Endocrine System

I. Endocrine Glands

A. 2 major types of glands

a. exocrine – discharge secretions through duct onto epithelial surface – lubricating or

digesting

b. endocrine – secretions go into blood and ECF hormones regulate metabolic processes

of other cells and are organic compounds

B. Major endocrine glands – pituitary, thyroid, parathyroid, adrenal, Isles of Langerhans

ovaries, testes

C. Organs with nonendocrine functions – kidneys  renin – blood pressure, GI – gastrin,

placenta

D. Hormone output

1. negative feedback

2. gland stimuli

a. humoral – secrete hormones in direct response to changing blood levels of

nutrients - insulin

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 b. neural – nerve stimuli – NE, EPI

c. hormonal – secrete hormones in response to hormones produced by other

endocrine organs

E. Hormone receptors

1. within target cells or on cell membranes

2. number change related to hormone levels

a. up-regulation – form more receptors in response to rising blood level of

hormone

b. down-regulation – loss of receptors to prevent target cells from overreacting

F. Half-Life – length of time a hormone remains in the blood/fraction of a minute to 30

minutes

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II. Hormone Structure and Mode of Action

A. Classified – according to structure with no reference to function because function can be

very diverse

B. Steroids – fat soluble, ring structure like cholesterol

1. organs and examples

a. adrenal cortex – aldosterone, cortisol

b. ovaries – estrogen

c. testis – testosterone

d. thyroid – thyroxin

2. activate transcription of specific genes – enter nucleus of target cell and form

hormone – receptor complex that interacts directly with cell’s DNA

C. Peptide hormones – short chain amino acids or large proteins attaches to exterior which

exerts intracellular effects

1. produced by – most endocrine glands

2. regulate activity from cell surface – can’t penetrate plasma membrane

a. bind to receptor proteins on target cell surface

b. causes cell to produce intracellular messenger molecules

c. second messengers – exert effect through these intracellular 2nd messengers

d. examples of 2nd messengers – cAMP and IP3

e. function – stimulate enzyme activity which produces a desired end product

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 3. phosphodiesterase degrades cAMP

III. Pituitary Gland

A. Connected to hypothalamus by infundibulum

B. Sella turcica

C. Hypothalamic control

1. produces regulatory hormones which affect anterior pituitary/released in portal

venous system

2. produces hormones stored in posterior

D. Anterior lobe – called master gland because products regulate the release of hormones by

other endocrine glands

1. cords of epithelial cells with granules of stored hormones

2. five cell types, six hormones

a. somatotropes—GH

b. mammotropes—prolactin

c. thyrotropes—TSH

d. corticotropes—ACTH

e. gonadotropes—FSH and LH

3. function

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 a. GH – stimulates body cell enlargement and long bone growth during formative

years – acromegaly

b. prolactin – initiates and maintains milk secretion via mammary glands

c. TSH – stimulates thyroid to synthesize and secrete thyroid hormone

d. ACTH – stimulates adrenal cortex to manufacture and secrete adrenocortical

hormones

e. FSH and LH – growth and development of gonads and control output of sex

hormones responsible for secondary sex characteristics

E. Posterior Lobe

1. pituicytes – specialized cells contained in meshwork of nerve fiber

2. pituitary stalk – bundles of nerve fibers, connect hypothalamus to posterior lobe not

linked by portal system

3. oxytocin – stimulates the following

a. uterus contraction

b. milk ejection—contractile cells – surround breast glands and ducts, nipple

stimulation initiates nerve impulses  hypothalamus  pos. pit.

4. ADH – main target is kidney

a. function causes cells of renal and collecting tubules to become more permeable

to water

b. ADH increases, water - absorption increases – creates less urine

c. ADH decreases – creates more urine, water absorption decreases

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IV. Thyroid Gland

A. Two lobes with isthmus

B. Spherical thyroid follicles with colloid surrounded by cuboidal follicular cells – stores

hormone as a colloid extracellularly then slowly released

C. Hormones T3 and T4 – secreted by follicular cells, general term for thyroid

1. function – regulate metabolic processes necessary for normal growth and

development – especially nervous system

D. Parafollicular cells – between follicles

1. calcitonin – regulates Ca metabolism Ca  calcitonin  stimulates blast  to take up Ca

E. Hormone synthesis

1. follicular cells – actively concentrate I and tyrosine to form T3 and T4

2. T3 and T4 and thyroglobulin = colloid

F. Hormone secretion—thyroxin

1. TSH—negative feedback

2. released from colloid

a. thyroid follicular cells

b. ingest colloid, pinocytosis

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 c. releases T3 and T4

d. degrades thyroglobulin

G. Metabolic effect

1. hyperthyroidism

a. too much

b. high body temperature, sweating, loose weight, high BP, oily skin, irritable

c. Graves disease

2. hypothyroidism

a. too little

b. weight gain, lethargy, very cold, dry skin

c. cretinism – severe hypo in infants, mentally retarded, short, thick tongue

3. goiter—lack of I2

a. enlarged thyroid

V. Parathyroid – secreted continuously not stored

A. Embedded in thyroid – 4 small glands

B. Secrete parathyroid hormone – PTH – principal regulator of Ca metabolism

C. Calcium requirements – cardiac, skeletal muscles, nerve impulse, blood coagulation

D. Reciprocal relationship between Ca and P

E. Sources of Ca and P – Ca – 1-2 g. per day but only 10% absorbed – P – abundant in diet,

absorbed easily with Ca

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 F. Parathyroid hormone and vitamin D and calcitonin regulate blood Ca

1. parathyroid hormone and vitamin D increase as Ca  therefore clast 

2. calcitonin increases as Ca  therefore blast 

3. changes Ca salts in bones

H. Calcium metabolism regulators

1. PTH – protein – increases Ca in blood

a. synthesized, secreted continuously

b. 3 sites of action

1. skeletal—releases Ca from bone

2. intestines—increases Ca absorption

3. kidneys—increases Ca reabsorption and inhibits

phosphate reabsorption

2. Vitamin D

a.  Ca absorption from intestines

b. UV converts sterol into intermediate

1. cholesterol derivative on skin

2. liver and kidneys metabolize intermediate to vitamin D

3. Calcitonin—antagonist –  Ca levels in blood

VI. Adrenal Glands – on top of kidneys

A. Cortex and medulla

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 B. Cortex—3 zones and main hormones – outer layer enclosed in fibrous capsule

1. outer zona glomerulosa—mineralocorticoids – aldosterone

2. middle zona fasciculata—glucocorticoids – cortisol

3. inner zona reticularis—androgens

C. Stimulated by ACTH to release adrenocortical hormones

D. Cortex glucocorticoids

1. cortisol—main

a. excess – Cushing’s syndrome – usually from medication

b. psychological and physiological stress can keep cortisol high

E. Cortex mineralocorticoids

+ +
1. Na absorption and K excretion – by kidneys

2. aldosterone—major – most important regulator of electrolytes and water balance

F. Sex hormones—estrogen, progesterone, testosterone

1. androgens

2. both sexes small amounts, more in gonads

3. sex drive in women

G. Medulla Hormones

1. norepinephrine and epinephrine – 2 hormones

2. stored in cytoplasmic granules – in medulla cells

3. postganglionic fibers—sympathetic

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 4. emotional stress  sympathetic  adrenal medulla – to release hormones in response

to fear, anger, anxiety

5. flight or fight

VII. Islets of Langerhans – 1 million cell clusters

A. Pancreas

B. Alpha – cells

1. 20%

2. release glucagon – increases blood glucose by causing liver to change glycogen 

glucose

C. Beta

1. 70%

2. release insulin

a. small protein – 51 amino acids

b. insulin released when blood glucose levels  causing  in blood glucose

c. stimulates conversion of glucose  glycogen

D. Insulin – major physiologic importance

1. ER  golgi  secretory granules – accumulate in cytoplasm of Beta cells

2. Insulin—2 short peptides joined by disulfide bonds

3. secretion

a. initial outpouring – from stored insulin in cytoplasmic granules of beta cells

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 b. slower, more sustained release – from synthesis and release of additional insulin

c. stimulus – blood glucose elevation which occurs after eating

d. ingested glucose more insulin secreted than intravenous glucose because

digestion triggers the release of hormones and amino acids that stimulate insulin

release

4. actions

a. multiple effects on metabolism – fat, carb, protein

b. chief sites – liver cells, muscle, adipose

c. promotes entry of glucose into cells and enhances its use as an energy source

d. promotes storage of glucose as glycogen in liver and muscles

e. promotes entry of amino acids into cells and stimulates protein synthesis

E. Diabetes

1. insulin absence restricts glucose uptake therefore secreted in urine – only hormone

that reduces blood sugar

2. diabetes mellitus

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 a. most common of all endocrine disorders – 6 m in US

b. forms

1. Type I—IDDM—before 20, – 10% autoimmune – immune system causes

B cell damage

c. Type II—NIDDM—after 40, target tissue loses responsiveness – not sure why

thought to be inherited, treat with diet, exercise and avoid foods that stimulate

insulin

VIII. Pineal Gland

A. Near center of brain

B. Secretes melatonin

1. modified amino acid

2. may regulate menstruation

3. involved in circadian rhythms

IX. Thymus – normal development of immune response is due in part to hormones produced here

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X. Ovaries and Testes

A. Ovaries—eggs, estrogen and progesterone

B. Testis—sperm, testosterone

C. Estrogen and progesterone

1. promote female sex traits – secondary sex traits at puberty

2. regulate menstrual cycle

3. maintain uterus for pregnancy

4. prepare mammary glands for lactation

D. Testosterone

1. promote male sex characteristics

2. sex drive – in males and females

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