1. 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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7. 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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10. 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 –
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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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The stimuli that produce imbalances in homeostasis are called stressors. These fall into three categories:
Physical
Psychological
Sociological
Stressors
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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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15. Control Systems Respond to and Regulate Imbalances in Homeostasis
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16. 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,
the thermostat (sensor) senses this change,
its equivalent of an integrating centre instructs the air conditioner (effector) to activate which lowers the temperature below the set point.
It reverses the temperature change.
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Negative feedback
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20. 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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21. Negative feedback loop
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22. 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 positive feedback, a rise in temperature would be amplified by the effector, thus the temperature would continue to increase
Another example is the release of oxytocin to intensify the contractions that take place during childbirth
Another example is the release of oxytocin to intensify the contractions that take place during childbirth
Marieb, Elaine N. & Hoehn, Katja (2007). Human Anatomy & Physiology (Seventh ed.). San Francisco, CA: Pearson Benjamin Cummings
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24. Positive feedback
An example of positive feedback occurs in child birth
Contractions cause uterine muscle stretch
Signals sent to posterior pituitory
Oxytocin (a hormone) is released
Stimulates further contractions
+ve
feedback

25. Homeostasis and Health
Homeostasis and Health / Ill health
Unsuccessful outcome
Successful outcome
Imbalance in homeostasis
Altered Function and Ill Health
Physical Stressors (e.g. Infection; Malnutrition)
Psychosocial Stressors (e.g. Loss, Mental stress)
Environmental Stressors (e.g. Toxins, Deprivation)
Interventions
Monitoring and regulating systems
Nervous Endocrine Immune
Adaptation via
Nervous e.g. maintenance of BP during postural changes
Endocrine e.g. increased insulin production with increased glucose intake
Immune e.g. creation of antibodies against specific disease
Cardiovascular e.g. increased heart rate during exercise
Cognitive & physical behaviour e.g. mental coping strategies
Gastrointestinal e.g. diarrhoea and vomiting to eliminate toxins
Evolution (Genetic) e.g. production of melanin for skin protection
Respiratory e.g. muco-ciliary clearance and cough
Genitourinary e.g. reduction of urine output to conserve fluid
Integumentary e.g. sweating to maintain body temperature
Homeostasis and Health
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26. 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.

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Summary
In this session we have briefly explored the
following :
Homeostasis
The internal and external environment
Stress in physiological terms
Set point/ set range
Negative feedback loops
Positive feedback
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