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Unit 4: Homeostasis
Chapter 9: The Endocrine System
Section 9.1: The Glands and Hormones of
the Endocrine System
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The functioning of the over
100 trillion diverse cells
making up the tissues and
organs in your body must be
regulated and controlled
In order for this to occur, the
cells must be able to
communicate with each other
The body systems that
facilitate cellular
communication and control
are the nervous and
endocrine systems
Section 9.1: The Glands and Hormones of
the Endocrine System
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Recall from Chap 8 that nervous system messages are
transmitted rapidly to precise locations in the body through
neurons
The body also secretes chemical messages from glands
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Endocrine glands secrete chemical messengers called hormones
directly into the bloodstream, which transports the hormones
throughout the body
Original Greek meaning of the word hormone is to “excite” or “set
in motion”
The endocrine glands and the hormones they secrete make up
the endocrine system
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Compared to the rapid actions of the nervous system, the endocrine
system typically has slower and longer acting effects, and affects a
broader range of cell types
The Endocrine Glands
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There are over 200
hormones or hormone-like
chemicals in the human body
They have a wide variety of
functions, such as:
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Regulating growth and
development
Speeding up or slowing down
the metabolism
Regulating blood pressure or
immune response
The Endocrine Glands
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Glands that function exclusively as
endocrine glands include the:
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Pituitary
Pineal
Thyroid
Parathyroid
Adrenal
Tissues and organs that secrete
hormones (but don’t function
exclusively as endocrine glands)
include the:
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Hypothalamus
Thymus
Pancreas
Testes
Ovaries
Hormone Activity on Target Cells
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When hormones are released, they act on target cells
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Target cells contain receptor proteins
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Cells whose activity is affected by a particular hormone
Circulating hormones bind to their specific receptor proteins, like a
key fits into a lock
Human growth hormone (hGH) can be used as a specific
example
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hGH circulates in the bloodstream and interacts with the liver,
muscle, and bone cells
Each of these cell types contains receptor proteins specifically
shaped to bind with hGH
When hGH binds to its receptor, this triggers other reactions in the
target cell
In other word, the target cell receives and responds to the chemical
message sent by the hormone
Steroid Hormones and Water-Soluble
Hormones
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Steroid hormones, such as
testosterone, estrogen, and
cortisol, are lipid-based
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They can easily diffuse
through the lipid bilayer of cell
membranes
Inside the target cell, steroid
hormones bind to their
receptor proteins
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This interaction activates
specific genes, causing changes
in the cell
Ex: Estrogen can trigger cell
growth
Steroid Hormones and Water-Soluble
Hormones
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Epinephrine, human growth
hormone (hGH), thyroxine
(T4), and insulin are watersoluble hormones
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Can’t diffuse across the cell
membrane
Water-soluble hormones
bind to a receptor protein
on the surface of the target
cell
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This starts a cascade of
reactions inside the target cell
Each reaction that occurs
triggers many other reactions
The impact of the hormone is
greatly amplified
Steroid Hormones and Water-Soluble
Hormones
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For example, a single molecule of epinephrine in the liver
can trigger the conversion of glycogen into about 1
million molecules of glucose
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When epinephrine reaches the liver, it stimulates the
conversion of ATP to cyclic adenosine monophosphate (cAMP)
cAMP triggers an enzyme cascade that results in many
molecules of glycogen being broken down into glucose
The glucose enters the bloodstream and will eventually be
used by cells for energy
Once a hormone’s message has been delivered, enzymes
inactivate the hormone
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Any lingering effect could potentially be very disruptive
Regulating the Regulators
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For many years, scientists referred to the pituitary gland
as the “master gland”
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Many of the hormones it secretes stimulate other endocrine
glands
Further research has shown that the pituitary gland is
actually controlled by the hypothalamus
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After receiving signals from various sensors in the body, the
hypothalamus secretes releasing hormones, which often travel
to the pituitary gland
Releasing hormones stimulate the pituitary gland to secrete
hormones that act on other endocrine glands
Regulating the Regulators
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Hormones that stimulate endocrine glands to release other
hormones are called tropic hormones
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Many of the hormones released from the hypothalamus and anterior
pituitary are tropic hormones
The hypothalamus and the pituitary gland control many
physiological processes that maintain homeostasis
Regulating the Regulators
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Figure 9.5A shows the general mechanism of action of tropic hormones
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The hypothalamus secretes a releasing hormone into the anterior pituitary
Causes the anterior pituitary to release a second tropic hormone into the
bloodstream
The second tropic hormone stimulates the target gland to release a third
hormone into the blood
This hormone travels to another target tissue and produces an effect
Regulating the Regulators
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Like many hormones, this system is controlled by a
negative feedback loop
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In this case, the third hormone prevents further release of the
first two hormones in the pathway
A specific example is the feedback system that controls
thyroid-stimulating hormone (TSH)
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Low blood levels of the thyroid hormone T4 initiate the
response from the hypothalamus
When blood levels of T4 increase, the release of TRH and TRH
is inhibited
Working Together to Maintain Homeostasis
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Homeostasis depends on the close relationship between the
nervous system and the endocrine system
The functions of these two systems often overlap:
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Some nervous system structures, such as cells in the hypothalamus,
secrete hormones
Several chemicals function as both neurotransmitters and hormones
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Epinephrine acts as a neurotransmitter in the nervous system, and as a
hormone in the fight-or-flight response
The endocrine and nervous systems are regulated by feedback loops
The regulation of several physicological processes involves the
nervous and endocrine systems acting together
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Ex: When a mother breastfeeds her baby, the baby’s suckling initiates a
sensory message in the mother’s neurons that travels to the
hypothalamus. This triggers the pituitary to release the hormone
oxytocin. Oxytocin travels to the mammary glands of the breast, causing
the secretion of milk
Section 9.2: Hormonal Regulation of
Growth, Development, and Metabolism
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You many have heard the expression “growing like a
weed” used to refer to an adolescent who has grown
several centimeters in just a few months
You may have heard people say they have a “fast
metabolism” meaning they can eat whatever they want
and not gain weight
The growth and development of muscles and bones are
controlled by hormones released by the pituitary gland
The rate of metabolism is controlled by hormones
released by the thyroid gland
The Pituitary Gland
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The pituitary gland has two
lobes and is about 1 cm in
diameter (about the size of a
pea)
It sits in a bony cavity attached
by a thin stalk to the
hypothalamus at the base of the
brain
Despite its small size, it releases
6 main hormones involved in
the body’s metabolism, growth,
development, reproduction, and
other critical life functions
The Pituitary Gland
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The anterior pituitary and posterior pituitary make up
the two lobes of the pituitary gland
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Each lobe is really a separate gland and they release different
hormones
The posterior pituitary is considered part of the
nervous system
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Don’t produce hormones
It stores and releases the hormones ADH and oxytocin, which
was produced by the hypothalamus and transferred to the
posterior pituitary by neurons
The Pituitary Gland
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The anterior pituitary is a true hormone-synthesizing gland
Its cells produce and release 6 major hormones
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Thyroid-stimulating hormone (TSH)
Adrenocorticotropic hormone (ACTH)
Prolactin (PRL)
Human growth hormone (hGH)
Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
A series of blood vessels called a portal system carries
releasing hormones from the hypothalamus to the anterior
pituitary
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These hormones either stimulate or inhibit release of hormones
from this gland
Human Growth Hormone
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The anterior pituitary regulates growth, development, and
metabolism through the production and secretion of human
growth hormone (hGH)
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This hormone ultimately affects almost every body tissue
It can affect some tissues by direct stimulation, but the majority of
the effects are tropic
hGH stimulates the liver to secrete hormones called growth
factors
hGH and the growth factors influence many physiological
processes. For example, they increase:
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Protein synthesis
Cell division and growth, especially the growth of cartilage, bone, and
muscle
Metabolic breakdown and release of fats stored in adipose (fat)
tissue
Human Growth Hormone
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hGH stimulates the growth of
muscles, connective tissue, and the
growth plates at the end of the
long bones, which causes
elongation of the bones
If the pituitary gland secrete
excessive amounts of hGH during
childhood, it can result in a
condition called gigantism
Insufficient gGH production during
childhood results in pituitary
dwarfism
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Will be of extremely small stature as
an adult, but have typical body
proportions
Human Growth Hormone
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When someone reaches
adulthood and skeletal growth is
completed, overproduction of
hGH can lead to a condition called
acromegaly
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Excess hGH can no longer cause an
increase in height, so the bones and
soft tissues of the body widen
Over time the face widens, the ribs
thicken, and the feet and hands
enlarge
Can also cause debilitating
headaches, an enlarged heart, liver,
and kidneys, fatigue, breathing
problems, cardiovascular diseases,
sugar intolerance leading to diabetes,
muscle weakness, and colon cancer
The Thyroid Gland: A Metabolic Thermostat
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The thyroid gland lies directly
below the larynx (voice box)
It has two lobes, one on either
side of the trachea (windpipe),
which are joined by a narrow
band of tissue
Millions of cells within the
thyroid secrete immature thyroid
hormones into the spaces
between the cells
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One of these hormones,
thyroxine (T4) will become
functional and be released into the
bloodstream
The Thyroid Gland: A Metabolic Thermostat
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The primary effect of thyroxine is to increase the rate at
which the body metabolizes fats, proteins, and
carbohydrates for energy
Doesn’t have one specific target organ’
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Stimulates the cells of the heart, skeletal muscles, liver, and
kidneys to increase the rate of cellular respiration
Also plays an important role in the growth and
development of children by influencing the organization of
various cells into tissues and organs
The Thyroid Gland: A Metabolic Thermostat
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If the thyroid fails to develop
properly during childhood, a
condition called cretinism can
result
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The thyroid produces extremely low
quantities of thyroxine and the
person is said to have severe
hypothyroidism
Individuals with this condition are
stocky and shorter than average,
and without hormonal injections
early on in life they will have
mental developmental delays
The Thyroid Gland: A Metabolic Thermostat
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Adults with hypothyroidism
tend to:
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Feel tired much of the time
Have a slow pulse rate and puffy
skin
Experience hair loss and weight
gain
Explains why someone with a
slow metabolism due to an
underactive thyroid may eat
very little, but still gain weight
The Thyroid Gland: A Metabolic Thermostat
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Overproduction of thyroxine is
called hyperthyroidism
Symptoms include:
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Anxiety
Insomnia
Heat intolerance
Irregular heartbeat
Weight loss
Graves’ disease is a severe form of
hyperthyroidism
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Results when body’s immune system
attacks the thyroid
Produces swelling of muscles around
the eyes, causing them to protrude and
interferes with vision
The Thyroid Gland: A Metabolic Thermostat
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Thyroxine secretion is controlled
by negative feedback
The anterior pituitary releases a
hormone called thyroidstimulating hormone (TSH)
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As thyroxine levels rise in the
blood, thyroxine itself feeds back
to the hypothalamus and anterior
pituitary
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Causes thyroid to secrete thyroxine
Suppresses secretion of TSH and,
therefore, thyroxine
When the body is at homeostasis,
the amount of thyroxine in the
bloodstream stays relatively
constant
The Thyroid Gland: A Metabolic Thermostat
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The thyroid requires iodine in order
to make thyroid hormones
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The short form of thyroxine, T4, refers
to the four iodine molecules in the
hormone
If there is insufficient iodine in the
diet, thyroxine can’t be made, and
there will be no signal to stop the
secretion of TSH by the anterior
pituitary
The continuous stimulation of the
thyroid gland by TSH causes a
goiter, an enlargement of the
thyroid gland
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Causes visible swelling in the neck
Also causes difficulty breathing and/or
swallowing, and coughing
The Thyroid Gland: A Metabolic Thermostat
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In the Great Lakes region in Canada, iodine is lacking in
the soil, and therefore in the drinking water
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Why don’t we all have goiters?
Salt refiners add iodine to salt, making it iodized
Other dietary sources of iodine include:
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Seafood
Fish (cod, haddock, and perch)
Kelp
Dairy products
The Thyroid Gland and Calcitonin
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Calcium (Ca2+) is essential for healthy teeth and skeletal
development
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Also plays crucial role in blood clotting, nerve conduction, and
muscle contraction
Calcium levels in the body are regulated, in part, by the
hormone calcitonin
When the concentration of calcium in the blood rises too
high, calcitonin stimulates the uptake of calcium into
bones
A different hormone, secreted by the parathyroid glands,
is release if blood calcium levels get too low
The Parathyroid Glands and Calcium
Homeostasis
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The parathyroid glands are four small glands attached to the
thyroid
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The body synthesizes and releases PTH in response to falling
concentrations of calcium in the blood
PTH stimulates bone cells to break down bone material
(calcium phosphate) and secrete calcium into the blood
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Produce the hormone called parathyroid hormone (PTH)
PTH also stimulates the kidneys to reabsorb calcium from the urine,
activating vitamin D in the process
Vitamin D, in turn, stimulates the absorption of calcium from food in
the intestine
These effects bring the concentration of calcium in the blood
back within a normal range so that the parathyroid glands no
longer secrete PTH
Section 9.3: Hormonal Regulation of the
Stress Response and Blood Sugar
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The stress response involves
many interacting hormone
pathways, including those that
regulate:
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Metabolism
Heart rate
Breathing
In this section we’ll focus on
the hormones of the adrenal
glands and their effects on the
body
Section 9.3: Hormonal Regulation of the
Stress Response and Blood Sugar
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The human body has two adrenal glands
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Each gland is composed of:
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Located on top of the kidneys
Named for two Latin words that mean “near the kidney”
An inner layer called the adrenal medulla
An outer layer called the adrenal cortex
The adrenal cortex produces hormones that are different in structure and function
from the hormones produced by the adrenal medulla
The Adrenal Medulla: Regulating the ShortTerm Stress Response
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The adrenal medulla produces two closely related hormones:
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These hormones regulate a short-term stress response
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Epinephrine (also called adrenaline)
Norepinephrine (also called noradrenaline)
Commonly called the flight-or-fight response
Effects are similar to those caused by stimulation of the sympathetic
nervous system
In the developing embryo, sympathetic neurons and adrenal medulla
cells are formed from nervous system tissue
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Why the adrenal medulla is considered a neuroendocrine structure
The Adrenal Medulla: Regulating the ShortTerm Stress Response
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In response to a stressor, neurons of the sympathetic nervous
system carry a signal from the hypothalamus to the adrenal
medulla
Stimulate adrenal medulla to secrete epinephrine and a small
amount of norepinephrine
These hormones trigger an increase in:
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Breathing rate
Heart rate
Blood pressure
Blood flow to the heart and muscles
Conversion of glycogen to glucose in the liver
In addition, pupils dilate and blood flow to extremities
decreases
The Adrenal Medulla: Regulating the ShortTerm Stress Response
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Epinephrine acts quickly
Epinephrine injections are used to treat life-threatening
conditions
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Can be used to stimulate the heart to start beating in someone
with cardiac arrest
In cases of anaphylactic shock caused by severe allergies (such
as nuts, bee stings, or certain medications), it will open up air
passages and restore breathing
Release of epinephrine and norepinephrine is rapid
because it is under nervous system control
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But their effects lat 10X longer than the sympathetic nervous
system’s effects
The Adrenal Cortex: Regulating the LongTerm Stress Response
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The adrenal cortex produce the stress hormones that
trigger the sustained physiological responses that make up
the long-term stress response
These hormones include:
Glucocorticoids
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Mineralcorticoids
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Increase blood sugar
Increase blood pressure
Gonadocorticoids
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Supplement the hormones produced by the gonads (testes and
overies)
The Adrenal Cortex: Regulating the LongTerm Stress Response
Cortisol
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Cortisol is the most abundant
glucocorticoid
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A steroid hormone synthesized from
cholesterol
When the brain detects danger, it
directs the hypothalamus to
secrete a releasing hormone
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The releasing hormone stimulates
the anterior pituitary gland to
secrete adrenocorticotropic
hormone (ACTH)
ACTH targets the adrenal cortex
Causes the release of the stress
hormone cortisol
Cortisol
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Cortisol works in conjunction with epinephrine, but is
longer lasting
Its main function is to raise blood glucose levels
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Also prompts the breakdown of fat cells
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Does this by promoting the breakdown of muscle protein into
amino acids
Amino acids are taken out of the blood by the liver, where they
are used to make glucose, which is then released back into the
blood
Also releases glucose
Increased cortisol levels in the blood cause negative
feedback on the hypothalamus and anterior pituitary
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Suppresses ACTH production and stops the release of cortisol
Cortisol
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Sustained high levels of cortisol (such
as chronic stress) can:
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Impair thinking
Damage the heart
Cause high blood pressure
Lead to diabetes
Increase susceptibility to infection
Even cause early death
In Japan…
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Long work hours and high-stress jobs
are common
So many business people have died from
heart attacks and strokes that the
phenomenon has been called “karoshi”,
which means “death from overwork”
Cortisol
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One of the ways the body fights disease is by
inflammation
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Cortisol is a natural anti-inflammatory
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Cells of the immune system attack foreign material, such as
invading bacteria
Suppresses the immune system
Probably why sustained high levels of cortisol makes people
more susceptible to infections
Synthesized cortisol is commonly used as a medication to
reduce inflammation associated with asthma, arthritis, or
joint injuries
Aldosterone
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The main mineralcorticoid is the hormone aldosterone
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If the adrenal cortex is damaged, Addison’s disease can
result
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Stimulates the kidneys to increase the absorption of sodium
into the blood
Increases the concentration of solutes in the blood, which
draws more water from the kidneys, raising blood pressure
The body secretes inadequate amounts of mineralcorticoids
and glucocorticoids
Symptoms include:
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Hypoglycemia (low blood sugar)
Sodium and potassium imbalances
Rapid weight loss
Aldosterone
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Low aldosterone results in a loss of sodium and water
from the blood
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Due to increase in urine output
As a result, blood pressure drops
A person with this condition needs to be treated within
days, or the severe electrolyte imbalance will be fatal
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Can be controlled with injections of glucocorticoids and
mineralcorticoids
The Hormones of the Pancreas
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The pancreas is located behind the stomach and is
connected to the small intestine by the pancreatic duct
Most of the pancreatic tissue secretes digestive enzymes
into the small intestine
The pancreas also functions as an endocrine gland,
secreting hormones directly into the bloodstream
Scattered throughout the pancreas are more than 2000
clusters of endocrine cells called the islets of
Langerhans
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Named for Paul Langerhans, the scientist who first described
them in 1869
The Hormones of the Pancreas
The Hormones of the Pancreas
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The islets of Langerhans secrete
two hormones, insulin and
glucagon
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The beta cells of the pancreas
secrete insulin
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They have opposite effects
(anatagonistic)
Decreases blood glucose levels
The alpha cells secrete
glucagon
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Increases blood glucose levels
The Hormones of the Pancreas
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Both insulin and glucagon are regulated by negative
feedback mechanisms
When you eat a meal, your digestive system breaks down
the food
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Releases a substantial amount of glucose into your
bloodstream
When blood glucose levels rise, pancreatic beat cells
secrete appropriate amounts of insulin
Insulin circulates throughout the body
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Acts on specific receptors to make target cells more
permeable to glucose
The Hormones of the Pancreas
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Insulin especially affects:
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Muscle cells
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Liver cells
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Use large amounts of glucose in
cellular respiration
Where glucose is converted into
glycogen for temporary storage
As glucose levels in the blood
return to normal, insulin
secretion slows
The Hormones of the Pancreas
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Rigorous exercise or fasting can cause blood glucose
levels to drop
Low blood sugar stimulates the alpha cells of the islets of
Langerhans to release glucagon
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Stimulates the liver to convert glycogen back into glucose,
which is released into the blood
Other hormones, such as hGH, cortisol, and epinephrine,
also contribute to increasing the level of blood glucose
The Effects of Glucose Imbalance
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Diabetes mellitus is a serious chronic condition with
no known cure
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Results when the body doesn’t produce enough insulin, or
does not respond properly to insulin
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Affects over 285 million people worldwide (as of 2009)
As a result, blood glucose levels tend to rise sharply after
meals, and remain and significantly elevated levels
This condition is called hyperglycemia, or high blood
sugar
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Derived from the Greek words “hyper” (too much), “glyco”
(sugar), and “emia” (condition of the blood)
The Effects of Glucose Imbalance
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Hyperglycemia has short-term and long-term effects
on the body
Without insulin, cells remain relatively impermeable to
glucose and can’t obtain enough from the blood
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The individual experiences fatigue as the cells become satrved
for glucose
The body compensates by switching to protein and fat
metabolism for energy
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Fats and proteins are less accessible and more difficult to break
down than glucose
Fat metabolism also releases ketones, such as acetone, as toxic
by-products, which can be smelled on the breath
The Effects of Glucose Imbalance
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The kidneys are incapable of reabsorbing all of the
glucose that’s filtered through them from the blood
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So glucose is excreted in urine
Due to the concentration gradient in the kidneys, large
volumes of water follow the glucose into the urine and get
excreted
People with untreated diabetes experience low energy
and great thirst, and produce large volumes of glucoserich urine
The Effects of Glucose Imbalance
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In the long term, continued high levels of blood glucose
can lead to:
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Blindness
Kidney failure
Nerve damage
Gangrene (severe infection) in the limbs
Diabetes remains one of the leading causes of death in
North America
Causes of Diabetes
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There are two major types of diabetes mellitus:
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Type 1 diabetes (also called juvenile diabetes or insulindependent diabetes)
Type 2 diabetes (also called adult-onset diabetes or noninsulin-dependent diabetes)
Causes of Diabetes
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In type 1, the immune system produces antibodies that
attack and destroy the beta cells of the pancreas
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As a result, the beat cells degenerate and are unable to
produce insulin
This condition is usually diagnosed in early childhood
People with type 1 must have daily insulin injections in order to
live
Type 2 diabetes tends to develop gradually
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Insulin receptors on the body’s cells stop responding to insulin
or the beta cells of the pancreas produce less and less insulin
over time
Causes of Diabetes
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People who are overweight have a greater chance of
developing type 2 diabetes
It is usually diagnosed in adulthood and often controlled
with diet, exercise, and oral medications
Most people with diabetes (about 90%) have type 2
Without proper care, type 2 diabetes can develop into
type 1, which is insulin-dependent
Toward a Cure for Diabetes
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In 1889, the physician Oscar
Minkowski removed the pancreas
from a healthy dog
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It developed the symptoms of diabetes
This established the relationship
between the pancreas and diabetes
For the next 2 decades, scientists
attempted to isolate a substance
from the pancreas that could be used
to treat diabetes, but were
unsuccessful
Toward a Cure for Diabetes
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In 1921, a research team from the University of Toronto, led by
Fredrick Banting and his assitant Charles Best, made a
breakthrough
By tying off a dog’s pancreatic duct with some string…
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
They were able to remove some islets of Langerhans from the dog’s
pancreas
Able to isolate the insulin from the islets
Banting and his team soon found a way to isolate insulin from
the pancreases of embryonic calves that were a by-product of
the beef industry
Working with a biochemist from the University of Alberta, J.B.
Collip, they further purified the extracted insulin

Used it to successfully treat a boy with diabetes
Toward a Cure for Diabetes



Today, synthetic insulin is produced by genetically
engineered bacteria and other organisms
Furthermore, The Edmonton Protocol, led by James
Shapiro at the University of Alberta, has pioneered the
first successful islet cell transplants to restore functioning
beta cells to the pancreas
The technology of blood glucose monitoring devices is
also improving


Many people with diabetes use digital blood glucose monitors
Advances in insulin injection technology have led to the
development of the insulin pump

Mimics the pattern of release of insulin from a healthy pancreas
Section 9.4: Hormonal Regulation of the
Reproductive System



The human reproductive system is adapted to unite a
single reproductive cell from a female parent with a single
reproductive cell from a male parent
The male and female reproductive systems have different
structures, functions, and hormones
The two systems also have many features in common
Section 9.4: Hormonal Regulation of the
Reproductive System

Both male and female reproductive systems include a pair
of gonads




Gonads (testes and ovaries) are the organs that produce
reproductive cells
Sperm in males, eggs in females
Male and female reproductive cells are also called gametes
The gonads also produce sex hormones

The chemical compounds that control the development and
functions of the reproductive system
Structures and Functions of the Male
Reproductive System

The male reproductive system consists of:



Organs that produce and store large numbers of sperm cells
Organs that help deposit these sperm cells within the female
reproductive tract
Some of these organs are located outside the body,
others are located inside the body
The Testes

The two male gonads are called the testes


The scrotum regulates the temperature of the testes



Held outside the body in a pouch of skin called the scrotum
In humans, sperm production is most successful at
temperatures around 35°C, which is a few degrees cooler than
normal body temperature
In cold conditions, the scrotum draws close to the body
so the testicles stay warm
In hot conditions, the scrotum holds the testicles more
loosely, allowing them to remain cooler than the body
The Testes

The testes are composed of:




Long, coiled tubes, called seminiferous tubules
Hormone-secreting cells, called interstitial cells, that lie
between the seminiferous tubules
The interstitial cells secrete the male hormone
testosterone
The seminiferous tubules are where sperm are produced


Each testis contains more than 250m of seminiferous tubules
Can produce more than 100 million sperm each day
The Testes

For each testis, sperm are transported to a nearby duct
called the epididymis


Within each epididymus, the sperm mature and become motile
The epididymus is connected to a storage duct called the
ductus deferens (plural: ductus deferentia)


Leads to the penis via the ejaculatory duct
The ductus deferens is also known by an older term, vas
deferens
The Penis

The penis is the male organ for sexual intercourse



Has a variable-length shaft with an enlarged tip called the glans penis
A sheath of skin, called the foreskin, surrounds and protects the
glans penis



Its primary reproductive function is to transfer sperm from the male to
the female reproductive tract
Doesn’t have any reproductive function
Circumcision, the surgical removal of the foreskin, is a common practice
in some cultures and families
During sexual arousal, the flow of blood increases to specialized
erectile tissues in the penis, causing them to expand



At the same time, the veins that carry blood away from the penis
becomes compressed
The penis engorges with blood and become erect
Sperm cells move out of each epididymus through the ductus deferencs
Seminal Fluid

As the sperm cells pass through the ductus deferens, they are
mixed with fluids from a series of glands





The combination of sperm cells and fluids is called semen
If sexual arousal continues, semen enters the urethra from the
ductus deferentia



Seminal vesicles
Prostate gland
Cowper’s gland
The urethra is the duct that carries fluid through the penis
The movement of semen is the result of a series of
interactions between the sympathetic, parasympathetic, and
somatic nerve system
Sensory stimulation, arousal, and coordinated muscular
contractions combine to trigger the release, or ejaculation, of
semen from the penis
Sex Hormones and the Male Reproductive
System


The development of the male sex organs begins before birth
In embryos that are genetically male, the Y chromosome
carries a gene called the testis-determining factor (TDF) gene


Triggers the production of the male sex hormones
Male sex hormones are also called androgens



“andro” comes from Greek word for “man” or “male”
The presence of androgens initiates the development of male
sex organs and ducts in the fetus
As the reproductive structures develop, they migrate within
the body to their final locations

Ex: Testes develop in the abdominal cavity, then migrate to the
scrotum
Maturation of the Male Reproductive System



Puberty is the period in which the reproductive system
completes its development and becomes fully functional
Most boys enter puberty between 10-13 years of age,
although the age of onset varies greatly
At puberty, a series of hormonal events lead to gradual
physical changes in the body

These changes include the final development of the sex organs
and the development of the secondary sex characteristics
Maturation of the Male Reproductive System


Puberty begins when the hypothalamus increases its
production of gonadotropin-releasing hormones (GnRH)
Acts on the anterior pituitary gland, causing it to release
two different sex hormones:



Follicle-stimulating hormone (FSH)
Leutinizing hormone (LH)
In males, these hormones cause the testes to begin
producing sperm and to release testosterone

Testosterone acts on various tissues to complete the
development of the sex organs and sexual characteristics
Hormonal Regulation of the Male
Reproductive System


The same hormones that trigger the events of puberty
also regulate the mature male reproductive system over a
person’s lifetime
Hormone feedback mechanisms control the process of
sperm production and maintain secondary sex
characteristics
Hormonal Regulation of the Male
Reproductive System
Hormonal Regulation of the Male
Reproductive System


The release of GnRH from the hypothalamus triggers the
release of FSH and LH from the anterior pituitary
FSH causes:



Inhibin acts on the anterior pituitary to inhibit the
production of FSH


Results in a negative feedback loop
As the level of FHS drops, the testes release less inhibin


The seminiferous tubules in the testes to produce sperm
Cells in the seminiferous tubules to release a hormones called
inhibin
A decrease in the level of inhibin causes the anterior pituitary to
release more FSH
This feedback loop keeps the level of sperm production
relatively constant over time
Hormonal Regulation of the Male
Reproductive System


A similar feedback loop maintains the secondary sex
characteristics
LH causes the interstitial cells in the testes to release
testosterone



Promotes changes such as muscle development and the
formation of facial hair
Acts on the anterior pituitary to inhibit the release of LH
This feedback loop keeps the testosterone levels
relatively constant in the body
Hormonal Regulation of the Male
Reproductive System



Reproductive function and secondary sex characteristics
both depend on the continued presence of male sex
hormones
Substances that interfere with the hormonal feedback
system can cause changes in the reproductive system
For example, anabolic steroids mimic the action of
testosterone in promoting muscle development


Some athletes illegally use steroids to increase their speed or
strength
Steroids also disrupt the reproductive hormone feedback
systems

Side effects include shrinking testicles, low sperm count, and
the development of breasts
Aging and the Male Reproductive System


A man in good health can remain fertile for his entire life
However, most men experience a gradual decline in their
testosterone level beginning around age 40



This condition is called andropause
May cause fatigue, depression, loss of muscle and bone mass,
and a drop in sperm production
Not all men experience andropause or its symptoms, and
symptoms vary widely

Difficult to diagnose accurately
Aging and the Male Reproductive System


Other hormonal changes associated with aging can affect
the male reproductive system
The prostate gland often begins to gradually grow in men
over age 40


Can lead to discomfort and urinary difficulties, because the
prostate squeezes on the urethra as it grows
Older men also have an increased risk of prostate cancer
Structures and Functions of the Female
Reproductive System


Unlike the male system, the female reproductive system
doesn’t mass-produce large numbers of gametes
The female gonads, or ovaries, produce only a limited
number of gametes


The other female sexual organs are adapted to:




Gametes are called eggs or ova (singular: ovum)
Provide a safe environment for fertilization
Support and nourish a developing fetus
Allow for birth of a baby
Most of the structures of the female reproductive system
are located inside the body
The Ovaries


The two ovaries are suspended
by ligaments within the
abdominal cavity
Site of oogenesis




The production of an ovum
Comes from two Greek words
meaning “egg-creation”
Ova are also called oocytes
The ovaries usually alternate so
that only one produces an egg
each month
The Ovaries

The ovary contains specialized cell structures called follicles


Each month, a follicle matures and ruptures, releasing the
ovum into the oviduct




A single ovum develops within each follicle
This event is called ovulation
Thread-like projections called fimbraie continually sweep over
the ovary
When an ovum is released, it is swept into a cilia-lined tube
about 10cm long called an oviduct
The oviduct carries the ovum from the ovary to the uterus

Within the oviduct, the beating cilia create a current that moves the
ovum toward the uterus
The Ovaries



A mature ovum is a non-motile,
sphere-shaped cell approximately
0.1mm in diameter (over 20X
larger than the head of a sperm
cell)
Contains a large quantity of
cytoplasm, which contains
nutrients for the first days of
development after fertilization
It’s encased in a thick membrane
that must be penetrated by a
sperm cell before fertilization can
take place
The Uterus and Vagina

The uterus is a muscular organ
that holds and nourishes a
developing fetus



Normally about the size and
shape of a pear
It expands to many times its size
as the fetus develops
The lining of the uterus is
called the endometrium

Richly supplied with blood
vessels to provide nutrients for
the fetus
The Uterus and Vagina





At its upper end, the uterus connects to the oviducts
At its base the uterus forms a narrow opening called the
cervix
The cervix, in turn, connects to the vagina
The vagina serves as an entrance for an erect penis to
deposit sperm during sexual intercourse
Also serves as an exit for the fetus during childbirth
The Uterus and Vagina

The ovum survives in the oviduct for up to 24 hours after
ovulation


The fertilized egg, now called a zygote, continues moving
through the oviduct for several days before reaching the
uterus



If a living egg encounters sperm in the oviduct, fertilization will take
place
During this time, the endometrium thickens as it prepares to receive
the zygote
The zygote implants itself in the endometrium, and
development of the embryo begins
If the egg is not fertilized, it doesn’t implant

The endometrium disintegrates, and its tissues and blood flow out of
the vagina in a process known as menustruation
The Uterus and Vagina

The vagina opens into the female external genital organs,
known together as the vulva


Includes labia majora and labia minora, two pairs of skin folds
that protect the vaginal opening
The vulva also includes the glans clitoris
Sex Hormones and the Female Reproductive
System


Our understanding of the specific factors that trigger the
development of female sex organs in a female embryo is
incomplete
Until recently scientists assumed that the development of
female sex organs was a “default” pattern


If there is no Y chromosome, then female organs will develop
Researchers now suspect that the processes of female
sex development are more complex and that specific
hormonal triggers cause female sex organs to develop
Sex Hormones and the Female Reproductive
System



Like a baby boy, a baby girl has a complete but immature set of
reproductive organs at birth
North American girls usually begin puberty between 9-13
years of age
The basic hormones and hormonal processes of female
puberty are similar to those of male puberty



A girl begins puberty when the hypothalamus increases its
production of GnRH
This hormone acts on the anterior pituitary to trigger the release of
LH and FSH
In girls, LH and FSH act on the ovaries to produce the female
sex hormones estrogen and progesterone


Stimulate the development of female secondary sex characteristics
Launch a reproductive cycle that will continue until about middle age
Hormonal Regulation of the Female
Reproductive System

In humans, female reproductive function follows a cyclical
pattern known as the menstrual cycle


Usually about 28 days long



Ensures that an ovum is released at the same time as the uterus is
most receptive to a fertilized egg
Can vary between woman and even between cycles for the same
woman
Cycle begins with menstruation and ends with the start of the next
menstrual period
The menstrual cycle is actually two separate but
interconnected cycles of event



One takes place in the ovaries and is known as the ovarian cycle
The other takes place in the uterus and is known as the uterine cycle
Both are controlled by the female sex hormones estrogen and
progesterone, which are produced by the ovaries
Hormonal Regulation of the Female
Reproductive System
The Ovarian Cycle

The ovary contains cellular structures called follicles, each
containing a single immature ovum




At birth, a baby girl has more the 2 million follicles
Many degenerate, leaving up to about 400,000 by puberty
During her lifetime, only ~400 of these follicles will mature to
release an ovum
In a single ovarian cycle, one follicle matures, releases an
ovum, and then develops into a yellowish, gland-like
structure known as a corpus luteum

The corpus leuteum then disintegreates
The Ovarian Cycle



The ovarian cycle can be roughly divided into two stages
The first stage is known as the follicular stage
Begins with an increase in the level of FSH released by the
anterior pituitary gland


As the follicle matures, it releases estrogen and some
progesterone



FSH stimulates one follicle to mature
The rising level of estrogen in the blood acts on the anterior
pituitary to inhibit the release of FSH
At the same time, the estrogen triggers a sudden release of GnRH
from the hypothalamus
Leads to a sharp increase in LH production by the anterior
pituitary triggering ovulation

The follicle bursts, releasing the ovum
The Ovarian Cycle


Ovulation marks the end of the follicular stage and the
beginning of the second stage, called the luteal stage
Once the ovum has been released, LH causes the follicle
to develop into a corpus luteum



The corpus luteum secretes progesterone and some estrogen
They act on the anterior pituitary to inhibit FSH and LH
production
The corpus luteum disintegrates, leading to a decrease in
the levels of estrogen and progesterone

Causes the anterior pituitary to increase its secretion of FSH,
and the cycle begins again
The Ovarian Cycle
The Ovarian Cycle

If the ovum is fertilized and implants in the
endometrium…



Blood hormone levels of progesterone and estrogen remain
high under stimulus of hormones released by embryosupporting membranes
The continued presence of progesterone maintains the
endometrium to support the developing fetus
The continued presence of estrogen stops the ovarian
cycle so no additional follicles mature
The Uterine Cycle

The uterine cycle is closely linked to the ovarian cycle






Ovulation takes place about halfway through the ovarian cycle,
around day 14
The ovum survives for up to 24 hours after voulation
If fertilization occurs, the fertilized egg completes the passage
through the oviduct and arrives at the uterus a few days later
The timing of the uterine cycle ensures that the uterus is
prepared to receive and nurture a new life
The events of the uterine cycle cause a build-up of blood
vessels and tissues in the endometrium
If fertilization doesn’t occur, the endometrium
disintegrates and menstruation begins
The Uterine Cycle

The uterine cycle begins on
the first day of menstruation
(which is also the first day of
the ovarian cycle)



On this day, the corpus
luteum had degenerated and
the levels of the sex
hormones in the blood are
low
Menstruation lasts for the
first 5 days of the uterine
cycle and by the end, the
endometrium is very thin
As a new follicle begins to
mature and release estrogen,
the level of estrogen in the
blood gradually increases
The Uterine Cycle


Beginning around the sixth day of the uterine cycle, the
estrogen level is high enough to cause the endometrium
to begin thickening
After ovulation, the release of progesterone by the
corpus luteum causes a more rapid thickening of the
endometrium


Between days 15 and 23 of the cycle, the thickness of the
endometrium may double or even triple
If fertilization doesn’t occur, the corpus luteum
degenerates

The level of sex hormones drop, the endometrium breaks
down, and menstruation begins again
Aging and the Menstrual Cycle

The number of functioning follicles in the female
reproductive system decreases with age


Leads to an overall decline in the amount of estrogen and
progesterone in the blood
As the hormone levels drop, a woman’s menstrual cycle
becomes irregular


Within a few years it stops altogether, known as menopause
The average age for menopause in North American women is
~50, but it can begin earlier or later
Aging and the Menstrual Cycle


A woman who has completed menopause no longer produces
ova and is no longer fertile
As well, the decrease in the sex hormones disrupts the
homeostasis of a number of hormone systems




Has a range of effects on the body
During menopause, blood vessels alternately constrict and
dilate, causing “hot flashes”
Some women also experience moodiness
Over the longer term, menopause is associated with:



Rising cholesterol levels
Diminishing bone mass
Increased risk of uterine cancer, breast cancer, and heart disease
Hormone Replacement Therapy

Hormone replacement therapy (HRT) is a
prescription of low levels of estrogen with or without
progesterone



HRT has been linked to:




Can ease some of the symptoms of menopause
Also carries a number of health risks
An increases risk of coronary heart disease, strokes, and blood
clots
An increased risk of breast cancer and colorectal cancer
An increased risk of demntia
Health Canada advises that a woman should not start
HRT without a thorough medical evaluation
Summarizing Reproductive Hormones