Keeping conditions steady: the internal environment and negative feedback
By the end of this lesson you will be able to
- Define homeostasis and the internal environment, and explain why temperature, glucose, water, carbon dioxide and pH must be held within narrow limits.
- Describe a control system in terms of set point, stimulus, receptor, coordination centre, effector and response.
- Explain negative feedback with named examples, and distinguish it from positive feedback.
- Distinguish physiological from behavioural responses and identify the body systems that interact in each.
- Analyse data showing a regulated variable fluctuating about a set point, quoting ranges and deviations.
You met this before
- Enzymes are proteins whose active site fits one substrate. Human enzymes work fastest near 37 °C; above about 45 °C most denature (the active site changes shape), and below 30 °C reactions slow sharply (Year 3, Unit 1).
- Aerobic respiration in every cell: glucose + oxygen → carbon dioxide + water, releasing energy. Carbon dioxide dissolves in the blood as a weak acid, so more respiration lowers blood pH (Year 3, Unit 1).
- Osmosis is the net movement of water across a partially permeable membrane from a dilute to a more concentrated solution. A red blood cell swells and bursts in pure water and shrinks in strong salt solution (Unit 1 of this course).
- The nervous system carries fast electrical impulses; the endocrine system releases hormones into the blood; the circulatory system carries heat, glucose, water, hormones and carbon dioxide to every tissue (Year 3, Unit 1).
- In Year 3 you met the idea of a feedback loop: a receptor detects a change, a coordinator decides, an effector reverses the change.
During a sprint the carbon dioxide concentration of your blood rises. Name the structure that detects the rise and two effectors that respond, and state what they do.
Reveal
Chemoreceptors in the brain stem (and in the aorta and carotid arteries) detect the rise in carbon dioxide, which they sense as a fall in blood pH.
The effectors are the diaphragm and the intercostal muscles between the ribs: they contract more often and more strongly, so breathing becomes faster and deeper and more carbon dioxide is breathed out.
Revise it in full: Year 3 · Body systems · Lesson 4 — Control and excretion: how the body holds itself steady · Year 3 · Body systems · Lesson 2 — Breathing and the lungs: how air reaches every cell · Year 3 · Body systems · Lesson 1 — Digestion: from a plate of pap to a molecule in the blood
The puzzle
In January 1774 the London physician Charles Blagden and a few friends walked into a small room heated by a stove until their thermometer read close to the boiling point of water — later trials went to about 127 °C.
They stayed for minutes at a time. Eggs and a piece of beef taken in with them cooked. Their own mouth temperatures, measured with the same kind of thermometer, stayed close to 37 °C.
A steak and a human arm are both made of wet protein. The steak cooked; the arm did not. Something in a living body was actively fighting the heat — and the same “something” keeps you at 37 °C on a frosty July morning in Gaborone and on a 42 °C afternoon in Maun.
What is it, how does it know what temperature to aim for, and why does the body bother?
Blagden noticed that he was dripping with sweat in the hot room, and that a dog taken in with him panted hard. Predict what the sweat and the panting have in common, and suggest why the steak, which was also wet, still cooked.
Compare your answer
Both put water on a surface where it evaporates. Evaporation takes energy from the surface, so it cools it — sweat on skin, saliva on a dog’s tongue and airways.
The steak had a little surface water, but nothing replaced it: once it had evaporated, the meat heated up. The living body kept supplying new sweat, and the blood carried heat from deep inside to the skin to be lost.
The body detected the heat, and responded to reverse its effect. That is homeostasis, and the pattern — detect, respond, reverse — is negative feedback.
The science — structured notes
1.1 The internal environment, and why it must be held steady
Your cells do not live in the air. Each one is bathed in tissue fluid, a thin layer of liquid squeezed out of the blood capillaries, and it is from the tissue fluid that a cell takes oxygen, glucose and water and into it that it releases carbon dioxide and wastes. The blood plasma and the tissue fluid together are the internal environment.
The French physiologist Claude Bernard, in the 1850s–1870s, realised that this internal environment stays remarkably constant even when the outside world changes wildly. Homeostasis is the maintenance of a constant internal environment within narrow limits, despite changes outside and inside the body.
Note what the definition does not say: it is not “keeping body temperature at 37 °C”. Temperature is one example. The definition is general.
Why does it matter so much? Three reasons run through this whole unit.
- Enzymes. Almost every reaction in a cell is catalysed by an enzyme whose shape depends on temperature and pH. Too cold and reactions are too slow; too hot, or the wrong pH, and the active site changes shape (denatures).
- Water and osmosis. If the tissue fluid becomes too dilute, water enters cells by osmosis and they swell; too concentrated, and they shrink. Brain cells, boxed in by the skull, are especially vulnerable.
- Supply of fuel. Brain cells respire glucose almost exclusively and store very little; if blood glucose falls far, they stop working within minutes.
| Regulated variable | Normal range (adult) | Why it matters | Main control |
|---|---|---|---|
| Core body temperature | 36.5–37.5 °C | Enzyme activity; above about 40 °C proteins begin to be damaged; below 35 °C is hypothermia | Hypothalamus → skin, sweat glands, muscles (Lesson 4) |
| Blood glucose concentration | about 4.0–7.8 mmol/L (fasting 4.0–5.4) | Fuel for respiration, especially the brain; too high damages vessels and draws water by osmosis | Pancreas → insulin and glucagon → liver, muscle (Lesson 3) |
| Water content of the blood | plasma concentration held within about ±1–2 % | Osmosis into and out of every cell | Hypothalamus → pituitary → ADH → kidney (Lesson 5) |
| Carbon dioxide and blood pH | pH 7.35–7.45 | Enzyme and protein shape; below pH 7.0 or above 7.8 is life-threatening | Brain stem → breathing muscles; kidney |
“Homeostasis means keeping the body at exactly 37 °C.”
Reveal answer
False, twice over. Homeostasis regulates many variables — glucose, water, carbon dioxide, pH, ion concentrations — not only temperature.
And even temperature is held within a range (about 36.5–37.5 °C), not at one exact value; it drifts through the day and oscillates slightly about its set point as corrections are made. In November 2023 examiners reported that many candidates defined homeostasis using temperature alone, and lost the mark.
1.2 A control system: set point, receptor, coordination centre, effector
Every homeostatic mechanism has the same parts, whether it controls temperature in your body or in a baby incubator.
- Set point — the value the system aims for (for core temperature, about 37 °C).
- Stimulus — a change in the variable away from the set point.
- Receptor — a cell or group of cells that detects the stimulus (thermoreceptors, chemoreceptors).
- Coordination centre — compares the input with the set point and sends out instructions; usually the brain (hypothalamus, brain stem), spinal cord or a gland such as the pancreas.
- Effector — a muscle or gland that carries out the response (sweat glands, the diaphragm, liver cells).
- Response — the action that brings the variable back towards the set point.
Messages travel from receptor to coordination centre, and from coordination centre to effector, either as nerve impulses (fast, precise — Lesson 2) or as hormones in the blood (slower, widespread, longer-lasting — Lesson 3).
During a 400 m race the carbon dioxide concentration in a runner’s blood rises. Identify the stimulus, receptor, coordination centre, effectors and response, and explain why this is negative feedback.
Stimulus: a rise in blood carbon dioxide concentration above the set point (sensed as a fall in blood pH).
Receptors: chemoreceptors in the brain stem and in the walls of the aorta and carotid arteries.
Coordination centre: the breathing centre in the brain stem (medulla).
Effectors: the diaphragm and intercostal muscles, reached by motor nerves.
Response: faster, deeper breathing, so more carbon dioxide is breathed out and blood carbon dioxide falls back towards the set point.
Negative feedback: the response opposes the original change — a rise causes a fall — and it switches off as the concentration returns to normal.
The May 2021 Integrated Sciences markscheme for this chain wanted three links in order: more carbon dioxide breathed out; so less carbon dioxide in the blood; so blood pH rises back to normal. “It regulates the pH” on its own earned nothing.
A school’s reptile tank has a heater, a thermometer probe and a controller set to 30 °C. Match each part to the terms receptor, coordination centre and effector, and state what the “response” is when the room cools at night.
Reveal answer
Receptor: the thermometer probe (detects the tank temperature). Coordination centre: the controller (compares the reading with the 30 °C set point and switches the heater). Effector: the heater.
Response: as the tank cools below the set point, the controller switches the heater on; the tank warms, and once it passes 30 °C the heater is switched off — so the temperature oscillates slightly about 30 °C. The body’s version uses nerve cells and muscles instead of wires and a heating element, but the logic is identical.
1.3 Negative feedback: the change switches off its own cause
Negative feedback is a control mechanism in which a change in a variable triggers a response that reverses the change, returning the variable towards its set point. “Negative” does not mean bad; it means the response has the opposite sign to the change.
Two features follow. First, the variable oscillates: because detection and response take time, each correction slightly overshoots, and the next correction pulls it back. Second, most variables have separate mechanisms for each direction.
| Variable | If it rises above the set point | If it falls below the set point |
|---|---|---|
| Core temperature | Sweating; vasodilation of skin arterioles; hairs lie flat | Shivering; vasoconstriction of skin arterioles; hairs raised; sweating stops |
| Blood glucose | Pancreas secretes insulin → liver and muscle store glucose as glycogen | Pancreas secretes glucagon → liver breaks glycogen down to glucose |
| Concentration of the blood plasma (solutes per unit of water) | More ADH → kidneys reabsorb more water; thirst | Less ADH → large volume of dilute urine |
| Blood carbon dioxide | Faster, deeper breathing | Slower, shallower breathing |
Having an “accelerator” and a “brake” gives much finer control than a single mechanism switched on and off: a car with both is easier to hold at 60 km/h than one with an accelerator alone.
On one October day in Maun the air temperature rose from 19 °C at dawn to 41 °C in the afternoon. A healthy resting adult’s core temperature, logged by a swallowed sensor, ranged from 36.6 °C to 37.3 °C. Compare the two ranges.
Air range = 41 °C − 19 °C
Air range = 22 °C
Core range = 37.3 °C − 36.6 °C
Core range = 0.7 °C
Ratio = 22 °C ÷ 0.7 °C
Ratio ≈ 31
The outside temperature varied about 31 times as much as the core. Notice too that 0.7 °C is not zero: part of that is the normal daily rhythm, and part is the oscillation of feedback. Examiners want both the calculation and the comment that the variable was held within a narrow range, not held fixed.
A student writes: “Negative feedback is bad because it stops the body working. When it is hot, the brain makes us sweat, which makes the temperature go up to 37 °C again. Glucagon is the negative feedback for insulin.”
Reveal answer
(1) “Negative” describes the direction of the response, not its value: negative feedback is what keeps the body working.
(2) Sweating makes the temperature go down towards 37 °C, not up. (3) Glucagon is not “feedback for insulin”; insulin and glucagon are two separate effect pathways of one negative-feedback system controlling blood glucose — insulin lowers it, glucagon raises it.
(4) Strictly the brain does not “make us sweat” by itself: the hypothalamus (coordination centre) sends nerve impulses to sweat glands (effectors).
1.4 Positive feedback: when a change is made bigger
In positive feedback the response increases the original change, so the variable moves further and further from where it started until some outside event stops the loop. Positive feedback cannot hold anything steady, so it is rare in homeostasis. It is used where the body needs a fast, all-or-nothing result.
- Childbirth. The baby’s head pressing on the cervix triggers release of the hormone oxytocin; oxytocin makes the uterus contract harder, which presses the head down more, which releases more oxytocin — until the baby is born and the loop breaks.
- Blood clotting. Platelets that stick at a wound release chemicals that attract more platelets, so the plug grows rapidly until the gap is sealed.
A fever is caused when chemicals released during an infection raise the hypothalamus’s set point from 37 °C to 39 °C. At the start of a fever a patient shivers and feels cold although her temperature is 37.5 °C. Explain this using the idea of a set point, and state whether the shivering is negative or positive feedback.
Reveal answer
The coordination centre compares the actual temperature (37.5 °C) with the new set point (39 °C). The body is now “too cold” relative to the set point, so the hypothalamus triggers the responses to cold: shivering and vasoconstriction, which also make her feel cold.
Her temperature rises towards 39 °C and is then held there. It is still negative feedback: the response reduces the gap between the actual value and the set point. What has changed is the set point, not the type of feedback. When the infection clears the set point returns to 37 °C and she sweats.
1.5 Physiological and behavioural responses
The mechanisms that restore a variable may be physiological — automatic changes in how organs work, which you do not decide on — or behavioural — actions the whole organism takes, often consciously.
| Variable | Physiological responses (automatic) | Behavioural responses (actions) |
|---|---|---|
| Temperature too high | Sweating; vasodilation; panting (dogs) | Moving into shade; removing clothing; resting at midday; fanning; a cold drink |
| Temperature too low | Shivering; vasoconstriction; hairs raised | Putting on a blanket; huddling; moving into the sun (lizards basking) |
| Water too low | ADH release, concentrated urine | Feeling thirsty and drinking |
| Glucose too low | Glucagon release; glycogen broken down | Feeling hungry and eating |
Behaviour matters most for animals with limited physiological control. A lizard cannot generate much heat from its metabolism, so it controls its temperature almost entirely by basking in the morning and hiding under rocks at noon. An elephant flaps its enormous ears, driving air over a dense network of blood vessels.
Humans combine both — and our behavioural responses are also cultural. Loose, layered robes in hot deserts, the heavy wool Basotho blanket in Lesotho’s cold highlands, a midday rest in the hottest months: these are ways communities have expressed, over generations, what their bodies needed.
Classify each response as physiological or behavioural: a dog panting; a person switching on a fan; the kidneys producing a small volume of dark urine; a camel turning to face the sun so that less of its body is lit; goosebumps.
Reveal answer
Physiological: panting (an automatic reflex controlled by the brain), the kidneys making concentrated urine (ADH), goosebumps (erector muscles contracting automatically).
Behavioural: switching on a fan; the camel’s orientation — an action of the whole animal that reduces the area exposed to direct sunlight.
1.6 Systems that interact: who detects, who decides, who acts
No single organ runs homeostasis. The nervous system detects most changes and coordinates fast responses; the endocrine system sends hormones for slower, longer changes; the circulatory system carries heat, glucose, water and hormones between them; and the effectors are organs of other systems — the skin, muscles, liver, kidneys and lungs.
The November 2021 Biology paper asked candidates to select two systems that interact to monitor and maintain body temperature: the nervous system to monitor it, and the circulatory (cardiovascular) or muscular system to maintain it.
| Nervous control | Hormonal (endocrine) control | |
|---|---|---|
| Signal | Electrical impulse along neurones; chemical across synapses | Hormone (chemical) carried in blood plasma |
| Speed | Very fast — milliseconds | Slower — seconds to hours |
| Targets | Precise: specific muscles or glands | Widespread: every cell with the right receptor |
| Duration | Short — stops when impulses stop | Longer — until the hormone is broken down |
| Homeostatic example | Sweating, shivering, breathing rate | Insulin and glucagon; ADH |
“Hormones act faster than nerve impulses because the blood flows round the body in about a minute.”
Reveal answer
False. Blood does circulate in about a minute, but a nerve impulse reaches its effector in milliseconds — thousands of times faster. A hormone must also be made or released, travel, bind to receptors and change the activity of its target cells, which can take seconds to hours. Hormones are slower but longer-lasting and widespread; that is why the body uses nerves for sweating and breathing, and hormones for blood glucose and water.
A person’s hypothalamus is damaged by a stroke, but their skin, sweat glands and muscles are healthy. Predict what happens to their body temperature on a hot day and on a cold night, and explain.
Reveal answer
Their temperature will drift towards the surroundings: rising on a hot day and falling on a cold night. The effectors still work, but the coordination centre that compares blood temperature with the set point and sends impulses to them is damaged, so they are not switched on at the right time.
The loop is broken in the middle. Patients with hypothalamic damage must rely on behavioural responses — clothing, air conditioning, carers checking their temperature — to do what the physiological loop no longer does.
Blood pH is held between 7.35 and 7.45. pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in mol/dm³. During a severe asthma attack a patient’s blood pH falls from 7.40 to 7.10 because carbon dioxide cannot be breathed out. Calculate [H⁺] at each pH and the factor by which it has increased, and explain why a “small” fall of 0.3 matters so much.
Reveal answer
[H⁺] = 10^(−pH)
At pH 7.40: [H⁺] = 10^(−7.40)
At pH 7.40: [H⁺] = 3.98 × 10⁻⁸ mol/dm³
At pH 7.10: [H⁺] = 10^(−7.10)
At pH 7.10: [H⁺] = 7.94 × 10⁻⁸ mol/dm³
Factor = 7.94 × 10⁻⁸ ÷ 3.98 × 10⁻⁸
Factor = 2.0
The pH scale is logarithmic, so a fall of 0.3 doubles the hydrogen-ion concentration. Enzymes and other proteins carry charged groups whose charge depends on [H⁺]; doubling it changes the charges on their surfaces and so their shape, and many enzymes stop working. A pH of 7.10 is a medical emergency — which is why the breathing loop acts within seconds.
Try it — exercise and breathing rate: a feedback loop you can count
Every stride of a runner adds carbon dioxide to the blood. Chemoreceptors detect it, the brain stem responds, and the breathing muscles work harder until the carbon dioxide leaving the lungs matches what the muscles make. This lab measures the effector’s response: breaths per minute at seven treadmill speeds, for an untrained or a trained student, aged 14 or 40.
Mean results for the untrained 14-year-old: 0 km/h → 16; 2 → 22; 4 → 27; 6 → 33; 8 → 38; 10 → 44; 12 → 50 breaths per minute. For the trained student: 14, 21, 25, 28, 30, 31, 32. Describe both relationships with values, calculate the gradient for the untrained student, state whether it is proportional, and explain the difference.
Reveal answer
Untrained: breathing rate rises steadily from 16 breaths/min at rest to 50 at 12 km/h, by about 5–6 breaths/min for every 2 km/h — a straight line.
Gradient = (50 − 16) breaths/min ÷ (12 − 0) km/h
Gradient = 34 ÷ 12
Gradient ≈ 2.8 breaths/min per km/h
The relationship is linear but not proportional: at 0 km/h the rate is 16, not 0, so the line does not pass through the origin (resting cells still respire).
Trained: the rate rises steeply at first (14 to 25 between 0 and 4 km/h) then levels off, gaining only 1 breath/min between 10 and 12 km/h. Training strengthens the breathing muscles and increases the volume of each breath, so the same carbon dioxide is removed with fewer, deeper breaths. Both students are regulating the same variable; the trained one has a more powerful effector.
Common mistakes to avoid
- Defining homeostasis with one example — “keeping temperature at 37 °C” loses the mark. Write “maintaining a constant internal environment (within narrow limits)”.
- “Negative feedback is harmful” — negative means the response opposes the change.
- Giving childbirth or clotting as negative feedback — both are positive feedback.
- Confusing receptor and effector — receptors detect (thermoreceptors, chemoreceptors); effectors respond (muscles, glands). Examiners in 2026 reported many candidates unable to name both.
- Stopping the chain too early — “breathing rate increases so pH is regulated” has one link; the markscheme wanted CO₂ breathed out → less CO₂ in blood → pH rises back to normal.
- Saying the brain “makes” the body do something — name the coordination centre and say it sends impulses (or a hormone) to a named effector.
- “Constant” without “within limits” — every regulated variable fluctuates about its set point.
Study smart: how this is examined
Study routine. Today: write the definition of homeostasis and of negative feedback from memory, then draw one feedback loop each for temperature and blood carbon dioxide, labelling all six terms.
In two days: without notes, fill in the “rises / falls” table in Section 1.3 for four variables. In a week: run the breathing lab as a full investigation, do the quiz, and explain to someone at home why a fever is still negative feedback.
How we found out — from the milieu intérieur to homeostasis
Blagden’s hot-room experiments of 1774–1775 showed that a living body resists heat actively. He concluded that the body had a power of “destroying heat”, and credited the evaporation of sweat — an idea that took another century to be fitted into a general theory.
That theory came from Claude Bernard (1813–1878), professor of physiology in Paris. Working on the liver, he discovered in the 1850s that it stores a starch-like substance he named glycogen and releases glucose into the blood, so that blood glucose stays steady even in a fasting animal.
From this and similar findings he built the idea of the milieu intérieur, the internal environment: “the constancy of the internal environment is the condition for a free and independent life” (published in his lectures of 1878). An animal that controls its inner fluid can live in deserts, in winters and at altitude.
The word homeostasis was coined in 1926 by the American physiologist Walter B. Cannon (1871–1945) of Harvard, and developed in his 1932 book The Wisdom of the Body. Cannon — who also coined the phrase “fight or flight” — chose “homeo” (similar) rather than “homo” (same) on purpose: the internal environment is kept similar, within limits, not identical.
He showed that the nervous system and the adrenal glands act together to hold blood glucose, temperature and blood volume within limits during stress.
Engineers had been building negative feedback into machines for longer. James Watt’s steam engines of the 1780s and 1790s used a spinning “governor” whose weights flew outwards as the engine sped up and closed the steam valve; James Clerk Maxwell analysed governors mathematically in 1868.
In 1948 Norbert Wiener, in his book Cybernetics, argued that animals and machines are controlled by the same logic of feedback. Physiology and engineering have borrowed from each other ever since — today’s insulin pumps and baby incubators are Cannon’s loops built in plastic and silicon.
Why did Cannon insist on the prefix “homeo-” (similar) rather than “homo-” (the same), and what evidence from this lesson supports his choice?
Reveal answer
Because regulated variables are held within a range, not at one fixed value. Evidence: core temperature varies by about 0.5–0.7 °C through the day; blood glucose rises after each meal and returns; every feedback loop oscillates about its set point because detection and correction take time; and set points themselves can move, as in fever.
“Same” would describe a system that never changes; “similar” describes one that is continually corrected — which is what the data show.
Where it is used now
- Intensive care. A patient in an intensive care unit has most of their homeostatic loops monitored and partly taken over: ventilators control carbon dioxide, drips control water and glucose, warming blankets control temperature. Clinicians work with the same set points and normal ranges as in Section 1.1.
- Newborn incubators. A premature baby has little fat and a large surface area for its mass, and cannot shiver. Incubators in neonatal units — in Gaborone, Francistown and hospitals worldwide — use a skin probe and a heater in a feedback loop set to about 36.5 °C.
- Kangaroo mother care. Where incubators are scarce, hospitals in many countries use skin-to-skin contact: the baby is held against the parent’s chest, and the parent’s own homeostasis warms the baby. The World Health Organization recommends it for small and preterm babies.
- Livestock in the heat. Cattle on Botswana’s ranches stop grazing and seek shade in the hottest hours; farmers plant shade trees and provide water points, supporting the animals’ behavioural thermoregulation, because heat-stressed cattle eat less and grow more slowly.
- Sport. Endurance athletes and their coaches monitor heart rate, sweat loss and, increasingly, core temperature from swallowed sensor capsules, to stay inside the limits of their own homeostasis.
Science now — and the scientist you need to be
What is happening in 2026
Homeostasis used to be measured in a clinic, one number at a time. It is now measured continuously on the body. Continuous glucose monitors report glucose every few minutes to a phone (Lessons 3 and 5); several smartwatches and rings sold since 2022 record overnight skin temperature, heart rate and blood oxygen; swallowable capsules report core temperature to athletes, soldiers and firefighters.
The result is a new kind of data: millions of people’s regulated variables, recorded every minute. Researchers are using this to see normal ranges more precisely — for example how much skin temperature rises before a fever, or how glucose varies in people without diabetes.
Machine-learning tools are also being tested in hospitals to spot when a patient’s vital signs are drifting out of range hours before a person would notice. Such tools support clinicians; they do not replace them, and whether they improve outcomes is still being tested in trials.
Over the next decade, expect “closed loops” to spread: devices that do not only measure but act — as the artificial pancreas already does for glucose (Lesson 5). Careers here include physiologist, biomedical engineer, clinical scientist, sports scientist, and data scientist in health.
The mindshift
What you need to be able to do
Define homeostasis generally; name the six parts of any control loop; explain a negative feedback chain to the words “back to normal”; tell negative from positive feedback; sort physiological from behavioural responses; and quote ranges from data. Examiners list homeostasis as a strength in some sessions and a weakness in others — the difference is precision of language.
See an example
A geyser: set point about 60 °C on the thermostat dial; receptor — the thermostat’s temperature sensor in the tank; coordination centre — the thermostat switch; effector — the heating element; response — the element switches on when the water cools below the set point and off when it passes it.
An assistant asked the same question named the thermostat as “the receptor and the control” in one step, merging two parts. That is acceptable for a simple device, but in the body receptor and coordination centre are separate structures, and examiners want both named.
Key terms
| Homeostasis | The maintenance of a constant internal environment within narrow limits. |
| Internal environment | The blood plasma and tissue fluid that bathe the body’s cells. |
| Set point | The value of a variable that a control system aims to maintain. |
| Normal range | The limits within which a regulated variable is kept (for example 36.5–37.5 °C). |
| Stimulus | A change in a variable that is detected by a receptor. |
| Receptor | A cell or group of cells that detects a stimulus. |
| Coordination centre | A structure (brain, spinal cord, gland) that receives information, compares it with the set point and sends out instructions. |
| Effector | A muscle or gland that carries out a response. |
| Negative feedback | A mechanism in which a change triggers a response that reverses the change. |
| Positive feedback | A mechanism in which a change triggers a response that increases the change (childbirth, clotting). |
| Physiological response | An automatic change in how an organ works (sweating, insulin release). |
| Behavioural response | An action of the whole organism that helps restore a variable (seeking shade, drinking). |
Think like a scientist
Criterion A Lerato runs a 10 km race in Gaborone on a hot afternoon. Before the race her core temperature is 37.0 °C and her blood glucose 5.0 mmol/L. During the race her core temperature reaches 38.6 °C, she sweats heavily, her breathing rate triples, and she drinks water at two stations.
After the race she sits in the shade and eats a banana. Twenty minutes later her core temperature is 37.4 °C.
(a) State the meaning of the term homeostasis. [1]
(b) For the control of her body temperature during the race, identify the stimulus, one receptor, the coordination centre and one effector. [4]
(c) Identify one physiological and two behavioural responses in the description, stating the variable each one helps to control. [3]
(d) Explain why her enzymes would be affected if her core temperature kept rising to 42 °C. [3]
(e) Her breathing rate tripled. Explain how this helped to keep her blood pH within its normal range. [3]
(f) Calculate the percentage by which her core temperature rose during the race, and explain why the fall from 38.6 °C to 37.4 °C after the race is an example of negative feedback. [3]
Model answer
(a) The maintenance of a constant internal environment (within narrow limits) [1]. (An answer about temperature only scores 0.)
(b) Stimulus: a rise in core (blood) temperature above the set point of about 37 °C [1]. Receptor: thermoreceptors in the hypothalamus that detect blood temperature, or in the skin [1].
Coordination centre: the hypothalamus (thermoregulatory centre) [1]. Effector: sweat glands, or the muscles in the walls of skin arterioles (vasodilation) [1].
(c) Physiological: sweating, controlling temperature [1]. Behavioural: drinking water at the stations, controlling the water content of the blood [1]; sitting in the shade (temperature), or eating a banana (blood glucose) [1].
(d) Enzymes are proteins with an active site of a specific shape that fits the substrate [1]. At 42 °C the bonds holding the enzyme’s shape begin to break, so the active site changes shape (the enzyme denatures) [1].
The substrate no longer fits, so the rate of enzyme-controlled reactions such as respiration falls; cells, especially brain cells, stop working normally [1]. (Do not accept “enzymes die”.)
(e) Her muscles respired faster, producing more carbon dioxide, which lowers blood pH; faster breathing removed more carbon dioxide from the blood in the lungs [1].
So the carbon dioxide concentration in the blood fell [1], and the pH rose back towards 7.4, within its normal range [1].
(f) Rise = 38.6 °C − 37.0 °C
Rise = 1.6 °C
Percentage rise = 1.6 ÷ 37.0 × 100
Percentage rise = 4.3 % [1]
After the race the high temperature was still detected by the hypothalamus, which kept the sweating and vasodilation going [1]; as heat was lost her temperature fell towards the set point, and the responses were reduced as it approached 37 °C — the change triggered a response that reversed it [1].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
State the meaning of the term homeostasis.
Reveal answer
The maintenance of a constant internal environment (within narrow limits) [1]. A definition that names only temperature, glucose or water scores 0.
The body controls temperature by negative feedback. State one other example of negative feedback in the body.
Reveal answer
Any one: blood glucose concentration; water content of the blood (water balance); blood carbon dioxide or pH; hormone levels [1]. Not childbirth or blood clotting (positive feedback).
Distinguish between a receptor, a coordination centre and an effector, giving an example of each from the control of blood carbon dioxide.
Reveal answer
Receptor — detects a change (stimulus): chemoreceptors in the brain stem or in the aorta and carotid arteries [1]. Coordination centre — compares the information with the set point and sends out instructions: the breathing centre in the brain stem (medulla) [1].
Effector — carries out the response: the diaphragm or intercostal muscles, which contract faster and harder [1].
Explain how an increase in breathing rate during exercise helps to regulate the pH of the blood.
Reveal answer
More carbon dioxide is breathed out (removed from the blood at the lungs) [1]; so the carbon dioxide concentration in the blood falls [1]; so the blood pH rises back to its normal value / the blood becomes less acidic [1]. (The third mark needs the second.)
Distinguish between negative and positive feedback, with one example of each.
Reveal answer
Negative feedback: the response reverses the change and returns the variable to its set point — for example sweating when core temperature rises [1]. Positive feedback: the response increases the change, moving further from the starting point until an outside event stops it — for example oxytocin causing stronger contractions during childbirth, or platelets attracting more platelets in clotting [1].
A desert lizard’s body temperature was recorded every hour: 22 °C at 06:00, 34 °C at 09:00, 36 °C at 12:00, 35 °C at 15:00 and 28 °C at 19:00, while the air went from 12 °C to 42 °C and back to 25 °C. Suggest two behaviours that could explain the data between 06:00 and 15:00, and outline why the lizard relies on behaviour more than a human does.
Reveal answer
Basking in the sun in the early morning (lying flat, side-on to the Sun, on warm rock) raised its temperature from 22 to 34 °C [1]; moving into shade or a burrow, or facing the Sun head-on, around midday kept it near 35–36 °C although the air reached 42 °C [1].
A lizard’s metabolism releases too little heat to warm its body, and it cannot sweat or shiver effectively, so it must use the environment’s heat sources and shelters — a human has powerful physiological effectors [1].
7. A shower mixer takes about 5 seconds for a change at the tap to reach the water at the shower head. Explain, using the ideas of set point, delay and negative feedback, why a person adjusting the tap often makes the water too hot, then too cold, and suggest why the body’s temperature loop shows the same pattern only very weakly.
Reveal answer
The person (receptor and coordination centre) feels water colder than their set point and turns the tap towards hot; because of the 5-second delay nothing changes, so they turn it further [1]. When the hot water arrives it overshoots the set point; they turn it back towards cold, again too far because of the delay — the temperature oscillates [1].
Negative feedback with a delay always oscillates; the larger the delay and the stronger each correction, the bigger the swings [1].
The body reduces the swings by having receptors in the skin that detect changes before they reach the core (early warning), by grading its responses to the size of the error, and by having opposing mechanisms (sweating and shivering) that act as brake and accelerator, so core temperature oscillates by only fractions of a degree [1].
8. In a severe bacterial infection a patient’s temperature is 39.5 °C. A doctor explains that the hypothalamus is “doing its job perfectly”. Using what you know about pathogens (Year 2) and set points, explain the doctor’s statement, and evaluate whether it is always wise to bring the fever down with medicine.
Reveal answer
White blood cells responding to the bacteria release chemical signals that raise the hypothalamus’s set point to about 39.5 °C [1]; the hypothalamus then uses negative feedback — shivering and vasoconstriction, later sweating — to hold the body at the new set point, so the loop is working correctly around a moved target [1].
For: a higher temperature can slow the multiplication of some pathogens and speed some immune responses, so a moderate fever may help fight the infection [1].
Against: fever raises metabolic rate and heart rate, causes discomfort and dehydration, and very high fevers (above about 41 °C) or fevers in young children with seizures, or in people with heart disease, can be dangerous — so lowering it is sensible when the risk outweighs the benefit; a judgement, not a rule [1].
Summary
- Homeostasis is the maintenance of a constant internal environment (blood plasma and tissue fluid) within narrow limits; it regulates temperature, blood glucose, water, carbon dioxide and pH, because enzymes, osmosis and the brain’s fuel supply depend on them.
- Every control loop has a set point, a stimulus, a receptor, a coordination centre, an effector and a response; messages travel as nerve impulses or hormones.
- Negative feedback: a change triggers a response that reverses it, so variables oscillate within a normal range; most have separate mechanisms for rising and falling. Positive feedback (childbirth, clotting) amplifies a change.
- Responses are physiological (sweating, insulin, ADH) or behavioural (shade, drinking, clothing) — and behavioural responses are shaped by culture.
- The nervous, endocrine and circulatory systems interact with effectors in the skin, muscles, liver, kidneys and lungs; in a fever the set point moves, but feedback still works.
Next time → The nervous system: how a neurone carries an impulse at up to 120 m/s, why you pull your hand from a hot pan before you feel the pain, what happens in the 20 nm gap of a synapse, and how to turn a falling ruler into a reaction time.