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Unit 4: Homeostasis
Chapter 9: The Endocrine System
Section 9.1: The Glands and Hormones of the Endocrine System 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 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 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 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 There are over 200
hormones or hormone-like chemicals in the human body
They have a wide variety of functions, such as: Regulating growth and
development Speeding up or slowing
down the metabolism Regulating blood pressure
or immune response
The Endocrine Glands Glands that function exclusively
as endocrine glands include the: Pituitary Pineal Thyroid Parathyroid Adrenal
Tissues and organs that secrete hormones (but don’t function exclusively as endocrine glands) include the: Hypothalamus Thymus Pancreas Testes Ovaries
Hormone Activity on Target Cells When hormones are released, they act on target cells
Cells whose activity is affected by a particular hormone Target cells contain receptor proteins
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 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 Steroid hormones, such
as testosterone, estrogen, and cortisol, are lipid-based They can easily diffuse
through the lipid bilayer of cell membranes
Inside the target cell, steroid hormones bind to their receptor proteins This interaction activates
specific genes, causing changes in the cell
Ex: Estrogen can trigger cell growth
Steroid Hormones and Water-Soluble Hormones Epinephrine, human growth
hormone (hGH), thyroxine (T4), and insulin are water-soluble hormones Can’t diffuse across the cell
membrane Water-soluble hormones bind
to a receptor protein on the surface of the target cell 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 For example, a single molecule of epinephrine in
the liver can trigger the conversion of glycogen into about 1 million molecules of glucose 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 Any lingering effect could potentially be very disruptive
Regulating the Regulators For many years, scientists referred to the
pituitary gland as the “master gland” Many of the hormones it secretes stimulate other
endocrine glands Further research has shown that the pituitary
gland is actually controlled by the hypothalamus 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 Hormones that stimulate endocrine glands to release
other hormones are called tropic hormones 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 Figure 9.5A shows the general mechanism of action of tropic
hormones 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 Like many hormones, this system is controlled
by a negative feedback loop 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) 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 Homeostasis depends on the close relationship between the
nervous system and the endocrine system The functions of these two systems often overlap:
Some nervous system structures, such as cells in the hypothalamus, secrete hormones
Several chemicals function as both neurotransmitters and hormones 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 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 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 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 The anterior pituitary and posterior pituitary
make up the two lobes of the pituitary gland Each lobe is really a separate gland and they
release different hormones The posterior pituitary is considered part of
the nervous system 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 The anterior pituitary is a true hormone-synthesizing
gland Its cells produce and release 6 major hormones
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 These hormones either stimulate or inhibit release of
hormones from this gland
Human Growth Hormone The anterior pituitary regulates growth, development,
and metabolism through the production and secretion of human growth hormone (hGH) 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: 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 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 Will be of extremely small stature
as an adult, but have typical body proportions
Human Growth Hormone When someone reaches adulthood
and skeletal growth is completed, overproduction of hGH can lead to a condition called acromegaly 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 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 One of these hormones,
thyroxine (T4) will become functional and be released into the bloodstream
The Thyroid Gland: A Metabolic Thermostat 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’ 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 If the thyroid fails to develop
properly during childhood, a condition called cretinism can result 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
Adults with hypothyroidism tend to: 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 Overproduction of thyroxine is
called hyperthyroidism Symptoms include:
Anxiety Insomnia Heat intolerance Irregular heartbeat Weight loss
Graves’ disease is a severe form of hyperthyroidism 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 Thyroxine secretion is controlled
by negative feedback The anterior pituitary releases a
hormone called thyroid-stimulating hormone (TSH) Causes thyroid to secrete thyroxine
As thyroxine levels rise in the blood, thyroxine itself feeds back to the hypothalamus and anterior pituitary 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
The thyroid requires iodine in order to make thyroid hormones 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 Causes visible swelling in the neck Also causes difficulty breathing and/or
swallowing, and coughing
The Thyroid Gland: A Metabolic Thermostat In the Great Lakes region in Canada, iodine is
lacking in the soil, and therefore in the drinking water Why don’t we all have goiters?
Salt refiners add iodine to salt, making it iodized
Other dietary sources of iodine include: Seafood Fish (cod, haddock, and perch) Kelp Dairy products
The Thyroid Gland and Calcitonin Calcium (Ca2+) is essential for healthy teeth
and skeletal development 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 The parathyroid glands are four small glands attached
to the thyroid Produce the hormone called parathyroid hormone (PTH)
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 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 The stress response
involves many interacting hormone pathways, including those that regulate: 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 The human body has two adrenal glands
Located on top of the kidneys Named for two Latin words that mean “near the kidney”
Each gland is composed of: 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 Short-Term Stress Response The adrenal medulla produces two closely related hormones:
Epinephrine (also called adrenaline) Norepinephrine (also called noradrenaline)
These hormones regulate a short-term stress response 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 Why the adrenal medulla is considered a neuroendocrine structure
The Adrenal Medulla: Regulating the Short-Term Stress Response 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: 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 Short-Term Stress Response Epinephrine acts quickly Epinephrine injections are used to treat life-
threatening conditions 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 But their effects lat 10X longer than the
sympathetic nervous system’s effects
The Adrenal Cortex: Regulating the Long-Term Stress Response 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
Increase blood sugar Mineralcorticoids
Increase blood pressure Gonadocorticoids
Supplement the hormones produced by the gonads (testes and overies)
The Adrenal Cortex: Regulating the Long-Term Stress Response
Cortisol Cortisol is the most abundant
glucocorticoid A steroid hormone synthesized
from cholesterol When the brain detects danger,
it directs the hypothalamus to secrete a releasing hormone 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 Cortisol works in conjunction with epinephrine, but is
longer lasting Its main function is to raise blood glucose levels
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 prompts the breakdown of fat cells Also releases glucose
Increased cortisol levels in the blood cause negative feedback on the hypothalamus and anterior pituitary Suppresses ACTH production and stops the release of
cortisol
Cortisol Sustained high levels of cortisol
(such as chronic stress) can: Impair thinking Damage the heart Cause high blood pressure Lead to diabetes Increase susceptibility to infection Even cause early death
In Japan… 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 One of the ways the body fights disease is by
inflammation Cells of the immune system attack foreign
material, such as invading bacteria Cortisol is a natural anti-inflammatory
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 The main mineralcorticoid is the hormone
aldosterone 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 If the adrenal cortex is damaged, Addison’s disease
can result The body secretes inadequate amounts of mineralcorticoids
and glucocorticoids Symptoms include:
Hypoglycemia (low blood sugar) Sodium and potassium imbalances Rapid weight loss
Aldosterone Low aldosterone results in a loss of sodium
and water from the blood 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 Can be controlled with injections of glucocorticoids
and mineralcorticoids
The Hormones of the Pancreas 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 Named for Paul Langerhans, the scientist who first
described them in 1869
The Hormones of the Pancreas
The Hormones of the Pancreas The islets of Langerhans
secrete two hormones, insulin and glucagon They have opposite effects
(anatagonistic) The beta cells of the
pancreas secrete insulin Decreases blood glucose
levels The alpha cells secrete
glucagon Increases blood glucose
levels
The Hormones of the Pancreas Both insulin and glucagon are regulated by
negative feedback mechanisms When you eat a meal, your digestive system
breaks down the food 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 Acts on specific receptors to make target cells
more permeable to glucose
The Hormones of the Pancreas Insulin especially affects:
Muscle cells Use large amounts of
glucose in cellular respiration
Liver cells 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 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 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 Diabetes mellitus is a serious chronic
condition with no known cure Affects over 285 million people worldwide (as of
2009) Results when the body doesn’t produce
enough insulin, or does not respond properly to insulin 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 Derived from the Greek words “hyper” (too much),
“glyco” (sugar), and “emia” (condition of the blood)
The Effects of Glucose Imbalance 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 The individual experiences fatigue as the cells
become satrved for glucose The body compensates by switching to protein
and fat metabolism for energy 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 The kidneys are incapable of reabsorbing all of
the glucose that’s filtered through them from the blood 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 glucose-rich urine
The Effects of Glucose Imbalance In the long term, continued high levels of
blood glucose can lead to: 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 There are two major types of diabetes
mellitus: Type 1 diabetes (also called juvenile diabetes or
insulin-dependent diabetes) Type 2 diabetes (also called adult-onset diabetes
or non-insulin-dependent diabetes)
Causes of Diabetes In type 1, the immune system produces
antibodies that attack and destroy the beta cells of the pancreas 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
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 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 In 1889, the physician Oscar
Minkowski removed the pancreas from a healthy dog 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 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… 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
Held outside the body in a pouch of skin called the scrotum
The scrotum regulates the temperature of the testes 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
Its primary reproductive function is to transfer sperm from the male to the female reproductive tract
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 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 Seminal vesicles Prostate gland Cowper’s gland
The combination of sperm cells and fluids is called semen If sexual arousal continues, semen enters the urethra from
the ductus deferentia 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:
The seminiferous tubules in the testes to produce sperm Cells in the seminiferous tubules to release a hormones
called inhibin 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 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 Gametes are called eggs or ova (singular: ovum)
The other female sexual organs are adapted to: 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 A single ovum develops within each follicle
Each month, a follicle matures and ruptures, releasing the ovum into the oviduct 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 If a living egg encounters sperm in the oviduct, fertilization
will take place The fertilized egg, now called a zygote, continues
moving through the oviduct for several days before reaching the uterus 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 Ensures that an ovum is released at the same time as the uterus
is most receptive to a fertilized egg Usually about 28 days long
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 FSH stimulates one follicle to mature
As the follicle matures, it releases estrogen and some progesterone 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 embryo-supporting 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 Can ease some of the symptoms of menopause Also carries a number of health risks
HRT has been linked to: 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
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