The Endocrine System

Chapter 9

9.1 - The Glands and Hormones of the Endocrine System 

Endocrine System

• Composed of hormone-producing glands and tissues of the body

• Works with nervous system to monitor and respond to body’s changing internal environment

• Functions: control of heart rate, growth/development, blood pressure. rate of metabolism, immune response to infection, reproduction, response to stress

• Slower and longer acting effects compared to the nervous system

Endocrine Glands

• Secrete chemical messengers (hormones) into the bloodstream and are transported throughout the body 

• Glands that function exclusively as endocrine glands: pituitary, pineal, thyroid, parathyroid and adrenal glands

• Glands with other functions: hypothalamus, thymus, pancreas, testes and ovaries

Diagram of glands 

Hormone

• Chemical messenger that circulates through blood and exerts control or effect on target cels, tissues or organs

• Secreted in small amounts

• Over 200 hormones in the human body 

Hormone Action on Target Cells

• A hormone targets a specific cell by binding to specific receptor proteins (lock-and-key fit)

• The target cell receives and responds to the chemical message sent by the hormone

Steroid Hormones

e.g. cortisol, testosterone, estrogen

• Lipid-based (hydrophobic): must combine with a protein carrier that transports them through bloodstream 

• can diffuse through cell membrane 

• In target cell they bind to a protein receptor molecule in nucleus 

• Hormone-receptor complex activates a gene and synthesis of a specific mRNA molecule 

• mRNA moves into cytoplasm for protein synthesis 

Steroid Hormones

Water-Soluble Hormones

e.g. epinephrine, hGH, thyroxine (T4), insulin

• Made of amino acids

• Cannot diffuse across cell membrane (large and polar)

• Binds to a receptor protein on the surface of a target cell

• Causes cascade of reactions within target cell

• Each reactions triggers many other reactions - impact of hormone is amplified

• One hormone can trigger release of hundreds of other molecules

Water-soluble Hormones

Regulating the Regulators

• Hypothalamus receives sensory info from nervous system

• Hypothalamus controls pituitary gland which produces hormones that stimulate endocrine glands in the body.

• Hormones that targets endocrine glands and stimulates them to release other hormones are tropic hormones. Hormones released from the Hypothalamus and Pituitary Gland are these types of hormones.

Regulation through negative feedback loops

Relationship between the Endocrine and Nervous System

• Hypothalamus: part of nervous system but also secretes hormones

• Some neurotransmitters are also hormones (eg. Epinephrine)

• Both systems are regulated by feedback loops

• Some physiological processes require both systems to work together. For example; lactation - sensory message in the neurons travels to hypothalamus, triggering the release of the hormone oxytocin, which travels through the bloodstream to the mammary glands, causing milk secretion

Hormones and their effect: Hypothalamus

• Hormone Secreted: hypothalamic releasing and inhibiting hormones

• Effect on target tissues/organs: regulates anterior pituitary hormones

Hormones and their effect: Anterior pituitary 

• Hormone secreted: Human growth hormone (hGH)

• Effect on target tissues/organs: stimulates cell division, bone and muscle growth and metabolic functions 

Hormones and their effect: Anterior pituitary 

• Hormone secreted: Thyroid-stimulating hormone (TSH)

• Effect on target tissues/organs: Stimulates the thyroid gland

Hormones and their effect: Anterior pituitary 

• Hormone secreted: Adrenocorticotropic hormone

• Effect on target tissues/organs: Stimulates the adrenal cortex to secrete glucocorticoids

Hormones and their effect: Anterior pituitary 

• Hormone secreted: Follicle-stimulating hormone (FSH) 

• Effect on target tissues/organs: Stimulates production of ova and sperm from the ovaries and testes

Hormones and their effect: Anterior pituitary 

• Hormone secreted: Luteinizing hormone (LH) 

• Effect on target tissues/organs: Stimulates sex hormone production from the ovaries and testes

Hormones and their effect: Anterior pituitary 

• Hormone secreted: Prolactin (PRL) 

• Effect on target tissues/organs: Stimulates milk production from the mammary glands

Hormones and their effect: Posterior pituitary 

• Hormone secreted: Antidiuretic hormone (ADH) 

• Effect on target tissues/organs: Promotes the retention of water by the kidneys

Hormones and their effect: Posterior pituitary

• Hormone secreted: Oxytocin (OCT) 

• Effect on target tissues/organs: Stimulates uterine muscle contractions and release of milk by the mammary glands

Hormones and their effect: Thyroid

• Hormone secreted: Thyroxine (T4) 

• Effect on target tissues/organs: Affects all tissues; increases metabolic rate and regulates growth and development

Hormones and their effect: Thyroid

• Hormone secreted: Calcitonin

• Effect on target tissues/organs: Targets bones and kidneys to lower blood calcium by inhibiting release of calcium from bone and reabsorption of calcium by kidneys

Hormones and their effect: Parathyroid

• Hormone secreted: Parathyroid hormone (PTH) 

• Effect on target tissues/organs: Raises blood calcium levels by stimulating the bone cells to release calcium, the intestine to absorb ca

Hormones and their effect: Adrenal Cortex

• Hormone secreted: Glucocorticoids (for example, cortisol) 

• Effect on target tissues/organs: Stimulates tissues to raise blood glucose and break down protein

Hormones and their effect: Adrenal Cortex

• Hormone secreted: Mineralocorticoids (for example,

  aldosterone)

• Effect on target tissues/organs: Promotes reabsorption of sodium and water by the kidneys

Hormones and their effect: Adrenal Cortex

• Hormone secreted: Gonadocorticoids 

• Effect on target tissues/organs: Promotes secondary sexual characteristics

Hormones and their effect: Adrenal Medulla

• Hormone secreted: Epinephrine and norepinephrine

• Effect on target tissues/organs: Fight-or-flight hormones

Hormones and their effect: Pancreas

• Hormone secreted: Insulin

• Effect on target tissues/organs: Lowers blood glucose levels and promotes the formation of glycogen in the liver

Hormones and their effect: Pancreas

• Hormone secreted: Glucagon 

• Effect on target tissues/organs: Raises blood glucose levels by converting glycogen in the liver to glucose

Hormones and their effect: Ovaries 

• Hormone secreted: Estrogen

• Effect on target tissues/organs: Stimulates uterine lining growth and promotes development of the female secondary sexual characteristics

Hormones and their effect: Ovaries

• Hormone secreted: Progesterone

• Effect on target tissues/organs: Promotes growth of the uterine lining and prevents uterine muscle contractions

Hormones and their effect: Testes

• Hormone secreted: Testosterone

• Effect on target tissues/organs: Promotes sperm formation and development of the male secondary sexual characteristics

9.2 Hormonal Regulation of Growth, Development, and Metabolism

Hypothalamus

• Regulates body’s internal environment to maintain homeostasis

• Responds to body signals like temperature, hunger, thirst, blood pressure, stress, emotions and

• Link between nervous system and endocrine system

• Produces hormones that are secreted into the pituitary gland and stimulate/inhibits production and secretion of anterior pituitary hormones

Hypothalamus diagram

Pituitary Gland 

• “Master Gland” - it secretes hormones into the bloodstream that control the secretions of other endocrine glands (thyroid gland, adrenal cortex, mammary glands, ovaries and testes) 

• 2 lobes - posterior and anterior 

• 1 cm in diamater 

Pituitary Gland

Posterior Pituitary

• Part of the nervous system

• Does NOT produce any hormones

• Stores and releases antidiuretic hormone (ADH) and oxytocin - both produced in the hypothalamus and transferred to the posterior pituitary by neurons

Anterior Pituitary

Produces and releases 6 major hormones:

• Thyroid-stimulating hormone (TSH) → targets thyroid gland

• Adrenocorticotropic hormone (ACTH) → targets adrenal cortex

• Prolactin (PRL) → targets mammary glands

• Human growth hormone (hGH) → targets most cells (bones, tissues)

• Follicle-stimulating hormone (FSH) → targets testes or ovaries

• Luteinizing hormone (LH) → targets testes or ovaries

• Hypothalamus sends message to anterior pituitary to stimulate/inhibit the release of hormones from this gland into the bloodstream

Human Growth Hormone (hGH)

• Affects almost every body tissue by direct simulation or tropic effects

• hGH stimulates liver to secrete growth factors which increase → protein synthesis, cell division and growth, and metabolic breakdown and release of fats stored in adipose tissue

Human Growth Hormone (hGH): Conditions

Gigantism: too much hGH secreted during childhood

Dwarfism: insufficient hGH production during childhood

Acromegaly: overproduction of hGH during adulthood (when skeletal growth is completed) bones and soft tissues widen

Thyroid Gland

• Located below the larynx and on top of the trachea

• Secretes immature thyroid hormones into the spaces between the cells of the thyroid gland

• Ex. Thyroxine (T4) once functional, will be released into the bloodstream, where it increases the rate at which the body metabolizes fats, proteins and carbohydrates for energy.

• T4 stimulates the cells of the heart, skeletal muscles, liver and kidneys to increase the rate of cellular respiration

Thyroid Gland

Thyroid Gland: Conditions

Hypothyroidism: failure of the thyroid to develop during childhood results in cretinism which results in low levels of T4. 

• Individuals are shorter than average, mental development delays

• Adults with this condition will feel tired, have slow pulse rate, and slow metabolism 

Thyroid Gland: Conditions

Hyperthyroidism: overproduction of T4 causes anxiety, insomnia, heat intolerance, irregular heartbeat and weight loss.

• Graves disease (severe state) results in the immune system attacking the thyroid

Thyroid Gland: Conditions

Goitre: Iodine is required to make thyroid hormones 

• If I2 levels are low, T4 cannot be made and no messages are sent to the anterior pituitary 

• Enlargement of thyroid gland result in Goitre which causes difficulty to breath, swallow and cough

The Thyroid Gland and Calcitonin

• Calcium is essential for healthy teeth/skeletal development

• It plays a crucial role in blood clotting, nerve conduction and muscle contraction

• If calcium levels are high, calcitonin is released and stimulates the uptake of calcium into the bones and lowering calcium levels in the blood

The Thyroid Gland and Calcitonin

Regulation of the thyroid gland by negative feedback

• Thyroid stimulating hormone (TSH) is released by anterior pituitary gland, which causes the thyroid gland to secrete thyroxine (T4)

• If T4 levels get too high, it signals the hypothalamus and anterior pituitary to decrease secretion of TSH which decrease T4 levels

Regulation of the thyroid gland by negative feedback

The Parathyroid Gland and parathyroid hormone (PTH)

• If calcium levels are low in the blood, parathyroid hormone (PTH) is produced and stimulates:

• Bone cells to break down bine material and secrete calcium into the blood

• The kidneys reabsorb calcium from the urine, activating vitamin D in the process

• Activated vitamin D stimulates the absorption of calcium from food in the intestines

• → all 3 of these processes increase the concentration of calcium in the blood back to normal levels

The Parathyroid Gland and parathyroid hormone (PTH)

9.3 Hormonal Regulation of Stress response and Blood Sugar

Adrenal Glands

• Located on top of the kidneys and are composed of two layers: Adrenal medulla (inner layer) and adrenal cortex (outer layer)

Adrenal Glands

Short term stress response

• Heart rate and blood pressure increase

• Blood flow to heart and muscles increases

• Breathing rate increases

• Blood glucose rises

• Rate of cellular metabolism increases

Long term stress response

• Kidney absorbs sodium ions and water, and blood volume and pressure increase 

• Protein and fat metabolism stimulated, which releases glucose 

• Inflammation is reduced and immune cells suppressed 

Adrenal Medulla

• Inner layer of adrenal gland

• Regulates short term stress response 

• Produces epinephrine (adrenaline) and norepinephrine (noradrenaline) which have a similar response as stimulation of sympathetic nervous system 

• Regulates short term stress response/fight or flight response by increasing metabolism  

• In response to stress:

• Sympathetic nervous system neurons send a signal from hypothalamus to adrenal medulla to produce epinephrine & norepinephrine

• Causes increase in breathing rate, heart rate, blood

  pressure, blood flow to the heart and muscles and conversion

  of glycogen to glucose in the liver, pupils dilate, decrease of

  blood to the extremities

• Epinephrine acts quickly-therefore used in life- threatening conditions

• Rapid release of the hormones since it is under nervous system control

Adrenal Cortex

• Outer layer of adrenal gland

• Regulates long-term stress response

• Produces:

• Glucocorticoids (e.g. cortisol - increases blood sugar)

• Mineralocorticoids (e.g. aldosterone-increase blood pressure)

• Gonadocorticoids (small amounts of female & male sex hormones

Cortisol

• Steroid hormone synthesized from cholesterol (glucocorticoid)

• If DANGER sensed:

→ hypothalamus secretes a releasing hormone which

→ stimulates the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH) which

→ targets the adrenal cortex

→ releases cortisol

• Causes an increase in blood glucose levels by promoting the breakdown of muscle protein into amino acids which get turned into glucose

• Promotes break down of fat cells

• Increased cortisol blood levels causes negative feedback on the hypothalamus and anterior pituitary

• Acts as anti-inflammatory by suppressing the actions of the immune system

• Sustained levels of cortisol can impair thinking, damage hear, increase blood pressure, lead to diabetes, increase susceptibility to infection, cause early death

Aldosterone

• Mineralocorticoid

• Stimulates kidneys to increase absorption of sodium into the blood, which increases the concentration of solutes in the blood which increases water retention in kidneys, increasing blood pressure 

Conditions

Addison’s disease-inadequate secretion of mineralocorticoids and glucocorticoids causes hypoglycemia (low blood sugar), sodium & potassium imbalances, rapid weight loss, increased urine output, decrease in blood pressure, may be fatal in a few days if not treated.

Hormones of the Pancreas

Much of the pancreas secretes digestive enzymes into the small intestine, but it can also function as an endocrine gland. 

→ Islets of Langerhans: pancreatic endocrine cells that secrete two antagonistic hormones:

• Insulin-secreted by beta cells- job is to decrease the level of blood glucose

• Glucagon-secreted by alpha cells- job is to increase the level of blood glucose

→ Both hormones are regulated by negative feedback mechanism

After Eating

• Glucose moves from intestinal tract into the blood

• Increases blood glucose levels

• Beta cells secrete insulin, which circulates and acts on specific receptors to make the target cells more permeable to glucose

• Especially effects muscle & liver cells where the glucose is converted into glycogen for storage, or other cells as an energy source

• Once levels of glucose in the blood decrease so too does the insulin release.

Exercise/Fasting

• Decrease in blood glucose levels

• Alpha cells release glucagon

• Stimulates the liver to convert glycogen back into glucose which moves into the blood

The Effects of Glucose Imbalance

Diabetes mellitus:

• serious chronic condition that results when the pancreas does not make enough insulin or the body does not respond properly to insulin

• Levels of blood glucose tend to rise after meals, called hyperglycemia, or high blood sugar

• Without insulin, cells remain impermeable to glucose and cells become starved for glucose

• The body compensates to some degree by switching to protein and fat

metabolism for energy

• Fat metabolism releases ketones, a toxic byproduct and can lead to

ketoacidosis (can be life-threatening)

Effects of Diabetes Mellitus:

• Kidneys are incapable of reabsorbing all the glucose that is filtered through them from the blood, therefore glucose is excreted in the urine

• Large volumes of water follow the glucose into the urine and get excreted

• People with untreated diabetes experience low energy, thirst, produce large volumes of glucose-rich urine

• Longer effects can lead to blindness, kidney failure, nerve damage, gangrene (a severe infection) in the limb

Causes of Diabetes Mellitus:

There are 2 types of diabetes:

→ Type 1 diabetes (also called juvenile diabetes and insulin-dependent diabetes

→ Type 2 diabetes (also called adult-onset or non-insulin-dependent diabetes

Type 1 diabetes:

• The immune system produces antibodies that attack and destroy the beta cells of the pancreas (cells that produce insulin)

• Must have daily insulin injections in order to live

Type 2 diabetes:

• The insulin receptors on the body’s cells stop responding to insulin and the beta cells of the pancreas produce less insulin over time

• People who are overweight have a greater chance of developing type 2 diabetes

• Can often be controlled with diet, exercise and oral medication

9.4 Hormonal Regulation of the Reproductive System

Gonads

• The organs that produce reproductive cells (sperm & egg) or gametes.

• Produce sex hormones- which are the chemical compounds that control the development and function of the reproductive system

Parts of the Male Reproductive System

Testes → seminiferous tubules - produce sperm (about 250m of tubules) and can produce more than 100 million sperm/day

• interstitial cells (in between the tubules)- produce testosterone-sperm then move to epididymis which is connected to a storage duct called the ductus deferens, which leads to-penis

Penis →Transfers sperm from male to female reproductive tract. Used for reproduction and urinary waste purposes

Seminal Fluid → (combination of fluid from seminal vesicles, prostate gland & Cowper's gland + sperm = semen)

Parts of the Male Reproductive System

Sex Hormones and the Male Reproductive System

• Development of male sex organs begins before birth.

• Y chromosome carries a gene called TDF (testis- determining factor), which leads to production of male sex hormones (androgens). This initiates development of male sex organs and ducts in fetus.

• Testis originate in abdominal cavity → then move down.

Puberty: 10-13 years old

• Hypothalamus increases production of GnRH (gonadotropin-releasing hormone) → goes to anterior pituitary gland  releases FSH & LH (follicle stimulating hormone and luteinizing hormone).

• These hormones cause the testes to produce sperm and release testosterone.

• Testosterone acts on various tissues to complete development of sex organs & sexual characteristic

Sex Hormones and the Male Reproductive System - REGULATION

FSH: Causes the seminiferous tubules to produce sperm &

release inhibin.

→ Inhibin acts on anterior pituitary to inhibit production of FSH (neg. feedback loop), as FSH drops, testes release less inhibin, therefore anterior pituitary releases more FSH, -therefore constant sperm production over time.

LH: Causes interstitial cells in testes to release testosterone. Testosterone promotes changes such as muscle development and formation of facial hair, and it acts on anterior pituitary to inhibit release of LH (neg. feedback loop).

→ Anabolic Steroids mimic action of testosterone in promoting muscle development… but disrupt reproductive hormone feedback system-shrinking testes, low sperm count and development of breast

How does aging affect the male reproductive system?

• Decline in testosterone at 40 (andropause) may be linked to symptoms such as fatigue, depression, loss of muscle and bone mass, decrease in sperm production.

• Low doses of testosterone can help.

• Prostate gland can get bigger. This leads to discomfort, urinary difficulties, can increase cancer risk.

Parts of the Female Reproductive System

• Ovaries: Two gonads contain the ova and connected to the uterus via the fallopian tubes. Each ovary has a follicle, where the singular ovum develops. Responsible for releasing a mature ovum (via cilia in the oviduct), to begin its journey to the uterus. Fallopian tubes is the site of fertilization.

• Uterus: Pear-shaped muscular organ. Inside it contains a lining made of layers of tissue designed to house and nurture the fertilized egg when it implants. If implantation does not occur the lining sheds. The opening of the uterus is called the cervix, this connects it to the birth canal (vagina).

• Vagina: Where the female reproductive system opens to the outside also known as the birth canal (where the fetus exits)

Parts of the Female Reproductive System

Parts of the Female Reproductive System

Sex Hormones and the Female Reproductive System

• Hypothalamus produces GnRH at puberty (9-13 years old), and causes anterior pituitary to release LH & FSH →

• These hormones act on the ovaries to produce estrogen & progesterone - stimulate development of female secondary sex characteristics and launch a reproductive cycle until about mid age

The Menstrual Cycle

• Period of roughly 28 days which hormones stimulate development of uterine lining, and egg released from ovary. If egg is not fertilized, uterine lining is shed (along with unfertilized egg)

• Split into 2 separate but interconnected cycles (occurring at the same time)

1) Ovarian Cycle → in the ovaries

2) Uterine Cycle → in the uterus

End result is that mature follicles release an ovum, so it is ready to be fertilized. At the same time, the uterus is preparing itself for the implantation of an egg

Ovarian Cycle

• Girls are born with 2 million follicles, 400k survive puberty, and 400 will mature and become an ovum (egg).

• Ovarian Cycle in 2 Stage

1. Follicular stage → inc. in the level of FSH – stimulates one follicle to mature, it releases estrogen and some progesterone. This acts on the anterior pituitary to inhibit release of FSH.

• At same time estrogen triggers a sudden release of GnRH from hypothalamus, this leads to a sharp inc. in LH production from anterior pituitary -triggering ovulation - follicle bursts releasing the ovum.

• Ovulation ends the follicular stage

Luteal stage - once ovum released, LH causes follicle to develop into Corpus Luteum → secretes progesterone and some estrogen. As levels of these hormones inc. in blood, they act on anterior pituitary to inhibit FSH & LH production. The Corpus Luteum degenerates, leading to a dec. in Estrogen & Progesterone, the low levels in blood cause anterior pituitary to inc. FSH & cycle begins again.

• if ovum fertilized- and implants in endometrium, blood levels of Estrogen & Progesterone remain high under stimulus of hormones released by embryo-supporting membrane. The continued presence of progesterone maintains the endometrium to support the developing fetus

Uterine cycle

• Begins on first day of menstruation - also first day of ovarian cycle-

• Corpus Leuteum has degenerated and levels of sex hormones in blood are low.

• Menstruation lasts for first 5 days of uterine cycle and by end endometrium (lining of the uterus) is very thin.

• As a new follicle begins to mature, and release Estrogen (inc. in blood) ~6th day of uterine cycle, Estrogen level is high enough to cause the endometrium to begin thickening.

• After ovulation, release of Progesterone by Corpus Luteum causes a more rapid thickening of endometrium. Between day 15 and 23 of cycle, thickness of endometrium may double or even triple.

• If fertilization doesn’t occur, Corpus Luteum degenerates, dec. sex hormones, endometrium breaks down and menstruation begins again

How does aging affect the female reproductive system?

• Number of viable follicles decreases with age, leading to a decrease in estrogen and progesterone

• Menstrual cycle becomes irregular at this point, and then stops altogether, known as menopause. This takes place around 50 years old.

• No ova produced anymore, dec. in sex hormones leads to: hot flashes, mood changes, rising cholesterol levels, diminishing bone mass, inc. risk of uterine cancer, breast cancer and heart disease

What is the purpose of HRT? What are some of the issues associated

with it?

• Low estrogen doses with or without Progesterone can ease symptoms of menopause

• Risks: Inc. coronary heart disease, strokes and blood clots, inc. risk of breast & colorectal cancer, inc. risk of dementia