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Chapter 37
Chemical Control of the
Animal Body: The
Endocrine System
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 37 At a Glance
 37.1 How Do Animal Cells Communicate?
 37.2 How Do Animals Hormones Work?
 37.3 What Are the Structures and Functions of
the Mammalian Endocrine System?
Biology: Life on Earth, 9e
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37.1 How Do Animal Cells Communicate?
 Individual cells of an animal’s body must
communicate with one another to ensure the
proper functioning of the whole
– Methods of communication between cells fall into
four broad categories:
–Direct
–Synaptic
–Paracrine
–Endocrine
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37.1 How Do Animal Cells Communicate?
 Methods of communication between cells fall
into four broad categories (continued)
– In direct communication, tissues such as heart
muscle have gap junctions that directly link the
insides of adjacent cells, allowing ions and
electrical signals to flow between them
–This type of communication is very fast, but
also has a very short range
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37.1 How Do Animal Cells Communicate?
 Methods of communication between cells fall into four
broad categories (continued)
– In the other three types of communication, “sending” cells
release messenger chemicals through their plasma
membranes
– The chemicals then move to “receiving” cells and alter
their physiology by binding to receptors, specialized
proteins on the surface or inside the receiving cells
– When the messenger binds to a receptor, the recipient
cell responds in a way that is determined by the
messenger, the receptor, and the type of cell
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37.1 How Do Animal Cells Communicate?
 Every cell has dozens of receptors, each capable to
binding a specific messenger and stimulating a
particular response
– Cells with receptors that bind a messenger molecule and
respond to it are target cells for that message
– Cells without the correct receptors cannot respond to
the messenger and are not target cells
– Therefore, a given cell can be a target cell for some
messenger molecules but not others, depending on
which receptors it has
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37.1 How Do Animal Cells Communicate?
 Synaptic, paracrine, and endocrine communication
differ in speed and distance
– Synaptic communication is used in the nervous system
– Electrical signals within individual nerve cells send
information to the farthest reaches of the body in a
fraction of a second
– Then, the nerve cell communicates with a small
number of other cells at junctions called synapses
– At a synapse, a nerve cell elicits responses from a
target cell by releasing chemicals, called
neurotransmitters, across a tiny space between the
nerve cell and its target
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37.1 How Do Animal Cells Communicate?
 Synaptic, paracrine, and endocrine communication
differ in speed and distance (continued)
– In paracrine communication, cells release chemicals that
diffuse through the extracellular fluid to other cells in the
immediate vicinity
– They influence only a small group of cells, but do so
quickly because the distances are very short
– Endocrine hormones, on the other hand, are released
into the bloodstream, move throughout the body in a few
seconds, and trigger responses that may last from a few
seconds to a lifetime
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37.1 How Do Animal Cells Communicate?
 Local hormones diffuse to nearby target cells
– Many cells engage in paracrine communication,
secreting local hormones into the extracellular fluid
– Local hormones include histamine, which is released
as part of the allergic and inflammatory responses,
and the cytokines by which cells of the immune
system communicate with one another
– Local hormones have only short range actions
because they are either degraded rapidly or taken up
by nearby cells and cannot get far from the cells that
secrete them
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37.1 How Do Animal Cells Communicate?
 Local hormones diffuse to nearby target cells
(continued)
– Prostaglandins are modified fatty acids that are
important local hormones secreted by cells throughout
the body
– They have diverse roles; for example, during
childbirth, they cause the cervix to dilate and help
stimulate the muscles of the uterus to contract
– Prostaglandins contribute to inflammation and pain
sensations
– Drugs such as aspirin, acetaminaphen, and ibuprofen
provide relief from these symptoms by blocking the
enzymes that synthesize prostaglandins
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37.1 How Do Animal Cells Communicate?
 Endocrine hormones are transported to target
cells throughout the body by the circulatory
system
– Endocrine hormones are messenger molecules
produced by the endocrine glands
– The secretory cells of an endocrine gland are
embedded within a network of capillaries, and
the cells secrete their hormones into the
extracellular fluid surrounding the capillaries
– The hormones diffuse into the capillaries and are
carried throughout the body by the bloodstream
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37.1 How Do Animal Cells Communicate?
 Endocrine hormones are transported to target cells
throughout the body by the circulatory system
(continued)
– The hormone oxytocin stimulates the contraction of
uterine muscles during childbirth because the muscle
cells have receptors that bind oxytocin
– Oxytocin, however, does not cause other muscles of the
body to contract because their cells do not have the
necessary receptors
– Uterine muscles, therefore, contain target cells for
oxytocin, whereas a woman’s biceps do not
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Hormone Release, Distribution, and Reception
1 Endocrine cells
release hormone
2 The hormone enters the
blood and is carried
throughout the body
(extracellular
fluid)
capillary
3 The hormone leaves
the capillaries and diffuses
to all tissues through
the extracellular fluid
biceps
uterus
4 The hormone affects
cells bearing receptors
to which the hormone
can bind
5 The hormone cannot affect
cells that only bear receptors to
which the hormone cannot bind
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Fig. 37-1
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37.1 How Do Animal Cells Communicate?
 The changes induced by hormones may be
prolonged and irreversible, as in the onset of
puberty or the transformation of a caterpillar into
a butterfly
 More typically, the changes are temporary and
reversible, and help to regulate the physiological
systems of the animal body within a course of
time of seconds to hours
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37.2 How Do Animal Hormones Work?
 Vertebrate endocrine hormones are often evolutionarily
ancient
– Insulin is found not only in vertebrates, but also in
protists, fungi, and bacteria, although the function of
insulin in most of these organisms is not known
– Thyroid hormones have been found in invertebrates such
as worms, insects, and mollusks, which do not have
thyroid glands
– Hormones appear to work similarly in the cells of
invertebrates as they do in humans and other vertebrates
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37.2 How Do Animal Hormones Work?
 There are three classes of vertebrate endocrine
hormones:
– Peptide hormones, which are chains of amino
acids
– Amino acid–derived hormones, which are
composed of one or two modified amino acids
– Steroid hormones, which are synthesized from
cholesterol
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Table 37-2
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37.2 How Do Animal Hormones Work?
 Hormones act by binding to receptors on or in
target cells
– Receptors for hormones are found in two general
locations on target cells:
–On the plasma membrane
–Inside the cell, within the cytoplasm or the
nucleus
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37.2 How Do Animal Hormones Work?
 Peptide and amino acid hormones usually bind to
receptors on the surfaces of target cells
– These hormones cannot diffuse through the phospholipid
bilayer of the plasma membrane and must bind to
receptors on the surface of the target cell’s plasma
membrane
– Hormone–receptor binding activates an enzyme that
synthesizes a molecule, called a second messenger,
inside the cell
– An example is cyclic adenosine monophosphate
(cyclic AMP), which regulates many cellular activities
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37.2 How Do Animal Hormones Work?
 Second messengers
– The second messenger transfers the signal from the first
messenger—the hormone—to other molecules within the
cell, often activating specific intracellular enzymes
– These activated enzymes then initiate a chain of
biochemical reactions that vary depending on the
hormone, the second messenger, and the target cell
– Epinephrine stimulates the synthesis of cyclic AMP in
both heart muscle and liver cells, but the result is
different in the two cell types
– Cyclic AMP causes heart muscle cells to contract
more strongly, while in liver cells, it activates
enzymes that breakdown glycogen to glucose
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Author Animation: The Action of Nonsteroid
Hormones
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Actions of Peptide and Amino Acid-Derived
Hormones on Target Cells
peptide or amino
acid-derived
hormone
(first messenger)
1 The hormone binds to
a receptor on the plasma
membrane of a target cell
2 Hormone–receptor binding
activates an enzyme that catalyzes
the synthesis of a second messenger,
such as cyclic AMP
cyclic AMPsynthesizing
enzyme
(extracellular
fluid)
receptor
(cytoplasm)
ATP
active
enzyme
product
cyclic AMP
(second messenger)
4 The activated enzymes
catalyze specific reactions
plasma membrane
inactive
enzyme
reactant
3 The second
messenger activates
other enzymes
nuclear
envelope
(nucleus)
Fig. 37-2
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37.2 How Do Animal Hormones Work?
 Steroid hormones usually bind to receptors inside
target cells
– Steroid hormones are lipid soluble and diffuse through
the plasma membrane of target cells
– These hormones bind to receptors inside target cells,
which are either in the nucleus or move into the
nucleus after hormone binding
– The hormone–receptor complex then binds to the
DNA of the promoter region of specific genes and
stimulates transcription of messenger RNA
– The mRNA travels to the cytoplasm and directs
protein synthesis
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Author Animation: The Action of Steroid Hormones
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Steroid Hormone Action on Target Cells
steroid hormone
(extracellular
fluid)
2 The hormone binds to a
receptor in the nucleus or to
a receptor in the cytoplasm
that carries it into the nucleus
3 The hormone–receptor
complex binds to DNA and
causes RNA polymerase to
bind to a nearby promoter
site for a specific gene
1 A steroid hormone
diffuses through the
plasma membrane
DNA
plasma
membrane
hormone receptor
ribosome
RNA polymerase
5 The mRNA leaves the
nucleus, then attaches to a
ribosome and directs the
synthesis of a specific protein
product
mRNA
4 RNA polymerase catalyzes
the transcription of DNA into
messenger RNA (mRNA)
gene
new protein
nuclear
envelope
(cytoplasm)
(nucleus)
Fig. 37-3
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37.2 How Do Animal Hormones Work?
 Although it is not a steroid, thyroid hormone also
acts intracellularly
– Thyroid hormone is actively transported into
many cell types
– Once inside the cell, thyroid hormone binds to
intracellular receptors and activates transcription
of specific genes
– Hormones that bind to intracellular receptors
may take several minutes or even days to exert
their full effects
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37.2 How Do Animal Hormones Work?
 Hormone release is regulated by feedback mechanisms
– Negative feedback is a response to a change that tends
to counteract a change and restore the system to its
original condition
– As an example, after jogging on a hot, sunny day, you
have lost a quart of water through perspiration
– Your pituitary releases antidiuretic hormone (ADH),
which causes increased water reabsorption by your
kidneys, concentrating your urine
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37.2 How Do Animal Hormones Work?
 Negative feedback (continued)
– If you then drink two quarts of water, your would
have excess blood volume
–Negative feedback would act to restore the
original condition by turning off ADH secretion,
and your kidneys would eliminate the excess
water
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37.2 How Do Animal Hormones Work?
 In a few cases, hormone release is temporarily
controlled by positive feedback
– In this case, the response to a change enhances the
change
– For example, contractions of the uterus early in
childbirth push the baby’s head against the cervix,
which causes the cervix to stretch
– Stretching the cervix sends nervous signals to the
mother’s brain, which in turn causes the release of
oxytocin
– Oxytocin stimulates continued contractions of the
uterus, pushing the baby harder against the cervix
until delivery is complete
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37.2 How Do Animal Hormones Work?
 Vertebrate and invertebrate endocrine
hormones often have similar mechanisms of
action
– Insects must molt occasionally in order to grow;
molting is controlled by the steroid hormone
ecdysone, or molting hormone
–Ecdysone acts on receptors located within the
nucleus and affects gene transcription,
initiating a complex process in which the
epithelial cells detach from the old cuticle and
secrete a soft new cuticle beneath it
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37.2 How Do Animal Hormones Work?
 Molting (continued)
– The insect then expands its body by pumping
itself full of air
– This splits open the old cuticle and stretches out
the new one to accommodate some future
growth
– As the insect emerges, it leaves an insectshaped cuticle behind
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Insect Molting
Emerging
cicada
Old cuticle
Fig. 37-4
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The mammalian endocrine system consists of
the endocrine hormones and the glands that
produce them
 The major endocrine glands and organs are:
– The hypothalamus–pituitary complex
– The thyroid gland
– The pancreas
– The sex organs
– The adrenal glands
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The Major Mammalian Endocrine Glands and
Their Hormones
Hypothalamus
ADH, oxytocin, and regulatory
hormones for the anterior pituitary
Pineal gland
melatonin
Pituitary gland
anterior pituitary:
ACTH, TSH, GH, PRL, FSH, LH
posterior pituitary:
oxytocin and ADH
Parathyroid glands (on the
posterior surface of the thyroid
gland) parathyroid hormone
Heart
atrial natriuretic peptide
Thyroid gland
thyroxine, calcitonin
Kidneys
erythropoietin
Thymus gland
thymosins
Digestive tract
several hormones (see
Chapter 34)
Adrenal glands (one on
each kidney)
medulla:
epinephrine, norepinephrine
cortex:
glucocorticoids (cortisol),
mineralocorticoids
(aldosterone), testosterone
Pancreas islet cells
insulin, glucagon
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Fat
leptin
Gonads
testes (male):
androgens, especially
testosterone
ovaries (female):
estrogens, progesterone
testis
ovary
Fig. 37-5
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Hormones of the hypothalamus and pituitary
gland regulate many functions throughout the
body
– The hypothalamus and pituitary gland coordinate
the action of many key hormonal systems
–The hypothalamus is a part of the brain that
contains clusters of specialized nerve cells
called neurosecretory cells
–Neurosecretory cells synthesize peptide
hormones, store them, and release them
when stimulated
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Table 37-3, 1 of 3
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Table 37-3, 2 of 3
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Table 37-3, 3 of 3
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The pituitary gland is pea-sized gland connected to
the hypothalamus by a stalk
– The pituitary consists of two distinct parts:
– The anterior pituitary, which is a true endocrine
gland, composed of several types of hormonesecreting cells enmeshed in a network of capillaries
– The posterior pituitary, consisting mainly of a
capillary bed and the endings of neurosecretory cells
whose cell bodies are in the hypothalamus
– The hypothalamus controls the release of hormones from
both parts of the pituitary
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Hypothalamic hormones control hormone
release in the anterior pituitary
– Neurosecretory cells of the hypothalamus
produce at least seven hormones that regulate
the release of hormones from the anterior
pituitary
– These hypothalamic hormones are called
releasing hormones or inhibiting hormones,
depending on whether they stimulate or inhibit
the release of a particular pituitary hormone
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Hypothalamic hormones control hormone release in the
anterior pituitary (continued)
– Releasing and inhibiting hormones are secreted into a
capillary bed in the stalk connecting the hypothalamus to
the pituitary, and travel through blood vessels to a
second capillary bed that surrounds the endocrine cells
of the anterior pituitary
– There, they diffuse out of the capillaries and bind to
receptors on the surfaces of the pituitary endocrine
cells
– Some of these hypothalamic hormones, such as growth
hormone-releasing hormone, stimulate the release of
pituitary hormones, while others inhibit the release of
pituitary hormones
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The anterior pituitary produces and releases several
hormones
– Four of these regulate hormone production in other
endocrine glands:
– Follicle-stimulating hormone (FSH) and luteinizing
hormone (LH) stimulate the production of sperm and
testosterone in males and the production of eggs,
estrogen, and progesterone in females
– Thyroid-stimulating hormone (TSH) stimulates the
thyroid gland to release its hormones
– Adrenocorticotropic hormone (ACTH) causes the
release of the hormone cortisol from the adrenal
cortex
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The remaining hormones of the anterior pituitary do not
act on other endocrine glands
– Prolactin, in conjunction with other hormones, stimulates
the development of milk-producing mammary glands in
the breasts during pregnancy
– Growth hormone acts on nearly all the body’s cells by
increasing protein synthesis, promoting the use of fats for
energy, and regulating carbohydrate metabolism
– During childhood, growth hormone stimulates bone
growth, which influences human height; too little
growth hormone results in dwarfism, and too much
results in gigantism
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When the Anterior Pituitary Malfunctions
Fig. 37-7
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The posterior pituitary releases hormones
synthesized by cells in the hypothalamus
– The hypothalamus contains two types of
neurosecretory cells that send axons into the
posterior pituitary
–These axons end in a capillary bed into which
they release hormones that are then carried
by the bloodstream to the rest of the body
– These neurosecretory cells synthesize and
release either antidiuretic hormone (ADH) or
oxytocin
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The posterior pituitary releases hormones
synthesized by cells in the hypothalamus
(continued)
– Antidiuretic hormone (ADH) helps prevent
dehydration by causing more water to be
reabsorbed from the urine and returned to the
bloodstream by the kidneys
–Alcohol inhibits the release of ADH and
increases urination, resulting in the loss of
more water than is consumed with dehydration
resulting
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Author Animation: Hypothalamic Control of the
Pituitary
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The Hypothalamus–Pituitary System
hypothalamus
1 Neurosecretory cells of
the hypothalamus produce
oxytocin and ADH
1 Neurosecretory cells
of the hypothalamus
produce releasing and
inhibiting hormones
2 Releasing or inhibiting hormones
(green circles) are secreted into
capillaries feeding the anterior lobe
of the pituitary
2 Oxytocin and ADH
(blue triangles) are
secreted into the blood
via capillaries in the
posterior pituitary
blood flow
pituitary
(anterior lobe)
endocrine
cell
pituitary
(posterior lobe)
capillary
bed
3 Endocrine cells of the
anterior pituitary secrete
hormones (red squares)
in response to releasing
hormones; the pituitary
hormones enter the
bloodstream
capillary
bed
blood
flow
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Fig. 37-6
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The posterior pituitary releases hormones
synthesized by cells in the hypothalamus
(continued)
– Oxytocin causes contractions of uterine muscles
during childbirth and triggers “milk letdown” in
nursing mothers by causing muscle tissue within
the mammary glands of breasts to contract in
response to stimulation by the suckling infant
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Hormones and Breast-Feeding
hypothalamus
2 Neurosecretory cells
of the hypothalamus
release oxytocin from
endings in the posterior
pituitary
posterior
pituitary
3 Oxytocin is
carried in the blood
to the breast
1 Suckling stimulates
sensory receptors in
the breast, which then
send nerve impulses to
the hypothalamus
4 Oxytocin binds to
receptors on the milk
gland muscles, causing
them to contract and
squeeze milk out of the
glands
milk
gland
milk
milkproducing
cells
duct
nipple
clusters of
milk glands
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muscle
cells
Fig. 37-8
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The posterior pituitary releases hormones synthesized
by cells in the hypothalamus (continued)
– Oxytocin also acts directly on the brain, causing
behavioral effects
– In rats, injecting oxytocin into the brain causes virgin
females to exhibit maternal behaviors, such as nest
building, licking other rats’ pups, and retrieving pups
that have strayed
– In humans, oxytocin may play a role in emotions,
including trust and both romantic and maternal love
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The thyroid and parathyroid glands influence
metabolism and calcium levels
– Lying in front of the neck, just below the larynx, is
the thyroid gland, which produces two
hormones: thyroxine and calcitonin
– The parathyroid gland consists of two pairs of
small disks of endocrine cells on each side of the
thyroid, and releases parathyroid hormone
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The Thyroid and Parathyroid Glands
larynx
thyroid gland
esophagus
parathyroid
glands
trachea
Fig. 37-9
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Thyroxine influences energy metabolism
– Thyroxine, or thyroid hormone, is an iodine-containing
amino acid derivative that works by binding to
intracellular receptors that regulate gene activity
– By stimulating glucose breakdown and providing the
resulting energy from it, thyroid hormone elevates the
metabolic rate of many body cells
– In juvenile animals, including humans, thyroxine helps
regulate growth by stimulating both metabolic rate and
nervous system development
– Undersecretion of thyroid hormone leads to cretinism,
a condition characterized by retardation
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Thyroxine influences energy metabolism (continued)
– An idodine-deficient diet can reduce the production of
thyroxine and trigger a feedback that attempts to restore
normal hormone levels by increasing the number of
thyroxine-producing cells
– The thyroid gland becomes enlarged, forming a
condition called goiter
– Iodine deficiency in pregnant women and young children
is the leading preventable cause of mental retardation
– Iodized salt is a simple, and cheap, solution to iodine
deficiency
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Goiter
Fig. 37-10
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Thyroxine release is controlled by the hypothalamus
and anterior pituitary
– Thyroid stimulating hormone-releasing hormone (TSHreleasing hormone) produced by neurosecretory cells in
the hypothalamus travels to the anterior pituitary and
causes the release of TSH
– TSH travels in the blood to the thyroid and stimulates the
release of thyroxine
– Secretion of TSH-releasing hormone and TSH are
regulated by negative feedback, whereby adequate
levels of thyroxine inhibit the secretion of both TSHreleasing hormone from the hypothalamus and TSH from
the anterior pituitary
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Negative Feedback in Thyroid Gland Function
1 Neurosecretory
cells of the hypothalamus
secrete TSH-releasing
hormone
4 Thyroxine inhibits
TSH-releasing hormone
and TSH release by
negative feedback
releasing
hormone
2 The releasing hormone causes
the anterior pituitary to secrete
thyroid-stimulating hormone (TSH)
TSH
endocrine
cells of the
anterior
pituitary
thyroxine
hormoneproducing
cells of the
thyroid
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thyroid
gland
3 TSH causes the thyroid to
secrete thyroxine, which
increases cellular metabolism
throughout the body
Fig. 37-11
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Thyroxine has varied effects in different
vertebrates
– In amphibians, thyroxine has the dramatic effect
of triggering metamorphosis
–In 1912, tadpoles were fed minced horse
thyroid and metamorphosed prematurely into
miniature adult frogs
– Thyroxine also regulates the seasonal molting of
most vertebrates from snakes to birds to the
family dog, where surges of thyroxine stimulate
the shedding of skin, feathers, and hair
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Parathyroid hormone and calcitonin regulate calcium
metabolism
– The proper concentration of calcium is essential to nerve
and muscle function, and the parathyroid hormone from
the parathyroid gland and calcitonin from the thyroid
gland work together to maintain nearly constant calcium
levels in the blood
– If blood calcium levels drop, parathyroid hormone
causes the bones to release calcium and the kidneys
to reabsorb more calcium from urine
– If blood calcium gets too high, calcitonin inhibits the
release of calcium from bone
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The pancreas has both digestive and endocrine
functions
– The pancreas produces bicarbonate and several
enzymes that are released into the small
intestine, promoting the digestion of food
– The endocrine portion of the pancreas consists
of clusters of islet cells that produce one of two
peptide hormones: insulin and glucagon
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Insulin and glucagon control glucose levels in
the blood
– Insulin and glucagon work in opposition to
regulate carbohydrate and fat metabolism
–Insulin reduces the blood glucose level
–Glucagon increases it
– When blood glucose rises after a meal, the
pancreas releases insulin, which causes body
cells to take up glucose and metabolize it, or
convert it to fat or glycogen
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Insulin and glucagon control glucose levels in the blood
(continued)
– When blood glucose drops—for example, after skipping
breakfast—insulin secretion is inhibited and glucagon
secretion is stimulated
– Glucagon activates an enzyme in the liver that breaks
down glycogen, releasing glucose into the blood
– Glucagon also promotes fat breakdown, which releases
fatty acids that can be metabolized for energy
– These actions increase blood glucose levels, which
inhibits glucagon secretion
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The Pancreas Controls Blood Glucose Levels
8 Blood
glucose is
increased
7 Glucagon stimulates
cells to burn fat instead of
glucose; the liver converts
glycogen to glucose
1 Eating
raises blood
glucose
high blood
glucose
2 High blood
glucose stimulates
insulin release and
inhibits glucagon
release
glucagon
3 Insulin stimulates
glucose uptake by
body cells; the liver
converts glucose to
glycogen
6 Low blood glucose
stimulates glucagon
release and inhibits
insulin release
5 Exercise
and fasting also
reduce blood
glucose
pancreas
insulin
liver
low blood
glucose
muscle
4 Glucose uptake into
cells and conversion of
glucose to glycogen
reduce blood glucose
Fig. 37-12
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Diabetes results from a malfunctioning insulin control
system
– Lack of insulin production or the failure of target cells to
respond to insulin results in diabetes mellitus
– In either case, blood glucose levels are high because
cells cannot take up glucose unless they are
stimulated by insulin, and they rely on fats as an
energy source, which leads to high levels of blood
lipids
– Many diabetics suffer from heart and blood vessel
disease caused by fat deposition
– Insulin replacement therapy improves the health of
diabetics
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The sex organs produce both gametes and sex
hormones
– Besides producing sperm or eggs, the testes in
males and ovaries in females are also important
endocrine organs
–The testes secrete several steroid hormones,
collectively called androgens, the most
important being testosterone
–The ovaries secrete two types of steroid
hormones: estrogen and progesterone
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Sex hormone levels increase during puberty
– Puberty is the phase of life during which the reproductive
systems of both sexes become mature and functional
– It begins when the hypothalamus starts to secrete
increasing amounts of releasing hormones, which in turn
stimulate the anterior pituitary to secrete more luteinizing
hormone (LH) and follicle-stimulating hormone (FSH)
– LH and FSH stimulate target cells in the testes and
ovaries to produce higher levels of sex hormones
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Sex hormone levels increase during puberty
(continued)
– Testosterone, secreted by the testes of males, promotes
sperm production and stimulates the development of
male secondary sexual characteristics, including body
and facial hair, broad shoulders, and muscle growth
– Estrogen from the ovaries of females stimulates breast
development and the maturation of the female
reproductive system, including egg production
– Progesterone prepares the reproductive tract to receive
and nourish the fertilized egg
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The adrenal glands secrete hormones that
regulate metabolism and responses to stress
– The adrenal glands consist of two very different
parts:
–The adrenal cortex
–The adrenal medulla
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The Adrenal Glands
The adrenal medulla secretes
epinephrine and norepinephrine
The adrenal cortex
secretes glucocorticoids,
mineralocorticoids,
and testosterone
kidney
Fig. 37-13
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The adrenal cortex produces steroid hormones
– The outer layer of the adrenal gland forms the
adrenal cortex, which secretes three types of
steroid hormones:
–Glucocorticoids, which help control glucose
metabolism
–Mineralocorticoids, which regulate salt
metabolism
–Small amounts of testosterone
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Glucocorticoid release is stimulated by
adrenocorticotropic hormone (ACTH) from the anterior
pituitary, which in turn is stimulated by releasing
hormones from the hypothalamus
– Glucocorticoids are released in response to stimuli such
as stress, trauma, or exposure to temperature extremes
– Cortisol is by far the most abundant glucocorticoid
– It increases blood glucose levels by stimulating
glucose production, inhibiting the uptake of glucose by
muscle cells, and promoting the use of fat for energy
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Many different hormones are involved in
glucose metabolism: thyroxine, insulin,
glucagon, epinephrine, and the glucocorticoids
– The reason for so many different hormones in
the regulation of glucose appears to be traced to
the metabolic requirements of the brain
–Most body cells can produce energy from fats
and proteins as well as from carbohydrates
–Brain cells can only metabolize glucose, so
glucose levels in the blood cannot be allowed
to fall too far or brain cells rapidly die
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Mineralocorticoid hormones regulate the mineral (salt)
content of the blood
– The most important mineralocorticoid is aldosterone,
which helps to control sodium concentrations
– A constant blood sodium concentration is crucial to
many cellular events, including the production of
electrical signals by nerve cells
– If blood sodium falls, the adrenal cortex releases
aldosterone, which causes the kidneys and sweat
glands to retain sodium
– When blood sodium returns to normal, aldosterone
secretion is turned off
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 In both women and men, the adrenal cortex also
produces the male sex hormone testosterone,
although in much smaller quantities than are
produced by the testes
– Tumors of the adrenal cortex can lead to
excessive testosterone release, causing
masculinization of women
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The adrenal medulla produces amino acidderived hormones
– The adrenal medulla is located in the center of
each adrenal gland and produces two hormones
in response to stress or exercise: epinephrine
and norepinephrine
–These hormones prepare the body for
emergency action by increasing the heart and
respiratory rates, increasing blood pressure,
causing blood glucose levels to rise, and
directing blood flow away from the digestive
tract and toward the brain and muscles
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Hormones are also produced by the pineal gland,
thymus, kidneys, heart, digestive tract, and fat cells
– The pineal gland is located between the two
hemispheres of the brain and produces the hormone
melatonin
– This hormone is secreted in a daily rhythm, which in
mammals is regulated by light entering the eyes
– The pineal appears to regulate the seasonal
reproductive cycles of many animals
– Despite years of research, the function of melatonin
and the pineal gland in humans is still not known
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The thymus is located in the chest cavity behind the
breastbone and produces the hormone thymosin,
which stimulates the development of specialized white
blood cells (T cells) that play crucial roles in the
immune response
– The thymus is large in infants, but under the influence of
sex hormones, decreases in size after puberty
– As a result, the elderly produce fewer new T cells than
adolescents do and are more susceptible to new
diseases
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The kidneys produce erythropoietin, a peptide
hormone that is released when the oxygen content of
the blood is low
– Erythropoietin stimulates the bone marrow to increase
red blood cell production
– The kidneys also produce an enzyme called renin in
response to low blood pressure, which catalyzes the
production of the hormone angiotensin from proteins
in the blood
– Angiotensin raises blood pressure by constricting
arterioles and stimulating the release of aldosterone
by the adrenal cortex, which leads to increased
sodium and water reabsorption by the kidney and
increased blood volume and pressure
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 The stomach and small intestine produce a number of
peptide hormones that help regulate digestion
– These hormones include gastrin, ghrelin, secretin, and
cholecytokinin
 The heart releases the hormone atrial natriuretic
peptide (ANP), which inhibits the release of ADH and
aldosterone and increases the excretion of sodium
– These actions of ANP lead to a drop in blood volume by
reducing reabsorption of water and salt by the kidneys
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37.3 What Are the Structures and Functions of the
Mammalian Endocrine System?
 Fat can act as an endocrine organ
– In 1995, Jeffrey Friedman at Rockefeller University
discovered the peptide hormone leptin, which is
released by fat cells
– Mice genetically engineered to lack the gene for leptin
became obese, and leptin injections caused them to
lose weight
– The researchers hypothesized that by releasing leptin,
fat tissue “tells” the body how much fat it has stored
and therefore how much to eat
– Leptin has other roles that include stimulating the
growth of capillaries and speeding wound healing
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