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The Endocrine System. Endocrine System: Overview. Endocrine system – the body’s second great controlling system which influences metabolic activities of cells by means of hormones Endocrine glands – pituitary, thyroid, parathyroid, adrenal, pineal, and thymus

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The Endocrine System

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The endocrine system

The Endocrine System

Endocrine system overview

Endocrine System: Overview

  • Endocrine system – the body’s second great controlling system which influences metabolic activities of cells by means of hormones

  • Endocrine glands – pituitary, thyroid, parathyroid, adrenal, pineal, and thymus

  • The pancreas and gonads produce both hormones and exocrine products

Endocrine system overview1

The hypothalamus has both neural functions and releases hormones

Other tissues and organs that produce hormones – adipose cells, pockets of cells in the walls of the small intestine, stomach, kidneys, and heart

Endocrine System: Overview

The endocrine system

  • Hormones are substances secreted by cells that act to regulate the activity of other cells in the body.

    • Hormones affect all cells in the body and are made and secreted by endocrine glands.

  • Endocrine glands are ductless organs that secret hormones either into the bloodstream or the fluid around cells.

The endocrine system

  • The endocrine glands can be found through out the body and are collectively known as the endocrine system.

  • Endocrine glands, such as the pancreas, can also be exocrine glands.

    • Exocrine glands secrete substances through ducts to specific locations inside and outside the body.

The endocrine system1

The Endocrine System

Types of hormones

Types of Hormones

  • Hormones can be grouped into two types based on their structure. Hormones can either be amino acid-based hormones or steroid hormones.

    • Amino acid based-hormones are made of amino acids, either a single modified amino acid or a protein made of 3-200 amino acids, and are water soluble.

    • Steroid hormones are lipid hormones that the body makes from cholesterol and are fat soluble.

      • Similar to steroid hormones are thyroid hormones.

The endocrine system

  • Regardless of which type of hormone is being activated, all hormones affect only their target cells.

    • Target cells are specific cells to which a hormone travels to produce a specific effect.

    • On the target cells are receptors. Receptors are proteins that bind to specific signal molecules, such as hormones, that cause a cell to respond.

The endocrine system

  • Amino acid-based hormones

    • Amino acid-based hormones bind to receptor proteins on the cell membrane and are called first messengers.

    • Second messengers are then activated. A second messenger is a molecule that initiates changes inside a cell in response to the binding of a specific substance to a receptor on the outside of a cell.

      • Changes to the cell through enzyme action then occur in a cascade fashion.

The endocrine system

  • Steroid and Thyroid Hormones

    • Steroid and thyroid hormones are similar because they are both fat soluble.

    • Because these hormones are fat soluble they can pass through the cell membrane.

    • Thus, these hormones can enter their target cells and bind directly to receptor sites in the cytoplasm or nucleus and directly activate enzymes.

The endocrine system

  • Two other types of chemical messengers that are classified as hormones are neuropeptides and prostaglandins.

    • Neuropeptides are hormones secreted by the nervous system and tend to affect many cells near the nerve cells that release them.

    • Prostaglandins are modified fatty acids that are secreted by most cells and tend to accumulate in areas where tissues are disturbed or injured.

Hypothalamus and pituitary gland

Hypothalamus and Pituitary Gland

  • Two organs, the hypothalamus and the pituitary gland, control the initial release of many hormones for the endocrine system.

    • The hypothalamus is the area of the brain that coordinates many activities of the nervous and endocrine systems.

The endocrine system

  • The hypothalamus secretes:

  • 1. ADH = Antidiuretic Hormone=stimulates reabsorption of water in the body.

  • 2. Oxytocin- Used during childbirth to bring on labor contractions.

The endocrine system

  • The nerve cells in the hypothalamus that secrete hormones are calledneurosecretory cells.

    • These cells secrete two types of hormones to the pituitary gland.

      • Releasing hormonesstimulate the anterior pituitary to make and secrete hormones.

      • Release-inhibiting hormonesinhibit production and secretion of anterior-pituitary hormones.

Hypothalamus video

Hypothalamus Video

The pituitary gland

1. Known as the master gland. Controls all other glands.

2. It is stimulated by hormones from the hypothalamus to target other glands.

The Pituitary Gland

The endocrine system

  • The six hormones of the adenohypophysis:

    • Are abbreviated as GH, TSH, ACTH, FSH, LH, and PRL

    • Regulate the activity of other endocrine glands

  • In addition, pro-opiomelanocortin (POMC):

    • Has been isolated from the pituitary

    • Is enzymatically split into ACTH, opiates, and MSH

Growth hormone gh

Produced by somatotropic cells of the anterior lobe that:

Stimulate most cells, but target bone and skeletal muscle

Promote protein synthesis and encourage the use of fats for fuel

Most effects are mediated indirectly by somatomedins

Growth Hormone (GH)



  • Widening of bones

Pituitary video

Pituitary Video

Thyroid stimulating hormone thyrotropin

Thyroid Stimulating Hormone (Thyrotropin)

  • Tropic hormone that stimulates the normal development and secretory activity of the thyroid gland

  • Triggered by hypothalamic peptide thyrotropin-releasing hormone (TRH)

  • Rising blood levels of thyroid hormones act on the pituitary and hypothalamus to block the release of TSH

Adrenocorticotropic hormone corticotropin

Adrenocorticotropic Hormone (Corticotropin)

  • Stimulates the adrenal cortex to release corticosteroids

  • Triggered by hypothalamic corticotropin-releasing hormone (CRH) in a daily rhythm

  • Internal and external factors such as fever, hypoglycemia, and stressors can trigger the release of CRH



  • Gonadotropins – follicle-stimulating hormone (FSH) and luteinizing hormone (LH)

    • Regulate the function of the ovaries and testes

    • FSH stimulates gamete (egg or sperm) production

    • Absent from the blood in prepubertal boys and girls

    • Triggered by the hypothalamic gonadotropin-releasing hormone (GnRH) during and after puberty

Functions of gonadotropins in females

Functions of GonadotropinsIn females

  • LH works with FSH to cause maturation of the ovarian follicle

  • LH works alone to trigger ovulation (expulsion of the egg from the follicle)

  • LH promotes synthesis and release of estrogens and progesterone

Functions of gonadotropins in males

Functions of GonadotropinsIn males

  • LH stimulates interstitial cells of the testes to produce testosterone

  • LH is also referred to as interstitial cell-stimulating hormone (ICSH)

Prolactin prl

Prolactin (PRL)

  • In females, stimulates milk production by the breasts

  • Triggered by the hypothalamic prolactin-releasing hormone (PRH)

  • Inhibited by prolactin-inhibiting hormone (PIH)

  • Blood levels rise toward the end of pregnancy

  • Suckling stimulates PRH release and encourages continued milk production

The posterior pituitary and hypothalamic hormones

The Posterior Pituitary and Hypothalamic Hormones

  • Posterior pituitary – made of axons of hypothalamic neurons, stores antidiuretic hormone (ADH) and oxytocin

  • ADH and oxytocin are synthesized in the hypothalamus

  • ADH influences water balance

  • Oxytocin stimulates smooth muscle contraction in breasts and uterus

  • Both use PIP-calcium second-messenger mechanism



  • Oxytocin is a strong stimulant of uterine contraction

  • Regulated by a positive feedback mechanism to oxytocin in the blood

  • This leads to increased intensity of uterine contractions, ending in birth

  • Oxytocin triggers milk ejection (“letdown” reflex) in women producing milk

Antidiuretic hormone adh

Antidiuretic Hormone (ADH)

  • ADH helps to avoid dehydration or water overload

    • Prevents urine formation

  • Osmoreceptors monitor the solute concentration of the blood

  • With high solutes, ADH is synthesized and released, thus preserving water

  • With low solutes, ADH is not released, thus causing water loss from the body

  • Alcohol inhibits ADH release and causes copious urine output

The endocrine system

  • The tropic hormones that are released are:

    • Thyroid-stimulating hormone (TSH)

    • Adrenocorticotropic hormone (ACTH)

    • Follicle-stimulating hormone (FSH)

    • Luteinizing hormone (LH)

The endocrine system

  • 3. It secretes:

    • A. GH= Growth Hormone

    • B. ACTH= Adrenocorticotrophic Hormone

    • C. FSH= Follicle Stimulating Hormone

    • D. LH=Luteinizing Hormone

    • E. TSH=Thyroid Stimulating Hormone

The thyroid gland

The Thyroid Gland

  • The thyroid gland is located near the larynx and helps maintain a normal heart rate, blood pressure, and body temperature by increasing or decreasing cellular metabolic rates.

  • It secretes thyroxine.

  • The thyroid gland is also important for development.

Thyroid gland

The largest endocrine gland, located in the anterior neck, consists of two lateral lobes connected by a median tissue mass called the isthmus

Composed of follicles that produce the glycoprotein thyroglobulin

Colloid (thyroglobulin + iodine) fills the lumen of the follicles and is the precursor of thyroid hormone

Other endocrine cells, the parafollicular cells, produce the hormone calcitonin

Thyroid Gland

Thyroid hormone

Thyroid Hormone

  • Thyroid hormone – the body’s major metabolic hormone

  • Consists of two closely related iodine-containing compounds

    • T4 – thyroxine; has two tyrosine molecules plus four bound iodine atoms

    • T3 – triiodothyronine; has two tyrosines with three bound iodine atoms

Effects of thyroid hormone

Effects of Thyroid Hormone

  • TH is concerned with:

    • Glucose oxidation

    • Increasing metabolic rate

    • Heat production

  • TH plays a role in:

    • Maintaining blood pressure

    • Regulating tissue growth

    • Developing skeletal and nervous systems

    • Maturation and reproductive capabilities



The endocrine system

  • Abnormal thyroid activity can result in hypothyroidism or hyperthyroidism.

  • Overproduction of thyroid hormones is calledhyperthyroidismand can have symptoms that vary from overactivity to high body temperature.

  • A deficiency in a thyroid hormone is known ashypothyroidismand can have symptoms that vary from weight gain to retardation.

Thyroid video

Thyroid Video

Parathyroid glands

Tiny glands embedded in the posterior aspect of the thyroid

Cells are arranged in cords containing oxyphil and chief cells

Chief (principal) cells secrete PTH

PTH (parathormone) regulates calcium balance in the blood

Parathyroid Glands

Effects of parathyroid hormone

PTH release increases Ca2+ in the blood as it:

Stimulates osteoclasts to digest bone matrix

Enhances the reabsorption of Ca2+ and the secretion of phosphate by the kidneys

Increases absorption of Ca2+ by intestinal mucosal cells

Rising Ca2+ in the blood inhibits PTH release

Effects of Parathyroid Hormone

The endocrine system

  • Parathyroid Gland

    • The parathyroid glands is made up of four glands embedded in the two thyroid glands.

      • These glands secrete theparathyroid hormone, (calcitonin) which stimulates the transfer of calcium ions from the bones to the blood.



  • Calcitonin targets the skeleton, where it:

    • Inhibits osteoclast activity (and thus bone resorption) and release of calcium from the bone matrix

    • Stimulates calcium uptake and incorporation into the bone matrix

  • Regulated by a humoral (calcium ion concentration in the blood) negative feedback mechanism

Parathyroid video

Parathyroid Video

The adrenal gland

The Adrenal Gland

  • Humans have an adrenal gland located above each kidney. Each adrenal gland has an inner core, the medulla, and an outer core, also called the cortex.

  • The medulla and the cortex function as separate endocrine glands.

    • The medulla is controlled by the nervous system, and the cortex is controlled by the anterior pituitary.

Adrenal cortex

Adrenal Cortex



  • Regulate the electrolyte concentrations of extracellular fluids

  • Aldosterone – most important mineralocorticoid

    • Maintains Na+ balance by reducing excretion of sodium from the body

    • Stimulates reabsorption of Na+ by the kidneys


Aldosterone secretion is stimulated by:

Rising blood levels of K+

Low blood Na+

Decreasing blood volume or pressure


Glucocorticoids cortisol

Glucocorticoids (Cortisol)

  • Help the body resist stress by:

    • Keeping blood sugar levels relatively constant

    • Maintaining blood volume and preventing water shift into tissue

  • Cortisol provokes:

    • Gluconeogenesis (formation of glucose from noncarbohydrates)

    • Rises in blood glucose, fatty acids, and amino acids

Excessive levels of glucocorticoids

Excessive Levels of Glucocorticoids

  • Excessive levels of glucocorticoids:

    • Depress cartilage and bone formation

    • Inhibit inflammation

    • Depress the immune system

    • Promote changes in cardiovascular, neural, and gastrointestinal function

Adrenal video

Adrenal Video

Cushing s syndrome 11month old baby

Cushing’s Syndrome -11month old baby

Cushing s syndrome

Cushing’s Syndrome

The endocrine system

  • Adrenal Medulla

    • The adrenal medulla secretes the hormones that stimulate a “flight-or-fight” response to a stress.

    • In this response, the hormones epinephrine, also called adrenaline, and norepinephrine are released.

      • These hormones increases heart rate, blood pressure, blood glucose levels, and blood flow into the heart and lungs so the body can respond to the initial stress.

The endocrine system

  • Adrenal Cortex

    • In the presence of some stresses the pituitary gland will secrete the adrenocorticotropic hormone (ACTH).

    • This hormone stimulates the adrenal cortex to produce the hormone cortisol.

      • Cortisol promotes the production of glucose from proteins to help cells make usable energy.

Adrenal medulla

Adrenal Medulla

  • Made up of chromaffin cells that secrete epinephrine and norepinephrine

  • Secretion of these hormones causes:

    • Blood glucose levels to rise

    • Blood vessels to constrict

    • The heart to beat faster

    • Blood to be diverted to the brain, heart, and skeletal muscle

Adrenal medulla1

Adrenal Medulla

  • Epinephrine is the more potent stimulator of the heart and metabolic activities

  • Norepinephrine is more influential on peripheral vasoconstriction and blood pressure

The gonads

The Gonads

  • Gonads are the gamete-producing organs that also produce a group of steroid sex hormones.

    • Gonads, ovaries in females and the testes in males, are regulated by sex hormones, which begin production at puberty.

  • Puberty is the adolescent stage during which the sex organs mature and secondary sex characteristics appear.

The endocrine system

  • The production of sex hormones is stimulated by the release of two hormones by the pituitary.

    • The first hormone is luteinizing hormone (LH). This hormone stimulates ovulation and the release ofprogesteronein females and the release of androgens, such astestosterone,in males.

    • The second hormone is follicle-stimulating hormone (FSH). This hormone stimulates the growth and maturation of the ovarian follicles in females and sperm production in males.

Gonadocorticoids sex hormones

Gonadocorticoids (Sex Hormones)

  • Most gonadocorticoids secreted are androgens (male sex hormones), and the most important one is testosterone

  • Androgens contribute to:

    • The onset of puberty

    • The appearance of secondary sex characteristics

    • Sex drive in females

  • Androgens can be converted into estrogens after menopause

Gonads female

Gonads: Female

  • Paired ovaries in the abdominopelvic cavity produce estrogens and progesterone

  • They are responsible for:

    • Maturation of the reproductive organs

    • Appearance of secondary sexual characteristics

    • Breast development and cyclic changes in the uterine mucosa

Gonads male

Gonads: Male

  • Testes located in an extra-abdominal sac (scrotum) produce testosterone

  • Testosterone:

    • Initiates maturation of male reproductive organs

    • Causes appearance of secondary sexual characteristics and sex drive

    • Is necessary for sperm production

    • Maintains sex organs in their functional state

The gonad video

The Gonad Video

The pancreas

The Pancreas

  • The pancreas contains both exocrine and endocrine cells. The endocrine cells are calledislets of Langerhans.

  • These cells secrete hormones that regulate the level of sugar in the blood. The hormone insulin is one of the hormones produced.

    • Insulin lowers the blood sugar level by stimulating body cells to store glucose or use it for energy.

The endocrine system

  • A condition called diabetes mellitus occurs when cells are unable to obtain glucose from the blood. This results in a high glucose level in the blood

  • There are two types of diabetes: Type I and Type II.

    • Type I occurs when immune cells attack and destroy the islet of Langerhans cells.

    • Type II occurs when cells don’t have sufficient insulin levels or when the organism’s cells have become less responsive.

The endocrine system

  • A condition called hypoglycemia occurs when excessive insulin is stored and not properly delivered to body cells.

  • This leads to a lowered blood glucose concentration, which can cause such symptoms as overactivity and dizziness.



  • A triangular gland, which has both exocrine and endocrine cells, located behind the stomach

  • Acinar cells produce an enzyme-rich juice used for digestion (exocrine product)

  • Pancreatic islets (islets of Langerhans) produce hormones (endocrine products)

  • The islets contain two major cell types:

    • Alpha () cells that produce glucagon

    • Beta () cells that produce insulin



  • A 29-amino-acid polypeptide hormone that is a potent hyperglycemic agent

  • Its major target is the liver, where it promotes:

    • Glycogenolysis – the breakdown of glycogen to glucose

    • Gluconeogenesis – synthesis of glucose from lactic acid and noncarbohydrates

    • Release of glucose to the blood from liver cells


A 51-amino-acid protein consisting of two amino acid chains linked by disulfide bonds

Synthesized as part of proinsulin and then excised by enzymes, releasing functional insulin


Lowers blood glucose levels

Enhances transport of glucose into body cells

Counters metabolic activity that would enhance blood glucose levels


Diabetes mellitus dm

Diabetes Mellitus (DM)

  • Results from hyposecretion or hypoactivity of insulin

  • The three cardinal signs of DM are:

    • Polyuria – huge urine output

    • Polydipsia – excessive thirst

    • Polyphagia – excessive hunger and food consumption

  • Hyperinsulinism – excessive insulin secretion, resulting in hypoglycemia

Pancreas video

Pancreas Video

Other glands

Other Glands

  • There are several other glands in the endocrine system, including thymus gland, the pineal gland and the parathyroid glands.

  • Thymus Gland

    • The thymus gland is located beneath the sternum and plays a role in the development of the immune system by secretingthymosin.

      • This amino acid-based hormone stimulates formation of T cells.



  • Lobulated gland located deep to the sternum in the thorax

  • Major hormonal products are thymopoietins and thymosins

  • These hormones are essential for the development of the T lymphocytes (T cells) of the immune system

Thymus video

Thymus Video

The endocrine system

  • Pineal Gland

    • The pineal gland is located near the base of the brain and helps regulate sleep patterns by secreting melatonin.

Pineal gland

Pineal Gland

  • Small gland hanging from the roof of the third ventricle of the brain

  • Secretory product is melatonin

  • Melatonin is involved with:

    • Day/night cycles

    • Physiological processes that show rhythmic variations (body temperature, sleep, appetite)

Pineal gland video

Pineal Gland Video

The endocrine system

  • Digestive Cells

    • Some of the digestive organs also act as endocrine cells by secreting a variety of hormones that control digestive processes. Two hormones that are secreted are gastrin and secretin.

      • Gastrinis released by the stomach when food is eaten.

      • Secretinis released by the small intestine and helps stimulates the release of various digestive fluids from the pancreas.

Other hormone producing structures

Other Hormone-Producing Structures

  • Heart – produces atrial natriuretic peptide (ANP), which reduces blood pressure, blood volume, and blood sodium concentration

  • Gastrointestinal tract – enteroendocrine cells release local-acting digestive hormones

  • Placenta – releases hormones that influence the course of pregnancy

Other hormone producing structures1

Other Hormone-Producing Structures

  • Kidneys – secrete erythropoietin, which signals the production of red blood cells

  • Skin – produces cholecalciferol, the precursor of vitamin D

  • Adipose tissue – releases leptin, which is involved in the sensation of satiety, and stimulates increased energy expenditure

The endocrine system

  • The endocrine system plays an important role in the maintenance of a stable internal environment, or homeostasis.

  • Maintenance of homeostasis is controlled by feedback mechanisms. A feedback mechanism is one in which the last step in a series of events controls the first.

  • Feedback mechanisms can be either negative or positive.

The endocrine system

  • Negative Feedback

    • When the final step in a series of events inhibits the initial signal in the series it is called negative feedback.

    • An example of negative feedback is the regulation of thyroid hormones.

Negative feedback

Negative Feedback

The endocrine system

  • Positive Feedback

    • Positive feedback occurs when the release of an initial hormone stimulates release or production of other hormones or substances.

    • An example of positive feedback is the stimulation and increase in luteinizing hormone by estrogen.

The endocrine system

  • Antagonistic Hormones

    • Antagonistic hormones work together in pairs to regulate the levels of critical substances.

    • Example: Both glucagon and insulin regulate blood sugar levels.

The endocrine system


1.What are the chemical messengers of the endocrine system called?

A. neurons

B. hormones

C. blood cells

D. carbohydrates

The endocrine system

  • Answer

  • D. Hormones

The endocrine system

2. X and Y are hormones. X stimulates the secretion of Y, which exerts negative feedback on the cells that secrete X. Suppose the level of Y decreases. What should happen immediately afterwards?

F. Less X is secreted.

G. More X is secreted.

H. Secretion of Y stops.

J. Secretion of X stops.

The endocrine system

  • Answer

  • G. More X is secreted

The endocrine system

3.Endocrine glands

A. function only after puberty.

B. function only before puberty.

C. release products through ducts.

D. release products into the bloodstream.

The endocrine system

  • Answer

  • D. Release products into the bloodstream

The endocrine system

4.What happens after food is eaten?

F. Blood glucose levels increase.

G. Blood glucose levels decrease.

H. Blood glucose levels remain the same.

J. Blood glucose levels decrease then increase.

The endocrine system

  • Answer

  • F. Blood glucose increases

The endocrine system

5. Which hormones are primarily responsible for the changes in blood glucose levels about 2 hours after food is eaten?

A. insulin

B. estrogen and progesterone

C. epinephrine and norepinephrine

D. aldosterone and parathyroid hormones

The endocrine system

  • Answer

  • A. insulin

The endocrine system

6.Neurotransmitters : nervous system :: hormones :

F. feedback system

G. endocrine system

H. circulatory system

J. respiratory system

The endocrine system

  • Answer

  • G. Endocrine system

The endocrine system

7. Thyroid stimulating hormone (TSH) is a hormone that stimulates the release of the thyroid hormones from the thyroid gland. At what time would you expect thyroid hormone levels to be at their lowest?

A. 0 hours

B. 4 hours

C. 8 hours

D. 12 hours

The endocrine system

  • Answer

  • D. 12 hours

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