Year 12 BiologyModule 8 · IQ1⏱ ~45 minPractice bank · 3 Short AnswerLesson 1 of 21
Homeostasis, Stimulus-Response, Feedback Loops and the Internal Environment
Your internal conditions stay remarkably stable while the world around you changes. What detects the change, and what decides the correction?
Today's hook: Walter Cannon introduced the term homeostasis in 1926 to describe dynamic internal stability, not a body frozen at one value. What feedback machinery keeps variables such as temperature and blood glucose within workable ranges while a person eats, moves and responds to stress?
0/5TASKS
Your guided path0 of 8 done · keep going
1
You’re here
Orient and predict
See the lesson destination, connect it to what your body is doing now, and commit to an initial explanation.
Why this lesson starts here
Claude Bernard developed the idea of a stable milieu intérieur (internal environment) in the nineteenth century. Walter Cannon later introduced the term homeostasis in 1926 for the coordinated regulation that maintains this dynamic stability. Disrupted regulation helps explain several Module 8 conditions, while sensory and kidney disorders also require you to understand the affected organ's structure and function.
Worksheets
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
Homeostasis, maintaining the internal environment within tolerance ranges
Your Body Right Now
warm-up
As you read this, your body temperature is approximately 37 °C. The room around you might be 22 °C. You are continuously losing heat to the environment, yet your temperature stays almost perfectly constant. Something is actively working against that heat loss at every moment.
Now consider this: when you sprint 400 metres, your muscle cells burn glucose rapidly. Your blood glucose should plummet, but in a healthy person it barely moves below the normal range before being corrected.
Before reading on, answer both questions:
Q1: What do you think the body needs to 'detect' before it can 'correct'? What happens first, the correction or the detection?
Q2: Think of a real-life thermostat in a house. How does it work, and how might the body work the same way? What would be the equivalent of the thermostat's temperature sensor and heating system in your body?
Learning Intentions
goals
Know
The definition of homeostasis and why it is essential for survival
The five components of the stimulus-response model
The difference between negative and positive feedback
Three specific examples of homeostatic variables in humans
Understand
Why negative feedback, not positive, is the primary mechanism of homeostasis
Why the direction of the response matters (corrective vs amplifying)
How disruption to homeostatic systems leads to disease
Why tolerance ranges exist rather than fixed single values
Can Do
Draw and label a complete stimulus-response pathway for a given variable
Classify any given feedback scenario as negative or positive feedback
Explain how a specific homeostatic system fails in a named disease
Distinguish homeostatic tolerance ranges from set points in exam responses
Scan these before reading
vocab
HomeostasisThe maintenance of a relatively stable internal environment despite changes in the external environment.
StimulusA detectable internal or external change that can trigger a response; it does not have to cause complete homeostatic failure.
ReceptorA cell or structure that detects a change and initiates signalling within the regulatory pathway.
Control centreProcesses the signal from the receptor and determines the appropriate response (often the brain or a gland).
EffectorThe organ, muscle, or gland that carries out the corrective response.
Negative feedbackA response that opposes the original stimulus, returning the variable toward its set point.
Positive feedbackA response that amplifies the original stimulus, moving the variable further from its set point.
Tolerance rangeThe acceptable range of values for a variable within which the body can function optimally.
Cross-lesson links: Homeostatic regulation directly explains the temperature, glucose and water-balance systems developed in L02–L04 and helps explain several later diseases. Other Module 8 disorders, especially hearing and vision disorders, are more directly explained through altered organ structure and function.
Core Content
Key Point
Use homeostasis when a condition involves failure to regulate an internal variable. Do not force every non-infectious disease or sensory disorder into the same feedback-loop explanation.
2
Define homeostasis and tolerance ranges
Learn why internal conditions vary within safe limits rather than staying fixed.
1
What Is Homeostasis and Why Does It Matter?
+5 XP
The foundation for understanding how organisms regulate internal variables
An ICU nurse checks a patient at 2 am: blood glucose 12.4 mmol/L, temperature 38.9 °C, blood pH 7.28. Each value sits outside its usual range and signals that regulation is under strain. What they share is a deviation the body would normally detect and correct.
The word comes from the Greek homoios (similar) and stasis (standing still), but the internal environment is never truly still. It oscillates around reference conditions, with corrective systems continually moving variables back toward a workable range.
Blood glucose
4.0 to 6.0 mmol/L
Core temperature
36.5 to 37.5 °C
Blood pH
7.35 to 7.45
Why do these ranges matter? Enzyme-controlled reactions depend on temperature, pH and substrate concentration. A change in temperature alters molecular motion and collision rate; a change in pH alters charges and bonding within an enzyme. Large or prolonged deviations reduce catalytic activity and, under extreme conditions, denature proteins.
Remember!
"Relatively stable" means kept within a tolerance range around a set point, not pinned to one fixed value. Markers reward this distinction.
Homeostasis in the context of Module 8
Homeostasis directly explains conditions such as diabetes (disrupted glucose regulation) and kidney dysfunction (disturbed water and ion balance). Later lessons examine hearing and vision disorders through organ structure and function instead. So homeostasis is a key Module 8 framework, but not the only explanation the module uses.
IQ Connection
The first inquiry question, "How is an organism's internal environment maintained in response to a changing external environment?", asks you to build a regulation framework that later questions apply and extend.
Homeostasis is the maintenance of a relatively stable internal environment within a workable range around a reference condition, not a fixed value. These ranges matter because cellular reactions, including enzyme-controlled reactions, depend on suitable internal conditions.
Pause, copy the highlighted definition into your book before moving on.
Homeostasis maintains a variable within a _____ range around its set point, not at a single fixed value.
Interactive · Feedback Loop Classifier
Unlock check: Which statement best describes homeostasis?
3
Map the stimulus–response pathway
Name what detects, decides and acts, and link all five components in order.
2
The Stimulus-Response Model, Five Components in Order
+5 XP
Every homeostatic response follows this sequence, memorise the order and the roles
The stimulus-response model is the universal template for all homeostatic regulation, from body temperature to blood glucose to blood pressure. Master this five-component sequence and you can describe any homeostatic system.
The Stimulus-Response Pathway
Step 1Stimuluse.g. body temp rises above 37.5°C
→
Step 2Receptore.g. thermoreceptors in hypothalamus detect heat
→
Step 3Control Centree.g. hypothalamus processes signal
Stimulus: A detectable internal or external change that can trigger a response, such as blood glucose rising after a meal or ambient temperature falling. In many classroom feedback diagrams the change is shown crossing a reference range, but detection and corrective responses can begin before complete homeostatic failure occurs.
Receptor: Specialised cells or structures that detect particular changes. Neural receptors can produce impulses after a threshold is reached; endocrine sensor cells can respond to changing chemical concentrations and release a hormone. Thermoreceptors detect temperature, osmoreceptors detect osmolarity, and chemoreceptors detect chemical conditions such as CO₂.
Control centre: Integrates the signal from receptors and determines the magnitude and type of response. In many homeostatic systems, the control centre is the hypothalamus (temperature, water balance) or the pancreas (glucose). The control centre compares the incoming signal against the set point and sends instructions to the effector.
Effector: The organ, muscle, or gland that carries out the physical response. Effectors include skeletal muscles (shivering), sweat glands (cooling), the liver (glucose release), the kidneys (water reabsorption), and blood vessels (vasodilation/vasoconstriction). One stimulus can activate multiple effectors simultaneously.
Response: The physical change produced by the effector that reduces the stimulus and returns the variable toward its set point. In negative feedback, the response opposes the stimulus, this is the mechanism by which the variable is returned to the tolerance range.
HSC Tip
In an exam question asking you to describe a homeostatic response, always name all five components in order and link them: "The stimulus is [x], detected by [receptor], which sends a signal to [control centre], which activates [effector], producing [response] that reduces [stimulus]." A response that names only two or three components will not earn full marks.
Common error Why does "the body detects the stimulus and responds" lose marks? +
It names only two of the five components and skips the receptor, control centre and effector.
The HSC marking scheme rewards the complete pathway: every component named with its specific role in your worked example.
Stimulus-response pathway (in order): Stimulus → Receptor (detects) → Control centre (processes) → Effector (acts) → Response (opposes stimulus). HSC answers must name all five components with their specific roles, receptors are variable-specific (thermoreceptors, osmoreceptors, chemoreceptors).
Add the highlighted pathway order to your notes before the check below.
Which is the correct order of the stimulus-response pathway?
Interactive · Thermoregulation Feedback Explorer
Active practice · sort the pathway+7 XP
Put the stages of a temperature response in biological order.
Sweat glands increase sweat production.
Thermoreceptors detect the change.
Heat loss increases and temperature moves back toward its usual range.
Body temperature rises.
The hypothalamus coordinates cooling actions.
Unlock check: Which sequence is correct?
4
Compare negative and positive feedback
Decide whether a response opposes the stimulus or amplifies it.
3
Negative vs Positive Feedback, The Critical Distinction
+5 XP
The direction of the response decides everything: corrective or amplifying.
The whole logic of homeostasis rests on one question about the response. Does it oppose the change (negative feedback) or amplify it (positive feedback)? That answer decides whether the system restores balance or drives the body toward an extreme.
Negative feedback
Response opposes the original stimulus
Returns the variable toward the set point
Self-limiting: the response weakens as the variable returns to normal
Like a thermostat: rise above the set point turns cooling on
Positive feedback
Response amplifies the original stimulus
Moves the variable further from the set point
Self-reinforcing: the response strengthens the stimulus
Drives a process to completion, not to balance
Examples: childbirth, blood clotting
Like a microphone near its speaker: it amplifies until it screeches
Why negative feedback is primary
Negative feedback is the mechanism of homeostasis because it is self-correcting. When a variable drifts from its set point, the response brings it back; as it returns to normal the stimulus fades, so the response fades too. That gives stable oscillation around the set point rather than runaway deviation.
Positive feedback is not for maintaining balance. It drives a process rapidly to completion. In childbirth, contractions stretch the cervix, which releases oxytocin, which strengthens contractions, which stretch the cervix further. The essential feature: the loop only stops when an external event ends it (here, delivery of the baby).
Watch out When does positive feedback become dangerous, and why is fever the wrong example? +
Uncontrolled positive feedback can be pathological. In circulatory shock, a severe fall in blood volume lowers blood pressure, which reduces heart and tissue perfusion, which weakens cardiac function, which lowers blood pressure further. This self-reinforcing spiral needs treatment to interrupt it.
Do not use ordinary fever as your example. Fever is a regulated rise in the hypothalamic set point, whereas hyperthermia is an uncontrolled rise with no set-point change.
Feature
Negative Feedback
Positive Feedback
Direction of response
Opposes stimulus
Amplifies stimulus
Effect on variable
Returns toward set point
Drives variable further from set point
Self-limiting?
Yes, response weakens as variable normalises
No, loop reinforces itself until external event stops it
Role in homeostasis
Primary mechanism
Not a homeostatic mechanism
Human examples
Temperature, glucose, blood pressure, water balance
Extreme deviation, pathological if homeostatic system
HSC Tip
When asked to distinguish negative from positive feedback, the key phrase is direction: "Negative feedback produces a response that opposes the original stimulus, returning the variable to its set point. Positive feedback produces a response that amplifies the original stimulus, driving the variable further from its set point." Always include an example for each.
Common Error
"Negative feedback is bad because the body is responding negatively." The word 'negative' refers to the direction of the response (opposing the change), not to a bad outcome. Negative feedback is the healthy, corrective mechanism. Positive feedback drives a process forward, it is not inherently harmful, but it is not a homeostatic mechanism.
Negative feedback: response opposes the stimulus → returns variable to set point (self-limiting; the primary homeostatic mechanism). Positive feedback: response amplifies the stimulus → drives a process to completion (childbirth, blood clotting, action potentials), it is pathological only when it hijacks a homeostatic system.
Pause, write the highlighted distinction into your book, making sure to give one example of each type.
Negative feedback amplifies the original stimulus, driving the variable further from its set point.
Homeostasis is the maintenance of a relatively stable internal environment despite changes in external conditions.
Positive feedback loops always work to restore homeostasis by counteracting deviations from the set point.
Interactive · Feedback Loop Builder
Unlock check: Body temperature rises, so sweating increases heat loss and temperature falls. What type of feedback is this?
5
Apply the model to named systems
Trace temperature, glucose and water balance as complete feedback pathways.
4
Three Homeostatic Systems Applied, Temperature, Glucose, Water
+5 XP
These three variables recur throughout Module 8, map each onto the stimulus-response model now
The same five-component model applies to every homeostatic system. Temperature, blood glucose, and water balance are the three most examined examples in Module 8, and each introduces a different organ as the key effector.
Two internal coordination routes
Homeostatic information and instructions move through two main coordination systems. Neural coordination uses electrical impulses along neurons and chemical transmission at synapses; it is usually rapid, targeted and brief. Hormonal coordination uses hormones released by endocrine tissue and carried in the bloodstream; only cells with the matching receptor respond, and effects are generally slower but more sustained. The systems often cooperate rather than operating separately.
Route
Signal and pathway
Typical pattern
Module application
Neural
Electrical impulse along neurons; neurotransmitter across a synapse
Rapid, targeted, usually brief
Thermoreceptors and hypothalamic outputs in L02
Hormonal
Chemical hormone transported in blood to receptor-bearing target cells
Generally slower, potentially widespread and sustained
Insulin/glucagon in L03 and ADH coordination in L04
Example 1, Temperature Regulation
Variable: Core body temperature | Set point: ~37°C | Tolerance range: 36.5–37.5°C
When temperature rises: Thermoreceptors in the hypothalamus (receptor) → hypothalamus (control centre) → sweat glands produce sweat (effector → evaporative cooling) + peripheral blood vessels dilate/vasodilate (effector → heat loss from skin) → temperature falls back toward 37°C (response). This is negative feedback.
When temperature falls: Hypothalamus → skeletal muscles shiver (generating heat) + peripheral vessels constrict/vasoconstrict (reducing heat loss) + metabolic rate increases → temperature rises back toward 37°C. Also negative feedback.
Temperature control will be explored in much greater depth in L02. The key point here is that two opposing responses exist, one for overcooling, one for overheating, both feeding back to return temperature to set point.
When blood glucose rises (e.g. after a meal): beta cells in the islets of Langerhans sense the rise and release insulin → body cells take up more glucose and the liver stores it as glycogen (glycogenesis) → blood glucose falls. This is negative feedback.
When blood glucose falls: alpha cells release glucagon → the liver converts glycogen back to glucose (glycogenolysis) → blood glucose rises. Also negative feedback.
The critical Module 8 connection: when insulin production fails (Type 1 diabetes) or cells respond inadequately to insulin (Type 2 diabetes), glucose regulation is disrupted and blood glucose can remain chronically elevated. L03 develops the regulatory mechanism; L14 examines the treatment and management of Type 2 diabetes.
Example 3, Water Balance (a preview of L04)
Variable: Blood osmolarity | Set point: ~285 to 295 mOsm/kg. The same five-step logic applies: osmoreceptors in the hypothalamus detect a rise, the posterior pituitary releases ADH, and the kidney (effector) reabsorbs more water to bring osmolarity back down. This is negative feedback.
Roadmap
You only need the outline here. L04 develops the full ADH and collecting-duct mechanism, so keep water balance light for now.
Preview
L02 applies fast neural coordination to thermoregulation. L03 develops hormonal glucose regulation. L04 compares and integrates neural and hormonal coordination in water balance. L14 later examines Type 2 diabetes management, and L20 applies water-balance knowledge to loss of kidney function.
Three key homeostatic variables: Temperature (~37°C; hypothalamus; sweat/vasodilation for heat, shiver/vasoconstriction for cold). Blood glucose (4.0–6.0 mmol/L; insulin from beta cells lowers it, glucagon from alpha cells raises it; liver = effector). Osmolarity (~285–295 mOsm/kg; ADH from posterior pituitary controls kidney collecting duct permeability).
Add the three variable systems to your notes, include the hormone and key effector organ for each.
When blood glucose falls below the set point, which hormone restores it by triggering glycogenolysis in the liver?
Unlock check: Which hormone restores blood glucose when it falls below the set point?
6
Transfer the model and construct an HSC response
Classify unfamiliar loops, apply all five components and connect failure to disease.
Choose the support you need
Same science, three routes into the task
Use one route to prepare, then complete the two required transfer activities below. Stretch never replaces the core outcome.
Supportedword bank + sentence frame
Word bank: temperature · thermoreceptors · hypothalamus · sweat glands · heat loss
Complete aloud or in your notes: “When body ______ rises, ______ detect the change. The ______ coordinates a response. ______ become more active, increasing ______. This is negative feedback because…”
Coreindependent HSC application
A runner’s body temperature rises during a race. Describe the complete stimulus–response pathway and justify why it is negative feedback.
StretchBand 5–6 evaluation
A student says, “Any rise above 37°C proves homeostasis has failed.” Evaluate the statement using tolerance range, negative feedback and set-point change.
Activity 1
ApplyBand 3
Negative or Positive Feedback?
For each scenario, identify negative or positive feedback and justify in one sentence by stating the direction of the response relative to the stimulus.
After a heavy meal, blood glucose rises. Insulin is released, cells take up glucose, and blood glucose returns to the normal range.
During childbirth, uterine contractions stretch the cervix. Stretching of the cervix causes more oxytocin to be released, which increases the strength and frequency of contractions.
During exercise, body temperature rises above 37.5°C. Sweat glands become more active and blood is directed toward the skin surface. After exercise stops, temperature returns to 37°C and sweating stops.
When blood vessel walls are damaged, platelets stick to the exposed collagen. This activates more platelets, which release chemicals that attract even more platelets to the wound site. The clot grows rapidly until the wound is sealed.
A person becomes severely dehydrated. Blood osmolarity rises above 295 mOsm/kg. The hypothalamus triggers ADH release. The kidneys reabsorb more water, producing concentrated urine. Blood osmolarity falls back to normal and ADH secretion decreases. (Also name all five components of the stimulus-response pathway.)
Activity 2
AnalyseBand 4
Applying the Stimulus-Response Model to a New System
The carotid bodies are chemoreceptors located in the carotid artery. They detect rising CO₂ levels (and falling O₂ levels) in the blood. When CO₂ rises above normal (e.g. during exercise), the carotid bodies send a signal to the medulla oblongata in the brain, which responds by increasing the rate and depth of breathing. The extra breathing removes more CO₂, reducing blood CO₂ levels back toward normal. When CO₂ returns to normal, breathing rate decreases.
Using this information, answer all seven parts:
🏥 REAL-WORLD ANCHOR
Why ICU Nurses Monitor Vital Signs Every 15 Minutes
In an intensive care unit, nurses check a patient's temperature, blood pressure, blood glucose, oxygen saturation, and blood pH every 15 minutes. These are not arbitrary checks, each one monitors a homeostatic variable that, if it breaches its tolerance range and is not corrected artificially, will cause organ failure within hours.
A post-surgical patient whose blood glucose exceeds 10 mmol/L (above the normal tolerance range) will receive an insulin infusion, the medical equivalent of the body's own negative feedback mechanism. A patient whose body temperature drops to 35°C (hypothermia, below tolerance range) will be placed on a warming blanket. In each case, the medical intervention is mimicking what the body's homeostatic systems are supposed to do but cannot do adequately in that clinical state.
This is precisely why homeostasis is the first topic in Module 8: every disease and disorder you study, from cancer to kidney failure to hearing loss, can be traced back to a biological system that has stopped maintaining the internal environment within its tolerance range. The clinician's job is to understand what went wrong and how to restore or replace the homeostatic function that has been lost.
PRIORITY MISCONCEPTIONS
Priority Misconceptions
✗ "Negative feedback is harmful because the body is reacting negatively."
✓ 'Negative' refers to the direction of the response (opposing the change), not a bad outcome. Negative feedback is the healthy, corrective mechanism keeping you alive. Positive feedback is the amplifying one.
✗ "Homeostasis means keeping the variable at a fixed constant value."
✓ Homeostasis maintains a variable within a tolerance range, not at a single fixed point. The internal environment oscillates continuously around a set point, it is dynamic, not static.
✗ "Positive feedback always causes disease."
✓ Positive feedback is a normal, healthy biological mechanism in appropriate contexts (blood clotting, childbirth, action potentials). It only becomes pathological when it operates in a system that is supposed to maintain homeostasis.
✗ "The receptor detects the response and sends it back to the brain."
✓ The receptor detects the original stimulus (the change in the variable), not the response. The feedback signal is different from the effector response.
✗ "The hypothalamus is both the receptor and the control centre for temperature."
✓ For core temperature, the hypothalamus contains both thermoreceptors (receptor function) AND processes the signal (control centre function), so this is partially correct. However, peripheral thermoreceptors in the skin are also receptors that send signals to the hypothalamus as control centre. Be precise about which component you are describing.
Fig. 1, Stimulus-response pathway for temperature regulation. Dashed red arrow = negative feedback returning body temperature to the 37°C set point.
Fig. 2, Blood glucose oscillates around the 5 mmol/L set point. Insulin corrects post-meal spikes; glucagon corrects lows during exercise and overnight. Green band = normal tolerance range (4–8 mmol/L).
Homeostasis Definition
Maintenance of a relatively stable internal environment
Despite changes in the external environment
Essential for optimal enzyme function
Maintained within a tolerance range (not a single value)
Stimulus-Response Pathway
Stimulus → change outside tolerance range
Receptor → detects the stimulus
Control centre → processes signal, decides response
Effector → carries out the response
Response → opposes stimulus (negative feedback)
Negative vs Positive Feedback
Negative: response opposes stimulus → returns to set point
Positive: response amplifies stimulus → drives to extreme
Negative = primary homeostatic mechanism
Positive = drives processes to completion (clotting, birth)
Three Key Variables
Temperature: 36.5–37.5°C; hypothalamus control centre
The Homeostasis tool shows that when blood glucose rises above the set point, the pancreas releases…
7
Independent practice
Show what you can do without prompts
Apply the lesson to HSC-style questions, then compare your reasoning with the model answers.
01
Multiple Choice
+5 XP
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
02
Short Answer, 13 marks
+5 XP
UnderstandBand 3(3 marks) 1. Define homeostasis and explain why it is essential for the normal functioning of enzymes in the human body. In your answer, refer to one specific homeostatic variable and its tolerance range.
ApplyBand 4(5 marks) 2. Using the stimulus-response model, describe the homeostatic response when a person becomes severely dehydrated on a hot day. Name all five components of the pathway and identify the type of feedback operating.
EvaluateBand 5–6(5 marks) 3. A student states: "Positive feedback is always dangerous and represents a failure of homeostasis." Evaluate this statement using two specific biological examples, one that supports and one that challenges the student's claim.
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1, Sort + Classify
1. Negative feedback. The response (insulin release → glucose uptake by cells) opposes the original stimulus (rising blood glucose), returning blood glucose toward the set point of ~5 mmol/L. The response is self-limiting, as glucose falls back to normal, insulin secretion decreases.
2. Positive feedback. The response (increased oxytocin → stronger contractions) amplifies the original stimulus (cervical stretch). Each cycle of contractions intensifies the next, the loop reinforces itself until delivery (an external event) removes the stimulus.
3. Negative feedback. The response (sweating and vasodilation → heat loss) opposes the original stimulus (rising temperature). As temperature returns to ~37°C, the hypothalamus detects the correction and sweating decreases, the response is self-limiting.
4. Positive feedback. Initial platelet adhesion triggers clotting factor release → more platelet recruitment → more clotting factors, the response amplifies the original stimulus (platelet activation). The loop is self-reinforcing until the wound is sealed (an external endpoint stops the loop).
5. Negative feedback. Five components: Stimulus = rising blood osmolarity during dehydration. Receptor = osmoreceptors in the hypothalamus. Control centre = hypothalamus integrates the change and triggers ADH release from the posterior pituitary. Effector = receptor-bearing collecting-duct cells in the kidney respond to ADH. Response = collecting ducts become more permeable to water → more water is reabsorbed → blood osmolarity moves toward its usual range and urine becomes more concentrated. As osmolarity falls, ADH release decreases, so the pathway is self-limiting negative feedback.
Activity 2, Stimulus-Response Application
(a) Stimulus: Rising blood CO₂ concentration above normal (e.g. during exercise).
(b) Receptor: Carotid bodies (chemoreceptors in the carotid artery), detect elevated blood CO₂.
(c) Control centre: Medulla oblongata in the brain, receives the signal, processes it, and sends motor signals to the diaphragm and intercostal muscles.
(d) Effector: Diaphragm and intercostal muscles, contract more frequently and deeply to increase breathing rate and depth.
(e) Response: Increased rate and depth of breathing removes more CO₂ from the blood via the lungs, reducing blood CO₂ concentration back toward normal.
(f) Negative feedback the response (increased breathing rate) opposes the original stimulus (rising CO₂), returning blood CO₂ to its set point. As CO₂ falls back to normal, breathing rate decreases, self-limiting.
(g) If this system failed long-term, CO₂ would accumulate in the blood, lowering blood pH (CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻, respiratory acidosis), disrupting enzyme activity. Relevant diseases include COPD and emphysema, where impaired gas exchange means CO₂ cannot be adequately removed even with increased breathing effort.
Short Answer Model Answers
SA1 (3 marks): Homeostasis is the maintenance of a relatively stable internal environment in response to changes in the external environment [1]. It is essential for enzyme function because enzymes have an optimal temperature, pH, and substrate concentration, straying outside these optima causes the enzyme's tertiary structure to denature, reducing or eliminating catalytic activity [1]. For example, blood pH must be maintained within 7.35–7.45; if pH falls below 7.35 (acidosis), the charged amino acid side chains on enzymes are disrupted, tertiary structure is altered, and metabolic reactions slow or cease, threatening cell survival [1].
SA2 (5 marks): Stimulus: dehydration raises blood osmolarity above its usual range [1]. Receptor: osmoreceptors in the hypothalamus detect the increased osmolarity [1]. Control centre and signal: the hypothalamus integrates this information and triggers ADH release from the posterior pituitary [1]. Effector and response: ADH acts on receptor-bearing collecting-duct cells in the kidney, increasing water permeability and reabsorption, so urine becomes more concentrated and blood osmolarity moves back toward its usual range [1]. This is negative feedback because the response opposes the original rise in osmolarity [1].
SA3 (5 marks): The statement is only partially correct [1]. Example supporting the claim, circulatory shock: a severe fall in blood volume can reduce cardiac output and blood pressure; reduced heart and tissue perfusion then weakens cardiac function, causing cardiac output and pressure to fall further. This self-reinforcing deterioration is dangerous [2]. Example challenging the claim, blood clotting: vessel damage triggers platelet activation and chemical release, recruiting more platelets until the wound is sealed. This positive-feedback process is normal and protective because an external endpoint stops it [1]. Therefore positive feedback amplifies change, but whether it is harmful depends on the biological context and whether an appropriate stopping event exists [1].
8
Final step
Retrieve, reflect and finish
Check what stuck, revisit your opening model and save the completed lesson.
Check what actually stuck
Take the full module quiz
quiz
A full module quiz covering every lesson in this module, not just this one. Set aside a decent block of time and treat it like a real assessment.
Take on the boss-battle arena for this lesson, rapid-fire questions on homeostasis and feedback.
How did your thinking change?
Return to your Think First responses and consider Cannon's central idea: internal stability is dynamic and coordinated, not perfectly motionless. Negative feedback explains how a detected deviation can produce a corrective response that moves a variable back toward its workable range.
Q1: Detection must precede correction. You can now name the component that detects a change (the receptor) and the one that carries out the correction (the effector), then explain how the control centre links them in the model.
Q2: Cannon's homeostasis concept maps directly onto a thermostat. In the body, the thermostat's sensor = thermoreceptors in the hypothalamus; the heating/cooling system = sweat glands, blood vessels, skeletal muscles; the 'set point dial' = the hypothalamus's temperature set point (~37°C). Was your original analogy close? What did you miss?
