Blood cholesterol, HDL and LDLSpec 1.18(i)
In short
Cholesterol is carried in the blood in lipoproteins. Low-density lipoproteins (LDL) carry cholesterol from the liver to body tissues and, at high levels, deposit it in artery walls. High-density lipoproteins (HDL) carry cholesterol from tissues back to the liver. High LDL and total cholesterol increase CVD risk; high HDL is protective.
Cholesterol is needed to make cell membranes and some hormones, but it is hydrophobic and cannot dissolve in plasma. It is carried in blood inside lipoproteins: particles with a surface of phospholipid and protein around a core of cholesterol and triglyceride.
| LDL (low-density lipoprotein) | HDL (high-density lipoprotein) | |
|---|---|---|
| Carries cholesterol | From the liver to the body tissues | From the tissues back to the liver |
| Effect on arteries | Excess LDL is taken into the artery wall and contributes to plaque formation | Removes excess cholesterol from the artery wall, so protective |
| Effect on CVD risk | Higher level = higher risk | Higher level = lower risk |
Doctors look at total cholesterol, LDL and HDL, and often the ratio of total cholesterol to HDL. The lower the ratio, the lower the risk. Typical UK guidance suggests a total cholesterol below about 5 mmol dm⁻³ and LDL below about 3 mmol dm⁻³ (you are not expected to learn these values). A diet high in saturated fat tends to raise LDL.
When analysing data, compare people using LDL and the total:HDL ratio, quote the values, and say a high LDL level with a low HDL level means the highest risk.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026