Topic 9 is the most drawable topic in the syllabus, and that is exactly where its difficulty hides. A heart diagram tempts you to read the picture instead of the biology — the side of the page, the colour of the ink, the size of a tube — and every one of those instincts is wrong at least once. The left side of the heart is drawn on the right. The pulmonary artery carries deoxygenated blood. The thicker left ventricle pumps the same volume at a higher pressure. A vein has the wider lumen. Valves are pushed, never opened. Platelets do not turn into fibrin. Twelve traps, six walkthroughs, six lookalike pairs, a concept map and ten full practice questions below — each one aimed at a place where a sensible-sounding sentence earns nothing.
Twelve traps that cost marks on Topic 9 challenge papers. Every one is an answer that sounds right and that mark schemes refuse.
Six challenge-level questions worked through in the order you should actually think about them. Try each part before revealing the next step.
Y is unmistakable: a wall of 0.001 mm is one cell thick and a lumen of 8 µm fits one red blood cell, so Y is a capillary. W and Z have thick walls relative to their lumens and no valves: both are arteries. X has a thin wall, the widest lumen and valves, so X is a vein. Notice that the valves alone settle X, because nothing else in the body has them.
Z has an artery’s structure and a vein’s oxygen content, and that combination has exactly one answer: the pulmonary artery, carrying deoxygenated blood from the right ventricle to the lungs. This is the whole point of the table. If you had identified vessels by the oxygen column you would have called Z a vein and lost the mark, and the paper would have caught precisely the misconception it was fishing for.
Y is the only vessel whose oxygen content changes, from 19 units where it starts to 13 where it ends. That is because a capillary is the exchange surface: oxygen diffuses out of the blood, down a concentration gradient, into the respiring tissue cells that have been using it up. Every other vessel is delivery, so its value is constant.
Use the structural columns — wall, lumen, valves — to decide what kind of vessel it is. Then use the oxygen column to decide where in the body it is. Doing it in that order makes tables like this one straightforward and it never fails.
One beat takes 0.8 s, so the number of beats in 60 s is 60 ÷ 0.8 = 75 beats per minute. Check the sense of it: shorter beat, faster rate. If you get a number smaller than 60 for a beat shorter than a second, you have divided the wrong way round.
The aorta’s lowest pressure, just before the ventricle contracts, is 10 kPa. The semilunar valve is held shut by that pressure from above. The moment the rising ventricle pressure exceeds it, the valve is pushed open and blood is ejected. So the answer is 10 kPa, and the explanation is that the pressure below has become greater than the pressure above.
The atrioventricular valve shuts when the ventricle pressure rises above the atrium pressure, which happens at about 1.1 kPa. It is a sudden, forceful closure, and it is what you hear as the first heart sound, “lub”. The second sound, “dup”, is the semilunar valve snapping shut later when ventricle pressure falls back below aorta pressure.
Between about 1.1 and 10 kPa the AV valve has already shut and the semilunar valve has not yet opened, so every valve is closed. The ventricle muscle is contracting hard on a fixed volume of blood that has nowhere to go, so the pressure rises very steeply without any blood leaving. It is the same reason squeezing a sealed bottle raises the pressure inside without emptying it.
Of the 75 cm³ leaving the ventricle, 30 cm³ goes the wrong way. Only 75 − 30 = 45 cm³ reaches the aorta. Multiply by the rate: 45 × 96 = 4320 cm³ per minute. Almost every mark lost on this question is lost by multiplying 75 by 96, which measures how hard the ventricle is working rather than how much blood the body receives.
The healthy value is 70 × 72 = 5040 cm³ per minute. So she delivers 4320 against 5040, about 14 % less, even though her heart is beating a third faster. Quote both numbers — a comparison question reserves a mark for using the data.
Her cells need the same amount of oxygen and glucose per minute as anyone else’s. If less blood is delivered per beat, the only way to keep the volume per minute anywhere near normal is to have more beats per minute. That is why her rate has risen to 96, and why it still is not enough.
Normally there are two clean sounds, “lub” as the atrioventricular valves shut and “dup” as the semilunar valves shut. A leaking valve does not seal, so blood is forced backwards through a gap and makes an extra hissing or whooshing sound — a murmur. Listening to the sounds of the valves closing is one of the three syllabus methods of monitoring the heart, and this is exactly what it is for.
Animals with a single circulation deliver blood to the body at 2.2–2.5 kPa; animals with a double circulation deliver it at 13–16 kPa, roughly six times higher. Quoting a range from each group and the ratio between them is what a describe mark is paying for. Do not explain yet.
In a fish, blood goes straight from the gill capillaries to the body, having lost most of its pressure squeezing through them. In a mammal it returns to the heart and is pumped a second time, so its pressure is restored before it is sent to the body. Higher pressure means faster flow, so oxygen and glucose are delivered faster and carbon dioxide is removed faster, which allows a higher rate of respiration — which is what the oxygen-use column is measuring.
Both mammals hold their temperature constant regardless of their surroundings; both fish do not. Keeping warm requires energy, and energy requires respiration, and respiration requires oxygen — which is a second reason the mammals use ten times as much. The septum contributes too: because the two sides never mix, the blood reaching the tissues is fully oxygenated.
A rabbit is much smaller than a human, so it has a much larger surface area relative to its volume and loses heat far more quickly. To hold 39 °C it must respire faster per kilogram, so it uses more oxygen per kilogram. This is the same ratio argument you met in Topic 3 for cells, applied to a whole animal — the sort of transfer a challenge paper is built to reward.
Glucose and amino acids are the two products of digestion that are absorbed into the blood at the small intestine. Only one vessel has both raised, and it is the hepatic portal vein. It has just come from the gut, where those nutrients were absorbed, and the liver has not yet had a chance to do anything with them.
Blood entering the liver has 9.4 mmol per dm³ and blood leaving it has 5.1, a fall of 4.3. The liver has removed the excess glucose and stored it, so that the blood going on to the rest of the body has a regulated concentration. Quote both numbers and the difference — the data mark is free.
Oxygen arrives by two routes: fully oxygenated in the hepatic artery at 19, and partly used in the hepatic portal vein at 13. Both then supply the liver cells, which are extremely active and respire rapidly, using oxygen up. The hepatic vein carries what is left of both, so its value, 8, is lower than either.
Every other vein carries blood from an organ back towards the heart. The hepatic portal vein carries blood from one organ directly to another — it begins in the capillaries of the small intestine and ends in the capillaries of the liver, without passing through the heart in between. That is the sentence to write.
Go along each row and mark anything outside its range. P is normal throughout. Q has red cells of 3.2 (below 4.5) and haemoglobin of 84 (below 120) — two independent pieces of evidence, and both are about oxygen transport, so Q is anaemic. R has white cells of 21 400, roughly double the top of the range. S has platelets of 42 thousand, far below 150.
S is short of platelets, and platelets are needed for clotting. So S’s blood will clot slowly: cuts bleed for much longer than they should and bruises appear easily. Notice you were never told what S has — you deduced the symptom from the job of the component, which is what suggest means.
R’s white cell count is about double the normal maximum and R has been ill for three days, which suggests an infection. White blood cells are produced in larger numbers to deal with the pathogen: phagocytes carry out phagocytosis, engulfing and digesting the pathogens, and lymphocytes produce antibodies. Naming both cells and both jobs is worth two of the three marks.
84 ÷ 120 × 100 = 70 %. And a note worth making in any evaluate part: this table shows that Q has too little haemoglobin, not why. A shortage of iron in the diet is the likeliest cause, but blood loss would give the same picture, and nothing here distinguishes them.
Six pairs that look almost identical and have different answers. The distinction is where the marks live.
Click each node to see how the sub-topics connect into one story: a pump, a set of pipes matched to their pressure, and a liquid carrying six kinds of cargo.
Six real student answers. Find the fault before you reveal it.
Ten Cambridge-style challenge questions. Write your answer first, then reveal the model answer and the examiner’s notes.