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The Endocrine System
CHAPTER 8
The endocrine system, interacts with the nervous
system to coordinate and integrate the activity of
body cells.
-The NS regulates the activity of muscles and
glands via electrochemical impulses and those
organs respond within milliseconds.
-The endocrine system influences metabolic
activity by means of hormones that typically occur
after a lag period of seconds or even days. But,
once initiated, those responses tend to be much
more prolonged than those induced by the nervous
system.
Hormonal targets
Include most cells of the body, but mainly:
-Reproduction;
- Growth and development;
- Mobilization of body defenses;
- Maintenance of electrolyte, H2O, &
nutrient balance of the blood; and
-Regulation of cellular metabolism and
energy balance.
Endocrinology: “scientific study of hormones
and the endocrine organs”
Compared with other organs, those of the endocrine
system are:
- small and unimpressive. (to collect 1 kg of hormoneproducing tissue, endocrine tissue from eight or nine
adults would be needed)
- no anatomical continuity typical of most organ
systems ,the endocrine organs are widely scattered
about the body.
Control of Hormone Release
The synthesis and release of most hormones are
regulated by some type of negative feedback system
In such a system, hormone secretion is triggered by
some internal or external stimulus.
As hormone levels rise, they cause target organ effects
and inhibit further hormone release.
 As a result, blood levels of many hormones vary only
within a narrow range.
Endocrine Organs
The endocrine glands include the pituitary, thyroid, parathyroid, adrenal,
pineal, and thymus glands .
-In addition, the pancreas and gonads (ovaries and testes), are also major
endocrine(mixed glands).
-The hypothalamus, along with its neural functions, produces and releases
hormones, so we can consider the hypothalamus a neuroendocrine organ .
Endocrine function of hypothalamus:
(a) regulates the hormonal output of the anterior pituitary via releasing and
inhibiting hormones .
(b) synthesizes two hormones (???????) that it exports to the posterior pituitary
for storage and later release.
Major Endocrine Organs
I-The Pituitary Gland (Hypophysis) It hangs from the
base of the brain by a stalk and is enclosed by
bone(Turk`s saddle).
It consists of a hormone-producing glandular portion
(anterior pituitary, adenohypophysis ) and a neural
portion (posterior pituitary , neurohypophysis).
Four of the six anterior pituitary hormones—thyroidstimulating hormone, adrenocorticotropic hormone,
follicle-stimulating hormone, and luteinizing
hormone—are tropins or tropic hormones (tropi =
turn on, change), which are hormones that regulate
the secretory action of other endocrine glands.
Hormones of Pitutary gland

 1-Growth hormone (GH) :
 - stimulates most body cells to increase in
size and divide, mainly bones and skeletal
muscles leading to long bone growth and
increased muscle mass.
 - Secretion of GH is regulated chiefly by two
antagonistic hypothalamic hormones:
 1- Growth hormone–releasing hormone
(GHRH) and
 2- Growth hormone– inhibiting hormone
(GHIH), (also called somatostatin).
Hormones of Pitutary gland
 2-Prolactin (PRL)
 promotes milk production in humans. Its
secretion is prompted by prolactin-releasing
hormone (PRH) and inhibited by prolactininhibiting hormone (PIH) from the
hypothalamus.
 3-Adrenocorticotropic hormone (ACTH)or
corticotropin stimulates the adrenal cortex to
release corticosteroid hormones. ACTH release,
elicited by hypothalamic corticotropin-releasing
hormone (CRH).
4- Thyroid-stimulating hormone (TSH), or thyrotropin, is
a tropic hormone that stimulates normal development
and secretory activity of the thyroid gland.
TSH release is triggered by the hypothalamic
thyrotropin-releasing hormone (TRH).
5&6-Gonadotropins : Follicle-stimulating hormone (FSH)
and luteinizing hormone (LH) referred to collectively as
gonadotropins, regulate the function of the gonads
(ovaries and testes)In both sexes:
-FSH stimulates gamete (sperm or egg) production
-LH promotes production of gonadal hormones.
*In females, LH works with FSH to cause maturation of
an egg-containing ovarian follicle. LH then
independently triggers ovulation and promotes
synthesis and release of ovarian hormones.
*In males, LH stimulates the interstitial cells of the
testes to produce the male hormone testosterone.
Gonadotropins are absent in prepubertal boys and
girls. At puberty, gonadotropin begins to rise, causing
the gonads to mature. In both sexes, gonadotropin
release by the adenohypophysis is prompted by
gonadotropin-releasing hormone (GnRH) produced
by the hypothalamus. Gonadal hormones, feed back
to suppress FSH and LH release.
The Posterior Pituitary
It stores antidiuretic hormone (ADH) and oxytocin that
have been synthesized by hypothalamus.
1-Oxytocin is released in higher amounts during
childbirth and breastfeeding women. As blood levels of
oxytocin rise, the expulsive contractions of labor
increases and finally end in birth.
2-Antidiuretic Hormone (ADH) It targets the kidney
tubules that respond by reabsorbing more water to the
bloodstream. As a result, less urine is produced and
blood volume increases. As the solute concentration of
the blood declines, the ADH release stops.
II-The Thyroid Gland
Butterfly-shaped located just inferior to the larynx
and is composed of:
-The follicles cells largest pure endocrine gland in
the body. It which produce thyroid hormones.
-The parafollicular cells, produce calcitonin, an
entirely different hormone.
1-Thyroid Hormone (TH)
Is the body’s major metabolic hormone.TH is
actually two : thyroxine, or T4, and triiodothyronine
or T3. The difference is the number of bound iodine
atoms.
TH affects virtually every cell in the body by
ncreasing basal metabolic rate and body heat
production.It is especially critical for normal skeletal,
nervous system development and maturation and
2-Calcitonin
Is produced by the parafollicular cells of the thyroid gland.
It is a direct antagonist of parathyroid hormone. Calcitonin targets the
skeleton,where it stimulates Ca2+ uptake and incorporation into bone matrix. Thus,
calcitonin has a bone-sparing effect.
Excessive blood levels of Ca2+ act as a stimulus for calcitonin release, whereas
declining blood Ca2+ levels inhibit its release.
Calcitonin appears to be important only in childhood, when the skeleton grows
quickly and the bones are changing dramatically in mass, size, and shape.
III-The Parathyroid Glands
The tiny, yellow-brown parathyroid glands are in the posterior aspect of the thyroid
gland .There are usually four of these glands, but the precise number varies( may
be eight, and some may be located in the neck or even in the thorax).
Discovery of the parathyroid glands was after partial thyroid gland removal, when
some patients suffered uncontrolled muscle spasms and severe pain, and
subsequently died.
III-The Parathyroid Glands
The tiny, yellow-brown ,four parathyroid glands are in the posterior aspect of the
thyroid gland .
Discovery of the parathyroid glands was after partial thyroid gland removal, when
some patients suffered uncontrolled muscle spasms and severe pain, and
subsequently died.
Parathyroid hormone (PTH), or parathormone, is the single most
important hormone controlling the calcium balance of the blood. PTH increases
Ca2+ levels in blood Leading to hypercalcemia.
*Because plasma calcium is essential for nerve impulses, muscle contraction, and
blood clotting, precise control of Ca2+ levels is critical.
IV-The Adrenal (Suprarenal) Glands
The paired adrenal glands are pyramid-shaped organs atop the kidneys .Each
adrenal gland is structurally and functionally two endocrine glands.
-The inner adrenal medulla, more like a knot of the sympathetic nervous system.
-The outer adrenal cortex, encapsulating the medullary region and forming the bulk
of the gland.
*Each region produces its own set of hormones but all adrenal hormones help us
cope with stressful situations.
A-The Adrenal Cortex
Its Steroid hormones( all are called corticosteroids),
are synthesized from cholesterol .
The cortical cells are arranged in three layers or zones:
-The superficial zone produce mineralocorticoids.
-The middle zone produce the metabolic hormones
called glucocorticoids.
-The innermost zone produce small amounts of
adrenal sex hormones, or gonadocorticoids.
1-Mineralocorticoids
Although there are many mineralocorticoids,
aldosterone accounts for more than 95% of them.
Aldosterone reduces excretion of Na from the kidney
tubules followed by water.
Aldosterone secretion is stimulated by low blood levels of
Na+, and decreasing blood volume and blood pressure
2- Glucocorticoids(cortisol) Essential to life, they :
-influence the energy metabolism of most body cells
- help us to resist stressors as hemorrhage, infection, or
physical or emotional trauma .
-keep blood glucose levels fairly constant (hyperglycemic)
by breaking down of fats and even
proteins(gluconeogenesis).
3-Gonadocorticoids (Sex Hormones)
The bulk of it are weak androgens, or male sex
hormones(it also makes small amounts of female
hormones). It is insignificant compared with the amounts
made by the gonads during late puberty and adulthood.
B-The Adrenal Medulla
It is considered as part of the autonomic
nervous system. They are modified ganglionic
sympathetic neurons that synthesize the
catecholamines epinephrine and norepinephrine
(NE) .
When the sympathetic nervous system is
mobilized, blood glucose levels rise, blood
vessels constrict and the heart beats faster
(together raising the blood pressure), blood is
diverted from temporarily nonessential organs
to the heart and skeletal muscles, and preganglionic sympathetic nerve endings to the
adrenal medulla signal for release of
catecholamines, which reinforce and prolong
the fight-or-flight response.
*Unlike the adrenocortical hormones, which 
promote long-lasting body responses to stressors,
catecholamines cause fairly brief responses.
VI-Pancreatic islets (islets of Langerhans)
The pancreas is a mixed gland . Scattered among the
enzyme-producing cells are approximately a million
pancreatic islets ,tiny cell clusters that produce
pancreatic hormones .The islets contain :
-the glucagon-synthesizing alpha (α) cells and
-the more numerous insulin-producing beta (β) cells.
Insulin and glucagon effects are antagonistic:
Insulin is a hypoglycemic hormone, whereas
glucagon is a hyperglycemic hormone.
1-Glucagon
Is an extremely potent hyperglycemic agent by breakdown of
glycogen in the liver to glucose (glycogenolysis).
-Insulin
Its main effect is to lower blood glucose levels by :
1-enhancing membrane transport of glucose (and other simple
sugars) into body cells, especially muscle and fat cells. (It does
not accelerate glucose entry into liver, kidney, and brain tissue,
all of which have easy access to blood glucose regardless of
insulin levels.)
2-Inhibition of the breakdown of glycogen to glucose and the
conversion of amino acids or fats to glucose; thus, it counters
any metabolic activity that would increase plasma levels of
glucose.
VII-The Gonads
The male and female gonads produce steroid sex
hormones, identical to those produced by adrenal
cortical cells. The major distinction is the source and
relative amounts produced.
Female gonads:
-The paired ovaries are small, oval organs located in the
female’s abdomino-pelvic cavity. Besides producing
ova, the ovaries produce several hormones, most
importantly estrogens and progesterone .
-the estrogens are responsible for maturation of the
reproductive organs and the appearance of the
secondary sex characteristics of females at puberty.
Acting with progesterone, estrogens promote
breast development and the menstrual cycle.
-Progesterone is the hormone of pregnancy
Male gonads:
The male testes, located in an extra-abdominal
skin pouch called the scrotum, produce sperm
and male sex hormones( androgens), primarily
testosterone
-During puberty, testosterone initiates the
maturation of the male reproductive organs and
the appearance of secondary sex characteristics
and sex drive.
 In addition, testosterone is necessary
for normal sperm production and
maintains the reproductive organs in
their mature functional state in adult
males.
The release of gonadal hormones is
regulated by gonadotropins(FSH and
LH).
VIII-pineal gland
The tiny, pineal gland hangs from the roof of the third
ventricle in the diencephalon .
The endocrine function of the pineal gland is still
somewhat of a mystery, its only major secretory
product is melatonin.
.
Peak levels occur during the night and make us
drowsy; lowest levels occur around noon.
 IX-The Thymus
Located deep to the sternum ,Large in infants and
children, the thymus diminishes in size throughout
adulthood.
By old age, it is composed largely of adipose and
fibrous connective tissues.
The major hormonal products appear to be
essential for the normal development of T
lymphocytes and the immune response
Other Hormone-Producing Structures
Other hormone-producing cells occur in various organs of the
body, including the following :
1. Heart. The atria contain some specialized cardiac muscle
cells that secrete atrial natriuretic peptide. By signaling the
kidneys to increase their production of salty urine and by
inhibiting aldosterone release by the adrenal cortex, ANP
reduces blood volume, blood pressure, and blood sodium
concentration .
2. Gastrointestinal tract. In the mucosa of the GI tract ,
there are scattered cells release several peptide hormones
that help regulate a wide variety of digestive functions.
Gastrin , secretin and cholecystokinin are examples of these
hormones.
3. Placenta. Besides sustaining the fetus
during pregnancy, the placenta secretes
several steroid and protein hormones that
influence the course of pregnancy.
Placental hormones include estrogens and
progesterone (hormones more often
associated with the ovary), and human
chorionic gonadotropin (hCG).
4.
Kidneys. Interstitial cells in the kidneys
secrete erythropoietin ,a protein hormone
that signals the bone marrow to increase
production of red blood cells. The kidneys
also release renin, the hormone that initiates
the renin-angiotensin mechanism .
5. Skin. The skin produces cholecalciferol, an
inactive form of vitamin D3 which is then
activated in liver and kidneys.
Prostglandins
Are locally acting hormones derived from fatty acid molecules
secreted from plasma membranes of virtually all body cells after
stimulation by local irritants or hormones, etc. There are several
groups known till now indicated by the letters A , B, C, till I
(PGA-PGI)
They have many targets , but act locally at site of release.
Examples of effects include:
-increase blood pressure by acting as vasoconstrictor
-constriction of respiratory passageways
-stimulate uterine muscles , promoting labor
-enhance blood clotting
-promote inflammation and pain
-increase gastric secretions
-cause fever