OCR GCSE Biology A: Gateway Science (J247)

B3.3 Maintaining internal environments

Homeostasis is the single biggest idea in human biology — and OCR Gateway Science Biology A (J247) topic B3.3 is where the rules are stated. You need to be able to define homeostasis, explain WHY it matters (enzymes, cells, optimum function despite external changes), name the three parts of every control system (receptors, coordination centres, effectors), and explain negative feedback well enough to apply it to blood glucose, body temperature and water balance — including any unfamiliar example the examiner throws at you. OCR B3.3 also expects you to know the named coordination centres (brain, spinal cord, pancreas, hypothalamus), the role of ADH and the kidney in water balance, and how thermoregulation works through sweat glands, skin blood flow and shivering. This page works through each piece with the exact OCR marking phrases, then ties it to Type 1 and Type 2 diabetes.

Why this matters

Your internal environment — blood glucose, body temperature, water content, ion concentration, pH — is kept inside a narrow range despite huge changes around you. You can eat a sugary meal, walk into a heatwave, run a 5k or fast for a day, and your blood glucose stays close to a set point, your core temperature stays close to 37 °C, and your blood water content barely moves. That is homeostasis. It matters because every chemical reaction in your body is catalysed by enzymes, and enzymes need a stable environment: the right temperature, the right pH and the right concentration of substrate. If your internal conditions wandered too far from the optimum, enzymes would slow down or denature, cells would dehydrate or burst by osmosis, and you would not survive long. Homeostasis is also what keeps the body responsive — without a stable baseline, you could not detect or react to anything new. Every control system that does this work has the same three-part architecture: a RECEPTOR that detects a change (a stimulus), a COORDINATION CENTRE that processes the information (the brain, spinal cord, pancreas or hypothalamus), and an EFFECTOR that produces a response (a muscle or a gland). And every one of these systems uses negative feedback — a change in a variable triggers a response that reverses the change. This idea is so general that once you see it in one example (blood glucose), you can apply it to every other one. OCR J247 B3.3 develops this across three named loops: blood glucose control (pancreas + insulin/glucagon + liver), thermoregulation (hypothalamus + sweat glands + skin blood flow + shivering) and osmoregulation (hypothalamus + ADH + kidney + collecting duct). Type 1 and Type 2 diabetes are the worked examples of homeostasis breaking down.

How to learn this topic

Build on what you already know

GCSE B1: enzymes work best at specific temperatures and pH — they denature outside their optimum.
GCSE B1: osmosis — water moves from dilute to more concentrated solutions across a partially-permeable membrane.
GCSE B2: the circulatory system carries glucose, oxygen and hormones around the body.
KS3: the body senses temperature and reacts (sweating, shivering).

Define homeostasis precisely — 'maintenance of constant internal conditions' — and link it to enzymes.
List the conditions controlled in humans: blood glucose, body temperature, water levels.
Introduce the three-part architecture: receptor → coordination centre → effector.
Name the four OCR coordination centres: brain, spinal cord, pancreas, hypothalamus.
Name the two coordination systems: nervous (fast, electrical) and endocrine (slower, chemical).
Define negative feedback and draw the generic graph (variable oscillating around a set point).
Apply the framework to blood glucose (pancreas + insulin/glucagon + liver).
Apply it to thermoregulation (hypothalamus + sweat glands + skin blood flow + shivering).
Apply it to osmoregulation (hypothalamus + ADH + kidney + collecting duct).
Drill the OCR marking phrases: 'optimum conditions for cell function', 'negative feedback reverses the initial change'.
Pre-empt common errors: positive feedback, set point vs set value, what counts as the effector.
Link to Type 1 and Type 2 diabetes as failures of homeostasis.

Key terms

homeostasis: The maintenance of constant internal conditions despite external changes, providing optimum conditions for cell function — including enzyme activity. (OCR examiners want the full phrase: 'maintenance of constant internal conditions / optimum conditions for cell function / enzyme activity / negative feedback'. Don't just say 'keeping the body balanced'.)
internal conditions: The conditions inside the body that homeostasis keeps stable — chiefly blood glucose, body temperature, water content, ion concentration and pH. (For OCR J247 B3.3 you must name blood glucose, body temperature and water levels (osmoregulation).)
set point: The 'normal' or target value of a controlled variable — for example ~37 °C body temperature or the normal blood-glucose range. Negative feedback brings the variable back to the set point.
negative feedback: A control mechanism in which a change in a variable triggers a response that reverses (opposes) the change, returning the variable to its set point. (The OCR marking-phrase finisher is 'negative feedback reverses the initial change' (or '…reverses the initial stimulus'). Memorise it word-for-word.)
receptor: A cell (or group of cells) that detects a stimulus — a change in the environment, internal or external — and converts it into a signal that the body can act on. (Receptors DETECT stimuli. They do not 'respond' — effectors do that. Be precise.)
stimulus: A detectable change in the environment, either inside the body (e.g. blood glucose rising) or outside it (e.g. ambient temperature falling).
coordination centre: A structure that receives and processes information from receptors and signals to effectors. In OCR J247 B3.3, named examples are the brain, the spinal cord, the pancreas and the hypothalamus. (OCR explicitly names brain, spinal cord, pancreas and hypothalamus. Quoting the relevant ones is safest.)
effector: A muscle or gland that brings about a response when signalled by a coordination centre. Muscles respond by contracting; glands respond by secreting hormones or other substances. (Always give the type: 'muscle (e.g. skeletal muscle)' or 'gland (e.g. pancreas releasing insulin, sweat gland secreting sweat)'.)
nervous system: The fast, electrical coordination system of the body — receptors send impulses along neurones to a coordination centre (brain/spinal cord), and the centre signals effectors via more neurones. (Fast, short-lasting, electrical, uses neurones.)
endocrine system: The slower, chemical coordination system of the body — glands secrete hormones into the blood, which carry the hormones to target organs (effectors). (Slower to start, longer-lasting, chemical, uses hormones in blood.)
hypothalamus: A region of the brain that acts as the coordination centre for body temperature and water balance. It contains thermoreceptors (for core temperature) and osmoreceptors (for blood water concentration). (OCR B3.3 names the hypothalamus explicitly. Use it whenever the question is about temperature or osmoregulation.)
ADH: Antidiuretic hormone — a hormone released by the pituitary gland that increases water reabsorption in the kidney by making the collecting duct more permeable to water. (Mark-scheme phrase: 'pituitary gland secretes/releases ADH · ADH increases water reabsorption in the kidney · blood water concentration rises back to normal · negative feedback.')
enzyme: A biological catalyst (a protein) that speeds up a specific reaction in the body. Enzymes need a narrow range of temperature and pH — outside that range they denature. (The reason homeostasis matters is to keep enzymes (and other proteins) working — that link earns marks on 'why' questions.)
denaturation: The change in the shape of a protein (especially the active site of an enzyme) when conditions move outside its optimum — the protein stops working.
Type 1 diabetes: A condition in which the pancreas does not produce insulin. Blood glucose levels rise after meals; cells cannot take up glucose, cannot respire properly, and water moves out of cells by osmosis. Treated with insulin injections.
Type 2 diabetes: A condition in which the body cells stop responding to insulin (insulin resistance). The pancreas still produces insulin, but it is less effective. Linked to obesity. Managed by diet, weight loss and exercise.

Notes

What homeostasis is

Homeostasis is the maintenance of constant internal conditions despite external changes, providing optimum conditions for cell function — including enzyme activity.

The wording matters for OCR exams. Examiners expect you to say:

It is maintenance of constant internal conditions — active control, not just stability.
Despite external changes — so the body can keep functioning when the environment around it shifts.
To provide optimum conditions for cell function, especially enzyme activity.
Through negative feedback, which reverses the initial change.

In humans, OCR B3.3 names three controlled variables you must know in detail:

Blood glucose concentration — coordinated by the pancreas (kept close to ~90 mg/100 cm³).
Body temperature — coordinated by the hypothalamus (kept close to ~37 °C).
Water levels (osmoregulation) — coordinated by the hypothalamus and pituitary, acting on the kidney.

Other conditions are controlled too — ion concentration, pH of the blood, oxygen and carbon dioxide levels — but those three are the named OCR examples.

Why homeostasis matters

Every reaction in your body is catalysed by an enzyme, and enzymes only work properly within a narrow range of temperature, pH and substrate concentration. Move too far away and the enzyme denatures — its active site changes shape, the substrate no longer fits, and the reaction stops. Cells also need a particular water and ion balance to avoid bursting or shrivelling by osmosis. Homeostasis keeps every cell inside the conditions in which it can function. Without it, even small changes in the environment would shut your body down.

OCR mark-scheme phrase to memorise: maintenance of constant internal conditions · optimum conditions for cell function / enzyme activity · negative feedback reverses the initial change.

The three parts of every control system

Whatever is being controlled — glucose, temperature, water — every control system in the body has the same three-part architecture. OCR expects you to name and describe each part.

Receptors — cells (or groups of cells) that detect stimuli. A stimulus is any change in the environment, internal or external. Examples: glucose-sensitive cells in the pancreas; thermoreceptors in the skin and hypothalamus; osmoreceptors in the hypothalamus.
Coordination centres — structures that receive and process information from the receptors and decide what to do. OCR B3.3 names the brain, the spinal cord, the pancreas and the hypothalamus. (The hypothalamus is the coordinator for temperature and water balance; the pancreas coordinates blood glucose; the brain and spinal cord coordinate nervous reflexes.)
Effectors — muscles or glands that bring about the response. A muscle contracts (e.g. shivering, moving away from a hot stove). A gland secretes a hormone (e.g. pancreas secretes insulin) or another product (e.g. sweat gland secretes sweat).

The flow is always the same: stimulus → receptor → coordination centre → effector → response.

The response changes the internal condition, which feeds back to the receptor, which closes the loop.

The two coordination systems

Receptors, coordination centres and effectors are linked by one of two systems:

The nervous system — uses electrical impulses along neurones. Signals are fast (milliseconds) and short-lasting. Used for reflexes, voluntary movement and rapid temperature responses.
The endocrine system — uses chemical hormones secreted into the blood. Signals are slower to start (seconds to minutes) but longer-lasting (minutes to hours). Used for blood glucose, water balance and reproductive cycles.

Many control loops use both: temperature regulation is mostly nervous (with some hormonal back-up); blood glucose is mostly hormonal; water balance combines a hormonal output (ADH) with nervous detection in the hypothalamus.

Negative feedback — the engine of homeostasis

Negative feedback is a control loop in which a change in a variable triggers a response that opposes (negates) the change, returning the variable to its set point. This is what keeps every controlled variable inside a narrow range.

The pattern is symmetric:

The variable moves above the set point → receptor detects → coordination centre signals → effectors bring the variable down.
The variable moves below the set point → receptor detects → coordination centre signals → effectors bring the variable up.

Because both directions are corrected, the variable oscillates in a tight range around the set point than wandering off.

OCR mark-scheme phrase to memorise: negative feedback reverses the initial change (or …reverses the initial stimulus).

The opposite — positive feedback — is when a change triggers a response that AMPLIFIES the change. This is rare in homeostasis (it shows up briefly in childbirth and blood clotting) but it is NOT what OCR B3.3 is about. Do not write 'positive feedback' in an answer about homeostasis.

Worked example: blood glucose (negative feedback)

Blood glucose is the classic example. It is coordinated by the pancreas and acts on the liver.

After a meal, blood glucose rises. The pancreas detects high blood glucose. Insulin is secreted/released by the pancreas into the bloodstream. Insulin causes the liver and other body cells to take up glucose, and glucose is converted to glycogen for storage in the liver. As glucose is removed from the blood, the response reverses the initial change — blood glucose returns to normal. Negative feedback.

When blood glucose falls (e.g. during exercise) the pancreas releases the opposite hormone, glucagon, which makes the liver break glycogen back down into glucose. Both directions are negative feedback — the response opposes the change.

Worked example: thermoregulation (negative feedback)

Body temperature is coordinated by the hypothalamus, with thermoreceptors in the skin and brain.

When you overheat (e.g. running on a hot day), the temperature rise is detected by the hypothalamus. The hypothalamus sends signals to several effectors:

Sweat glands secrete sweat onto the skin surface; as the sweat evaporates, it removes heat energy and cools the body.
Blood vessels supplying the skin capillaries dilate (vasodilation) — more blood flows close to the surface, so more heat is lost by radiation.
Skeletal muscles relax — no shivering.

The result: body temperature falls back towards 37 °C. Negative feedback reverses the initial stimulus.

When you are cold, the opposite happens: blood vessels constrict (vasoconstriction), sweat production stops, and skeletal muscles contract rapidly — shivering — which releases heat from respiration. Both directions are negative feedback.

Worked example: osmoregulation and ADH (negative feedback)

Water balance is coordinated by the hypothalamus (which monitors blood water concentration via osmoreceptors) and the pituitary gland (which releases the hormone). The effector is the kidney, the collecting duct.

When the blood becomes too concentrated (dehydration — e.g. after exercise, hot weather or salty food), osmoreceptors in the hypothalamus detect the change. The pituitary gland secretes/releases ADH (antidiuretic hormone) into the blood. ADH increases water reabsorption in the kidney — it makes the collecting duct walls more permeable to water, so more water moves out of the kidney tubule and back into the blood, and less water is lost in urine. Blood water concentration rises back to normal. Negative feedback.

When the blood becomes too dilute (you have drunk a lot of water), less ADH is released, the collecting duct becomes less permeable, less water is reabsorbed, and more dilute urine is produced. Same architecture, opposite direction.

Adrenaline — a non-feedback example to contrast

Not every hormonal response is part of a homeostatic loop. Adrenaline is released by the adrenal glands during shock, danger or excitement (the 'fight or flight' response). It increases heart rate, increases breathing rate, and raises blood sugar/glucose levels by causing the liver to convert glycogen to glucose. This is the OPPOSITE direction to the insulin loop — adrenaline overrides normal blood-glucose homeostasis temporarily so the body can fuel a sudden burst of activity.

Diabetes — what goes wrong if homeostasis fails

A broken control loop has consequences. In Type 1 diabetes the pancreas fails to produce insulin. After a meal, blood glucose levels rise uncontrollably. Without insulin, cells cannot take up glucose. Because cells are starved of fuel, cells cannot respire properly. And because blood plasma now has a high concentration of glucose (a high solute concentration), water moves out of cells by osmosis — leaving the cells dehydrated and producing the thirst and frequent urination that characterise untreated diabetes. Type 1 is treated with insulin injections.

Type 2 diabetes is different — the pancreas still produces insulin, but the body cells stop responding to it properly (insulin resistance). It is linked to obesity and is managed primarily by diet, weight loss and exercise.

Exam tips

On any 'what is homeostasis' question, give all four pieces: maintenance of constant internal conditions; despite external changes; optimum conditions for cell function (enzyme activity); negative feedback reverses the initial change. Each is worth a mark.
Memorise the four named OCR coordination centres exactly: brain, spinal cord, pancreas, hypothalamus. Quote the relevant one(s) for the question.
Effectors are MUSCLES OR GLANDS — write both options to be safe, then give one named example per type (e.g. sweat gland, pancreas, kidney).
Always finish a feedback answer with 'negative feedback reverses the initial change' — OCR examiners look for this exact wording.
For blood-glucose answers, build the scaffold: pancreas detects high blood glucose · insulin is secreted/released by the pancreas · glucose converted to glycogen · response reverses the initial change.
For thermoregulation: temperature rise detected by the hypothalamus · sweat glands secrete sweat · body temperature falls back towards 37 °C · negative feedback reverses the initial stimulus.
For ADH/water balance: pituitary gland secretes/releases ADH · ADH increases water reabsorption in the kidney · blood water concentration rises back to normal · negative feedback.
Receptors DETECT, coordination centres PROCESS, effectors RESPOND. Never write 'receptors respond' — it is the most common slip on this topic.
Adrenaline is NOT part of a negative-feedback loop — it overrides normal homeostasis. If a question asks about adrenaline, mention: increases heart rate, increases breathing rate, raises blood sugar/glucose levels, glycogen is converted to glucose.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What is homeostasis in one sentence?

Homeostasis is the maintenance of constant internal conditions despite external changes, providing optimum conditions for cell function — including enzyme activity. All control involves receptors that detect stimuli, coordination centres (brain, spinal cord, pancreas, hypothalamus) that process the information, and effectors (muscles or glands) that bring about a response. Every loop uses negative feedback — a change in a variable triggers a response that reverses the initial change.

Why does homeostasis matter — what would happen without it?

Every chemical reaction in your body is catalysed by enzymes, and enzymes only work properly within a narrow range of temperature, pH and substrate concentration. Outside that range they denature — the active site loses its shape and the reaction stops. Cells also need a particular water and ion balance to avoid swelling or shrivelling by osmosis. Homeostasis keeps internal conditions inside the band in which enzymes work and cells survive. Without it, the body could not function — and could not respond to changes in the environment, because it would have no stable baseline to return to.

What are receptors, coordination centres and effectors?

These are the three components of every control system. RECEPTORS are cells that DETECT stimuli — changes in the environment, internal or external (e.g. glucose-sensitive cells in the pancreas, thermoreceptors in the skin, osmoreceptors in the hypothalamus). COORDINATION CENTRES receive and PROCESS the information from receptors and decide what response is needed — OCR names the brain, spinal cord, pancreas and hypothalamus. EFFECTORS are MUSCLES or GLANDS that BRING ABOUT the response — muscles contract, glands secrete hormones or other substances. The flow is always: stimulus → receptor → coordination centre → effector → response.

What is negative feedback?

Negative feedback is a control loop in which a change in a variable triggers a response that REVERSES the change, returning the variable to its set point. If blood glucose rises above the set point, the response (insulin release, glucose → glycogen) brings it back down. If body temperature rises, the response (sweating, vasodilation) brings it back down. If blood water concentration falls, ADH is released and the kidney reabsorbs more water, raising it back up. Because both directions are corrected, the variable oscillates within a narrow range around the set point rather than wandering off. The word 'negative' refers to the response being the OPPOSITE of the change — not to the direction of the change itself.

How does ADH control water balance?

ADH (antidiuretic hormone) is released by the pituitary gland when osmoreceptors in the hypothalamus detect that blood water concentration has fallen (the blood is too concentrated). ADH travels in the blood to the kidneys, where it makes the walls of the collecting duct more permeable to water. More water is reabsorbed from the kidney tubule back into the blood, less water is lost in urine (the urine becomes more concentrated), and blood water concentration rises back to normal. When blood is too dilute, less ADH is released, the collecting duct is less permeable, less water is reabsorbed, and more dilute urine is produced. Classic negative feedback.

How does the body control its temperature?

The coordination centre is the hypothalamus, which contains thermoreceptors that monitor blood temperature. When you overheat, the hypothalamus signals the sweat glands to secrete sweat (which cools the body as it evaporates) and the blood vessels supplying the skin to dilate (vasodilation — more heat lost by radiation). When you are cold, blood vessels constrict (vasoconstriction), sweating stops, and skeletal muscles contract rapidly — shivering — which releases heat from respiration. Both responses reverse the initial change. Negative feedback.

What is the difference between Type 1 and Type 2 diabetes?

Type 1 diabetes is caused by the pancreas failing to produce insulin — usually because the insulin-producing cells have been destroyed by the immune system. It typically appears in childhood and is treated with insulin injections. Type 2 diabetes is different: the pancreas still produces insulin, but the body cells stop responding to it properly (insulin resistance). It usually develops in adulthood, is linked to obesity, and is managed by diet, weight loss and exercise. Both conditions cause blood glucose levels to rise; cells cannot take up glucose; cells cannot respire properly; and water moves out of cells by osmosis.