IB Biology HLTransport in Animals & PlantsPaper 1 & 2~13 min read
Coronary Heart Disease
The heart pumps blood to everything, including itself. Its own supply comes down two narrow coronary arteries on its surface. Block one of those and the muscle behind the blockage starts to die within minutes – and it takes years of slow damage to get there.
📚 What you need to know
Occlusion is the narrowing of an artery due to a blockage.
Atherosclerosis begins with damage to the artery wall, often caused by high blood pressure.
Fatty deposits called atheromas build up under the endothelium and narrow the lumen.
Narrowing raises blood pressure further, causing more damage. Fibrous tissue repairs it but is not elastic, so the wall becomes stiffer, and lesions called plaques form.
A clot formed inside a vessel is a thrombus; once it breaks free and circulates it is an embolus.
If an embolus blocks a coronary artery, heart muscle cells are starved of oxygen and glucose and die – a myocardial infarction. Blockages can be treated with bypass surgery.
Evidence linking risk factors to CHD comes from epidemiological and clinical studies, which show correlation, not proof of causation.
How an artery gets blocked
Atherosclerosis is a slow chain of events, and each step makes the next one more likely.
🧩 The sequence
The endothelium is damaged, often by persistently high blood pressure.
Fatty deposits (atheromas), mainly cholesterol and white blood cells, build up under the lining.
The deposits narrow the lumen, so there is less space for blood and pressure rises even further.
Higher pressure causes more damage, and fibrous tissue forms to repair it. Fibrous tissue is not elastic, so the wall loses elasticity.
The smooth lining breaks down into rough lesions called plaques.
A plaque can rupture the vessel wall, triggering blood clotting. The clot is a thrombus; if it breaks off and travels, it is an embolus.
The thinner red arrow in panel B is the point: less blood gets past, so the tissue beyond receives less oxygen and glucose.
What happens next
When an embolus blocks a small artery or arteriole, the tissue beyond it stops receiving enough oxygen and nutrients. Cell functions are inhibited and the cells die.
If this happens in a coronary artery, part of the heart muscle dies. The heart may stop pumping properly – a myocardial infarction, or heart attack.
A blocked coronary artery can be treated with bypass surgery: a blood vessel taken from the patient’s leg is used to make an alternative route around the blockage.
Why the heart is so vulnerable. Cardiac muscle cannot rest and cannot respire anaerobically for long. It also has very few alternative routes for blood, so one blocked coronary artery cuts off a whole region with nothing to take over.
Evaluating the evidence
Claims that a risk factor such as a diet high in saturated fat causes coronary heart disease rest on two kinds of study, and both have limits.
Type of study
What it gives you
Limitation
Epidemiological studies of whole populations
Patterns linking a factor to disease rates across many people
Gives correlation only, not a definite causal link
Clinical studies of individual patients
Detailed data on real cases
Small samples, so results may not be representative; usually no controlled experiment, so causation cannot be established
Why not simply run a controlled experiment, with one group on a normal diet and one group on a high-fat diet for years? Ethics. Deliberately exposing people to a factor you believe causes disease is not acceptable, so researchers have to work with the imperfect evidence they can gather.
What to check when evaluating a study
Sample size – larger samples are more likely to represent the population.
Representative sample – not all from one demographic group, and human rather than animal subjects if the conclusion is about human physiology.
Controlled variables – age, biological sex, prior health, exercise and stress all affect heart disease. If they are not controlled the results are not valid.
A control group – when testing a drug, a group who do not receive it shows whether the effect is really due to the drug.
Repeats – one study is never enough. Results that other researchers can reproduce are reliable.
Statistical significance – are the differences bigger than you would expect by chance? Look for error bars or a statistical test.
Bias – who funded the work, and did they want a particular answer?
Data collection – questionnaires about diet and exercise rely on people telling the truth.
Correlation and causation
A correlation is an association between two variables. Causation means one variable actually influences the other. A correlation is a reason to investigate; it is not proof.
Positive correlation: as variable A increases, variable B increases.
Negative correlation: as variable A increases, variable B decreases.
The correlation coefficient (r) measures the strength of the relationship. r = 1 or r = −1 means every point lies on a straight line; r = 0 means no correlation. The closer to 1 or −1, the stronger the relationship.
Even a very strong correlation does not prove causation – but a low or zero coefficient is good evidence against a hypothesis.
The y-axis on each graph is variable B. A correlation coefficient puts a number on how tightly the points follow the trend.
Worked examples
WE 1
From damage to heart attack
Outline how atherosclerosis in a coronary artery can lead to a myocardial infarction. (4 marks)
Step 1: the damage
High blood pressure damages the endothelium, and fatty deposits build up under the lining.
Step 2: the narrowing
These atheromas narrow the lumen and form plaques, reducing blood flow and raising pressure further.
Step 3: the clot
A ruptured plaque triggers clotting; the thrombus, or an embolus that breaks away from it, blocks the artery.
Step 4: the consequence
Heart muscle beyond the blockage receives no oxygen or glucose, so those cells cannot respire and die.
Damage → atheroma → plaque → clot → muscle diesuse the terms thrombus and embolus correctly; a clot only becomes an embolus once it travels
WE 2
Evaluating a study
A study of 480 non-smoking adults living with a smoker reported a relative risk of heart disease of 1.00 for no exposure, 1.18 for light exposure and 1.34 for heavy exposure. Evaluate the validity of these data. (4 marks)
Point 1: sample
480 people is a fairly small sample and may not represent a whole population.
Point 2: repeats
This is a single study, so it would need to be repeated, or supported by other studies, before firm conclusions are drawn.
Point 3: other variables
Other risk factors are not accounted for – age, diet, biological sex and exercise may all differ between the groups and could be interacting with smoke exposure.
Point 4: statistics
No statistical test is reported, so we cannot say whether the differences between 1.00, 1.18 and 1.34 are significant.
A correlation worth investigating, not proof of a cause“evaluate” means give limitations and say what the data can and cannot show
WE 3
Reading a correlation coefficient
Two studies report correlation coefficients between blood cholesterol and risk of CHD: study 1 gives r = 0.82 and study 2 gives r = 0.06. Explain what each value tells you. (3 marks)
Study 1
r = 0.82 is close to 1, so there is a strong positive correlation: as cholesterol rises, risk rises.
Study 2
r = 0.06 is close to 0, so there is effectively no correlation in that data set.
The caution
Even the strong correlation does not prove that cholesterol causes CHD; a value near zero, however, is evidence against the hypothesis.
Strong association in one, none in the other, causation in neitherthe sign gives the direction, the size gives the strength
💡 Exam tips
Learn the sequence in order and use the terms atheroma, plaque, thrombus, embolus precisely.
Say a coronary blockage starves cardiac muscle of oxygen so it cannot respire aerobically.
For evaluation questions, use a checklist: sample size, representativeness, controls, repeats, statistics, bias.
Write the sentence “this shows a correlation but does not prove causation” whenever data links a risk factor to a disease.
Mention ethics as the reason a controlled human experiment is not carried out.
Do not confuse reliability (repeats give similar results) with validity (the study measures what it claims to).
⚠ Common mistakes
Saying fat blocks the artery by floating in the blood. The atheroma builds up within the wall, under the endothelium.
Using thrombus and embolus interchangeably.
Claiming a study proves a cause. Epidemiological data shows correlation.
Saying the heart muscle runs out of blood so the person cannot breathe. The problem is oxygen supply to cardiac muscle, not to the lungs.
Forgetting that fibrous repair tissue is inelastic, which is why the artery stiffens.
Treating a large sample as automatically valid. A big but unrepresentative sample is still biased.
Up next: The Transpiration Stream – over to plants, and the pulling force that lifts water to the top of a tree with no pump at all.
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