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