The document discusses the concept of homeostasis, defining it as the body's ability to maintain stable internal conditions despite external changes. It explains the roles of various body systems, particularly the nervous and endocrine systems, in regulating homeostasis through feedback mechanisms, both negative and positive. Additionally, it addresses the significance of maintaining homeostasis for health and the potential consequences of homeostatic imbalance.

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Gerry Lennon
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Learning Outcomes
1. Define homeostasis and explain it’s importance to
health
2. Identify the role of the nervous, endocrine and
circulatory systems in homeostasis
3. Describe the components of a feedback system
4. Compare positive and negative feedback loops and
give examples of each.
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Definition
‘Homeostasis is the body’s ability to maintain relatively stable
internal conditions even though the outside world is
continuously changing’
(Marieb, 2000)
The body is able to ‘buffer’ fluctuations in the environment
which ensures cell functioning and ultimately survival.
Changes include;
•Nutrients eg.Glucose
•Level of toxins/waste
•Salinity (osmotic pressure)
•Water
•pH
•Oxygen
•Temperature
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Why is homeostasis so important?
The body cells are extremely fussy about intracellular
conditions (inside the cell) and it is the fluid ‘outside’
the cell (extracellular/interstitial fluid) which
determines whether the inside of the cell is kept
constant. For continued health, chemical balance
must be maintained inside and outside of the cells
glucose carbon
dioxide
oxygen
ICF=Intracellular Fluid
cell
Extracellular
fluid (ECF)
ICF
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Maintenance of Homeostasis
Occurs via adaptive mechanisms under the control of
the NERVOUS SYSTEM (neural responses) and the
ENDOCRINE SYSTEM (hormonal responses).
Neural and hormonal responses ensure that the
following are kept at optimum levels;
•Body temperature
•Blood pressure (BP)
•Balance of body salts (eg. NaCl)
•Balance of O2 and CO2
•Concentration of substance in blood eg. H2O
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Body Systems and Homeostasis
Practically everything that goes on in the body helps
maintain homeostasis;
 Kidneys – removes a carefully regulated amount of
H2O and wastes
 Lungs – work with heart, blood vessels, blood to
distribute O2 and remove CO2
 Digestive system- nutrients pass through the small
intestine into the blood and then to all body parts
Coordinated feedback mechanisms ensure that
homeostasis takes place and appropriate adaptations
by body made in response to changes or STRESSORS
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Imbalance
 Most disease is seen as being due to the result of
homeostasis being disturbed i.e. homeostatic
imbalance
 With increasing age the control systems are less
efficient and the stability of internal environment is
weakened
Enabling Homeostasis to Occur
 Need communication within the body:
 Nervous system - uses electrical & chemical
information that travels through nerves
 Endocrine system - uses hormones (chemicals
released from endocrine organs) which travel in the
bloodstream to their target areas
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The Control System
Receptor
 Is a sensor that monitors the environment and
responds to the change (stimuli)
The control centre
 Determines the ‘set point’ i.e. the range or level at
which the variable has to be maintained
 Information is now analyzed and an appropriate
response
The effector
 This provides the way that something can happen
 The results then get fed back to influence the stimulus
so that it is repressed/ reduced (negative feedback) or
enhanced (positive feedback)
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Variables such as
temperature, blood
sugar, and BP are
regulated by the body’s
response to deviations
away from the norm
and which is called
‘Homeostasis’.
Changes in a variable
will stimulate a
receptor to send
messages to the brain
or control centre which
triggers a response to
bring it back to normal

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Negative Feedback
 The variable changes in a direction OPPOSITE to that
of the initial change returning it to its ‘ideal’ value
 The output of the system shuts off the original
stimulus or reduces its intensity
 The endocrine, cardiovascular and neurological
system are all important for this process
 One example is that of temperature control…
 Can you think of any others?
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Example of Negative Feedback
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Positive Feedback
 The result or response ENHANCES OR
EXAGGERATES the original stimulus so that the
activity (output) is accelerated
 It is ‘positive’ as the change that occurs is in the SAME
direction as the initial disturbance and so the variable
deviates further and further from its original value or
range
 They control infrequent events that do not need
continuous adjustments
 Amplifies something and usually triggers a cascade
system
 Example: clotting cascade and contractions when
giving birth
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Types of Stressors Affecting
Homeostasis
 Physical: heat, noise
 Chemical: food, hormones
 Microbiological: viruses, bacteria
 Physiological: tumour
 Developmental: age, genetic factors
 Psychological: emotional and mental disturbances.
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Illness and Death
•If all systems are functioning adequately then
homeostasis can be restored quickly via the feedback
mechanisms.
•If a system breaks down then person may feel ill.
•When the body is unable to restore homeostasis then
external interventions may be required in order to avoid
or delay death.
•Can you suggest such interventions you may have seen
in clinical practice?
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Group Work!
1. Split into groups of three and identify yourselves as A, B or C
2. Those who are As should explain the components included
in a ‘homeostatic feedback’ system and how it works- try
drawing a diagram to help you.
3. Those who are Bs need to give an example of a negative
feedback mechanism in our body and describe how it works
– including stating ‘where’ the control centre is located.
(choose a simple example!)
4. Finally, those of you who are the Cs need to give an example
of a positive feedback mechanism, and explain why this is
not as common as negative feedback systems. (you do not
need to explain the detail of your example)
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