Cholesterol, LDL and the Artery Wall
Your liver packs fat and cholesterol into VLDL particles; an enzyme called lipoprotein lipase strips the fat out until what is left is an LDL particle — each one wrapped in exactly one ApoB protein. Watch those particles circulate, get pulled back out of the blood by LDL receptors on the liver, and watch what happens to the ones that instead get stuck in the artery wall: they oxidise, monocytes arrive, macrophages eat them, and the wall fills with foam cells until a plaque bulges into the lumen.
Try this: run Normal for a minute, then switch to Familial hypercholesterolaemia and watch the receptors break and LDL pile up. Then hit Statin — receptors multiply and particles get vacuumed out of the blood. Press Dietary cholesterol at any time and notice how little the numbers move: it is particle number and retention time, not the cholesterol on your plate, that builds plaque.
Live lipid panel
What's happening
The Science in Plain Language
Cholesterol is cargo, not poison
Every cell you own needs cholesterol. It stiffens and seals your cell membranes, and it is the raw material for bile acids (which let you absorb fat), vitamin D, and the steroid hormones — cortisol, testosterone, oestrogen. Your body will not risk running out, so your liver and other tissues make most of it: on a typical diet the body synthesises the large majority of its cholesterol and absorbs only a minority from food.
The problem is logistics. Cholesterol and fat do not dissolve in blood, so they cannot simply float there. They have to be packed into lipoproteins — tiny spheres with a fatty core and a water-friendly shell, addressed by proteins called apolipoproteins. What we casually call “LDL cholesterol” is not a substance in your blood. It is a measurement of the cholesterol carried inside one class of delivery van.
From VLDL to LDL: the shrinking particle
Watch the top of the animation. The liver loads triglyceride (fat) plus cholesterol into a large particle called VLDL (very-low-density lipoprotein) and posts it into the blood. Wrapped around every VLDL is exactly one molecule of apolipoprotein B-100 (ApoB) — and that ApoB stays with the particle for its whole life. One particle, one ApoB. Remember that; it is the key to the whole page.
As the VLDL passes through the capillaries of muscle and fat tissue, an enzyme anchored to the capillary wall — lipoprotein lipase (LPL) — cleaves the triglycerides out of it and hands the fatty acids to the tissue to burn or store. The particle deflates. It passes through an intermediate stage (IDL) and ends up as a smaller, denser, now cholesterol-rich particle: LDL. Nothing new was created. The same particle, still carrying its single ApoB, simply lost its fat.
The LDL receptor: how LDL leaves the blood
An LDL particle circulates for a couple of days. It ends when a LDL receptor — mostly on liver cells — recognises the ApoB, binds it, and pulls the whole particle inside the cell, where it is broken down. That is the exit door, and it is the single biggest determinant of your LDL level.
The number of doors is regulated by a sensor called SREBP-2. If a liver cell is short of cholesterol, it builds more LDL receptors and hoovers more LDL out of the blood. If it is already loaded with cholesterol, it builds fewer. A protein called PCSK9 works in the opposite direction: it tags LDL receptors for destruction, so fewer are recycled back to the surface.
This is exactly where the drugs act, and why the animation’s scenario buttons do what they do:
- Statins block HMG-CoA reductase, the enzyme the liver uses to make its own cholesterol. The liver cell senses the shortfall and puts more LDL receptors on its surface — so it clears more LDL from the blood. Press Statin and watch the receptors multiply and the particles get vacuumed out. High-intensity statin therapy typically lowers LDL-C by about half.
- Familial hypercholesterolaemia (FH) is the mirror image: an inherited fault in the LDL-receptor pathway. In the common heterozygous form, roughly half the receptors are non-functional, LDL cannot be cleared at a normal rate, and LDL-C sits enormously high from birth — commonly in the 190–400 mg/dL range untreated. Press Familial hyperchol. and watch receptors break and particles stack up. FH causes heart attacks decades early, and it is common enough (on the order of 1 in 250 people) that it is badly under-diagnosed.
- PCSK9 inhibitors and ezetimibe lower LDL by the same final route — more working receptors, fewer circulating ApoB particles.
Why plaque starts: retention, not just level
Now the artery. Its lining is a single layer of cells — the endothelium. LDL particles are small enough to slip between and through those cells into the space underneath, the sub-endothelial intima. Most drift straight back out. The dangerous ones are the ones that get stuck: the ApoB on the particle binds to sticky sugar-protein scaffolding (proteoglycans) in the wall and holds it there. This is the response-to-retention model of atherosclerosis, and it is the mainstream explanation today.
Two things therefore control how fast plaque forms:
- How many ApoB particles are bumping into the wall — because each one is an independent chance of being retained. This is particle number, and ApoB measures it directly.
- How long they linger — residence time. Slow clearance (few receptors, as in FH) means each particle gets far more chances to be trapped.
This is why ApoB is a better predictor than LDL-C. LDL-C tells you how much cholesterol is inside the particles; ApoB tells you how many particles there are. Usually they agree — but when triglycerides are high, or in diabetes and metabolic syndrome, particles become small and cholesterol-poor, so you can have a “fine” LDL-C carried in a large and dangerous number of particles. That mismatch is called discordance, and when the two disagree, risk follows the ApoB.
Foam cells: how a trapped particle becomes a plaque
Trapped LDL gets chemically modified and oxidised. Oxidised LDL is an alarm signal: the endothelium above it switches on adhesion molecules, and monocytes roll out of the blood, squeeze through the lining and become macrophages — the immune system’s cleanup crew.
Here is the cruel part. A macrophage eats oxidised LDL through scavenger receptors (SR-A, CD36), and unlike the LDL receptor, scavenger receptors are not switched off by cholesterol. The macrophage keeps eating until it is bloated with cholesterol droplets and becomes a foam cell — so called because under a microscope it looks foamy. Foam cells die, spill their lipid, and the debris builds a necrotic core under a fibrous cap of smooth-muscle cells and collagen. That is a plaque, and it starts far earlier in life than most people imagine — fatty streaks are already present in the arteries of many teenagers and young adults.
Turn on Oxidative stress (the state produced by smoking, high blood pressure, high blood sugar and chronic inflammation) and you will see the trapped particles oxidise faster, more monocytes get recruited, and the plaque accelerate — without changing a single lipid number. Lipids load the gun; inflammation and oxidation help pull the trigger.
Why the lumen looks fine long after the wall is diseased
Watch the artery carefully in the animation: at first the wall bulges outward and the channel stays wide open. This is real, and it is called compensatory (Glagov) remodelling — the artery expands to accommodate the plaque, so the lumen is preserved until the plaque occupies roughly 40% of the vessel. It is the reason an angiogram can look reassuringly “clean” while a great deal of disease is already sitting in the wall.
Only once that buffer is used up does the lumen start to close. Flow holds up remarkably well: maximal flow (what you need when you climb stairs) begins to fall at roughly 50% diameter stenosis — that is the exertional chest pain of stable angina — while resting flow is usually preserved until about 85–90%. Symptoms are a late signal, not an early one.
And the biggest misconception of all: most heart attacks are not caused by a plaque slowly pinching an artery shut. They are caused by a plaque with a thin, inflamed cap — often one that was not severely narrowing the artery — suddenly rupturing or eroding, exposing its contents to the blood and triggering a clot that blocks the vessel in minutes. That is why lowering ApoB matters even when your arteries are “not blocked enough” to need a stent, and why stabilising plaque is as important as opening it.
HDL and reverse transport — the honest version
The green particles are HDL. They do the opposite job: they accept cholesterol from cells — including from the very foam cells inside a plaque, via transporters called ABCA1 and ABCG1 — and ferry it back to the liver for disposal in bile. That process is reverse cholesterol transport, and it is genuinely protective. Press High HDL and watch cholesterol come back out of the wall.
But be careful with the popular version of this story. A high HDL-C number is associated with lower risk, yet drugs that raised HDL-C substantially — niacin, and the CETP inhibitors — failed to prevent heart attacks in large randomised trials, and genetic studies do not support HDL-C as a direct cause of protection. What appears to matter is HDL function — how well the particles actually pull cholesterol out of cells (“cholesterol efflux capacity”) — not the number printed on your lab report. Treat a high HDL-C as a favourable sign, not as a licence to ignore a high ApoB.
So what about the cholesterol in your food?
Press Dietary cholesterol and watch the readouts. They barely move — and that is the honest science, not a trick. When you eat cholesterol, your liver senses it and simply makes less of its own; absorption is also limited. Across most people, adding dietary cholesterol changes LDL-C by only a few mg/dL, which is why the 2015–2020 US Dietary Guidelines dropped the old 300 mg/day limit. A minority are genuine “hyper-responders” whose LDL rises more, so an individual response can still be checked with a lipid panel.
What does move ApoB is different: saturated and trans fats (which suppress LDL receptors), excess calories, alcohol and refined carbohydrate (which raise triglycerides and drive small, dense, numerous particles), insulin resistance, smoking (oxidation and endothelial injury), and above all your genes. Replacing saturated fat with unsaturated fat — olive oil, nuts, oily fish — and adding soluble fibre lowers LDL; exercise, weight loss and cutting alcohol lower triglycerides.
The one-sentence version: plaque is built by how many ApoB particles are in your blood and how long they stay there, in a wall that is inflamed enough to keep them — not by the cholesterol on your plate.