Homeostasis
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Homeostasis
• Organisms use homeostasis to maintain a “steady state” or
internal balance regardless of external environment
• In humans, body temperature, blood pH, and blood glucose
concentration are each maintained at a constant level
• What are some other conditions (variables) your body
maintains?
•
Blood Gas (O2, CO2) Concentration
•
Blood Calcium (Ca+2) Concentration
• And more….
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Theme: Feedback Mechanisms Regulate
Biological Systems
• Feedback mechanisms allow biological processes to selfregulate
• Negative feedback means that as more of a product
accumulates, the process that creates it slows and less of
the product is produced
• Positive feedback means that as more of a product
accumulates, the process that creates it speeds up and
more of the product is produced
Animation: Negative
Feedback
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Animation: Positive
Feedback
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• Mechanisms of homeostasis
moderate changes in the
internal environment
• For a given variable,
fluctuations above or below a
set point serve as a stimulus;
these are detected by a
sensor/receptor, transmitted
to the control center and then
triggers a response
• The response returns the
variable to the set point
• The response may be through
hormones (Endocrine
System)or muscles (Muscular
System)
Mechanisms of Homeostasis
Set point
Feedback
Loop
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Feedback Control in Homeostasis
• The dynamic equilibrium of homeostasis is maintained
by negative feedback, which helps to return a variable
to a normal range
• Most homeostatic control systems function by negative
feedback, where buildup of the end product shuts the
system off
• Positive feedback amplifies a stimulus and does not
usually contribute to homeostasis in animals
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The regulation process is a constant ”battle” to bring the variable back towards the set point, as a stress or
stimulus causes the variable to deviate from the set point, homeostatic mechanisms work to bring the
variable back (up or down) to the set point. (dynamic equilibrium)
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Response: Foot
off Gas Pedal
Maintaining
The Speed Limit
Control Center (Brain)
deviation above set pointupper feedback loop
Cars Slows
Stimulus: Too fast
Set Point:
Speed Limit 40 mph
Stimulus: Too Slow
Cars Speeds Up
deviation below set pointlower feedback loop
Control Center( Brain)
Response: Foot pushes
on Gas Pedal
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Complete the
Heating System Homeostasis
worksheet now.
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Figure 40.8
Heating System
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Does the temperature of the home heated by this
furnace remain exactly at 20°C all day long?
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Temperature
Time
Throughout the day,
The temperature
fluctuates, but the
thermostat and
furnace work
together to keep the
house’s temperature
within a narrow
range, above or
below the set
point.
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Physiological an Behavioral Responses In Thermoregulation
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Blood Glucose
Homeostasis
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Complete the Body
Temperature Homeostasis
worksheet now.
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Figure 40.16
Body Temperature
Homeostasis
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Positive Feedback
A ripening apple releases the volatile plant
hormone ethylene (the stimulus). Ethylene
accelerates the ripening of unripe fruit in
its vicinity so nearby fruit also ripens,
releasing more ethylene (the response).
All the fruit quickly becomes ripe together.
("One 'bad' apple has ruined the whole lot."
The biological explanation for an old saying
- positive feedback!)
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