Underneath every diagnosis is a simple, powerful question:
What has disrupted the body’s ability to maintain balance?
That balance is called homeostasis.
What do we mean by homeostasis?
Homeostasis refers to the body’s ability to maintain a relatively stable internal environment despite constant internal and external change.
Importantly, relatively stable does not mean fixed.
Walter Cannon deliberately chose the term homeostasis (meaning similar condition), rather than implying a rigid, unchanging state. Your body is constantly adjusting — breathing, sweating, excreting, secreting hormones — to keep key variables within safe ranges.
This dynamic stability is what allows cells to survive.
The internal environment: a “sea within”
Most cells in the human body are not directly exposed to the outside world. Instead, they live in a carefully regulated internal environment known as the extracellular fluid (ECF).
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Intracellular fluid (ICF): fluid inside cells
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Extracellular fluid (ECF): fluid surrounding cells
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Cell membranes act as selective barriers between the two
Cells are remarkably intolerant of change. Even small disturbances in temperature, osmolarity, pH, or electrolyte concentration can impair function or cause injury.
Maintaining the composition of the ECF is therefore one of the body’s highest priorities.
Homeostasis does not mean equilibrium
A common early misconception is that homeostasis means “everything is equal everywhere”.
It doesn’t.
For example:
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Sodium (Na⁺) is much higher in the ECF
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Potassium (K⁺) is much higher inside cells
These compartments are not at equilibrium — they exist in a state of steady-state disequilibrium, actively maintained by energy-dependent processes.
Life depends on this imbalance.
Mass balance: why inputs must equal outputs
The body is an open system — matter and energy constantly move in and out.
To maintain homeostasis, the total amount of a substance in the body must obey the law of mass balance:
Change in body content = intake + production − excretion − metabolism
Water, sodium, oxygen, glucose, and waste products are all regulated this way.
If intake exceeds output, levels rise.
If output exceeds intake, levels fall.
When balance is lost, compensatory mechanisms activate. If compensation succeeds, health is restored. If it fails, disease develops.
This is the foundation of pathophysiology.
How the body controls variables: control systems
To keep variables within acceptable ranges, the body relies on control systems.
All control systems share three core components:
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Sensor – detects change
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Integrating centre – compares to a setpoint
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Effector – produces a response
There are two broad patterns:
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Local control – confined to a tissue (e.g. local vasodilation in hypoxic muscle)
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Reflex (long-distance) control – coordinated via nervous and/or endocrine systems (e.g. blood pressure regulation)
Most clinically relevant regulation relies on reflex control.
Negative vs positive feedback
Most homeostatic systems operate via negative feedback.
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The response opposes the original disturbance
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Once the variable returns to range, the response switches off
This is why body temperature, blood glucose, and blood pressure oscillate gently around a setpoint rather than staying perfectly flat.
Positive feedback, in contrast, amplifies change.
It is uncommon, tightly controlled, and usually requires an external signal to stop (e.g. labour, blood clotting).
When homeostasis fails
Disease can arise when:
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a regulated variable is pushed beyond its compensatory capacity
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a control system fails
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or competing demands overwhelm normal regulation
Importantly, symptoms are often consequences of compensation, not the original problem.
Understanding this reframes clinical reasoning:
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Tachycardia is often compensatory
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Sweating is protective
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Thirst is a signal, not a defect
Why this matters in Week 1
Homeostasis is not a single topic you “tick off”.
It is the organising principle of physiology.
Every system you study — cardiovascular, respiratory, renal, endocrine, nervous — exists to:
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sense change
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restore balance
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protect cells
If you keep asking:
What variable is being regulated?
What disrupted it?
What is the body trying to do in response?
You will find medicine makes far more sense.
A final reassurance
You are not expected to master all of this immediately.
Early learning in medicine often feels uncomfortable because you’re building frameworks before details. That’s normal — and necessary.
Understanding how the body maintains balance will help everything else you learn fall into place.
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