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CHAPTER 18The Endocrine System -Summary Notes
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ENDOCRINE SYSTEM GLANDS Glands that secrete endocrine hormones into
the bloodstream are called endocrine
glands
They are one of two major types of glands
in the body, the other being exocrine
glands (which
secrete their products into ducts )
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INTRODUCTION The nervous and endocrine systems
coordinate all of the body systems
The nervous system does so through the
action of neurons, and the
neurotransmitters they secrete
The endocrine
system uses
hormones produced
by endocrine structures
to produce their
effects
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The nervous system releases
neurotransmitters and is fast acting (millisec),
but its effects tend to be of shorter duration
(millisec)
The endocrine system releases hormones,
and is slower acting (sec/hrs/days), but the
effects are much longer lasting (sec/days)
Most hormones circulate through the blood and
bind to receptors on “target cells.”
COMPARISON OF CONTROL BY THE NERVOUS AND ENDOCRINE
SYSTEMS
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Endocrine glands secrete hormones. They
do not have ducts and secrete their
hormones directly into the interstitial fluid
that surrounds them and then they enter the
blood stream.
The hormones diffuse into the blood stream
through capillaries and are carried to target
cells throughout the body.
ENDOCRINE GLANDS
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Hormones are simply chemicals called
mediator molecules that have effects on
cells in the local environment, or in a distant
part of the body
Some hormones, called autocrine
hormones are local hormones that are
secreted into the interstitial fluid, and then
bind to the same cell.
TYPES OF HORMONES
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TYPES OF HORMONES Paracrine hormones are local hormones
that are secreted into interstitial fluid and
act on nearby cells
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Endocrine hormones are secreted into
interstitial fluid and then absorbed into the
bloodstream to be carried systemically
to any cell that
displays the
appropriate type
of receptor
TYPES OF HORMONES
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Hormones traveling throughout the body will only
affect target cells that possess specific protein
receptors. When stimulated, an endocrine gland will release
its hormone in frequent bursts, increasing the
concentration of the hormone in the blood,
consequently protein receptors are continually
being synthesized or broken down Receptors may be down-regulated in the
presence of high concentrations of hormone. Receptors may be up-regulated in the presence
of low concentrations of hormone.
HORMONE ACTIVITY
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SOLUBILITY OF HORMONES Hormones can be divided into two broad
chemical classes. This chemical
classification is useful because the two
classes exert their effects differently
Lipid soluble hormones bind to receptors
in the cytoplasm or nucleus of the cell
Water soluble hormones bind to
receptors on the surface of the cell
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SOLUBILITY OF HORMONES Lipid soluble hormones require a carrier
protein for transport in the watery environment of
the blood
Once they arrive at their
destination, however, they
are able to freely pass
through the plasma
membrane to bind to
receptors located in the cytoplasm
or the nucleus of the target cell
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SOLUBILITY OF HORMONES Water soluble hormones are easy to
transport in the watery blood. The plasma
membrane of target cells, however, is
impermeable to them
Water soluble hormones exert their effects by
binding to receptors exposed to the
interstitial fluid on the surface of target cells
• the hormone binding to its receptor acts as
the first messenger in a cascade of
signaling in the cell
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The first messenger (the hormone) then
causes production of a second messenger
inside the cell, where specific hormone-
stimulated responses take place
SOLUBILITY OF HORMONES
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CONTROL OF HORMONES Hormone secretion is regulated by signals
from the nervous system, chemical
changes in the blood, and other
hormones
• Most hormonal regulatory systems work
via negative feedback, but a few
operate via positive feedback
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CONTROL OF HORMONES In a negative feedback system the hormone
output reverses a particular stimulus. For example:
Blood Ca2+ level is controlled by the parathyroid
hormone (PTH). If blood Ca2+ is low, there is a
stimulus for the parathyroid glands to release
more PTH. PTH then exerts its effects in the body
until the Ca2+ level returns to normal. If the level
gets too high the body will cease PTH production
and the parafollicular cells of the thyroid gland
will secrete calcitonin (CT) to lower the Ca2+
levels.
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CONTROL OF HORMONES In a positive feedback system
the hormone output reinforces
and encourages the stimulus.
For example, during childbirth,
the hormone oxytocin stimulates
contractions of the uterus, and
uterine contractions in turn
stimulate more oxytocin release,
a positive feedback effect
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THE ENDOCRINE SYSTEM The endocrine system consists of the pituitary,
thyroid, parathyroid, adrenal and pineal
glands
Some of the most important glands of the
endocrine system are not exclusively endocrine
glands: The hypothalamus, thymus,
pancreas, ovaries, and testes are paramount;
the kidneys, stomach, liver,
small intestine, skin, heart,
and placenta also contribute
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THE ENDOCRINE SYSTEM
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THE HYPOTHALAMUS The hypothalamus is the major link
between the nervous and endocrine systems
It receives input from several regions in the
brain
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THE PITUITARY GLAND The hypothalamus now known as the master
control gland and mainly controls the pituitary
gland. The pituitary hangs down from the
hypothalamus
on a stalk called the infundibulum
The gland is divided into an anterior
adenohypophysis and a posterior
neurohypophysis - the anterior pituitary
accounts
for about 75% of the total
weight of the gland
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Specialized neurosecretory cells in the
hypothalamus secrete releasing hormones
into the hypophyseal portal system
that supplies blood to the
anterior pituitary
gland
THE ADENOHYPOPHYSIS
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THE ADENOHYPOPHYSIS The second capillary system of the
hypophyseal portal system delivers the
hypothalamic releasing hormones to the
anterior pituitary
5 types of anterior pituitary cells secrete
seven hormones
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ANTERIOR PITUITARY HORMONES
Hypothalamus Hormone
Hormone released from Adenohypoph
ysis
Major Function/ Target
Growth hormone releasing hormone (GHRH)
Human Growth Hormone (hGH)
Stimulates secretion of insulin-like growth factors (IGFs) that promote growth in many tissues
Thyrotropin releasing hormone (TRH)
Thyroid Stimulating Hormone (TSH)
Stimulates synthesis and secretion of thyroid hormones by the thyroid gland
Prolactin releasing hormone (PRH)
Prolactin (PRL)
Stimulates breast growth, and development of the mammary glands
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ANTERIOR PITUITARY HORMONES
Hypothalamus Hormone
Hormone released
from Adenohypop
hysis
Major Function/ Target
Gonadotropic releasing hormone (GnRH)
Follicle Stimulating hormone (FSH)
Ovaries initiate development of oocytes (immature egg cell); testes initiate development of spermatozoa
Gonadotropic releasing hormone (GnRH)
Luteinizing hormone (LH)
Ovaries stimulate ovulation (release of eggs from ovaries); testes stimulate testosterone production
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ANTERIOR PITUITARY HORMONES
Hypothalamus Hormone
Hormone released from Adenohypoph
ysis
Major Function/ Target
Corticotropin releasing hormone (CRH)
Adrenocorticotropic Hormone (ACTH)
Stimulates release of steroid hormones, including cortisol from the adrenal cortex
Corticotropin releasing hormone (CRH)
Melanocyte Stimulating hormone (MSH)
Stimulate the production and release of melanin by melanocytes in skin and hair. MSH signals to the brain have effects on appetite and sexual arousal
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THE NEUROHYPOPHYSIS The posterior pituitary (neurohypophysis)
releases, but does not synthesize any
hormones
When stimulated, neurosecretory
cells in the hypothalamus
release oxytocin and
ADH from their axon
terminals located in
the posterior pituitary
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THE NEUROHYPOPHYSIS Oxytocin targets smooth muscle in the uterus
and breasts. In the uterus, oxytocin stimulates
uterine contractions, and in response to the
sucking from an infant, oxytocin stimulates
“milk letdown” in the breasts
ADH targets the collecting ducts in the kidney
and sweat glands in the skin to minimize
water loss. It also directly causes arterioles to
constrict thereby increasing blood pressure
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THE THYROID GLAND The butterfly-shaped thyroid gland is
located inferior to the larynx and anterior to
the trachea. It has two laterally placed lobes
separated by a bridge-like isthmus
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The two hormones released are:
thyroxine or T4
(tetraiodothyronine)
and T3
(triiodothyronine) and need
iodine for their synthesis
THYROID HORMONES
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THYROID HORMONES Together with hGH and insulin, thyroid
hormones accelerate body growth, particularly
the growth of the nervous and skeletal systems
Thyroid hormones regulate oxygen use and
metabolic rate, cellular metabolism, and growth
and development. Secretion is controlled by
TRH from the hypothalamus and thyroid-
stimulating hormone from the anterior pituitary
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A goiter is an enlargement of the thyroid gland
and may be associated with hyperthyroidism,
hypothyroidism, or euthyroidism
In many third-word countries
dietary iodine intake is inadequate;
the resultant low level of thyroid
hormone in the blood stimulates
secretion of TSH, which causes
thyroid gland enlargement
THYROID HORMONES
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THE PARATHYROID GLANDS The parathyroid glands are small, round
masses of tissue attached to the posterior
surface of the lateral lobes of the thyroid
gland
There are usually two
parathyroid glands
attached to each
lobe of the thyroid,
one superior and one inferior
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PARATHYROID GLANDS The parathyroid glands are embedded on
the posterior surfaces of the thyroid.
Parathyroid hormone (PTH) regulates the
homeostasis of calcium, and PTH secretion is
controlled by the level of calcium in the
blood.
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THE ADRENAL GLANDS There are two adrenal glands, one superior
to each kidney (also called the suprarenal
glands). During embryonic development,
the adrenal glands differentiate into two
structurally and functionally distinct regions
• the adrenal cortex
• the adrenal medulla Catecholamines like epinephrine
Steroid hormones like cortisol
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THE ADRENAL GLANDS
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THE ADRENAL CORTEX The adrenal cortex is peripherally located
and makes up 80-90% of the total weight of
the gland
The cortex is subdivided into three zones,
each of which secretes a different group of
steroid hormones, all formed
from the cholesterol
molecule
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THE ADRENAL MEDULLA The inner region of the adrenal gland, the
adrenal medulla.
The catecholamines epinephrine (80%),
and norepinephrine (20%), are secreted
at the adrenal medulla and serve to
prolong the sympathetic response
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THE PANCREAS The pancreas is both an endocrine and an
exocrine gland. It is located posterior and
inferior to the stomach. We will discuss its
endocrine functions here and its exocrine
functions
in detail in chapter 24
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Most of the exocrine cells of the pancreas
are arranged in clusters called acini and
produce digestive enzymes which flow
through ducts into the GI tract
Distributed among the acini are clusters of
endocrine tissue
called pancreatic
islets (islets of
Langerhans)
THE PANCREAS
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Each pancreatic islet contains four types of
hormone-secreting cells: alpha (A), beta (B),
delta (D), and F cells
Alpha cells secrete glucagon which
increases blood glucose levels by acting
on the liver to
convert glycogen
to glucose
Beta cells secrete
insulin
PANCREATIC HORMONES
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PANCREATIC HORMONES Insulin is an anabolic hormone - it decreases
blood glucose levels by acting on
the liver to convert glucose
to glycogen and then facilitating
diffusion of glucose into the cells
Insulin and glucagon are counter-
regulatory hormones in that
their actions act to balance one
another in terms of blood glucose
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GONADAL HORMONES The ovaries are paired oval bodies located in
the female pelvic cavity. They produce several
steroid hormones including estrogens,
progesterone, relaxin, and inhibin
Estrogens, along with FSH and
LH from the anterior pituitary,
regulate the menstrual cycle,
maintain pregnancy, and prepare
the mammary glands for lactation
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GONADAL HORMONES Ovarian hormones also promote
enlargement of the breasts and widening of
the hips at puberty, and help maintain these
female secondary sex characteristics
Progesterone prepares the uterus lining for
implantation of a fertilized ovum
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GONADAL HORMONES The male gonads, the testes, are oval
glands that lie in the scrotum. The main
hormone produced and secreted by the
testes is testosterone, an androgen (male
sex hormone)
Testosterone is needed for
production of sperm and
maintenance of male
secondary sex characteristics
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THE PINEAL GLAND The pineal gland is a small
endocrine gland
attached to the roof of the
third ventricle – it is part of
the epithalamus
The pineal gland secretes the hormone
melatonin, which contributes to maintaining
the biological clock (seasonal and daily cycles)
• more melatonin is secreted in darkness; the
pineal gland is very developed in nocturnal
animals
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THE THYMUS GLAND The thymus gland secretes thymosin, which
promotes the proliferation and maturation of
T cells
T cells are a type of white blood cell
(lymphocyte) that destroys microorganisms
and foreign substances
through direct
cellular contact