Key Idea: A single cell can rely on diffusion, but a large animal or plant is far too big for that — so it needs a transport system to carry materials between the places that supply them and the places that use them. This topic asks the same question on two sides of life. In animals: how does the blood travel in vessels (2.7.1), how does the heart pump it (2.7.2), and what keeps the circulation healthy (2.7.3)? In plants (which have no heart at all): how is water carried up the xylem by transpiration (2.7.4), and how is sugar carried in the phloem by translocation (2.7.5)? Two ideas tie the whole topic together. First, structure follows function — every wall, valve and tube is built for the job it does. Second, things move down a gradient: animal blood is pumped under pressure, but plants move materials by pressure and pull, with no pump at all. Transport appears across all papers — Paper 1A (a quick MCQ on a vessel, chamber or xylem feature), Paper 1B / Paper 3 (read a cardiac-cycle pressure graph, an ECG, or a ringing / tracer result) and Paper 2 (explain how a structure is adapted, or trace a pathway in order).
🩸 Blood vessels — arteries, veins & capillaries
Blood travels in three kinds of vessel, and each wall is built to suit its job. Arteries carry blood away from the heart at high pressure; veins return it to the heart at low pressure; and capillaries are where substances are exchanged with the tissues. The artery wall is thick, muscular and elastic with a narrow lumen: it stretches as each surge of blood passes and then recoils, which withstands the high pressure and keeps the blood moving. The vein wall is thin with a wide lumen, and because the pressure is so low it has valves that close to stop backflow. The capillary wall is only one cell thick — a very short diffusion distance — and capillaries form a huge branching network, giving a large surface area close to every cell. The golden rule of this micro is to state the structure AND the function it allows — 'thick wall' alone is weak; 'thick elastic wall to withstand high pressure' scores.
The three vessels in cross-section. The artery has a thick, muscular and elastic wall with a narrow lumen for high pressure; the vein has a thin wall, a wide lumen and valves for low pressure; the capillary wall is just one cell thick, giving a short diffusion distance for exchange. Wall thickness always matches the pressure the vessel carries.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction of blood | away from the heart | back to the heart | through the tissues |
| Wall | thick — muscular and elastic | thin | one cell thick |
| Lumen | narrow | wide | very narrow (one cell wide) |
| Pressure | high | low | low (and falling) |
| Valves? | no | yes — stop backflow | no |
| Main job | carry blood at high pressure | return blood at low pressure | exchange materials with tissues |
Arteries carry blood Away from the heart; veins bring it back. Match the wall to the pressure: thick wall = high pressure (artery); thin wall + valves = low pressure (vein); one cell thick = exchange (capillary). On a 'Distinguish' question, pair each feature with 'whereas' — describing only one vessel scores nothing.
❤️ The heart, cardiac cycle & double circulation
The heart is really two pumps side by side. The right side handles deoxygenated blood and sends it to the lungs; the left side handles oxygenated blood and sends it to the body. A wall called the septum keeps the two kinds of blood from mixing. The two upper atria are thin-walled and receive blood; the two lower ventricles are thick-walled and pump it out. The left ventricle has the thickest wall of all, because it must generate a high enough pressure to reach the whole body. Each beat is the cardiac cycle: the atria contract (pushing blood into the ventricles), then the ventricles contract (the AV valves shut — the 'lub' sound — and the semilunar valves open, so blood is forced into the pulmonary artery and aorta), then everything relaxes (the semilunar valves shut — the 'dub' sound — and the chambers refill). Valves are pushed open and shut by pressure differences — a valve opens when the pressure behind it exceeds the pressure in front of it — which is exactly what a cardiac-cycle pressure graph shows. Mammals have double circulation: blood passes through the heart twice per body circuit. After it loses pressure in the lungs, the left ventricle re-pressurises it, so the body always receives high-pressure, fully oxygenated blood.
The four chambers and four great vessels. Blue = deoxygenated blood (the right side pumps it to the lungs); red = oxygenated blood (the left side pumps it to the body). The left ventricle has the thickest wall because it must generate a high pressure to reach the whole body.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Stage of the cardiac cycle | What contracts / relaxes | Valves | Effect on blood |
|---|---|---|---|
| Atrial systole | atria contract | AV valves open | blood pushed from atria into ventricles |
| Ventricular systole | ventricles contract | AV valves shut ('lub'), semilunar valves open | blood forced out into pulmonary artery and aorta |
| Diastole | atria and ventricles relax | semilunar valves shut ('dub'), AV valves open | chambers refill with returning blood |
Trace the double circulation. Right side (deoxygenated): vena cava → right atrium → right ventricle → pulmonary artery → lungs. Left side (oxygenated): pulmonary vein → left atrium → left ventricle → aorta → body. Blood passes through the heart twice for one full circuit.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Usually 'artery = oxygenated', but the pulmonary artery carries deoxygenated blood (heart → lungs) and the pulmonary vein carries oxygenated blood (lungs → heart). These are the only two vessels that break the rule. When tracing a pathway, name the structures in order and use 'deoxygenated' for the right side, 'oxygenated' for the left — the vessel leaving the heart is always an artery.
🫀 Circulatory health — pressure, cholesterol & exercise
Keeping the circulatory system healthy comes down to two big numbers: blood pressure and blood cholesterol. Blood pressure is written as two values: the higher systolic pressure (when the ventricles contract) and the lower diastolic pressure (when they relax between beats). Eating too much salt (sodium) raises blood pressure — hypertension — which strains the heart and damages arteries. Cholesterol is carried by two particles. LDLs carry it to the tissues and deposit the excess in artery walls (the 'bad' carrier); HDLs carry the excess away to the liver for disposal (the 'good' carrier). Too much LDL cholesterol is deposited in an artery wall, building into a fatty plaque (atherosclerosis) that narrows the artery. If this narrows a coronary artery — the vessels that supply the heart muscle — the heart muscle gets less oxygen: coronary heart disease, and a full blockage causes a heart attack. Exercise is the protective factor. Short term, the heart rate and stroke volume (and so cardiac output) rise to deliver more oxygen to working muscles. Long term, the heart muscle grows stronger, so stroke volume rises and the resting heart rate falls — a fitter, more efficient heart.
| Idea | What it means | Why it matters to health |
|---|---|---|
| Systolic pressure | the higher number — ventricles contract | high values strain the heart and arteries |
| Diastolic pressure | the lower number — ventricles relax between beats | stays high in hypertension |
| LDL ('bad') | deposits cholesterol in artery walls | drives plaque / atherosclerosis |
| HDL ('good') | hauls excess cholesterol away to the liver | protects the arteries |
| Atherosclerosis | plaque narrows and hardens an artery | narrowed coronary artery → CHD |
| Factor | Raises your risk | Lowers your risk |
|---|---|---|
| Cholesterol balance | high LDL | high HDL, low LDL |
| Diet | high saturated fat and salt | low saturated fat and salt |
| Lifestyle | smoking, obesity, lack of exercise | regular exercise |
| Result in arteries | plaque / atherosclerosis | clear, wide arteries |
Systolic = Squeeze (the higher number, heart contracting); Diastolic = Down-time (the lower number, heart relaxing). LDL = Lousy (it Leaves cholesterol in your arteries); HDL = Healthy (it Hauls it back to the liver). You want low LDL and high HDL. For exercise, give one short-term effect AND one long-term effect — two of the same kind only scores once.
🌿 Plant transport — xylem & transpiration
A plant has no heart, yet it must lift water from its roots to its highest leaves. Water enters at the root hairs, travels up dead tubes called xylem vessels, and evaporates from the leaves as water vapour — this loss is transpiration, and it is the very thing that pulls the water up. The mechanism is cohesion-tension. Water evaporates from the leaf and diffuses out through the stomata, so the leaf cells pull water from the xylem. This puts the whole water column under tension (a pull, like sucking a straw). Because water molecules stick to one another (cohesion), the column does not break — the pull is passed all the way down, and water is pulled up from the roots to replace what was lost. This only works because the xylem is adapted: the vessels are dead, hollow cells stacked end to end with their end walls removed, forming one continuous pipe, and their walls are strengthened with lignin so they do not collapse under the tension. The rate of transpiration is fastest in hot, dry, windy and bright conditions — exactly the weather that dries washing on a line — while high humidity slows it down.
The transpiration stream: water enters at the root hairs, is pulled up the xylem and evaporates from the leaf as water vapour. Evaporation at the top creates tension; because water molecules stick together (cohesion), the unbroken column is pulled all the way up — the cohesion-tension mechanism.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Idea / adaptation | What it is | Why it matters |
|---|---|---|
| Transpiration | water vapour lost at the leaf (via stomata) | the engine that pulls water up |
| Tension | the pull on the water column | transmitted down from the leaf |
| Cohesion | water molecules stick together | keeps the column unbroken |
| No end walls | vessels join into one continuous tube | water flows up with nothing blocking it |
| Lignified walls | walls strengthened with lignin | stops the vessel collapsing under tension |
| Factor | Increasing it does what to the rate? | Why |
|---|---|---|
| Light | increases | opens the stomata, so more vapour escapes |
| Temperature | increases | gives water molecules more energy to evaporate |
| Wind / air movement | increases | blows away humid air, keeping the difference large |
| Humidity | decreases | moist outside air means less water diffuses out |
Think of drying the washing: it dries fastest when it is hot, dry and windy — the same conditions that speed up transpiration. For the mechanism, name both key words: transpiration → tension (the pull) → cohesion (water sticks together) → water rises. Always tie lignin to not collapsing under the tension.
🍯 Plant transport — phloem & translocation
Plants make sugar in their leaves but need it everywhere — in growing roots, flowers, fruits and stores. The tissue that carries it is the phloem, and the movement of dissolved sugar through it is translocation. It always runs from a source (where sugar is made or released) to a sink (where sugar is used or stored) — and because a store can switch from sink to source, translocation can run up or down the plant. There are four steps: load sugar into the sieve tube at the source by active transport (this uses ATP, supplied by the companion cell); water follows in by osmosis, which raises the pressure; the high pressure pushes the sugary sap along by bulk flow; and at the sink the sugar is unloaded (used or stored as starch) so the pressure drops and the sap keeps flowing. Each conducting cell is a sieve tube with sieve plates (perforated end walls), little cytoplasm and no nucleus, kept alive by a neighbouring companion cell. This is the mirror image of the xylem: phloem is living and two-way and moves sugar by active loading; xylem is dead and one-way and moves water by the passive transpiration pull.
Translocation in the phloem: sugar made at a SOURCE (leaf) is actively loaded into the living sieve tube using ATP, water follows by osmosis, the raised pressure pushes the sugary sap by bulk flow, and the sugar is unloaded at a SINK (root or fruit). Phloem is living and two-way; xylem is dead and one-way.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Step of translocation | What happens | Key point |
|---|---|---|
| 1. Load at the source | sugar moved into the sieve tube by active transport | uses ATP from the companion cell |
| 2. Water follows | water enters the sieve tube by osmosis | raises the pressure at the source |
| 3. Bulk flow | the sugary sap is pushed along the sieve tube | driven by the pressure difference |
| 4. Unload at the sink | sugar removed and used or stored as starch | pressure drops → sap keeps flowing |
| Feature | Phloem | Xylem |
|---|---|---|
| Mainly transports | dissolved sugar (sucrose) | water + mineral ions |
| Direction | two-way — source to any sink (up or down) | one-way — roots up to the leaves |
| Living or dead cells | living sieve tubes (kept alive by companion cells) | dead, hollow vessels |
| What drives flow | active loading → pressure → bulk flow (uses ATP) | transpiration pull / evaporation (no ATP) |
Load, follow, flow, unload. Sugar is loaded at the source, water follows in, pressure drives bulk flow, and sugar is unloaded at the sink — the sap moves because of pressure, not because the plant pumps it. Phloem is living and two-way; xylem is dead and one-way.
✍️ Worked examples
IB-style question — distinguish artery and vein walls [2.7.1]
Distinguish between the wall structure of an artery and that of a vein. [2]
How to score both marks:
Contrast the wall thickness. An artery has a thick wall, with a lot of muscle and elastic tissue, whereas a vein has a thin wall.
Contrast a second feature. An artery has a narrow lumen and no valves, whereas a vein has a wide lumen and valves to stop backflow. (1 mark per clear paired contrast, max 2.)
An artery has a thick, muscular wall with a narrow lumen and no valves, whereas a vein has a thin wall with a wide lumen and valves to prevent backflow.
IB-style question — trace blood to the lungs [2.7.2]
Describe the circulation of deoxygenated blood from the heart to the lungs. [2]
How to score both marks:
Start where the deoxygenated blood arrives. It returns from the body in the vena cava and enters the right atrium, then passes into the right ventricle.
Follow it out to the lungs. The right ventricle pumps the blood into the pulmonary artery, which carries it to the lungs to be oxygenated. (Mark 1: right atrium → right ventricle. Mark 2: pulmonary artery → lungs.)
Deoxygenated blood enters the right atrium, passes into the right ventricle, and is pumped through the pulmonary artery to the lungs.
IB-style question — cholesterol and coronary heart disease [2.7.3]
Outline how high blood cholesterol can contribute to coronary heart disease. [3]
How to score all three marks:
Start with the deposit. Excess (LDL) cholesterol is deposited in the wall of an artery.
Name the build-up. The deposits form a fatty plaque (atherosclerosis), which narrows the artery and reduces blood flow.
Link it to the heart. If this narrows a coronary artery, the heart muscle receives less oxygen, causing coronary heart disease. (Mark 1: cholesterol deposited. Mark 2: plaque narrows the artery. Mark 3: less oxygen to the heart muscle.)
Excess cholesterol is deposited in the artery wall, forming a plaque (atherosclerosis) that narrows the coronary arteries, so the heart muscle receives less oxygen — coronary heart disease.
IB-style question — how transpiration moves water up [2.7.4]
Explain how transpiration drives the movement of water up through a plant. [3]
How to score all three marks:
Start at the leaf. Water evaporates from the leaf and is lost as vapour through the stomata (transpiration), so the leaf cells pull water from the xylem.
Name the force. This pulling places the water column under tension (it is pulled, not pushed), and the pull is transmitted down the xylem.
Use cohesion. Because water molecules stick together (cohesion), the column stays unbroken, so water is pulled up from the roots to replace what was lost. (Mark 1: evaporation/transpiration. Mark 2: creates tension. Mark 3: cohesion keeps the column continuous so water rises.)
Water evaporates from the leaf (transpiration), creating tension that pulls on the water column; because the molecules cohere, the column stays unbroken and water is pulled up the xylem from the roots.
IB-style question — translocation source to sink [2.7.5]
Explain how dissolved sugar is moved from a leaf to a growing root by translocation in the phloem. [4]
How to score all four marks:
Load at the source. In the leaf, sugar is actively loaded into the sieve tube using ATP supplied by the companion cell.
Water follows. The high sugar concentration draws water in by osmosis, raising the pressure at the source end.
Bulk flow to the sink. The high pressure pushes the sap by bulk flow along the sieve tubes towards the root, where the pressure is lower.
Unload at the sink. At the root the sugar is unloaded and used for growth (or stored), so the pressure drops and the sap keeps flowing. (Mark 1: active loading. Mark 2: water follows → pressure. Mark 3: bulk flow. Mark 4: unloading at the sink.)
Sugar is actively loaded into the sieve tube at the leaf using ATP from the companion cell; water follows by osmosis, raising the pressure; the sap is pushed by bulk flow to the root, where the sugar is unloaded and used for growth.
✅ Quick self-check
Tap each card to check yourself.
How does the wall of each blood vessel match its job? Artery: thick, muscular, elastic wall and narrow lumen to withstand high pressure. Vein: thin wall, wide lumen and valves for low-pressure return (valves stop backflow). Capillary: wall one cell thick (short diffusion distance) and a large surface area for exchange.
Why does the left ventricle have the thickest wall, and what is double circulation? The left ventricle pumps blood to the whole body, so it needs the most muscle to generate a high pressure. Double circulation = blood passes through the heart twice per circuit (pulmonary to the lungs, systemic to the body), so the left ventricle re-pressurises blood after the lungs.
How does high cholesterol lead to coronary heart disease, and what do HDLs do? Excess LDL cholesterol is deposited in artery walls, forming plaque (atherosclerosis) that narrows the coronary arteries, so the heart muscle gets less oxygen. HDLs do the opposite — they carry excess cholesterol away from the arteries back to the liver.
What pulls water up the xylem, and how is the xylem adapted? Evaporation at the leaf (transpiration) creates tension; because water molecules cohere, the unbroken column is pulled up — the cohesion-tension mechanism. Xylem vessels are dead, hollow tubes with no end walls (one continuous pipe) and lignified walls that stop them collapsing under tension.
How is sugar moved in the phloem, and how does phloem differ from xylem? Sugar is actively loaded into the sieve tube at the source (uses ATP from the companion cell); water follows by osmosis, raising the pressure, so the sap moves by bulk flow to the sink, where the sugar is unloaded. Phloem is living and two-way (sugar); xylem is dead and one-way (water).
When does a heart valve open or close? A valve opens when the pressure behind it exceeds the pressure in front of it, and closes when the pressure in front becomes higher. The valves closing make the heart sounds: AV valves = 'lub', semilunar valves = 'dub'.
Exam Tips
- Always state the STRUCTURE and the FUNCTION it allows — 'thick wall' alone is weak; 'thick elastic wall to withstand high pressure' scores.
- On a 'Distinguish' question, pair each feature with 'whereas' (artery thick wall whereas vein thin wall) — describing only one vessel scores nothing.
- Capillary 'adapted for exchange' marks: wall one cell thick → short diffusion distance, plus a large surface area.
- When tracing a pathway, name the structures IN ORDER; the vessel leaving the heart is always an artery. The pulmonary ARTERY carries deoxygenated blood, the pulmonary VEIN oxygenated.
- On a pressure graph, a valve opens when the pressure behind it exceeds the pressure in front; curves crossing is the cue. AV = 'lub', semilunar = 'dub'.
- For 'why pump twice?': blood loses pressure in the lungs, so the left ventricle re-pressurises it — double circulation delivers high-pressure, separated blood to the body.
- Systolic = higher (ventricles contract); diastolic = lower (ventricles relax). Too much salt → high blood pressure (hypertension).
- HDL hauls cholesterol AWAY to the liver; LDL leaves it in arteries. The CHD chain must reach the heart muscle / oxygen: deposit → plaque → narrowed coronary artery → less oxygen.
- Exercise: give ONE short-term effect (heart rate / output up) AND ONE long-term effect (stronger heart, lower resting rate) — not two of the same.
- For 'how transpiration drives water up', name BOTH tension (the pull) and cohesion (water sticking together) — not just 'water rises'. Tie lignin to NOT collapsing under tension.
- For translocation, name the SOURCE and the SINK and say sugar is actively loaded (uses ATP); water follows → pressure → bulk flow → unload. Phloem is living and two-way; xylem is dead and one-way.
Key Idea: Big bodies need transport systems, and across both animals and plants the rule is structure follows function. In animals, blood travels in three vessels: arteries carry it away at high pressure (thick, muscular, elastic wall; narrow lumen), veins return it at low pressure (thin wall, wide lumen, valves stop backflow), and capillaries (wall one cell thick, large surface area) are where exchange happens. The heart is two pumps in one: the right side sends deoxygenated blood to the lungs, the left side sends oxygenated blood to the body, and the left ventricle has the thickest wall. Each beat is the cardiac cycle (atria contract → ventricles contract, 'lub' → relax, 'dub'), with valves driven by pressure differences; double circulation pumps blood through the heart twice, so the body always gets high-pressure, oxygenated blood. Health depends on blood pressure (systolic high / diastolic low; salt raises it) and cholesterol (LDL deposits → plaque / atherosclerosis → narrowed coronary arteries → CHD; HDL hauls it away; exercise protects). Plants have no heart: water is pulled up the xylem by transpiration — evaporation at the leaf creates tension, cohesion holds the column unbroken, and the dead, hollow, lignified vessels carry it (rate up in hot, dry, windy, bright conditions). Sugar moves in the living phloem by translocation from source to sink: actively loaded (uses ATP) → water follows → pressure → bulk flow → unloaded. Animal blood is pumped; plant sap moves by pressure and pull — but in every case, structure follows function.