Nat Sci 179—Saladin Ch. 17 Endocrine System

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Overview of Endocrine System
-consists of...

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endocrine glands & hormones they secrete

-we will look at: hypothalamus, pituitary, thyroid, parathyroid, pancreas, adrenal, sex organs

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hormones

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-chemical messengers
-neurotransmitters are another chem. messenger
-nt.’s are secreted by neurons
-secreted into the bloodstream
-act only on target cells

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Hypothalamus

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-releases several h’s that control the ant. pit.
-names end with RH (releasing hormone)
-exs: GHRH = growth h. releasing h.
PRH = prolactin releasing h.

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Posterior pituitary

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-makes no h’s
-receives them from hypothal. & stores them

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oxytocin

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-promotes contraction of uterine muscles during childbirth

-promotes milk release from breas

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ADH

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(antidiuretic h.)
-acts on kidneys to increase water retention

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Anterior pituitar

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several h’s are tropins (aka trophins) (stimulate release of another h. from another endocrine gland)

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FSH

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(follicle-stimulating h.)
-stimulates egg & sperm development

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LH

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(luteinizing h.)
-stimulates ovulation & cells of testes

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TSH

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(thyroid-stimulating h.)
-stimulates release of thyroid hormone from thyroid gland

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ACTH

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(adrenocorticotropic h.)
-stimulates release of h. by adrenal glands

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Prolactin

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-stimulates milk production
-in males, supports testes

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GH

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(growth hormone)
-aka somatostatin
-promotes cell division
-promotes growth in childhood
-increases metabolism
-secretion increases in sleep & exercise

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Thyroid Gland

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-T3 (triiodothyronine)
-T4 (thyroxine)

calcitonin

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-T3 (triiodothyronine)
-T4 (thyroxine)

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-contain iodine
-secreted in response to TSH from ant. pit.
-increase metabolic rate
-increases greatly in animals in winter

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calcitonin

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-maintains calcium ion homeostasis:
-increases calcium deposition in bone

-decreases calcium ion in blood
-increases activity of osteoblasts
-decreases activity of osteoclasts

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Parathyroid glands

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located behind the thyroid gland

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PTH (parathyroid hormone)

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maintains calcium ion homeostasis:
-decreases calcium deposition in bone
-increases calcium ion in blood
-increases activity of osteoclasts
-decreases activity of osteoblasts

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Adrenal Glands

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located on top of kidney

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adrenal medulla

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-secretes epinephrine & norepinephrine
-produces the sympathetic NS “fight-or-flight” response
-increases HR, BP, BR, blood glucose
-decreases other functions, such as digestion

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adrenal cortex

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secrete over 25 steroid h’s called corticosteroids

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aldosterone

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promotes water retention

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glucocorticoids

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-main one is cortisol
-promotes metabolism for stress & repair
-also has anti-inflammatory effect
-long-term secretion in stress can suppress

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cortisol

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-promotes metabolism for stress & repair
-also has anti-inflammatory effect
-long-term secretion in stress can suppress the immune system

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sex steroids

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androgens & estrogens estradiol

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androgens & estrogens

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produce secondary sex characteristics, maintain gonads
-main androgen is DHEA
-is converted to testosterone in both sexes
-testosterone is made is greater amts in

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estradiol

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main estrogen is estradiol
-source of estrogen after menopause

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Pancreas

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-also is an exocrine gland
-makes digestive enzymes that are carried to small intestine

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insulin

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-decreases blood glucose
-promotes uptake of blood glucose by cells

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glucagon

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promotes increase in blood glucose

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Gonads (1 of 2)

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-ovaries make an estrogen called estradiol, progesterone
-produce 2-dary sex characteristics
-regulate menstrual cycle
-regulate pregnancy

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Gonads (2 of 2)

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-testes produce testosterone, other androgens, and estrogen
-produce 2-dary sex characteristics
-maintain gonads & sperm production

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Hormone Receptors & Modes of Action

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-hormones act only on target cells
-cells with the right receptors
-specific

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two main mechanisms of action of hormones

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steroid and thyroid hormones

peptide hormones

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steroid and thyroid hormones

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these h’s are hydrophobic (lipid-soluble)
-can pass through cell membranes
-enter nucleus & bind to DNA-associated receptors; this hormone-receptor complex then binds to DNA
-affects protein synthesis (transcription & translation)
-misuse associated with cancers

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thyroid h’s can act in three ways

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-bind to mitochondria to increase cellular respiration
-increases metabolic rate
-bind to ribosomes to increase translation (&protein synth)
-bind to DNA to increase transcription (& protein synth.)
-one protein increased is the Na-K pump, which increases heat generation

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peptide hormones

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-cannot enter cell
-bind to receptors on outside of target cell
-this stimulates a second-messenger system
-starts a cascade of reactions
-cyclic AMP is a second messenger

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-how glucagon increases blood glucose:

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hormone glucagon binds to liver cell receptors -> activates G protein -> activates cAMP -> other enzymes activated that promote breakdown of glycogen in liver cells

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Methods of Hormone Regulation

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Enzyme Amplification

Hormone clearance

Modulation of Target Cell Sensitivity

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Enzyme Amplification

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E.A. means that one h. molecule can lead to production of LOTS of protein molecule

-result: great amplification of the final effect
-means body only needs small amts. of h.
-also target cells need small #’s of receptors

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Hormone clearance

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-body removes h’s from blood when effect is no longer needed
-length of h effect varies widely
-can be minutes, hours, days, weeks
-h’s are often destroyed by liver and kidneys, excreted in bile or urine

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Modulation of Target Cell Sensitivity

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-target cells can change their sensitivity to a h
-done by changing the # of receptors

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up-regulation

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-increase in # of receptors on target cells

-in late pregnancy, uterus produces receptors for oxytocin (will cause uterine contractions)

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down-regulation

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-decrease in number of receptors on target cells
-decreases the response of the target cell
-can happen with long-term exposure to the h.

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other methods of changing target cell sensitivity

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-ex: use of steroids by atheletes
-causes feminization in males due to two things:
-excess h. can bind to related receptors

(such as estrogen receptors)
-excess h. can be converted to estrogen

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Hormone Interactions

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-some target cells have receptors for more than one h.
-three possible effects:
-one h. increases effects of a second h.
-one h. decreases effects of another h.
-two h.’s are required to be present together to achieve an effect

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pituitary

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GH (growth hormone): dwarfism, gigantism

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thyroid

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-goiter: lack of iodine in diet
-hypothyroidism in childhood leads to abnormal nervous system development (mental retardation)
-Grave’s disease
-too much thyroid hormones
-bulging of eyes
-first President Bush & Mrs. Bush & dog

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adrenal

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-Cushing syndrome
-excess cortisol
-increased protein & carb. metabolism
-hyperglycemia, muscle loss
-characteristic thin limbs, but thick torso

-Addison’s disease
-too little adrenal hormones
-President Kennedy
-darkening of skin
-metabolic imbalance

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pancreas

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-diabetes mellitus (Type I and II)
-glucose cannot be utilized, so fat and muscle break down; ketoacidosis results from fatty acids in blood