The Endocrine System

A. Overview• Second messenger system of the body (first is

nervous)• Uses chemical messages (hormones) that are

released into blood• Hormones control several major processes–Reproduction–Growth and development–Mobilization of body defenses against stress–Maintenance of much of homeostasis–Regulation of metabolism

B. Hormones• Produced by specialized

cells in glands• Cells secrete hormones

into extracellular fluids• Blood transfers hormones to target sites or

effectors• Hormones regulate activity of other cells in

effector

• Three chemical classifications of hormones–Amino acid-based hormones•Proteins•Peptides•Amines

–Steroids – made from cholesterol–Prostaglandins – made from highly

active lipids

C. Hormones in Action• Affect only target tissues/organs via receptors– (that’s why heart muscle won’t produce

sperm – no receptors for that action)• Once hormone binds to receptor, alters

cellular activity – Increases or decreases normal metabolic

process

• Binding of receptor causes up to four changes–Changes in plasma membrane permeability

or electrical state (chemicals in/out)– Synthesis of proteins, such as enzymes

(digestion)–Activation or inactivation of enzymes– Stimulation of mitosis (growth)

• If steroid hormone:–Hormones diffuse through the plasma

membrane of target cells (steroids are lipid-based)– Enters the nucleus–Bind to a specific protein within the nucleus–Hormone-protein complex binds to specific

sites on the cell’s DNA–Activate genes that result in transcription &

translation to synthesize new proteins

Steroid Hormone Action

Figure 9.1a

• If non-steroid hormone (amino-acid or prostaglandins) …– Hormone binds to a membrane receptor (can’t

pass through membrane itself)– Hormone does not enter the cell– Sets off a series of reactions that activates an

enzyme– Catalyzes a reaction that produces a second

messenger molecule (either cAMP, Ca2+, or protein)– Messenger molecule oversees additional

intracellular changes to promote a specific response

Non-steroid Hormone Action

Figure 9.1b

D. Control of Hormone Release• Hormone levels in blood are maintained by negative

feedback• External or internal stimulus or low hormone levels

in blood triggers release of more hormone– Stimuli types: • Hormonal – hormones trigger endocrine glands• Humoral (blood chemistry) – levels of ions &

nutrients• Neural – nerve fibers trigger release (“flight or

fight”)• Hormone release stops once appropriate level in

blood is reached

• Endocrine glands are activated by other hormones

Figure 9.2a

Hormonal Stimuli

Humoral Stimuli• Changing blood levels

of certain ions stimulate hormone release

Figure 9.2b

Neural Stimuli• Nerve impulses

stimulate hormone release

• Most are under control of the sympathetic nervous system – “flight or fight”

Figure 9.2c

E. Major Endocrine Organs

Figure 9.3

1.1. Pituitary GlandPituitary Gland• Size of a grape• Hangs by a stalk from the

hypothalamus• Protected by the sphenoid bone area called

“Turk’s saddle”• Has two functional lobes– Anterior pituitary – glandular tissue– Posterior pituitary – nervous tissue

• Six anterior pituitary hormones– Two affect non-endocrine targets• Growth hormone (GH) – overall growth (synthesizing

protein), breaks down fats• Prolactin – stimulates breast milk production (lactation)

– Four stimulate other endocrine glands (tropic hormones)• Thyroid stimulating hormone (TSH)- thyroid gland• Adrenocorticotropic hormone (ACTH) - cortex of adrenal

gland• Follicle stimulating hormone (FSH) – follicles in ovaries

and sperm in testes• (women) Luteinizing hormone (LH) – ovulation of egg• (men) Interstitial cell-stimulating hormone (ICSH) –

testosterone production in testes

• Characteristics of all anterior pituitary hormones– Proteins (or peptides)– Act through second-messenger systems– Regulated by hormonal stimuli, mostly negative

feedback

• Posterior pituitary is not strictly an endocrine gland so it doesn’t MAKE hormones

• Posterior pituitary releases two hormones made by hypothalamus:– Oxytocin• Stimulates contractions of uterus during labor• Causes milk ejection triggered by suckling

– Antidiuretic hormone (ADH)• inhibits urine production• In large amounts, causes vasoconstriction leading

to increased blood pressure (as a result sometimes called vasopressin)

Hormones of the Posterior Pituitary

Figure 9.5

• produces the two hormones that are transported to neurosecretory cells of the posterior pituitary (ADH & oxytocin)

2.2. HypothalamusHypothalamus

3.3. Thyroid GlandThyroid Gland• Found at base of throat• Consists of two lobes and a

connecting isthmus• Produces two hormones:

– Thyroid hormone (TH) - two active iodine-containing hormones • Thyroxine (T4) – secreted by

thyroid follicles• Triiodothyronine (T3) –

conversion of T4 at target tissues

– Calcitonin - Decreases blood calcium levels by causing it to be deposited on bone

4.4. Parathyroid GlandsParathyroid Glands• Tiny masses on posterior

of thyroid• Secrete parathyroid

hormone (PTH)– Stimulate osteoclasts to

remove calcium from bone when levels in blood are too low

– Stimulate kidneys & intestine to absorb more calcium– Raise calcium levels in the blood (opposite calcitonin)

Calcitonin vs PTH

Figure 9.9

5.5. Adrenal GlandsAdrenal Glands• Two glands that sit on top of

kidneys• Divided into two areas:–Cortex – outer glandular

region in three layers–Medulla – inner neural tissue

region• Cortex produces three major groups of steroid

hormones collectively called corticosteroids

• CORTICOSTEROIDS:– Mineralocorticoids (mainly aldosterone)• Produced in outer adrenal cortex• Regulate mineral content in blood, water, and electrolyte

balance (target organ is kidney)• Production stimulated by renin and aldosterone• Production inhibited by atrial natriuretic peptide

– Glucocorticoids (including cortisone and cortisol)• Produced in middle layer of adrenal cortex• Promote normal cell metabolism• Help resist long-term stressors (work, family, health stress)• Released in response to increased blood levels of ACTH

– Sex hormones• Produced in inner layer of adrenal cortex• Androgens (male) and some estrogen (female)

Hormones of the Adrenal Cortex

Figure 9.10

• Two hormones produced by medulla called catecholamines which are triggered by sympathetic nervous system– Epinephrine (adrenaline)– Norepinephrine (noradrenaline)

• prepare the body to deal with short-term stress as in fight-or-flight scenarios

Figure 9.12

Adrenal Glands in Stress Response

6.6. Pancreas (Pancreatic Islets)Pancreas (Pancreatic Islets)• pancreas is a mixed gland• islets of pancreas produce hormones– Insulin – allows glucose to cross plasma

membranes into cells from beta cells• Blood glucose delivered to cells for energy

– Glucagon – allows glucose to enter blood from alpha cells• Converts stored glucose (glycogen) back into glucose

– these hormones are antagonists that maintain blood sugar homeostasis

Pancreatic Islets

Figure 9.13

Blood Sugar Hormones

Figure 9.14

Glucose ($)

Glycogen (€)

blood

liver

glycogenesis

Fats/protein (£)

gluconeogenesis

7.7. Pineal GlandPineal Gland• Found on third

ventricle of brain• Secretes melatonin– Helps establish body’s

wake/sleep cycles– May have other as-

yet-unsubstantiated functions

8.8. ThymusThymus• Located posterior to

sternum• Largest in infants and

children• Produces thymosin– Matures some types of

white blood cells– Important in developing the

immune system

9.9. OvariesOvaries• Produce estrogens– Produced by Graafian follicles

in ovaries or placenta– Stimulates development of

secondary female characteristics (

– Matures female reproductive organs

– Helps prepare uterus to receive a fertilized egg

– Helps maintain pregnancy– Prepares breasts to produce

milk

• Also produce progesterone– Produced by corpus luteum– Acts with estrogen to bring about menstrual cycle– Helps in implantation of an embryo in uterus

10.10.TestesTestes• Interstitial cells of testes are

hormone-producing• Produce several androgens• Testosterone is the most

important androgen– Responsible for adult male

secondary sex characteristics– Promotes growth and

maturation of male reproductive system

– Required for sperm cell production

11.11.PlacentaPlacenta• Produces hormones that

maintain pregnancy• Some hormones play a

part in baby delivery • Produces human chorionic

gonadotropin (hCG) in addition to estrogen, progesterone, and other hormones

• Home pregnancy tests check for presence of hCG

12.12.OthersOthers• Duodenum (first part) of small intestine– Gastrin – delivered to stomach to inhibit HCl– Secretin – stimulates pancreas to release high pH

juice; stimulates release of bile from liver– Cholecystokinin (CCK) – stimulates pancreas to

release enzymes; gallbladder to release stored bile

• Stomach– Gastrin – stimulates stomach glands to release HCl

• Adipose tissue– Leptin – stimulates brain to suppress appetite &

increase energy usage

Hormone Producing Agent

Function

Growth h. (GH) Ant. Pituitary Overall growth; synthesize proteins; breakdown fats

Prolactin Ant. Pituitary Stimulates lactation

Thyroid stimulating h. (TSH) Ant. Pituitary

Adrenocorticotropic h. (ACTH) Ant. Pituitary

Follicle stimulating h. (FSH) Ant. Pituitary

Luteinizing h. (LH) Ant. Pituitary

Interstitial cell-stimulating h. (ICSH) Ant. Pituitary

Oxytocin (released by post. pituitary) Hypothalamus

Antidiuretic h. (ADH) (released by post. pituitary) Hypothalamus

F. Diseases/Disorders • Long-term imbalance of particular

hormones leads to diseases or disorders of endocrine system–Acromegaly (gigantism)–Goiter–Addison’s disease–Diabetes mellitus (Type I & Type II)–Sterility (male & female)

Acromegaly• Hypersecretion (excess) of

pituitary growth hormone (GH) even after long bones have grown results in acromegaly or gigantism.– Facial bones, hands and feet

enlarge tremendously– Height can reach between 8 – 9

feet!– Results from tumor on pituitary

gland that secretes GH too.

Sandy Allen7’7 ¼”

Goiter• Enlargement of thyroid gland as a result of

deficiency of iodine– Iodine is needed to make thyroxine– When TSH stimulates thyroxine production, it’s

produced, but it’s disfunctional– TSH continues to stimulate thryoid, so in turn, it

grows to meet demands (although demands are never met)

• Iodine only found naturally in seafood, so USA added iodine to salt– Other land-locked areas continue to produce

goiters

Addison’s Disease• Hyposecretion (deficiency) of all

adrenal hormones results in Addison’s disease, and a bronzing of skin is the effect (melanocyte-stimulating hormone overactive).– Low aldosterone leads to water &

electrolyte loss– Low glucocorticoids leads to

hypoglycemia (too little glucose in blood), and suppression of immune system• No glucocorticoids would mean

death

Diabetes Mellitus• Hyperglycemia or excess glucose (sugar) in

blood– Literally from Latin “something sweet is being

siphoned from body”

• Normal blood glucose 80 – 120 mg/100 mL blood– Diabetes is higher than 126 mg (121-125 is

borderline)– Pancreas does not produce insulin (Type I) or cells’

receptors don’t recognize it (Type II)

• No glucose means no fast energy– Body must then break down proteins or fats for

energy, blood becomes acidic• Acidosis or ketosis results which can lead to coma or

death

• Three main signs of diabetes:– Polyuria = excessive urination to rid body of

glucose/ketones– Polydipsia = excess thirst from lack of water– Polyphagia = hunger since cells don’t receive

glucose energy

“Juvenile Diabetes”No insulin produced

“Adult-Onset Diabetes”Insulin produced, receptors weaker

• Long-term effects of uncontrolled diabetes (I or II):– Cardiovascular disease• Excess glucose in blood causes heart to beat faster

(hypertension/high blood pressure), circulate slower leading to tissue damage (possible gangrene), eventually heart attack

– Chronic renal (kidney) failure• Kidneys have to filter so much glucose, eventually shut

down from overwork

– Blindness (retinal damage)• Blood vessels in retina get damaged & retina can’t

process image

Sterility• Hormonal imbalances (too much or too little)

leads to sterility in both males and females– Males• Less FSH – little/no sperm production• Less LH – less testosterone• Less testosterone – little/no sperm production

– Females• Excess prolactin – no ovulation• Less FSH and/or LH – no ovulation• Less estrogen – less FSH/LH – no ovulation• Less progesterone - miscarriage

G. Development• In utero– Up to 11 weeks pregnant, both males &

female fetuses have same genital makeup– At 12 weeks, Y chromosome triggers

testosterone to form male genitals; absence of Y chromosome triggers estrogen to form female genitals

– If hormone levels are off, can result in intersex (formerly hermaphrodite) individual• Chromosomes show male or female

but genitalia something in between

Embryo at 11 weeks

• 46 XY Intersex– Male chromosomes, external

genitals incomplete or female-like

• 46 XX Intersex– Female chromosomes, ovaries,

male-like external genitalia

• Gonadal Intersex– Both ovarian and testicular tissue– May be either XY or XX

• Complex/Undetermined Intersex– XO, XXX, XXY, XYY

• Puberty– Increase in hormone production by ovaries or

testes to initiate secondary sex characteristics and be able to reproduce

– Girls: begins at 10-11 and complete by 15-17– Boys: begins at 12-13 and complete by 16-18

• Adulthood– Most endocrine organs operate smoothly until old

age

• Late Adulthood– Menopause in women is brought about by lack of

efficiency of the ovaries• Problems associated with reduced estrogen are

common (hot flashes, insomnia, moodiness)– Growth hormone production declines with age– Many endocrine glands decrease output with age

including testes