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Anatomy & Physiology
Chapter 18: Endocrine System
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Organization of the
Endocrine System
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The endocrine and nervous systems function to
achieve and maintain homeostasis (Table 18-1)
When the two systems work together, referred
to as the neuroendocrine system, they perform
the same general functions: communication,
integration, and control
In the endocrine system, secreting cells send
hormone molecules by way of the blood to
specific target cells contained in target tissues or
target organs
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Organization of the
Endocrine System
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Hormones—carried to almost every point in the body;
can regulate most cells; effects work more slowly and
last longer than those of neurotransmitters
Endocrine glands are “ductless glands”; many are
made of glandular epithelium whose cells
manufacture and secrete hormones; a few endocrine
glands are made of neurosecretory tissue
Glands of the endocrine system are widely scattered
throughout the body (Figure 18-2; Table 18-2)
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Hormones

Classification of hormones
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Classification by general function
 Tropic hormones—hormones that
target other endocrine glands and
stimulate their growth and secretion
 Sex hormones—hormones that target
reproductive tissues
 Anabolic hormones—hormones that
stimulate anabolism in target cells
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Hormones

Classification of hormones (cont)

Classification by chemical
structure (Figure 18-3; Table
18-3)
Steroid hormones
 Nonsteroid hormones

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Hormones

Classification of hormones (cont)
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Steroid hormones (Figure 18-4)
Synthesized from cholesterol
(Figure 18-5)
 Lipid soluble and can easily pass
through the phospholipid plasma
membrane of target cells
 Examples of steroid hormones:
cortisol, aldosterone, estrogen,
progesterone, and testosterone

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Hormones

Classification of hormones (cont)
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Nonsteroid hormones (Figure 18-6)
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Synthesized primarily from amino acids
Protein hormones—long, folded chains of
amino acids; e.g., insulin, parathyroid
hormone
Glycoprotein hormones—protein hormones
with carbohydrate groups attached to the
amino acid chain
Peptide hormones—smaller than protein
hormones; short chain of amino acids; e.g.,
oxytocin, antidiuretic hormone (ADH)
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Hormones

Nonsteroid hormones (cont)
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Amino acid derivative hormones—each
is derived from a single amino acid
molecule
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Amine hormones—synthesized by
modifying a single molecule of tyrosine
or tryptophan; produced by
neurosecretory cells and by neurons;
e.g., epinephrine, norepinephrine
Amino acid derivatives produced by the
thyroid gland; synthesized by adding
iodine to tyrosine
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Hormones

How hormones work

General principles of hormone
action
Hormones signal a cell by binding to
the target cell’s specific receptors in
a “lock-and-key” mechanism (Figure
18-7)
 Different hormone-receptor
interactions produce different
regulatory changes within the target
cell through chemical reactions

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Hormones
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General principles of hormone action (cont)
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Combined hormone actions
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Synergism—combinations of hormones acting
together have a greater effect on a target cell than
the sum of the effects that each would have if acting
alone
Permissiveness—when a small amount of one
hormone allows a second one to have its full effects
on a target cell
Antagonism—one hormone produces the opposite
effects of another hormone; used to “fine tune” the
activity of target cells with great accuracy
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Hormones

General principles of hormone action
(cont)
Most hormones have primary effects that
directly regulate target cells and many
secondary effects that influence or modulate
other regulatory mechanisms in target cells
 Endocrine glands produce more hormone
molecules than are actually needed; the
unused hormones are quickly excreted by
the kidneys or broken down by metabolic
processes

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Hormones

How hormones work (cont)

Mechanism of steroid hormone
action (Figure 18-8)
Steroid hormones are lipid soluble,
and their receptors are normally
found in the target cell’s cytosol
 After a steroid hormone molecule has
diffused into the target cell, it binds to
a receptor molecule to form a
hormone-receptor complex

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Hormones

Mechanism of steroid hormone action (cont)
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Mobile-receptor model—the hormone passes into the
nucleus, where it binds to a mobile receptor and
activates a certain gene sequence to begin transcription
of mRNA; newly formed mRNA molecules move into the
cytosol, associate with ribosomes, and begin
synthesizing protein molecules that produce the effects
of the hormone
Steroid hormones regulate cells by regulating production
of certain critical proteins
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Hormones

Mechanism of steroid hormone action
(cont)
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The amount of steroid hormone present
determines the magnitude of a target cell’s
response
Because transcription and protein synthesis
take time, responses to steroid hormones are
often slow
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Hormones

How hormones work (cont)
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Mechanism of nonsteroid hormone action
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The second messenger mechanism—also known as the
fixed-membrane-receptor model (Figure 18-9)
 A nonsteroid hormone molecule acts as a “first
messenger” and delivers its chemical message to
fixed receptors in the target cell’s plasma
membrane
 The “message” is then passed by way of a Gprotein–coupled receptor (GPCR) into the cell
where a “second messenger” triggers a G protein,
which leads to the appropriate cellular changes
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Hormones

The second messenger mechanism
(cont)

Second messenger mechanism produces target
cell effects that differ from steroid hormone
effects in several important ways
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The effects of the hormone are amplified by the
cascade of reactions
There are a variety of second messenger
mechanisms—e.g., IP3, cGMP, calciumcalmodulin mechanisms (Figure 18-10)
Second messenger mechanism operates much
more quickly than the steroid mechanism
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Hormones

Mechanism of nonsteroid hormone
action
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The nuclear-receptor mechanism—
small iodinated amino acids (T4 and T3)
enter the target cell and bind to
receptors associated with a DNA
molecule in the nucleus; this binding
triggers transcription of mRNA and
synthesis of new enzymes
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Hormones
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Regulation of hormone secretion
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Control of hormonal secretion is usually part of a
negative feedback loop and is called endocrine
reflexes (Figure 18-11)
Simplest mechanism—when an endocrine gland
is sensitive to the physiological changes produced
by its target cells
Endocrine gland secretion may also be regulated
by a hormone produced by another gland
Endocrine gland secretions may be influenced by
nervous system input; this fact emphasizes the
close functional relationship between the two
systems
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Hormones
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Regulation of target cell sensitivity
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Sensitivity of target cell depends in part on
number of receptors (Figure 18-12)
Up-regulation—increased number of
hormone receptors increases sensitivity
 Down-regulation—decreased number of
hormone receptors decreases sensitivity
Sensitivity of target cell may also be regulated
by factors that affect signal transcription or
gene transcription
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
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Prostaglandins
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Unique group of lipid hormones (20-carbon
fatty acid with 5-carbon ring) that serve
important and widespread integrative
functions in the body but do not meet the
usual definition of a hormone (Figure 18-13;
Table 18-4)
Called tissue hormones because the
secretion is produced in a tissue and
diffuses only a short distance to other cells
within the same tissue; PGs tend to
integrate activities of neighboring cells
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Prostaglandins
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Many structural classes of prostaglandins have been
isolated and identified
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Prostaglandin A (PGA)—intraarterial infusion resulting in an
immediate fall in blood pressure accompanied by an increase in
regional blood flow to several areas
Prostaglandin E (PGE)—vascular effects: regulation of red blood
cell deformability and platelet aggregation; inflammation (which
can be blocked with drugs that inhibit PG-producing enzymes
such as COX-1 and COX-2); gastrointestinal effects: regulates
hydrochloric acid secretion
Prostaglandin F (PGF)—especially important in reproductive
system, causing uterine contractions; also affects intestinal
motility and is required for normal peristalsis
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Prostaglandins
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Many tissues are known to secrete
PGs
PGs have diverse physiological
effects
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The Big Picture: The Endocrine
System and The Whole Body
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Nearly every process in the human
organism is kept in balance by the
intricate interaction of different
nervous and endocrine regulatory
chemicals
The endocrine system operates with
the nervous system to finely adjust the
many processes they regulate
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