,
endocrine organs are well vascularized.

4. The Endocrine System
4
 A system of ductless glands
Secrete messenger molecules called
hormones
 Interacts closely with the nervous
system
 Endocrinology – study of hormones
and endocrine glands

5. Endocrine system
 Endocrine system and the nervous
system often work together to bring
about homeostasis.
 Both use specific communication
methods and affect specific target
organs.
 Their methods and effects differ.

7. The major glands of the
endocrine system are the
hypothalamus, pituitary,
thyroid, parathyroid, adrenals,
pineal body, and the
reproductive organs (ovaries
and testes ). The pancreas is
also a part of this system; it
has a role in hormones

9.  The endocrine system is regulated by
feedback in much the same way that a
thermostat regulates the temperature in
a room. For the hormones that are
regulated by the pituitary gland, a signal
is sent from the hypothalamus to the
pituitary gland in the form of a "releasing
hormone," which stimulates the pituitary
to secrete a "stimulating hormone" into
the circulation . The stimulating hormone
then signals the target gland to secrete
its hormone.

10.  As the level of this hormone rises in
the circulation, the hypothalamus and
the pituitary gland shut down
secretion of the releasing hormone
and the stimulating hormone, which in
turn slows the secretion by the target
gland. This system results in stable
blood concentrations of the hormones
that are regulated by the pituitary
gland. The concentration of hormone
as seen by target cells is determined

11. 1-Rate of production: Synthesis and secretion of
hormones are the most highly regulated aspect of
endocrine control. Such control is mediated by
positive and negative feedback circuits.
2-Rate of delivery: An example of this effect is blood
flow to a target organ or group of target cells - high
blood flow delivers more hormone than low blood
flow.
3-Rate of degradation and elimination: Hormones,
like all bimolecular, have characteristic rates of
decay, and are metabolized and excreted from the
body through several routes. Shutting off secretion
of a hormone that has a very short half life causes
circulating hormone concentration to plummet, but

12. Feedback Control of Hormone
Production
Feedback circuits are at the
root of most control
mechanisms in physiology, and
are particularly prominent in
the endocrine system.
Instances of positive feedback
certainly occur, but negative
feedback is much more

13.  Feedback loops are used extensively
to regulate secretion of hormones in
the hypothalamic pituitary axis.
 An important example of a negative
feedback loop is seen in control of
thyroid hormone secretion. The
thyroid hormones thyroxin and
triiodothyronine ("T4 and T3") are
synthesized and secreted by thyroid
glands and affect metabolism
throughout the body. The basic

14. 1-Neurons in the hypothalamus secrete thyroid
releasing hormone (TRH), which stimulates
cells in the anterior pituitary to secrete
thyroid-stimulating hormone (TSH).
2-TSH binds to receptors on epithelial cells in
the thyroid gland, stimulating synthesis and
secretion of thyroid hormones, which affect
probably all cells in the body.
3-When blood concentrations of thyroid
hormones increase above a certain
threshold, TRH-secreting neurons in the
hypothalamus are inhibited and stop

15. Endocrine gland and hormones
 Exocrine glands secretions released into ducts
opening onto an epithelial surface.
 Endocrine glands ductless organs that secrete
their molecules directly into the bloodstream.
 All endocrine cells are located within highly
vascularized areas to ensure that their products
enter the bloodstream immediately.

16. Overview of hormones
 Molecules that have an effect on specific organs.
 Only cells with specific receptors for the hormone
respond to that hormone called target cells, and
the organs that contain them are called target
organs.
 Organs, tissues, or cells lacking the specific
receptor do not respond to its stimulating effects.

17. Classification of hormones
 Peptide hormones
formed from chains of amino acids
most of our body’s hormones are peptide
hormones
longer chains are called protein
hormones
example is growth hormone
 Steroid hormones
type of lipid derived from cholesterol
example is testosterone
 Biogenic amines
small molecules produced by altering the
structure of a

19. Negative feedback loop
 A stimulus starts a process, and eventually either
the hormone that is secreted or a product of its
effects causes the process to slow down or turn
off.
 Many hormonal systems work by negative
feedback mechanisms.
 one example is the regulation of the blood
glucose level in the body.

21. Positive feedback loop
 Accelerates the original process, either to ensure
that the pathway continues to run or to speed up
its activities.
 Only a few positive feedback loops occur in the
human endocrine system.
 one example is the process of milk release from
the mammary glands

23. Hypothalamus
As the master control center of the endocrine
system the hypothalamus oversees most
endocrine activity.
special cells in the hypothalamus secrete
hormones that influence the secretory activity of
the anterior pituitary gland called regulatory
hormones because they are secreted into the
blood to regulate secretion of most anterior
pituitary hormones
releasing hormones (RH)
inhibiting hormones (IH)
Hypothalamus has indirect control over these
endocrine organs.

25. Hypothalamus
 The hypothalamus shaped like a
flattened funnel , forms the floor and
walls of the third ventricle of the brain .
 It regulates primitive functions of the
body ranging from water balance to sex
drive and carries out many of its roles by
way of the pituitary gland.

26. Cont.....
 Hypothalamus produces two hormones that are
transported to and stored in the posterior pituitary.
oxytocin
antidiuretic hormone (ADH)
 Hypothalamus directly oversees the stimulation
and hormone secretion of the adrenal medulla.
 An endocrine structure that secretes its
hormones in response to stimulation by the
sympathetic nervous system.
 Some endocrine cells are not under direct control
of hypothalamus

28. Function of the hypothalamus
 The main function of the hypothalamus is to
regulate homeostasis .
 The hypothalamus helps to maintain many
basic physiological functions, such as body
temperature, blood pressure, fluid and
electrolyte balance, and the regulation of
digestion.
 The hypothalamus also helps regulate
complex behaviors such as appetite, thirst,
sleep, general arousal, and reproductive
behaviors.
 It plays a role in emotional reactions, including

30. Pituitary gland (hypophysis)
 The pituitary gland is located on the inferior
aspect of the brain in the region of the
diencephalon and is attached to the brain by a
structure called the pituitary stalk .
 The pituitary gland is a rounded, pea-shaped
gland measuring about 1.3 cm (0.5 in.) in
diameter. It is covered by the dura mater and is
supported by the sella turcica of the sphenoid
bone.
 The cerebral arterial circle surrounds the highly
vascular pituitary gland, providing it with a rich
blood supply.

32. Pituitary gland (hypophysis)
 The pituitary gland is structurally and functionally
divided into :
(1) anterior pituitary gland
(2) posterior pituitary gland

34. Anterior pituitary gland
 Anterior pituitary gland is controlled by
regulatory hormones secreted by the
hypothalamus.
 Hormones reach the anterior pituitary
via hypothalamohypophyseal portal
system.
 essentially a “shunt” that takes venous
blood carrying regulatory hormones from
the hypothalamus directly to the anterior
pituitary before the blood returns to the
heart.

36. Pituitary hormones
 The pituitary gland releases nine important
hormones.
 The first seven in the following list are
secreted by the anterior pituitary.
 The seventh is secreted by certain cells that
are remnants of the pars intermedia .
 The last two are produced in the
hypothalamus,
 transported through axons in the infundibulum
to the posterior pituitary, and released from
the posterior pituitary into the blood.

37. Anterior pituitary hormones
 Follicle stimulating hormones (FSH)
 Luteinizing hormones (LH)
 Thyroid-stimulating hormone (TSH)
 Adrenocorticotrophic hormone (ACTH)
 Prolactin hormone (PRL)
 Growth hormone (GH)
 Melanocyte-stimulating hormone (MSH).

39. Posterior pituitary gland
 The two posterior lobe hormones
are :
1. Antidiuretic hormone (ADH)
2. Oxytocin (OT)
 Both are synthesized the
hypothalamus transported down the
salk, stored in the posterior pituitary
and released on demand.

41. Anterior pituitary hormones
1. Growth hormone or somatotropin regulates the rate
of growth of all body cells and promotes mitotic
activity.
2.Thyroid stimulating hormone is produced and
released into the bloodstream by the pituitary gland
. It controls reduction of the thyroid
hormones, thyroxine and triiodothyronine, by
the thyroid gland by binding to receptors located on
cells in the thyroid gland. Thyroxine and
triiodothyronine are essential to maintaining the
body’s metabolic rate, heart and digestive functions,
muscle control, brain development and maintenance
of bones.
3.luteinizing hormones (LH) and follicle stimulating

42. Cont......
4.prolactin:hormones that stimulates milk production
in females .
5. Melanocyte-stimulating hormone (MSH): The exact
action of MSH in humans is unknown, but it can
cause darkening of the skin by stimulating the
dispersion of melanin granules within melanocytes.
6.Adrenocorticotropin hormones (ACTH):its important
in regulating the body's response to stress . Its
named for its effect on the adrenal cortex, the outer
layer of an endocrine gland near the kidney .
ACTH stimulates the adrenal cortex to secrete
hormones called glucocorticoids.

43. Posterior pituitary hormones
 Oxytocin
Oxytocin is responsible for uterine contractions,
both before and after delivery.
Oxytocin is also responsible for milk ejection during
breastfeeding.
 Antidiuretic Hormone (ADH)
The function of ADH is to inhibit or prevent the
formation of urine.

44. Thyroid gland
 The thyroid gland is located in the neck, just
below the larynx .
 It has two lobes, each about 5 cm (2 in.) long,
are positioned on either lateral side of the
trachea and connected anteriorly by a bridge
of tissue called the isthmus.
 The thyroid is the largest of the endocrine
glands, weighing between 20 and 25 g.
 It receives an abundant blood supply (80–
120 ml/min) through the paired superior
thyroid branches of the external carotid
arteries and the paired inferior thyroid

46. Functions of the Thyroid Gland
 The thyroid gland produces two
major hormones:
1. thyroxine (T4) and
2. triiodothyronine (T3), and
 the minor hormone calcitonin
(thyrocalcitonin).

47. Cont...
 The release of thyroxine and
triiodothyronine is controlled by the
hypothalamus and by the TSH secreted
from the anterior of the pituitary gland.
 Thyroxine and triiodothyronine are stored
in the thyroid follicles and released as
needed to control the metabolic rate of the
body.
 Calcitonin works in concert with
parathyroid hormone to regulate calcium

48. Parathyroid Glands
 The small, flattened parathyroid glands are
embedded in the posterior surfaces of the
lateral lobes of the thyroid gland .
 There are usually four parathyroid glands: a
superior and inferior pair.
 Each parathyroid gland is a small yellow
brown body 3–8 mm (0.1–0.3 in.) long, 2–5 mm
(0.07–0.2 in.) wide, and about 1.5 mm (0.05 in.)
deep.

50. Parathyroid hormone
The parathyroid glands secrete one
hormone called parathyroid hormone
(PTH).
This hormone promotes a rise in blood
calcium levels by acting on the bones,
kidneys, and small intestine thus, it
opposes the effects of calcitonin,
released by the thyroid gland.

51. Pancreas
 The pancreas is both an endocrine and an
exocrine gland.
 The gross structure of this gland and its
exocrine functions in digestion .
 The endocrine portion of the pancreas
consists of scattered clusters of cells
called pancreatic islets .
 These endocrine structures are most
common in the body and tail of the
pancreas and mainly consist of two types

53. Function of the pancreas
The endocrine function of the
pancreas is to produce and
secrete the hormones glucagon
and insulin.
 The alpha cells of the
pancreatic islets secrete
glucagon and the beta cells
secrete insulin.

54. Cont...
 Glucagon stimulates the liver to convert
glycogen into glucose, which causes the
blood glucose level to rise.
 Insulin has a function opposite to that of
glucagon: it decreases the level of blood
sugar.
 Insulin promotes the movement of
glucose through cell membranes,
especially in muscle and adipose cells

55. Adrenal gland
 The adrenal glands (suprarenal glands)
are paired organs that cap the superior
borders of the kidneys .
 The adrenal glands, along with the
kidneys, are retroperitoneal and are
embedded against the muscles of the
back in a protective pad of fat.

57. Adrenal gland
 Each of the pyramid-shaped adrenal
glands is about 50 mm (2 in.) long, 30 mm
(1.1 in.) wide, and 10 mm (0.4 in.) deep.
 Each consists of an outer adrenal cortex
and inner adrenal medulla that function as
separate glands.
 The adrenal cortex is red to light brown in
colour and is composed of three zones. From
the outer to inner, the layers are;
1. zona glomerulosa
secretes mineralocorticoids
2. zona fasciculata secretes glucocorticoids
3. zona reticularis secretes sex steroids
or androgens

58. Functions of the Adrenal
Glands
 Over 30 hormones have been identified
as being produced by the adrenal
cortex. These hormones are called
corticosteroids or corticoids, for short.
 The adrenal corticoids are grouped into
three functional categories:
1. Mineralocorticoids
2. Glucocorticoids and
3. Gonadocorticoids .

59. Cont......
 Mineralocorticoids the three hormones
secreted by this layer, aldosterone is the most
important. Aldosterone affects the kidneys,
causing them to reabsorb sodium and
increase potassium excretion. At the same
time, it promotes water reabsorption and
reduces urine output.
 The glucocorticoids also promote
vasoconstriction, act as anti-inflammatory
compounds, and help the body resist stress.
 Gonadocorticoids are the sex hormones that
are secreted by the zona reticularis of the
adrenal cortex.

60. Gonads
 The gonads are the male and
female primary sex organs.
 The male gonads are called testes
and the female gonads are called
ovaries.
 The gonads are mixed glands in
that they produce both sex
hormones and sex cells, or
gametes.

61. Gonads
Testes
 The interstitial cells of the testes produce and
secrete the male sex hormone testosterone.
Testosterone controls the development and
function of the male secondary sex organs.
Ovaries
 The endocrine function of the ovaries is the
production of the female sex hormones,
estrogens and progesterone. Estrogens are
produced in the ovarian follicles and corpus
luteum of the ovaries.

62. Cont.....
 Estrogens are responsible for
(1) development and function of the
secondary sex organs,
(2) menstrual changes of the uterus
(3) development of the female secondary sex
characteristics
(4) regulation of the sex drive.
 Progesterone is produced by the corpus
luteum and is primarily associated with
pregnancy in preparing the uterus for
implantation and preventing abortion of the

63. Pineal Gland
 The pineal gland is located behind the
third cerebral ventricle in the midline
(between the two cerebral hemispheres)
of the brain. Its name is derived from its
shape, which is like that of a pine cone .
 In adult humans it is about 0.8 cm (0.3
inch) long and weighs approximately 0.1
gram (0.004 ounce).
 The gland is relatively large in children
and begins to shrink with the onset

65. Pineal gland
 The pineal gland has several critical
functions including secretion of melatonin the
hormone that causes sleepiness and
regulation of certain endocrine functions. The
gland also helps the body to convert signals
from the nervous system to signals in the
endocrine system.
 the pineal gland controls the sex drive,
hunger, thirst and the biological clock which
determines the body’s normal aging process.

66. Thymus
 The thymus is located in the upper
anterior (front) part of the chest directly
behind the sternum and between the
lungs. The pinkish-gray organ has two
thymic lobes.
 The thymus reaches its maximum
weight (about 1 ounce) during puberty
 the thymus of adults becomes infiltrated
with strands of fibrous and fatty
connective tissue.

68. function of the thymus
 . The principal function of the thymus is
associated with the lymphatic system in
maintaining body immunity through the
maturation and discharge of a
specialized group of lymphocytes called
T cells (thymus-dependent cells).
 The thymus also secretes a hormone
called thymosin, which is believed to
stimulate the T cells after they leave the
thymus