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Endocrine System
Hormones
& Homeostasis
Regents Biology
2009-2010
Homeostasis

Homeostasis

maintaining internal balance in the body



organism must keep internal conditions
stable even if environment changes
also called “dynamic equilibrium”
example: body temperature

humans:
 too cold = shiver
 too warm = sweat

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lizard:
 too cold = bask in sun
 too warm = hide in shade
Regulation

How we maintain homeostasis

nervous system


nerve signals control body functions
endocrine system


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hormones
chemical signals control body functions
Nervous System Control
Feedback
Controlling Body Temperature
nerve signals
brain
sweat
high
body temperature
low
brain
constricts surface shiver
blood vessels
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nerve signals
dilates surface
blood vessels
Hormones

Why are hormones needed?
chemical messages from one body part
to cells in other parts of body
 communication needed
to coordinate whole body
 maintaining homeostasis

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growth hormones
Endocrine System

Endocrine system releases hormones

glands which secrete (release)
chemical signals into blood

chemicals cause changes
in other parts of body
 growth hormones
 sex hormones
 response hormones
 metabolism hormones
 and more….
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Responding to hormones

Lock and key system

hormone fits receptor on “target” cell
target
cell
secreting
cell
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can’t
read
signal
nontarget
cells
can’t
read
signal
Glands

Pineal


Pituitary


insulin, glucagon
Ovary


adrenaline
Pancreas


thyroxine
Adrenal


many hormones:
master gland
Thyroid


melatonin
estrogen
Testes

testosterone
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Positive vs. Negative Feedback

Positive- Causes the variable to
accelerate the change in the same
direction of the initial disturbance.
or

Negative- Causes the variable to
change in a direction opposite to
that of the initial change.
or
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Feedback
Maintaining homeostasis
hormone 1
lowers
body condition
gland
high
specific body condition
low
raises
body condition
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gland
hormone 2
Negative Feedback

Response to changed body condition

if body is high or low from normal level


signal tells body to make changes that will
bring body back to normal level
once body is back
to normal level,
signal is
turned off
gland
hormone 1
lowers
body condition
high
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specific body condition
Endocrine System Control
Regulation of Blood Sugar
Feedback
insulin
liver stores
sugar
body
cells take
up sugar
from blood
pancreas
high
liver
blood sugar level
(90mg/100ml)
low
triggers
hunger
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liver
releases
sugar
liver
pancreas
glucagon
reduces
appetite
Positive or Negative Feedback?
Oxytocin Production during birth
Baby pushes on the
cervix
 Nervous system sends a
signal to the
hypothalamus
 Hypothalamus
manufactures oxytocin
 Oxytocin is transported
to the posterior pituitary
gland and released
 Oxytocin stimulates
uterine contraction
 Loop stops when the
baby leaves the birth
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canal

Positive or Negative Feedback?
Thyroid Gland and TRH production
1.
2.
3.
4.
5.
Hypothalamus senses that the
thyroxine level in the blood is low
Hypothalamus secretes thyrotropinreleasing hormone (TRH)
TRH stimulates the anterior pituitary
to secrete thyroid-stimulating
hormone (TSH)
TSH stimulates the release of thyroxine
by the thryoid gland
High levels of thyroxine in the blood
inhibit the secretion of TRH and TSH
(stops the release of additional
thyroxine)
*This
loop keeps the level of thyroxine in the
blood relatively constant.
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Feedback

Suppose the secretion of a certain hormone
causes an increase in the concentration of
substance X in the blood. A low concentration
of X causes the hormone to be released. What
is the effect on the rate of hormone secretion if
an abnormal condition causes the level of X in
the blood to remain very low?
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