introduction to homeostasis september 2006 clinical science team cardiffuniversitysonms©csanteam
TRANSCRIPT
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 feedbackCardiffUniversitySONMS©CSANTeam
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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HomeostasisThe 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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• 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/litreArterial plasma pH – 7.35 -7.45
Set Point or Set Homeostatic Range
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STRESS
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In physiological terms stress is defined as:
any stimulus that creates an imbalance (above or below the set homeostatic range), within the internal environment.
Stress
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• The stimuli that produce imbalances in homeostasis are called stressors. These fall into three categories:
1. Physical
2. Psychological
3. Sociological
Stressors
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Control SystemsThe 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 LoopsFor 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 feedbackThe 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 an antagonistic effector
• This affords a finer degree of control
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Negative feedback loopCardiffUniversitySONMS©CSANTeam
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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Positive feedback• An example of positive feedback occurs in
child birth
1. Contractions cause uterine muscle stretch
2. Signals sent to posterior pituitory
3. Oxytocin (a hormone) is released
4. Stimulates further contractions
+vefeedback
Unsuccessful Return to
homeostasis
Nervous Endocrine Immune Learned Behavioural Responses
Physical & Mental StressorsInternal & External
Microbiology
Malnutrition
INTERVENTIONSPharmacologyNutritionNursingMDT
Successful Return to homeostasis & health
Altered Physiological function - Ill Health
Adaptation
Monitoring & Regulating systems
Imbalance in homeostasis
Homeostasis and HealthCardiffUniversitySONMS©CSANTeam
ConclusionIn 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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