1. Introduction to Homeostasis
September 2006
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
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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 an 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
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
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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
Unsuccessful Return to homeostasis
Nervous
Endocrine
Immune
Learned Behavioural Responses
Physical & Mental Stressors
Internal & External
Microbiology
Malnutrition
INTERVENTIONS
Pharmacology
Nutrition
Nursing
MDT
Successful Return to homeostasis & health
Altered Physiological function - Ill Health
Adaptation
Monitoring & Regulating systems
Imbalance in homeostasis
Homeostasis and Health
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Conclusion
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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