HOMEOSTASIS is the maintenance of a stable internal environment despite changes in external conditions or internal activity.
The body must keep the following conditions within very narrow limits:
BODY TEMPERATURE — maintained at ~37°C for optimal enzyme activity.
BLOOD GLUCOSE CONCENTRATION — maintained at ~5 mmol/L for cell function.
WATER CONTENT of blood and tissue fluid — controlled to prevent cells shrinking or swelling.
If any of these deviate significantly from their set points, cells and organs begin to malfunction — potentially causing death.
Homeostasis is what allows warm-blooded animals like humans to survive in very different environments, from Arctic tundra to tropical desert.
Negative Feedback
All homeostatic control systems use NEGATIVE FEEDBACK.
Negative feedback means: when a variable moves AWAY from the set point, the system responds to bring it BACK.
The word 'negative' means the response OPPOSES the change — it works against the deviation, not with it.
Example: if body temperature rises, the response is to COOL the body (opposing the rise). If body temperature falls, the response is to WARM the body (opposing the fall).
Negative feedback keeps variables oscillating slightly around a set point — rarely exactly at it, but always returning to it.
The Three Components of Homeostatic Control
Every homeostatic control system has the same three components:
1. RECEPTOR (sensor):
Detects changes in the variable being controlled.
Generates a signal proportional to the degree of change.
Examples: thermoreceptors in skin (detect temperature), osmoreceptors in hypothalamus (detect blood water content), glucose receptors in pancreas.
2. COORDINATION CENTRE (control centre):
Receives information from receptors.
Compares the actual value to the set point.
Determines the appropriate response.
Examples: the hypothalamus in the brain (temperature, water), the pancreas (blood glucose).
3. EFFECTOR:
Carries out the response that corrects the deviation.
Can be a MUSCLE (e.g. skeletal muscles shivering) or a GLAND (e.g. sweat glands, endocrine glands releasing hormones).
The effector's response reduces the original stimulus — this completes the negative feedback loop.
Why Homeostasis is Essential
Enzymes control almost every biochemical reaction in the body — and enzymes are extremely sensitive to temperature, pH and concentration.
IF TEMPERATURE RISES TOO HIGH:
Enzymes denature → active site shape changes permanently → reactions stop → cells die.
Body temperature above ~40°C causes proteins to denature → organ failure → death.
IF BLOOD GLUCOSE FALLS TOO LOW (hypoglycaemia):
Brain cells are starved of glucose → confusion, unconsciousness, brain damage, coma.
Too little water → cells shrink → dehydration of tissues → organ failure.
Too much water → cells swell → can burst (lysis) in animal cells.
Homeostasis protects all these systems from harmful fluctuations.
⚠️ Common Mistake
Negative feedback does NOT mean the response is negative or harmful. It means the response OPPOSES the change — it is a corrective response. If temperature rises, the negative feedback brings it back DOWN. If temperature falls, it brings it back UP. The 'negative' refers to the direction of the response relative to the change.
📌 Key Note
Homeostasis: stable internal environment via negative feedback. Receptor → coordination centre → effector. Controls: temperature (37°C), blood glucose (~5 mmol/L), water content. Essential because enzymes are sensitive to these conditions.
🎯 Matching Activity — Match the Homeostatic Component
Match each component to its role in the homeostatic control system. — drag the symbols on the right to match the component names on the left.
Receptor
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Coordination centre
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Effector
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Negative feedback
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Set point
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Receives the signal, compares to set point, decides the response
Detects a change in the variable — generates a signal
Carries out the response — a muscle or gland
The target value the body aims to maintain — e.g. 37°C for temperature
The response opposes the original change — brings the variable back to the set point
🎯 Test Yourself
Question 1 of 3
1. What is homeostasis?
2. Why is maintaining body temperature at 37°C so important?
3. A person's blood glucose level drops too low. What type of response will occur?
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