What you'll be able to do by the end
- Define homeostasis and explain why it is necessary
- Describe the components of a control system — receptor, coordination centre, effector
- Explain negative feedback
- Describe the structure and function of the nervous system
- Explain the reflex arc and why reflexes are rapid
- Describe how body temperature is controlled
- Describe how blood glucose is controlled, and explain diabetes
- Describe the role of the kidneys in water and ion balance
- Name the main hormones and their functions
- Describe the menstrual cycle and methods of contraception
- Compare nervous and hormonal communication
Part 1 — What homeostasis is
Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external change. That's the definition, and it's worth learning word for word — but the idea underneath it is simple.
Why the body must control conditions
Enzymes. Every reaction in your body is controlled by an enzyme, and enzymes are fussy.
| Condition | Why it must be controlled |
|---|---|
| Temperature | Too cold and reactions are slow; too hot and enzymes denature and stop working permanently |
| pH | Enzymes have an optimum pH; outside it they denature |
| Water and ion concentration | Cells swell or shrink by osmosis if the surrounding concentration is wrong |
| Blood glucose | Cells need glucose for respiration; too much damages tissues, too little starves them |
Enzymes are not alive. They cannot die.
Denaturing means the enzyme's shape changes so the substrate no longer fits the active site. It is permanent, and it is why a very high fever is dangerous. Writing "the enzymes die" loses the mark every time.
The three parts of every control system
Every homeostatic system, without exception, has these three:
| Part | Job | Example |
|---|---|---|
| Receptor | Detects the change | Temperature receptors in the skin |
| Coordination centre | Processes the information and decides | Brain, spinal cord, pancreas |
| Effector | Carries out the response | Muscle or gland |
Any homeostasis question can be answered by naming the receptor, the coordination centre and the effector. Learn the three, apply them to each system, and the topic collapses from four things to one.
Negative feedback
When a condition moves away from its normal level, the body responds in the opposite direction to bring it back.
Level rises above normal → Receptor detects the change → Coordination centre processes it → Effector responds to LOWER it → Level returns to normal → Response switches off
Negative doesn't mean bad. It means the response is in the opposite direction to the change.
A thermostat. Room gets cold → thermostat detects it → boiler switches on → room warms → thermostat detects that → boiler switches off. Your body is a thermostat that also sweats.
Part 2 — The nervous system
The nervous system allows humans to react to their surroundings and coordinate behaviour.
Structure
| Part | Contains |
|---|---|
| Central nervous system (CNS) | Brain and spinal cord |
| Peripheral nervous system | All the nerves connecting the CNS to the body |
The pathway
Learn this sequence. It appears on every paper in some form.
Types of neurone
| Neurone | Carries impulses |
|---|---|
| Sensory | From receptor to CNS |
| Relay | Within the CNS |
| Motor | From CNS to effector |
Synapses
A synapse is the gap between two neurones. How the signal crosses:
- The electrical impulse reaches the end of the first neurone
- Chemicals (neurotransmitters) are released into the gap
- They diffuse across
- They bind to receptors on the next neurone
- A new electrical impulse is triggered
Diffusion is slower than an electrical impulse. Every synapse adds a delay. This is exactly why the reflex arc has as few synapses as possible.
The reflex arc
A reflex is rapid and automatic — it does not involve the conscious part of the brain.
Worked example — touching something hot:
| Stage | What happens |
|---|---|
| Stimulus | Heat |
| Receptor | Temperature and pain receptors in the skin |
| Sensory neurone | Carries impulse to the spinal cord |
| Relay neurone | In the spinal cord — bypasses the brain |
| Motor neurone | Carries impulse to the muscle |
| Effector | Biceps muscle |
| Response | Arm pulls away |
Why reflexes are fast: they do not involve the conscious brain, and they have fewer synapses, so less delay. Why they matter: they protect the body from damage before you have consciously noticed anything.
You do feel the pain — the message goes to the brain too. But your hand has already moved by then. The reflex happens first. The awareness catches up.
Part 3 — Controlling body temperature
Normal human body temperature: 37°C. Controlled by the thermoregulatory centre in the hypothalamus of the brain.
The control system
| Part | What |
|---|---|
| Receptors | In the hypothalamus (monitoring blood temperature) and in the skin |
| Coordination centre | Thermoregulatory centre in the hypothalamus |
| Effectors | Sweat glands, blood vessels, muscles, hair erector muscles |
Too hot
| Response | How it helps |
|---|---|
| Sweating | Water evaporates from the skin, taking heat energy with it |
| Vasodilation | Blood vessels supplying skin capillaries dilate, so more blood flows near the surface and more heat is lost by radiation |
Too cold
| Response | How it helps |
|---|---|
| Shivering | Muscles contract rapidly; respiration in muscle cells releases heat |
| Vasoconstriction | Blood vessels supplying skin capillaries constrict, so less blood flows near the surface and less heat is lost |
| Sweating stops | No heat lost by evaporation |
| Hairs stand up | Traps a layer of air as insulation (minimal effect in humans) |
Blood vessels do not move. They do not "come to the surface" or "go deeper." The arterioles supplying the skin capillaries dilate or constrict, changing how much blood flows through vessels that are already there.
Write: "the arterioles supplying the skin capillaries dilate, so more blood flows through them and more heat is lost." Never: "the blood vessels move closer to the skin."
Connecting to adaptation
Endotherms — mammals and birds — control body temperature internally. Costly in energy, but they can be active in any conditions. Ectotherms — reptiles, amphibians, fish — rely on behaviour: basking, moving to shade, burrowing.
A bearded dragon basking in the morning and retreating to shade at midday is doing exactly what your hypothalamus does — just with behaviour instead of sweat glands.
Part 4 — Controlling blood glucose
Blood glucose concentration is monitored and controlled by the pancreas.
Too high
- The pancreas detects the rise
- It secretes insulin
- Insulin causes glucose to move from the blood into cells
- In the liver and muscles, excess glucose is converted to glycogen for storage
- Blood glucose falls to normal
Too low
- The pancreas detects the fall
- It secretes glucagon
- Glucagon causes the liver to convert glycogen back into glucose
- Glucose is released into the blood
- Blood glucose rises to normal
Glucagon is the hormone. Glycogen is the stored carbohydrate. One letter apart, and swapping them makes the answer nonsense. Write them slowly.
Type 1 diabetes
Cause: the pancreas produces little or no insulin, so blood glucose rises to dangerously high levels. Treatment: insulin injections, usually before meals. Also managed by: monitoring blood glucose, controlling carbohydrate intake, exercise. Usually diagnosed in childhood. Not caused by lifestyle.
Type 2 diabetes
Cause: body cells stop responding properly to insulin — insulin resistance. Risk factor: obesity. Treatment: carbohydrate-controlled diet, exercise, weight management; medication or insulin if needed. Usually develops in adulthood, though increasingly in younger people.
| Type 1 | Type 2 | |
|---|---|---|
| Cause | No insulin produced | Cells don't respond to insulin |
| Onset | Usually childhood | Usually adulthood |
| Main risk factor | Not lifestyle-related | Obesity |
| Treatment | Insulin injections | Diet, exercise, sometimes medication |
Students in the room may have Type 1 diabetes, or a family member with either type. Type 1 is not caused by anything anyone did. Say so explicitly. Lessons that discuss "diabetes" as a lifestyle disease without distinguishing the two do real harm to children who inject insulin four times a day through no fault of their own.
Part 5 — The kidneys and water balance
What the kidneys do
- Filter the blood, removing urea, excess water and excess ions
- Selectively reabsorb useful substances — all the glucose, and the water and ions the body needs
- Produce urine from what's left
Where urea comes from
Excess amino acids cannot be stored. In the liver they undergo deamination — the nitrogen part is removed, producing ammonia, which is toxic. Ammonia is immediately converted to urea, which is less toxic, and transported to the kidneys for excretion.
Water loss
Water leaves the body through the lungs (in exhaled breath), the skin (in sweat, along with ions and urea) and the kidneys (in urine). Only urine production is controlled. You cannot decide to sweat less.
ADH — anti-diuretic hormone
Water content is controlled by ADH, released from the pituitary gland.
More ADH released → kidney tubules more permeable → more water reabsorbed → small volume of concentrated urine
Less ADH released → tubules less permeable → less water reabsorbed → large volume of dilute urine
This is negative feedback, controlled by the pituitary.
Kidney failure
| Treatment | How it works | Advantages | Disadvantages |
|---|---|---|---|
| Dialysis | Blood passes through a machine with a partially permeable membrane; urea and excess ions diffuse out into dialysis fluid | Available immediately; no surgery; no rejection | 3–4 sessions weekly, several hours each; restricted diet; expensive long-term; affects quality of life |
| Transplant | A donor kidney is surgically implanted | Long-term solution; normal diet and lifestyle; cheaper over time | Donor shortage; major surgery; risk of rejection; immunosuppressants for life, increasing infection risk |
Because diffusion only moves substances down a concentration gradient. If dialysis fluid contained no glucose, all the patient's glucose would diffuse out and be lost. By matching healthy glucose and ion concentrations, useful substances stay put while urea — absent from the fluid — diffuses out. This is a favourite exam question and it's pure diffusion.
Part 6 — Hormones and the endocrine system
The endocrine system is made of glands that secrete hormones directly into the bloodstream.
The main glands and hormones
| Gland | Hormone | Effect |
|---|---|---|
| Pituitary | FSH, LH, ADH, growth hormone | The "master gland" — controls other glands |
| Thyroid | Thyroxine | Controls metabolic rate |
| Pancreas | Insulin, glucagon | Controls blood glucose |
| Adrenal | Adrenaline | Prepares the body for "fight or flight" |
| Ovaries | Oestrogen, progesterone | Female reproduction, menstrual cycle |
| Testes | Testosterone | Male reproduction, sperm production |
Adrenaline
Produced by the adrenal glands in times of fear or stress. Effects: increases heart rate; increases breathing rate; diverts blood to muscles; converts glycogen to glucose. Purpose: delivers more oxygen and glucose to the muscles and brain, preparing the body for "fight or flight." Not controlled by negative feedback — it's a response to an external situation, not an internal level.
Thyroxine
Controls the basal metabolic rate — the rate of chemical reactions in cells at rest. Important for growth and development. Controlled by negative feedback:
Nervous versus hormonal
| Nervous | Hormonal | |
|---|---|---|
| Transmitted by | Electrical impulses along neurones | Chemicals in the blood |
| Speed | Very fast | Slower |
| Duration | Short-lived | Longer-lasting |
| Target | Precise — a specific muscle or gland | General — anywhere with the right receptors |
Nervous: a text message to one person. Instant, specific, over quickly. Hormonal: an announcement over a tannoy. Slower, everyone with the right ears hears it, effects last longer.
Part 7 — Reproduction and the menstrual cycle
Puberty
Secondary sexual characteristics develop, driven by testosterone in males — produced by the testes, stimulates sperm production — and oestrogen in females — produced by the ovaries, involved in the menstrual cycle.
The four hormones of the menstrual cycle
| Hormone | Released by | Role |
|---|---|---|
| FSH | Pituitary | Causes an egg to mature in the ovary; stimulates oestrogen production |
| Oestrogen | Ovaries | Causes the uterus lining to thicken; inhibits FSH; stimulates LH |
| LH | Pituitary | Triggers ovulation — release of the egg, around day 14 |
| Progesterone | Ovaries | Maintains the uterus lining; inhibits FSH and LH |
The cycle
| Days | What happens |
|---|---|
| 1–4 | Menstruation — the uterus lining breaks down |
| 4–14 | FSH matures an egg; oestrogen rebuilds the lining |
| 14 | LH surge triggers ovulation |
| 14–28 | Progesterone maintains the lining. If no fertilisation, progesterone falls and the cycle restarts |
Contraception
| Method | How it works | Type |
|---|---|---|
| Combined pill | Oestrogen and progesterone inhibit FSH, so no egg matures | Hormonal |
| Progesterone-only pill / implant / injection | Thickens cervical mucus, inhibits ovulation | Hormonal |
| Condom | Physical barrier preventing sperm reaching the egg | Barrier |
| Diaphragm | Barrier over the cervix, used with spermicide | Barrier |
| IUD (coil) | Prevents implantation; some release hormones | Device |
| Sterilisation | Surgical cutting or tying of tubes | Surgical |
| Abstinence | Avoiding intercourse when an egg may be present | Behavioural |
Condoms are the only method that also protects against sexually transmitted infections.
Treating infertility
Fertility drugs — FSH and LH stimulate egg maturation. Can result in multiple births. IVF — eggs collected, fertilised in a laboratory, embryos grown and implanted.
| For IVF | Against IVF |
|---|---|
| Enables people to have children who otherwise could not | Emotionally and physically stressful |
| Success rates improving | Success rate still relatively low |
| Expensive, and not equally available | |
| Multiple births carry risk to mother and babies | |
| Ethical objections to the destruction of unused embryos |
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 71 marks.
Question 1
(Total 8 marks)
Question 2
A person touches a hot surface and their hand pulls away before they feel pain.
(Total 10 marks)
Question 3
(Total 8 marks)
Question 4
(Total 10 marks)
Question 5
(Total 12 marks)
Question 6
(Total 9 marks)
Question 7
(Total 8 marks)
Question 8 — extended response
Evaluate the use of IVF as a treatment for infertility. (6)
(Total 6 marks)
TOTAL FOR PAPER: 71 MARKS
Mark scheme
Question 1
| (a) 2. The regulation of internal conditions | 1 |
| to maintain optimum conditions for function, in response to internal and external change | 1 |
| (b) 3. Receptor: detects the change | 1 |
| Coordination centre: processes the information | 1 |
| Effector: brings about the response | 1 |
| (c) 3. Enzymes control the reactions of the body | 1 |
| At low temperatures reactions are too slow | 1 |
| At high temperatures enzymes denature and stop working permanently | 1 |
Question 2
(a) 1 — A reflex / reflex action.
(b) 4 — award 1 each for four correct stages in order: stimulus (heat) → receptor in skin → sensory neurone → relay neurone in spinal cord → motor neurone → effector (muscle) → response (hand withdraws).
| (c) 2. Does not involve the conscious brain | 1 |
| Fewer synapses, so less delay from diffusion | 1 |
| (d) 3. The impulse causes neurotransmitter chemicals to be released into the gap | 1 |
| They diffuse across the synapse | 1 |
| They bind to receptors on the next neurone, triggering a new electrical impulse | 1 |
Question 3
(a) 3 — any three: shivering; vasoconstriction of arterioles supplying skin capillaries; sweating stops; hairs stand up.
| (b) 3. Arterioles supplying the skin capillaries dilate | 1 |
| More blood flows through the capillaries near the skin surface | 1 |
| More heat is transferred to the surroundings by radiation | 1 |
| (c) 2. Blood vessels do not move within the skin | 1 |
| It is the arterioles that dilate or constrict, changing the volume of blood flowing through capillaries that are already there | 1 |
Question 4
(a) 1 — The pancreas.
| (b) 4. The pancreas detects the rise in blood glucose | 1 |
| It secretes insulin | 1 |
| Insulin causes glucose to move from the blood into cells | 1 |
| Excess glucose is converted to glycogen in the liver and muscles for storage | 1 |
| (c) 3. Type 1: the pancreas produces little or no insulin | 1 |
| Type 2: the body's cells no longer respond properly to insulin | 1 |
| Type 2 is associated with obesity; Type 1 is not caused by lifestyle | 1 |
| (d) 2. Insulin is a protein | 1 |
| It would be digested by protease enzymes in the stomach and small intestine before reaching the blood | 1 |
Question 5
| (a) 3. Filtration — the blood is filtered, removing urea, water and ions | 1 |
| Selective reabsorption — all the glucose is reabsorbed | 1 |
| Plus the water and ions the body needs; the rest is excreted as urine | 1 |
| (b) 4. The blood is too concentrated / has too little water | 1 |
| The pituitary gland releases more ADH | 1 |
| The kidney tubules become more permeable to water | 1 |
| More water is reabsorbed, producing a small volume of concentrated urine | 1 |
| (c) 3. Substances move by diffusion, down a concentration gradient | 1 |
| If the fluid had no glucose, glucose would diffuse out of the blood and be lost | 1 |
| Matching the concentration means there is no net movement of glucose, while urea — absent from the fluid — diffuses out | 1 |
(d) 2 — any two: long-term solution rather than repeated treatment; normal diet and lifestyle; cheaper over time; far less disruptive to daily life.
Question 6
(a) 3 — any three: nervous uses electrical impulses, hormonal uses chemicals in the blood; nervous is very fast, hormonal is slower; nervous effects are short-lived, hormonal effects last longer; nervous acts on a precise target, hormonal effects are more general.
(b) 3 — Adrenal gland (1) plus any two effects: increases heart rate; increases breathing rate; diverts blood to muscles; converts glycogen to glucose (2).
| (c) 3. If thyroxine falls, the pituitary releases TSH | 1 |
| TSH stimulates the thyroid to release more thyroxine | 1 |
| As thyroxine rises, TSH release is inhibited, so levels return to normal | 1 |
Question 7
(a) 2 — LH / luteinising hormone (1), produced by the pituitary gland (1).
| (b) 3. Produced by the ovaries | 1 |
| Causes the lining of the uterus to thicken | 1 |
| Inhibits FSH and stimulates the release of LH | 1 |
| (c) 3. Contains oestrogen and progesterone | 1 |
| These inhibit the production of FSH | 1 |
| So no egg matures in the ovary and ovulation does not occur | 1 |
Question 8 — extended response (6 marks, levels-marked)
Level 3 (5–6): balanced evaluation with arguments on both sides, explained rather than listed, reaching a justified conclusion. Level 2 (3–4): both sides present with limited explanation, or one side developed and the other acknowledged. Level 1 (1–2): relevant points listed, largely one-sided.
Indicative — for: enables people who cannot conceive naturally to have a child; success rates continue to improve; allows screening for serious inherited disorders. Against: physically and emotionally demanding, involving hormone treatment and surgery; success rate remains relatively low, so repeated attempts are common; expensive and unevenly available, raising fairness concerns; multiple births carry increased risk to mother and babies; ethical objections to the creation and destruction of unused embryos. Conclusion: any justified position, e.g. that IVF is justified where the alternative is childlessness, provided the risks are clearly explained and access is equitable.
Common mistakes in this topic
"The enzymes die." Enzymes aren't alive. They denature.
"Blood vessels move closer to the skin." They don't move. Arterioles dilate or constrict.
Confusing glucagon and glycogen. One is the hormone, one is the store.
Saying insulin "turns glucose into glycogen" without mentioning the liver. The liver and muscles are where it's stored.
Describing Type 2 diabetes as "when you eat too much sugar." It is insulin resistance, with obesity as a risk factor — not a direct cause.
Forgetting the spinal cord in the reflex arc. The relay neurone is in the CNS, and that's what makes it fast.
Saying hormones are "faster." They're slower than nerves, but longer-lasting.
Teaching notes
Teach the three-part control system first and use it for everything. Receptor, coordination centre, effector. Once students can identify those three in temperature, glucose and water balance, they've learned one thing rather than three.
The thermostat analogy earns its place. Negative feedback is abstract; a boiler switching on and off is not. Come back to it every time.
The vasodilation error should be corrected the first time it appears and every time after. It's the most common wrong answer in the whole topic, it's in every mark scheme as an unacceptable response, and once a student has written it three times it's difficult to shift.
Diabetes needs care. Distinguish Type 1 and Type 2 explicitly, and say clearly that Type 1 is not caused by anything anyone did. There will be students in the room for whom this is personal, and a badly handled lesson does lasting damage.
The dialysis fluid question is the best in the unit. It looks like recall and is actually pure diffusion. Students who can explain it have understood concentration gradients properly.
Link back to adaptation. A bearded dragon basking and retreating to shade is doing thermoregulation behaviourally, exactly as the hypothalamus does it physiologically. Students who see the connection understand both topics better.