Unit 1:
Organization of
the Body
DLT #’s 5 – 7
Chapter 1: The Human Body: An Orientation
• What is homeostasis?
• The tendency of an organism or cell to
regulate its internal environment and
maintain equilibrium.
• Usually by a system of feedback controls
• To stabilize health and function
• The body is generally is homeostasis when its
needs are met and it is functioning properly.
DLT 5: I can define homeostasis
and explain its significance.
Homeostatic Control
System
• The results of the response from the effector feed
back to influence the stimulus, either depressing
it (negative feedback) so that the whole control
mechanism is shut off or enhancing it (positive
feedback) so that the reaction continues and is
amplified.
Negative Feedback
Positive Feedback
• Almost all homeostatic
control mechanisms.
• Change the variable back
to its original state or
“ideal value”.
• The exact opposite of a
negative feedback
mechanism.
• The output enhances the
original stimulus.
DLT 6: I can describe how negative
and positive feedback maintain
body homeostasis.
Negative Feedback
Examples
Positive Feedback
Examples
• A home thermostat. The thermostat
•
contains the receptor (thermometer)
and control center. If the heating
system is set at 70 degrees
Fahrenheit, the heat (effector) is turned
on if the temperature drops below 70
degrees Fahrenheit. After the heater
heats the house to 70 degrees
Fahrenheit, it shuts off effectively
maintaining the ideal temperature.
• The control of blood sugar (glucose) by •
insulin. When blood sugar rises,
receptors in the body sense a change .
In turn, the control center (pancreas)
secretes insulin into the blood
effectively lowering blood sugar levels.
Once blood sugar levels reach
homeostasis, the pancreas stops
releasing insulin.
Child birth. During labor, a
hormone called oxytocin is
released that intensifies and
speeds up contractions. The
increase in contractions causes
more oxytocin to be released and
the cycle goes on until the baby is
born. The birth ends the release of
oxytocin and ends the positive
feedback mechanism.
Blood clotting. Once a vessel is
damaged, platelets start to cling to
the injured site and release
chemicals that attract more
platelets. The platelets continue to
pile up and release chemicals until
a clot is formed.
• Most diseases can be regarded as a result of a disturbance
to homeostasis, and this is called homeostatic
imbalance.
• With age our body’s control systems become less
efficient, and as a result our internal environments
become less stable.
• This increases our risk for illness, and produces the changes
we associate with aging.
DLT 7: I can describe the
relationship between homeostatic
imbalance and disease.
• 1. What process allows us to adjust to either
extreme heat or extreme cold?
Review
• 2. When we begin to get dehydrated, we usually
get thirsty, which causes us to drink fluids. Is
thirst part of a negative or a positive feedback
control system? Be ready to defend your choice.
• 3. Why is the control
mechanism in this
figure called a
positive feedback
mechanism? What
event ends it?
• The response leads to
an even greater
response.
• When the clot seals
the break in the
vessel, the cascade
ends.