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Denny Agustiningsih
Dept. of Physiology
WHAT IS HOMEOSTASIS
 Concept
of homeostasis
was formulated by a
French Physiologist Claude
Bernard in 1865
 He noticed that La fixité
du milieu intérieur est la
condition de la vie libre (it
is the fixity of milieu
intérieur which is the
condition of free and
independent life)
 From
Greek words
 Homeo= same
 Stasis= to stand or
stay
 Term proposed by
American
Physiologist Walter
B Cannon, 1933
 Sensing
and responding to changes in
surrounding environment
 Control exchange of materials between cell and
its surrounding environment


Obtain nutrients and oxygen from surrounding
environment
Eliminate carbon dioxide and other wastes to
surrounding environment
 Perform
chemical reactions that provide energy
for the cell
 Synthesize needed cellular components
1.
2.
3.
DYNAMIC EQUILIBRIUM
MULTIPLE STIMULI
MAINTAINED BY NEGATIVE FEEDBACK
 Homeostasis
involves dynamic mechanisms
that detect and respond to deviations in
physiological variables from their “set point”
values by initiating effector responses that
restore the variables to the optimal
physiological range.
 Nervous


system
Controls and coordinates bodily activities that
require rapid responses
Detects and initiates reactions to changes in
external environment
 Endocrine


system
Secreting glands of endocrine regulate
activities that require duration rather than
speed
Controls concentration of nutrients and, by
adjusting kidney function, controls internal
environment’s volume and electrolyte
composition
Concentration of oxygen
and carbon dioxide
 pH of the internal
environment
 Concentration of
nutrients and waste
products
 Concentration of salt
and other electrolytes
 Volume and pressure of
extracellular fluid


Homeostasis is continually being
disrupted by
 External stimuli


heat, cold, lack of oxygen, pathogens,
toxins
Internal stimuli




Body temperature
Blood pressure
Concentration of water, glucose, salts,
oxygen, etc.
Physical and psychological distresses
Disruptions can be mild to severe
 If homeostasis is not maintained,
pathological condition even death
may result

 In
order to maintain homeostasis, control
system must be able to



Detect deviations from normal in the internal
environment that need to be held within narrow
limits
Integrate this information with other relevant
information
Make appropriate adjustments in order to restore
factor to its desired value
The organs and tissues which maintain homeostasis
are called control systems.
 Autoregulation (local control)



Intrinsic control


when cells, tissues, organs or systems automatically change in
response to signals within themselves.
controls that generate a response to change that built into the
organ.
Extrinsic control


changes stimulated by signals from outside of the cell, organ, or
system
Mediated by


Nervous system:


brain & spinal cord signals cause change;
response is rapid but short term.


glands produce hormones that cause changes;
response is slower, but lasts longer
Endocrine system
 Feedforward
- term used for responses made
in anticipation of a change
 Feedback - refers to responses made after
change has been detected

Types of feedback systems


Negative
Positive



Stimulus:

The change from ideal or resting conditions.

The cells or tissue which detects the change due to the
stimulus
Receptor:
Relay:


Effector:


The cells or tissue, usually a gland or muscles, which cause the
response to happen.
Response:


The transmission of the message, via nerves or hormones or
both, to the effector.
An action, at cell, tissue or whole organism level which would
not have occurred in the absence of the stimulus
Feedback:


The consequence of the response on the stimulus.
May be positive or negative.
A REGULATORY MECHANISM IN
WHICH A CHANGE IN A CONTROLLED
VARIABLE TRIGGERS A RESPONSE
THAT OPPOSES THE CHANGE.

Most common homeostatic feedback mechanism


responses that resists any change from normal range.
Restoration of homeostasis because
Negative feedback causes action that a value is falling too
low.
 Negative feedback causes action that a value is rising too
high.






Values fluctuate within Set limits,
Values change from hour to hour, sometimes with
regular diurnal cycles ( circadian rhythms).
Elevated or reduced changes are corrected before
they become too extreme.
Can continue forever
Maintains homeostasis
 EXAMPLE:
body
temperature - Set
point = 37 C, 98.6 F
 Normal limits 36.537.5 C, 97.7-99.5 F
Examples:
.Body Temperature – Nervous System
.Blood Calcium Levels – Endocrine System
When blood calcium level drops, parathyroid glands
sense that and secrete hormones that cause release
of calcium stores from bone. Blood calcium levels
return to normal, secretion stops.
Baroreceptors in walls of
blood vessels detect an
increase in BP
 Brain receives input and
signals blood vessels and
heart
 Blood vessels dilate, HR
decreases
 BP decreases

A REGULATORY MECHANISM IN
WHICH THE RESPONSE TO A
STIMULUS, IN A CONTROL SYSTEM,
CAUSES THE CONTROLLED VARIABLE
TO MOVE FARTHER FROM THE
SET POINT.



Rare homeostatic feedback
mechanism
The response enhances the original
stimulus
Positive feedback promotes
changes that are
developing, so they become
more extreme
 Positive feedback causes
action that a value is falling
causing it to fall farther.
 Positive feedback causes
action that a value is rising
causing it to rise higher.
 Positive feedback in disease
- ex. Hypothalamus
increases set point.
Development of a fever.

Examples:
.Hemorrhage
.Childbirth
.Lactation
 Unstable
system
 It is used to trigger a sudden event or
phenomenon
 Can not continue forever, always has a limit
 Does not result in homeostasis
 Beneficial only in special circumstances

Example in childbirth, stretching of uterus elicits
production of hormones that stimulate contraction
of uterine muscles; this leads to more stretching of
uterus, more hormone secretion, more muscle
contractions; loop is broken upon birth of a baby
 Stretch
receptors in walls of uterus send signals to
the brain
 Brain induces release of hormone (oxytocin) into
bloodstream
 Uterine smooth muscle contracts more forcefully
 More stretch, more hormone, more contraction etc.
 Cycle ends with birth of the baby & decrease in
stretch