SCHOOL OF NURSING
ANATOMY AND PHYSIOLOGY: ENDOCRINE SYSTEM
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OUTLINE
Principles of Chemical Communication
Functions of the Endocrine System
Characteristics of the Endocrine System
Hormones
Control of Hormone Secretion
Hormone Receptors and Mechanisms of Action
Endocrine Glands and Their Hormones
Other Hormones
Effects of Aging on the Endocrine System
PRINCIPLES OF CHEMICAL COMMUNICATION
➢ The body has a remarkable capacity of maintaining
homeostasis despite having to coordinate the
activities of over 75 trillion cells.
➢ The principal means by which this coordination
occurs is through chemical messengers, some are
produced by the nervous system and others are
produced by the endocrine system.
➢ Chemical Messengers – allow cells to communicate
with each other to regulate body activities.
The study of endocrine system includes several of the
following categories:
1. Autocrine Chemical Messengers – stimulates the
cell that originally secreted it, and sometimes nearby
cells of the same type.
• Good examples of autocrine chemical
messengers are those secreted by white blood
cells during infection. Several types of white
blood cells can stimulate their own replication so
that the total number of white blood cells
increases rapidly.
2. Paracrine Chemical Messengers – the messengers
are local messengers. These messengers are
secreted by one cell type but affect neighboring cells
of a different type. Paracrine chemical messengers
do not travel in the general circulation; instead, they
are secreted into the extracellular fluid.
• Histamine – is an example of paracrine chemical
messenger and is released by certain white
blood cells during allergic reactions. Histamine
stimulated vasodilation in nearby blood vessels.
3. Neurotransmitters – are chemical messengers
secreted by neurons that activate an adjacent cell,
whether it is another neuron, a muscle cell, or
glandular cell. Neurotransmitters are secreted into a
synaptic cleft, rather than into the bloodstream.
4. Endocrine Chemical Messengers – are secreted into
the bloodstream by certain glands and cells, which
together constitute the endocrine system. These
chemical messengers affect cells that are distant
from their source.
FUNCTIONS OF ENDOCRINE SYSTEM
The main regulatory functions of the endocrine system
are the following:
1. Metabolism – the endocrine system regulates the
rate of metabolism, the sum of the chemical changes
that occur in tissues.
2. Control of Food Intake and Digestion – the
endocrine system regulates the level of satiety
(fullness) and the breakdown of food into individual
nutrients.
3. Tissue Development – the endocrine system
influences the development of tissues, such as those
of the nervous system.
4. Ion Regulation – the endocrine system regulates the
solute concentration of the food.
5. Water Balance – the endocrine system regulates the
water balance by controlling solutes in the blood.
6. Heart Rate and Blood Pressure Regulation – the
endocrine system helps regulate the heart and blood
pressure and helps prepare the body for physical
activity.
7. Control of Blood Glucose and other Nutrients – the
endocrine system regulates the levels of blood
glucose and other nutrients in the food.
8. Control of Reproductive Functions – the endocrine
system controls the development and functions of
the reproductive systems in males and females.
9. Uterine Contractions and Milk Release – the
endocrine system regulates uterine contractions
during delivery and stimulates milk release from the
breasts in lactating females.
10. Immune System Regulation – the endocrine system
helps control the production and functions of
immune cells.
CHARACTERISTICS OF THE ENDOCRINE SYSTEM
The endocrine system is composed of endocrine glands
and specialized endocrine cells located throughout the
body.
➢ Hormones – are chemical messengers secreted
minute amounts by endocrine glands and cells.
Hormones then travel through the general blood
circulation to target tissues or effectors.
basis of a hormone’s metabolism, its transport in the
blood, its interaction with its target, and its removal
from the body, is dependent on the hormone’s
chemical name.
➢ Within the two chemical categories, hormones can
be subdivided into groups based on their chemical
structures.
• Steroid Hormones – are those derived from
cholesterol.
• Thyroid Hormones – are derived from the amino
acid tyrosine.
• Other hormones are categorized as amino acid
derivatives, peptides, or proteins.
LIPID-SOLUBLE HORMONES
Lipid-soluble hormones are nonpolar, and include
steroid hormones, thyroid hormones, and fatty acid
derivative hormones, such as certain eicosanoids.
The term endocrine, derived from the Greek word endo,
meaning within, and krino, means to secrete,
appropriately describes this system.
➢ Endocrine glands are not to be confused with
exocrine glands. Exocrine glands have ducts that
carry their secretions to the outside of the body, or
into hollow organs, such as the stomach or
intestines.
• Examples of exocrine secretion are saliva, sweat,
breast milk, and digestive enzymes.
➢ Endocrinology – study of endocrine system.
HORMONES
The word hormone is derived from the Greek word
hormone, which means to set into motion. Hormones
regulate almost every physiological process in our body.
CHEMICAL NATURE OF HORMONES
➢ Hormones fit into one of two chemical categories:
1. Lipid-soluble hormones
2. Water-soluble hormones
➢ A distinction based on their chemical composition,
which influences their chemical behavior. The entire
➢ Transport of Lipid-Soluble Hormones – lipid-soluble
hormones are small molecules and are insoluble in
water-based fluids, such as the plasma of blood.
• For these reasons, lipid-soluble hormones travel
in the bloodstream attached to binding proteins.
• Binding proteins transport and protect
hormones.
• Lipid-soluble hormones are degraded slowly and
are not rapidly eliminated from the circulation.
• The life span of lipid-soluble hormones ranges
from a few days to as long as several weeks.
• Without the binding proteins, the lipid-soluble
hormones would quickly diffuse out capillaries
and be degraded by enzymes of the liver and
lungs or be removed from the body by the
kidneys.
• Circulating hydrolytic enzymes can also
metabolize free lipid-soluble hormones.
• The breakdown products are then excreted in
the urine or the bile.
WATER-SOLUBLE HORMONES
Water-soluble hormones are polar molecules; they
include protein hormones, peptide hormones, and most
amino acid derivative hormones.
➢ Transport of Water-Soluble Hormones – because
water-soluble hormones can dissolve in blood, many
circulate as free hormones, meaning that most of
them dissolve directly into the blood and are
delivered to their target tissue without attaching to
a binding protein.
• Because many water-soluble hormones are
quite large, they do not readily diffuse through
the walls of all capillaries; therefore, they tend to
diffuse from the blood into tissue spaces more
slowly.
• Organs regulated by some protein hormones
have very porous, or fenestrated, capillaries to
aid in delivery of these hormones to individual
cells.
• On the other hand, other water-soluble
hormones are quite small. To avoid being filtered
out of the blood, these hormones require
attachment to a binding protein.
• Water-soluble hormones have relatively short
half-lives because they are rapidly degraded by
enzymes, called proteases, within the
bloodstream.
• The kidneys then filter the hormone breakdown
products from the blood. Hormone target cells
also destroy water-soluble hormones. Some
target cells take up the hormone through
endocytosis, thus terminating their effect.
• Once the hormones are inside the target cell,
lysosomal enzymes degrade them. Often, the
target cell recycles the amino acids of peptide
and protein hormones and uses them to
synthesize new proteins.
• Hormones with short half-lives normally have
concentrations that change rapidly within the
blood and tend to regulate activities that have a
rapid onset and short duration.
• However, some water-soluble hormones are
more stable in the blood than others. In many
instances, protein and peptide hormones have a
carbohydrate attached to them, or their terminal
ends are modified.
• These modifications protect them from protease
activity to a greater extent than water-soluble
hormones lacking such modification. In addition,
some water-soluble hormones also attach to
binding proteins and therefore circulate in the
blood longer than free water-soluble hormones.
CONTROL OF HORMONE SECRETION
Three types of stimuli regulate hormone release:
1. Humoral Stimuli
2. Neural Stimuli
3. Hormonal Stimuli
No matter what stimulus releases the hormone,
however, the blood level of most hormones fluctuates
within a homeostatic range through negative-feedback
mechanisms. In a few-instances, positive-feedback
systems also regulate blood hormone levels.
STIMULATION OF HORMONE RELEASE
➢ Control by Humoral Stimuli – blood-borne chemical
can directly stimulate the release of some hormones.
These chemicals are referred to as humoral stimuli
because they circulate in blood, and the word
humoral refers to body fluids, including blood.
• These hormones are sensitive to the blood levels
of a particular substance, such as glucose,
calcium, or sodium.
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•
•
When the blood level of a particular chemical
changes (calcium), the hormone PTH is released
in response to the chemical’s concentration.
Another example, if a runner has just finished a
long race during hot weather, he may not
produce urine for up to 12 hours after the race
because his elevated concentration of blood
solutes stimulates the release of a waterconservation hormone called antidiuretic
hormone (ADH).
Similarly, elevated blood glucose levels directly
stimulate insulin secretion by the pancreas, and
elevated blood potassium levels directly
stimulate aldosterone release by the adrenal
cortex.
➢ Control by Neural Stimuli – the second type of
hormone regulation involves neural stimuli of
endocrine glands.
• Following action potentials, neurons release a
neurotransmitter into the synapse with the cells
that produce the hormone.
• In some cases, the neurotransmitter stimulates
the cells to increase hormone secretion.
•
•
•
For example, the sympathetic nervous system
stimulates the secretion of epinephrine and
norepinephrine from the adrenal gland during
exercise.
• Epinephrine and norepinephrine increase heart
rate and, in turn, increase blood flow through
the exercising muscles. When the exercise stops,
the neural stimulation declines and the secretion
of epinephrine and norepinephrine decreases.
• Some neurons secrete chemical messengers
directly into the blood when they are stimulated,
making these chemical messengers hormones,
which are called neuropeptides.
• Specialized neuropeptides stimulate hormone
secretion from other endocrine cells and are
called releasing hormones, a term usually
reserved for hormones from the hypothalamus.
➢ Control by Hormonal Stimuli – the third type of
regulation uses hormonal stimuli. It occurs when a
hormone is secreted that, in turn, stimulates the
secretion of other hormones.
• The most common examples are hormones from
the anterior pituitary gland, called tropic
hormones.
•
Tropic hormones are hormones that stimulate
secretion of another hormone. These hormones
are part of a complex process in which a
releasing hormone from the hypothalamus
stimulates the release of a tropic hormone from
the anterior pituitary gland.
The anterior pituitary tropic hormone then
travels to another endocrine gland and
stimulates the release of its hormone.
For example, hormones from the hypothalamus
and anterior pituitary regulate the secretion of
thyroid hormones from the thyroid gland.
INHIBITION OF HORMONE RELEASE
Stimulating hormone secretion is important but
inhibiting hormone release is also important. This
process involves the same three types of stimuli:
humoral, neural, and hormonal.
➢ Inhibition of Hormone Release by Humoral Stimuli
• Often when a hormone’s release is sensitive to
the presence of a humoral stimulus, there exists
a companion hormone whose release is
inhibited by the same humoral stimulus.
• Usually, the companion hormone’s effects
oppose those of the secreted hormone and
counter interact the secreted hormone’s action.
• For example, to raise blood pressure, the adrenal
cortex secretes the hormone atrial natriuretic
peptide (ANP), which lowers blood pressure.
• Therefore, aldosterone and ANP work together
to maintain homeostasis of blood pressure.
➢ Inhibition of Hormone Release by Neural Stimuli
• Neurons inhibit targets just as often as they
stimulate targets. If the neurotransmitter is
inhibitory, the target endocrine gland does not
secrete its hormone.
➢ Inhibition of Hormone Release by Hormonal Stimuli
• Some hormones prevent the secretion of other
hormones, which is a common mode of hormone
regulation.
• For example, hormones from the hypothalamus
that prevent the secretion of tropic hormones
from the anterior pituitary gland are called
inhibiting hormones.
• Thyroid hormones can control their own blood
levels by inhibiting their anterior pituitary tropic
hormone.
• Without the original stimulus, less thyroid
hormone is released.
REGULATION OF HORMONE LEVELS IN THE BLOOD
Two major mechanisms maintain hormone levels in
blood within a homeostatic range:
1. Negative Feedback – most hormones are regulated
by a negative-feedback mechanisms, whereby the
hormone’s secretion is inhibited by the hormone
itself once blood levels have reached a certain point
and there is adequate hormone to activate the target
cell.
• The hormone may inhibit the action of other,
stimulatory hormones prevent the secretion of
the hormone in question, thus it is a self-limiting
system.
• Example, thyroid hormones inhibit the secretion
of their releasing hormone from the
hypothalamus and their tropic hormone from
the anterior pituitary.
2. Positive Feedback – some hormones, when
stimulated by a tropic hormone, promote the
synthesis and secretion of the tropic hormone in
addition to stimulating their target cell.
• In turn, this stimulates further secretion of the
original hormone. Thus, it is a self-propagating
system.
• Example, prolonged estrogen stimulation
promotes a release of the anterior pituitary
hormone responsible for stimulating ovulation.
HORMONE RECEPTORS AND MECHANISMS OF ACTION
➢ Receptors – hormones exert their actions by binding
to proteins.
➢ Receptor Site – is a portion of each receptor
molecule where a hormone binds. The shape and
chemical characteristics of each receptor site allow
only a specific type of hormone to bind to it.
➢ Specificity – the tendency for each type of hormones
to bind to one type of receptor, and not to others.
CLASSES OF RECEPTORS
The lipid-soluble and water-soluble hormones bind to
their own classes of receptors.
1. Lipid-soluble hormones bind to nuclear receptors –
lipid-soluble hormones tend to be relatively small
and are all nonpolar. Lipid-soluble hormones diffuse
through the cell membrane and bind to nuclear
receptors, which are mostly found in the cell nucleus.
2. Water-soluble hormones bind to membrane-bound
receptors – water-soluble hormones are polar
molecules and cannot pass through the cell
membrane. Instead, they interact with membranebound receptors, which are proteins that extend
across the cell membrane, with their hormonebinding sites exposed on the cell membrane’s outer
surface.
ACTION OF NUCLEAR RECEPTORS
Primarily, lipid-soluble hormones stimulate protein
synthesis. After lipid-soluble hormones diffuse across the
cell membrane and bind to their receptors, the hormonereceptor complex binds to DNA to produce new proteins.
➢ Hormone-Response Elements – are the receptors
that bind to DNA have a fingerlike projection that
recognize and bind to specific nucleotide sequences
in the DNA.
➢ The combination of the hormone and its receptor
forms a transcription factor because, when the
hormone-receptor complex binds to the hormoneresponse element, it regulates the transcription of
specific messenger ribonucleic acid (mRNA).
M-BOUND RECEPTORS AND SIGNAL AMPLIFICATION
Membrane bound receptors are examples of membrane
proteins. These receptors activate responses in two
ways:
1. Some receptors alter the activity of G proteins at the
inner surface of the cell membrane.
2. Other receptors directly alter the activity of
intracellular enzymes.
➢ Activation of G proteins, or intracellular enzymes,
elicit specific responses in cells, including the
production of molecules called second messengers.
➢ In some cases, this coordinated set of events is
referred to as a second-messenger system.
• For example, cyclic adenosine monophosphate
(cAMP) is a common second messenger
produced when a ligand binds to its receptor.
Rather than the ligand (first messenger) entering
the cell to directly activate a cellular process, the
ligand stimulates cAMP production.
• It is a cAMP that then stimulates the cellular
process. This mechanism is usually employed by
water-soluble hormones that are unable to cross
the target cell’s membrane.
• It has also been demonstrated that some lipidsoluble hormones activate second messenger
systems, which are consistent with actions via
membrane-bound receptors.
➢ Membrane-Bound Receptors That Activate G
Proteins – many membrane-bound receptors
produce responses through the action of G proteins.
G proteins consist of three subunits, from largest to
smallest, they are called:
1. Alpha
2. Beta
3. Gamma
➢ The G proteins are so named because one of the
subunits binds to guanine nucleotides. In the inactive
state, a guanine diphosphate (GDP) molecule is
bound to the alpha subunit of each G protein. In the
active state, guanine triphosphate (GTP) is bound to
the alpha subunit.
➢ G Proteins That Interact with Adenylate Cyclase –
activated alpha subunits of G proteins can alter the
activity of enzymes inside the cell.
• For example, activated a subunits can influence
the rate of cAMP formation by activating or
inhibiting adenylate cyclase, an enzyme that
coverts ATP to cAMP.
• Protein Kinases – are enzymes that, in turn,
regulate the activity of other enzymes.
Depending on the other enzyme, protein kinases
can increase or decrease its activity. The amount
of time cAMP is present to produce a response
in a cell is limited.
• Phosphodiesterase – an enzyme in cytoplasm, it
breaks down cAMP to AMP. Once cAMP levels
drop, the enzymes in the cell are no longer
stimulated.
➢ Signal Amplification – nuclear receptors work by
activating protein synthesis, which for some
hormones can take several hours. However,
hormones that stimulate the synthesis of second
messengers can produce an almost instantaneous
response because the second messenger influences
existing enzymes.
• Additionally, each receptor produces thousands
of second messengers, leading to a cascade
effect and ultimately amplification of the
hormonal signal.
ENDOCRINE GLANDS AND THEIR HORMONES
The endocrine system consists of ductless glands that
secrete hormones into the interstitial fluid. The
hormones then enter the blood. The organs in the body
with the richest blood supply are endocrine glands, such
as the adrenal gland and thyroid gland.
membrane bound receptors involved with regulating
anterior pituitary hormone secretion.
➢ Hypothalamic-Pituitary Portal Systems – are the
capillary beds and veins that transport the releasing
and inhibiting hormones.
➢ Hypothalamic Neuropeptides – functions as either
releasing hormones or inhibiting hormones.
PITUITARY GLAND
DIRECT INNERVATION OF THE POSTERIOR PITUITARY
➢ Pituitary Gland – is also called hypophysis, it is a
small gland about the size of a pea. It rests in a
depression of the sphenoid bone inferior to the
hypothalamus of the brain. The pituitary gland is
divided into two parts:
• Anterior Pituitary – is made up of epithelial cells
derived from embryonic oral cavity.
• Posterior Pituitary – is an extension of the brain
and is composed of nerve cells.
➢ Hormones in the pituitary gland control the
functions of many other glands in the body, such as:
• Ovaries
• Testes
• Thyroid Gland
• Adrenal Cortex
➢ The pituitary gland also secretes hormones that
influence growth, kidney function, birth, and milk
production by the mammary glands.
➢ Historically, the pituitary gland was known as the
body’s master gland because it controls the function
of so many other glands.
➢ Hypothalamus controls the pituitary glands in two
ways:
1. Hormonal Control
2. Direct Innervation
The posterior pituitary is a storage location for two
hormones synthesized by a special neuron in the
hypothalamus.
➢ Stimulation of neurons within the hypothalamus
controls the secretion of the posterior pituitary
hormones.
➢ Within the hypothalamus and pituitary, the nervous
and endocrine systems are closely interrelated.
➢ Emotions such as joy and anger, as well as chronic
stress, influence the endocrine system through
hypothalamus. Conversely, system can influence the
functions of the hypothalamus and other parts of the
brain.
HORMONAL CONTROL OF THE ANTERIOR PITUITARY
The anterior pituitary gland synthesizes hormones,
whose secretion is under the control of the
hypothalamus.
➢ Neurons
of
the
hypothalamus
produce
neuropeptides and secrete them into a capillary bed
in the hypothalamus.
➢ The neuropeptide is then transported through veins
to a second capillary bed in the anterior pituitary.
➢ Once the neuropeptides arrive at the anterior
pituitary gland, they leave the blood and bind to
HORMONES OF THE ANTERIOR PITUITARY
➢ Growth Hormone (GH) – stimulates growth of
bones, muscles, and other organs by increasing gene
expression. It also resists protein breakdown during
periods of food deprivation and favors lipid
breakdown.
• Pituitary Dwarf – is a condition of young person
that suffers from deficiency of growth hormone
remains small, although normally proportioned.
This condition can be treated by administering
growth hormones.
• Giantism – is a condition when a person
becomes abnormally tall. If excess growth
hormones are present before bones finish
growing in length, exaggerated bone growth
occurs.
• Acromegaly – a condition if excess hormone is
secreted after growth in bone length is
complete, growth continues in bone diameter
only. As a result, the facial features and hands
become abnormally large.
➢ The secretion of growth hormone is controlled by
two hormones from the hypothalamus. A releasing
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➢
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hormone stimulates growth hormone secretion, and
an inhibiting hormone inhibits its secretion.
Insulin-like Growth Factors (IGFs) – or
somatomedins, are part of the effect of growth
hormone is influenced by a group of protein
hormones.
Thyroid-stimulating Hormone (TSH) – binds to
membrane bound receptors on cells of the thyroid
gland and stimulates the secretion of thyroid
hormone. TSH can also stimulate the growth of the
thyroid gland.
Adrenocorticotropic Hormone (ACTH) – binds to
membrane–bound receptors on adrenal cortex cells.
ACTH increases the secretion of a hormone from the
adrenal cortex called cortisol, also called
hydrocortisone.
Gonadotropins – bind to membrane-bound
receptors on the cells of the gonads. The
gonadotropins regulate the growth, development,
and functions of the gonads.
• Luteinizing Hormone – in females, it stimulates
ovulation, promotes the secretion of
reproductive hormones such as estrogen and
progesterone. In males, it stimulates interstitial
cells of the testes to secrete the reproductive
hormone testosterone, and thus called
interstitial cell-stimulating hormone (ICSH).
Follicle-Stimulating Hormone (FSH) – stimulates the
development of follicles in the ovaries and sperm
cells in the testes. Without the LH and FSH, the
ovaries and testes decrease in size, no longer
produce oocytes or sperm cells, and no longer
secrete hormones.
Prolactin – binds to membrane-bound receptors in
cells of the breast, where it helps promote
development of the breast during pregnancy and
stimulates the production of milk following the
pregnancy.
Melanocyte-Stimulating Hormone (MSH) – binds to
membrane–bound receptors on melanocytes and
causes them to synthesize melanin. The structure of
MSH is similar to that of ACTH, and over-secretion of
either hormones causes the skin to darken.
HORMONES OF THE POSTERIOR PITUITARY
➢ Antidiuretic Hormone (ADH) – binds to membranebound receptors and increases water reabsorption
by kidney tubules. This results in less water lost as
urine. ADH can also cause blood vessels to constrict
when released in large amounts. ADH is also called
vasopressin. A lack of ADH secretion causes diabetes
insipidus, which is the production of a large amount
of dilute urine.
➢ Oxytocin – binds to membrane-bound receptors and
causes contraction of the smooth muscle cells of the
uterus as well as milk letdown from the breasts in
lactating women. Commercial preparations of
oxytocin, known as Pitocin are given under certain
conditions to assist in childbirth and to constrict
uterine blood vessels following child birth.
THYROID GLAND
The thyroid gland is made up of two lobes connected by
a narrow band called the isthmus. The lobes are located
on each side of the trachea, just inferior to the larynx.
The thyroid is one of the largest endocrine glands. It
appears redder than the surrounding tissues because it
is highly vascular. It is surrounded by a connective tissue
capsule.
➢ Thyroid Hormones – secreted by thyroid gland,
which bind to nuclear receptors in cells and regulate
the rate of metabolism in the body.
➢ Thyroid Follicles – where thyroid hormones
synthesized and stored within. These are small
spheres with walls composed of simple cuboidal
epithelium. Each thyroid follicle is filled with the
thyroglobulin, to which thyroid hormones are
attached.
➢ TSH-Releasing Hormone (TRH) – secreted by
hypothalamus, TRH travels to the anterior pituitary
to stimulate the secretion of thyroid-stimulating
hormone. In turn, TSH stimulates the secretion of
thyroid hormones from the thyroid gland.
• The thyroid hormones have negative feedback
effect on the hypothalamus and pituitary, so that
increasing levels of thyroid hormones inhibit the
secretion of TRH from the hypothalamus and
inhibit TSH secretion from the anterior pituitary
gland.
• A loss of negative feedback will result in excess
TSH, which causes the thyroid gland to enlarge,
a condition called goiter.
• Hypothyroidism – a lack of thyroid hormone.
• In infants, hypothyroidism can result in
cretinism, which characterized by mental
retardation, and abnormally formed skeletal
muscles.
• Hypothyroidism can cause myxedema, which is
the accumulation of fluid and other molecules in
the subcutaneous tissue of the skin.
• Hyperthyroidism – too much of thyroid
hormone, cases an increased metabolic rate,
extreme nervousness, and chronic fatigue.
• Graves Disease – is an autoimmune disease that
causes hyperthyroidism. This disease occurs
when the immune system produces abnormal
proteins that are similar in structure and
function to TSH, which overstimulates the
thyroid gland.
- Exophthalmia – a condition that
accompanied graves diseases, which is a
bulging of eyes.
➢ A thyroid gland requires iodine to synthesize two
separate thyroid hormones. Without iodine, thyroid
hormones are neither produced or secreted.
• Thyroxine – or tetraiodothyronine, which
contains 4 iodine atoms and is abbreviated T4.
• Triiodothyronine – contains 3 iodine atoms and
is abbreviated T3.
➢ Calcitonin – are the parafollicular cells of the thyroid
gland. It is secreted if the blood concentration of
Ca2+ becomes too high. Calcitonin lowers blood
Ca2+ levels to their normal range.
PARATHYROID GLANDS
Four tiny parathyroid glands are embedded in the
posterior wall of the thyroid gland.
➢ Parathyroid Hormone (PTH) – secreted by
parathyroid gland, which is essential for the
regulation of blood calcium levels. In fact, PTH is
more important than the calcitonin in regulating
blood Ca2+ levels. PTH has many effects:
1. PTH increases active vitamin D formation
through effects on membrane-bound receptors
of renal tubule cells in the kidneys.
2. PTH secretion increases blood Ca2+ levels. PTH
binds to receptors on osteoblasts. In turn,
osteoblasts secrete substances that stimulate
osteoclasts to reabsorb bone.
3. PTH decreases loss of Ca2+ in the urine.
➢ Hyperparathyroidism – an abnormally high rate of
PTH secretion.
➢ Hypoparathyroidism – an abnormally low rate of
PTH secretion.
ADRENAL GLANDS
The adrenal glands are two small glands located superior
to each kidney. Each adrenal gland has an inner part:
1. Adrenal Medulla
2. Adrenal Cortex
The adrenal medulla and the adrenal cortex function as
separate endocrine glands.
ADRENAL MEDULLA
➢ Epinephrine – or adrenaline, is the principal
hormone released from adrenal medulla. Small
amounts of norepinephrine are also released.
• Epinephrine and Norepinephrine – are called
the fight or flight hormones. They prepare the
body for intense physical activity. The major
effects of the hormones released from the
adrenal medulla are:
1. Release of stored energy sources to support
increased physical activity.
2. Increased heart rate, which raises blood
pressure.
3. Increased smooth muscle contraction in
internal organ and skin blood vessels (called
vasoconstriction), which also raises blood
pressure.
4. Increased blood flow to skeletal muscle.
5. Increased metabolic rate of several tissues,
especially skeletal muscle, cardiac muscle,
and nervous tissue.
Response to hormones from the adrenal medulla
reinforce the effect of the sympathetic division of the
autonomic nervous system. The adrenal medulla and the
sympathetic division prepares the body for physical
activity to produce fight-flight response and many other
responses to stress.
ADRENAL CORTEX
The adrenal cortex secretes three classes of steroid
hormones:
1. Mineralocoticoids
2. Glucocorticoids
3. Androgens
The molecules of all three classes of steroid hormones
enter their target cells and bind to nuclear receptor
molecules.
➢ Androgens – the third class of hormones, secreted
by the inner layer of the adrenal cortex, it stimulates
the development of male secondary sex
characteristics.
• Small amounts of androgens are secreted from
the adrenal cortex in both males and females.
• In adult males, most androgens are secreted by
testes.
• In adult females, the adrenal androgens
influence the female sex drive.
PANCREAS, INSULIN AND DIABETES
➢ Mineralocorticoids – first class of hormones
secreted by the outer layer of the adrenal cortex, it
helps regulate blood volume and blood vessels of K+
and Na+.
• Aldosterone is the major hormone of this class,
it primarily binds to receptor molecules in the
kidney, but it also affects the intestine, sweat
glands, and salivary glands.
• Renin – low blood pressure causes the release of
protein molecule called renin, it acts as an
enzyme, causes a blood protein called
angiotensinogen, to be converted to
angiotensin I. Then a protein called angiotensinconverting enzyme causes angiotensin I to be
converted to angiotensin II. Angiotensin
II causes smooth muscle in blood vessels to
constrict, and acts on the adrenal cortex to
increase aldosterone secretion.
➢ Glucocorticoids – the second class of hormones,
secreted by the middle layer of the adrenal cortex, it
help regulate blood nutrient levels.
• Cortisol – major corticoid hormone, which
increases the breakdown of proteins and lipids
and increases their conversion to forms of
energy the body can use. Cortisol also causes
proteins to be broken down to amino acids,
which are then released into the blood.
The endocrine part of the pancreas consists of pancreatic
islets, which are dispersed through-out the exocrine
portion of the pancreas. The islets consist of three cell
types:
1. Alpha Cells – secrete glucagon.
2. Beta Cells – secrete insulin.
3. Delta Cells – secrete somatostatin.
These three hormones regulate the blood vessels of
nutrients, especially glucose.
➢ A below normal blood glucose level causes the
nervous system to malfunction because glucose is
the nervous system’s main source of energy.
➢ When blood glucose decreases, other tissues rapidly
break down lipids and proteins to provide an
alternative energy source.
➢ As lipids are broken down, the liver converts some of
the fatty acids to acidic ketones, which are released
into the blood.
➢ When blood glucose levels are very low, the
breakdown of lipids can cause the release of enough
fatty acids and ketones to reduce the pH of the body
fluids below normal, a condition called acidosis.
➢ Elevated blood glucose levels stimulate beta cells to
secrete insulin.
➢ The decrease in insulin levels allows blood glucose to
be conserved to provide the brain with adequate
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glucose and to allow other tissues to metabolize fatty
acids and glycogen stored in the cells.
The major target tissues for insulin are the liver,
adipose tissue, muscles, and the area of the
hypothalamus that controls appetite, called the
satiety center.
Diabetes Mellitus – is the body’s inability to regulate
blood glucose levels within the normal range. There
are two types of diabetes mellitus:
1. Type 1 – occurs when too little insulin is secreted
from the pancreas.
• In type 1 diabetes mellitus, tissues cannot
take up glucose effectively, causing blood
glucose levels to become very high, a
condition called hyperglycmia.
2. Type 2 – diabetes mellitus is caused by either too
few insulin receptors on target cells or defective
receptors on target cells. The defective receptors
do not respond normally to insulin.
Glucagon – is released from the alpha cells when
blood glucose levels are low. Glucagon binds to
membrane-bound receptors primarily the liver,
causing the glycogen stored in the liver to be
converted to glucose.
Somatostatin – is released by the delta cells in
response to food intake. Somatostatin inhibits the
secretion of insulin and glucagon and inhibits gastric
tract activity.
TESTES AND OVARIES
The testes of the male and the ovaries of the female
secrete reproductive hormones, in addition to producing
sperm cells or oocytes, respectively.
➢ Testosterone – main reproductive hormone for
males, which is secreted by the testes. It is
responsible for the growth and development of the
male reproductive structures, muscle enlargement,
the growth of body hair, voice changes, and the male
sexual drive.
➢ Estrogen and Progesterone – two main classes of
reproductive hormones for females. Together, these
hormones contribute to the development and
function of female reproductive structures and other
female sexual characteristics.
THYMUS
The thymus lies in the upper part of the thoracic cavity.
It is important in the function of the immune system.
➢ Thymosin – hormones secreted by thymus, which
aids the development of white blood cells called T
cells. T cells help protect the body against infection
by foreign organisms. If an infant is born without a
thymus, the immune system does not develop
normally, and body is less capable of fighting
infections.
PINEAL GLAND
The pineal gland is a small, pinecone-shaped structure
located superior and posterior to the thalamus of the
brain.
➢ Melatonin – a hormone produced by pineal gland.
Melatonin is thought to inhibit the reproductive
hypothalamic-releasing hormone, gonadotropinreleasing hormone. Animal studies have
demonstrated that the amount of available light
controls the rate of melatonin secretion.
OTHER HORMONES
Cells in the lining of the stomach and small intestine
secrete hormones that stimulates the production of
digestive juices from the stomach, pancreas, and liver.
➢ Prostaglandins – are widely distributed in tissues of
the body, where they function as intercellular
signals. It is also responsible for inflammation.
Prostaglandins produced by platelets appear to be
necessary for normal blood clotting.
➢ Atrial Natriuretic Hormone (ANH or ANP) – secreted
by right atrium of the heart, in response to elevated
blood pressure. ANH inhibits Na+ reabsorption in the
kidneys. This causes more urine to be produced,
reducing blood volume. Lowered blood volume
lowers blood pressure.
➢ Erythropoietin – secreted by the kidneys, in
response to reduced oxygen levels in the kidney. It
acts on bone marrow to increase the production of
red blood cells.
➢ In pregnant women, the placenta is an important
source of hormones that maintain pregnancy and
stimulate breast development. These hormones are
estrogen, progesterone, and human chorionic
gonadotropin, which is similar in structure and
function to LH.
EFFECTS OF AGING ON THE ENDOCRINE SYSTEM
➢ Age related changes to the endocrine system include
a gradual decrease in the secretion of some, but not
all, endocrine glands.
➢ Some of the decreases in secretion may be due to the
fact that older people commonly engage in less
physical activity.
➢ GH secretion decreases as people age. However,
regular exercise offsets this decline. Older people
who do not exercise have significantly lower GH
levels than older people who exercise regularly.
➢ Decreasing GH levels may explain the gradual
decrease in bone and muscle mass and the increase
in adipose tissue seen in many older people.
➢ So far, administering GH to slow or prevent the
consequences of aging has not been found to be
effective, and unwanted side effects are possible.
➢ A decrease in melatonin secretion may influence
age-related changes in sleep patterns, as well as the
decreased secretion of some hormones, such as GH
and testosterone.
➢ The secretion of thyroid hormones decreases slightly
with age. Age related damage to the thyroid gland by
the immune system can occur.
➢ Approximately 10% of elderly women experience
some reduction in thyroid hormone secretion, this
tendency is less common in men.
➢ The kidneys of the elderly secretes less renin,
reducing the ability to respond to decrease in blood
pressure.
➢ Reproductive hormone secretion gradually declines
in elderly men, and women experience menopause.
➢ Secretion of thymosin from the thymus decreases
with age. Fewer functional lymphocytes are
produced, and the immune system becomes less
effective in protecting the body against infections
and cancer.
➢ Parathyroid hormone secretion increases to
maintain blood calcium levels of dietary Ca2+ and
vitamin D levels decreases, as they often do in the
elderly. Consequently, a substantial decrease in
bone matrix may occur.
➢ In most people, the ability to regulate blood glucose
does not decrease with age. However, there is an
age-related tendency to develop type 2 diabetes
mellitus for those who have a familial tendency, and
it is correlated with age-related increases in body
weight.