biology 30 endocrine system
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Hormonal Regulation and Homeostasis
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Overview: The Bodys Long-Distance
Regulators
` Animal hormones are chemical signals that are secretedinto the circulatory system and communicate regulatorymessages within the body
` Hormones reach all parts of the body, but only targetcells are equipped to respond
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The endocrine system and the nervous system act individually and
together in regulating an animals physiology
y Animals have two systems of internal communicationand regulation: the nervous system and the endocrinesystem
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` The nervous system conveys high-speed electricalsignals along specialized cells called neurons
` The endocrine system secretes hormones that
coordinate slower but longer-acting responses
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Overlap Between Endocrine and
Nervous Regulation
y The endocrine and nervous systems function together inmaintaining homeostasis, development, andreproduction
y Specialized nerve cells known as neurosecretory cellsrelease neurohormones into the blood
y Both endocrine hormones and neurohormones functionas long-distance regulators of many physiologicalprocesses
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Control Pathways and Feedback
Loops
y There are three types of hormonal control pathways:
simple endocrine, simple neurohormone, and simpleneuroendocrine
y Acommon feature is a feedback loop connecting theresponse to the initial stimulus
y
Negative feedback regulates many hormonal pathwaysinvolved in homeostasis
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Targeteffectors
Response
Simple endocrine pathway
Glycogenbreakdown,glucose releaseinto blood
Liver
Blood
vessel
Pancreassecretesglucagon ( )
Endocrinecell
Low bloodglucose
Receptorprotein
Stimulus
Pathway Example
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Targeteffectors
Response
Simple neurohormone pathway
Stimulus
Pathway Example
Suckling
Milk release
Smooth musclein breast
Neurosecretorycell
Bloodvessel
Posterior pituitarysecretes oxytocin( )
Hypothalamus/posterior pituitary
Sensoryneuron
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Targeteffectors
Response
Simple neuroendocrine pathway
Stimulus
Pathway Example
Milk production
Bloodvessel
Hypothalamus
Sensoryneuron
Mammary glands
Endocrinecell
Bloodvessel
Anteriorpituitarysecretesprolactin ( )
Hypothalamussecretes prolactin-releasinghormone ( )
Neurosecretorycell
Hypothalamicneurohormonereleased inresponse toneural and
hormonalsignals
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Hormones and other chemical signals bind to target cell
receptors, initiating pathways that culminate in specific cell
responses.
y Hormones convey information via the bloodstream to
target cells throughout the body
y Three major classes of molecules function as hormonesin vertebrates:
y Proteins and peptidesy Amines derived from amino acids
y Steroids
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y Signaling by any of these hormones involves three keyevents:
y Reception
y Signal transduction
y Response
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SECRETORYCELL
Hormonemolecule
Signal receptor
VIABLOOD
VIABLOOD
TARGETCELL
TARGET
CELLSignaltransductionpathway
OR
Cytoplasmicresponse
DNA
NUCLEUS
Nuclearresponse
Receptor in plasma membrane Receptor in cell nucleus
DNA
NUCLEUS
mRNA
Synthesis ofspecific proteins
Signal
transductionand response
Signalreceptor
Hormonemolecule
SECRETORYCELL
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y Binding of a hormone to its receptor initiates a signal
transduction pathway leading to responses in thecytoplasm or a change in gene expression
y The same hormone may have different effects on target
cells that havey Different receptors for the hormone
y Different signal transduction pathways
y Different proteins for carrying out the response
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` The hormone epinephrine has multiple effects inmediating the bodys response to short-term stress
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Different receptors different cell responses
Epinephrine
E receptor
Epinephrine
F receptor
Epinephrine
F receptor
Vesselconstricts
Vesseldilates
Intestinal bloodvessel
Skeletal muscleblood vessel
Liver cell
Different intracellular proteins different cell responses
Glycogendeposits
Glycogen
breaks down
and glucose
is released
from cell
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Intracellular Receptors for Lipid-Soluble Hormones
y Steroids, thyroid hormones, and the hormonal form of
vitamin D enter target cells and bind to protein receptorsin the cytoplasm or nucleus
y Protein-receptor complexes then act as transcription
factors in the nucleus, regulating transcription ofspecific genes
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Paracrine Signaling by Local Regulators
y In paracrine signaling, nonhormonal chemical signalscalled local regulators elicit responses in nearby targetcells
y Types of local regulators:
y Neurotransmitters
y Cytokines and growth factors
y Nitric oxide
y Prostaglandins
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` Prostaglandins help regulate aggregation of platelets, an
early step in formation of blood clots
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Testis(male)
Ovary(female)
Adrenal glands
Pancreas
Parathyroid glands
Thyroid gland
Pituitary gland
Pineal gland
Hypothalamus
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The hypothalamus and pituitary integrate many functions of the
vertebrate endocrine system
y The hypothalamus and the pituitary gland control muchof the endocrine system
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Relation Between the Hypothalamus and
Pituitary Gland
The hypothalamus, a region
of the lower brain, contains
neurosecretory cells
The posterior pituitary is anextension of the
hypothalamus
Hormonal secretions fromneurosecretory cells are
stored in or regulate the
pituitary gland
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Mammary glands,
uterine muscles
Hypothalamus
Kidney tubules
OxytocinHORMONE
TARGET
ADH
Posteriorpituitary
Neurosecretorycells of thehypothalamus
Axon
Anteriorpituitary
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y
Other hypothalamic cells produce tropic hormones,hormones that regulate endocrine organs
y Tropic hormones are secreted into the blood and
transported to the anterior pituitary
y The tropic hormones of the hypothalamus controlrelease of hormones from the anterior pituitary
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Neurosecretory cellsof the hypothalamus
Endocrine cells of theanterior pituitary
Portal vessels
Pituitary hormones(blue dots)
Pain receptorsin the brain
Endorphin Growth hormone
BonesLiver
MSH
Melanocytes
Prolactin
Mammaryglands
ACTH
Adrenalcortex
TSH
ThyroidTestes orovaries
FSH and LH
TARGET
HORMONE
Hypothalamicreleasinghormones(red dots)
Tropic Effects OnlyFSH, follicle-stimulating hormone
LH, luteinizing hormoneTSH, thyroid-stimulating hormone
ACTH, adrenocorticotropic hormone
Nontropic Effects OnlyProlactin
MSH, melanocyte-stimulating hormoneEndorphin
Nontropic and Tropic Effects
Growth hormone
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Posterior Pituitary Hormones
y The two hormones released from the posterior pituitaryact directly on nonendocrine tissues
y Oxytocin induces uterine contractions and milk ejection
y Antidiuretic hormone (ADH) enhances waterreabsorption in the kidneys
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Anterior Pituitary Hormones
y The anterior pituitary produces both tropic andnontropic hormones
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Tropic Hormones
yThe four strictly tropic hormones arey Follicle-stimulating hormone (FSH)
y Luteinizing hormone (LH)
y Thyroid-stimulating hormone (TSH)
y Adrenocorticotropic hormone (ACTH)
Each tropic hormone acts on its target endocrine tissueto stimulate release of hormone(s) with direct metabolic
or developmental effects
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Nontropic Hormones
y Nontropic hormones produced by the anterior pituitary:y Prolactin
y Prolactin stimulates lactation in mammals but has diverse effectsin different vertebrates
y Melanocyte-stimulating hormone (MSH)
y MSH influences skin pigmentation in some vertebrates and fatmetabolism in mammals
y F-endorphin
y Endorphins inhibit pain
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Growth Hormone
y Growth hormone (GH) has tropic and nontropic actions
y It promotes growth directly and has diverse metaboliceffects
y It stimulates production of growth factors
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Nonpituitary hormones help regulate metabolism,homeostasis, development, and behavior
y
Many nonpituitary hormones regulate various functionsin the body
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Thyroid Hormones
y
The thyroid gland consists oftwo lobes on the ventralsurface of the trachea
y It produces two iodine-containing hormones:triiodothyronine (T3) andthyroxine (T4)
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y The hypothalamus and anterior pituitary controlsecretion of thyroid hormones through two negativefeedback loops
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Hypothalamus
TRH
Anteriorpituitary
TSH
Thyroid
T3 T4
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` Thyroid hormones stimulate metabolism and influencedevelopment and maturation
` Hyperthyroidism, excessive secretion of thyroid
hormones, can cause Graves disease in humans
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y The thyroid gland also produces calcitonin, whichfunctions in calcium homeostasis
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Parathyroid Hormone and Calcitonin: Control of
Blood Calcium
y Two antagonistichormones, parathyroid
hormone (PTH) andcalcitonin, play the majorrole in calcium (Ca2+)homeostasis in mammals
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STIMULUS:Rising blood
Ca2+ level
Thyroid glandreleasescalcitonin.
Calcitonin
StimulatesCa2+ depositionin bones
ReducesCa2+ uptakein kidneys
Blood Ca2+
level declinesto set point
Homoeostasis:Blood Ca2+ level(about 10 mg/100 mL)
STIMULUS:Falling blood
Ca2+ level
Blood Ca2+
level risesto set point
StimulatesCa2+ releasefrom bones
PTH
Parathyroidgland
Stimulates Ca2+
uptake in kidneysActivevitamin D
IncreasesCa2+ uptakein intestines
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y
Calcitonin stimulates Ca2+
deposition in bones andsecretion by kidneys, lowering blood Ca2+ levels
y PTH, secreted by the parathyroid glands, has theopposite effects on the bones and kidneys, and thereforeraises Ca2+ levels
y PTH also has an indirect effect, stimulating the kidneysto activate vitamin D, which promotes intestinal uptake
of Ca2+ from food
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Insulin and Glucagon: Control of Blood Glucose
y The pancreas secretes insulinand glucagon, antagonistichormones that help maintain
glucose homeostasis
y Glucagon is produced byalpha cells
y Insulin is produced by betacells
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Beta cells ofpancreasrelease insulininto the blood.
Insulin
Liver takesup glucoseand stores itas glycogen.
STIMULUS:Rising blood glucose
level (for instance, aftereating a carbohydrate-
rich meal)
Blood glucose leveldeclines to set point;stimulus for insulinrelease diminishes.
Homeostasis:Blood glucose level
(about 90 mg/100 mL)
STIMULUS:Dropping blood glucoselevel (for instance, after
skipping a meal)
Blood glucose levelrises to set point;
stimulus for glucagonrelease diminishes.
Liver breaksdown glycogenand releasesglucose into theblood.
Body cellstake up moreglucose.
Alpha cells of pancreasrelease glucagon intothe blood.
Glucagon
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Target Tissues for Insulin andG
lucagon
y Insulin reduces blood glucose levels by
y Promoting the cellular uptake of glucose
y Slowing glycogen breakdown in the livery Promoting fat storage
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y Glucagon increases blood glucose levels byy Stimulating conversion of glycogen to glucose in the liver
y Stimulating breakdown of fat and protein into glucose
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Diabetes Mellitus
y Diabetes mellitus is perhaps the best-known endocrinedisorder
y It is caused by a deficiency of insulin or a decreased responseto insulin in target tissues
y It is marked by elevated blood glucose levels
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` Type I diabetes mellitus (insulin-dependent) is anautoimmune disorder in which the immune systemdestroys pancreatic beta cells
` Type II diabetes mellitus (non-insulin-dependent)involves insulin deficiency or reduced response of targetcells due to change in insulin receptors
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Adrenal Hormones: Response to
Stress
y The adrenal glands areadjacent to the kidneys
y Each adrenal gland actuallyconsists of two glands: theadrenal medulla and adrenalcortex
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.from the Adrenal Medullay The adrenal medulla secretes epinephrine (adrenaline)
and norepinephrine (noradrenaline)
y They are secreted in response to stress-activatedimpulses from the nervous system
y They mediate various fight-or-flight responses
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` Cortisol triggers the metabolism of proteins and fats to produceglucose. Cortisol also suppresses the immune system, which is
probably one reason that chronic stress is unhealthy.` Cortisol secretion is suppressed by negative feedback to the
hypothalamus and anterior pituitary.
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` The adrenal cortex also secretes aldosterone, which stimulatesthe kidneys to absorb sodium and thus water. This increasesthe blood pressure. When homeostasis is reached, negativefeedback shuts off the secretion of aldosterone.
` Low aldosterone causes low blood pressure, an imbalance ofelectrolytes (sodium and potassium ions) in the blood, andunhealthy weight loss, which are symptoms ofAddisons
disease.
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Stress Hormones from the Adrenal
Cortexy Hormones from the adrenal cortex also function in
response to stress
y They fall into three classes of steroid hormones:
y Glucocorticoids, such as cortisol, influence glucosemetabolism and the immune system
y Mineralocorticoids, such as aldosterone, affect salt andwater balance
y Sex hormones are produced in small amounts
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Stress
Hypothalamus
Anterior pituitary
Blood vessel
ACTH
ACTH
Releasing
hormone
Nervecell
Nerve cell
Long-term stress response
Effects of
mineralocorticoids:
1. Retention of sodiumions and water bykidneys
2. Increased bloodvolume and bloodpressure
Effects of
glucocorticoids:
1. Proteins and fatsbroken down andconverted to glucose,leading to increasedblood glucose
2. Immune system maybe suppressed
Short-term stress response
Effects of epinephrine and norepinephrine:
1. Glycogen broken down to glucose;increased blood glucose
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
5. Change in blood flow patterns, leading to
increased alertness and decreased
digestive and kidney activity
NervesignalsSpinal cord
(cross
section)
Adrenalgland
Kidney
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Gonadal Sex Hormones
yThe gonads, testes andovaries, produce most of thesex hormones:androgens(testosterone),
estrogens, and progestins
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` The testes primarily synthesize androgens, mainlytestosterone, which stimulate development andmaintenance of the male reproductive system
` Testosterone causes increase in muscle and bone massand is often taken as a supplement to cause musclegrowth, which carries health risks
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yEstrogens, most importantly estradiol, are responsiblefor maintenance of the female reproductive system andthe development of female secondary sex characteristics
y In mammals, progestins, which include progesterone,are primarily involved in preparing and maintaining theuterus
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Melatonin and Biorhythms
y The pineal gland, located inthe brain, secretesmelatonin
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Light/dark cycles control release of melatonin
y Primary functions of melatonin appear to relate tobiological rhythms associated with reproduction