HookA coin-sized patch that never stops watching your blood sugar
Since 2022 the NHS in England has offered a small round patch, worn on the back of the upper arm, to everyone living with Type 1 diabetes. The sensor under it reads the glucose in the fluid beneath the skin every few minutes and streams the number to a phone, so a graph of blood sugar scrolls across the screen all day and night. Before it, people managed by pricking a finger a handful of times a day and guessing at the hours in between. What the patch makes visible is something a healthy body does silently, thousands of times a day, and never shows you: it holds blood glucose inside a narrow band whatever you eat, whether you run for a bus or sleep for nine hours. When that automatic control breaks, you can see on the screen exactly how hard it was working — and that is the idea this whole section is built on.
That automatic control is homeostasis: the regulation of the body's internal conditions — temperature, water, blood glucose — within narrow limits, so that cells and especially their enzymes keep working. The body coordinates in two ways and you need both. The nervous system is fast and electrical, carrying impulses along neurones for split-second responses such as a reflex. The endocrine system is slower and chemical, releasing hormones into the blood for longer, whole-body changes such as the menstrual cycle or the control of blood sugar. This section teaches both systems, then applies them to glucose, reproduction and fertility.
ModelThe control loop — receptors, coordination centres, effectors
Every homeostatic system in the body is built from the same three parts, and naming them is the backbone of a good six-mark answer. Receptors are cells that detect a change in the environment — a stimulus. A coordination centre, such as the brain, spinal cord or pancreas, receives and processes the information and decides on a response. Effectors, which are muscles or glands, carry the response out — a muscle by contracting, a gland by secreting a hormone.
The body must do this because its chemistry only works in a narrow band. Enzymes have an optimum temperature and pH; too much or too little water swells or shrinks cells by osmosis; the brain starves within minutes if blood glucose falls too far. Holding those conditions steady keeps every other process possible.
The control is achieved by negative feedback: whenever a condition moves away from its set point, the system triggers a response that pushes it back — and once the level returns to normal, the response switches off. If a level rises too high the correction brings it down; if it falls too low a different correction brings it up. A thermostat holding a room at a set temperature is the everyday analogy, and every example that follows, from blood glucose to thyroxine, is a version of the same loop.
MechanismThe nervous system and the reflex arc
The nervous system lets you react to your surroundings and coordinate your behaviour. Its centre is the central nervous system (CNS) — the brain and spinal cord. Information travels as electrical impulses along cells called neurones, and the full pathway is worth learning in order: a stimulus is detected by a receptor, which sends an impulse along a sensory neurone to the CNS; from there an impulse passes along a motor neurone to an effector, which produces the response. Where two neurones meet there is a tiny gap, the synapse, which the impulse crosses when a chemical (a neurotransmitter) is released, diffuses across, and triggers a new impulse in the next neurone.
A reflex is an automatic, rapid response that does not involve the conscious part of the brain — snatching your hand off a hot pan before you feel the pain. The reflex arc runs receptor → sensory neurone → relay neurone in the CNS → motor neurone → effector, skipping the conscious brain precisely so that it can be fast and protective. That is the point examiners test: reflexes are quick because they bypass conscious thought. Required practical 6 measures how quickly the nervous system can respond, and the worked example shows how to turn a caught ruler into a reaction time.
Required practical 6 investigates a factor affecting reaction time. In the ruler-drop test, one student holds a 30 cm ruler and lets it fall without warning between another student's thumb and finger; the distance it drops before it is caught converts to a reaction time through \(t = \sqrt{2d/g}\), because the ruler accelerates under gravity at \(g = 9.8\ \mathrm{m/s^2}\). Testing the dominant against the non-dominant hand, a student catches the ruler at 15.0, 13.0 and 14.0 cm with the right (dominant) hand — a mean of 14.0 cm, or 0.140 m — and at 20.0, 18.4 and 19.2 cm with the left, a mean of 19.2 cm, or 0.192 m. Converting the means: dominant hand \(t = \sqrt{(2 \times 0.140)/9.8} = \sqrt{0.0286} = 0.169\ \mathrm{s}\); non-dominant \(t = \sqrt{(2 \times 0.192)/9.8} = \sqrt{0.0392} = 0.198\ \mathrm{s}\). The dominant hand is about 0.03 s quicker. The result only means something because the control variables were fixed: the same ruler, no warning, no practice runs, the same starting gap and the catcher looking away until the drop. Taking a mean of three repeats to reduce random error, and stating those controls, is where the marks sit — not in the raw distance.
ModelThe endocrine system — chemical messengers
The endocrine system is made of glands that secrete hormones — chemical messengers — directly into the bloodstream, which carries them to their target organs. Compared with the nervous system it is slower to act, but its effects are more general and longer-lasting, and that speed-versus-duration contrast is a guaranteed exam comparison. The pituitary gland in the brain is the 'master gland': it secretes several hormones that in turn control other glands, so a signal from the pituitary can set off a chain of responses elsewhere.
The other glands worth naming are the thyroid, the pancreas, the adrenal glands, and the reproductive organs — the ovaries in women and the testes in men. Each releases its own hormones for its own job, and the rest of this section is really four case studies of those glands at work: the pancreas controlling glucose, the ovaries and pituitary running the menstrual cycle, and — at higher tier — the thyroid and adrenal glands. Keep the contrast with the nervous system sharp: nerves carry fast electrical impulses along neurones for short, targeted responses; hormones travel slowly in the blood for slower, wider, longer-lasting ones.
MechanismControlling blood glucose — insulin, glucagon and diabetes
Blood glucose control is the model homeostatic case, and the patch in the introduction is watching exactly this loop. The pancreas monitors and controls the concentration of glucose in the blood. When blood glucose rises too high — after a meal — the pancreas secretes insulin, which makes liver and muscle cells take up glucose and store the excess as glycogen, bringing the level back down. Higher-tier students also need the opposite direction: when blood glucose falls too low, the pancreas secretes glucagon, which makes the liver convert glycogen back into glucose and release it into the blood, raising the level. That is negative feedback run by two opposing hormones.
Around 4.4 million people in the UK are diagnosed with diabetes, and the distinction between its two forms is heavily tested. Type 1 diabetes is a disorder in which the pancreas produces little or no insulin, so blood glucose can climb to dangerous levels; it is normally treated with insulin injections, and it is the form the flash monitor was designed for. Type 2 diabetes is where the body's cells stop responding properly to insulin; it is strongly linked to obesity and is usually controlled first through a carbohydrate-controlled diet and exercise. Fewer than one in ten people with diabetes has Type 1, so assuming every diabetic injects insulin is a classic lost mark.
CaseHormones in human reproduction and the menstrual cycle
At puberty, reproductive hormones trigger the secondary sex characteristics. The main male hormone, testosterone from the testes, stimulates sperm production; the main female hormone, oestrogen from the ovaries, drives the menstrual cycle. Four hormones control that monthly cycle, and you must know each source and role. FSH (follicle stimulating hormone), from the pituitary, matures an egg in the ovary. Oestrogen, from the ovary, builds up the lining of the uterus and switches FSH off. LH (luteinising hormone), from the pituitary, triggers the release of the egg — ovulation — around day 14. Progesterone, from the ovary, maintains the uterus lining through the second half of the cycle, and when it falls the lining breaks down.
Getting the sources right is the discriminating skill: FSH and LH come from the pituitary; oestrogen and progesterone come from the ovary. Muddling them is one of the most common avoidable errors on the paper. The same four hormones are the levers that both contraception and fertility treatment pull, which is where this section goes next.
CaseControlling fertility — contraception and IVF
Contraception works by interrupting the hormone system that controls reproduction. Hormonal methods — the pill, implant, injection and patch — use oestrogen and/or progesterone to inhibit FSH, so that no egg matures; they are very effective, but they can have side effects and do not protect against infection. Non-hormonal methods include condoms and diaphragms (barriers that stop sperm reaching the egg), intrauterine devices, spermicides, surgical sterilisation, and abstaining from intercourse when an egg may be present. Each is a trade-off between reliability, side effects and protection, and a good answer weighs them rather than just listing them.
Hormones also treat the opposite problem. If a woman does not make enough FSH to mature her own eggs, a 'fertility drug' containing FSH and LH can stimulate maturation (higher tier). In IVF (in vitro fertilisation), FSH and LH are given to mature several eggs, which are collected and fertilised by sperm in the laboratory; the resulting embryos develop for a few days before one or two are inserted into the uterus. IVF gives many couples a child who could not otherwise have one, but the balanced evaluation examiners reward notes the costs too: it is emotionally and physically demanding, the success rate is low — the birth rate per embryo transferred is under a third even for younger patients — and stimulating several eggs can lead to risky multiple births.
ModelFeedback beyond glucose — thyroxine and adrenaline (higher tier)
Two more hormones round out the higher-tier picture and make a sharp contrast. Thyroxine, from the thyroid gland, sets the basal metabolic rate — the base rate at which the body's reactions tick over — and is important for growth and development. It is controlled by negative feedback: when the level of thyroxine in the blood falls too low, the pituitary releases a hormone that stimulates the thyroid to release more; when thyroxine rises back to normal, that stimulation is switched off. It is the same loop as blood glucose, just holding a different quantity steady.
Adrenaline, from the adrenal glands, is the exception that proves the rule. It is released in times of fear or stress and prepares the body for 'fight or flight': it raises the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles. Crucially, adrenaline is not controlled by negative feedback — it is a surge released in response to a threat, not a level held at a set point. Being able to say that adrenaline is a boost while thyroxine is a regulated set point is exactly the kind of distinction the higher-tier mark scheme rewards.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Whenever a question hands you a homeostatic situation, structure the answer as receptor → coordination centre → effector, and describe the correction in both directions of negative feedback — what happens when the level is too high and when it is too low. Answers that describe only one direction cap at half marks. This is AO2: applying the loop to the exact context in the question, not reciting it in the abstract.
For the nervous system, write the reflex pathway in full and in order, and stress that it does not involve the conscious brain — that phrase is the discriminating point. For the endocrine system, be ready to compare it with the nervous system on transmission (blood versus neurones), speed (slow versus fast) and duration (long versus short). Name hormones and their glands precisely: FSH and LH from the pituitary, oestrogen and progesterone from the ovary; muddling the sources is a frequent, avoidable loss.
On the higher-tier content, give glucagon as well as insulin for the two directions of glucose control, present thyroxine as a negative-feedback loop, and flag that adrenaline is the exception. On required practical 6 the marks are in the control variables and the mean of repeats (AO3), so state them explicitly rather than just quoting a result. Watch the command word: 'describe' wants the sequence of events, 'explain' wants the reason behind each step, and a six-marker is marked by levels of response, so cover the whole loop coherently rather than in fragments.