Chapter 20: Endocrine
System
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Endocrine Glands
The endocrine system is made of glands
and tissues that secrete hormones.
Hormones are chemicals that influence
metabolism of cells, the growth and
development of body parts, and
homeostasis.
Hormones can be classified as peptides
or steroids.
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There is a close association between the
endocrine and nervous systems.
Hormone secretion is usually controlled
by either negative feedback or
antagonistic hormones that oppose
each other’s actions, and results in
maintenance of a bodily substance or
function within normal limits.
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The endocrine system
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Hypothalamus and Pituitary
Gland
The hypothalamus regulates the internal
environment through the autonomic
system and also controls the
secretions of the pituitary gland.
The pituitary has two portions: the
anterior pituitary and the posterior
pituitary.
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Posterior Pituitary
The posterior pituitary stores and releases
the antidiuretic hormone (ADH) and
oxytocin produced by the hypothalamus.
ADH is secreted during dehydration and
causes more water to be reabsorbed by
the kidneys; the secretion of ADH is
regulated by negative feedback.
Oxytocin causes uterine contractions and
milk release, and is controlled by positive
feedback.
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Anterior Pituitary
The hypothalamus controls the anterior
pituitary by producing hypothalamicreleasing hormones and hypothalamicinhibiting hormones.
The anterior pituitary produces six
hormones.
Three of these six hormones have an effect
on other endocrine glands:
1) Thyroid-stimulating hormone (TSH)
stimulates the thyroid to produce thyroid
hormones;
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2) adrenocorticotropic hormone (ACTH)
stimulates the adrenal cortex to
produce cortisol;
3) the gonadotropic hormones (FSH and
LH) stimulate the gonads to produce
sex cells and hormones.
In these three instances, the blood level of
the last hormone exerts negative
feedback control over the secretion of
the first two hormones.
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The next three anterior pituitary
hormones do not effect other endocrine
glands.
After childbirth, prolactin (PRL) causes
mammary glands to produce milk.
Growth hormone (GH) promotes skeletal
and muscular growth.
Melanocyte-stimulating hormone (MSH)
causes skin color changes in fishes,
amphibians, and reptiles.
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Hypothalamus and the pituitary
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Effects of Growth Hormone
The quantity of GH is greatest during
childhood and adolescence; GH
promotes bone and muscle growth.
Pituitary dwarfism results from too little
GH during childhood.
Giants result from too much growth
hormone during childhood.
If growth hormone is overproduced in an
adult, it causes acromegaly.
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Effect of growth hormone
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Acromegaly
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Thyroid and Parathyroid Glands
The thyroid gland is a large gland located
in the neck, where it is attached to the
trachea just below the larynx.
The four parathyroid glands are
embedded in the posterior surface of
the thyroid gland.
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Thyroid Gland
The thyroid gland requires iodine to
produce thyroxine (T4) which contains
four iodine atoms, and triiodothyronine
(T3) which contains three iodine atoms.
Thyroid hormones increase the
metabolic rate, and stimulate all body
cells to metabolize and use energy at a
faster rate.
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Effects of Thyroid Hormones
If iodine is lacking in the diet, a simple
goiter develops.
Use of iodized salt helps prevent simple
goiters.
Hypothyroidism in childhood produces
cretinism; in adulthood it causes
myxedema.
If the thyroid is overactive (Grave’s
disease) an exophthalmic goiter
develops.
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Simple goiter
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Cretinism
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Calcitonin
The thyroid gland also produces
calcitonin, which helps lower the blood
calcium level when it is too high.
The primary effect of calcitonin is to
bring about the deposit of calcium in
the bones; it does this by temporarily
reducing the activity and number of
osteoclasts.
When the blood level of calcium is
returned to normal, the release of
calcitonin is inhibited.
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Parathyroid Glands
Parathyroid glands secrete parathyroid
hormone (PTH), which raises the blood
calcium when it is insufficient, and
decreases the blood phosphate level.
PTH acts by stimulating the activity of
osteoclasts, thus releasing calcium from
bone, and stimulates the reabsorption of
calcium by the kidneys and intestine.
Insufficient parathyroid hormone will
cause serious loss of blood calcium and
cause tetany.
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Regulation of blood calcium
level
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Adrenal Glands
Adrenal glands sit atop the kidneys and
have an inner adrenal medulla and an
outer adrenal cortex.
The hypothalamus uses ACTH-releasing
hormone to control the anterior
pituitary’s secretion of ACTH that
stimulates the adrenal cortex.
The hypothalamus regulates the medulla
by direct nerve impulses.
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The adrenal medulla secretes
epinephrine and norepinephrine, which
bring about responses we associate
with emergency situations.
On a long-term basis, the adrenal cortex
produces glucocorticoids similar to
cortisone and mineralocorticoids to
regulate salt and water balance.
The adrenal cortex also secretes both
male and female sex hormones in both
sexes.
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Adrenal glands
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Glucocorticoids
Cortisol promotes breakdown of muscle
proteins to amino acids; the liver then
breaks the amino acids into glucose.
Cortisol also promotes metabolism of
fatty acids rather than carbohydrates,
which spares glucose.
Both actions raise the blood glucose
level.
High levels of blood glucocorticoids can
suppress immune system function.
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Mineralocorticoids
Aldosterone causes the kidneys to
reabsorb sodium ions (Na+) and excrete
potassium ions (K+).
When blood sodium levels and blood
pressure are low, the kidneys secrete
renin; the effect of the renin-angiotensinaldosterone system is to raise blood
pressure.
The hormone atrial natriuretic hormone
produced by the heart inhibits the
secretion of aldosterone, thus reducing
blood pressure.
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Regulation of blood pressure
and volume
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Malfunction of the Adrenal
Cortex
Addison disease develops when the
adrenal cortex hyposecetes hormones.
A bronzing of the skin follows low levels
of cortisol, and mild infection can lead to
death; aldosterone is also hyposecreted,
and dehydration can result.
Cushing syndrome develops when the
adrenal cortex hypersecretes cortisol.
The trunk and face become round; too
much aldosterone results in fluid
retention.
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Addison disease
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Cushing syndrome
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Pancreas
The pancreas is between the kidneys and
the duodenum and provides digestive
juices and endocrine functions.
Pancreatic islets secrete insulin, which
lowers the blood glucose level, and
glucagon, which has the opposite
effect, together regulating the glucose
level in the blood.
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Regulation of blood glucose
level
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Diabetes Mellitus
The most common illness due to
hormonal imbalance is diabetes
mellitus.
Diabetes is due to the failure of the
pancreas to produce insulin or the
inability of the body cells to take it up.
Hyperglycemia symptoms develop, and
glucose appears in the urine.
Diabetes is diagnosed using a glucose
tolerance test.
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Glucose tolerance test
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Type I diabetes mellitus occurs when the
pancreas does not produce insulin and
the patient requires insulin injections.
Most people with diabetes have Type II
diabetes mellitus where the pancreas
produces insulin but the body cells do
not respond.
Both types lead to long-term serious
complications.
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Other Endocrine Glands
Testes and Ovaries
Testes, located in the scrotum, produce
the male hormone testosterone.
Ovaries in the female produce estrogens
and progesterone.
Secretions from the gonads are controlled
by the anterior pituitary hormones.
These sex hormones maintain the sex
organs and secondary sex
characteristics.
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The effects of anabolic steroid
use
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Thymus Gland
The thymus under the sternum produces
thymosins that stimulate T lymphocyte
production and maturation.
The thymus decreases in size with age
and becomes fatty.
There is hope that thymosins can be
injected into AIDS or cancer patients
where they would enhance T
lymphocyte function.
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Pineal Gland
The pineal gland in the brain produces
melatonin which is involved in
circadian rhythms and the timing of
development of the reproductive
organs.
Children whose pineal gland has been
destroyed due to a brain tumor
experience early puberty.
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Hormones from Other Tissues
Leptin
Adipose tissue (fat) produces leptin that
acts on the hypothalamus where it
signals satiety—that the individual
feels “full” and has had enough to eat.
It is possible that obese people have
ineffective leptin due to a genetic
mutation or that their cells lack
adequate leptin receptors.
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Growth Factors
A number of different types of organs and
cells produce peptide growth factors,
which stimulate cell division and growth:
granulocyte and macrophage colonystimulating factor to fight infection,
platelet-derived growth factor for wound
healing,
epidermal growth factor and nerve growth
factor, both for wound healing, and
tumor angiogenesis factor that causes
blood vessels to grow near tumor cells.
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Prostaglandins
Prostaglandins are produced within cells
from arachidonate, a fatty acid.
Prostaglandins act close to where they
are produced.
They cause uterine muscle contraction
and are involved in the pain of
menstrual cramps; aspirin is effective
against the pain by countering
prostaglandins.
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Chemical Signals
A chemical signal is any substance that
affects cell metabolism or behavior of
the individual.
Chemical signals can be used between
body parts, between cells, and between
individual organisms (pheromones).
Underarm secretions may be slightly
attractive and may be involved in
synchronizing the menstrual cycles of
women who live together.
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Chemical signals
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The Action of Hormones
Steroid hormones enter the nucleus and
combine with a receptor protein, and
the hormone-receptor complex
attaches to DNA and activates certain
genes.
Transcription and translation lead to
protein synthesis.
Peptide hormones are usually received
by a hormone receptor protein located
in the plasma membrane.
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Action of a steroid hormone
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Most often the reception of a peptide
hormone leads to activation of an
enzyme that changes ATP to cyclic
AMP (cAMP).
cAMP, as a second messenger, then
activates an enzyme cascade.
Calcium is also a common second
messenger.
Hormones work in small quantities
because their effect is amplified by
enzymes.
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Action of a peptide hormone
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Chapter Summary
Hormones are chemical signals that
affect the activity of target glands or
tissues.
Endocrine glands are ductless and
distribute hormones by the
bloodstream.
The hypothalamus is a part of the brain
that controls the functioning of the
pituitary gland.
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The anterior pituitary produces several
hormones, some of which control other
endocrine glands.
Growth hormone is produced by the
anterior pituitary; giants are due to
overproduction of growth hormone
during childhood, and pituitary dwarfs
are due to underproduction of growth
hormone.
The thyroid produces two hormones that
speed metabolism and another hormone
that lowers the blood calcium level.
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The distinct parathyroid glands produce
a hormone that raises blood calcium
level.
Adrenal glands produce hormones that
help us respond to stress.
Malfunction of the adrenal cortex leads to
the symptoms of Addison disease and
Cushing disease.
The pancreas secretes hormones that
regulate the blood glucose level.
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Diabetes mellitus occurs when cells are
unable to take up glucose and it spills
over into the urine.
The gonads produce sex hormones that
control secondary sex characteristics.
Many other tissues, although not
traditionally considered endocrine
glands, secrete hormones.
Hormones influence the metabolism of
their target cells.
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