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Chapter 16
Endocrine System
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Slide 1
Introduction
• The endocrine and nervous systems function to achieve
and maintain homeostasis (Table 16-1)
• When the two systems work together as one system,
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 via the blood to specific target cells contained
in target tissues or target organs
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Slide 2
Introduction
• 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 16-2; Table 16-2)
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Slide 3
Hormones
• Classification of hormones

Classification by general function
• Tropic hormones—target other endocrine glands and
stimulate their growth and secretion
• Sex hormones—target reproductive tissues
• Anabolic hormones—stimulate anabolism in target cells

Classification by chemical structure
(Figure 16-3; Table 16-3)
• Steroid hormones
• Nonsteroid hormones
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Slide 4
Hormones
• Classification of hormones (cont.)

Steroid hormones (Figure 16-4)
• Synthesized from cholesterol (Figure 16-5)
• 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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Slide 5
Hormones
• Classification of hormones (cont.)

Nonsteroid hormones (Figure 16-6)
• Synthesized primarily from amino acids
• Protein hormones—long, folded chains of amino acids; e.g., insulin and
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 and antidiuretic hormone (ADH)
• Amino acid derivative hormones—each is derived from a single amino
acid molecule

Amine hormones—synthesized by modifying a single molecule of tyrosine;
produced by neurosecretory cells and by neurons; e.g., epinephrine and
norepinephrine

Amino acid derivatives produced by the thyroid gland; synthesized by
adding iodine to tyrosine
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Slide 6
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 16-7)
• Different hormone-receptor interactions produce different regulatory changes
within the target cell through chemical reactions
• Combined hormone actions:



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 permits, or enables,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
• 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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Slide 7
Hormones
• How hormones work (cont.)

Mechanism of steroid hormone action (Figure 16-8)
• 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
• Mobile-receptor model—hormone passes into nucleus, where it binds
to 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
• The amount of steroid hormone present determines 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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Slide 8
Hormones
• How hormones work (cont.)

Mechanisms of nonsteroid hormone action
• The second messenger mechanism—also known as the fixed-membranereceptor model (Figure 16-9)

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 G protein into the cell, where a “second
messenger” triggers the appropriate cellular changes

Second messenger mechanism—produces target cell effects that differ from steroid
hormone effects in several important ways:
– Effects of the hormone are amplified by the cascade of reactions
– There are a variety of second messenger mechanisms—examples: IP3, GMP, calciumcalmodulin mechanisms (Figure 16-10)
– The second messenger mechanism operates much more quickly than the steroid mechanism
• 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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Slide 9
Hormones
• Regulation of hormone secretion

Control of hormonal secretion is usually part of a negative
feedback loop and is called endocrine reflexes
(Figure 16-11)

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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Slide 10
Hormones
• Regulation of target cell sensitivity

Sensitivity of target cell depends in part on number of
receptors (Figure 16-12)
• 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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Slide 11
Prostaglandins (PGs)
• 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 16-13; Table 16-4)
• 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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Slide 12
Prostaglandins
• Many structural classes of prostaglandins have been
isolated and identified:

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
• Many tissues are known to secrete PGs
• PGs have diverse physiological effects
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Slide 13
Pituitary Gland
• Structure of the pituitary gland

Formerly known as hypophysis

Size: 1.2 to 1.5 cm (about 1⁄2 inch) across; weight: 0.5 g
(1⁄60 ounce)

Located on the ventral surface of the brain within the
skull (Figure 16-14)

Infundibulum—stemlike stalk that connects pituitary to
the hypothalamus

Made up of two separate glands, the adenohypophysis
(anterior pituitary gland) and the neurohypophysis
(posterior pituitary gland)
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Slide 14
Pituitary Gland
• Adenohypophysis (anterior pituitary)

Divided into two parts:
• Pars anterior—forms the major portion of adenohypophysis
• Pars intermedia
 Tissue is composed of irregular clumps of secretory
cells supported by fine connective tissue fibers and
surrounded by a rich vascular network
 Three types of cells can be identified according to their
affinity for certain stains (Figure 16-15):
• Chromophobes—do not stain
• Acidophils—stain with acid stains
• Basophils—stain with basic stains
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Slide 15
Pituitary Gland
• Adenohypophysis (anterior pituitary) (cont.)

Five functional types of secretory cells exist:
• Somatotrophs—secrete GH
• Corticotrophs—secrete ACTH
• Thyrotrophs—secrete TSH
• Lactotrophs—secrete prolactin (PRL)
• Gonadotrophs—secrete LH and FSH
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Slide 16
Pituitary Gland
• Adenohypophysis (anterior pituitary) (cont.)

Growth hormone (GH) (Figure 16-16; Table 16-6)
• Also known as somatotropin (STH)
• Promotes growth of bone, muscle, and other tissues by
accelerating amino acid transport into the cells
• Stimulates fat metabolism by mobilizing lipids from storage in
adipose cells and speeding up catabolism of the lipids after
they have entered another cell
• GH tends to shift cell chemistry away from glucose catabolism
and toward lipid catabolism as an energy source; this leads to
increased blood glucose levels
• GH functions as an insulin antagonist and is vital to maintaining
homeostasis of blood glucose levels
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Slide 17
Pituitary Gland
• Adenohypophysis (anterior pituitary) (cont.)

Prolactin (PRL) (Table 16-6)
• Produced by acidophils in the pars anterior
• Also known as lactogenic hormone
• During pregnancy, PRL promotes development of the
breasts, anticipating milk secretion; after the baby is
born, PRL stimulates the mother’s mammary glands to
produce milk
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Slide 18
Pituitary Gland
• Adenohypophysis (anterior pituitary) (cont.)

Tropic hormones—have a stimulating effect on other endocrine glands; four
principal tropic hormones are produced and secreted by the basophils of
the pars anterior (Table 16-6):
• Thyroid-stimulating hormone (TSH), or thyrotropin—promotes and maintains
growth and development of thyroid; also causes thyroid to secrete its hormones
• Adrenocorticotropic hormone (ACTH), or adrenocorticotropin—promotes and
maintains normal growth and development of cortex of adrenal gland; also
stimulates adrenal cortex to secrete some of its hormones
• Follicle-stimulating hormone (FSH)—in female, stimulates primary graafian
follicles to grow toward maturity; also stimulates follicle cells to secrete
estrogens; in male, FSH stimulates development of seminiferous tubules of
testes and maintains spermatogenesis
• Luteinizing hormone (LH)—in female, stimulates formation and activity of corpus
luteum of ovary; corpus luteum secretes progesterone and estrogens when
stimulated by LH; LH also supports FSH in stimulating maturation of follicles; in
male, LH stimulates interstitial cells in testes to develop and secrete
testosterone; FSH and LH are called gonadotropins because they stimulate
growth and maintenance of gonads
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Slide 19
Pituitary Gland
• Adenohypophysis (anterior pituitary) (cont.)

Control of secretion in the adenohypophysis
• Hypothalamus secretes releasing hormones into the blood, which are then
carried to hypophyseal portal system (Figure 16-17; Table 16-5)
• Hypophyseal portal system carries blood from hypothalamus directly to
adenohypophysis where target cells of releasing hormones are located
(Figure 16-18)
• Releasing hormones influence secretion of hormones by acidophils and
basophils
• Through negative feedback, hypothalamus adjusts secretions of
adenohypophysis, which then adjusts secretions of target glands that, in turn,
adjust activity of their target tissues (Figure 16-19)
• Minute-by-minute variations in hormone secretion can exhibit occasional large
peaks, caused by pulse in releasing hormone secretion by hypothalamus
(Figure 16-20)
• In stress, hypothalamus translates nerve impulses into hormone secretions by
endocrine glands, basically creating a mind-body link
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Slide 20
Adrenal Glands
• Adrenal cortex (cont.)

Mineralocorticoids
• Have an important role in regulatory process of sodium in the body
• Aldosterone

Only physiologically important mineralocorticoid in the human; primary
function is maintenance of sodium homeostasis in the blood by
increasing sodium reabsorption in the kidneys

Aldosterone also increases water retention and promotes loss of
potassium and hydrogen ions

Aldosterone secretion is controlled by the renin-angiotensinaldosterone system (RAAS) and by blood potassium concentration
(Figure 16-32)
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Slide 21
Adrenal Glands
• Adrenal cortex (cont.)

Glucocorticoids
• Main glucocorticoids secreted by the zona fasciculata are cortisol,
cortisone, and corticosterone, with cortisol the only one secreted in
significant quantities
• Affect every cell in the body
• Are protein-mobilizing, gluconeogenic, and hyperglycemic
• Tend to cause a shift from carbohydrate catabolism to lipid
catabolism as an energy source
• Essential for maintaining normal blood pressure by aiding
norepinephrine and epinephrine to have their full effect, causing
vasoconstriction
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Slide 22
Adrenal Glands

Glucocorticoids (cont.)
• High blood concentration causes eosinopenia and marked
atrophy of lymphatic tissues
• Act with epinephrine to bring about normal recovery from injury
produced by inflammatory agents
• Secretion increases in response to stress
• Except during stress response, secretion is mainly controlled by
a negative feedback mechanism involving ACTH from the
adenohypophysis
• Secretion is characterized by several large pulses of increased
hormone levels throughout the day—the largest occurring just
before waking (Figure 16-33)

Gonadocorticoids—sex hormones (androgens) that are
released from the adrenal cortex
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Slide 23
Adrenal Glands
• Adrenal medulla

Neurosecretory tissue—composed of neurons specialized to
secrete their products into the blood

Adrenal medulla secretes two important hormones—epinephrine
and norepinephrine; they are part of the class of nonsteroid
hormones called catecholamines

Both hormones bind to the receptors of sympathetic effectors to
prolong and enhance the effects of sympathetic stimulation by the
ANS (Figure 16-34)
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Slide 24
Pancreatic Islets
• Structure of the pancreatic islets (Figure 16-35)

Elongated gland, weighing approximately 100 g
(3.5 ounces); its head lies in the duodenum, extends
horizontally behind the stomach, and then touches
the spleen

Composed of endocrine and exocrine tissues
• Pancreatic islets (islets of Langerhans)—endocrine portion
• Acini—exocrine portion—secretes a serous fluid containing
digestive enzymes into ducts draining into the small intestine
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Slide 25
Pancreatic Islets
• Structure of the pancreatic islets (cont.)

Pancreatic islets—each islet contains four primary
types of endocrine glands joined by gap junctions
• Alpha cells (A cells)—secrete glucagon (Figure 16-36)
• Beta cells (B cells)—secrete insulin; account for up to 75% of
all pancreatic islet cells
• Delta cells (D cells)—secrete somatostatin
• Pancreatic polypeptide cells (F, or PP, cells)—secrete
pancreatic polypeptides
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Slide 26
Pancreatic Islets
• Pancreatic hormones (Table 16-9)—work collaboratively
to maintain homeostasis of food molecules (Figure 16-37)

Glucagon—produced by alpha cells; tends to increase blood
glucose levels; stimulates gluconeogenesis in liver cells

Insulin—produced by beta cells; lowers blood concentration of
glucose, amino acids, and fatty acids and promotes their
metabolism by tissue cells

Somatostatin—produced by delta cells; primary role is regulating
the other endocrine cells of the pancreatic islets

Pancreatic polypeptide—produced by F (PP) cells; influences the
digestion and distribution of food molecules to some degree
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Slide 27
Gonads
• Testes (Figure 16-2; Table 16-10)

Paired organs within the scrotum in the male

Composed of seminiferous tubules and a
scattering of interstitial cells

Testosterone is produced by the interstitial cells
and is responsible for the growth and maintenance
of male sexual characteristics

Testosterone secretion is mainly regulated by
gonadotropin levels in the blood
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Slide 28
Gonads
• Ovaries (Figure 16-2; Table 16-10)

Primary sex organs in the female

Set of paired glands in the pelvis that produce several types
of sex hormones
• Estrogens—steroid hormones secreted by ovarian follicles;
promote development and maintenance of female sexual
characteristics
• Progesterone—secreted by corpus luteum; maintains the lining
of the uterus necessary for successful pregnancy
• Ovarian hormone secretion depends on the changing levels of
FSH and LH from adenohypophysis
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Slide 29
Placenta
• Tissues that form on the lining of the uterus as
a connection between the circulatory systems
of the mother and the developing child
• Serves as a temporary endocrine gland that
produces human chorionic gonadotropin,
estrogens, and progesterone (Table 16-10)
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Slide 30
Thymus (Figure 16-2)
• Gland located in the mediastinum just
beneath the sternum
• Thymus is large in children, begins to atrophy
at puberty, and, by old age, the gland is a
vestige of fat and fibrous tissue
• Considered to be primarily a lymphatic organ,
but the hormone thymosin has been isolated
from thymus tissue (Table 16-10)
• Thymosin—stimulates development of T cells
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Slide 31
Gastric and Intestinal Mucosa
• The mucous lining of the GI tract contains cells that produce
both endocrine and exocrine secretions
(Table 16-10)
• GI hormones such as gastrin, secretin, and cholecystokinin
(CCK) play regulatory roles in coordinating the secretory and
motor activities involved in the digestive process
• Ghrelin—hormone secreted by endocrine cells in gastric
mucosa; stimulates hypothalamus to boost appetite;
slows metabolism and fat burning; may be a contributor
to obesity
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Slide 32
Heart
• The heart has a secondary endocrine role
• Hormone-producing cells produce several atrial
natriuretic peptides (ANPs), including atrial
natriuretic hormone (ANH) (Table 16-10)
• ANH’s primary effect is to oppose increases in
blood volume or blood pressure; also an
antagonist to ADH and aldosterone
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Slide 33
Other Endocrine Glands and Organs
(Table 16-10)
• Major endocrine glands produce more hormones
that are outlined in this book (e.g., inhibin
secreted by the ovaries)
• Many tissues (perhaps all tissues) produce
hormones, most of which are beyond the scope
of this book (e.g., leptin and resistin secreted by
adipose tissue)
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Slide 34
Cycle of Life: Endocrine System
• Endocrine regulation begins in the womb
• Many hormones are active from gestational period

Evidence that a hormonal signal from fetus to mother signals
the onset of labor
• Hormones related to reproduction begin at puberty
• Secretion of male reproductive hormones—continuous
production from puberty, slight decline in late adulthood
• Secretion of female reproductive hormones declines
suddenly and completely in middle adulthood
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Slide 35
The Big Picture: The Endocrine
System and the Whole Body
• 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
• Neuroendocrine system adjusts nutrient supply
• Calcitonin, parathyroid hormone, and vitamin D balance
calcium ion use
• The nervous system and hormones regulate reproduction
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Slide 36