Homeostasis is the body's negative-feedback machinery for holding internal conditions within narrow ranges. The three systems AQA examines are blood glucose, blood water potential, and core body temperature. Each runs the same loop: detect the deviation, signal the correction, return the variable to the set point.
Read this topic as an analogy.
Picture the body as a large multi-storey building. At the top is a control room (the hypothalamus, and the pancreas for glucose work). Around the building are sensors of every kind: thermostats, humidity meters, glucose gauges, water-level floats. The control room watches the sensors continuously. When any one of them drifts above or below its set point, the control room dispatches a courier.
The courier carries an envelope through the building's internal post system (the bloodstream). The envelope is addressed to a specific receiving desk, found only at certain offices around the building. The desk has an intake slot shaped to accept only one kind of envelope; the courier's envelope fits because its outer shape is complementary to the slot. Once the envelope is in, the desk acts. Some desks send maintenance workers to install new pipes inside the wall, opening a new flow path that wasn't there before. Other desks pass the message to an internal relay team, which doubles the signal at each handover and ends with a furnace stoking the building's heat (or releasing a stored fuel).
The action drives the original sensor back toward its set point. As the sensor steadies, the control room stops sending couriers, the maintenance workers retire, the relay team falls quiet. The system is self-correcting and self-limiting: the bigger the deviation, the louder the signal; the closer to set point, the quieter it gets. Occasionally the building uses the opposite logic, where a single alarm escalates rather than calms (positive feedback), but this is rare and is reserved for situations with a clear termination event, such as the dilation of the cervix during childbirth.
The building also has a water-treatment plant on a lower floor (the kidneys). Water from the main pipes arrives at a high-pressure coarse filter; small molecules are forced through the mesh while large items stay in the main pipe. A salvage worker on the other side pulls back useful materials from the filtered stream: glucose, salts, most of the water. Below the salvage section, the pipe loops down into a pre-conditioned salt bath in the floor of the plant, then loops back up; while the pipe runs through the salt bath, water can be coaxed out of the descending side by osmosis, and the bath itself is maintained by salt pumps on the rising side. Finally, the pipe runs through that same salt bath on its way out. Whether water gets pulled out of the final stretch is decided on demand by a plumber (ADH) who installs extra valves in the pipe wall when the courier brings instructions to do so.
Things go wrong when the dispatch desk shuts down (the courier office for glucose can be lost in Type I diabetes), or when the receiving desks stop reading their envelopes properly (the desks become less responsive in Type II diabetes). Either way, the control loop is interrupted, and the variable drifts.
Mapping back to formal vocabulary. When you write an exam answer, drop the building. The control room is the hypothalamus (or the pancreas for glucose). The couriers are hormones: insulin, glucagon, ADH, adrenaline. The envelope shape that fits the receiving desk is the hormone's tertiary structure, complementary to the receptor binding site. The maintenance worker installing new pipes is the vesicle insertion of channel proteins (GLUT4 for insulin, aquaporins for ADH). The internal relay team that doubles the signal is the second messenger cascade with adenyl cyclase, cAMP, and protein kinase. The pre-conditioned salt bath is the medullary gradient generated by the loop of Henle. The high-pressure coarse filter is ultrafiltration; the salvage worker is the proximal convoluted tubule reabsorbing glucose and water. The corrective principle is negative feedback: detect the change, send the opposite response.
Negative feedback corrects deviations by triggering responses opposite in direction to the change.
Homeostasis is the maintenance of a constant internal environment despite changes outside the body and metabolism inside it. The three controlled variables AQA examines are blood glucose, blood water potential, and core body temperature. Each must stay within a narrow range because enzymes and membrane transport proteins lose their tertiary structure outside their operating window, and cellular function collapses with them.
A receptor detects a deviation from the set point. A coordinator (nervous or hormonal) relays the information. An effector produces a corrective response that opposes the original change. As the variable returns toward the set point, the deviation shrinks, and the response diminishes. The system is self-correcting and self-limiting.
Positive feedback is the opposite arrangement: a detected change triggers a response that amplifies the change rather than reversing it. The AQA standard example is the dilation of the cervix during childbirth, where stretching triggers contractions that stretch the cervix further, until delivery completes and the loop terminates. Positive feedback is rare in physiology because amplifying loops are inherently unstable; it appears only in contexts that have a clear termination event.
Rising blood glucose triggers insulin, which inserts new glucose channels into target cells.
Blood glucose rises after meals and falls between them, but it must be held within a narrow range so that respiring cells, particularly brain neurones, have a continuous supply. The pancreas contains the sensors and dispatchers (the alpha and beta cells of the islets of Langerhans); the liver is the main storage site; muscle and adipose cells are the peripheral targets that take glucose from blood.
The hormone's tertiary structure is complementary to the binding site on the receptor of the target cell. Same mechanism for insulin, glucagon, and ADH. AQA's two-mark answer requires both terms: tertiary structure and complementary. Either alone caps at one out of two.
- Beta cells in the islets of Langerhans detect rising blood glucose and secrete the hormone insulin into the bloodstream.
- Insulin binds to receptors on the membrane of target cells (hepatocytes, muscle cells, adipose cells), where the hormone-receptor binding follows the standard complementary tertiary-structure pattern.
- Binding triggers intracellular vesicles carrying glucose channel proteins (GLUT4) to fuse with the cell-surface membrane, inserting new channels into the membrane.
- Glucose enters the cell by facilitated diffusion through the new channels; inside hepatocytes and muscle cells, glucose is converted to glycogen (glycogenesis) for storage.
Write
tertiary structureandcomplementarytogether on every hormone-receptor question. Don't writeactive site,substrate,induced fit, orantigen— those are enzyme or immunology imports and they score zero here. Writenew channel proteins inserted into the membranefor the insulin mechanism, notinsulin opens existing channels; the mechanism is vesicle insertion of new GLUT4, not gating of pre-existing transporters. Glucose uptake under insulin isfacilitated diffusion, notactive transport.
Falling blood glucose triggers glucagon, and adrenaline acts through a second messenger cascade that amplifies its signal.
When blood glucose falls, the alpha cells of the islets of Langerhans detect the drop and secrete glucagon. Glucagon also targets hepatocytes via the standard hormone-receptor binding pattern, but its effect is opposite to insulin's: it raises blood glucose by switching on the enzymes that release stored glucose. Glucagon does not catalyse anything itself; it activates intracellular pathways.
Glycogenolysis is the hydrolysis of stored glycogen to glucose. Gluconeogenesis is the synthesis of new glucose from non-carbohydrate substrates such as amino acids, glycerol, and lactate. Both raise blood glucose; both are activated by glucagon; the two are not synonyms.
Write
glycogenolysisfor glycogen to glucose. AQA rejectsglycolysis(wrong pathway; that converts glucose to pyruvate in respiration),glucolysis, andglucogenesis— none of which are biological terms. Writeglucagon activates enzymes, notglucagon convertssubstrates orglucagon produces enzymes. Glucagon is a hormone, not an enzyme; it switches on pre-existing enzymes via the cascade.
The cascade's purpose is amplification. One adrenaline molecule activates many adenyl cyclase molecules; each produces many cAMP molecules; each cAMP activates many protein kinase molecules; and so on down the chain. A small hormonal signal at the outside of the cell triggers a much larger metabolic response inside. cAMP is the second messenger — the first messenger is the hormone outside the membrane; cAMP carries the signal inside the cytoplasm.
On any inhibition question about the cascade, every step needs
lessornoafter the blocked point. "Less adenyl cyclase activated, less cAMP produced, less protein kinase activated, less glycogen phosphorylase activated, less glucose released" earns each step. The same chain without the negative qualifiers earns nothing.
Diabetes is the failure of the insulin response system, in two distinct types.
Diabetes mellitus is the chronic failure of the blood glucose control system; persistently elevated blood glucose (hyperglycaemia) is its diagnostic signature. AQA tests the two main types directly because each involves a distinct cell-level failure.
Type I and Type II diabetes side by side.
| Property | Type I | Type II |
|---|---|---|
| Cellular cause | Autoimmune destruction of beta cells | Receptors less sensitive (target cells less responsive) |
| Insulin production | Little or none (beta cells destroyed) | Continues (beta cells intact) |
| Typical age of onset | Early life | Adulthood (falling) |
| Strong risk factors | None established | Weight, diet, exercise |
| Management | Exogenous insulin matched to intake | Diet and exercise first; medication added later |
For Type II, write
receptors less sensitiveortarget cells less responsive. AQA rejectsthe body is less sensitive to insulin— it lacks the cellular referent and scores zero. For Type II risk factors, name them specifically:weight,diet,exercise.Lifestylealone is rejected.
Pitfall — Beta cells destroyed equals Type I, not Type II
Beta cells destroyed equals Type I, not Type II.
Some research papers and Evaluate-stem questions describe animal models in which beta cells are surgically or chemically destroyed. That models Type I, because insulin production is eliminated.
Type II requires beta cells intact and producing insulin, with the failure at the target cell's receptor side. On Evaluate stems involving rat or mouse models, the model-mismatch is itself a credited limitation.
The kidney filters blood at the glomerulus, then selectively reabsorbs useful solutes in the proximal convoluted tubule.
The kidneys are the primary osmoregulation organs, controlling both the volume and the solute composition of plasma. Each kidney contains around one million nephrons, the functional filtration units. The cortex contains the Bowman's capsules and the convoluted tubules; the medulla contains the loops of Henle and the collecting ducts.
Ultrafiltration is the first step. Blood arrives at the glomerulus, a knot of capillaries inside the cup of the Bowman's capsule, via the afferent arteriole. The afferent arteriole is wider than the efferent arteriole leaving, and the narrower exit creates high hydrostatic pressure inside the glomerular capillaries. The pressure forces water and small solutes (water, glucose, urea, mineral ions, amino acids) through the filtration barrier into the lumen of the Bowman's capsule. Large molecules — plasma proteins, red blood cells, white blood cells — are too large to pass and remain in the blood.
PCT epithelial cells are adapted for the high rate of reabsorption they perform. Many microvilli on the apical surface increase the area for transporter proteins. Many mitochondria supply ATP for active transport. Many co-transporter proteins in the membrane carry solutes across. These are features of the cells, not of the tubule.
The Na⁺-K⁺ pump on the basolateral surface actively transports sodium out of the PCT cell into the blood, maintaining a low intracellular sodium concentration. Sodium then flows from filtrate into the cell down its gradient via a co-transporter that simultaneously brings glucose with it, against glucose's own gradient. Glucose then diffuses from cell to blood by facilitated diffusion. Under normal blood glucose, all filtered glucose is recovered.
Write
high hydrostatic pressurefor the ultrafiltration driving force. AQA rejectshigher,increased, andraised— the pressure is absolute, not comparative. Writemany microvilli, many mitochondria, many carrier proteinsand keep the wordmanyin every clause; omitting it loses the mark. And these are features of the cells, not of the tubule —one cell thickandgood blood supplydescribe the tubule wall and score zero on a PCT-cell-features question.
The loop of Henle generates a medullary osmotic gradient; ADH controls how much water the collecting duct reabsorbs from it.
The loop of Henle's job is not direct water reabsorption. It is the generation of a low water potential (a high solute concentration) in the medullary interstitium, which the collecting duct then exploits. The descending limb is permeable to water but largely impermeable to sodium ions; as filtrate descends into the increasingly hypertonic medulla, water moves out of the descending limb by osmosis, concentrating the filtrate downward. The ascending limb is impermeable to water but actively transports sodium ions out into the medullary interstitium. The counter-current arrangement multiplies the effect: pumped sodium concentrates the same interstitium the descending limb is exposed to, amplifying the gradient.
A longer loop of Henle reaches deeper into the medulla and generates a steeper sodium ion concentration gradient. The mechanism is the gradient, not a longer diffusion pathway (an explicit reject). Animals in arid environments have longer loops and produce more concentrated urine, because the steeper gradient draws more water out of the collecting duct.
Write
sodium ions, notionsalone, for what the ascending limb pumps out. AQA rejects the unspecified version. And write water movingout ofthe descending limb andout ofthe collecting duct, notinto. Direction is the most consistent slip on counter-current questions; water leaves both, it does not enter them.
The collecting duct passes through this medullary gradient on its way to the renal pelvis. Whether water is reabsorbed from the collecting duct — and how much — is controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released from the posterior pituitary gland. The hypothalamus is the production site; the posterior pituitary is the release site; confusing the two is a high-frequency one-mark loss.
- Osmoreceptors in the hypothalamus detect a fall in blood water potential (a rise in solute concentration); the osmoreceptor cells lose water by osmosis and shrink, triggering nerve signals.
- The hypothalamus signals the posterior pituitary, which releases ADH into the bloodstream. ADH travels in the blood to the collecting duct.
- ADH binds to receptors on the collecting duct cells; vesicles containing aquaporin channel proteins fuse with the membrane, inserting new aquaporins and increasing water permeability.
- Water moves out of the collecting duct by osmosis down the water potential gradient established by the loop of Henle; concentrated urine results; blood water potential is restored.
When blood water potential returns to normal, the osmoreceptors stop signalling, ADH secretion declines, the aquaporins are removed from the membrane by endocytosis, and the collecting duct returns to its low resting water permeability. Negative feedback completed. The ADH mechanism is mechanistically identical to insulin's: vesicle insertion of new channel proteins, not gating of pre-existing ones.
ADH is produced in the hypothalamus and released from the posterior pituitary. Don't name the hypothalamus as the release site; it is the production site. Write
new aquaporins inserted(vesicles fuse with the membrane), notADH opens existing channels. Writeosmosisfor water movement; AQA rejectsosmotic movement of fluid— osmosis is the movement of water specifically, andfluidis rejected in this context.
Pitfall — The nephron water-movement chain is three steps in fixed order
The nephron water-movement chain is three steps in fixed order.
Step 1: state the water potential change — either the filtrate's water potential changes, or the medulla has a low water potential due to high sodium ion concentration.
Step 2: water moves out by osmosis.
Step 3: name the region — descending limb of the loop of Henle, collecting duct, or proximal convoluted tubule. The ascending limb does not appear in water-movement answers because it is impermeable to water. Step 1 is the consistently dropped mark; most students jump to osmosis without setting up the water potential change.
Thermoregulation runs the same negative-feedback loop on body temperature, with hypothalamic control of vasodilation, sweating, vasoconstriction, and shivering.
Body temperature is held near the set point at which enzymes operate optimally. The hypothalamus integrates information from two thermoreceptor sources: peripheral thermoreceptors in the skin (external temperature) and central thermoreceptors in the hypothalamus itself (core blood temperature). The hypothalamus acts as the thermostat and triggers effector responses when temperature drifts from the set point.
Cooling responses are activated. Vasodilation in skin capillaries increases blood flow to the surface, promoting heat loss by radiation and convection. Sweating removes latent heat as water evaporates from the skin. Metabolic rate may decrease, reducing internally generated heat.
Warming responses are activated. Vasoconstriction in skin capillaries reduces surface blood flow, conserving heat in the core. Shivering generates heat as a by-product of ATP hydrolysis in rapid involuntary muscle contractions. Metabolic rate may rise, generating more heat. Piloerection traps a layer of insulating air against the skin.
The control loop is the same negative-feedback pattern as the blood glucose and water systems: detection by thermoreceptors, coordination by the hypothalamus, effector response that opposes the deviation, and reduced signalling as the set point is restored.
Key terms
- insulin
- glucose
- blood glucose
- glycogenolysis
- gluconeogenesis
- water potential
- osmosis
- filtrate
- loop of Henle
- collecting duct
- channel proteins
- tertiary structure