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Homeostasis
Chapter 1
Mr. Knowles
Anatomy and Physiology
Liberty Senior High School
Homeostasis is a balancing act!
Homeostasis
• “Homeo”- same.
• “-stasis”- standing or
status.
• To maintain a
relatively constant
internal environment.
• External environment
has many variablestemp., pH, amount of
energy, etc.
What is a stimulus and a
response?
• Stimulus- an
environmental change
that signals the body in
some way.
• Response- the change
in cellular activity the
body makes in order to
maintain homeostasis.
Homeostasis is a Balancing
Act!
• Chemical reactions within cells
work most effectively within a
certain range of conditions (temp.,
pH, salt, energy).
• Organ systems work together to
maintain this stable internal
environment.
Homeostatic Regulation
• Two Mechanisms:
1. Autoregulation- activities of a cell,
tissue or organ change automatically when
faced with environmental variation.
Ex. Cells in a certain tissue need more
O2. Cellsrelease chemicalsdilate
(open) blood vessels nearbylocal cells
receive more O2 . Only localized changes.
Homeostatic Regulation
• Two Mechanisms:
2. Extrinsic Regulation-activities of several
systems, such as the nervous and endocrine, work
together to adjust or change the internal
environment.
Ex. Touching a hot stove nervous system
respondsprocesses informationmuscle
contraction .
Ex. Endocrine system releases chemical
messengers (hormones)  affect many systems
for a long period, last for hours, days.
Homeostasis Has Three Parts
• Receptor- a sensor that is sensitive to a
particular environmental change or
stimulus.
• Control Center- receives and process
the information supplied by the receptor.
• Effector- a cell or organ that responds
to the commands of the control center
and whose activity opposes or enhances
the original stimulus.
Homeostasis in Your House!
Two Types of Feedback
1. Negative Feedback- when the
effector(s) activated by the control
center oppose or eliminate
stimulus. Most common feedback
system; thermoregulation, salt,
energy, pH, etc.
Another Perspective!
Negative Feedback-Ignores Minor
Changes
Homeostasis and Negative
Feedback
• It is range in which
there is a set point.
• The set point is not
a fixed value.
• The set point is
dynamic and can
change for the time
of day or the
individual.
Blood Sugar Regulation
Osmoregulation
Water Increase
The Second Kind of
Feedback
2. Positive Feedback- initial
stimulus produces a response
that exaggerates or enhances
its effects. Less common.
Ex. Labor contractions, Blood
clotting
Positive Feedback
A Physiological Example
When positive feedback
doesn’t stop…
Prolapsed Uterus in a Horse
Homeostasis is a Balancing
Act!
• Organ systems work
together to keep
within the range
around the set point.
• The systems are
interdependent.
• All are necessary for
survival.
Show Me an Example of
Homeostasis!
The New Living BodyHomeostasis, 1995, VT 574.1
HOM
Push Here for
Another Example