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LECTURE \ GALLARDO JR. 01

MODULE 9: ENDOCRINE SYSTEM

OUTLINE • The endocrine system also controls body activities by

I Nervous System vs Endocrine System releasing mediators, called hormones, but the means of
II Exocrine vs Endocrine control of the two systems are very different.
III Endocrine Glands • A hormone is a mediator molecule that is released in one
IV Hormone Activity part of the body but regulates the activity of cells in other
V Chemical Classes of Hormones parts of the body. Most hormones enter interstitial fluid
A Lipid-soluble hormones and then the bloodstream.
B Water-soluble hormones
VI Control of Hormone Secretions • The circulating blood delivers hormones to cells
VII Hypothalamus throughout the body.
VIII Pituitary Gland • Both neurotransmitters and hormones exert their effects
IX Hypophyseal Portal System by binding to receptors on or in their “target” cells.
X Anterior Pituitary Cells • Responses of the endocrine system often are slower
XI HGH and IGF than responses of the nervous system; although some
XII Thyroid-Stimulating Hormone
hormones act within seconds, most take several minutes
XIII Follicle-Stimulating Hormone
XIV Luteinizing Hormone or more to cause a response.
XV Prolactin • The effects of nervous system activation are generally
XVI Adrenocorticotropic Hormone briefer than those of the endocrine system.
XVII Posterior Pituitary • The nervous system à acts on specific muscles and
XVIII Oxytocin glands.
XIX ADH (Vasopressin)
• The influence of the endocrine system is much broader;
XX Endocrine System Part 2
it helps regulate virtually all types of body cells.
NERVOUS SYSTEM ENDOCRINE SYSTEM
• Circulating or local hormones of the endocrine system
contribute to homeostasis by regulating the activity and • acts through nerve • also controls body
growth of target cells in your body. impulses (action activities by
• Hormones also regulate your metabolism. potentials) conducted releasing mediators,
along axons of neurons. called hormones
• The Endocrine System plays a very important role in
regulating metabolic body functions in order to maintain • Trigger the release of • HORMONES =
homeostasis. mediator (messenger) circulating blood
• Perhaps no other period in life so dramatically shows the molecules called o mediator
impact of the endocrine system in directing development neurotransmitters molecule that is
and regulating body functions. released in one
• In girls, estrogens promote accumulation of adipose part of the body
tissue in the breasts and hips, sculpting a feminine shape. but regulates
• At the same time or a little later, increasing levels of the activity of
testosterone in boys begin to help build muscle mass and cells in other
enlarge the vocal cords, producing a lower pitched voice. parts of the
• These changes are just a few examples of the powerful body
influence of endocrine secretions. • NEUROTRANSMITTERS • Slower than
= in between synapse responses of the
OBJECTIVES nervous system
• Compare control of body functions by the nervous system • Most take several
and endocrine system minutes or more to
• Distinguish between exocrine and endocrine glands. cause a response
• Describe how hormones interact with target-cell • Effectors are generally • Influence is much
receptors. briefer broader
• Compare the two chemical classes of hormones based • Acts on specific muscles • It helps regulate
on their solubility and glands virtually all types of
• Describe the mechanisms of hormone action. body cells
• Describe the mechanisms of control of hormone
secretion.
• Nervous system à uses mediators called
• Describe the locations of and relationships between the neurotransmitters (released locally in response to nerve
hypothalamus and pituitary gland. impulses)
• Describe the location, histology, hormones, and functions • Endocrine system à has hormones for mediators & are
of the anterior and posterior pituitary. delivered to tissues throughout the body by the blood
• Try to memorize the table in the next page
NERVOUS SYSTEM VS ENDOCRINE SYSTEM
• Recall that the nervous system acts through nerve
impulses (action potentials) conducted along axons of
neurons and targeted towards the effector organ thus
eliciting their motor response.
• At synapses, nerve impulses trigger the release of
mediator (messenger) molecules called
neurotransmitters.

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ENDOCRINE VS EXOCRINE GLANDS • For example, thyroid-stimulating hormone (TSH) binds

• The body contains two kinds of glands: to receptors on cells of the thyroid gland, but it does not
o exocrine glands bind to cells of the ovaries because ovarian cells do not
o endocrine glands have TSH receptors.
• Exocrine glands (exo- = outside) secrete their products • Receptors, like other cellular proteins, are constantly
into ducts that carry the secretions into body cavities, into being synthesized and broken down.
the lumen of an organ, or to the outer surface of the body. • Hormones influence their target cells through chemical
• Exocrine glands include: process.
o sudoriferous (sweat) • Generally, a target cell has 2000 to 100,000 receptors for
o sebaceous (oil) a particular hormone.
o mucous
o digestive glands
DOWN-REGULATION
• If a hormone is present in excess, the number of target-
• Endocrine glands (endo- = within) secrete their products
cell receptors may decrease—an effect called down-
(hormones) into the interstitial fluid surrounding the
regulation.
secretory cells rather than into ducts.
• For example, when certain cells of the testes are exposed
• From the interstitial fluid, hormones diffuse into blood
to a high concentration of luteinizing hormone (LH), the
capillaries and blood carries them to target cells
number of LH receptors decreases.
throughout the body.
• Down-regulation makes a target cell less sensitive to a
• Because most hormones are required in very small
hormone.
amounts, circulating levels typically are low.
• The endocrine glands include the:
(1) pituitary
UP-REGULATION
(2) thyroid • When a hormone is deficient, the number of receptors
(3) parathyroid may increase.
(4) adrenal • This phenomenon, known as up-regulation, makes a
(5) pineal glands target cell more sensitive to a hormone.

• In addition, several CIRCULATING HORMONES

organs and tissues are • Most endocrine hormones are circulating hormones—
not exclusively they pass from the secretory cells that make them into
classified as interstitial fluid and then into the blood.
endocrine glands but
contain cells that
secrete hormones.
These include the:
o Hypothalamus
o Thymus
o Pancreas
o Ovaries
o Testes
o Kidneys
o Stomach
o Liver
o small intestine
o skin
o heart
o adipose tissue
o placenta • Local hormones à the other hormones, act locally on
• Taken together, all endocrine glands and hormone- neighboring cells or on the same cell that secreted them
secreting cells constitute the endocrine system. without first entering the bloodstream.
• Paracrines
HORMONE ACTIVITY o Local hormones that act on neighboring cells are
called paracrines (para- = beside or near)
• Although a given hormone travels throughout the body in
the blood, it affects only specific target cells. • Autocrines
o Local hormones that act on the same cell that
secreted them are called autocrines (auto- = self).
HORMONE RECEPTORS
• Local hormones usually are inactivated quickly;
• Hormones, like neurotransmitters, influence their target
circulating hormones may linger in the blood and exert
cells by chemically binding to specific protein receptors.
their effects for a few minutes or occasionally for a few
• Only the target cells for a given hormone have receptors hours.
that bind and recognize that hormone.

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• In time, circulating hormones are inactivated by the liver

and excreted by the kidneys. In cases of kidney or liver
failure, excessive levels of hormones may build up in the
blood.

CHEMICAL CLASSES OF HORMONES

• Hormones are divided into two broad classes:
(1) Lipid-soluble Hormones
(2) Water-soluble Hormones

LIPID-SOLUBLE HORMONES
1) STEROID HORMONES
• are derived from cholesterol
• Each steroid hormone is unique due to the presence of
different chemical groups attached at various sites on the
four rings at the core of its structure.
• These small differences allow for a large diversity of
functions.

2) THYROID HORMONES (T3 AND T4)

• Two thyroid hormones (T3 and T4) are synthesized by
attaching iodine to the amino acid tyrosine.
• T3 à triiodothyronine
• T4 à thyroxine
• The benzene ring of tyrosine plus the attached iodines
make T3 and T4 very lipid soluble.

3) NITRIC OXIDE (NO)

• The gas nitric oxide (NO) is both a hormone and a
neurotransmitter.
• Its synthesis is catalyzed by the enzyme nitric oxide
synthase.

WATER-SOLUBLE HORMONES
1) AMINE HORMONES
• Amine hormones are synthesized by decarboxylating
(removing a molecule of CO2) and otherwise modifying
certain amino acids.
• They are called amines because they retain an amino
group (–NH3+).
• The catecholamines—epinephrine, norepinephrine, and
dopamine—are synthesized by modifying the amino acid
tyrosine.
• Histamine is synthesized from the amino acid histidine
by mast cells and platelets.
• Serotonin and melatonin are derived from tryptophan.

2) PEPTIDE HORMONES & PROTEIN HORMONES

• Peptide hormones and protein hormones are amino acid
polymers.
• The smaller peptide hormones consist of chains of 3 to
49 amino acids; the larger protein hormones include 50 HORMONAL ACTION MECHANISMS
to 200 amino acids. • The response to a hormone depends on both the
• Examples of peptide hormones are: hormone and the target cell.
o antidiuretic hormone and oxytocin; • Various target cells respond differently to the same
o protein hormones include human growth hormone hormone. Insulin, for example, stimulates synthesis of
and glycogen in liver cells and synthesis of triglycerides in
o insulin adipose cells.
• Several of the protein hormones, such as: • The response to a hormone is not always the synthesis
o thyroid-stimulating hormone, have attached of new molecules, as is the case for insulin.
carbohydrate groups and thus are glycoprotein • Other hormonal effects include changing the permeability
hormones. of the plasma membrane, stimulating transport of a
substance into or out of the target cells, altering the rate
3) EICOSANOID HORMONES of specific metabolic reactions, or causing contraction of
• The eicosanoid hormones are derived from arachidonic smooth muscle or cardiac muscle.
acid, a 20-carbon fatty acid. • In part, these varied effects of hormones are possible
• The two major types of eicosanoids are prostaglandins because a single hormone can set in motion several
and leukotrienes. different cellular responses.
• The eicosanoids are important local hormones, and they • However, a hormone must first “announce its arrival” to a
may act as circulating hormones as well. target cell by binding to its receptors.

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• The receptors for lipid-soluble hormones à are located • The action of a typical water-soluble hormone occurs as
inside target cells. follows:
• The receptors for water-soluble hormones à are part 1. A water-soluble hormone (the first messenger)
of the plasma membrane of target cells. diffuses from the blood through interstitial fluid and
then binds to its receptor at the exterior surface of a
ACTION OF LIPID-SOLUBLE HORMONES target cell’s plasma membrane. The hormone–
• Lipid-soluble hormones, including steroid hormones and receptor complex activates a membrane protein
thyroid hormones, bind to receptors within target cells. called a G protein. The activated G protein in turn
• Their mechanism of action is as follows: activates adenylate cyclase.
1. A free lipid-soluble hormone molecule diffuses from 2. Adenylate cyclase converts ATP into cyclic AMP
the blood, through interstitial fluid, and through the (cAMP). Because the enzyme’s active site is on the
lipid bilayer of the plasma membrane into a cell. inner surface of the plasma membrane, this reaction
occurs in the cytosol of the cell.
2. If the cell is a target cell, the hormone binds to and
activates receptors located within the cytosol or 3. Cyclic AMP (the second messenger) activates one
nucleus. The activated receptor–hormone complex or more protein kinases, which may be free in the
then alters gene expression: It turns specific genes cytosol or bound to the plasma membrane. A protein
of the nuclear DNA on or off. kinase is an enzyme that phosphorylates (adds a
phosphate group to) other cellular proteins (such as
3. As the DNA is transcribed, new messenger RNA enzymes). The donor of the phosphate group is ATP,
(mRNA) forms, leaves the nucleus, and enters the which is converted to ADP.
cytosol. There, it directs synthesis of a new protein,
often an enzyme, on the ribosomes. 4. Activated protein kinases phosphorylate one or more
cellular proteins. Phosphorylation activates some
4. The new proteins alter the cell’s activity and cause of these proteins and inactivates others, rather like
the responses typical of that hormone. turning a switch on or off.
5. Phosphorylated proteins in turn cause reactions that
produce physiological responses. Different protein
kinases exist within different target cells and within
different organelles of the same target cell. Thus, one
protein kinase might trig- ger glycogen synthesis, a
second might cause the break- down of triglyceride,
a third may promote protein synthesis, and so forth.
As noted in step 4, phosphorylation by a protein
kinase can also inhibit certain proteins. For example,
some of the kinases unleashed when epinephrine
binds to liver cells inactivate an enzyme needed for
glycogen synthesis.
6. After a brief period, an enzyme called
phosphodiesterase inactivates cAMP. Thus, the
cell’s response is turned off unless new hormone
molecules continue to bind to their receptors in the
plasma membrane.

ACTION OF WATER-SOLUBLE HORMONES

• Because amine, peptide, protein, and eicosanoid
hormones are not lipid-soluble, they cannot diffuse
through the lipid bilayer of the plasma membrane and
bind to receptors inside target cells.
• Instead, water-soluble hormones bind to receptors that
protrude from the target cell surface.
• The receptors are integral trans-membrane proteins in
the plasma membrane.
• When a water-soluble hormone binds to its receptor at
the outer surface of the plasma membrane, it acts as the
first messenger.
• The first messenger (the hormone) then causes
production of a second messenger inside the cell, where
specific hormone-stimulated responses take place.
o One common second messenger is cyclic AMP
(cAMP).
o Neurotransmitters, neuropeptides, and several
sensory transduction mechanisms also act via
second-messenger systems.

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• The binding of a hormone to its receptor activates many PITUITARY GLAND

G-protein molecules, which in turn activate molecules of • The pituitary gland is a pea-shaped structure that
adenylate cyclase (as noted in step 1). measures 1–1.5 cm (0.5 in.) in diameter
• Unless they are further stimulated by the binding of more • It lies in the hypophyseal fossa of the sella turcica of the
hormone molecules to receptors, G proteins slowly sphenoid bone.
inactivate, thus decreasing the activity of adenylate • It attaches to the hypothalamus by a stalk, the
cyclase and helping to stop the hormone response. infundibulum
• G proteins are a common feature of most second- • It has two anatomically and functionally separate
messenger systems. portions:
o the anterior pituitary
CONTROL OF HORMONE SECRETIONS o the posterior pituitary
• The release of most hormones occurs in short bursts,
with little or no secretion between bursts.
• When stimulated, an endocrine gland will release its
hormone in more frequent bursts, increasing the
concentration of the hormone in the blood.
• In the absence of stimulation, the blood level of the
hormone decreases. Regulation of secretion normally
prevents overproduction or underproduction of any given
hormone.
• Hormone secretion is regulated by:
(1) signals from the nervous system
§ e.g. nerve impulses to the adrenal medullae
regulate the release of epinephrine
(2) chemical changes in the blood, and
§ e.g. blood Ca2+ level regulates the secretion of
parathyroid hormone
(3) other hormones • The anterior pituitary (anterior lobe), also called the
§ e.g. a hormone from the anterior pituitary adenohypophysis, accounts for about 75% of the total
(adrenocorticotropic hormone) stimulates the weight of the gland.
release of cortisol by the adrenal cortex • The anterior pituitary consists of two parts in an adult:
o The pars distalis is the larger portion, and
• Most hormonal regulatory systems work via negative o the pars tuberalis forms a sheath around the
feedback, but a few operate via positive feedback. infundibulum.
o For example, during childbirth, the hormone • The posterior pituitary (posterior lobe), also called the
oxytocin stimulates contractions of the uterus, and neurohypophysis, also consists of two parts:
uterine contractions in turn stimulate more oxytocin o the pars nervosa, the larger bulbar portion, and
release, a positive feedback effect. o the infundibulum
o Example of negative feedback is overstimulation of • A third region of the pituitary gland called the pars
the thyroid gland, increasing the release of your TSH. intermedia atrophies during human fetal development
So when there’s already enough thyroid hormones and ceases to exist as a separate lobe in adults.
that are already being circulated or used by the body, However, some of its cells migrate into adjacent parts of
the negative response would be the body/thyroid the anterior pituitary, where they persist.
gland would signal the hypothalamus to decrease
the secretion of the thyroid stimulating hormone.

HYPOTHALAMUS
• The hypothalamus is also known as “the master
endocrine gland”
• It is the major link between the nervous and endocrine
systems
• It is a small region in the brain below the thalamus
• Cells in the hypothalamus synthesizes at least 9 different
hormones and the pituitary gland secretes seven.
• Together, these hormones play important roles in the
regulation of virtually all aspects of growth, development,
metabolism, and homeostasis.

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HYPOPHYSEAL PORTAL SYSTEM o They synthesize the hypothalamic releasing and

• What is a portal system? inhibiting hormones in their cell bodies and package
o When blood flows from one capillary network into a the hormones inside vesicles, which reach the axon
portal vein, and then into a second capillary network terminals by axonal transport.
without passing through the heart. o Nerve impulses stimulate the vesicles to undergo
• Hypophyseal Portal System exocytosis.
o Blood flows from capillaries in the hypothalamus into o The hormones then diffuse into the primary plexus
portal veins that carry blood to capillaries of the of the hypophyseal portal system.
anterior pituitary. • Quickly, the hypothalamic hormones flow with the blood
o In the portal system, the heart is being bypassed. through the portal veins and into the secondary plexus.
• This direct route permits hypothalamic hormones to act
immediately on anterior pituitary cells, before the
hormones are diluted or destroyed in the general
circulation.
• Hormones secreted by anterior pituitary cells pass into
the secondary plexus capillaries, which drain into the
anterior hypophyseal veins and out into the general
circulation.
• Anterior pituitary hormones then travel to target tissues
throughout the body.
• Those anterior pituitary hormones that act on other
endocrine glands are called tropic hormones or tropins.

ANTERIOR PITUITARY CELLS

• Five types of anterior pituitary cells—somatotrophs,
thyrotrophs, gonadotrophs, lactotrophs, and
corticotrophs—secrete seven hormones

• Hypothalamic hormones reach the anterior pituitary 1) SOMATOTROPHS

through a portal system.
• Secrete: human growth hormone (hGH) or
• Usually, blood passes from the heart through an artery to
somatotropin
a capillary to a vein and back to the heart.
• Human growth hormone in turn stimulates several
• In a portal system, blood flows from one capillary
tissues to secrete insulin-like growth factors à
network into a portal vein, and then into a second capillary
hormones that stimulate general body growth and
network without passing through the heart.
regulate aspects of metabolism.
• The name of the portal system indicates the location of
the second capillary network.
• In the hypophyseal portal system, blood flows from
capillaries in the hypothalamus into portal veins that carry
blood to capillaries of the anterior pituitary.
• The superior hypophyseal arteries, branches of the
internal carotid arteries, bring blood into the
hypothalamus.
• At the junction of the median eminence of the
hypothalamus and the infundibulum, these arteries divide
into a capillary network called the primary plexus of the
hypophyseal portal system.
• From the primary plexus, blood drains into the
2) THYROTROPHS
hypophyseal portal veins that pass down the outside of
the infundibulum. • Secrete: thyroid-stimulating hormone (TSH) or
• In the anterior pituitary, the hypophyseal portal veins thyrotropin
divide again and form another capillary network called the • TSH controls the secretions and other activities of the
secondary plexus of the hypophyseal portal system. thyroid gland.

3) GONADOTROPHS
• Secrete two gonadotropins:
o follicle-stimulating hormone (FSH)
o luteinizing hormone (LH).
• FSH and LH both act on the gonads.
• They stimulate secretion of estrogens and
progesterone and the maturation of oocytes in the
• NEUROSECRETORY CELLS ovaries.
o Near the median eminence and above the optic • They also stimulate sperm production and secretion of
chiasm are clusters of specialized neurons, called testosterone in the testes.
neurosecretory cells.

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4) LACTOTROPHS
• Secrete: prolactin (PRL) HORMONES SECRETED BY THE PITUITARY
o which initiates milk production in the mammary GLAND
glands.
HUMAN GROWTH HORMONE AND INSULIN-LIKE
GROWTH FACTORS
• Human Growth Hormone (hGH) are produced by
somatotrophs
• The most numerous cells in anterior pituitary
• The most plentiful anterior pituitary hormone in the
anterior pituitary gland
• Main function: promote synthesis and secretion of small
protein hormones called insulin-like growth factors
(IGFs) or somatomedins.

INSULIN-LIKE GROWTH FACTORS

5) CORTICOTROPHS • IGF or somatomedins
• In response to human growth hormone, cells in the
• Secrete: adrenocorticotropic hormone (ACTH) or liver, skeletal muscles, cartilage, bones, and other tissues
corticotropin secrete IGFs, which may either enter the bloodstream
o which stimulates the adrenal cortex to secrete from the liver or act locally in other tissues as autocrines
glucocorticoids such as cortisol or paracrines.
• Some corticotrophs, remnants of the pars intermedia, • The functions of IGFs include the following:
also secrete melanocyte-stimulating hormone (MSH). 1. IGFs cause cells to grow and multiply by increasing
uptake of amino acids into cells and accelerating
protein synthesis. IGFs also decrease the
breakdown of proteins and the use of amino acids for
ATP production.
§ Due to these effects of the IGFs, human growth
hormone increases the growth rate of the
skeleton and skeletal muscles during childhood
and the teenage years. In adults, human growth
hormone and IGFs help maintain the mass of
muscles and bones and promote healing of
injuries and tissue repair.
2. IGFs also enhance lipolysis in adipose tissue,
which results in increased use of the released fatty
acids for ATP production by body cells.
3. In addition to affecting protein and lipid metabolism,
human growth hormone and IGFs influence
carbohydrate metabolism by decreasing glucose
uptake, which decreases the use of glucose for ATP
production by most body cells à causes the liver to
release glucose.
§ This action spares glucose so that it is available
to neurons for ATP production in times of
glucose scarcity. IGFs and human growth
hormone may also stimulate liver cells to
release glucose into the blood.

• Somatotrophs in the anterior pituitary release bursts of

human growth hormone every few hours, especially
during sleep. Their secretory activity is controlled mainly
by two hypothalamic hormones:
(1) growth hormone–releasing hormone (GHRH)
promotes secretion of human growth hormone, and
(2) growth hormone–inhibiting hormone (GHIH)
suppresses it.
• A major regulator of GHRH and GHIH secretion is the
blood glucose level:

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1. Hypoglycemia, an abnormally low blood glucose • Release of TRH in turn depends on blood levels of T3 and
concentration, stimulates the hypothalamus to T4; high levels of T3 and T4 inhibit secretion of TRH via
secrete GHRH, which flows toward the anterior negative feedback.
pituitary in the hypophyseal portal veins. • There is no thyrotropin-inhibiting hormone.
2. Upon reaching the anterior pituitary, GHRH
stimulates somatotrophs to release human growth FOLLICLE-STIMULATING HORMONE
hormone. • In females, the ovaries are the targets for follicle-
3. Human growth hormone stimulates secretion of stimulating hormone (FSH).
insulin-like growth factors, which speed up • Each month FSH initiates the development of several
breakdown of liver glycogen into glucose, causing ovarian follicles àsaclike arrangements of secretory
glucose to enter the blood more rapidly. cells that surround a developing oocyte.
4. As a result, blood glucose rises to the normal level • FSH also stimulates follicular cells to secrete estrogens
(about 90 mg/100 mL of blood plasma). (female sex hormones).
5. An increase in blood glucose above the normal level
• In males, FSH stimulates sperm production in the
inhibits release of GHRH.
testes.
6. Hyperglycemia, an abnormally high blood glucose
• Gonadotropin-releasing hormone (GnRH) from the
concentration, stimulates the hypothalamus to
hypothalamus stimulates FSH release.
secrete GHIH (while inhibiting the secretion of
GHRH). • Release of GnRH and FSH is suppressed by estrogens
7. Upon reaching the anterior pituitary in portal blood, in females and by testosterone (the principal male sex
hormone) in males through negative feedback systems.
GHIH inhibits secretion of human growth hormone by
somatotrophs. • Through a negative feedback mechanism, if there are
8. A low level of human growth hormone and IGFs high levels of estrogen and testosterone, there will be a
slows breakdown of glycogen in the liver, and decrease in your GnRH and FSH.
glucose is released into the blood more slowly. • If there is a decrease in the levels of estrogen and
9. Blood glucose falls to the normal level. testosterone, the opposite will be true. There will be
10. A decrease in blood glucose below the normal level increased levels of GnRH and FSH.
(hypoglycemia) inhibits release of GHIH. • There is no gonadotropin-inhibiting hormone.

LUTEINIZING HORMONE
• In females, luteinizing hormone (LH) triggers ovulation
à the release of a secondary oocyte (future ovum) by an
ovary.
• LH stimulates formation of the corpus luteum (structure
formed after ovulation) in the ovary and the secretion of
progesterone (another female sex hormone) by the
corpus luteum.
• Together, FSH and LH also stimulate secretion of
estrogens by ovarian cells.
• Estrogens and progesterone prepare the uterus for
implantation of a fertilized ovum and help prepare the
mammary glands for milk secretion.
• In males, LH stimulates cells in the testes to secrete
testosterone.
• Secretion of LH, like that of FSH, is controlled by
gonadotropin-releasing hormone (GnRH).

PROLACTIN
• Prolactin (PRL) initiates and maintains milk secretion by
the mammary glands.
• By itself, prolactin has only a weak effect.
• Only after the mammary glands have been primed by
estrogens, progesterone, glucocorticoids, human growth
hormone, thyroxine, and insulin, which exert permissive
effects, does PRL bring about milk secretion.
• Ejection of milk from the mammary glands depends on
the hormone oxytocin, which is released from the
posterior pituitary.
• Milk secretion and ejection = constitute lactation.
• The hypothalamus secretes both inhibitory and excitatory
hormones that regulate prolactin secretion.
• In females: prolactin-inhibiting hormone (PIH), which
is DOPAMINE, inhibits the release of prolactin from the
anterior pituitary most of the time.
o Each month, just before menstruation begins, the
THYROID-STIMULATING HORMONE secretion of PIH diminishes and the blood level of
• Thyroid-stimulating hormone (TSH) stimulates the prolactin rises, but not enough to stimulate milk
production.
synthesis and secretion of the two thyroid hormones:
o triiodothyronine (T3) o Breast tenderness just before menstruation may be
caused by elevated prolactin.
o thyroxine (T4)
(both produced by the thyroid gland) o As the menstrual cycle begins anew, PIH is again
secreted and the prolactin level drops.
• Thyrotropin-releasing hormone (TRH) from the
hypothalamus controls TSH secretion.

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o During pregnancy, the prolactin level rises, • The paraventricular nucleus à synthesizes the
stimulated by prolactin-releasing hormone (PRH) hormone oxytocin.
from the hypothalamus. • The supraoptic nucleus à produces antidiuretic
o The sucking action of a nursing infant causes a hormone, also called vasopressin.
reduction in hypothalamic secretion of PIH. • The axon terminals in the posterior pituitary are
• The function of prolactin is not known in males, but its associated with specialized neuroglia called pituicytes.
hypersecretion causes erectile dysfunction (impotence, o These cells have a supporting role similar to that of
the inability to have an erection of the penis). astrocytes.
• In females, hypersecretion of prolactin causes • After their production in the cell bodies of neurosecretory
galactorrhea (inappropriate lactation) and amenorrhea cells, oxytocin and antidiuretic hormone are packaged
(absence of menstrual cycles). into secretory vesicles, which move by fast axonal
transport to the axon terminals in the posterior pituitary,
ADRENOCORTICOTROPIC HORMONE where they are stored until nerve impulses trigger
• Corticotrophs secrete mainly adrenocorticotropic exocytosis and release of the hormone.
hormone (ACTH).
• ACTH controls the production and secretion of cortisol
and other glucocorticoids by the cortex (outer portion) of
the adrenal glands.
• Corticotropin-releasing hormone (CRH) from the
hypothalamus stimulates secretion of ACTH by
corticotrophs.
• Stress-related stimuli, such as low blood glucose or
physical trauma, and interleukin-1, a substance produced
by macrophages, also stimulate release of ACTH.
• Glucocorticoids inhibit CRH and ACTH release via
negative feedback.

OXYTOCIN
• During and after delivery of a baby, oxytocin affects two
target tissues:
o the mother’s uterus and breasts
• During delivery, oxytocin enhances contraction of
smooth muscle cells in the wall of the uterus;
• After delivery, it stimulates milk ejection (“letdown”) from
the mammary glands in response to the mechanical
stimulus provided by a suckling infant.
• The function of oxytocin in males and in nonpregnant
females is not clear.
• Experiments with animals have suggested that it has
actions within the brain that foster parental caretaking
behavior toward young offspring.
• It may also be responsible, in part, for the feelings of
sexual pleasure during and after intercourse.

ANTIDIURETIC HORMONE OR VASOPRESSIN

• As its name implies, an antidiuretic is a substance that
decreases urine production.
POSTERIOR PITUITARY • ADH causes the kidneys to return more water to the
• Posterior pituitary gland aka Neurohypophysis blood, thus decreasing urine volume.
• It consists of axons and axon terminals of more than • In the absence of ADH, urine output increases more than
10,000 hypothalamic neurosecretory cells. tenfold, from the normal 1 to 2 liters to about 20 liters a
• It does not synthesize hormones, it does store and day.
release two hormones: • If this hormone is not working properly in our body, it
o Oxytocin – synthesized by the paraventricular would cause an imbalance or a pathologic condition
nucleus called diabetes insipidus.
o Antidiuretic hormone (ADH) aka vasopressin – • Too much would cause SIAD à Syndrome of
synthesized by the supraoptic nucleus Inappropriate Antidiuresis, prevents the patient to urinate
• The cell bodies of the neurosecretory cells are in the (excess fluids in the body)
paraventricular and supraoptic nuclei of the • Drinking alcohol often causes frequent and copious
hypothalamus; their axons form the urination because alcohol inhibits secretion of ADH.
hypothalamohypophyseal tract. • ADH also decreases the water lost through sweating and
o This tract begins in the hypothalamus and ends near causes constriction of arterioles, which increases blood
blood capillaries in the posterior pituitary. pressure.

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 TRANS: Endocrine System

• This hormone’s other name, vasopressin, reflects this 5. Low osmotic pressure of blood (or increased blood
effect on blood pressure. volume) inhibits the osmoreceptors.
• The amount of ADH secreted varies with blood osmotic
pressure and blood volume. 6. Inhibition of osmoreceptors reduces or stops ADH
secretion. The kidneys then retain less water by
forming a larger volume of urine, secretory activity of
sweat glands increases, and arterioles dilate. The
blood volume and osmotic pressure of body fluids
return to normal.

End of Endocrine System Part 1

1. High blood osmotic pressure—due to dehydration or

a decline in blood volume because of hemorrhage,
diarrhea, or excessive sweating—stimulates
osmoreceptors, neurons in the hypothalamus that
monitor blood osmotic pressure. Elevated blood
osmotic pressure activates the osmoreceptors
directly; they also receive excitatory input from other
brain areas when blood volume decreases.

2. Osmoreceptors activate the hypothalamic

neurosecretory cells that synthesize and release
ADH.

3. When neurosecretory cells receive excitatory input

from the osmoreceptors, they generate nerve
impulses that cause exocytosis of ADH-containing
vesicles from their axon terminals in the posterior
pituitary. This liberates ADH, which diffuses into
blood capillaries of the posterior pituitary.

4. The blood carries ADH to three target tissues: the

kidneys, sudoriferous (sweat) glands, and smooth
muscle in blood vessel walls. The kidneys respond
by retaining more water, which decreases urine
output. Secretory activity of sweat glands decreases,
which lowers the rate of water loss by perspiration
from the skin. Smooth muscle in the walls of
arterioles (small arteries) contracts in response to
high levels of ADH, which constricts (narrows) the
lumen of these blood vessels and increases blood
pressure.

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 TRANS: Endocrine System

ENDOCRINE SYSTEM PART 2

OBJECTIVES SYNTHESIS, STORAGE, AND RELEASE OF

THYROID HORMONE
1. Describe the location, histology, hormones, and functions
of the thyroid gland.
2. Describe the synthesis and secretion of thyroid
hormones.
3. Describe the location, histology, hormone, and functions
of the parathyroid glands.
4. Describe the location, histology, hormones, and functions
of the adrenal glands.
5. Describe the location, histology, hormones, and functions
of the pancreatic islets.
6. Describe the location, hormones, and functions of the
male and female gonads.
7. Describe the location, histology, hormone, and functions
of the pineal gland.
8. Briefly describe the mechanisms of stress response.

THYROID GLAND

• Butterfly-shaped
• Located just inferior to the larynx (voice box).
• Composed of:
o Lateral lobes (right and left)
o Isthmus
• Pyramidal-shaped lobe (sometimes extends upward from
the isthmus)
• Wt: 30 g (1 oz).
• Highly vascularized - receives 80–120 mL of blood per • The thyroid gland is the only endocrine gland that stores
minute its secretory product in large quantities—normally about
a 100-day supply. Synthesis and secretion of T3 and T4
MICROSCOPIC FEATURES: occurs as follows:
• Thyroid follicles - spherical sacs; make up most of the
thyroid gland (1) IODINE TRAPPING
o Follicular cells – cells that line the wall of each • Thyroid follicular cells trap iodide ions (I) by actively
follicle; most of which extend to the lumen (internal transporting them from the blood into the cytosol.
space) of the follicle.
• As a result, the thyroid gland normally contains most of
§ INACTIVE = their shape is low cuboidal to
the iodide in the body.
squamous
§ ACTIVE (under TSH influence) = cuboidal to low
columnar
(2) SYNTHESIS OF THYROGLOBULIN
o Basement membrane - surrounds each follicle • While the follicular cells are trapping I, they are also
• The follicular cells produce two hormones: synthesizing thyroglobulin (TGB), a large glycoprotein
o Tetraiodothyronine or T4 (aka Thyroxine) packaged into secretory vesicles n that is produced in the
§ contains four atoms of iodine rough endoplasmic reticulum, modified in the Golgi
o Triiodothyronine or T3 complex, and packaged into secretory vesicles.
§ contains three atoms of iodine • The vesicles then undergo exocytosis, which releases
• T3 and T4 together are also known as thyroid hormones. TGB into the lumen of the follicle.
• Parafollicular cells or C cells lie between follicles. (3) OXIDATION OF IODIDE
Produces: • Some of the amino acids in TGB are tyrosines that will
o Calcitonin - helps regulate calcium homeostasis. become iodinated.
• Negatively charged iodide ions cannot bind to tyrosine
(an amino acid) until they undergo oxidation (removal of
electrons) to iodine: 2 I- à I2
• As the iodide ions are being oxidized, they pass through
the membrane into the lumen of the follicle.

(4) IODINATION OF TYROSINE.

• Iodine molecules (I2) react with tyrosines that are part of
thyroglobulin molecules.
o 1 iodine + Tyrosine = monoiodotyrosine (T1)
o 2 iodine + Tyrosine = diiodotyrosine (T2)

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 TRANS: Endocrine System

• TGB with attached iodine atoms, a sticky material that 3. Stimulate protein synthesis and increase the use of
accumulates and is stored in the lumen of the thyroid glucose and fatty acids for ATP production. They also
follicle, is termed colloid. increase lipolysis and enhance cholesterol excretion.

(5) COUPLING OF T1 AND T2. 4. Enhance some actions of the catecholamines

(norepinephrine and epinephrine) because they up-
• Two T2 molecules = T4
regulate beta (β) receptors.
• One T1 + one T2 = T3
o Hyperthyroidism à symptoms include increased
heart rate, more forceful heartbeats, and increased
• During the last step in the synthesis of thyroid hormone, blood pressure.
two T2 molecules join to form T4 or one T1 and one T2
join to form T3. 5. Together with human growth hormone and insulin, thyroid
hormones accelerate body growth, particularly the growth
(6) PINOCYTOSIS AND DIGESTION OF COLLOID. of the nervous and skeletal systems. Deficiency of thyroid
• Droplets of colloid reenter follicular cells by pinocytosis hormones during fetal development, infancy, or childhood
and merge with lysosomes. causes severe mental retardation and stunted bone
• Digestive enzymes in the lysosomes break down TGB, growth.
cleaving off molecules of T3 and T4.

(7)SECRETION OF THYROID HORMONES.

• T3 and T4 diffuse through the plasma membrane into
interstitial fluid and then into the blood.
• T4 - secreted in greater quantity
• T3 - several times more potent.

• Because T3 and T4 are lipid-soluble, they diffuse through

the plasma membrane into interstitial fluid and then into
the blood.
• T4 normally is secreted in greater quantity than T3, but
T3 is several times more potent.
• Moreover, after T4 enters a body cell, most of it is CONTROL AND SECRETION
converted to T3 by removal of one iodine.

(8) TRANSPORT IN THE BLOOD.

• More than 99% of both the T3 and the T4 combine with
transport proteins in the blood, mainly thyroxine-binding
globulin (TBG).

THYROID HORMONES ACTIONS

• Because most body cells have receptors for thyroid
hormones, T3 and T4 exert their effects throughout the
body
1. Increase basal metabolic rate (BMR).
- Thyroid hormones increase basal metabolic rate
(BMR), the rate of oxygen consumption under
standard or basal conditions (awake, at rest, and
fasting), by stimulating the use of cellular oxygen to
produce ATP.
- When the basal metabolic rate increases, cellular
metabolism of carbohydrates, lipids, and proteins
increases.
2. Stimulate synthesis of additional sodium-potassium
pumps (Na+/K+ ATPase)
o As cells produce and use more ATP, more heat is
given off, and body temperature rises. à calorigenic
effect
o An important role in the maintenance of normal body
temperature.

- A second major effect of thyroid hormones is to

stimulate synthesis of additional sodium-potassium
pumps (Na+/K+ ATPase), which use large amounts • Thyrotropin-releasing hormone (TRH) from the
of ATP to continually eject sodium ions (Na+) from hypothalamus and thyroid-stimulating hormone (TSH)
the cytosol into the extracellular fluid and potassium from the anterior pituitary stimulate synthesis and release
ions (K+) from the extracellular fluid into the cytosol. of thyroid hormones
- As cells produce and use more ATP, more heat is
given off, and body temperature rises. 1. Low blood levels of T3 and T4 or low metabolic rate
- This phenomenon is called the calorigenic effect. stimulate the hypothalamus to secrete TRH.
- In this way, thyroid hormones play an important role 2. TRH enters the hypophyseal portal veins and flows
in the maintenance of normal body temperature. to the anterior pituitary, where it stimulates
- Normal mammals can survive in freezing thyrotrophs to secrete TSH.
temperatures, but those whose thyroid glands have
been removed cannot.

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 TRANS: Endocrine System

3. TSH stimulates virtually all aspects of thyroid • PTH also acts on the kidneys.
follicular cell activity, including iodide trapping, • First, it slows the rate at which Ca2+ and Mg2+ are lost from
hormone synthesis and secretion, and growth of the blood into the urine.
follicular cells. • Second, it increases loss of HPO42- from blood into the
4. The thyroid follicular cells release T3 and T4 into the urine.
blood until the metabolic rate returns to normal. • Because more HPO42- is lost in the urine than is gained
5. An elevated level of T3 inhibits release of TRH and from the bones, PTH decreases blood HPO42- level and
TSH (negative feedback inhibition). increases blood Ca2+ and Mg2+ levels.
• A third effect of PTH on the kidneys is to promote
• Conditions that increase ATP demand—a cold formation of the hormone calcitriol, the active form of
environment, hypoglycemia, high altitude, and vitamin D.
pregnancy—also increase the secretion of the thyroid • Calcitriol, also known as 1, 25-dihydroxy vitamin D3,
hormones. increases the rate of Ca2+, Mg2+ , and HPO42- absorption
CALCITONIN from the gastrointestinal tract into the blood.
• Produced by the parafollicular cells of the thyroid gland • The blood calcium level directly controls the secretion of
• CT can decrease the level of calcium in the blood by both calcitonin and parathyroid hormone via negative
inhibiting the action of osteoclasts, the cells that break feedback loops that do not involve the pituitary gland.
down bone extra-cellular matrix.
• The secretion of CT is controlled by a negative feedback
system.
• When blood level is high:
o Calcitonin lowers the amount of blood calcium and
phosphates by inhibiting bone resorption (breakdown
of bone extracellular matrix) by osteoclasts and by
accelerating uptake of calcium and phosphates into
bone extracellular matrix.
• Calcitonin promotes calcium deposition in your bones.

PARATHYROID GLANDS

1. A higher-than-normal level of calcium ions (Ca2+) in the

blood stimulates parafollicular cells of the thyroid gland to
release more calcitonin.
2. Calcitonin inhibits the activity of osteoclasts, thereby
decreasing the blood Ca2+ level.
3. A lower-than-normal level of Ca2+ in the blood stimulates
• Small, round masses of tissue partially embedded in the
chief cells of the parathyroid gland to release more PTH.
posterior surface of the lateral lobes of the thyroid gland
4. PTH promotes resorption of bone extracellular matrix,
• Each has a mass of about 40 mg (0.04 g)
which releases Ca2+ into the blood and slows loss of Ca2+
• One superior and one inferior parathyroid gland are in the urine, raising the blood level of Ca2+.
attached to each lateral thyroid lobe for a total of four. 5. PTH also stimulates the kidneys to synthesize calcitriol,
the active form of vitamin D.
6) MICROSCOPIC FEATURES: 6. Calcitriol stimulates increased absorption of Ca2+ from
• Contains two kinds of epithelial cells: foods in the gastrointestinal tract, which helps increase
o Chief (principal) cells the blood level of Ca2+.
§ more numerous
§ produce parathyroid hormone (PTH), also called ADRENAL GLANDS
parathormone.
o Oxyphil cell – function is not known

Chief (principal) cells

• Adrenal (suprarenal) glands, one of which lies superior

to each kidney in the retroperitoneal space
Oxyphil cell • Flattened pyramidal shape

• In adults:
o 3–5 cm in height
o 2–3 cm in width
PARATHYROID HORMONE o less than 1 cm thick
• Parathyroid hormone is the major regulator of the levels o mass of 3.5–5 g
of calcium (Ca2+), magnesium (Mg2+), and phosphate • Only half its size at birth.
(HPO42-) ions in the blood. • During embryonic development, the adrenal glands
• The specific action of PTH is to increase the number and differentiate into TWO STRUCTURALLY AND
activity of osteoclasts. (somehow opposite to the calcitonin) FUNCTIONALLY DISTINCT REGIONS:
• The result is elevated bone resorption, which releases o Adrenal cortex
ionic calcium (Ca2+), and phosphates (HPO42-) into the § large, peripherally located;
blood. § 80– 90% of the gland

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 TRANS: Endocrine System

o Adrenal medulla • The RENIN-ANGIOTENSIN-ALDOSTERONE (RAA)

§ small, centrally located PATHWAY controls secretion of aldosterone

• A connective tissue capsule covers the gland.

• The adrenal glands, like the thyroid gland, are highly
vascularized.
• The adrenal cortex produces steroid hormones that are
essential for life.
• Complete loss of adrenocortical hormones leads to death
due to dehydration and electrolyte imbalances in a few
days to a week, unless hormone replacement therapy
begins promptly.
• The adrenal medulla produces three catecholamine
hormones—norepinephrine, epinephrine, and a small
amount of dopamine.

1. Stimuli that initiate the renin–angiotensin–aldosterone

pathway include dehydration, Na+ deficiency, or
hemorrhage.
2. These conditions cause a decrease in blood volume.
3. Decreased blood volume leads to decreased blood
pressure.
4. Lowered blood pressure stimulates certain cells of the
kidneys, called juxtaglomerular cells, to secrete the
enzyme renin.
5. The level of renin in the blood increases.
ADRENAL CORTEX 6. Renin converts angiotensinogen, a plasma protein
Subdivided into three zones: produced by the liver, into angiotensin I.
• ZONA GLOMERULOSA 7. Blood containing increased levels of angiotensin I
o Outer zone, just deep to the connective tissue capsule circulates in the body.
o Its cells, which are closely packed and arranged in 8. As blood flows through capillaries, particularly those of
spherical clusters and arched columns, secrete the lungs, the enzyme angiotensin-converting enzyme
hormones called mineralocorticoids (ACE) converts angiotensin I into the hormone
§ affect mineral homeostasis angiotensin II.
• ZONA FASICULATA 9. Blood level of angiotensin II increases.
o Widest of the three zones and consists of cells 10. Angiotensin II stimulates the adrenal cortex to secrete
arranged in long, straight columns. aldosterone.
o The cells of the zona fasciculata secrete mainly 11. Blood containing increased levels of aldosterone
glucocorticoids circulates to the kidneys.
§ affect glucose homeostasis. 12. In the kidneys, aldosterone increases reabsorption of Na+
• ZONA RETICULARIS and water so that less is lost in the urine. Aldosterone also
o The cells of the inner zone, the zona reticularis are stimulates the kidneys to increase secretion of K+ and H+
arranged in branching cords. into the urine.
o They synthesize small amounts of weak androgens 13. With increased water reabsorption by the kidneys, blood
§ steroid hormones that have masculinizing effects volume increases.
14. As blood volume increases, blood pressure increases to
Zona Glomerulosa normal.
15. Angiotensin II also stimulates contraction of smooth
muscle in the walls of arterioles. The resulting
Zona Fasiculata vasoconstriction of the arterioles increases blood
pressure and thus helps raise blood pressure to normal.
16. Besides angiotensin II, a second stimulator of
Zona Reticularis aldosterone secretion is an increase in the K+
concentration of blood (or interstitial fluid). A decrease in
the blood K+ level has the opposite effect.

ADRENAL CORTEX: GLUCOCORTICOIDS

ADRENAL CORTEX: MINERALOCORTICOIDS • regulate metabolism and resistance to stress,
• (ZONA FASICULATA)
• (ZONA GLOMERULOSA) • Cortisol (hydrocortisone)
• Aldosterone o most abundant; 95% of the glucocorticoid activity
o is the major mineralocorticoid • Corticosterone
o Regulates homeostasis of two mineral ions: • Cortisone
§ SODIUM (Na+)
§ POTASSIUM (K+)
• CONTROL AND SECRETION
o Helps adjust blood pressure and blood volume.
o Via a negative feedback system.
o Also promotes excretion of H in the urine; this removal
o Low blood levels of glucocorticoids, mainly cortisol,
of acids from the body can help prevent acidosis
stimulate hypothalamus to secrete
o 9blood pH below 7.35)
corticotropinreleasing hormone (CRH).

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 TRANS: Endocrine System

o CRH (together with a low level of cortisol) promotes § For this reason, glucocorticoids are prescribed for
the release of ACTH from the anterior pituitary. organ transplant recipients to retard tissue
o ACTH flows in the blood to the adrenal cortex, where rejection by the immune system.
it stimulates glucocorticoid secretion.
ADRENAL CORTEX: ANDROGENS
• The adrenal cortex secretes small amounts of weak
androgens in both males and females.
• Major androgen: Dehydroepiandrosterone (DHEA)
o MALES
§ Amount of androgens secreted by the adrenal
gland are usually so low and effects are
insignificant in MALES after puberty due to greater
quantity of TESTOSTERONE release by the
testes
o FEMALES
§ In FEMALES à promote libido (sex drive) and are
converted into ESTROGENS (feminizing sex
steroids) by other body tissues.
§ After menopause, when ovarian secretion of
estrogens ceases, all female estrogens come
from conversion of adrenal androgens.
• Also stimulate growth of axillary and pubic hair in boys
and girls and contribute to the prepubertal growth spurt.
• Although control of adrenal androgen secretion is not fully
understood, the main hormone that stimulates its
secretion is ACTH.
• GLUCOCORTICOID EFFECTS:
1. Protein breakdown. ADRENAL MEDULLA
à increase rate of protein breakdown • The inner region of the adrenal gland
(mainly in muscle fibers) à increase amino is a modified sympathetic ganglion of
acids into the bloodstream à amino acids the autonomic nervous system (ANS)
may be used by body cells for synthesis of • It develops from the same embryonic
new proteins or for ATP production. tissue as all other sympathetic
2. Glucose formation. ganglia, but its cells, which lack
à liver cells convert certain amino acids or axons, form clusters around large
lactic acid to glucose à (neurons and other blood vessels.
cells can use for…) ATP production à • Rather than releasing a
gluconeogenesis (derivation of glucose neurotransmitter, the cells of the
from other substance other that glycogen) adrenal medulla secrete hormones.
(conversion of a substance other than • Its hormone-producing cells are called chromaffin cells
glycogen or another monosaccharide into o Are innervated by sympathetic preganglionic neurons
glucose) of the ANS.
3. Lipolysis. o Because the ANS exerts direct control over the
à breakdown of triglycerides and release chromaffin cells, hormone release can occur very
of fatty acids from adipose tissue into the quickly.
blood.
4. Resistance to stress. • Two major hormones synthesized are:
§ Glucocorticoids work in many ways to provide o Epinephrine (Adrenaline) – secreted 80%
resistance to stress. o Norepinephrine (Noradrenalin) – 20%
• Unlike the hormones of the adrenal cortex, the hormones
à Additional glucose supplied by the liver cells of the adrenal medulla are not essential for life since they
provides tissues with a ready source of ATP to only intensify sympathetic responses in other parts of the
combat a range of stresses (exercise, fasting, body.
fright, temperature extremes, high altitude,
bleeding, infection, surgery, trauma, and disease). • In stressful situations (e.g. exercise) à impulses from the
à (Glucocorticoids make blood vessels more hypothalamus stimulate sympathetic preganglionic
sensitive to other hormones that…) cause neurons à stimulate the chromaffin cells to secrete
vasoconstriction à raise blood pressure epinephrine and norepinephrine à two hormones greatly
§ This effect would be an advantage in cases of augment the fight-or-flight response
severe blood loss, which causes blood pressure o Increasing heart rate and force of contraction à
to drop. increase the output of the heart à increases blood
pressure.
5. Anti-inflammatory effects. o They also increase blood flow to the heart, liver,
àThey inhibit white blood cells that skeletal muscles, and adipose tissue
participate in inflammatory responses à o Dilate airways to the lungs
also retard tissue repair à slow wound o Increase blood levels of glucose and fatty acids.
healing
§ Although high doses can cause severe mental
disturbances, glucocorticoids are very useful in
the treatment of chronic inflammatory disorders
such as rheumatoid arthritis.
6. Depression of immune responses.
à High doses depress immune responses.

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 1. Alpha or A cells – secrete
TRANS: Endocrine System
glucagon (increases
glucose)
PANCREATIC2.
ISLETS
Beta or B cells – secrete
• Pancreas is both an endocrine gland insulin
and an (lowers
exocrineglucose)
gland. 3. Delta or D cells – secrete
• A flattened organ that measures about somatostatin
12.5–15 cm (4.5– (inhibits both
insulin and glucagon)
6 in.) in length, the pancreas is located in the curve of the
duodenum, the first part of4.the Fsmall cells – secrete
intestine, and
pancreatic polypeptide
consists of a head, a body, and a tail.
(inhibits somatostatin)

• 99% of the cells of the pancreas are arranged in clusters

called acini à secretes digestive enzymes (exocrine REGULATION OF GLUCAGON AND INSULIN
function) (which flow into the gastrointestinal tract through SECRETION
a network of ducts. • The principal action of glucagon is to increase blood
• Scattered clusters of endocrine tissue are 1-2 million tiny glucose level when it falls below normal.
clusters of endocrine tissue called pancreatic islets or • Via negative feedback mechanism
islets of Langerhans • Glucagon – increase blood glucose level when it falls
• Abundant capillaries serve both the exocrine and below normal
endocrine portions of the pancreas. • Insulin – helps lower blood glucose level when it is too
high.
1. Low blood glucose level (hypoglycemia) stimulates
secretion of glucagon from alpha cells of the pancreatic
islets.
2. Glucagon acts on hepatocytes (liver cells) to accelerate
the conversion of glycogen into glucose (glycogenolysis)
and to promote formation of glucose from lactic acid and
certain amino acids (gluconeogenesis).
PANCREATIC ISLETS: CELL TYPES
3. As a result, hepatocytes release glucose into the blood
more rapidly, and blood glucose level rises.
7) FOUR TYPES OF HORMONESECRETING 4. If blood glucose continues to rise, high blood glucose
CELLS: level (hyperglycemia) inhibits release of glucagon
1. Alpha or A cells (negative feedback).
o 17% of pancreatic islet cells 5. High blood glucose (hyperglycemia) stimulates secretion
o secrete glucagon (increases glucose) of insulin by beta cells of the pancreatic islets.
2. Beta or B cells 6. Insulin acts on various cells in the body to accelerate
o 70% of pancreatic islet cells facilitated diffusion of glucose into cells; to speed
o secrete insulin (lowers glucose) conversion of glucose into glycogen (glycogenesis); to
3. Delta or D cells increase uptake of amino acids by cells and to increase
o 7% of pancreatic islet cells protein synthesis; to speed synthesis of fatty acids
o secrete somatostatin (identical to the growth (lipogenesis); to slow the conversion of glycogen to
hormone-inhibits both insulin and glucagon) glucose (glycogenolysis); and to slow the formation of
4. F cells glucose from lactic acid and amino acids
o remainder of pancreatic islet cells (gluconeogenesis).
o secrete pancreatic polypeptide (inhibits 7. As a result, blood glucose level falls.
somatostatin) 8. If blood glucose level drops below normal, low blood
glucose inhibits release of insulin (negative feedback)
• The interactions of the four pancreatic hormones are and stimulates release of glucagon.
complex and not completely understood.
• We do know that glucagon raises blood glucose level,
and insulin lowers it.
• Somatostatin acts in a paracrine manner to inhibit both
insulin and glucagon release from neighboring beta and
alpha cells.
• It may also act as a circulating hormone to slow
absorption of nutrients from the gastrointestinal tract.
• Pancreatic polypeptide inhibits somatostatin secretion,
gallbladder contraction, and secretion of digestive
enzymes by the pancreas.

EATIC ISLETS:
YPES
hormone-
s: A cells

A cells – secrete
(increases B cells

D cells F cell
cells – secrete
owers glucose)
D cells – secrete
atin (inhibits both PANANGHID SA DAAN SA GAHIMO ^-^ | 1D-MT 16
 TRANS: Endocrine System

OVARIES AND TESTES postganglionic neurons of the superior cervical ganglion,

which in turn stimulate the pinealocytes of the pineal
gland to secrete melatonin in a rhythmic pattern, with low
levels of melatonin secreted during the day and
significantly higher levels secreted at night.
• During sleep, plasma levels of melatonin increase tenfold
and then decline to a low level again before awakening.
• Small doses of melatonin given orally can induce sleep
• Gonads - organs that produce gametes and reset daily rhythms, which might benefit workers
o sperm = males; whose shifts alternate between daylight and nighttime
o oocytes = females hours.
• In addition to their reproductive function, the gonads • Melatonin also is a potent antioxidant that may provide
secrete hormones. some protection against damaging oxygen free radicals.

OVARIES OTHER ENDOCRINE TISSUES AND ORGANS

• Paired oval bodies located in the female pelvic cavity
GASTROINTESTINAL TRACT
• Produce several sterod hormones: two estrogens Gastrin Promotes secretion of gastric
(estradiol and estrone) and progesterone juice and increases movements
o (Along with FSH and LH from the anterior pituitary) of the stomach.
regulate menstrual cycle, maintain pregnancy, and
prepare the mammary glands for lactation
Glucose-dependent Stimulates release of insulin by
o Promote enlargement of the breasts and widening of
isulinotropic peptide pancreatic beta cells.
the hips at puberty
(GIP)
o Help maintain these female secondary sex
Secretin Stimulates secretion of pancreatic
characteristics.
juice and bile.
Cholecystokinin Stimulates secretion of pancreatic
• The ovaries also produce inhibin, a protein hormone that
(CCK) juice, regulates release of bile
inhibits secretion of follicle-stimulating hormone (FSH).
from the gallbladder, and brings
• During pregnancy, the ovaries and placenta produce a
about a feeling of fullness
peptide hormone called relaxin, which increases the
after eating.
flexibility of the pubic symphysis during pregnancy and
helps dilate the uterine cervix during labor and delivery.
PLACENTA
• These actions help ease the baby’s passage by enlarging
Human chorionic Stimulates the corpus luteum in
the birth canal.
gonadotropin (hCG) the ovary to continue the
TESTES production of estrogens and
• Oval glands that lie in the scrotum. progesterone to maintain
pregnancy.
• Main hormone produced and secreted: testosterone
Estrogens Maintain pregnancy and help
o stimulates descent of the testes before birth
o regulates production of sperm and progesterone prepare mammary glands to
secrete milk.
o stimulates the development and maintenance of male
secondary sex characteristics (beard growth and Human chorionic Stimulates the development of
deepening of the voice) somatomammotropin the mammary glands for
(hCS) lactation.
• The testes also produce inhibin, which inhibits secretion KIDNEYS
of FSH. Renin Part of a sequence of reactions
that raises blood pressure by
PINEAL GLAND bringing about vasoconstriction
and secretion of aldosterone.
Erythropoietin (EPO) Increases rate of red blood cell
formation.
Calcitriol* (active Aids in the absorption of dietary
form of vitamin D) calcium and phosphorus.
HEART
Atrial natriuretic Decreases blood pressure.
peptide (ANP)
ADIPOSE TISSUE
• A small endocrine gland attached to the roof of the third Leptin Suppresses appetite and may
ventricle of the brain at the midline increase the activity of FSH and
• Part of the epithalamus, it is positioned between the two LH.
superior colliculi
• A mass of 0.1–0.2 g STRESS RESPONSE
• Covered by a capsule formed by the pia mater. • It is impossible to remove all of the stress from our
• Consists of masses of neuroglia and secretory cells everyday lives.
called pinealocytes. • Eustress – prepares us to meet certain challenges and
• secretes the hormone MELATONIN (an amine hormone thus is helpful.
derived from seratonin) • Distress – harmful stress
o Contribute to the setting of the body’s biological clock, • Stressor – any stimulus that produces a stress response;
which is controlled by the suprachiasmatic nucleus of any disturbance of the human body, e.g.:
the hypothalamus o heat or cold
o As more melatonin is liberated during darkness à this o environmental poisons
hormone is thought to promotes sleepiness o toxins given off by bacteria
• In response to visual input from the eyes (retina), the o heavy bleeding from a wound or surgery
suprachiasmatic nucleus stimulates sympathetic o a strong emotional reaction
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• The responses to stressors may be pleasant or • A third hypothalamic releasing hormone, TRH, stimulates
unpleasant, and they vary among people and even within the anterior pituitary to secrete thyroid-stimulating
the same person at different times. hormone (TSH).
• TSH promotes secretion of thyroid hormones, which
• The body’s homeostatic mechanisms normally attempt to stimulate the increased use of glucose for ATP
COUNTERACT STRESS. production.
• When they are successful, the internal environment • The combined actions of hGH and TSH supply additional
remains within normal physiological limits. ATP for metabolically active cells throughout the body.
• If stress is extreme, unusual, or long lasting, the normal
mechanisms may not be enough. • The resistance stage helps the body continue fighting a
stressor long after the fight-or-flight response dissipates.
• The stress response or general adaptation syndrome • This is why your heart continues to pound for several
(GAS) minutes even after the stressor is removed.
o controlled mainly by the hypothalamus. • Generally, it is successful in seeing us through a stressful
o are a variety of stressful conditions or noxious agents episode, and our bodies then return to normal.
that elicit a similar sequence of bodily changes • Occasionally, however, the resistance stage fails to
o occurs in three stages: combat the stressor, and the body moves into the state of
(1) an initial fight-or-flight response exhaustion.
(2) a slower resistance reaction
(3) exhaustion EXHAUSTION
• The resources of the body may eventually become so
FIGHT-OR-FLIGHT RESPONSE depleted that they cannot sustain the resistance stage,
• The fight-or-flight response, initiated by nerve impulses and exhaustion ensues.
from the hypothalamus to the sympathetic division of the • Prolonged exposure to high levels of cortisol and other
autonomic nervous system (ANS), including the adrenal hormones involved in the resistance reaction causes
medulla, quickly mobilizes the body’s resources for wasting of muscle, suppression of the immune system,
immediate physical activity. ulceration of the gastrointestinal tract, and failure of
• It brings huge amounts of glucose and oxygen to the pancreatic beta cells.
organs that are most active in warding off danger: • In addition, pathological changes may occur because
o the brain, which must become highly alert; resistance reactions persist after the stressor has been
o the skeletal muscles, which may have to fight off an removed.
attacker or flee;
o and the heart, which must work vigorously to pump STRESS AND DISEASE
enough blood to the brain and muscles. • Although the exact role of stress in human diseases is not
• During the fight-or-flight response, nonessential body known, it is clear that stress can lead to particular
functions such as digestive, urinary, and reproductive diseases by temporarily inhibiting certain components of
activities are inhibited. the immune system.
• Reduction of blood flow to the kidneys promotes release • Stress-related disorders include gastritis, ulcerative
of renin, which sets into motion the renin–angiotensin– colitis, irritable bowel syndrome, hypertension, asthma,
aldosterone pathway. rheumatoid arthritis (RA), migraine headaches, anxiety,
• Aldosterone causes the kidneys to retain Na+, which and depression.
leads to water retention and elevated blood pressure. • People under stress are at a greater risk of developing
• Water retention also helps preserve body fluid volume in chronic disease or dying prematurely. Interleukin-1, a
the case of severe bleeding. substance secreted by macrophages of the immune
system, is an important link between stress and immunity.
THE RESISTANCE REACTION
• One action of interleukin-1 is to stimulate secretion of
• The second stage in the stress response is the resistance ACTH, which in turn stimulates the production of cortisol.
reaction.
• Not only does cortisol provide resistance to stress and
• Unlike the short-lived fight-or-flight response, which is inflammation, but it also suppresses further production of
initiated by nerve impulses from the hypothalamus, the interleukin-1.
resistance reaction is initiated in large part by • Thus, the immune system turns on the stress response,
hypothalamic releasing hormones and is a longer-lasting
and the resulting cortisol then turns off one immune
response.
system mediator.
• The hormones involved are: • This negative feedback system keeps the immune
o corticotropin-releasing hormone (CRH), response in check once it has accomplished its goal.
o growth hormone–releasing hormone (GHRH), and
• Because of this activity, cortisol and other glucocorticoids
o thyrotropin-releasing hormone (TRH).
are used as immunosuppressive drugs for organ
• CRH stimulates the anterior pituitary to secrete ACTH, transplant recipients.
which in turn stimulates the adrenal cortex to increase
release of cortisol.
• Cortisol then stimulates gluconeogenesis by liver cells,
breakdown of triglycerides into fatty acids (lipolysis), and REFERENCE
catabolism of proteins into amino acids.
• Tissues throughout the body can use the resulting Tortora – Principles of Anatomy and Physiology, 12th
glucose, fatty acids, and amino acids to produce ATP or edition Chapter 18 pp. 642 - 688
to repair damaged cells.
• Cortisol also reduces inflammation. Lecture and PPT of Danilo Gallardo Jr., MD, FM, FMCP

• A second hypothalamic releasing hormone, GHRH,

causes the anterior pituitary to secrete human growth
hormone (hGH).
• Acting via insulin-like growth factors, hGH stimulates
lipolysis and glycogenolysis, the breakdown of glycogen
to glucose, in the liver.
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