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

• The pancreas and gonads produce both hormones and exocrine products

Endocrine System: Overview

• 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

• 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 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 System

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.

• 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.

• 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.

• 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.

• 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

• 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 hypothalamus secretes:

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

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

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

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

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

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

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 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

http://www.ghorayeb.com/TransSphenoidAnatomy.html

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

Acromegaly

• Widening of bones

Pituitary Video

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)

• 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

• 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 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 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)

• 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

• 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

• 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)

• 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 tropic hormones that are released are:– Thyroid-stimulating hormone (TSH) – Adrenocorticotropic hormone (ACTH)– Follicle-stimulating hormone (FSH) – Luteinizing hormone (LH)

• 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 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 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

• 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

Goiter

• Abnormal thyroid activity can result in hypothyroidism or hyperthyroidism.

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

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

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

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

• Parathyroid Gland

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

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

Calcitonin

• 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

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

Mineralocorticoids

• 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

Mineralocorticoids

• Aldosterone secretion is stimulated by:– Rising blood levels of

K+

– Low blood Na+

– Decreasing blood volume or pressure

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:– Depress cartilage and bone formation– Inhibit inflammation– Depress the immune system– Promote changes in cardiovascular, neural,

and gastrointestinal function

Adrenal Video

Cushing’s Syndrome -11month old baby

Cushing’s Syndrome

• 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.

• 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

• 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 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

• 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 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 of progesterone in females and the release of androgens, such as testosterone, 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)

• 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

• 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

• 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 Pancreas

• The pancreas contains both exocrine and endocrine cells. The endocrine cells are called islets 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.

• 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.

• 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.

Pancreas

• 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

Glucagon

• 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

Insulin

• 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

• Insulin:– Lowers blood glucose levels– Enhances transport of glucose

into body cells– Counters metabolic activity

that would enhance blood glucose levels

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

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 secreting thymosin.

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

Thymus

• 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

• Pineal Gland

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

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

• 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.

• Gastrin is released by the stomach when food is eaten.

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

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 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 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.

• 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

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.

• Antagonistic Hormones

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

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

Quiz

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

A. neurons

B. hormones

C. blood cells

D. carbohydrates

• Answer

• D. Hormones

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.

• Answer

• G. More X is secreted

3.Endocrine glands

A. function only after puberty.

B. function only before puberty.

C. release products through ducts.

D. release products into the bloodstream.

• Answer

• D. Release products into the bloodstream

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.

• Answer

• F. Blood glucose increases

5. Which hormones are primarily responsible for the changes in blood glucose levels about 2 hours after food is eaten?A. insulinB. estrogen and progesteroneC. epinephrine and norepinephrineD. aldosterone and parathyroid hormones

• Answer

• A. insulin

6.Neurotransmitters : nervous system :: hormones :

F. feedback system

G. endocrine system

H. circulatory system

J. respiratory system

• Answer

• G. 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 hoursD. 12 hours

• Answer

• D. 12 hours