Hi Tara. Here is the shape of Topic 9 and where the marks really are. Sub-topic 9.1 is the overview — what a circulatory system is for, how the heart can be monitored, what exercise does to it, and coronary heart disease with its list of risk factors. It looks like easy marks and it is, provided you say “coronary arteries” and not “arteries in the heart”. Sub-topic 9.2 is the heart itself, and it is where the diagram marks live: four chambers, a septum, two kinds of valve, and one convention about left and right that catches out more candidates than anything else in the topic. Sub-topic 9.3 is three tubes with three jobs, and every mark in it can be reasoned out from one idea — the wall matches the pressure. Sub-topic 9.4 is a list, but a list with mechanisms attached: haemoglobin, phagocytosis, antibodies, and fibrinogen turning into fibrin. Finish with 9.5, which is the checklist to read the night before a paper.
Two warnings before you start, because they are worth several marks each. First: an artery is defined by direction, not by oxygen. An artery carries blood away from the heart. The pulmonary artery carries deoxygenated blood and is still an artery. Second: blood is never “pure” or “impure”. It is oxygenated or deoxygenated, and mark schemes accept nothing else.
Why an Animal Your Size Cannot Manage Without One
You already know from Topic 3 that substances move into and out of cells by diffusion, and that diffusion is fast over very short distances and hopelessly slow over long ones. A single-celled organism has no transport system because it does not need one — every part of it is within a fraction of a millimetre of the surrounding water. You are about half a metre thick. Oxygen arriving at your lungs would take years to diffuse to your toes.
So a transport system does not replace diffusion. It shortens the distance over which diffusion has to work. Blood carries oxygen most of the way at high speed, and diffusion only has to cross the last few micrometres from a capillary into a cell. Every feature of the system in this topic exists to serve that idea.
The circulatory system is a system of blood vessels with a pump and valves to ensure one-way flow of blood.
Three parts, and the last five words are the ones people leave out. A pump alone would slosh the blood backwards and forwards. It is the valves that turn a pump into a circulation.
The Rule That Has No Exceptions
Blood is pumped away from the heart in arteries and returns to the heart in veins. That is the whole definition. It says nothing about oxygen.
Most arteries do happen to carry oxygenated blood and most veins do happen to carry deoxygenated blood, which is why the wrong version survives. But the pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart. If your rule cannot survive those two vessels, it is not a rule.
Here is every named vessel in Topic 9 on one map. You will meet the individual pieces again in 9.2 and 9.3; for now just notice the two blue arrows at the top, because they are the two that break the habit.
Watching a Heart Without Opening Anyone
Cambridge names three ways the activity of the heart may be monitored, and a question asking for them is worth easy marks provided you know what each one actually measures.
| Method | What it detects | What it is good for |
|---|---|---|
| ECG (electrocardiogram) | The tiny electrical impulses that spread through the heart muscle and make it contract, picked up by electrodes on the skin | The rhythm and the pattern of contraction — it can show that the atria and ventricles are contracting out of step |
| Pulse rate | The surge of pressure that travels along an artery each time the ventricles contract, felt where an artery runs near the surface (wrist, neck) | Heart rate. Simple, needs no equipment, which is why every exercise investigation uses it |
| Listening to the heart sounds | The noise made by the valves closing — “lub” as the atrioventricular valves shut, “dup” as the semilunar valves shut | Whether a valve is leaking or narrowed — a damaged valve makes an extra hissing sound called a murmur |
When you feel your pulse you are not feeling blood go past. You are feeling the artery wall stretch and recoil as a pressure wave passes. That is why you can only feel it in an artery: by the time blood reaches a vein the pressure surges have been smoothed away completely, which is a fact you will use again in 9.3.
What Exercise Does, and Why
Ask a class why the heart beats faster during exercise and almost everyone says “because you need more oxygen”. That is true and it is worth about half a mark. The full answer has a chain in it.
1. The muscles are contracting more, so they are respiring faster.
2. So they need more oxygen and more glucose delivered, and produce more carbon dioxide that has to be taken away.
3. The heart beats faster and more strongly, so more blood is pumped per minute.
4. So the rate of delivery and of removal rises to match the rate at which the muscles are using and making things.
Notice that the last link is a rate matching a rate. That is what earns the final mark.
If you are asked to design the investigation: the independent variable is the level or duration of activity; the dependent variable is the pulse rate in beats per minute; and you must control the person (or use the same person each time), their fitness, the time allowed to settle before the resting reading, the temperature of the room, and how long after stopping you take the reading. Take a resting rate first, repeat each measurement and take a mean, and count for a full minute or count for 15 s and multiply by four. One person on one afternoon is not enough to generalise from — and saying so is very often the evaluation mark.
Why the heart rate stays high after you stop
During hard exercise the heart cannot deliver oxygen fast enough for all the respiration the muscles need. So some muscle cells also respire anaerobically, and this makes lactic acid. The lactic acid builds up in the muscles and the blood. This is the oxygen debt: oxygen is needed afterwards to get rid of the lactic acid.
- The heart keeps beating fast to carry lactic acid in the blood from the muscles to the liver.
- Breathing stays fast and deep to take in extra oxygen.
- In the liver, the lactic acid is broken down by aerobic respiration, using that oxygen.
- As the lactic acid is removed, the heart rate falls back to the resting rate.
The fitter someone is, the less lactic acid they make for the same exercise, so the shorter the recovery time.
Coronary Heart Disease
The heart is a muscle several centimetres thick, and it is full of blood every second of your life. It cannot use a drop of it. Blood in the chambers is rushing past far too fast, and the muscle in the wall is far too thick for oxygen to diffuse in from the inside. So the heart wall has its own blood supply: the coronary arteries, which branch off the aorta immediately above the semilunar valve and spread over the outside of the heart.
Fatty material builds up inside the wall of a coronary artery, narrowing the lumen. Less blood, and therefore less oxygen and glucose, reaches part of the heart muscle. That region cannot respire fast enough, so it cannot contract properly — which causes pain on exertion. If a coronary artery becomes completely blocked, usually by a clot forming at the narrowing, the muscle beyond it is starved of oxygen and dies. That is a heart attack.
A narrowed coronary artery may deliver quite enough blood while you are sitting down. The problem appears the moment the heart is asked to work harder, because demand rises and supply cannot. This is the single most common data stem in the sub-topic, and the answer is always about the gap between demand and supply, not about the artery suddenly getting narrower.
The Seven Risk Factors, and the Two Things You Can Do
Learn all seven. A three-mark question will ask for three, and the two that people forget are the two you cannot change.
| Risk factor | Can it be changed? | Why it matters |
|---|---|---|
| Diet — high in saturated fat and salt | Yes | More fatty material available to be deposited in artery walls; salt raises blood pressure, which damages the lining |
| Lack of exercise | Yes | Heart muscle stays weak, body mass rises, blood pressure rises |
| Stress | Partly | Raises heart rate and blood pressure over long periods |
| Smoking | Yes | Damages the lining of the arteries and raises blood pressure |
| Genetic predisposition | No | Some families inherit a strong tendency to deposit fatty material |
| Age | No | Deposits accumulate over a lifetime, so risk rises with age |
| Sex | No | Men are at higher risk than women of the same age until later life |
Cambridge also asks you to discuss the roles of diet and exercise in reducing the risk, which is a different question from listing risk factors. A good answer gives a mechanism for each:
Diet: eating less saturated fat means less fatty material is deposited in the artery walls, so the lumen stays wide; eating less salt keeps blood pressure lower, so the artery lining is damaged less; keeping the body mass sensible reduces the work the heart has to do.
Exercise: regular exercise strengthens the heart muscle, so it pumps more blood per beat and can beat more slowly at rest; it lowers blood pressure; and it helps keep body mass down. It also improves the balance of fats in the blood.
The increase is 148 − 58 = 90 beats per minute. The original is the resting value, 58. So 90 ÷ 58 × 100 = 155 %. The commonest error is dividing by 148, which answers a different question — a percentage change is always taken of the starting value.
Man B is fitter on every measure. His resting rate is lower (58 against 78), which means his heart pumps more blood per beat so it needs fewer beats. His maximum is lower for the same work, so the same task costs him less effort. And his recovery time is much shorter (4 minutes against 11), which is the single best indicator of fitness in a data question.
You cannot say from this that Man A has coronary heart disease, or that Man B exercises more. The data show a difference in fitness; they do not show its cause. Saying so is often worth a mark on an evaluate question, and never costs you one.
Read This Before You Look at the Diagram
A heart diagram is drawn as though the person is facing you. Their left hand is on your right. So the left side of the heart appears on the RIGHT of the diagram, and the right side appears on the left.
Every year thousands of candidates label the thick-walled chamber “right ventricle” because it is on the right of the page, and lose two or three marks in a single stroke. There is a check that never fails: the thick-walled ventricle is always the left one, whichever side of the paper it is drawn on. Find the thick wall first, then label everything else from it.
The Structures You Must Be Able to Identify
| Structure | What it is | What it does |
|---|---|---|
| Atria (one atrium, two atria) | The two thin-walled chambers at the top | Receive blood arriving from the veins and push it a short distance down into the ventricles |
| Ventricles | The two thick-walled chambers below | Pump blood out of the heart into the arteries |
| Muscular wall | Cardiac muscle, thick in the ventricles and thin in the atria | Contracts to squeeze the chamber and raise the pressure of the blood inside it |
| Septum | The solid wall of muscle down the middle, separating right from left | Keeps oxygenated and deoxygenated blood completely apart |
| Atrioventricular valves | Between each atrium and its ventricle | Prevent blood being pushed back up into the atria when the ventricles contract |
| Semilunar valves | At the base of the pulmonary artery and of the aorta | Prevent blood falling back into the ventricles from the arteries when the ventricles relax |
| Coronary arteries | Small arteries branching off the aorta and running over the surface of the heart | Supply the heart muscle itself with oxygen and glucose |
Atrio-ventricular tells you exactly where it is: between the atrium and the ventricle. Semilunar means “half-moon” and describes the shape of the three pockets, and those are the ones at the exits into the arteries.
So: going down from atrium to ventricle you pass an AV valve. Going out into an artery you pass a semilunar valve. Never write “flap” or “trapdoor” — the mark scheme wants the name.
Why the Walls Are the Thicknesses They Are
This is examined constantly, in two halves, and they have two different answers.
The right ventricle pumps blood only to the lungs, which are next door and full of extremely delicate capillaries. It needs a modest pressure — too much would damage them.
The left ventricle pumps blood to the whole body, all the way to your toes and back, against much greater resistance. It needs a far higher pressure, so it has a much thicker layer of muscle, which contracts more powerfully.
An atrium only has to push blood a few centimetres down into the ventricle below it. That needs almost no force, so the wall is thin.
A ventricle has to push blood out of the heart and around an entire circuit. That needs a lot of force, so the wall is thick.
A very common wrong answer is that the left ventricle is thicker “because it pumps more blood”. It does not. Both ventricles pump exactly the same volume with every beat — they have to, or blood would pile up in the lungs within a minute. The left ventricle pumps the same volume at a higher pressure. Get that word right and the mark is yours.
The Septum, and Why It Is Worth a Mark of Its Own
The septum is not decoration. It is the entire reason a mammal can run. Because the septum is complete, the deoxygenated blood on the right and the oxygenated blood on the left never mix. Every drop of blood leaving the aorta is fully oxygenated, so the tissues receive the most concentrated oxygen supply possible, and a steep concentration gradient exists between the blood and the respiring cells.
If a baby is born with a hole in the septum, some deoxygenated blood crosses into the left side. The blood pumped to the body then carries less oxygen than it should, the child tires quickly, and the heart has to work harder to deliver the same amount of oxygen. That scenario appears in papers constantly, and every mark in it comes from the word mixing.
One Beat, Three Stages
1. The atria contract. Pressure in the atria rises above that in the ventricles, so the atrioventricular valves are pushed open and blood flows down into the ventricles. The semilunar valves stay shut.
2. The ventricles contract. Pressure in the ventricles rises sharply. It rises above the pressure in the atria, so the atrioventricular valves are forced shut — that is the first heart sound, “lub”. It then rises above the pressure in the arteries, so the semilunar valves are pushed open and blood is ejected into the pulmonary artery and the aorta.
3. Everything relaxes. Ventricle pressure falls below artery pressure, so the semilunar valves snap shut — the second heart sound, “dup”. Blood flows in from the veins and the atria fill again.
Valves have no muscle and no nerve supply. They are pushed open and pushed shut by the blood itself, and which way they move depends only on which side has the higher pressure. Writing “the valve opens to let the blood through” sounds fine and gets nothing, because it makes the valve the cause instead of the consequence. Write “the pressure in the ventricle rises above the pressure in the atrium, so the atrioventricular valve is forced shut”.
One Circulation or Two?
The fish heart has one atrium and one ventricle. Blood is pumped from the heart to the gills, where it is oxygenated, and from the gills it travels straight on to the body before returning to the heart. Blood passes through the heart once in each complete circuit.
Two separate circuits sharing one heart. The pulmonary circulation takes blood from the right side of the heart to the lungs and back to the left side. The systemic circulation takes blood from the left side to the body and back to the right side. Blood passes through the heart twice in each complete circuit — that is what “double” means. It does not mean two hearts.
Now the part that is actually worth the marks: why double is better.
Blood loses a great deal of pressure squeezing through the tiny capillaries of a gas exchange surface. In a fish, blood goes from the gill capillaries directly on to the body, so it arrives at the muscles slowly and at low pressure.
In a mammal, blood returns from the lungs to the heart and is pumped a second time. Its pressure is restored before it is sent to the body, so it flows faster, delivers oxygen and glucose more quickly, and removes carbon dioxide more quickly. That supports a much higher rate of respiration and a higher, steadier body temperature.
The second advantage is the septum: because the two circuits are separate, the blood sent to the body is fully oxygenated rather than partly mixed.
The right ventricle pumps only to the lungs, which are close by and contain very thin-walled capillaries that a high pressure would burst. The left ventricle pumps to the whole body, against far greater resistance, so it needs a much higher pressure — and therefore a much thicker muscular wall. Notice that the answer names the destination, the resistance and the wall: three ideas, three marks.
All the blood the right ventricle sends to the lungs comes back to the left side, and all the blood the left ventricle sends to the body comes back to the right side. If one ventricle pumped even slightly more than the other, blood would accumulate in one circuit and drain from the other within a few minutes. So the difference between the two sides is pressure, not volume — which is exactly why “the left ventricle pumps more blood” is refused.
The semilunar valve in the aorta opens when the pressure in the left ventricle rises just above the pressure in the aorta, so at 10 kPa. Nothing decides this and nothing signals it; the valve is simply pushed open by the blood. Before that moment the ventricle is contracting hard with every valve shut and no blood is going anywhere.
The counting is correct: in a fish the blood passes through the heart once per circuit, and in a mammal twice. That is the definition of single and double circulation, and it is worth stating clearly.
“Twice as much oxygen” is meaningless. The blood is not oxygenated twice per circuit — it passes the gas exchange surface once either way. Going through the heart twice does not add oxygen; it adds pressure.
Blood loses most of its pressure passing through the capillaries of the lungs. Returning it to the heart lets it be pumped again, so it reaches the body at high pressure and therefore flows faster. Oxygen and glucose are delivered and carbon dioxide removed more rapidly, supporting a higher metabolic rate. And because the septum keeps the two sides apart, the blood sent to the body is fully oxygenated.
One Idea Explains Everything in This Sub-topic
If you learn one sentence from 9.3, learn this: the wall of a vessel matches the pressure of the blood inside it. Every structural difference between an artery, a vein and a capillary falls out of that, and you can reason your way to any of them in an exam even if you have forgotten the table.
| Artery | Vein | Capillary | |
|---|---|---|---|
| Direction | Away from the heart | Back to the heart | Links the smallest arteries to the smallest veins |
| Wall | Thick, with a lot of muscle and elastic fibres | Thin, with little muscle or elastic tissue | One cell thick, nothing else at all |
| Lumen | Narrow relative to the wall | Wide relative to the wall | So narrow that red blood cells pass in single file |
| Valves | None | Present | None |
| Pressure | High, and in surges | Low and steady | Low and falling |
| Pulse? | Yes | No | No |
An artery receives blood straight from a contracting ventricle, so the pressure inside is high and arrives in surges. A thick, muscular, elastic wall is needed to withstand that pressure without bursting, and the elastic fibres stretch and recoil with each surge, which smooths the flow and pushes the blood onwards. No backflow is possible, because the pressure behind the blood is enormous — so no valves are needed.
A vein receives blood that has already been through capillaries and has almost no pressure left. There is nothing to withstand, so the wall can be thin, and a wide lumen offers little resistance to a weak flow. But a weak flow could easily run backwards, so veins have valves.
A capillary is not a plumbing pipe at all — it is the exchange surface. Everything about it is sacrificed to make exchange fast, which is why the wall is only one cell thick.
The Pressure Story, as a Graph
1. The wave. The oscillation in the arteries is the ventricle contracting and relaxing. It is what you feel as a pulse, and it disappears completely by the time blood reaches the veins.
2. The cliff. The steepest fall is across the arterioles — the smallest arteries, whose muscular walls narrow the lumen. That is where most of the pressure is lost.
3. Where the pressure went. It was not “used up by the heart”. It was lost as friction against the walls of very narrow vessels. Saying so correctly is often the difference between two marks and four.
Why Capillaries Are Built the Way They Are
Capillaries are where the entire circulatory system finally does its job. Everything else is delivery. So the argument here is the same one you met with the villus in Topic 7 and with the leaf in Topic 6 — and Cambridge expects you to recognise it.
| Feature | How it helps exchange |
|---|---|
| Wall only one cell thick | A very short diffusion distance between the blood and the tissue cells, so diffusion is fast |
| Enormous numbers, forming dense networks in every tissue | A very large total surface area for exchange, and no cell is more than a fraction of a millimetre from one |
| Very narrow lumen | Red blood cells are squeezed against the wall in single file, so the oxygen they carry has almost no distance to travel |
| Blood flows very slowly (the total cross-sectional area of all the capillaries is far greater than that of the aorta) | More time for substances to diffuse in and out |
| Walls are permeable and slightly leaky | Water and dissolved substances can pass out to bathe the cells, and waste can pass back in |
Out of the blood into the cells: oxygen, glucose, amino acids, mineral ions, water.
Out of the cells into the blood: carbon dioxide, urea, water.
The direction is always down a concentration gradient, and the gradient is maintained because respiring cells keep using oxygen and keep producing carbon dioxide while the blood keeps flowing past. That last clause is the mark most people miss.
Valves in Veins, and the Muscle That Does the Real Work
By the time blood reaches the veins its pressure is under 1 kPa. The heart is far behind and there is almost nothing pushing. Two things get blood back:
1. Skeletal muscles. The veins run between the large muscles of your limbs. Every time those muscles contract they squeeze the veins and force the blood along. This is why standing perfectly still for a long time makes blood pool in your legs, and why moving your feet stops it.
2. Valves. The squeeze pushes blood in both directions equally. The valves make sure only the direction towards the heart succeeds: forward flow presses the pockets flat against the wall, and backward flow fills them so that they swell, meet in the middle and seal the vein shut.
An answer with only the valves in it explains why blood cannot go backwards but not what makes it go forwards. Give both.
The Named Vessels
You must be able to identify these on a diagram. Learn them in pairs, because a pair is one journey out and back.
| Organ | Vessel in (artery) | Vessel out (vein) |
|---|---|---|
| Heart (to the body and back) | Aorta — oxygenated | Vena cava — deoxygenated |
| Lungs | Pulmonary artery — deoxygenated | Pulmonary vein — oxygenated |
| Kidney | Renal artery | Renal vein |
| Liver | Hepatic artery | Hepatic vein |
| Gut → liver | Hepatic portal vein — the odd one out | |
“Renal” means of the kidney; “hepatic” means of the liver; “pulmonary” means of the lungs. Put the organ word in front and then decide artery or vein by direction: leaving the heart, it is an artery; heading back, it is a vein.
The Liver, and the One Vessel That Breaks the Pattern
Hepatic artery — branches from the aorta and brings oxygenated blood to supply the liver cells themselves. Without it the liver could not respire.
Hepatic portal vein — carries blood from the small intestine to the liver. The blood is deoxygenated, but it is loaded with the glucose and amino acids that have just been absorbed from a meal. This is the vessel Topic 7 was building towards.
Hepatic vein — carries blood out of the liver into the vena cava, and so back to the heart. Its glucose concentration has been regulated by the liver on the way through.
Every other vein in your body carries blood from an organ back towards the heart. The hepatic portal vein carries blood from one organ to another organ — it begins in the capillaries of the gut and ends in the capillaries of the liver, and never goes near the heart in between.
The reason is a good one. After a meal, the glucose concentration in blood leaving the intestine is enormous. Sending it straight round the body would be dangerous. Routing it through the liver first lets the liver take the excess out and store it, so the blood that finally reaches the rest of you has a regulated glucose concentration. It is still a vein, by the way, because it carries blood towards the heart in the general sense — but a question asking “which vessel begins and ends in capillaries?” has exactly one answer.
P and Q have similar overall sizes, so compare wall to lumen. In P the wall is thick relative to the lumen and there are no valves: P is an artery. In Q the wall is thin, the lumen is wider and valves are present: Q is a vein. The valves alone settle it, because only veins have them. R has a wall a thousandth of a millimetre thick and a lumen of 8 µm, about the width of one red blood cell: R is a capillary.
1.0 ÷ 0.5 = 2 times. Check the units match before dividing — both are in millimetres, so they do.
A capillary is where substances are exchanged with the tissues. A wall one cell thick gives a very short diffusion distance, so oxygen, glucose, carbon dioxide and urea diffuse quickly between the blood and the cells. It can afford to be that fragile because the blood pressure by then is very low.
Blood in the veins of the legs is at very low pressure, so there is almost nothing pushing it back up towards the heart against gravity.
Normally the skeletal muscles of the legs contract and squeeze the veins, forcing blood along, and the valves stop it slipping back down. Standing perfectly still removes the squeeze, so the pump stops working.
Blood pools in the veins of the legs, so less returns to the heart, so less is pumped out, and the blood supply to the brain falls. Flexing the calf muscles restarts the muscle pump, blood is squeezed upwards past the valves, venous return rises and the faint is avoided.
What Blood Is Made Of
Blood is a tissue: a straw-coloured liquid called plasma with three kinds of solid floating in it. If you spin a sample in a centrifuge the solids sink and you can see the proportions — a little over half of blood by volume is plasma.
Red blood cells — transport of oxygen, using haemoglobin.
White blood cells — phagocytosis and antibody production.
Platelets — clotting.
Plasma — transport of blood cells, ions, nutrients, urea, hormones and carbon dioxide.
Learn the plasma list as a list. A question saying “name three substances transported in the plasma” is three marks in ten seconds, and answers that say “oxygen” get none, because oxygen is carried by the red cells.
Red Blood Cells — a Cell Stripped of Everything Except Its Job
A red blood cell is one of the most specialised cells in your body, and Cambridge examines it as a structure-and-function pairing. Every feature answers the same question: how do you carry as much oxygen as possible, as fast as possible?
| Feature | How it helps |
|---|---|
| Contains haemoglobin, a red protein containing iron | Haemoglobin combines with oxygen where the concentration is high, forming oxyhaemoglobin, and releases it again where the concentration is low |
| No nucleus | More room inside for haemoglobin, so each cell carries more oxygen |
| Biconcave disc shape (dented on both sides) | A larger surface area for oxygen to diffuse across, and a shorter distance from the surface to the centre of the cell |
| Small and flexible | Can squeeze in single file through capillaries narrower than itself, pressed right against the wall |
Haemoglobin combines with oxygen — it does not “absorb” it or “attract” it. The product is oxyhaemoglobin, one word, and in the lungs the reaction goes forwards while in the respiring tissues it goes backwards.
And red blood cells have no nucleus. If a diagram shows a cell with a big dark nucleus in it, it is a white blood cell, whatever colour the printer used.
The Two White Blood Cells You Must Be Able to Tell Apart
How it looks: a very large, round nucleus filling almost the whole cell, with only a thin rim of cytoplasm around it.
What it does: antibody production. Antibodies are proteins that bind to pathogens, leading to their destruction or marking them out to be destroyed.
How it looks: a lobed nucleus — several blobs joined by thin strands — and plenty of granular cytoplasm.
What it does: phagocytosis. It changes shape, flows around a pathogen, engulfs it and digests it. The verb Cambridge wants is engulf, never “eat” and never “absorb”.
Work down this list and you will never get it wrong.
1. No nucleus, and much the most numerous → red blood cell.
2. Nucleus fills the cell and is round → lymphocyte.
3. Nucleus is in several lobes → phagocyte.
4. Tiny fragment, far smaller than any cell, no nucleus → platelet.
Platelets are not cells and are not white blood cells. They are fragments broken off larger cells, and calling them white blood cells is one of the standard distractors.
Clotting: One Soluble Protein Becomes One Insoluble Protein
1. A vessel is damaged. Platelets and the damaged tissue release enzymes at the wound.
2. These convert the soluble plasma protein fibrinogen into the insoluble protein fibrin.
3. Fibrin forms a mesh of threads across the wound, and red blood cells become trapped in it. The mesh is the clot; when it dries it becomes a scab.
1. It prevents further loss of blood from the damaged vessel.
2. It prevents the entry of pathogens through the break in the skin.
If a question is worth two marks, it is worth them because there are two roles. And notice the neat logic of the mechanism: fibrinogen is soluble so it can circulate everywhere harmlessly, and it only becomes insoluble at the wound, which is exactly where a solid is wanted. If fibrin circulated ready-made, you would clot everywhere at once.
Plasma — the Delivery Service
Plasma is over 90 % water, and water is a solvent — the point Topic 3 made about osmosis and Topic 7 made about digestion. Anything that dissolves can be transported.
| Carried in the plasma | From | To |
|---|---|---|
| Blood cells and platelets | — | Everywhere (they float in it) |
| Nutrients — glucose, amino acids | The small intestine (via the hepatic portal vein and the liver) | All respiring cells |
| Mineral ions | The small intestine | Wherever they are needed |
| Urea | The liver | The kidneys, to be removed |
| Hormones | Glands | Their target organs |
| Carbon dioxide | Respiring cells | The lungs |
Oxygen is carried by haemoglobin inside the red blood cells. Carbon dioxide is carried dissolved in the plasma. A question that offers “oxygen and carbon dioxide are both transported in the plasma” is testing exactly this, and it is wrong.
Increase = 6.0 − 4.8 = 1.2 million per mm³. As a percentage of the original: 1.2 ÷ 4.8 × 100 = 25 %. The sanity check is that 6.0 is a quarter more than 4.8, which it is.
More red blood cells means more haemoglobin, so more oxyhaemoglobin can be formed and more oxygen can be carried in each cubic millimetre of blood. That partly compensates for the lower concentration of oxygen in the air, so the respiring cells still receive enough oxygen. Three ideas: more cells, more haemoglobin, more oxygen delivered.
White blood cells deal with pathogens and platelets deal with clotting. Neither has anything to do with the transport of oxygen, so there is no reason for altitude to change them. Answering this part properly is really a test of whether you know what each component is for, and it also warns you off the lazy assumption that all the counts rise together.
Platelets do not turn into fibrin. Fibrin is made from fibrinogen, a protein already dissolved in the plasma. The platelets release the enzymes that cause the conversion. The answer has fused two different things into one and has also given only one of the two roles of clotting.
“The damaged tissue and the platelets release enzymes [1]. These convert the soluble plasma protein fibrinogen into insoluble fibrin [1], which forms a mesh of threads across the wound, trapping red blood cells to form a clot [1]. The clot prevents further loss of blood and prevents pathogens entering through the break in the skin [1].”
Keeping fibrinogen and fibrin distinct is what lets you answer the harder version of this question: why is the starting protein soluble? Because a soluble protein can circulate everywhere without doing harm, and only becomes a solid mesh at the site of the damage.
The Eight Errors That Cost the Most Marks
Every one of these is a sentence that sounds sensible, gets written by thousands of candidates every year, and earns nothing. Eliminate these eight and you are several marks up before you have learned anything new.
| Never write | Write instead | Why |
|---|---|---|
| “Arteries carry oxygenated blood” | “Arteries carry blood away from the heart” | The pulmonary artery carries deoxygenated blood. Direction defines the vessel, not oxygen |
| “Pure blood and impure blood” | “Oxygenated and deoxygenated blood” | Blood is never impure. Mark schemes accept only the two proper terms |
| “The left ventricle pumps more blood” | “The left ventricle pumps at a higher pressure” | Both ventricles pump the same volume per beat — they must, or blood would pile up in the lungs |
| “The valve opens to let the blood through” | “The pressure on one side rises above the other, so the valve is pushed open” | Valves have no muscle and no nerves. They are consequences, not causes |
| “The white blood cell eats the germ” | “The phagocyte engulfs the pathogen — phagocytosis” | Three separate words the mark scheme names explicitly |
| “Platelets turn into fibrin” | “Fibrinogen is converted into fibrin” | Platelets release the enzymes; the protein that changes was already dissolved in the plasma |
| “Double circulation means two hearts” | “Blood passes through the heart twice per circuit” | One heart, divided by the septum into two pumps side by side |
| “Oxygen and carbon dioxide are carried in the plasma” | “Oxygen is carried by haemoglobin; carbon dioxide is carried in the plasma” | The two gases travel by different routes, and questions are built on the assumption that you think they do not |
Reading the Command Word
State / Name — one word or one short phrase. No explanation needed, and adding one wastes time you will want later.
Identify — usually from a diagram. Give the name, and if the question says “using the diagram”, quote the letter or the feature you used.
Describe — say what happens or what something looks like. For the heart this means the sequence of events, not the reasons for them.
Explain — say why. Every mark needs a because. This is where “higher pressure”, “shorter diffusion distance” and “maintains the concentration gradient” live.
Suggest — apply what you know to something you have not met. There is usually more than one acceptable answer, and the mark is for the reasoning.
Compare — every sentence must mention both things. “Arteries have thick walls” is not a comparison; “arteries have thicker walls than veins” is.
Evaluate — say what the evidence does support, what it does not, and reach a judgement. An answer that only agrees is half an answer.
Calculate — show the working. Working earns a mark even when the final number is wrong.
How to Attack a Topic 9 Data Question
This topic is full of numbers: pressures in kPa, rates in beats per minute, wall thicknesses in mm, cell counts per mm³, percentages of people with a disease. Work through them in the same order every time.
1. Read the headings and the units first. A rate is not a total. A wall thickness in µm is not comparable with one in mm until you convert.
2. Say which way each column points — rising, falling, peaking — before you choose anything.
3. Describe with figures. Quoting two numbers from the table is very often a mark on its own, and it costs you five seconds.
4. Use ratios, not raw sizes, to identify vessels. An artery is not the widest vessel; it is the one whose wall is thick relative to its lumen.
5. Check the direction of your prediction. A narrowed artery delivers less blood. A leaking valve means less blood goes forwards, so the heart must beat faster.
6. Do not claim more than the data allow. A table showing that smokers have more heart disease shows a correlation. It does not prove cause on its own, and saying so is almost always the last mark on an evaluate question.
Three Scenarios to Test Yourself On
The volume of blood ejected into the aorta with each beat has fallen, because some of it went the wrong way. The heart is doing the same amount of work but delivering less.
The body’s cells still need the same amount of oxygen and glucose per minute. If less is delivered per beat, the only way to keep the total per minute up is to have more beats per minute. So the heart rate rises.
Many answers say the heart beats faster “to push the blood past the leaky valve”, which makes the leak sound like a blockage. It is the opposite: the problem is not resistance, it is that blood is escaping backwards. Say volume per beat and you cannot go wrong.
The diagram shows the heart of a person facing you, so their left side appears on the right of the page. The chamber is the left ventricle. The check that never fails is to find the thickest wall first — that is always the left ventricle, whichever way round the diagram is printed.
A vessel leaving a ventricle carries blood away from the heart, so it must be an artery. Leaving the left ventricle it is the aorta. A vein can never leave a ventricle, whatever colour it is drawn.
Both mistakes came from reading the picture instead of the biology — the position on the page and the colour of the ink. Diagram questions are marked on structure and direction, and the two rules above will get you through every one of them.
There is a clear association: the incidence among smokers is three times as high (12 % against 4 %). The sample is large, which makes it unlikely to be chance. Always quote the figures and the ratio — that is a mark.
A correlation is not by itself a cause. The smokers may also have differed in diet, exercise, stress or age, any of which is an independent risk factor, and the study as described did not control for them. The study is also only of men, so it says nothing about women.
The evidence is consistent with smoking increasing the risk, and combined with a known mechanism — smoking damages the lining of the arteries and raises blood pressure — the conclusion is reasonable. But this study alone does not prove it. Notice that the judgement is not “the newspaper is wrong”; it is “the newspaper has overstated what one study can show”.
The Night-Before Checklist
The definition of a circulatory system, in three parts. Arteries away, veins back. The three ways of monitoring the heart, and what each detects. The four-link chain explaining why exercise raises the heart rate. Coronary heart disease as a blockage of the coronary arteries, and all seven risk factors. Two mechanisms each for diet and exercise reducing risk. The four chambers, the septum, both kinds of valve and the coronary arteries. Left side drawn on the right. Why the left ventricle wall is thicker than the right, and why the atria are thinner than the ventricles — two different answers. The three stages of one beat, with which valves are open and shut in each, and which valves make which sound. Single circulation of a fish. Double circulation of a mammal, and the two advantages. The full artery / vein / capillary table. Where blood pressure falls most and why. Five features of a capillary that suit it to exchange. The muscle pump and the valves. Aorta, vena cava, pulmonary artery, pulmonary vein, renal artery, renal vein, hepatic artery, hepatic vein, hepatic portal vein. What is unusual about the hepatic portal vein. The four components of blood and the function of each. Four adaptations of a red blood cell. Lymphocyte versus phagocyte, by appearance and by job. Fibrinogen to fibrin, and both roles of clotting. Six things carried in the plasma.
That list is the entire topic. If you can produce it out loud in five minutes, you are ready.