Key Idea: Topic D3.3 is about homeostasis โ keeping the body's internal environment roughly constant even when the outside world changes. Every example uses the same control loop: a level drifts from its set point, receptors detect the change, a control centre signals effectors, and the response opposes the change. That self-correcting loop is negative feedback. From there the topic applies the loop to four real systems: temperature (thermoregulation), blood glucose (insulin and glucagon), blood pH (COโ and ventilation), and appetite (leptin, ghrelin, insulin). It is heavily tested on Paper 1 (graph and identify MCQs) and on Paper 2 (the 4- and 7-mark 'explain how X is regulated' questions).
โ๏ธ Homeostasis & negative feedback (4.9.1)
Homeostasis is maintaining the internal environment within narrow limits around a set point. It runs on negative feedback: any change is detected and a response reverses it, so the level swings gently up and down around the set point rather than drifting away. Positive feedback does the opposite โ it amplifies a change. It is not how homeostasis works; it is used for one-off events that need to run quickly to completion, such as the LH surge before ovulation or contractions during childbirth.
The control loop behind every example in this topic: a change shifts a level away from its set point, receptors detect it, a control centre signals effectors, and the response OPPOSES the change to bring the level back to normal. Temperature, glucose and blood pH are all corrected this way.
๐ Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Negative feedback | Positive feedback |
|---|---|---|
| What it does to a change | Reverses it โ pushes the level back toward the set point | Amplifies it โ pushes the level further in the same direction |
| Effect on stability | Keeps the internal environment stable | Drives a process rapidly to completion, then stops |
| Homeostatic? | Yes โ this is how homeostasis works | No โ it is not used to hold a level steady |
| Examples in the body | Temperature, blood glucose, blood pH, water balance | The LH surge before ovulation; clotting; childbirth contractions |
Key Idea: Whatever the system, answer a 'how is X regulated?' question with the same four steps: Receptor โ detects the change away from the set point. Control centre โ usually the brain (hypothalamus) or an endocrine gland; processes the signal. Effector โ the muscle, gland or organ that acts. Response โ opposes the change and restores the set point. Naming these four steps is what turns a vague answer into a full-mark one.
Negative feedback = No! โ it cancels the change. Positive feedback = 'more!' โ it pushes the change further. Homeostasis is always the No! loop.
๐ก๏ธ Thermoregulation (4.9.2)
Humans are endotherms: we hold a stable core temperature (~37ยฐC). The hypothalamus is the thermostat โ it detects the change in blood temperature and switches the skin's effectors on or off. If too hot: skin arterioles vasodilate (more blood near the surface, more heat radiated away) and sweat glands sweat more (evaporation cools the skin). If too cold: arterioles vasoconstrict (less blood to the surface, heat conserved), sweating stops, and muscles shiver to generate heat; brown adipose tissue adds extra heat by non-shivering thermogenesis.
| If core temperature is... | Skin blood vessels | Sweat glands | Muscles / other |
|---|---|---|---|
| Too high (cool down) | Vasodilation โ more blood to the skin, more heat lost | Sweat more โ evaporation removes heat | No shivering |
| Too low (warm up) | Vasoconstriction โ less blood to the skin, heat conserved | Sweat less / stop | Shivering (muscles make heat); non-shivering thermogenesis in brown fat |
It is the arterioles in the skin that widen or narrow โ the capillaries themselves do not move. Vasodilation = vessels widen โ more blood to the skin surface โ more heat lost (cooling). Vasoconstriction = vessels narrow โ less blood to the skin surface โ heat conserved (warming). Saying blood 'moves up and down' loses the mark โ name the process and the heat effect.
Hot โ dilate and drip (sweat). Cold โ constrict and convulse (shiver). Brown fat = the body's built-in heater for the cold.
๐ฉธ Blood glucose regulation (4.9.3)
Blood glucose must stay within narrow limits โ too low starves cells of fuel, too high damages tissues. The pancreas is both the receptor and the control centre here, releasing two opposing hormones. After a meal glucose rises, so the pancreas releases insulin: the liver and muscle take up glucose and store it as glycogen, lowering blood glucose. Between meals glucose falls, so the pancreas releases glucagon: the liver breaks glycogen back down into glucose and releases it, raising blood glucose. The two hormones pull in opposite directions to hold the level steady.
| When blood glucose is... | Hormone released | Made by | Effect that corrects it |
|---|---|---|---|
| Too high (after a meal) | Insulin | Pancreas (ฮฒ-cells) | Liver and muscle take up glucose and store it as glycogen โ glucose falls |
| Too low (between meals) | Glucagon | Pancreas (ฮฑ-cells) | Liver breaks glycogen back into glucose and releases it โ glucose rises |
Key Idea: Insulin is released when glucose is IN excess (high) and stores it away โ glucose down. Glucagon sounds like 'glucose-gone' โ it is released when glucose has gone low, and releases stored glucose โ glucose up. Both come from the pancreas; both work by negative feedback to defend the same set point.
| Feature | Type 1 diabetes | Type 2 diabetes |
|---|---|---|
| Core problem | Pancreas makes little or no insulin | Cells stop responding properly to insulin (insulin resistance) |
| Typical onset | Often in childhood | Often later in life; linked to diet and body mass |
| Usual treatment | Insulin injections | Diet, exercise, and drugs (e.g. metformin) that lower blood glucose |
A falling glucose curve after a meal means insulin is acting (glucose being stored). A rising curve between meals means glucagon is acting (stored glucose released). A drug like metformin lowers blood glucose, so on a before/after bar chart the 'after' bar is lower โ that is the effect to describe.
๐ซ Blood pH & ventilation control (4.9.4)
Blood pH must be held near 7.4. The key variable is carbon dioxide: COโ dissolves in blood to form carbonic acid, so more COโ means a lower (more acidic) pH. Chemoreceptors (in the brainstem and major arteries) detect the rise in COโ / fall in pH. The brain then increases the ventilation rate (breathing faster and deeper), so more COโ is exhaled, COโ falls, and pH rises back toward 7.4 โ a textbook negative-feedback loop.
The chain when blood pH falls below normal
- Blood COโ rises (e.g. during exercise), forming more carbonic acid, so blood pH falls.
- Chemoreceptors detect the higher COโ / lower pH and signal the breathing centre in the brainstem.
- The breathing centre increases ventilation rate โ breaths become faster and deeper.
- More COโ is exhaled, so blood COโ falls and pH rises back toward 7.4 (the set point).
- As COโ returns to normal the receptors stop firing and ventilation settles โ negative feedback.
More COโ โ more acid โ lower pH. The body's reply is simply 'breathe it out' โ raise the ventilation rate to blow off COโ and pull pH back up. The trigger is COโ level, not oxygen level.
๐ฝ๏ธ Appetite & hormonal control of feeding (4.9.5)
Long-term energy balance is regulated by hormones acting on the hypothalamus, the brain's appetite centre. The main signals are leptin, ghrelin and insulin. Leptin is released by fat (adipose) tissue โ the more fat stored, the more leptin, telling the brain to reduce appetite. Ghrelin is the 'hunger hormone' from an empty stomach that increases appetite. Insulin (from the pancreas) also signals fullness after a meal. Each works by naming where it is made, what it acts on (the hypothalamus), and its effect โ the exact three things examiners ask for.
| Hormone | Secreted by | Acts on | Effect on appetite |
|---|---|---|---|
| Leptin | Adipose (fat) tissue | Hypothalamus (brain) | Signals 'enough stored energy' โ reduces appetite |
| Ghrelin | Stomach (empty) | Hypothalamus (brain) | The 'hunger hormone' โ increases appetite |
| Insulin | Pancreas | Hypothalamus (brain) | After a meal, also helps signal fullness โ reduces appetite |
Key Idea: Leptin ('Leptin = Less appetite') comes from fat and tells the brain energy stores are full โ appetite down. Ghrelin ('Ghrelin = Growling/hunger') comes from the empty stomach โ appetite up. Because leptin tracks fat stores, faulty leptin signalling is studied in the context of obesity: the body keeps feeling hungry even with plenty of energy stored.
Answer any appetite-hormone question as a three-part chain: where it is secreted โ where it acts (the hypothalamus) โ what it does to appetite. 'Identify two appetite hormones and where they are secreted' = leptin (adipose tissue) and ghrelin (stomach).
โ๏ธ Worked examples
IB-style question โ cooling down on a hot day
A student sits still on a very hot afternoon and their skin becomes flushed and damp. Explain how negative feedback returns their core temperature to normal. [4]
Model answer:
Detect the change. The blood is warmer than the set point; thermoreceptors / the hypothalamus detect the rise in core temperature.
First effector โ vasodilation. Skin arterioles vasodilate, so more blood flows near the skin surface and more heat is radiated away (the flushed look).
Second effector โ sweating. Sweat glands release more sweat; as it evaporates it removes heat from the skin (the damp skin).
Close the loop. Heat loss lowers core temperature back toward the set point, the receptors stop firing, and the responses ease off โ the change has been reversed (negative feedback). (1 mark each: detect / vasodilation / sweating-evaporation / opposes the change.)
The hypothalamus detects the raised blood temperature; skin arterioles vasodilate so more heat is lost from the surface; sweating increases and evaporation cools the skin; this lowers core temperature back to the set point โ the response opposes the change (negative feedback).
IB-style question โ glucose after a meal
After breakfast a person's blood glucose rises sharply and then falls back to normal over the next two hours. Describe the role of one named hormone in producing the fall. [4]
Model answer:
Name the hormone and its source. The fall is caused by insulin, secreted by the pancreas when blood glucose is high.
Name the target organs. Insulin acts on the liver and muscle cells, increasing their uptake of glucose from the blood.
Name the storage step. These cells convert the glucose into glycogen for storage, removing glucose from the blood.
Link to the graph and feedback. As glucose is taken up and stored, blood glucose falls back to the set point; this is negative feedback correcting the rise. (1 mark each: insulin from pancreas / acts on liver & muscle / glucose โ glycogen / glucose falls.)
Insulin from the pancreas is released when glucose is high; it makes the liver and muscle take up glucose and store it as glycogen, so blood glucose falls back to normal โ negative feedback.
IB-style question โ contrast the two feedback types
Distinguish between negative and positive feedback, giving one example of each from the human body. [3]
Model answer:
Negative feedback โ what it does. It reverses a change, pushing the level back toward the set point, so it keeps the internal environment stable.
Positive feedback โ what it does. It amplifies a change, pushing the level further in the same direction until a process is complete.
One example of each. Negative feedback: control of blood glucose (or temperature / pH). Positive feedback: the LH surge before ovulation (or contractions in childbirth). (1 mark: negative reverses/restores; 1 mark: positive amplifies; 1 mark: one correct example of each.)
Negative feedback reverses a change to restore the set point and maintain stability (e.g. blood glucose control); positive feedback amplifies a change to drive a process to completion (e.g. the LH surge before ovulation).
โ Quick self-check
Tap each card to check yourself.
What are the four steps of a negative-feedback control loop? A receptor detects the change away from the set point; a control centre (brain or gland) processes it; an effector acts; and the response opposes the change to restore the set point.
How does the body cool down when it is too hot? Skin arterioles vasodilate so more heat is lost from the surface, and sweat glands release more sweat whose evaporation cools the skin; this lowers core temperature back to the set point.
How does the body warm up when it is too cold? Skin arterioles vasoconstrict to conserve heat, sweating stops, muscles shiver to generate heat, and brown adipose tissue produces heat by non-shivering thermogenesis.
Which hormones control blood glucose, and how? Insulin (high glucose) makes the liver and muscle store glucose as glycogen, lowering it; glucagon (low glucose) makes the liver break glycogen back into glucose, raising it. Both come from the pancreas.
How is blood pH controlled by ventilation? Rising CO2 forms carbonic acid and lowers pH; chemoreceptors detect this and the brain raises the ventilation rate, so more CO2 is exhaled and pH rises back toward 7.4.
Name two appetite hormones and what they do. Leptin from adipose tissue reduces appetite (signals stored energy); ghrelin from the empty stomach increases appetite (the hunger signal). Both act on the hypothalamus.
Exam Tips
- The master idea of the whole topic: homeostasis = negative feedback. Any change is detected and the response opposes it to restore the set point.
- Name all four steps โ receptor, control centre, effector, response โ for full marks on any 'how is X regulated?' question.
- Negative feedback reverses a change (stable internal environment); positive feedback amplifies it (e.g. the LH surge). Homeostasis only uses negative feedback.
- Thermoregulation wording: vasodilation = wider vessels, more heat lost (cooling); vasoconstriction = narrower vessels, heat conserved (warming). It is the arterioles, not capillaries.
- Cold-response heat sources: shivering muscles AND brown adipose tissue (non-shivering thermogenesis) โ examiners reward naming brown fat.
- Glucose: insulin (high โ store as glycogen โ down), glucagon (low โ release glucose โ up); both from the pancreas. On a graph, a falling curve = insulin, a rising curve = glucagon.
- Blood pH: the trigger is CO2, not O2. More CO2 โ more carbonic acid โ lower pH โ chemoreceptors raise ventilation to breathe CO2 out and restore pH 7.4.
- Appetite hormones: leptin (adipose tissue, less appetite), ghrelin (empty stomach, more appetite), insulin (pancreas, fullness). Always give source โ target (hypothalamus) โ effect.