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Introduction to Homeostasis
Clinical Science Team
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Learning Outcomes
•Define the term homeostasis
•Define the term internal environment
•Explain the principle of homeostatic set range
•Define the term stress as applied to physiological
systems
•Define the term stressor
•Define the two control systems
•Describe and illustrate a typical physiological control loop
•Explain the principle of negative feedback
•Explain the principle of positive feedback
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Homeostasis
‘The regulatory mechanisms of the body can be
understood in terms of a single shared function: that of
maintaining constancy of the internal environment. A
state of relative constancy of the internal environment is
known as homeostasis, and it is maintained by effectors
that are regulated by sensory information from the
internal environment (Fox 2002, p.5)’
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Homeostasis
The Body in Balance
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External Environment
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Threats from the External Environment
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Internal Environment
Nutrients &
Oxygen
etc
Waste
Products
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Homeostasis
• A condition in which the body’s
internal environment remains
within set physiological limits
(homeo = same; stasis = standing
still).
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Set Point or Set Homeostatic Range
Physiological set homeostatic points, or
the set homeostatic range:
• refer to the normal range of values for
given physiological factors
• equate with normal function and health of
both the cell and the individual, for example:
–Plasma glucose – 4 -7 mmol/litre
Arterial plasma pH – 7.35 -7.45
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STRESS
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Stress
In physiological terms stress is
defined as:
any stimulus that creates an
imbalance (above or below the
set homeostatic range), within
the internal environment.
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Stressors
• The stimuli that produce
imbalances in homeostasis are
called stressors. These fall into
three categories:
1. Physical
2. Psychological
3. Sociological
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Control Systems
The body detects and responds to homeostatic
imbalances via two complementary control
systems:
The Nervous System
The Endocrine System
These two control systems work together to
maintain homeostatic balance
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Control Systems Respond to and
Regulate Imbalances in Homeostasis
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Negative Feedback Loops
For constancy of the internal environment to be maintained,
the body must have:
 Sensors (receptors) that are able to detect deviations
from a set homeostatic point or range.
 An integrating centre that receives information from
the sensor (particular region of the brain/spinal cord, or
distinct cells within an endocrine gland). The integrating
centre responds by influencing the action of effectors.
 Effector cells or organs function to re-establish the
normal homeostatic range.
An analogy of this control loop is seen in temperature control
via a house thermostat:
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Negative feedback
The house thermostat…..
Imagine, the thermostat in your house is
set to 20°C (set point), it’s a warm day
and the temperature soon exceeds 20°C,
1) the thermostat (sensor) senses this
change,
2) its equivalent of an integrating centre
instructs the air conditioner (effector) to
activate which lowers the temperature
below the set point.
3) It reverses the temperature change.
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Negative feedback
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Antagonistic effectors
• Most factors are controlled by several
effectors
• These often have antagonistic (opposite)
effects
• Control by antagonistic effectors can be
described as ‘push-pull’
• Increasing activity of one effector is
accompanied by decreasing activity of the
corresponding antagonistic effector
• This affords a finer degree of control
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Negative feedback loop
Negative feedback loops
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Positive feedback
• Works in the opposite direction to negative
feedback
• Positive feedback amplifies the effect of the
change to the set point (i.e. the output that
another stimulus has activated)
• Think of the thermostat, if the mechanism was
Another example is the release of
positive feedback, a rise in temperature would
oxytocin
to by
intensify
the contractions
that
be
amplified
the effector,
thus the
takewould
placecontinue
during to
childbirth
temperature
increase
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Positive feedback
•
An example of positive feedback occurs in
child birth
1. Contractions cause uterine muscle
stretch
+ve
2. Signals sent to posterior pituitory
feedback
3. Oxytocin (a hormone) is released
4. Stimulates further contractions
Homeostasis and Health / Ill health
Physical Stressors (e.g. Infection; Malnutrition)
Psychosocial Stressors (e.g. Loss, Mental stress)
Unsuccessful outcome
Interventions
Successful outcome
Environmental Stressors (e.g. Toxins, Deprivation)
Altered Function and Ill
Health
Imbalance in homeostasis
Monitoring and regulating systems
Nervous
Endocrine
Immune
Successful outcome
Adaptation via
Nervous
Endocrine
Immune
Cardiovascular
Cognitive & physical behaviour
Gastrointestinal
Evolution (Genetic)
Respiratory
Genitourinary
Integumentary
e.g. maintenance of BP during postural changes
e.g. increased insulin production with increased glucose intake
e.g. creation of antibodies against specific disease
e.g. increased heart rate during exercise
e.g. mental coping strategies
e.g. diarrhoea and vomiting to eliminate toxins
e.g. production of melanin for skin protection
e.g. muco-ciliary clearance and cough
e.g. reduction of urine output to conserve fluid
e.g. sweating to maintain body temperature
Homeostasis and Health
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Unsuccessful outcome
Homeostasis is a process where by
physiological systems maintain a
reliable internal environment
1. Conditions remain stable even when the
environment is contantly changing.
2. Involves a dynamic state of equilibrium
(balance).
3. Control requires:
a. Receptor: picks up signal (stimulus)
b. Control Centre: determines set point.
analyzes/determines response.
c. Effector: causes increase or decrease in
activity to change initial stimulus.
Summary
In this session we have briefly explored the
following :
1. Homeostasis
2. The internal and external environment
3. Stress in physiological terms
4. Set point/ set range
5. Negative feedback loops
6. Positive feedback
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