IB Biology SLTopic 2 — Transport in Animals & PlantsPaper 1 & 2Practical skill~13 min read
Coronary Heart Disease (Skills)
Your heart muscle does not take oxygen from the blood sitting inside it. It has its own delivery pipes running across its surface, and they are narrow. Block one and a piece of the heart starts to die within minutes.
📚 What you need to know
Occlusion means the narrowing of an artery because of a blockage.
Atherosclerosis is the process: damage to the lining, then fatty deposits (atheromas) build up underneath it.
The lumen narrows, the wall loses elasticity, and the smooth lining breaks down into rough lesions called plaques.
A clot that forms inside a vessel is a thrombus. Once it breaks free and travels, it is an embolus.
Block a coronary artery and heart muscle is starved of oxygen and glucose, so those cells die — a myocardial infarction (heart attack).
Evidence linking risk factors to heart disease is mostly correlation, and correlation is not causation.
How an artery slowly closes
It starts with damage. High blood pressure batters the inner lining of an artery, and where the lining is damaged, fatty material starts to collect underneath it. These fatty deposits are atheromas.
Now a nasty loop begins. The deposit bulges into the lumen, so the space for blood is smaller. A narrower tube raises the pressure in that artery, and higher pressure damages the wall further. The body repairs the damage with fibrous tissue, but fibrous tissue is not elastic — so the wall becomes stiffer and even worse at coping with pressure surges.
The dark circle is a thrombus — a clot formed inside the vessel. If it breaks off and travels, it becomes an embolus.
🤔 Why the clot is the dangerous bit
A narrowed artery is a slow problem. People can live with one for years, feeling chest pain only when they exercise and the muscle asks for more oxygen than the narrow pipe can deliver.
The sudden problem comes when the plaque surface breaks. Blood clots on rough, damaged surfaces — that is how clotting is supposed to protect you from bleeding. Here it works against you: a clot forms right where the tube is already narrow, and the artery shuts in minutes rather than years.
The chain from damage to heart attack
The same chain explains a stroke, except the blocked vessel is in the brain rather than the heart.
What happens downstream of a blockage
When an embolus lodges in a small artery or arteriole, everything beyond it is cut off. Those tissues stop receiving oxygen and glucose, so aerobic respiration slows and then stops. Without enough ATP the cells cannot carry out their normal functions, and they die.
In the coronary arteries that means heart muscle dies. If enough dies, the heart cannot pump properly — this is a myocardial infarction, or heart attack.
Treatment worth knowing: in heart bypass surgery, a blood vessel taken from the patient’s own leg is attached above and below the blockage. It creates a new route for blood to reach the heart muscle, going round the blocked section rather than clearing it.
🧠 Thrombus or embolus?
A thrombus stays put where it formed. An embolus is on the move. Think of “embolus” as the one that has embarked on a journey.
Judging the evidence: correlation is not causation
Nearly every claim about heart disease risk factors — saturated fat, smoking, exercise — comes from studies that watch large groups of people and look for patterns. That kind of study can show a strong link. It cannot, on its own, prove that one thing causes the other.
The distinction to learnCorrelation = two variables change together. Causation = one variable actually influences the other.
Relative risk of 1.00 is the baseline group. A value of 1.34 means the risk is 34% higher than in that baseline group.
What to say when asked to evaluate a study
Examiners want specific criticisms, not “the sample was too small” on its own. Work through this list and pick the ones that actually apply to the data in front of you.
Check
What to write
Sample size
A few hundred people may not represent a whole population; a larger sample covers more of the variation
Who was sampled
If everyone came from one age, sex or region, the results may not apply to anyone else
Other risk factors
Age, diet, exercise and stress may differ between the groups and could be causing the difference instead
Statistics
Without error bars or a significance test you cannot say the difference is real rather than chance
Repeats
One study is not enough; results need replicating before conclusions are drawn
Bias
Researchers hoping for a particular result, or funded to find one, may influence the outcome
Data collection
People misreport their own diet and exercise on questionnaires, so the input data may be wrong
🤔 Why we cannot just run a proper experiment
The clean way to prove causation is a controlled experiment: take two matched groups, feed one a high saturated fat diet for twenty years, and compare. Nobody will ever run that study, because deliberately putting a group of people at risk of heart disease is not ethical.
That is why the evidence stays correlational. It is not sloppy science — it is the best evidence that can be gathered without harming people. Saying this in an evaluation question shows the examiner you understand why the limitation exists.
Correlation coefficients
The correlation coefficient (r) measures how strong the relationship is, from −1 to +1.
+1 means perfect positive correlation — every point lies on a straight upward line.
−1 means perfect negative correlation — as one goes up, the other goes down.
0 means no correlation at all.
The closer to +1 or −1, the stronger the relationship. But even r = 0.98 still does not prove causation.
A low or zero coefficient is useful too: it is evidence against a hypothesis.
Worked examples
WORKED EXAMPLE
Explain how atherosclerosis in a coronary artery can lead to a heart attack. [4 marks]
Build it as a chain, one link per markDo not describe the whole disease. Answer the question asked, in order.Link 1
Fatty deposits build up under the damaged lining of the coronary artery, so the lumen narrows.
Link 2
The plaque surface is rough and can rupture, so a thrombus forms and blocks the artery.
Link 3
Heart muscle beyond the blockage no longer receives oxygen and glucose, so aerobic respiration cannot continue.
Link 4
Without ATP those muscle cells die, so that part of the heart stops contracting and cannot pump blood.
4 clear links = 4 marks
WORKED EXAMPLE
Using the bar chart above, calculate how much higher the risk is in the heaviest exposure group than in the unexposed group. [1 mark]
Relative risk is already scaled so the baseline is 1.001.34 − 1.00 = 0.340.34 × 100 = 34%The risk is 34% higherDo not write “34 times higher”. It is 1.34 times, which is 34% more.
WORKED EXAMPLE
Evaluate the validity of the conclusion that living with a smoker causes heart disease, using the data above. [3 marks]
Point 1: the sample
Only 480 people were studied, which is a small sample and may not represent a whole population.
Point 2: other variables
The groups may differ in age, diet or exercise, and any of these could be causing the difference rather than the smoke exposure.
Point 3: statistics and repeats
No error bars or significance test are shown, so the differences could be down to chance, and a single study needs replicating.
The data show a correlation, but do not establish a causeNotice the pattern: sample, confounding variables, statistics. Three solid points, three marks.
💡 Exam tip
Learn the chain in order. Most marks in this topic are for sequence, not for extra detail.
Use the exact words: atheroma, plaque, thrombus, embolus, myocardial infarction.
When you evaluate data, always quote a figure from the data. It is often worth a mark by itself.
“Suggest a reason for the correlation” is asking for a biological mechanism. “Evaluate the conclusion” is asking for study limitations. Different questions.
Mention the ethics point if the question asks why a controlled experiment was not done. It is a quick, reliable mark.
Never write that a study “proves” anything. Use supports or is consistent with.
⚠ Common mix-up
Thrombus and embolus swapped. Formed and stayed = thrombus. Travelling = embolus.
Saying the plaque “blocks the vein”. Atherosclerosis is an artery disease, driven by high pressure.
“Fibrous tissue makes the wall stronger.” It repairs the damage but removes elasticity, which makes things worse.
Treating a strong correlation as proof. Even a coefficient of 0.99 is still only correlation.
Writing “small sample” with no number. Quote the actual figure from the data.
Forgetting the heart has its own blood supply. The heart muscle is fed by the coronary arteries, not by the blood inside its chambers.
Up next: The Transpiration Stream — we swap animals for plants. No pump at all this time, and yet water still climbs to the top of a tall tree.
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