The Endocrine System

Chapter 13 (pp. 434-471)

 Introduction
• Like the nervous system, the endocrine system is involved in
maintaining the body’s homeostasis (ie. a constant internal
environment despite an ever changing external environment).

• However, unlike the nervous system which allows the body to

make quick adjustments to stimuli, the endocrine maintains
homeostatic control in a slower fashion but over a longer
duration.

• The endocrine system achieves this through a series of glands

throughout the body that secrete chemical messengers known
as hormones into the bloodstream.

• These hormones then target specific cells in the body that have
receptors for them. Once bound to the cell these regulators
either speed up or slow down certain processes there.
• Can you name any one of the six hormones we’ve already
covered in Biology 20?

• There are two main classes of hormones:

• Lipid-based (steroid) hormones are made primarily of

cholesterol and are not soluble in water. They combine
with receptor molecules in the cytoplasm of their target
cells. The hormone-receptor complex then moves into the
nucleus where they activate specific genes, causing changes
in the cell. Examples of these hormones include
testosterone, estrogen & cortisol.

• Protein-based hormones combine with receptors on the

cell membrane. This causes a series of reactions to occur
inside the target cells. This greatly amplifies its impact.
Examples of these hormones include epinephrine,
thyroxine & insulin.
• Many hormones are regulated by negative feedback mechanisms or
loops.

• It is called this, because it causes a system to move in the opposite direction

from which it is going.

• A classic example of a negative feedback loop in your home is the

thermostat/furnace system in your home.
• Glands in the body are broken into two main categories:

• Exocrine glands are ducted and their materials end up

on the outside of the body (ie. salivary glands, sweat
glands & mammary glands)

• Endocrine glands are non-ducted and their materials go

into extracellular fluid (ECF) where they will eventually
be picked up by surrounding capillaries.
• The endocrine system is comprised of the endocrine glands
located around the body and the hormones they secrete.
Hypothalamus/Pituitary Gland
• At the base of your brain is the pea-sized pituitary gland.

• The hypothalamus controls the pituitary via a thin stalk that attaches it (ie. via neurons
and hormones that run through it)

• Many of the hormones released from the pituitary are called tropic hormones because they
target other endocrine glands to produce other hormones (hormone cascade). For this
reason, it is often called “the master gland”.
• The pituitary gland is
divided into two parts:

• Posterior pituitary is
considered part of the
nervous system.
Although it doesn’t
produce hormones, it
stores and releases those
produced by the
hypothalamus. It’s under
neural control by the
hypothalamus.

• Anterior pituitary is a
true gland, in that it
produces its own
hormones. It’s controlled
by hormonal secretions by
the hypothalamus.
• The anterior pituitary produces human growth
hormone (hGH), which affects almost every body
tissue and most of its affects are tropic.

• Triggered by age and the release of hGH-releasing

hormone (GHRH) by the hypothalamus, hGH:

• Stimulates the liver to secrete hormones such as

growth factor 1 (GF1).

• Promotes cell division and growth.

• Increases calcium uptake in bone & protein

synthesis in muscles.

• Causes the metabolic breakdown of fat stored in

adipose tissue.
• If excessive amounts of hGH
are produced during
childhood, it can result in
gigantism.

• Insufficient hGH production

during childhood results in
pituitary dwarfism
(hypopituitarism).

• It is worth noting that

individuals with this
condition will have typical
body proportions unlike the
70% of dwarfs, who suffer
from the genetic disorder
achondroplasia (ie. Peter
Dinklage from Game of
Thrones).
• In some cases, individuals
reach adulthood and still
produce too much hGH (ie.
once bone growth is
complete). This can lead to a
condition known as
acromegaly.

• Bones and soft tissues will

begin to widen and feet and
hand will enlarge.

• If left untreated, it can lead to

cardiovascular diseases,
diabetes, breathing problems,
muscle weakness and colon
cancer.
• Using the diagram
below as a guide,
sketch a labelled
hiliting a properly
functioning negative
feedback loop for
hGH using the
following terms: releasing gland

• Anterior pituitary
gland

• Growth factor 1
(GF1) stimulating gland

• Human growth
hormone (hGH)

• hGH-releasing
hormone (GHRH)
target gland
• Hypothalamus

• Liver
• Antidiuretic hormone (ADH) is produced by the hypothalamus and
stored by the posterior pituitary.

• When osmoreceptors in the hypothalamus detect a drop of water

concentration in the blood (ie. a drop in blood pressure and dehydration),
the hypothalamus signals the posterior pituitary to secrete ADH.

• Once it reaches the kidneys, ADH makes nephrons more permeable to

water causing increased water reabsorption into the blood (ie. decreased
urine production).
• Hyposecretion of ADH
results in diabetes
insipidus.

• Individuals suffering from

this condition excrete large
amounts of severely
diluted urine, regardless of
how much they drink. A
loss of potassium is also a
major concern.

• This results in extreme

thirst and increased fluid
intake.

• In children, this condition

can interfere with appetite,
weight gain and growth.
What If You Stopped Drinking Water?
• The pituitary gland also releases two hormones involved in breast
feeding:

• Prolactin (PRL) is produced and released by the anterior pituitary and

causes mammary glands to produce milk.

• Oxytocin (OCT) is produced by the hypothalamus and released by

the posterior pituitary. It stimulates the release of milk from
mammary glands and helps to initiate uterine contractions during
labour. Oxytocin is one of the only hormones that exhibits positive
feedback loops.
• The hypothalamus and the pituitary produce other hormones
involved what are the referred to as the HPT, HPA & HPG axises.

• These hormone cascades will be discussed later in the unit.

 Pineal Gland
• The pineal gland is located near the
centre of the brain, tucked between the
cerebral hemispheres in a groove
where the two halves of the thalamus
join.

• It produces the hormone melatonin

which modulates wake/sleep patterns
and seasonal cycles.

• Light affects how much melatonin

your body produces. During the
shorter days of winter, your body may
produce it either earlier or later in the
day than usual. This change can lead
to symptoms of seasonal affective
disorder (SAD) or winter depression.
 Adrenal Glands
• There are two
adrenal glands
located on top of
the kidneys.

• Like the kidneys,

the glands are
composed of an
inner layer (adrenal
medulla) and an
outer layer (adrenal
cortex). Each layer
produces different
hormones.
• The adrenal medulla produces two hormones involved in
regulating your short-term stress (fight-or-flight)
response: epinephrine (adrenaline) and norepinephrine
(noradrenaline).

• These hormones have similar effects on the body to those

caused by stimulation of your sympathetic nervous
system (SNS) (ie. increased heart rate, blood pressure &
breathing rate; dilation of bronchioles & blood vessels in
skeletal muscles, brain, heart & lungs; constriction of
blood vessels in the reproductive & digestive systems;
dilation of pupils; inhibition of urination; increased
conversion of glycogen to glucose by the liver).

• In stress situations these hormones are released quickly,

because the SNS carries a signal directly from the
hypothalamus to the adrenal medulla.
• The adrenal cortex produces two groups of
hormones involved in regulating your long-
term stress (fight-or-flight) response:
• Glucocorticoids (increase blood sugar)
• Mineralocorticoids (increase blood
pressure)
• As well, the adrenal cortex also secretes a
small amount of female and male sex
hormones, called gonadocorticoids, which
supplement hormones produced by the
gonads.
• The most abundant glucocorticoid is the
steroid hormone cortisol. It’s regulated
through a negative feedback loop.

• When the body detects danger, the

hypothalamus produces corticotropin-
releasing hormone (CRH), which targets
the anterior pituitary.

• The anterior pituitary then produces

adrenocorticotropic hormone (ACTH),
which targets the adrenal cortex.

• The adrenal cortex then produces cortisol,

which increases blood glucose levels.

• High level of cortisol in the bloodstream

cause negative feedback on the
hypothalamus and the anterior pituitary
which shut down production of CRH and
ACTH.

• This is referred to as the HPA

(hypothalamus pituitary adrenal) axis.
• Cortisol raises blood glucose levels by
promoting the break down of fat cells and
muscle proteins.
• These fats and amino acids are taken to the
liver to be converted into carbohydrates via a
process called gluconeogenesis.
• Cortisol weakens activity of the immune
system (ie. to conserve energy for the fight-
or-flight response). This allows it to act as a
natural anti-inflammatory in the body.
• This is why people who are under long-term
stress are more likely to get sick.
• Besides stimulating uterine muscles during labour and releasing milk from
the mammary glands while nursing, oxytocin is also known as the “cuddle
hormone”.

• It seems to play a role in counteracting the effects of long-term stress (ie.

relaxes your blood vessels). It’s released during positive social interactions.
• The principal
mineralocorticoid is the
hormone called
aldosterone.

• It stimulates the tubules of

the kidneys to increase
reabsorption of sodium into
the blood and potassium
secretion into the nephrons.
This increases water
reabsorption as well,
causing an increase in blood
pressure.

• Why is this control of salt so

important?
• In some cases, the body can
attack the adrenal cortex,
resulting in the autoimmune
condition Addison’s disease.

• This hypofunction of the adrenal

cortex results in low blood sugar,
sodium/potassium imbalances,
rapid weight loss and low blood
pressure (due to increased urine
production).

• Former U.S. President John F.

Kennedy suffered from this
condition.
• Cushing’s syndrome is a
debilitating disorder
resulting from the
hyperfunction of the adrenal
cortex. It often results from
a tumour of the pituitary
and/or adrenal glands.

• Symptoms include
hypertension, wide mood
swings, decreased sex drive,
loss of muscle, weakness/
fatigue, skin changes (ie.
easy bruising, purplish
stretch marks and red
cheeks) and weight gain in
the face and/or neck.
Let’s Quickly Review…
The Islets of Langerhans
• Although mainly
considered an
exocrine gland
(ie. it secretes
pancreatic juice
through ducts
into the GI tract),
the pancreas also
serves an
endocrine
function.

• It houses clusters
of cells called the
islets of
Langerhans,
which secrete
two hormones.
• The alpha cells secrete the hormone glucagon, which serves to
increase the level of blood glucose. It does this by:

• Stimulating the liver to break down glycogen into glucose.

• Stimulating adipose tissue to break down fat into glucose.

• The beta cells secrete the hormone insulin, which serves to decrease
the level of blood glucose. It does this by

• Stimulating the liver to convert glucose to glycogen for storage.

• Stimulating muscle cells to store glycogen and build protein.

• Stimulating adipose tissue to use glucose to form fat.

• What other two hormones that we’ve discussed in this unit play a role
in blood glucose regulation?

• Epinephrine & cortisol!

• So does thyroxine (we will discuss this hormone later in the unit).
• Glucagon and insulin are known as antagonistic hormones because
they perform opposing functions to balance glucose levels in the
blood to an optimal 90mg/100mL.
• If an individual produces too much insulin and/or not
enough glucagon, a drop in blood glucose levels can
occur. This is known as hypoglycemia.

• Diabetes mellitus is a metabolic disorder where the

person doesn’t produce enough insulin or the body
does not react properly to insulin. This causes levels of
blood glucose to rise quickly, resulting in
hyperglycemia.

• Short-term effects include: low energy, thirst and

passing more urine (with glucose present in it).

• Long-term effects include: blindness, kidney failure,

nerve damage and gangrene in the limbs.

• Diabetes mellitus is one of the leading causes of

death in North America.
• There are two main types of diabetes mellitus:

• Type 1 (juvenile/insulin dependent) diabetes

is an autoimmune condition where the body’s
immune system attacks the beta cells of the
pancreas. As a result, they are not longer able
to produce insulin. Daily insulin injections are
required to live.

• Type 2 (adult onset/non insulin dependent)

diabetes develops gradually, often because
insulin receptors on body cells stop responding
to insulin. One of the biggest risk factors of
this condition is obesity. About 90% of
individuals have type 2. Without proper care,
however, type 2 can develop into type 1.
Thyroid/Parathyroid Glands
• The thyroid gland is located directly below the larynx and produces
thyroxine (T4) and calcitonin.

• The parathyroid glands are four small glands attached to the thyroid
and produces parathyroid hormone (PTH).
• The first hormone the thyroid releases
is thyroxine (T4).
• It targets cells all over the body and
increases body metabolism by
increasing body temperature, increases
the rate of cellular respiration and
increasing the body’s ability to burn
fat.
• If the thyroid gland fails to work
properly there are several conditions
that can occur.
• If an individual’s thyroid
doesn’t produce enough
thyroxine, a condition called
hypothyroidism can occur.
Individuals are constantly
tired and experience weight
gain and hair loss.

• If the thyroid fails to

develop in childhood,
severe hypothyroidism or
cretinism can occur.
Individuals are shorter and
stockier than average and
can have mental
developmental delays.
• If an individual’s thyroid
produces too much thyroxine, a
condition called
hyperthyroidism can occur.
Symptoms can include anxiety,
insomnia, heat intolerance,
irregular heartbeat and weight
loss.

• A severe form of
hyperthyroidism is called
Graves’ disease. Symptoms
include swelling of the neck and
the muscles around the eyes,
which causes them to protrude
and interferes with vision.
• The thyroid requires
iodine to make
thyroxine.
• If there is insufficient
iodine in one’s diet, the
thyroid can become
enlarged causing a
goitre.
• In Canada, it’s
uncommon to get goitres
because iodine is added
to salt.
• Like many other parts of the body, the
thyroid gland can become cancerous.

• Although it can be caused by genetic

factors, exposure to ionizing radiation
is suspected to play a major role.

• In 1986, exposure to radioactive

iodine-131 after the Chernobyl nuclear
accident in the former Soviet Union led
to 4000 cases of thyroid cancer in the
areas surrounding the power plant.

• Recent studies have indicated that

thyroid cancer rates have spiked in
areas of Japan close to the Fukushima
power plant that released radioactive
material in the days following a
massive earthquake and tsunami in
2011.
• The regulation of the thyroid gland is
through a negative feedback loop.

• The hypothalamus produces thyrotropin-

releasing hormone (TRH), which targets
the anterior pituitary.

• The anterior pituitary then releases

thyroid-stimulating hormone (TSH),
which targets the thyroid gland.

• The thyroid gland then produces thyroxine

(T4), which increases cellular respiration in
target cells throughout the body.

• High level of thyroxine in the bloodstream

cause negative feedback on the
hypothalamus and the anterior pituitary
which shut down production of TRH and
TSH.

• This is referred to as the HPT

(hypothalamus pituitary thyroid) axis.
• The thyroid and parathyroid glands work together to regulate
calcium levels in the blood.

• This mineral plays a huge role in blood clotting, nerve

conduction and muscle contraction, but too much can be bad
for the body (ie. interferes with iron absorption and increases
the risk of heart attack and stroke).

• If too much calcium is present, the thyroid releases the

hormone calcitonin, which stimulates its uptake by bones
throughout the body.

• Parathyroid hormone (PTH) is released by the parathyroid

glands when calcium levels drop. It stimulates bones to
breakdown calcium phosphate, causing calcium to be
reabsorbed into the blood system.

• PTH also stimulates the kidneys to reabsorb calcium from the

urine and activates Vitamin D, which stimulates the
absorption of calcium from food in the intestine.
• Like glucagon and insulin produced by the pancreas, calcitonin and
PTH are antagonistic hormones. They perform opposing functions to
balance calcium levels in the blood to an optimal 10mg/100mL.
Let’s Quickly Review…
 Thymus Gland
• The thymus gland is
located behind the
sternum and between the
lungs.

• It produces the hormone

thymosin, which
stimulates the
development of disease-
fighting T-lymphocytes.

• It’s only active until

puberty, when it slowly
begins to shrink and is
replaced by fat.
 The Gonads
• As part of their reproductive
system, both males and females
have a pair of gonads. In males
they’re called testes and in females
they’re called ovaries.
• These are the organs that produce
reproductive cells (ie. sperm and
eggs), known as gametes.
• The development of the gonads and the production of gametes at puberty are
stimulated by hormones produced in the hypothalamus and the anterior
pituitary.

• The secretion of gonadotropin-releasing hormone (GnRH) from the

hypothalamus signals the anterior pituitary to release two gonadotropins
(hormones that act on the gonads) called follicle stimulating hormone (FSH)
and luteinizing hormone (LH).

• FSH stimulates the development of the sex organs and gamete production.

• LH stimulates the gonads to produce a series of steroid-based hormones:

• Testes produce testosterone which stimulates the development of male

secondary sex characteristics (ie. males only).

• Ovaries produce estrogen which stimulates the development of the

female reproductive tract and secondary sex characteristics.

• Ovaries also produce progesterone which causes the walls of the uterus
to thicken.

• This is referred to as the HPG (hypothalamus pituitary gonadal) axis.

• We’ll discuss these hormones more in depth during an upcoming unit on the
human reproductive system.
 Gland/Hormone Target(s) Effect(s)

Hypothalamus (4)

hGH-Releasing Hormone (GHRH) Anterior pituitary Release of hGH

Corticotropin-Releasing Hormone (CRH) Anterior pituitary Release of ACTH

Thyrotropin-Releasing Hormone (TRH) Anterior pituitary Release of TSH

Gonadotropin-Releasing Hormone (GnRH) Anterior pituitary Release of FSH and LH

Pituitary Gland (Anterior) (6)

Liver, bones, muscles & Release of GF1, increases calcium uptake, increase
Human Growth Hormone (hGH) adipose tissue protein synthesis, metabolic breakdown of fat

Prolactin (PRL) Mammary glands Production of milk after childbirth

Adrenocorticotropic Hormone (ACTH) Adrenal cortex Release of glucocorticoids (ie. cortisol)

Thyroid Stimulating Hormone (TSH) Thyroid gland Release of thyroxine

Follicle Stimulating Hormone (FSH) Gonads Development of sex organs & gamete production

Luteinizing Hormone (LH) Gonads Release of testosterone, estrogen & progesterone

 Gland/Hormone Target(s) Effect(s)

Pituitary Gland (Posterior) (2) NOTE: Hormones released by the posterior

pituitary are produced by the hypothalamus.

Increases water retention (ie. decreases urine

Antidiuretic Hormone (ADH) Kidneys output)

Mammary glands, uterus & Release of milk, promotes uterine contractions &
Oxytocin (OCT) blood vessels relaxes blood vessels.

Pineal Gland (1)

Unclear (possibly pigment
Melatonin cells and/or gonads) Modulates wake/sleep patterns & seasonal cycles

Adrenal Gland (Medulla) (2)

Heart, blood vessels, HR increases, BP increases, bronchioles dilate,
Ephinephrine (Adrenaline) bronchioles, liver & muscles glycogen to glucose & metabolic rate increases

Heart, blood vessels, HR increases, BP increases, bronchioles dilate,

Norepinephrine (Noradrenaline) bronchioles, liver & muscles glycogen to glucose & metabolic rate increases

Adrenal Gland (Cortex) (3)

Breakdown of fat, breakdown of proteins,
Glucocorticoids (Cortisol) Adipose tissue, muscle & liver conversion of fats/proteins into carbohydrates

Increases sodium/water reabsorption and

Mineralocorticoids (Aldosterone) Kidneys
potassium secretion

They supplement hormones produced by the

Gonadocorticoids Entire body gonads (ie. testosterone, estrogen & protesterone)
 Gland/Hormone Target(s) Effect(s)
Islets of Langerhans (2)
Increases blood glucose by converting glucose to
Glucagon Liver & adipose tissue
glycogen and by breaking down fat

Liver, adipose tissue & muscle Decreases blood glucose by converting glycogen to
Insulin cells glucose, forming fat and building protein

Thyroid Gland (2)

Increases metabolic rate and regulates growth and
Thyroxine (T4) Entire body development

Controls calcium levels in the blood by depositing it

Calcitonin Bones & kidneys in bones or secreting it into the kidneys

Parathyroid Glands (1)

Raises calcium levels in the blood by releasing it
Parathyroid Hormone (PTH) Bones & kidney from bones or reabsorbing it from the kidneys

Thymus Gland (1)

Thymosin White blood cells Stimulates the development of T-lymphocytes

Gonads (3)
Stimulates the development of the male
Testosterone (testes) Entire body reproductive tract and secondary sex characteristics

Stimulates the development of the female

Estrogen (ovaries) Entire body reproductive tract and secondary sex characteristics

Progesterone (ovaries) Uterus Causing uterine thickening