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Chapter 9: The Endocrine System

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1 Chapter 9: The Endocrine System
Unit 4: Homeostasis Chapter 9: The Endocrine System

2 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

3 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

4 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

5 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

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7 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

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9 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

10 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

11 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

12 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

13 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

14 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

15 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

16 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

17 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

18 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

19 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

20 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

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22 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

23 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

24 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

25 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

26 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

27 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

28 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

29 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

30 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

31 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

32 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

33 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

34 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

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36 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

37 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

38 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

39 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

40 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

41 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)

42 The Adrenal Cortex: Regulating the Long-Term Stress Response

43 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

44 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

45 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”

46 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

47 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

48 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

49 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 clusters of endocrine cells called the islets of Langerhans Named for Paul Langerhans, the scientist who first described them in 1869

50 The Hormones of the Pancreas

51 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

52 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

53 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

54 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

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56 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)

57 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

58 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

59 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

60 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)

61 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

62 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

63 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

64 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

65 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

66 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

67 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

68 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

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70 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

71 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

72 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

73 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

74 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

75 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

76 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 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

77 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

78 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

79 Hormonal Regulation of the Male Reproductive System

80 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

81 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

82 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

83 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

84 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

85 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

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87 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

88 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

89 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

90 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

91 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

92 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

93 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

94 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

95 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 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

96 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

97 Hormonal Regulation of the Female Reproductive System

98 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

99 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

100 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

101 The Ovarian Cycle

102 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

103 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

104 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

105 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

106 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

107 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

108 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

109 Summarizing Reproductive Hormones


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