IB Biology · Theme D Continuity and change · Organisms
D3.3 Homeostasis
Homeostasis keeps internal variables within narrow limits around a set point, whatever happens outside. Negative feedback does the correcting: blood glucose by insulin and glucagon, temperature by the hypothalamus. At HL, the kidney excretes urea and regulates osmotic concentration with ADH and aquaporins.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
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Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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D3.3.1 What homeostasis keeps steady
Homeostasis maintains the internal environment within preset limits despite outside change.
Human examples: body temperature, blood pH, blood glucose concentration, blood osmotic concentration.
Each has a set point, such as about 37 °C or pH 7.4.
The value oscillates within a narrow range; it is not held perfectly fixed.
Students often think any measured change means homeostasis has failed. In fact the variable fluctuates within limits as corrections switch on and off.
Students often count skin temperature or food eaten as homeostatic variables. In fact homeostasis is about internal variables such as core temperature and blood glucose.
D3.3.2 Why negative feedback, not positive
Negative feedback: a change away from the set point triggers responses that reverse it.
It works from above and from below, with opposite responses for each direction.
Positive feedback amplifies a change until an end point, as in childbirth contractions.
Homeostasis uses negative feedback because it is self-correcting and stabilising.
Students often think negative feedback only lowers a high value and positive feedback raises a low one. In fact negative feedback corrects both directions.
Students often think positive means beneficial. In fact positive feedback drives a variable away from its start, which is the opposite of control.
D3.3.3 Insulin and glucagon control blood glucose
Beta cells in the pancreatic islets sense high glucose and secrete insulin.
Insulin travels in the plasma; target cells with receptors take up glucose and make glycogen.
Alpha cells sense low glucose and secrete glucagon, which makes liver cells break glycogen down.
The two hormones are antagonistic, returning glucose to the set point from either side.
Students often think insulin breaks down glucose in the blood. In fact it makes cells take glucose up and store or respire it.
Students often mix up glucagon and glycogen. In fact glucagon is the hormone; glycogen is the stored polysaccharide it releases.
D3.3.4 Type 1 and type 2 diabetes
Type 1: the immune system destroys beta cells, so little or no insulin is made.
It usually starts young, is largely genetic, and needs insulin by injection or pump.
Type 2: insulin is made but target cells become insulin resistant; output may fall later.
Risk factors: obesity, sugary and fatty diet, inactivity, age, family history.
Type 2 is largely prevented and first treated by diet and exercise, with drugs added when needed.
Students often think both types are a lack of insulin. In fact type 2 begins as cells failing to respond to insulin that is present.
Students often think type 1 is caused by diet. In fact it is autoimmune; diet manages it but does not cause it.
D3.3.5 Thermoregulation: sensors, control centre, hormone, effectors
Peripheral thermoreceptors in the skin warn the hypothalamus of heat loss or gain early.
The hypothalamus also monitors blood temperature and compares it with about 37 °C.
It sends nerve impulses to effectors and hormonal signals via the pituitary.
Thyroxin from the thyroid raises metabolic rate: a slow, lasting response to prolonged cold.
Effectors include skeletal muscle, arteriole smooth muscle, arrector pili muscles, brown adipose tissue and sweat glands; white adipose tissue insulates.
Students often think skin receptors measure core temperature. In fact the hypothalamus reads the blood; skin receptors give early warning.
Students often think thyroxin acts in seconds like shivering. In fact it is a slow hormonal increase in heat production.
Shivering: skeletal muscle contracts rhythmically and releases heat from respiration.
Sweating cools only when the water evaporates, absorbing latent heat.
Brown adipose tissue makes heat by uncoupled respiration: protons bypass ATP synthase.
Hair erection by arrector pili muscles insulates furred mammals but does little in humans. Birds and mammals also regulate by behaviour.
Students often think blood vessels move nearer the skin. In fact arterioles widen or narrow; capillaries neither move nor constrict.
Students often think sweat cools because it is cold or because it runs off. In fact only evaporation removes heat.
D3.3.7 Excretion and osmoregulation are different jobs HL
Excretion removes metabolic waste such as urea and carbon dioxide; egestion is different.
Osmoregulation regulates the osmotic concentration of the blood, in osmol L⁻¹.
Plasma sits near 0.3 osmol L⁻¹; the kidney adjusts water reabsorption to hold it.
Each dissolved particle counts, so 0.15 mol L⁻¹ NaCl is 0.30 osmol L⁻¹.
Students often think defecation is excretion. In fact faeces are undigested food; excretion is metabolic waste such as urea.
Students often equate osmoles with moles. In fact a salt that splits into two ions gives two osmoles per mole.
D3.3.8 Ultrafiltration, then selective reabsorption HL
In the glomerulus, high pressure forces water and small solutes into Bowman's capsule.
Pressure is high because the efferent arteriole is narrower than the afferent.
Blood cells and plasma proteins are too large to pass; glucose, salts and urea do.
The proximal convoluted tubule actively reabsorbs all glucose, all amino acids and most sodium.
Water follows by osmosis, returning about 80% of the filtrate; urea stays and is excreted.
Students often think the glomerulus picks out toxins and keeps glucose. In fact filtration is by size only; glucose is recovered afterwards.
Students often think filtration is osmosis or diffusion. In fact it is hydrostatic pressure forcing plasma through the filter.
D3.3.9 The loop of Henle salts the medulla HL
The ascending limb actively transports sodium ions out into the medulla.
It is impermeable to water, so the medulla stays at high osmotic concentration.
That concentration lets water leave the collecting ducts by osmosis later.
Students often think the loop pumps water out. In fact it pumps sodium ions out; water leaves elsewhere, from the collecting ducts.
D3.3.10 ADH and aquaporins set the urine volume HL
Osmoreceptors in the hypothalamus detect a rise in blood osmotic concentration.
ADH, made in the hypothalamus, is released from the posterior pituitary.
ADH makes collecting duct cells move aquaporins from vesicles into the membrane.
Water leaves by osmosis into the medulla; urine is small in volume and concentrated.
When ADH falls, aquaporins are withdrawn by endocytosis and dilute urine is produced.
Students often think ADH increases urine output. In fact it increases water reabsorption, so urine volume falls.
Students often think ADH makes cells pump water or build new aquaporins. In fact it moves existing aquaporins between vesicles and membrane.
D3.3.11 Blood is redirected as activity changes HL
Arteriole vasodilation and vasoconstriction redistribute cardiac output between organs.
In vigorous activity, muscle takes most of a much larger output; gut and kidney flow falls.
In wakeful rest, gut and kidneys take large shares; in sleep, muscle flow is minimal.
Brain blood flow stays nearly constant in all three states.
Students often think brain flow rises or falls with exercise. In fact its volume barely changes; only its share of the total falls.
Students often think the kidneys get more blood in exercise to clear waste. In fact their flow falls as blood is diverted to muscle.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Which statement about homeostasis in humans is correct?
Answer and reasoning
Blood glucose concentration fluctuates within narrow preset limits around a set point. — Homeostasis maintains the internal environment by keeping variables such as blood glucose concentration within preset limits. The value is not fixed: it rises after a meal and falls during fasting, and negative feedback keeps these swings within a narrow range around the set point.
Core body temperature is held at exactly 37 °C with no variation at all. — A student who thinks homeostasis means perfect constancy picks this. Core temperature normally varies by a few tenths of a degree through the day; homeostasis keeps it within preset limits, not at one exact value.
Skin temperature is kept within preset limits whatever the air temperature. — A student who confuses the body's surface with its internal environment picks this. Skin temperature is allowed to vary widely; it is core temperature, blood pH, blood glucose and blood osmotic concentration that are the regulated variables.
Blood pH is corrected only when it rises above its set point of about 7.4. — A student who thinks homeostatic control acts in one direction only picks this. Blood pH is returned to the set point from values above and from values below it.
2 Why are homeostatic variables controlled by negative feedback rather than by positive feedback?
Answer and reasoning
Negative feedback responds to harmful changes, while positive feedback responds to beneficial ones. — A student who reads 'positive' and 'negative' as value judgements picks this. The words describe the direction of the response relative to the change, not whether the change is good or bad for the organism.
Negative feedback lowers a variable that has risen, which is the usual direction of deviation. — A student who equates 'negative' with 'decrease' picks this. Negative feedback also raises a variable that has fallen; it is defined by opposing the change, not by lowering.
Negative feedback opposes a change, returning the variable towards its set point. — In negative feedback the response reverses the deviation that triggered it, so the variable is pushed back towards the set point from either side. Positive feedback amplifies a change and would drive the variable ever further away, so it cannot maintain a set point.
Negative feedback keeps the variable fixed at the set point with no fluctuation. — A student who thinks homeostasis means perfect constancy picks this. Negative feedback produces small oscillations around the set point, because a response is only triggered after a deviation has occurred.
3 Blood glucose concentration rises after a meal. What happens next?
Answer and reasoning
Beta cells of the pancreatic islets secrete insulin into the blood, and target cells take up more glucose. — The beta cells of the pancreatic islets detect the rise directly and secrete insulin into the blood. Insulin is carried in the plasma to target cells, which take up glucose and convert it to glycogen or respire it, so the concentration falls.
The hypothalamus detects the rise and sends nerve impulses telling the pancreas to release insulin. — A student who models every homeostatic loop on thermoregulation picks this. In blood glucose control the beta cells of the pancreatic islets are themselves the sensors and secrete insulin without instructions from the brain.
Insulin is released into the blood, where it breaks down the excess glucose molecules. — A student who treats insulin as an enzyme picks this. Insulin is a hormone: it acts on target cells so that glucose leaves the blood by uptake into cells; it does not digest glucose in the plasma.
The pancreas secretes glycogen, which stores the excess glucose in the cells of the liver. — A student who has merged glycogen and glucagon picks this. Glycogen is the storage polysaccharide made inside liver and muscle cells; the hormone secreted after a meal is insulin.
4 What is the physiological change that underlies type 2 diabetes?
Answer and reasoning
Beta cells are destroyed by the immune system. — A student who files both types of diabetes under 'no insulin' picks this. Autoimmune destruction of beta cells is the basis of type 1; in type 2 the cells are present and insulin is secreted, but its targets respond poorly.
Beta cells are damaged by dietary sugar. — A student who thinks sugar itself injures the pancreas picks this. A sugar-rich diet is a risk factor for type 2 because it contributes to obesity and insulin resistance, not because it destroys beta cells.
Insulin in the blood stops digesting glucose. — A student who treats insulin as an enzyme picks this. Insulin never digests glucose; it signals to cells to take glucose up. In type 2 diabetes the signal is sent but the cells respond weakly.
Target cells respond less to insulin. — In type 2 diabetes insulin is secreted, at least in the early years, but liver, muscle and adipose cells have become insensitive to it, so glucose uptake falls and blood glucose stays high. This insulin resistance is why diet and exercise, which restore sensitivity, are the first treatments.
5 What is the role of the hypothalamus in thermoregulation?
Answer and reasoning
It passes on the core temperature measured by thermoreceptors in the skin to the effectors. — A student who thinks the skin receptors measure the core picks this. Peripheral thermoreceptors detect skin and external temperature; the core temperature is measured by the hypothalamus itself from the blood.
It generates heat itself whenever the blood flowing through it becomes too cold. — A student who reads 'controls temperature' as 'does the warming' picks this. The hypothalamus is a control centre; heat is generated by effectors such as skeletal muscle and brown adipose tissue.
It monitors blood temperature and coordinates the responses of the effectors. — The hypothalamus contains thermoreceptors that measure the temperature of the blood flowing through it and also receives impulses from peripheral thermoreceptors in the skin. It compares these with the set point and sends nerve impulses to effectors and hormonal signals via the pituitary gland.
It secretes thyroxin directly into the blood to raise the metabolic rate. — A student who has attached thyroxin to the wrong gland picks this. Thyroxin is secreted by the thyroid gland; the hypothalamus acts on the pituitary gland, which stimulates the thyroid.
6 How does vasodilation of the arterioles in the skin lower body temperature?
Answer and reasoning
More blood flows through the capillaries near the surface, so more heat is lost to the air. — Relaxation of smooth muscle in the arteriole walls widens them, so a greater volume of warm blood passes through the capillary beds close to the skin surface, where heat is transferred to the surroundings by radiation, conduction and convection.
The blood vessels move closer to the surface of the skin, so heat escapes more easily. — A student who reads the 'hot' and 'cold' diagrams literally picks this. Vessels are fixed in position; what changes is the diameter of the arterioles and therefore the flow of blood through the surface capillaries.
Warm blood at the surface is cooled by the cold sweat that lies on top of the skin. — A student who thinks sweat is cold picks this. Sweat is secreted at body temperature; it cools the skin only as it evaporates, and vasodilation works independently by delivering more warm blood to the surface.
The capillaries contract their walls to squeeze warm blood towards the surface. — A student who gives capillaries a muscular wall picks this. Capillaries have no muscle and cannot constrict or squeeze; flow through them is set upstream by the smooth muscle of the arterioles.
7 Which statement correctly distinguishes osmoregulation from excretion? HL
Answer and reasoning
Osmoregulation removes surplus water; excretion removes surplus solutes of every other kind. — A student who hears 'regulation' as 'removal' picks this. Osmoregulation may conserve water as well as discard it, and excretion concerns metabolic waste products specifically, not every solute.
Osmoregulation is carried out by the kidneys; excretion includes egestion of faeces by the gut. — A student who counts all waste leaving the body as excretion picks this. Faeces are mainly undigested food that never entered metabolism; their removal is egestion, not excretion.
Osmoregulation regulates the concentration of urea; excretion removes water from the body in urine. — A student who has swapped the two roles picks this. Urea removal is excretion; the regulated variable in osmoregulation is the osmotic concentration of the blood, not urea.
Osmoregulation controls blood osmotic concentration; excretion removes metabolic waste. — Osmoregulation is the regulation of the osmotic concentration of the blood (about 0.3 osmol L−1), achieved by adjusting water reabsorption. Excretion is the removal of the waste products of metabolism, such as urea. The kidney does both, but they are separate roles.
8 Which statement about ultrafiltration in the glomerulus is correct? HL
Answer and reasoning
Only urea and other toxins pass into Bowman's capsule, while glucose is retained in the blood. — A student who imagines a filter that recognizes waste picks this. Ultrafiltration is not selective for toxins; glucose passes freely and is recovered afterwards by reabsorption in the proximal convoluted tubule.
Glucose, urea and salts all pass into Bowman's capsule, but plasma proteins do not. — The glomerular filter (fenestrated capillary wall, basement membrane and podocyte slits) discriminates by size only. High blood pressure forces water and all small solutes through; blood cells and plasma proteins are retained, so the filtrate is plasma minus its proteins.
Water and solutes enter Bowman's capsule by osmosis and diffusion down gradients. — A student who assigns every membrane crossing to diffusion or osmosis picks this. The filtrate has the same small-solute concentration as plasma, so there is no gradient; filtration is bulk flow driven by blood pressure.
The filtrate is almost all excreted as urine after small changes in the tubule. — A student who thinks urine is essentially the filtrate picks this. About 180 litres are filtered a day and about 1.5 litres of urine are produced; the great majority of the filtrate is reabsorbed.
9 How does the loop of Henle make it possible for the kidney to produce concentrated urine? HL
Answer and reasoning
Its ascending limb actively transports water out of the filtrate and into the tiny blood vessels of the medulla. — A student who thinks the loop removes water directly picks this. Water is never actively transported, and the ascending limb is impermeable to water; it pumps sodium ions, and the water leaves later from the collecting ducts.
It absorbs sodium ions into the filtrate so that the urine leaving the loop is already highly concentrated. — A student who has the direction of transport reversed picks this. Sodium ions are pumped out of the ascending limb into the medulla, so the fluid leaving the loop is actually dilute; the concentrating step happens in the collecting ducts.
Its cells actively pump water into the collecting ducts, from which the blood then reabsorbs it directly. — A student who reaches for active transport to move water picks this. Water crosses the collecting duct wall by osmosis through aquaporins, driven by the high osmotic concentration the loop maintains in the medulla.
Its ascending limb pumps sodium ions into the medulla, so water leaves the collecting ducts by osmosis. — Active transport of sodium ions out of the ascending limb keeps the interstitial fluid of the medulla at a high osmotic concentration. The collecting ducts pass through this medulla, so when they are permeable to water it is drawn out of them by osmosis, concentrating the urine.
10 A person has drunk very little water for a day and their blood osmotic concentration has risen. Which sequence of events follows? HL
Answer and reasoning
Hypothalamic osmoreceptors detect the rise; the pituitary secretes less ADH, so that water is retained rather than excreted in urine. — A student who has the direction of ADH's effect reversed picks this. ADH is anti-diuretic: more of it means more water reabsorbed. Dehydration increases, not decreases, ADH secretion.
Hypothalamic osmoreceptors detect the rise; the pituitary secretes more ADH; aquaporins move into the collecting duct membranes. — Osmoreceptors in the hypothalamus respond to the higher osmotic concentration, the rate of ADH secretion from the pituitary gland increases, and ADH causes vesicles carrying aquaporins to fuse with the membranes of collecting duct cells. More water is reabsorbed and a small volume of concentrated urine is produced.
Cells in the kidney detect the rise and secrete ADH, and the collecting ducts then reabsorb more water from the filtrate. — A student who places the sensor in the organ that responds picks this. The osmoreceptors are in the hypothalamus and ADH is released from the pituitary gland; the kidney is the effector, not the detector.
The pituitary secretes more ADH, and ADH makes the collecting duct cells actively pump water back into the blood. — A student who thinks water is pumped picks this. ADH inserts aquaporins into the membrane; water then moves by osmosis into the concentrated medulla. There is no active transport of water.
Read the ones marked not yet in Learn, then Verify.
Verify confirm before you go
15 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A person's core temperature was recorded at intervals through a day on which the air temperature ranged from 5 °C to 30 °C. The readings were 36.6, 36.9, 37.2, 36.8 and 37.1 °C. What do these readings show?
Answer and reasoning
Homeostasis has failed, because the core temperature was not held at one constant value through the day. — A student who expects a homeostatic variable to be perfectly constant picks this. Small oscillations around the set point are the normal signature of negative feedback, not evidence of failure.
Core temperature is being kept within preset limits despite large changes in the external environment. — The air temperature varied by 25 °C but the core temperature stayed within 0.4 °C of 37 °C (a total range of only 0.6 °C). That is homeostasis: the internal variable fluctuates only within narrow preset limits while the external environment changes far more.
The readings cannot show homeostasis: skin temperature, not the core, is what is kept within limits. — A student who takes the regulated variable to be the body's surface rather than its internal environment picks this. Skin temperature is allowed to vary widely; it is the core temperature, along with blood pH, blood glucose and blood osmotic concentration, that is held within preset limits (D3.3.1).
Only the readings above 37 °C were brought back by negative feedback; the lower ones were not. — A student who thinks negative feedback corrects only rises picks this. The readings below 37 °C were also followed by a return towards the set point, because negative feedback acts from both directions.
2 During a long run, a person's blood glucose concentration falls to 3.5 mmol L−1. Glucagon secretion increases and the concentration returns to 5 mmol L−1. Which description of this sequence is correct?
Answer and reasoning
Positive feedback, because the response raised the variable, not lowered it. — A student who thinks negative feedback can only lower a variable picks this. The direction that matters is relative to the deviation: the fall was reversed, so the feedback is negative.
Positive feedback, because the response was beneficial and restored health. — A student who reads 'positive' as beneficial picks this. Positive feedback means a response that increases the original change; a beneficial correction that reverses a change is negative feedback.
A breakdown of homeostasis, because glucose left its set point of 5 mmol L−1. — A student who expects a regulated variable to stay fixed picks this. A deviation followed by a corrective return is homeostasis working, not failing; variables must move away from the set point before feedback can act.
Negative feedback correcting a fall of the variable below its set point. — The variable dropped below its set point and the response (glucagon) raised it back. Because the response opposed the change, this is negative feedback; it operates from below the set point just as insulin operates from above it.
3 Insulin is secreted into the blood, yet only cells such as liver, skeletal muscle and adipose cells respond to it. Why?
Answer and reasoning
Insulin is carried by the blood directly to these organs and to no other part of the body. — A student who pictures hormones travelling point-to-point like a nerve impulse picks this. The blood delivers insulin everywhere; there is no directed route to particular organs.
Insulin reaches every tissue, but only these cells have receptors that bind it. — Hormones are transported in the plasma to all parts of the body. Target specificity comes from receptor proteins: cells carrying insulin receptors respond, while cells without them are exposed to the same insulin and are unaffected.
Insulin breaks down glucose only in tissues where glucose is concentrated. — A student who thinks insulin is an enzyme acting on glucose picks this. Insulin acts on cells, not on glucose, and the cells that respond are those with insulin receptors.
The hypothalamus directs the insulin only to those organs that need it most. — A student who inserts a brain control centre into glucose regulation picks this. The hypothalamus plays no part in routing insulin; distribution is by the blood and specificity is by receptors.
4 Which option correctly pairs a hormone with its effect on target cells?
Answer and reasoning
Glycogen: liver cells release glucose when its concentration in the blood falls too low. — A student who has merged the two similar words picks this. Glycogen is the polysaccharide store inside liver cells, not a hormone; the hormone that triggers its breakdown is glucagon.
Insulin: enzymes in the blood plasma digest glucose so that its concentration falls. — A student who thinks insulin works like an enzyme picks this. Insulin's effects are on cells: increased glucose uptake, glycogen synthesis and respiration. Glucose is not digested in the plasma.
Glucagon: liver cells hydrolyse glycogen and release glucose into the blood. — Glucagon, secreted by alpha cells when blood glucose is low, binds to receptors on liver cells and stimulates the breakdown of stored glycogen to glucose, which is released into the blood to raise its concentration.
Insulin: liver cells alone receive it, as the pancreas sends it there. — A student who thinks hormones are delivered to one target picks this. Insulin is carried in the blood to all tissues and acts on every cell type with insulin receptors, including muscle and adipose cells.
5 A lean 12-year-old develops very high blood glucose within a few weeks; antibodies against pancreatic islet cells are found in the blood and blood insulin is very low. A 55-year-old with obesity has had slowly rising fasting blood glucose for years, with a high blood insulin concentration. Which interpretation is best supported?
Answer and reasoning
The child has type 1 diabetes from autoimmune loss of beta cells; the adult has type 2, with insulin secreted but target cells resistant to it. — Islet-cell antibodies, rapid onset in childhood and very low insulin point to autoimmune destruction of beta cells (type 1). Years of rising glucose despite high insulin, with obesity, show that insulin is present but its targets are not responding (type 2 insulin resistance).
Both have the same disorder, a pancreas that has stopped making insulin, seen at an early and at a late stage of the disease. — A student who believes all diabetes is an insulin shortage picks this. The adult's blood insulin is high, which rules out a failure to make insulin; the problem is the response of the target cells.
The child's diabetes must have been caused by a sugary diet, because it appeared in childhood; the adult's by the ageing of the pancreas. — A student who transfers the lifestyle explanation of type 2 onto type 1 picks this. The islet-cell antibodies show an autoimmune cause in the child; diet does not cause type 1 and cannot prevent it.
Both need insulin injections at once, because a high blood glucose concentration shows that insulin is lacking in each case. — A student who equates diabetes treatment with insulin injection picks this. The child does need insulin. The adult has plenty of insulin already; the first treatments are weight loss, diet and exercise to restore sensitivity, with drugs added if needed.
6 Which change would do most to reduce a middle-aged adult's risk of developing type 2 diabetes?
Answer and reasoning
Beginning insulin injections before any symptoms appear. — A student who thinks insulin injection is the answer to any diabetes picks this. Type 2 is a failure of cells to respond to insulin, not a shortage of it; extra insulin does not prevent insulin resistance developing.
Reducing body fat through diet and regular physical activity. — Obesity, a diet rich in sugar and fat, and inactivity are the main modifiable risk factors for type 2 diabetes because they reduce the sensitivity of target cells to insulin. Losing fat and exercising restore sensitivity, so this is the prevention the guide expects.
Avoiding the viral infections thought to trigger loss of beta cells. — A student who treats both types as the same failure of the pancreas picks this. Autoimmune destruction of beta cells, possibly triggered by infection, underlies type 1, not type 2.
Taking enzyme supplements that digest glucose in the blood. — A student who thinks blood glucose is lowered by digesting it picks this. Glucose leaves the blood by uptake into cells under the control of insulin; no enzyme in the plasma removes it.
7 A person moves to a cold climate. After several weeks their resting metabolic rate has risen and they produce more heat even when not shivering. Which explanation is correct?
Answer and reasoning
The pituitary gland is secreting thyroxin, which acts as quickly as a nerve impulse. — A student who has merged the gland sequence and time scale picks this. The pituitary releases thyroid-stimulating hormone; thyroxin comes from the thyroid gland, and its effect on metabolic rate develops over days, not seconds.
The hypothalamus has started to produce extra heat itself as the blood cools. — A student who thinks the control centre is also the heater picks this. The hypothalamus coordinates responses; the extra heat comes from the raised respiration of body tissues under the influence of thyroxin.
A thicker layer of white fat is making extra ATP, and the ATP is released as heat. — A student who confuses insulating white fat with heat-generating brown fat, and who thinks heat comes from ATP, picks this. White adipose tissue insulates; the raised metabolic rate here comes from thyroxin.
More thyroxin is being secreted, raising the rate of cell respiration. — Prolonged cold exposure increases signalling from the hypothalamus to the pituitary gland, which stimulates the thyroid gland to secrete more thyroxin. Thyroxin raises the metabolic rate of most cells, so more heat is released; this hormonal response builds up over days, unlike shivering.
8 On a hot, very humid day a runner sweats heavily, but their body temperature keeps rising. Why is sweating less effective in these conditions?
Answer and reasoning
The sweat produced is warmer than usual, so it cannot chill the skin as it runs off. — A student who thinks sweat cools by being a cool liquid picks this. Sweat is at body temperature whatever the weather; it is evaporation, not the temperature of the liquid, that removes heat.
Little of the sweat evaporates, so little latent heat is taken from the skin. — Sweat cools the body only when its water evaporates, absorbing latent heat from the skin. In humid air the rate of evaporation is low, so most sweat drips off unevaporated and removes almost no heat.
Humid air stops the blood vessels from moving up to the surface of the skin. — A student who thinks vessels relocate picks this. Blood vessels do not move, and humidity has no effect on vasodilation; the problem is the low rate of evaporation.
Sweat cools mainly as it drips off, and humid air makes it drip slowly. — A student who thinks heat leaves in the liquid sweat picks this. Sweat that drips off has removed almost no heat; only the fraction that evaporates cools the skin, and that fraction is small in humid air.
9 Brown adipose tissue in a newborn baby releases heat when the baby is cold. Which statement describes the mechanism?
Answer and reasoning
The mitochondria make ATP unusually fast, and the ATP is then broken down to release heat. — A student who cannot picture respiration without ATP as its product picks this. In uncoupled respiration ATP synthase is bypassed; the heat is released directly from the dissipation of the proton gradient, not from ATP hydrolysis.
The tissue forms a thick insulating layer of fat, so heat generated elsewhere in the body is retained. — A student who thinks all fat is insulation picks this. That is the role of white adipose tissue; brown adipose tissue is an active heat generator packed with mitochondria.
Protons re-enter the mitochondrial matrix without passing through ATP synthase, releasing heat. — An uncoupling protein in the inner mitochondrial membrane of brown adipose cells lets protons flow back into the matrix without driving ATP synthase, so the energy of the proton gradient built by the electron transport chain is released as heat rather than stored in ATP. This is uncoupled respiration.
Thyroxin from the hypothalamus stimulates the tissue, raising its metabolic rate within seconds. — A student who attaches thyroxin to the wrong gland and the wrong time scale picks this. Thyroxin is secreted by the thyroid gland and changes heat production over days; brown adipose tissue in a cold baby is activated by sympathetic nerve impulses and generates heat by uncoupled respiration in its mitochondria (D3.3.6).
10 Blood plasma has an osmotic concentration of about 0.30 osmol L−1. Red blood cells are placed in a 0.15 mol L−1 solution of sodium chloride, which dissociates fully into ions. What happens to the cells? HL
Answer and reasoning
Nothing visible: the solution is 0.30 osmol L−1, the same as plasma, so there is no net osmosis. — Each mole of NaCl gives two moles of osmotically active particles, so 0.15 mol L−1 NaCl is 0.30 osmol L−1. That equals the osmotic concentration of plasma, so water enters and leaves the cells at equal rates. This is why 0.15 mol L−1 (0.9%) saline is used clinically.
The cells swell, because 0.15 osmol L−1 is half the osmotic concentration of the plasma. — A student who equates osmotic concentration with molar concentration picks this. NaCl dissociates into two ions per formula unit, so the osmotic concentration is 0.30 osmol L−1, not 0.15.
The cells shrink, as water leaves them for the more dilute 0.15 osmol L−1 solution outside. — A student who thinks water moves towards the more dilute solution, and who has also ignored dissociation, picks this. Water moves towards higher osmotic concentration, and in any case the solution is 0.30 osmol L−1, equal to plasma.
The cells shrink, because water moves to where there is more water, and the saline has more. — A student who thinks water moves towards the more dilute side picks this. Osmosis is net movement of water from lower to higher osmotic concentration; here the saline is 0.30 osmol L−1 once its ions are counted, equal to the cell contents and the plasma, so there is no net movement (D3.3.7).
11 Fluid was sampled from Bowman's capsule and from the end of the proximal convoluted tubule of a healthy kidney. Glucose was 5 mmol L−1 in the capsule and undetectable at the end of the tubule. Urea was 5 mmol L−1 in the capsule and higher at the end of the tubule. Which explanation is correct? HL
Answer and reasoning
Glucose diffuses back into the blood down its own concentration gradient, whereas urea is unable to diffuse. — A student who thinks reabsorption is passive picks this. Glucose in the filtrate starts at the same concentration as in plasma, so there is no gradient for diffusion; complete recovery requires active transport by tubule cells with many mitochondria.
Urea is actively secreted into the tubule fluid, while the glucose is respired by the tubule cells. — A student who has misassigned the transport processes picks this. Urea is not actively pumped in the proximal tubule, and the glucose is returned to the blood, not consumed by the tubule cells.
Glucose is reabsorbed by active transport; urea is left behind and concentrates as water is reabsorbed. — Tubule cells actively transport all the glucose out of the filtrate and back to the blood. Urea is not actively reabsorbed, and because most of the water leaves the tubule by osmosis the urea that remains becomes more concentrated in the smaller volume.
The 5 mmol L−1 of glucose in the capsule shows that the glomerular filter of this kidney has been damaged. — A student who believes glucose is never filtered picks this. Glucose is small enough to pass a healthy glomerular filter and appears in the filtrate at plasma concentration; its absence from urine is due to reabsorption.
12 In a healthy kidney the osmotic concentration of the interstitial fluid rises from 0.3 osmol L−1 in the cortex to about 1.2 osmol L−1 in the deep medulla. A drug blocks the active transport of sodium ions in the ascending limb of the loop of Henle. Which prediction is best supported? HL
Answer and reasoning
The medullary gradient decays towards 0.3 osmol L−1, so less water leaves the collecting ducts and the urine becomes more dilute. — The high osmotic concentration of the medulla is maintained by sodium ions pumped out of the ascending limb. Without that pumping the gradient dissipates, the osmotic force drawing water out of the collecting ducts weakens, and a larger volume of more dilute urine is produced (this is how loop diuretics act).
The loop can no longer reabsorb water directly, but the collecting ducts are unaffected, so the urine is unchanged. — A student who thinks the loop's job is to remove water itself picks this. The loop's contribution is the medullary gradient; losing it does affect the collecting ducts, because there is no longer a strong osmotic gradient to draw water out of them.
Water reabsorption in the collecting ducts continues normally, because it is driven by pumps that ADH switches on. — A student who believes ADH activates water pumps picks this. ADH only inserts aquaporins; the driving force is osmosis into the concentrated medulla. With the gradient gone, aquaporins alone cannot move much water.
Urea can no longer be excreted, so it accumulates in the blood while the volume of urine stays about the same. — A student who merges osmoregulation with excretion picks this. Urea is removed by filtration and left in the filtrate; blocking sodium transport in the loop changes water balance, not the excretion of urea.
13 After drinking a litre of water, a person produces a large volume of dilute urine within an hour. What has happened in the cells of the collecting ducts? HL
Answer and reasoning
Aquaporin genes have been switched off, so no new water channels are being made for the membrane. — A student who thinks the number of channels is set by protein synthesis picks this. The change within an hour is far too fast for that; aquaporins already made are shuttled between vesicles and the membrane in response to ADH.
More ADH has been secreted, and ADH increases the volume of urine that the ducts must release. — A student who reads ADH as a diuretic picks this. ADH reduces urine volume; after drinking water its secretion falls, which is what allows the large volume of dilute urine.
Aquaporins have been withdrawn into vesicles, so the membrane is far less permeable to water. — The fall in blood osmotic concentration reduces osmoreceptor signalling, ADH secretion falls, and the aquaporins are removed from the collecting duct cell membranes by endocytosis into intracellular vesicles. Water can no longer leave the duct by osmosis, so dilute urine is produced.
The duct cells sensed the dilute blood themselves and stopped pumping water inward. — A student who places the sensor in the kidney picks this. The osmoreceptors are in the hypothalamus; the duct cells simply respond to the fall in ADH, and water is never pumped in either direction.
14 At wakeful rest a person's cardiac output is 5 dm3 per minute, of which skeletal muscle receives 1.0, gut 1.2, kidneys 1.1 and brain 0.75 dm3 per minute. During vigorous exercise cardiac output is 20 dm3 per minute: skeletal muscle 16, gut 0.6, kidneys 0.6 and brain 0.75 dm3 per minute. Which conclusion do these figures support? HL
Answer and reasoning
Brain blood flow falls sharply, since its share of cardiac output drops from 15% to under 4%. — A student who confuses share with amount picks this. The brain's percentage falls only because the total rose; the absolute flow it receives, 0.75 dm3 per minute, is exactly the same.
The kidneys receive more blood in exercise, because 0.6 is a larger part of 20 than 1.1 is of 5. — A student who expects the kidneys to work harder in exercise picks this, but the arithmetic is wrong: 0.6 of 20 is 3%, whereas 1.1 of 5 is 22%. Kidney flow falls in both absolute and relative terms.
Brain blood flow rose by the same factor as cardiac output, keeping its share of the total constant. — A student who assumes the brain must be supplied in proportion to activity picks this. The figures show brain flow held constant at 0.75 dm3 per minute while cardiac output quadrupled.
Brain blood flow is unchanged, while gut and kidney flow fall as muscle flow rises. — The brain receives 0.75 dm3 per minute in both states. Muscle flow rises sixteen-fold and takes 80% of the enlarged output, while vasoconstriction halves the flow to the gut and kidneys. This is the redistribution of blood supply with activity.
15 During sleep, cardiac output falls to about 4 dm3 per minute. Which pattern of blood supply to the organs is expected? HL
Answer and reasoning
Muscle flow is minimal and brain flow is maintained, while the gut and kidneys still receive large shares. — With no muscular work, skeletal muscle arterioles are constricted and muscle takes little of the output. The brain's flow is held nearly constant in all states. The gut and kidneys, no longer competing with muscle, continue to receive large shares for digestion and excretion.
Every organ's blood flow falls by the same fraction, because the heart is simply pumping less blood. — A student who thinks cardiac output is shared out in fixed proportions picks this. Arterioles in each organ set its share, so a fall in output is not spread evenly: muscle loses most, the brain loses almost nothing.
Brain blood flow falls to a small fraction of its waking value, since the brain itself is resting. — A student who assumes brain blood flow tracks how busy the brain feels picks this. Brain flow is kept nearly constant during sleep, wakeful rest and exercise alike; the sleeping brain remains highly active.
Kidney blood flow falls to near zero, since so little metabolic waste is produced during sleep. — A student who reasons from imagined need picks this. Metabolism, and so urea production, continues during sleep, and the kidneys keep receiving a large share of cardiac output; it is during vigorous exercise that their flow is cut.
That was your twenty minutes. Real practice on D3.3 is past-paper questions marked against the mark scheme.
What the exam asks of D3.3
Paper 1A asks which hormone, receptor or effector fits a described response, and whether feedback is positive or negative. Paper 1B gives glucose or temperature traces over time, or blood-flow tables across activity states, and asks you to interpret them. Paper 2 uses *outline* and *explain*: name the change detected, the sensor, the control centre, the effector, then the correction. HL questions use *explain* for the nephron: state where each substance moves, by what process, and why.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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