D3.3 Homeostasis · Educerie Biology

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Educerie · IB Diploma · Biology

Theme D Continuity and change · D3.3 Homeostasis

Level: SL and HL. Sections 8 to 12 are HL only. If you are SL, skip them; nothing in your papers tests them.
Themes (key concepts): continuity and change, at the level of organisms. Outside the body, temperature, food supply and water intake change all the time; inside, the blood stays almost the same from hour to hour. Homeostasis is the set of mechanisms that turns constant change outside into continuity inside.
The question this unit answers: how are constant internal conditions maintained in humans, and what are the benefits to organisms of maintaining them?
Where it is examined: Paper 1A multiple choice on which hormone, which cell or which effector; Paper 1B data questions, often a graph of blood glucose after a meal or of skin temperature in the cold; Paper 2 Section A, where "outline the control of blood glucose" (4 marks) and "distinguish type 1 from type 2 diabetes" (3 marks) are regular; and Paper 2 Section B, where "explain how body temperature is regulated" or, at HL, "explain the role of the kidney in osmoregulation" are extended-response parts worth 5 to 8 marks.

What you must be able to do

You must be able to	Level	What it looks like in the exam
Define homeostasis and name four homeostatic variables in humans	SL, HL	"State two variables that are kept constant by homeostasis" (2 marks)
Explain negative feedback, and why homeostasis uses negative rather than positive feedback	SL, HL	"Explain the role of negative feedback in homeostasis" (3 marks)
Explain the control of blood glucose by insulin and glucagon, from pancreatic cells to target cells	SL, HL	"Outline how blood glucose concentration is reduced after a meal" (4 marks)
Compare type 1 and type 2 diabetes: causes, risk factors, prevention and treatment	SL, HL	"Distinguish between type 1 and type 2 diabetes" (3 marks); Paper 1B glucose data
Explain thermoregulation as negative feedback: thermoreceptors, hypothalamus, pituitary, thyroxin, effectors	SL, HL	"Explain the role of the hypothalamus in thermoregulation" (4 marks)
Describe and explain the human responses to heat and to cold	SL, HL	"Describe how the body responds to a fall in temperature" (4 marks)
Distinguish excretion from osmoregulation; use osmol L⁻¹	HL only	"Distinguish between excretion and osmoregulation" (2 marks)
Explain ultrafiltration and reabsorption in the glomerulus, Bowman's capsule and proximal convoluted tubule	HL only	"Explain how the structure of the glomerulus…" (3–4 marks)
Explain how active transport of sodium ions in the ascending limb of the loop of Henle keeps the medulla hypertonic	HL only	2–3 marks, often linked to the collecting duct
Explain osmoregulation by ADH and aquaporins in the collecting duct	HL only	"Explain the role of ADH in osmoregulation" (4–5 marks)
Describe and explain changes in blood supply to muscles, gut, brain and kidneys in sleep, rest and vigorous activity	HL only	Paper 1B or 2: read a table of blood flows and explain (3 marks)

Before you start

You need B3.2 (blood, arterioles and capillaries), C1.2 (cell respiration and ATP synthase), and C3.1 on how hormones and nerves coordinate the body, since homeostasis uses both. You also need B2.1 on osmosis and on aquaporins as channel proteins for water, which the HL kidney sections use in full.

1The idea in one paragraph

Your cells work best in a narrow range of conditions, so the body keeps its internal environment, the blood and tissue fluid that bathe every cell, almost constant. It does this by negative feedback: a receptor detects a change, a control centre compares it with a set point, and effectors act to reverse the change. Blood glucose is held steady by two hormones from the pancreas, insulin and glucagon, and diabetes is what happens when that system fails. Body temperature is held steady by the hypothalamus, using sweating, shivering, the diameter of skin arterioles, hair erection, uncoupled respiration in brown fat, and the hormone thyroxin. At HL, the kidney removes wastes and controls the water content of the blood, with ADH switching water channels in and out of the collecting duct, and the circulation sends blood where it is needed most.

2Homeostasis and the internal environment

Homeostasis is the maintenance of the internal environment of an organism within preset limits, despite fluctuations in the external environment. "Preset limits" matters: nothing is held at exactly one value. Each variable drifts up and down in a narrow band around a set point.

The guide names four homeostatic variables in humans:

Variable	Kept at about	Why it matters
Body temperature	37 °C in the core	Enzyme activity; too hot denatures enzymes, too cold slows metabolism
Blood pH	7.4, within about 7.35 to 7.45	Enzyme and protein shape depend on pH
Blood glucose concentration	about 5 mmol L⁻¹ when fasting	Cells need a steady fuel supply; too much glucose draws water out of cells by osmosis
Blood osmotic concentration	about 0.29 osmol L⁻¹	Cells gain or lose water by osmosis if it changes

The benefit of all this is that cells can be specialised for a stable environment. Enzymes can run near their optimum all the time; cells neither swell nor shrink; the brain always has glucose. An animal that holds its internal state constant can stay active in the cold of night or a hot afternoon, where an animal whose body follows its surroundings is at their mercy.

3Negative feedback

Every homeostatic mechanism has the same parts. Figure 1 lays them out.

Figure 1 · The parts of a negative feedback loop

A receptor detects a change in the variable.
A control centre (often in the brain) compares the value with the set point.
An effector, a muscle or a gland, acts to bring the value back.
The response removes the stimulus, so the effector's action then stops or is reduced.

Negative feedback is feedback that reverses the change that triggered it. A rise causes a response that lowers the variable; a fall causes a response that raises it. Notice both directions. Homeostasis corrects deviations above and below the set point, often with different effectors for each direction, as Figure 2 shows.

Figure 2 · Negative feedback holds a variable near its set point

Why negative and not positive feedback? Positive feedback amplifies a change: a rise causes a response that raises the variable further. That drives the variable away from the set point, faster and faster, until something outside the loop stops it. For a variable that must stay constant, that is the opposite of what is needed. The body does use positive feedback, but only for events that must run to completion, such as the contractions of childbirth. It never uses it to hold a variable steady.

Negative feedback: the response reverses the change, from above or below the set point. Positive feedback: the response increases the change.

4Blood glucose: two hormones, one variable

Glucose enters the blood from the gut after a meal and leaves it whenever cells take it up for respiration. Without control it would swing widely. The control centre and the receptors are the same cells: groups of endocrine cells in the pancreas called islets of Langerhans. They contain two types of cell.

β (beta) cells detect a rise in blood glucose and secrete insulin.
α (alpha) cells detect a fall in blood glucose and secrete glucagon.

Both hormones are secreted into the blood and transported dissolved in the plasma all round the body, but they only affect target cells, which have receptors for them on their plasma membranes. Figure 3 follows both directions.

Figure 3 · Insulin and glucagon keep blood glucose near its set point

After a meal (glucose high). β cells secrete more insulin. Its target cells respond in three ways:

Liver and skeletal muscle cells take up more glucose and convert it to glycogen, a storage polysaccharide.
Skeletal muscle and adipose cells move glucose transporter proteins from vesicles into their plasma membranes, so they take up glucose from the blood faster, and cells use more glucose in respiration.
Adipose (fat) cells convert glucose into fat for storage.

Blood glucose falls back towards the set point, and insulin secretion falls with it.

Between meals or in exercise (glucose low). α cells secrete more glucagon. Its main target is the liver, where it stimulates the breakdown of glycogen into glucose, which is released into the blood. Blood glucose rises back towards the set point.

Two hormones with opposite effects give finer control than one: the body can push glucose actively down and actively up.

5Type 1 and type 2 diabetes

Diabetes is a condition in which blood glucose is not controlled and stays high. High glucose damages blood vessels over years, affecting the eyes, kidneys, nerves and heart, and glucose appears in the urine. The two main types fail at different points in the loop in Figure 3.

	Type 1	Type 2
What goes wrong	The immune system destroys the β cells, so little or no insulin is secreted	Target cells respond less to insulin (insulin resistance); β cells may later secrete less as well
Typical onset	Usually in childhood or adolescence, often quickly	Usually in adults, developing slowly over years; now increasingly seen in young people
Risk factors	Genetic predisposition; an autoimmune process whose triggers are not fully understood	Obesity, especially fat around the abdomen; physical inactivity; a diet high in sugars and refined carbohydrates; age; family history
Prevention	No reliable method is known	Largely preventable: a healthy body mass, regular exercise, a diet low in sugars and high in fibre
Treatment	Insulin injections or an insulin pump, matched to food and exercise, with frequent blood glucose testing	Diet and exercise first; then drugs that improve the response to insulin or reduce glucose release; insulin if other treatment fails

The single contrast worth a mark in any answer: type 1 is a lack of insulin; type 2 is a lack of response to insulin.

A glucose tolerance test shows the difference in the data. After fasting, a person drinks a measured dose of glucose and blood glucose is measured every 30 minutes. Figure 4 shows invented results for two people.

Figure 4 · A glucose tolerance test for two people (invented data)

Person A's glucose rises to 8.1 mmol L⁻¹ and is back near the start in two hours: insulin was secreted and the target cells responded. Person B starts higher, rises further, to 14.0 mmol L⁻¹, and is still at 11.1 mmol L⁻¹ after two hours. In clinical practice, a two-hour value of 11.1 mmol L⁻¹ or more is one of the criteria for diagnosing diabetes. A Paper 1B question would ask you to describe the difference with figures, then suggest a reason.

6Thermoregulation as negative feedback

Birds and mammals are endotherms: they generate heat by metabolism and control their body temperature, by physiological means (changes inside the body) and behavioural means (seeking shade, huddling, putting on clothes). The guide asks for the physiological details in humans only.

The loop has the parts from section 3.

Receptors. Peripheral thermoreceptors in the skin detect the temperature of the body surface and send impulses along sensory neurons. Thermoreceptors in the hypothalamus itself monitor the temperature of the blood flowing through the brain.
Control centre. The hypothalamus, at the base of the brain, compares the information with the set point of about 37 °C and sends signals to the effectors, by nerves and by hormones.
The hormonal route. The hypothalamus controls the pituitary gland just beneath it. In prolonged cold, the pituitary secretes a hormone that stimulates the thyroid gland to release thyroxin. Thyroxin raises the metabolic rate of most body cells, so more respiration happens and more heat is released. This route is slower than the nervous one and is used for sustained cold.
Effectors. Skeletal muscle (shivering), brown adipose tissue (heat production), arterioles in the skin, sweat glands, and the tiny muscles attached to hairs.

7The responses to heat and to cold

Figure 5 sets the two sets of responses side by side.

Figure 5 · How the hypothalamus corrects a rise and a fall in temperature

When the body is too cold:

Vasoconstriction. The arterioles that supply the skin surface contract, so less blood flows near the surface and less heat is lost from it by radiation. The skin goes pale.
Shivering. Skeletal muscles contract and relax rapidly and involuntarily. Muscle contraction needs ATP; making ATP by respiration releases heat.
Uncoupled respiration in brown adipose tissue. Brown fat cells are packed with mitochondria. They contain a protein in the inner mitochondrial membrane that lets protons flow back across it without passing through ATP synthase. The energy of the proton gradient is released directly as heat instead of being stored in ATP. Brown fat is plentiful in newborn babies, who cannot shiver effectively, and present in smaller amounts in adults.
Hair erection. Small muscles at the base of each hair contract and the hairs stand up. In furry mammals this traps a thicker layer of still air, which insulates. In humans the hair is too sparse to trap much; the response survives as goose bumps.
Thyroxin, in prolonged cold, raises the metabolic rate.

When the body is too hot:

Vasodilation. The arterioles supplying the skin surface widen, so more warm blood flows close to the surface and more heat is lost by radiation. The skin goes red.
Sweating. Sweat glands secrete sweat onto the skin. As the water evaporates it absorbs a large amount of heat from the skin, because of water's high latent heat of vaporisation (A1.1).
Shivering stops, hairs lie flat, and heat production is not increased.

Figure 6 shows the arteriole response, the one students most often describe wrongly.

Figure 6 · Vasodilation and vasoconstriction in the skin

The vessels that change width are arterioles, which have muscle in their walls. Capillaries have no muscle and cannot constrict, and blood vessels do not move up or down in the skin. Write "arterioles supplying the skin capillaries dilate", never "capillaries move closer to the surface".

8HLThe kidney: excretion and osmoregulation

SL students can skip to section 13.

The kidney does two jobs, and the guide wants them kept apart.

Excretion is the removal from the body of the waste products of metabolism, and of toxins. The main nitrogenous waste in humans is urea, made in the liver from excess amino acids. Drugs and their breakdown products are removed the same way.
Osmoregulation is the regulation of the osmotic concentration of the body fluids: the concentration of dissolved solutes, which decides which way water moves by osmosis. It is measured in osmoles per litre (osmol L⁻¹). Human blood plasma is held at about 0.29 osmol L⁻¹.

Both happen in about a million nephrons in each kidney. Figure 7 labels one.

Figure 7 · A nephron and its blood supply (HL)

The kidney has an outer cortex and an inner medulla. Each nephron starts in the cortex with a glomerulus, a knot of capillaries, inside a cup, the Bowman's capsule. The tubule then runs through the proximal convoluted tubule, down into the medulla and back as the loop of Henle, through the distal convoluted tubule, and into a collecting duct, which carries urine through the medulla to the ureter.

9HLUltrafiltration and reabsorption

Ultrafiltration in the glomerulus and Bowman's capsule. Blood arrives in the glomerulus through the afferent arteriole and leaves through the narrower efferent arteriole. Because blood leaves through a narrower vessel than it enters, the blood pressure in the glomerular capillaries is high. That pressure forces fluid out of the blood through three layers:

the capillary wall, which has pores (it is fenestrated);
the basement membrane, a fine mesh of proteins that acts as the actual filter;
the inner wall of the capsule, made of cells called podocytes with finger-like extensions and gaps between them.

Water and small solutes, such as glucose, amino acids, urea, ions and many toxins, pass through. Blood cells and plasma proteins are too large and stay in the blood. The fluid in the capsule is the glomerular filtrate. It is filtration under pressure at the scale of molecules, which is why it is called ultrafiltration. It is not selective beyond size: useful and useless small solutes go through together. An adult produces roughly 180 litres of filtrate a day but only about 1.5 litres of urine, so almost all of it must be taken back.

Selective reabsorption in the proximal convoluted tubule. The walls of the PCT are one layer of cells, with microvilli on the side facing the filtrate, which give a large surface area, and many mitochondria to supply ATP for active transport.

All the glucose and amino acids are reabsorbed, by cotransport with sodium ions.
Most of the sodium ions are pumped out of the tubule cells into the blood by active transport, and other ions follow.
About two thirds of the water follows by osmosis, because the solutes that have been reabsorbed make the blood side more concentrated.

What is left in the filtrate is mostly the things the body does not reabsorb: urea, toxins and other unwanted solutes, in a smaller volume of water. They pass on and are excreted in urine. The design is to filter almost everything out, then take back what is useful, which lets the kidney remove any small toxin without needing a separate transporter for each.

10HLThe loop of Henle

The guide limits this to one idea. The ascending limb of the loop of Henle actively transports sodium ions (with chloride ions following) out of the filtrate into the tissue fluid of the medulla. The ascending limb is impermeable to water, so water cannot follow. The result is that the tissue fluid of the medulla has a high osmotic concentration, rising deeper into the medulla to several times that of blood plasma.

Why this matters: the collecting ducts pass through this salty medulla on their way to the ureter. If the collecting duct wall lets water through, water leaves the urine by osmosis into the concentrated medulla and is returned to the blood. The loop of Henle builds the gradient; the collecting duct uses it. Figure 8 shows the arrangement.

Figure 8 · The loop of Henle makes the medulla salty, the collecting duct uses it (HL)

11HLOsmoregulation: ADH and aquaporins

How much water leaves the collecting duct is the variable the body controls, and it is a negative feedback loop, set out in Figure 9.

Figure 9 · ADH and osmoregulation (HL)

Receptors. Osmoreceptors in the hypothalamus detect the osmotic concentration of the blood.
Blood too concentrated (after sweating, or not drinking). The hypothalamus signals the pituitary gland to secrete more antidiuretic hormone (ADH) into the blood. In the cells of the collecting duct walls, ADH causes vesicles containing aquaporins, water channel proteins, to fuse with the plasma membrane. With more aquaporins in the membrane, the wall becomes more permeable to water. Water moves out of the collecting duct by osmosis into the concentrated medulla and back into the blood. A small volume of concentrated urine is produced, and the osmotic concentration of the blood falls back.
Blood too dilute (after drinking a lot). Osmoreceptors detect the fall; the pituitary secretes less ADH. Aquaporins are taken back out of the membrane by endocytosis into intracellular vesicles. The collecting duct wall becomes much less permeable to water, little water is reabsorbed, and a large volume of dilute urine is produced.

The neat part is that the aquaporins are not destroyed and remade each time. They are moved: into the membrane when ADH is high, back into vesicles when ADH is low. That makes the response fast and reversible.

12HLBlood supply changes with activity

The heart's output is shared out among the organs, and the share each gets changes with what the body is doing. The mechanism is the same one as in the skin: arterioles supplying each organ dilate or constrict, under the control of the nervous system and of local chemical signals from the tissues, so more blood flows where arterioles are wide.

Figure 10 shows illustrative values for an adult in three states. The numbers are rounded and invented to show the typical pattern; real values vary a great deal between people.

Figure 10 · Where the blood goes in sleep, rest and vigorous exercise (HL, illustrative values)

Organ	Sleep	Wakeful rest	Vigorous exercise	Why
Skeletal muscles	lowest	low	very high	Working muscle needs far more oxygen and glucose, and produces carbon dioxide and heat to remove
Gut	high	high	reduced	Digestion can wait; arterioles to the gut constrict to divert blood to muscle
Brain	constant	constant	constant	The brain has no energy store and must be supplied steadily in every state
Kidneys	high	high	reduced	Filtration can slow for a short time; blood is diverted to muscle

Three features to explain in an answer. The total flow (the cardiac output) rises several-fold in vigorous exercise, from about 5 L min⁻¹ to over 20 L min⁻¹ in a fit adult. The share to skeletal muscle rises from about a fifth to about three quarters or more. And the brain's absolute flow stays the same, so its share falls, though it is not starved of anything. In sleep the skeletal muscles are relaxed and the cardiac output is lowest, so muscle flow falls further, while the gut and kidneys carry on.

13Linking questions

For what reasons do organisms need to distribute materials and energy? Every cell respires, so every cell needs glucose and oxygen and must lose carbon dioxide and heat. The circulation distributes them, and section 12 shows it redistributes them as demand changes; thermoregulation uses the same blood flow to move heat to or from the skin.

What biological systems are sensitive to temperature changes? Enzymes above all (C1.1), because their rate rises with temperature until the active site denatures. Membranes change fluidity with temperature (B2.1). That sensitivity is the reason thermoregulation exists.

14Where marks are lost

Missing "within limits". Homeostasis does not keep a variable at one exact value; it keeps it within narrow limits around a set point.

Describing negative feedback in one direction only. Say that it corrects a rise and a fall, and that the response reduces the stimulus that caused it.

Mixing up glucagon and glycogen. Glucagon is a hormone from α cells; glycogen is the storage polysaccharide in liver and muscle. Insulin turns glucose into glycogen; glucagon turns glycogen into glucose.

Saying type 2 diabetes is caused by eating sugar. It is insulin resistance: target cells stop responding properly. Diet, obesity and inactivity are risk factors, not the physiological change.

Moving capillaries. Blood vessels do not move towards or away from the skin surface, and capillaries do not constrict. Arterioles dilate or constrict and change the flow.

Saying sweat cools because it is cold. Sweat is at body temperature. It cools because the evaporation of water absorbs heat from the skin.

HL: calling excretion "getting rid of waste". Faeces are not excreted, because undigested food was never made by metabolism. Excretion is the removal of metabolic waste such as urea.

HL: saying ADH makes aquaporins. ADH causes existing aquaporins stored in vesicles to be inserted into the membrane; when ADH falls they are withdrawn into vesicles again.

15Draw it right

A feedback loop: stimulus → receptor → control centre → effector → response, with an arrow from the response back to the variable labelled "reduces the stimulus".
Set-point graphs: a horizontal dashed line for the set point, the variable oscillating around it, and a labelled correction on both sides.
Blood glucose: show both hormones, name the cells that secrete them (β and α cells of the islets) and name the target organ (liver) for glucagon.
Thermoregulation: name the effector and say what it does ("arterioles in the skin constrict, less blood flows near the surface, less heat lost by radiation"); an effector without its effect scores little.
HL, nephron: label glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle (both limbs), distal convoluted tubule, collecting duct, afferent and efferent arterioles; show the afferent wider than the efferent; mark cortex and medulla.
HL, ADH: show osmoreceptors in the hypothalamus, ADH from the pituitary, aquaporins moving between vesicles and the membrane, and water moving into the medulla.
Data graphs (glucose, blood flow): axes with units, and quote values from the graph in your answer, with units.

16Try it

Marks in brackets. Answers and marker's notes are at the end.

Q1. Explain why homeostasis relies on negative feedback rather than positive feedback. 3 marks

Q2. Outline how the blood glucose concentration is returned to normal after a meal rich in carbohydrate. 4 marks

Q3. Two adults took a glucose tolerance test. The table shows their blood glucose concentration, in mmol L⁻¹ (invented data).

Time after drinking glucose / min	0	30	60	90	120
Person A	5.0	8.1	6.9	5.8	5.2
Person B	7.2	12.4	14.0	12.8	11.1

(a) Calculate the percentage increase in blood glucose for person B from 0 minutes to the peak. 2 marks

(b) Person B has type 2 diabetes. Using the data, explain the difference between the two people. 3 marks

Q4. Describe how the human body responds to a fall in core body temperature. 4 marks

Q5 (HL). Explain the role of antidiuretic hormone in osmoregulation. 5 marks

Q6 (HL). During vigorous exercise, the blood flow to the skeletal muscles of an adult rose from 1.0 to 18.0 L min⁻¹ and the flow to the gut fell from 1.4 to 0.5 L min⁻¹ (illustrative values). Calculate the percentage change in blood flow to the gut, and explain the changes. 4 marks

17In one breath

Homeostasis keeps the internal environment, including body temperature, blood pH, blood glucose and blood osmotic concentration, within narrow limits around a set point, so enzymes and cells always work in the conditions they are built for. It uses negative feedback, in which receptors, a control centre and effectors reverse a change from above or below; positive feedback would amplify the change and is kept for events like childbirth. When glucose rises, β cells in the islets secrete insulin, which makes liver and muscle cells take up glucose and store it as glycogen; when it falls, α cells secrete glucagon, which makes the liver break glycogen down. Type 1 diabetes is a lack of insulin after β cells are destroyed, treated with insulin; type 2 is a lack of response to insulin, linked to obesity and inactivity, and largely preventable. Thermoreceptors report to the hypothalamus, which acts through nerves and, via the pituitary and thyroxin, on metabolic rate: in the cold, vasoconstriction, shivering, uncoupled respiration in brown fat and hair erection; in the heat, vasodilation and sweating. HL: the kidney excretes urea and regulates osmotic concentration; high pressure in the glomerulus forces a filtrate into Bowman's capsule, the PCT reabsorbs all glucose, most ions and most water, the ascending limb of the loop of Henle pumps sodium into the medulla, and ADH from the pituitary puts aquaporins into collecting duct membranes so water is reabsorbed; arterioles redirect blood to muscles in exercise, away from gut and kidneys, while the brain's supply stays constant.

Answers

Q1. Negative feedback reverses a change: a rise in the variable causes a response that lowers it, and a fall causes a response that raises it, so the variable is returned to the set point from either side; positive feedback amplifies a change, pushing the variable further from the set point; this would make the internal environment unstable, which is the opposite of homeostasis. 1 for negative feedback reversing the change, 1 for positive feedback amplifying it, 1 for the link to keeping the variable near a set point or within limits. "Negative feedback is better" with no mechanism scores 0.

Q2. A rise in blood glucose is detected by β cells in the islets of Langerhans of the pancreas; the β cells secrete insulin into the blood; insulin is carried in the plasma to target cells with insulin receptors; liver and muscle cells take up glucose and convert it to glycogen; body cells take up more glucose and increase respiration, and adipose cells convert glucose to fat; the blood glucose concentration falls to the set point and insulin secretion falls. any four of these points. Glucagon in place of insulin scores 0 for that point.

Q3. (a) Increase = 14.0 − 7.2 = 6.8 mmol L⁻¹; 6.8 ÷ 7.2 × 100 = 94% (94.4%). M1 for the change divided by the starting value, A1 for 94%. (b) Person B's fasting glucose is higher (7.2 against 5.0 mmol L⁻¹); B's glucose rises higher (peak 14.0 against 8.1) and peaks later (60 against 30 minutes); after two hours B is still at 11.1 while A has returned to 5.2; this is because B's target cells respond less to insulin (insulin resistance), so less glucose is taken up from the blood. 1 for a comparison with figures, 1 for the slow return with figures, 1 for insulin resistance as the reason. "B has no insulin" scores 0 for the last mark: that is type 1.

Q4. Peripheral thermoreceptors in the skin and receptors in the hypothalamus detect the fall; arterioles supplying the skin constrict (vasoconstriction), reducing blood flow near the surface and heat loss by radiation; skeletal muscles shiver, and the respiration that fuels contraction releases heat; brown adipose tissue carries out uncoupled respiration, releasing energy as heat instead of making ATP; hair erector muscles contract, raising hairs; in prolonged cold, the pituitary stimulates release of thyroxin, which raises the metabolic rate. any four, each response with its effect. A list of responses with no effects is capped at 2.

Q5 (HL). Osmoreceptors in the hypothalamus detect a rise in the osmotic concentration of the blood; the pituitary gland secretes more ADH into the blood; ADH acts on cells of the collecting duct walls; vesicles containing aquaporins fuse with the plasma membrane, increasing its permeability to water; water moves out of the collecting duct by osmosis into the medulla, which has a high osmotic concentration maintained by the loop of Henle; a small volume of concentrated urine is produced and the blood's osmotic concentration falls; when the blood is too dilute, less ADH is secreted, aquaporins are withdrawn into vesicles, and a large volume of dilute urine is produced. any five. "ADH makes aquaporins" scores 0 for that point.

Q6 (HL). Percentage change to the gut = (0.5 − 1.4) ÷ 1.4 × 100 = −64%, a fall of 64%. Skeletal muscles need more oxygen and glucose for increased aerobic respiration, and more carbon dioxide and heat must be removed, so arterioles supplying the muscles dilate; arterioles supplying the gut constrict, because digestion is not urgent during exercise, diverting blood to the muscles. M1 for the change over the original, A1 for −64% or a 64% decrease, 1 for the muscles' need with a reason, 1 for vasoconstriction or vasodilation of arterioles as the mechanism.

Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section D3.3 Homeostasis. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.

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