The big idea: Keeping the circulatory system healthy comes down to two big numbers: your blood pressure and your blood cholesterol.
Blood pressure is the force the blood pushes on the artery walls. It is written as two numbers — a higher systolic pressure and a lower diastolic pressure.
Cholesterol is a lipid the body needs in small amounts, but too much in the blood can build up inside arteries. Cholesterol is carried around the blood by two kinds of particle: LDLs and HDLs.
- Blood pressure
- The force that flowing blood exerts on the walls of the arteries. It is given as two numbers, e.g. 120/80.
- Systolic pressure
- The HIGHER blood-pressure number, measured when the ventricles contract and push blood into the arteries.
- Diastolic pressure
- The LOWER blood-pressure number, measured when the ventricles relax between beats and the heart refills.
- Cholesterol
- A lipid found in the blood. The body needs a little, but too much in the blood is a health risk.
- LDL (low-density lipoprotein)
- A particle that carries cholesterol from the liver to the tissues; excess is deposited in artery walls. The 'bad' carrier.
- HDL (high-density lipoprotein)
- A particle that carries excess cholesterol away from the tissues back to the liver for disposal. The 'good' carrier.
| Measurement | When it happens | Pressure |
|---|---|---|
| Systolic pressure | When the ventricles CONTRACT and push blood into the arteries | The HIGHER of the two numbers |
| Diastolic pressure | When the ventricles RELAX between beats and the heart refills | The LOWER of the two numbers |
Systolic vs diastolic — a quick fix: Systolic = Squeeze. The higher number is when the heart squeezes (ventricles contract).
Diastolic = Down-time. The lower number is when the heart relaxes between beats.
Cholesterol becomes a problem when there is too much of it in the blood — especially too much carried by LDLs.
The job of HDLs is to do the opposite: they carry excess cholesterol away from the arteries and back to the liver, where it can be removed. So a healthy balance has low LDL and high HDL.
| Carrier | What it does | Nickname & why |
|---|---|---|
| LDL (low-density lipoprotein) | Carries cholesterol FROM the liver TO the body's tissues; excess is deposited in artery walls | 'Bad' cholesterol — it builds up in arteries |
| HDL (high-density lipoprotein) | Carries excess cholesterol AWAY from the tissues and artery walls BACK to the liver for disposal | 'Good' cholesterol — it removes cholesterol from arteries |
How high cholesterol damages arteries: When there is too much LDL cholesterol, it is deposited in the wall of an artery.
These deposits build into a fatty plaque — a process called atherosclerosis — which hardens the artery wall and narrows the space inside (the lumen).
A narrower artery lets less blood through, raising the risk of a blockage.
| Step | What happens |
|---|---|
| 1. Cholesterol deposits | Excess LDL cholesterol is deposited in the wall of an artery |
| 2. Plaque forms (atherosclerosis) | The deposits build into a fatty plaque that hardens the artery wall |
| 3. Artery narrows | The plaque narrows the lumen, so less blood can flow through |
| 4. Coronary arteries affected | If this happens in the coronary arteries, less blood reaches the heart muscle |
| 5. Reduced oxygen to heart | The heart muscle receives less oxygen, causing chest pain or, if a vessel is blocked, a heart attack |
Why this causes coronary heart disease: The coronary arteries are the small arteries that supply the heart muscle itself with oxygen.
If atherosclerosis narrows a coronary artery, the heart muscle gets less oxygen. This is coronary heart disease (CHD).
If a coronary artery becomes fully blocked, part of the heart muscle is starved of oxygen and dies — a heart attack.
What raises cholesterol — and blood pressure: High blood cholesterol can be caused by a diet high in saturated fat, smoking, a lack of exercise, obesity, or an inherited (genetic) tendency.
Separately, eating too much salt (sodium) raises blood pressure (hypertension), which also damages arteries and strains the heart.
Exercise — the protective factor: Regular exercise protects the circulatory system.
Short term (while you exercise), the heart rate and stroke volume rise, so cardiac output increases to deliver more oxygen to working muscles.
Long term (after weeks of training), the heart muscle grows stronger, so it pumps more blood per beat and the resting heart rate falls — a fitter, more efficient heart.
| Effect of exercise | Short term (during exercise) | Long term (after weeks of training) |
|---|---|---|
| Heart rate | Rises — the heart beats faster | Resting heart rate falls (the heart is more efficient) |
| Stroke volume | Rises — more blood pumped per beat | Increases permanently — a stronger heart pumps more per beat |
| Cardiac output | Rises — more blood delivered each minute to working muscles | Can rise higher at maximum effort |
| Heart muscle | Works harder temporarily | Grows stronger and thicker (more muscular) |
Raises your risk
- High LDL cholesterol (diet high in saturated fat)
- Smoking and obesity
- Too much salt (sodium) → high blood pressure
- A lack of exercise
Lowers your risk
- High HDL cholesterol (carries cholesterol away)
- Regular exercise
- A balanced diet, low in saturated fat and salt
- A stronger heart with a lower resting heart rate
LDL vs HDL — a memory hook: LDL = Lousy (it Leaves cholesterol in your arteries). HDL = Healthy (it Hauls cholesterol back to the liver). You want low LDL and high HDL.
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How this is tested: On Paper 3 this topic is a favourite for short, marked points.
A 1-mark Outline question asks how high blood cholesterol leads to coronary heart disease — you need the cause→effect chain (cholesterol deposited → plaque / atherosclerosis → coronary artery narrows → heart muscle gets less oxygen).
Other formats ask you to Identify causes of high cholesterol (2 marks = two causes), State the role of HDLs, or Distinguish systolic from diastolic blood pressure. A Suggest question may ask for a short-term and a long-term effect of exercise on the heart.
IB-style question — high cholesterol and coronary heart disease
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 in artery wall. Mark 2: plaque / atherosclerosis narrows the artery. Mark 3: reduced blood / oxygen to the heart muscle.)
Final answer
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.
✓ Why this scores full marks: It is a chain of causes, not a single statement: deposit → plaque → narrowed coronary artery → less oxygen to the heart.
A 3-mark Outline needs three linked steps, and the answer must reach the heart muscle / oxygen — the reason it counts as coronary heart disease.
| Step | What happens |
|---|---|
| 1. Cholesterol deposits | Excess LDL cholesterol is deposited in the wall of an artery |
| 2. Plaque forms (atherosclerosis) | The deposits build into a fatty plaque that hardens the artery wall |
| 3. Artery narrows | The plaque narrows the lumen, so less blood can flow through |
| 4. Coronary arteries affected | If this happens in the coronary arteries, less blood reaches the heart muscle |
| 5. Reduced oxygen to heart | The heart muscle receives less oxygen, causing chest pain or, if a vessel is blocked, a heart attack |