IB Biology · Theme B Form and function · Organisms
B3.2 Transport
Blood vessels are shaped by pressure: thick elastic arteries, thin valved veins, one-cell capillaries. Plants move water by pulling from the top; the xylem is built to survive the tension. At HL, tissue fluid, the heart's own cycle, root pressure and phloem complete the picture.
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
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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B3.2.1 Capillaries are built for exchange
A capillary wall is one endothelial cell thick, so the diffusion distance is short.
The lumen is so narrow that red blood cells pass in single file.
Repeated branching gives a huge total surface area reaching every tissue.
Fenestrations are pores through the wall where exchange must be fastest, as in the glomerulus.
Students often think thin walls are for letting blood flow quickly. In fact they shorten the distance substances must diffuse.
Students often think all capillaries have fenestrations wide enough for cells. In fact only some do, and the pores pass water and small solutes, not cells.
B3.2.2 Telling arteries from veins in a micrograph
Compare the wall thickness with the diameter of the lumen, the central space.
An artery has a thick wall and a narrow, round lumen.
A vein has a thin wall and a wide, often irregular lumen, with valves.
Students often pick the vessel with the bigger lumen as the artery. In fact the artery has the narrow lumen and the thick wall.
B3.2.3 How arteries handle high pressure
Arteries carry blood away from the heart, whatever its oxygen content.
Elastic tissue stretches as the ventricle contracts and recoils as it relaxes, keeping pressure up.
Smooth muscle helps withstand pressure and adjusts the lumen: narrower or wider.
The heart supplies the force; artery muscle does not pump blood along.
Students often define arteries by oxygenated blood. In fact the pulmonary artery carries deoxygenated blood; direction defines an artery.
Students often imagine a rigid pipe. In fact the wall stretches and recoils, so blood keeps flowing between beats.
B3.2.4 Taking a pulse
The pulse is a pressure wave along an artery wall with each left ventricular contraction.
Feel it with fingertips at the radial artery (wrist) or carotid artery (neck).
Count for a known time to get beats per minute; longer counts are more accurate.
Veins have no pulse: the pressure wave is lost in the capillaries.
Students often think the pulse is blood rushing past the fingers. In fact it is a pressure wave in the artery wall, one per heartbeat.
Students often think 10 seconds times six equals a 60-second count. In fact one miscount becomes six beats per minute.
B3.2.5 How veins get blood back
Venous blood is at low pressure, too low to prevent backflow by itself.
Valves are pocket flaps: open towards the heart, shut when blood slips back.
Thin, flexible walls are squeezed by surrounding skeletal muscles.
Muscle squeeze plus one-way valves moves blood towards the heart.
Students often think veins pump with thick muscular walls. In fact skeletal muscles around them do the squeezing.
Students often think valves actively push blood. In fact they are passive flaps that only close on backflow.
B3.2.6 Blocked coronary arteries, and what correlations can prove
Coronary arteries branch from the aorta and supply the heart wall itself.
Atheroma in the wall narrows the lumen; a clot there can block it.
Muscle beyond the block is starved of oxygen and dies: a myocardial infarction.
A correlation coefficient runs from −1 to +1; sign gives direction, size gives strength.
Even the strong correlation between saturated fat intake and coronary heart disease does not prove cause.
Students often think a strong correlation proves cause. In fact other factors varying alongside could be responsible.
Students often think +0.2 beats −0.7 because it is positive. In fact −0.7 is the stronger relationship; the sign only shows direction.
B3.2.7 Water is pulled up the xylem, not pushed
Water evaporates from wet mesophyll cell walls into air spaces, then out through stomata.
Capillary action draws water from the xylem through the cell walls to replace it.
This puts the xylem column under tension (negative pressure), which pulls water up.
Cohesion between water molecules, by hydrogen bonding, keeps the column continuous.
Students often think water is pumped up from the roots. In fact it is pulled from above by tension generated in the leaves.
Students often call sticking to xylem walls cohesion. In fact cohesion is water to water; that is what stops the column snapping.
B3.2.8 Xylem vessels are built for flow under tension
A xylem vessel is a column of dead cells with no contents.
End walls are incomplete or absent, so flow is unimpeded.
Lignin in the walls withstands tension without collapsing and supports the plant.
Pits are gaps in the lignin where water enters and leaves sideways.
Students often think xylem pumps water using ATP. In fact vessels are dead; the energy comes from the sun evaporating water.
Students often think water enters only at the bottom. In fact pits let water in from the root cortex and out into leaf cells.
B3.2.9 Tissues in a stem section
The epidermis is the outer layer, under a waxy cuticle.
The cortex is packing and storage parenchyma; not a transport tissue.
Vascular bundles form a ring near the outside, around a central pith.
In each bundle, xylem is inner and phloem is outer.
Students often draw the bundles down the centre. In fact they form a ring near the outside.
Students often think the cortex carries water and sugars. In fact it stores; transport is in the bundles.
B3.2.10 Tissues in a root section
A root has one central vascular cylinder, not a ring of bundles.
Xylem forms a star or cross at the centre.
Phloem sits in groups between the xylem arms.
Cortex surrounds the cylinder; the epidermis has root hairs and no cuticle.
Students often draw a root like a stem, with a ring of bundles. In fact the root has one central star of xylem.
B3.2.11 Tissue fluid leaves and returns (HL) HL
Tissue fluid bathes cells; it is plasma without most proteins or cells.
Pressure filtration: high pressure at the arteriole end forces water and small solutes out.
Proteins and cells stay behind; they are too large to pass.
Pressure is lower at the venule end, so fluid drains back in.
Students often think tissue fluid forms by osmosis. In fact hydrostatic pressure forces it out through gaps and fenestrations.
Students often think pressure is constant along a capillary. In fact it falls, which is why fluid returns at the venule end.
B3.2.12 Cells trade with tissue fluid, not blood (HL) HL
Plasma is water with glucose, amino acids, ions, gases, urea, hormones and plasma proteins.
Tissue fluid has the same solutes but almost no plasma proteins and no cells.
Oxygen and glucose diffuse from tissue fluid into cells; carbon dioxide and wastes diffuse out.
Students often think tissue fluid is identical to plasma. In fact the proteins are missing; they cannot cross the wall.
Students often think each cell touches a capillary. In fact cells exchange with the tissue fluid around them.
B3.2.13 Lymph drains the excess (HL) HL
Excess tissue fluid enters lymph ducts through gaps in their thin walls.
Valves stop backflow; surrounding muscle movement squeezes lymph along.
Lymph returns to the blood through ducts emptying into veins near the heart.
Students often think lymph never rejoins the blood. In fact it drains into veins near the heart.
Students often think lymph ducts pump like arteries. In fact they have thin walls, no pump, and rely on muscle and valves.
B3.2.14 Single and double circulation (HL) HL
Bony fish: single circulation; blood passes the heart once per circuit: heart, gills, body, heart.
Pressure lost in the gill capillaries is not restored, so body tissues get low-pressure blood.
Mammals: double circulation; blood passes the heart twice: right to lungs, left to body.
The left side repressurises blood before it goes to the body.
Students often call the fish circuit double because it visits two places. In fact it passes through the heart only once.
Students often think gills leave pressure unchanged. In fact narrow gill capillaries drop it, so body flow is slow.
B3.2.15 The heart delivers blood under pressure (HL) HL
Branched, joined cardiac muscle spreads the signal fast; the right-atrium pacemaker starts it.
Thin-walled atria collect from veins; ventricles pump out, the left thickest for systemic pressure.
Atrioventricular and semilunar valves open and shut by pressure differences alone, keeping flow one-way.
The septum separates the sides; coronary vessels supply the wall itself.
Trace the flow: venae cavae, right atrium, right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium, left ventricle, aorta.
Students often think the left ventricle is thicker because it pumps more blood. In fact both sides pump the same volume; the left needs higher pressure.
Students often think nerves trigger each beat. In fact the sinoatrial node fires on its own; nerves only adjust the rate.
B3.2.16 The cardiac cycle and blood pressure readings (HL) HL
Systole is contraction; diastole is relaxation and filling.
Atrial systole first, then a short delay, then ventricular systole, then all chambers in diastole.
Systolic pressure is peak arterial pressure; diastolic is the minimum, held by elastic recoil.
A reading such as 120/80 mm Hg is arterial pressure, not chamber pressure.
Students often think atria and ventricles squeeze together. In fact atria contract first to top up the ventricles.
Students often think arterial pressure drops to zero between beats. In fact recoiling artery walls hold it near 80 mm Hg.
B3.2.17 Root pressure pushes from below (HL) HL
Root cells actively transport mineral ions into the xylem, lowering its water potential.
Water follows by osmosis and builds a positive pressure potential that pushes upwards.
It matters when transpiration is too weak: high humidity, or spring before deciduous leaves open.
Students often think roots pump water molecules directly. In fact ions are pumped; water follows osmotically.
Students often think root pressure is another name for transpiration pull. In fact it is a push from below, used when the pull is insufficient.
B3.2.18 Phloem: sieve tubes and companion cells (HL) HL
2028 guide: scope reduced — Secondary source reports that symplastic and apoplastic pathways are not required from first assessment 2028. The 2025 guidance for this statement does not name these pathways either, so nothing in this bank depends on them and every item remains valid for both guides. Candidates sitting May/Nov 2026 or 2027 exams still need the fuller 2025 scope.
A sieve tube element is alive but has no nucleus and little cytoplasm: little resistance.
Sieve plates are perforated end walls linking elements into a continuous tube.
A companion cell has a nucleus and many mitochondria for loading and unloading sucrose.
Plasmodesmata connect the two cells; sap flows from source to sink, up or down.
Students often think sieve tube elements are dead like xylem. In fact they are living, kept going by their companion cells.
Students often think companion-cell ATP pushes sap along. In fact ATP powers loading and unloading; the resulting pressure difference drives flow.
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 Why is the wall of a capillary only one cell thick?
Answer and reasoning
It keeps the distance over which substances diffuse between blood and tissue cells very short. — Exchange across the capillary wall is by diffusion, and the rate of diffusion falls steeply as distance increases. A wall of a single endothelial cell keeps that distance to a minimum, so exchange is rapid.
It lets blood pass through the capillary faster, so that more blood reaches the tissue cells. — A student who thinks a thin wall is an easy-flow pipe picks this. In fact blood flows more slowly in capillaries than in any other vessel; the thin wall shortens the diffusion distance, it does not speed the flow.
It leaves room for a lumen wide enough for red blood cells to pass through side by side. — A student who believes a wider vessel is better for exchange picks this. In fact capillaries are narrow, with red blood cells passing in single file, which is what brings every cell close to the wall.
It allows red blood cells to squeeze out through the wall and into the tissues. — A student who pictures capillaries as full of large holes picks this. Red blood cells stay inside the capillary; the thin wall is about diffusion distance, and the fenestrations found in some capillaries are pores far too small for cells.
2 A micrograph shows two blood vessels in transverse section. Vessel X has a wall about as thick as the diameter of its small, round lumen. Vessel Y has a wall that is very thin compared with its wide, irregular lumen. Which identification is correct?
Answer and reasoning
Y is an artery, because its wide lumen carries the large volume of blood that is at high pressure. — A student who expects the high-pressure vessel to be the wider one picks this. An artery has a thick wall relative to its lumen; the thin-walled, wide, irregular vessel Y is a vein.
X is a vein, because a vein needs a thick muscular wall to pump blood back towards the heart. — A student who thinks veins have thick muscular walls that pump blood back picks this. Vein walls are thin and rely on compression by surrounding muscle, so the thick-walled vessel X is the artery.
X is an artery, because its wall is thick relative to its lumen to withstand high pressure. — The guide's criterion is the thickness of the wall relative to the diameter of the lumen. A thick muscular and elastic wall around a narrow, round lumen is the section of an artery; the thin wall and collapsed, wide lumen of Y are typical of a vein.
Neither can be identified, because a micrograph cannot show whether blood is oxygenated. — A student who defines arteries by the oxygen content of their blood picks this. Vessels are identified in section by wall structure and wall thickness relative to the lumen, not by oxygenation, so X is the artery and Y the vein.
3 Why can heart rate be determined by feeling the carotid or radial pulse with the fingertips?
Answer and reasoning
Because blood is pushed past the fingertips in one surge each time the muscular artery wall contracts. — A student who thinks the artery wall pumps picks this. The artery does not contract to push each surge; it is stretched by the pressure wave from the ventricle.
Because the blood flowing through the artery can be felt as it moves past the fingertips. — A student who thinks the pulse is the movement of blood picks this. Blood flows continuously; what is felt is the intermittent pressure wave that stretches the wall.
Because a pressure wave passes along the artery wall each time the left ventricle contracts. — Each ventricular contraction forces blood into the aorta and sends a pressure wave along the arteries. Where an artery lies close to the skin, such as the carotid or radial artery, the fingertips feel the wall expanding once per heartbeat, so counting pulses gives the heart rate.
Because these are the places where large veins close to the surface of the skin carry the heartbeat. — A student who thinks the pulse is found in veins picks this. The carotid and radial pulses are arterial; the pressure wave has been lost by the time blood reaches the veins.
4 A blood clot completely occludes a branch of a coronary artery. What is the direct consequence?
Answer and reasoning
Blood can no longer pass through the chambers of the heart, so the heart stops beating almost at once. — A student who thinks the coronary arteries are the route blood takes through the heart picks this. Blood continues to flow through the chambers; it is the muscle of the wall that loses its supply.
The region of cardiac muscle supplied by that branch is deprived of oxygen and its cells die. — The coronary arteries supply the cardiac muscle of the heart wall. Occlusion cuts off the oxygen supply to the muscle beyond the block, so the cells there die: a myocardial infarction. The damaged region may then fail to contract properly.
Fat from the diet keeps building up on the surface of the clot until the artery bursts open. — A student who holds the grease-in-a-drainpipe model picks this. Atheroma forms within the artery wall, and the consequence of occlusion is oxygen starvation of muscle, not bursting.
The muscle in the artery wall contracts to force the clot along, tearing the artery wall. — A student who thinks artery walls pump picks this. Artery muscle does not push contents along; the harm done by occlusion is to the cardiac muscle downstream.
5 Why are the walls of xylem vessels thickened with lignin?
Answer and reasoning
To seal the vessel completely so that no water can escape through the side walls into surrounding cells. — A student who pictures a sealed pipe picks this. Lignified walls contain many pits through which water enters and leaves the vessel; sealing the wall would prevent water reaching leaf cells.
To provide the mitochondria and cytoplasm of the vessel with a surface on which to generate ATP for transport. — A student who assumes xylem transport is active picks this. Mature xylem vessels have no cytoplasm or mitochondria; they are empty tubes and transport is passive.
To withstand the tension in the water column so that the vessel is not pulled inwards and collapsed. — The negative pressure potential generated by transpiration would suck a soft-walled tube flat. Lignified walls are rigid enough to withstand these tensions, keeping the vessel open for unimpeded flow; they also contribute to supporting the plant.
To hold the plant upright, which is the only function that lignin has anywhere in the xylem. — A student who remembers lignin only as the support material picks this. Support is a role, but for transport the essential function is resisting the tension that would otherwise collapse the vessel.
6 In a transverse section of a dicotyledonous root, where are the xylem and the phloem found?
Answer and reasoning
In separate vascular bundles arranged in a ring near the outside of the root, in just the same way as in the stem. — A student who applies the stem pattern to the root picks this. A root has no ring of separate bundles; its vascular tissue is grouped in a central cylinder.
At the centre of the root, with the xylem forming a star shape and groups of phloem between its arms. — The dicotyledonous root has a central vascular cylinder: xylem forms a star or cross shape in the middle and phloem lies in separate groups between the arms of the xylem. This is surrounded by the cortex and then the epidermis bearing root hairs.
At the centre of the root, with phloem forming a star shape and xylem in groups between its arms. — A student who has swapped the two tissues picks this. It is the xylem that forms the central star in a root, with phloem between the arms.
Spread throughout the cortex, which conducts water and sugars across the width of the root. — A student who thinks the cortex is a transport tissue picks this. Water does cross the cortex to reach the xylem, but the cortex is parenchyma for storage and packing; xylem and phloem are confined to the centre.
7 Blood pressure in a capillary is about 4.7 kPa at the arteriole end and about 1.3 kPa at the venule end. How do these values explain the release and reuptake of tissue fluid? HL
Answer and reasoning
Plasma diffuses out at the arteriole end because its concentration is higher there, and diffuses back in at the venule end where it is lower. — A student who reaches for diffusion to explain every movement picks this. Plasma is not a solute with a concentration; tissue fluid is formed by pressure filtration, with fluid forced out through the capillary wall by hydrostatic pressure.
The pressure at the two ends is effectively the same, so fluid must be pumped back into the capillary by the venule. — A student who assumes pressure is uniform along a vessel picks this. The values given differ by more than three times, and it is this fall in pressure, not a pump, that reverses the net movement of fluid.
At 4.7 kPa the pressure forces fluid out through the capillary wall; at 1.3 kPa it cannot, so tissue fluid drains back in. — Tissue fluid is formed by pressure filtration of plasma, promoted by the high pressure of blood arriving from arterioles. As blood flows along the capillary its pressure falls; at the venule end the lower pressure allows tissue fluid to drain back into the capillary.
The higher pressure at the arteriole end forces out blood cells and plasma proteins, which then return with the fluid at the venule end. — A student who thinks filtration lets everything through picks this. Cells and almost all plasma proteins are too large to pass the capillary wall and stay in the blood; only water and small solutes leave.
8 Which statement describes how excess tissue fluid is drained and what happens to it? HL
Answer and reasoning
It enters lymph ducts through gaps in their thin walls, valves prevent backflow, and the lymph is returned to the blood. — Not all tissue fluid drains back into the capillaries. The excess enters lymph ducts, which have thin walls with gaps for entry and valves that keep the lymph moving one way, and the lymph ducts eventually empty into veins so the fluid rejoins the blood.
It enters lymph ducts, which pump it along with their thick muscular walls to the kidneys, where it is excreted. — A student who expects lymph ducts to be muscular pumps picks this. Lymph ducts have thin walls, are compressed by surrounding muscle, and return lymph to the blood, not to the kidneys.
It stays in the lymphatic system as lymph, a separate fluid that circulates without rejoining the blood. — A student who sees the lymphatic system as a closed circuit picks this. Lymph is returned to the blood circulation; if it were not, fluid would accumulate in the tissues.
It is pushed back into the capillaries by the valves in the lymph ducts, which force fluid towards the blood. — A student who gives valves an active pushing role picks this. Valves are passive and only prevent backflow; lymph is moved by compression of the ducts and returned to the blood via veins, not pushed into capillaries.
9 Which sequence correctly traces deoxygenated blood from the body through the heart and back to the body? HL
Answer and reasoning
Venae cavae, right atrium, right ventricle, pulmonary vein, lungs, pulmonary artery, left atrium, left ventricle, aorta — A student who assumes the vein is the vessel that carries deoxygenated blood picks this. The pulmonary artery carries blood away from the heart to the lungs; the pulmonary vein returns oxygenated blood to the left atrium.
Vena cava, left atrium, left ventricle, pulmonary artery, lungs, pulmonary vein, right atrium, right ventricle and aorta — A student who has read the sides of the heart from the page rather than from the body picks this. Deoxygenated blood enters the right atrium and the right ventricle sends it to the lungs; the left side receives it back and pumps it to the aorta.
Vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta — Deoxygenated blood returns in the venae cavae to the right atrium, passes through the atrioventricular valve into the right ventricle and is pumped through the semilunar valve into the pulmonary artery. Oxygenated blood from the lungs returns in the pulmonary veins to the left atrium, enters the left ventricle and leaves in the aorta.
Vena cava, right atrium, right ventricle, coronary artery, cardiac muscle, coronary vein, left atrium, left ventricle, aorta — A student who thinks the coronary vessels are part of the route blood takes through the heart picks this. The coronary vessels supply the heart wall itself; blood in the chambers goes to the lungs, not through the coronary circulation.
10 A cut stem of a well-watered plant exudes sap from the xylem for several hours. When the roots are treated with a respiratory inhibitor, the exudation stops. What does this show about how the sap was being pushed out? HL
Answer and reasoning
Root cells were actively transporting water into the xylem, and this active transport of water molecules needs ATP from respiration. — A student who thinks water itself is actively transported picks this. Water is never actively transported; it is mineral ions that are pumped into the xylem, and water follows by osmosis.
The exudation was caused by tension pulling water up the xylem, and the inhibitor stopped the transpiration that generates it. — A student who merges root pressure with transpiration pull picks this. A cut stem has no leaves to transpire, and tension would draw water in rather than push it out; the positive pressure was generated in the roots.
Root cells were using ATP to pump mineral ions into the xylem, so water entered by osmosis and produced a positive pressure. — Root pressure is generated by active transport of mineral ions into the xylem. This lowers the water potential of the xylem sap, water follows by osmosis, and the accumulating water produces a positive pressure potential that pushes sap up the stem. Inhibiting respiration removes the ATP for ion transport, so the pressure collapses.
The xylem vessels themselves were using ATP from respiration to push the sap upwards, and the inhibitor stopped them. — A student who thinks xylem is living and transports water actively picks this. Mature xylem vessels are dead and have no cell contents; the ATP-dependent step is the active transport of mineral ions by living root cells around the xylem (B3.2.17, B3.2.8).
Read the ones marked not yet in Learn, then Verify.
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20 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A muscle receives blood through a network of thousands of capillaries rather than through a single vessel of the same total volume. What is the advantage of this arrangement for exchange?
Answer and reasoning
A single wide vessel would have more wall area than thousands of narrow ones, so exchange inside it would be faster. — A student who compares one wide vessel with one narrow vessel picks this. Summed over the network, the thousands of narrow capillaries have a far larger total wall area and a far higher surface area to volume ratio.
The total surface area of the many narrow, branching vessels is far larger, and all the blood is close to a wall. — Branching and narrow diameters are the adaptations the guide names for surface area. Dividing the blood among many narrow tubes multiplies the wall area available for exchange and ensures no part of the blood is more than a few micrometres from the wall.
The narrow capillaries force the blood through faster, so more oxygen is delivered to the muscle each second. — A student who thinks capillary design is about speed of flow picks this. Flow is actually slowest in capillaries, which gives more time for exchange; the advantage is surface area, not speed.
Each capillary has fenestrations, which are needed so that cells can pass out of the blood into the muscle. — A student who believes all capillaries are fenestrated and that fenestrations pass cells picks this. Muscle capillaries are continuous, without fenestrations, and fenestrations do not pass cells in any case.
2 What is the role of the elastic tissue in the wall of an artery?
Answer and reasoning
It contracts in waves behind the blood to push it along the artery towards the tissues. — A student who thinks the artery wall pumps like the gut wall picks this. Elastic tissue is not muscle and generates no propulsive wave; the heart provides the force that moves blood.
It makes the wall stiff and rigid so that the high pressure cannot burst the artery. — A student who equates a strong wall with a rigid wall picks this. A rigid artery would let pressure fall to zero between beats; elastic tissue is there precisely because the wall must stretch and recoil.
It forms flaps that actively open and shut to push blood forwards along the artery. — A student who gives valves an active pushing role picks this. Elastic tissue is not valve tissue and no valve, venous or cardiac, generates force; the artery's elastic tissue stretches and recoils to maintain pressure (B3.2.3).
It stretches during systole and recoils during diastole, maintaining pressure. — The layers of muscle and elastic tissue allow the artery to withstand and maintain high pressure. Stretching during ventricular contraction stores energy that the recoil returns during relaxation, keeping blood moving and pressure high between beats.
3 The pulmonary artery carries blood from the right ventricle to the lungs. Which statement about this vessel is correct?
Answer and reasoning
It is an artery because it carries blood away from the heart, whatever the oxygen content. — Arteries are adapted for transporting blood away from the heart, whatever the oxygen content of that blood. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs and has the thick, elastic wall of an artery.
It carries oxygenated blood, because by definition every artery carries oxygenated blood. — A student who has turned the red-and-blue diagram convention into a definition picks this. The pulmonary artery carries deoxygenated blood; arteries are defined by direction of flow, not oxygen content.
Its wall is thick and rigid so that pressure from the right ventricle cannot burst it. — A student who equates a strong wall with a rigid wall picks this. The pulmonary artery, like every artery, has an elastic wall that stretches and recoils to maintain pressure between beats (B3.2.3).
Its muscular wall contracts to pump the deoxygenated blood along to the lungs. — A student who thinks artery walls pump picks this. The right ventricle provides the force; the muscle and elastic tissue of the wall withstand the pressure and regulate the diameter.
4 A student counts 12 beats in 10 seconds at the radial pulse. Immediately afterwards a digital heart rate monitor on the same person reads 74 beats per minute. What is the most reasonable conclusion?
Answer and reasoning
The fingertip method gives 72 beats per minute but cannot be compared with the monitor, because it measures blood flow rather than heart rate. — A student who thinks the pulse is the blood flowing past picks this. The pulse is a pressure wave produced by each ventricular contraction, so pulse rate and heart rate are the same quantity and can be compared directly.
The values agree within the error of a 10-second count, about 6 beats per minute; a 60-second count would allow a fairer comparison. — 12 beats in 10 s gives 12 x 6 = 72 beats per minute. A miscount of one beat in 10 s is scaled to 6 beats per minute, so 72 and 74 are consistent. Counting for a full minute reduces this error and makes the traditional and digital methods comparable.
The fingertip count of exactly 72 beats per minute is precise, so the digital monitor must be reading 2 beats per minute too high. — A student who trusts the multiplied count as exact picks this. Any miscount of one beat in 10 s is multiplied by 6, so the fingertip value carries an uncertainty of about 6 beats per minute and the monitor is not shown to be faulty.
The student must have counted at a vein instead of an artery, which gives a slightly lower reading than the monitor does. — A student who thinks a pulse can be felt in a vein picks this. No pulse exists in veins, so a vein could not have given a count at all; the small difference is explained by counting error.
5 Which combination of features adapts a vein for returning blood to the heart at low pressure?
Answer and reasoning
A thick layer of muscle that contracts rhythmically, and a narrow lumen that keeps the pressure high. — A student who models veins on arteries picks this. Veins have thin walls with little muscle and a wide lumen; the pressure in them is low and they are not self-pumping.
Valves that open and shut actively to push blood along, and a wall of elastic tissue that recoils. — A student who gives the valves an active role picks this. Venous valves are passive flaps that only prevent backflow, and veins have little elastic tissue.
A pulse that travels along the wall and squeezes blood forwards, and a thick wall to resist it. — A student who thinks veins carry a pulse picks this. The pressure wave has been damped out by the capillaries; there is no pulse in veins and no thick wall.
Valves that prevent backflow, and a flexible wall compressed by surrounding muscle. — These are the adaptations the guide names. Skeletal muscles around a vein squeeze its flexible wall, pushing blood along, and the pocket valves close whenever blood begins to move backwards, so the net movement is towards the heart.
6 A soldier standing motionless on parade for a long time feels faint, but the feeling passes as soon as she flexes her leg muscles. What is the best explanation?
Answer and reasoning
Standing still removes the muscle contractions that squeeze the leg veins, so less blood returns to the heart until she moves. — Return of blood through veins depends on their flexible walls being compressed by surrounding muscle action, with valves preventing backflow. Without movement, blood pools in the leg veins, less returns to the heart, and blood supply to the brain falls. Flexing the muscles squeezes the veins and restores venous return.
Standing still tires the muscular walls of the leg veins, so they can no longer contract to pump blood up to the heart. — A student who thinks veins pump with their own muscular walls picks this. Vein walls are thin and are not what propels blood; it is the surrounding skeletal muscles that must contract.
Standing still stops the valves in the leg veins from opening and shutting, so they no longer push blood up to the heart. — A student who thinks the valves drive the flow picks this. Valves are passive and only prevent backflow; they cannot push blood, so the loss of muscle compression is the real cause.
Standing still stops the artery walls from contracting, so blood can no longer be pumped from the heart to the brain. — A student who thinks artery walls pump picks this. Arteries never pump, and the heart continues to beat; the problem is reduced return of blood to the heart through the veins.
7 In a study of 20 countries, the correlation coefficient between mean saturated fat intake and the death rate from coronary heart disease was +0.85. What can be concluded from this result alone?
Answer and reasoning
Saturated fat in the diet is shown to cause coronary heart disease, because the coefficient is so close to +1, the maximum. — A student who treats a strong correlation as proof of cause picks this. The guide is explicit that even the strong correlation between saturated fat intake and coronary heart disease does not prove a causal link; other factors could vary alongside fat intake.
The relationship is weak, because a correlation calculated from only 20 countries cannot be a strong one. — A student who takes sample size as the measure of strength picks this. The strength of the relationship is given by the coefficient itself; +0.85 is a strong positive correlation. A small sample lowers confidence in the value, which is a separate question (B3.2.6 NOS).
There is a strong positive correlation between the variables, but this does not prove that one causes the other. — A coefficient of +0.85 quantifies a strong positive relationship: countries with higher fat intake tend to have higher death rates. Correlation coefficients assess the strength of a relationship, but a causal link can only be established with other evidence, since confounding variables may be responsible.
Saturated fat from food coats the inside of the coronary arteries, which is why the two variables rise together in step. — A student who thinks dietary fat lines the artery like grease in a pipe, and who reads a mechanism into a correlation, picks this. The data give no mechanism, and atheroma forms within the artery wall rather than as fat coating the lumen.
8 Researchers calculated correlation coefficients between the incidence of coronary heart disease and four factors: smoking (2000 people) +0.72, hours of exercise per week (500 people) -0.65, dietary fibre intake (3000 people) +0.08, and blood cholesterol (400 people) +0.61. Which factor shows the weakest relationship with coronary heart disease?
Answer and reasoning
Hours of exercise per week, because a negative coefficient shows a weaker relationship than any positive one. — A student who reads a negative sign as meaning a weak relationship picks this. -0.65 is a strong negative correlation: more exercise is associated with less disease. The sign gives direction, not strength.
Blood cholesterol, because it was measured in the fewest people, so its relationship is the weakest. — A student who confuses the reliability of a study with the strength of a correlation picks this. Sample size affects how much confidence to place in a coefficient, but the strength of the relationship is given by the coefficient itself; +0.61 is a moderately strong correlation.
Smoking, because even a strong correlation of +0.72 cannot prove that smoking causes the disease. — A student who confuses the question of causation with the question of strength picks this. Whether or not it proves cause, +0.72 is the strongest relationship in the list, not the weakest.
Dietary fibre intake, because +0.08 is closest to zero, showing little or no correlation. — The size of a correlation coefficient, ignoring its sign, measures the strength of the relationship. +0.08 is very close to zero, so fibre intake shows almost no correlation with the disease in these data; such a low coefficient could count as evidence against a hypothesis that fibre affects coronary heart disease.
9 What generates the force that draws water up the xylem from the roots to the leaves?
Answer and reasoning
Evaporation of water from leaf cell walls draws water out of the xylem, putting the water column under tension. — Loss of water by transpiration from the cell walls of leaf cells causes water to be drawn out of the xylem vessels and through cell walls by capillary action. This generates tension, a negative pressure potential, and it is this tension that draws water up the xylem.
Cells in the root pump water into the base of the xylem, pushing the whole column up to the leaves. — A student who locates the force at the bottom, like a pump, picks this. Root pressure is too small to lift water up a tall plant; the water is pulled from above by the tension that transpiration generates.
The living xylem cells use ATP from respiration to move water molecules upwards against gravity. — A student who assumes uphill movement needs cellular energy picks this. Xylem vessels are dead and have no cell contents; the energy comes from the sun evaporating water in the leaf.
Water evaporates from the open ends of the xylem vessels at the leaf surface, sucking the column up. — A student who thinks evaporation happens straight from the xylem picks this. Xylem vessels do not open at the leaf surface; water leaves them through pits, moves through the cell walls of leaf cells, and evaporates from those walls into the air spaces.
10 In a tall tree the column of water in the xylem is under considerable tension, yet it does not break into separate lengths with gaps between them. Which property of water explains this?
Answer and reasoning
Water molecules are attracted to the lignin of the vessel wall, which holds each part of the column in place. — A student who assigns the column-holding role to sticking to walls picks this. Adhesion to walls does not stop the column snapping in the middle; it is the attraction of water molecules to each other that keeps it continuous.
Water molecules are held to one another by hydrogen bonds, so the whole column moves as an unbroken unit. — This is cohesion. Hydrogen bonding between water molecules means that when the topmost molecules are drawn out into leaf cell walls, they pull the molecules below with them, so a continuous column is maintained under tension all the way from the roots.
Water is pushed steadily upwards from the roots, so the column is kept under compression, not tension. — A student who thinks water is pushed from below picks this. The column in a transpiring plant is under tension, not compression, as the stem's shrinking diameter during the day shows; a push would not explain why it holds together.
Water molecules are pulled upwards by active transport in the xylem, faster than any gaps could form. — A student who expects a cellular energy input picks this. Xylem vessels have no cell contents and carry out no active transport; the continuity of the column is a physical property of water.
11 A xylem vessel in a leaf vein is blocked by an air bubble. Water still reaches the leaf cells beyond the blockage. Which feature of xylem makes this possible?
Answer and reasoning
Living xylem cells actively transport water around the blockage using ATP from respiration. — A student who expects the xylem to solve the problem by active transport picks this. Xylem vessels are dead, with no cell contents, and cannot carry out active transport.
The lignified side walls are completely sealed, so water can only move through the open ends. — A student who thinks lignin seals the vessel picks this. If the side walls were sealed, a blocked vessel would cut off everything beyond it; it is the openings in the walls that allow a bypass.
The end walls of the vessel are absent, so water is pushed past the bubble from below. — A student who imagines water being pushed from below picks this. Missing end walls give unimpeded flow along one vessel, but they cannot get water past a bubble in that vessel; water must leave sideways.
Pits in the lignified walls let water pass sideways into a neighbouring vessel. — Pits are gaps in the lignified wall for entry and exit of water. Water below the bubble can pass through pits into an adjacent vessel, bypass the blockage, and re-enter or continue on to the leaf cells, so a single blocked vessel does not stop supply.
12 A student is drawing a plan diagram from a micrograph of a transverse section of a dicotyledonous stem. Which description matches what should be drawn?
Answer and reasoning
A ring of vascular bundles near the outside, each with xylem towards the centre and phloem towards the epidermis, and cortex outside the ring. — In a dicotyledonous stem the vascular bundles are arranged in a ring. Within each bundle the xylem lies on the inner side and the phloem on the outer side; the cortex lies between the ring and the single-layered epidermis, and pith fills the centre.
A single central mass of vascular tissue, with xylem forming a star and phloem between its arms, surrounded by a wide cortex. — A student who expects transport tissue to run down the middle picks this. A central star of xylem is the arrangement in a root; a stem has a ring of separate vascular bundles.
A ring of vascular bundles near the outside, each with phloem towards the centre and xylem towards the epidermis, and pith in the middle. — A student who has reversed the two tissues within the bundle picks this. Xylem is on the inner side of each bundle, towards the centre; phloem is on the outer side.
A wide cortex carrying water and sugars up the stem, with a thin ring of xylem and phloem that provides support only. — A student who thinks the cortex is a transport tissue picks this. The cortex is storage and packing tissue; xylem and phloem in the vascular bundles carry out transport.
13 Samples of blood plasma and of tissue fluid from the same tissue were analysed. Both contained about 5 mmol per litre of glucose and similar concentrations of sodium ions, but plasma contained about 70 grams per litre of protein while tissue fluid contained only a small fraction of that amount. Which explanation of these results is correct? HL
Answer and reasoning
Tissue fluid should have exactly the same composition as plasma, so the low protein value must be an error in the analysis. — A student who thinks tissue fluid is unchanged plasma picks this. The result is expected: plasma proteins are retained in the capillary because they are too large to be filtered out.
The cells of the tissue took the protein directly from the blood inside the capillaries before the tissue fluid formed. — A student who thinks cells draw substances straight from the blood picks this. Cells exchange with tissue fluid, not directly with blood, and the missing protein never left the capillary in the first place.
Glucose and ions diffused out of the capillary but proteins cannot diffuse, so tissue fluid formed by diffusion is protein-free. — A student who explains tissue fluid formation by diffusion picks this. Tissue fluid is formed by pressure filtration through gaps in the capillary wall; small solutes pass with the water while proteins are held back by their size.
Pressure filtration lets water and small solutes through the capillary wall but retains the large plasma proteins. — Glucose and sodium ions are small enough to pass through the gaps and fenestrations in the capillary wall along with the water, so their concentrations match. Plasma proteins such as albumin are too large to pass and stay in the plasma, which is the main difference in composition between plasma and tissue fluid.
14 In a bony fish, mean blood pressure was measured at about 5 kPa in the artery leaving the heart and about 3.5 kPa in the vessel leaving the gills. In a mammal, mean pressure in the aorta was about 13 kPa. What do these data show about the two types of circulation? HL
Answer and reasoning
Both circulations are double, because in each case the blood passes through two capillary beds, one for gas exchange and one for the rest of the body. — A student who counts capillary beds instead of passes through the heart picks this. The fish circulation is single: blood passes through the heart once per circuit, going heart, gills, body, heart.
In the fish, blood loses pressure in the gill capillaries and goes on to the body at that lower pressure; in the mammal it is pumped again after the lungs. — The drop from 5 kPa to 3.5 kPa is the pressure lost in the gill capillaries, and in a single circulation this reduced-pressure blood goes straight on to the body without being pumped again. In the double circulation of a mammal, blood from the lungs returns to the left side of the heart and is pumped a second time, so the aorta carries blood at high pressure.
The fish data must be unreliable, because blood leaves a capillary bed at the same pressure as it had when it entered, so the two fish values should be equal. — A student who believes pressure is constant around a circuit picks this. Flow through narrow capillaries always causes a large pressure drop; the fish data are exactly what a single circulation predicts.
The mammalian value is higher because the muscular wall of the aorta contracts to pump blood onwards, adding to the pressure produced by the heart. — A student who thinks artery walls pump picks this. Arterial pressure comes from the ventricle; the high aortic pressure in a mammal is because blood returning from the lungs is pumped a second time by the left ventricle in a double circulation (B3.2.14).
15 The wall of the left ventricle is about three times thicker than the wall of the right ventricle. Which statement correctly explains this difference? HL
Answer and reasoning
The left ventricle pumps a larger volume of blood at each beat than the right ventricle, so it needs more muscle. — A student who links thickness to volume picks this. Both ventricles eject the same volume per beat; otherwise blood would accumulate in one circuit. The difference is the pressure each must generate.
The left ventricle pumps blood to the lungs, which offer more resistance to flow than the rest of the body does. — A student who has reversed the two sides of the heart picks this. The left ventricle pumps to the body, and it is the systemic circulation that has the greater resistance and needs the higher pressure.
The left ventricle gets a stronger nerve impulse from the brain at each beat, so it has built up more muscle. — A student who thinks the brain drives each contraction picks this. The beat is initiated by the sinoatrial node in the right atrium and spreads through the heart; ventricular wall thickness reflects the pressure the chamber must generate, not the strength of any nervous input (B3.2.15).
The left ventricle must generate a much higher pressure to drive blood through the whole systemic circulation. — This is a form-function adaptation of cardiac muscle: the thick wall of the left ventricle can develop the high pressure needed to push blood through the long, high-resistance systemic circuit, while the thinner right ventricle produces the lower pressure suited to the nearby lungs.
16 The sinoatrial node fires. Which sequence of events then follows in the left side of the heart? HL
Answer and reasoning
The atrium contracts, filling the ventricle; the ventricle then contracts, shutting the atrioventricular valve and opening the semilunar valve; then both relax. — The impulse spreads across the atria first, so atrial systole completes ventricular filling. After a brief delay the ventricle contracts: its rising pressure shuts the atrioventricular valve and, once it exceeds aortic pressure, opens the semilunar valve. Diastole follows, with both chambers relaxed and filling.
The atrium and the ventricle contract at the same moment, and the atrioventricular and semilunar valves open together to let blood into the aorta. — A student who pictures the heart as squeezing all at once picks this. Atrial systole precedes ventricular systole, and the atrioventricular valve must be shut while the semilunar valve is open.
Muscles attached to the valves pull them open one after the other, and the ventricle contracts only after both valves have been opened. — A student who thinks the valves are operated by muscles picks this. Valves have no muscle; they open and shut passively as the pressure differences across them change during the cycle.
A nerve impulse from the brain reaches the ventricle first, so the ventricle contracts and then the atrium contracts to refill it. — A student who thinks the brain initiates each beat picks this. The beat is initiated by the sinoatrial node in the heart itself, and its impulse reaches the atria before the ventricles.
17 A person's blood pressure is recorded as 120/80 mm Hg. Which interpretation of the two values is correct? HL
Answer and reasoning
120 is the pressure inside the left ventricle when it contracts and 80 is the pressure inside the left atrium when it relaxes. — A student who assumes the reading is taken inside the heart picks this. Both values are pressures measured in an artery of the arm, not in the chambers of the heart.
120 is the arterial pressure at the peak of ventricular systole and 80 is the arterial pressure during ventricular diastole. — Blood pressure is measured in an artery. The systolic value is the maximum pressure reached when the left ventricle contracts and forces blood into the arteries; the diastolic value is the minimum, maintained by elastic recoil of the artery walls while the ventricle relaxes and refills.
120 is the pressure during systole and 80 is the pressure the artery wall itself generates as it contracts to push blood. — A student who thinks artery walls pump picks this. The diastolic pressure is maintained by passive elastic recoil, not by contraction of the artery wall.
120 is the pressure during systole; the true diastolic pressure is zero, and 80 is an average over the whole cycle. — A student who thinks pressure vanishes when the heart relaxes picks this. Arterial pressure never falls to zero, because the stretched elastic walls recoil and keep the blood under pressure; 80 is the actual diastolic pressure.
18 Droplets of water appear at the leaf tips of a grass seedling overnight, when the air is saturated with water vapour, but not during a dry, sunny day. Which explanation is correct? HL
Answer and reasoning
At night the roots generate a tension that pulls water up the plant and out of the leaf tips, taking over from the pull of transpiration. — A student who thinks root pressure is a pull picks this. Root pressure is a positive pressure potential generated in the root; a tension could not force water out of a leaf.
At night root cells use ATP to actively transport water molecules into the xylem, and this pumping of water forces it out of the leaf tips. — A student who thinks water itself is actively transported picks this. Root pressure arises from active transport of mineral ions into the xylem; water follows by osmosis and is never pumped directly (B3.2.17).
Root pressure alone can lift water up the plant at any time of day, so droplets should appear whether or not the air is humid. — A student who thinks water is always pushed up from the roots picks this. During the day the rapid loss of water by transpiration removes the water as fast as it arrives, so no excess is forced out.
Humid air stops transpiration, so root pressure generated by active ion transport pushes water up and out of the leaf tips. — Root pressure is generated when transport in the xylem due to transpiration is insufficient, such as when high humidity prevents transpiration. Active transport of mineral ions into the root xylem draws in water by osmosis, and the resulting positive pressure forces water up the small seedling and out at the leaf tips.
19 Which feature of a sieve tube element helps sap to flow along the phloem? HL
Answer and reasoning
It has lost its nucleus and most of its cytoplasm and organelles, so little inside it obstructs the sap. — Sieve tube elements are living cells adapted for flow: reduced cytoplasm, few organelles and no nucleus leave the lumen largely clear, and perforated sieve plates at the ends let sap pass to the next element. The neighbouring companion cell provides the functions the element has given up.
It is dead and empty like a xylem vessel, so there is nothing inside the cell to slow the flow of sap. — A student who models phloem on xylem picks this. Sieve tube elements are alive, kept alive by companion cells; a dead phloem cell could not be loaded or unloaded.
Its end walls are solid plates that hold the sap under pressure so that it flows faster along the tube. — A student who takes plate to mean a solid barrier picks this. Sieve plates are perforated; solid end walls would stop the flow altogether.
It contains many mitochondria, whose ATP is used to push the sap along the tube towards the sink. — A student who thinks ATP propels the sap picks this. It is the companion cell that has many mitochondria, and their ATP is used for loading and unloading sugars, not for pushing sap along.
20 In spring, sugars stored in the roots of a deciduous tree are converted to sucrose and carried in the phloem to the opening buds. Which statement about this transport is correct? HL
Answer and reasoning
Mitochondria in the companion cells provide the ATP that physically drives the sap along the sieve tubes up to the buds. — A student who thinks ATP pushes the sap picks this. ATP from companion cells is used to load sucrose into the sieve tubes at the root and unload it at the buds; the flow along the tube is by pressure.
Companion cells in the root use ATP to load sucrose into the sieve tubes, and the sap flows up to the buds, the sinks. — Phloem sap flows from source to sink. Here the root is the source: its companion cells, rich in mitochondria, actively load sucrose into the sieve tube elements through plasmodesmata, and the sap moves up to the buds, the sinks, where sugars are unloaded for growth.
Sap cannot move upwards in the phloem, so the sugars must instead be carried up to the buds in the xylem. — A student who believes phloem sap only flows downwards picks this. Sap flows from any source to any sink, so in spring it moves upwards from the storage root to the buds.
The sieve tube elements must first be killed and emptied, as in xylem, before any sap is able to flow through them. — A student who thinks sieve tube elements are dead picks this. They are living cells with reduced contents; loading sucrose requires living companion cells and sieve tube elements connected by plasmodesmata.
That was your twenty minutes. Real practice on B3.2 is past-paper questions marked against the mark scheme.
What the exam asks of B3.2
Paper 1A asks you to identify a vessel or tissue from a micrograph, or to name the structure that does a stated job. Paper 1B often gives epidemiological data with a correlation coefficient and asks what it shows and what it cannot show, or blood pressure traces to read systolic and diastolic values. Paper 2 uses *draw* for stem and root plan diagrams, *outline* for capillary or vein adaptations, and *explain* for transpiration pull, where tension, cohesion and evaporation must all appear. At HL, *describe* the cardiac cycle in order, *distinguish* single from double circulation, and *explain* tissue fluid formation in terms of pressure.
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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