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The Endocrine SystemThe Endocrine System
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HormonesHormonesRecall that neurons have the
ability to communicateHowever, this is merely cell-to-cellWhat if a signal needed to be sent
to more than one cell?This is the role of the endocrine
system◦Releases chemicals into bloodstream
to rest of body
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These chemicals are hormonesTwo major classes based on
chemical structure◦Can be water soluble or lipid soluble◦And can be exist as a polypeptide,
steroid, or amineReleased by glands, they have
one or two possible destinations: 1.Onto body surface or into body
cavities (exocrine)2.Into the bloodstream (endocrine)
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Although the effects of hormones can be long lasting, they can also take a period of time to reach target cells
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Local RegulatorsLocal RegulatorsSimilar to hormones, but have no
need to enter bloodstreamTarget cells are nearbyTwo types:
◦Paracrine – secreting cell is next to target
◦Autocrine – molecules act on secreting cell itself
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Neurotransmitters and Neurotransmitters and NeurohormonesNeurohormonesLastly, we have already seen
secreting action through neurotransmitters
Neurohormones are a special class where the hormone is released from a neuron instead of a gland
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Major GlandsMajor Glands
10 in total
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HypothalamusHormones both used by, and
stored in, pituitary gland
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Pituitary GlandOften called master glandControls other glands
Pineal GlandInvolved in circadian rhythm
ThyroidRegulates metabolism with
thyroxine
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Parathyroid glands4 in totalParathyroid hormone controls
calcium levels
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ThymusReleases hormones to stimulate
immune system
PancreasProduces insulin and glucagonHave opposite effects on blood
sugar levels
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Adrenal glandsReleases epinephrineAlso site of cortisol release
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Ovaries and TestesProduce sex-specific hormonesResponsible for secondary sex
characteristics.
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Maintaining HomeostasisMaintaining HomeostasisThe body can respond to internal
conditions◦Insulin released when blood glucose
levels are highHowever, what is released can
have a effect on the pathway that created it
These are feedback mechanismsHave both positive and negative
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Negative (more common) feedbacks involve inhibition of pathways by end products
Positive (less common) feedbacks stimulate pathways by end products
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Metabolism ControlMetabolism ControlIncreased thyroxine levels spike
metabolismBut, how is it released?Lowered thyroxine stimulates
hypothalamus to release thyrotropin-releasing hormone (TRH)
Travels to pituitary, releases thyroid-stimulating hormone (TSH)
Now thyroid releases thyroxine
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The negative feedback comes in play when high levels of thyroxine inhibits release of both TRH and TSH
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Insulin and GlucagonInsulin and GlucagonSometimes hormones act in
opposing mechanismsBlood glucose level needs to be
kept at 90 mg/100mLWhen it drops, alpha cells of
pancreas release glucagonAbsorbed by liver, glycogen
broken down into glucoseLevels go up, alpha cells shut off
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When blood glucose levels are high, beta cells release insulin
Body cells absorb glucose, liver synthesizes glycogen
Levels drop, beta cells shut offProblems in these systems lead
to diabetes and hypoglycemia
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Homework!Homework!Determine the difference
between the pathways for water-soluble and lipid-soluble hormones
Find and describe an example of a positive feedback mechanism