Plaque builds up in your arteries through a slow, multi-stage process that starts with damage to the inner lining of the artery wall and gradually layers cholesterol, immune cells, calcium, and scar tissue into a growing deposit. This process, called atherosclerosis, begins surprisingly early. Over 50% of American children aged 10 to 14 already show some evidence of early changes in their arteries, and the buildup typically progresses silently for decades before causing symptoms.
It Starts With Damage to the Artery Wall
Your arteries are lined with a single layer of cells called the endothelium. When healthy, this lining acts as a smooth, protective barrier. It produces nitric oxide, a molecule that keeps blood vessels relaxed and prevents blood cells from sticking. But several forces can injure this lining: high blood pressure, high blood sugar, smoking, and chronic inflammation all take a toll over time.
When the endothelium is damaged, the cells enlarge and the gaps between them widen. The lining shifts from a protective, anti-inflammatory state to a pro-inflammatory one. It starts producing sticky molecules on its surface that attract white blood cells, and it loses much of its ability to produce nitric oxide. This is the critical first step, because a compromised lining allows cholesterol particles to slip through into the artery wall.
Why Plaque Forms in Specific Spots
Plaque doesn’t form evenly throughout your arteries. It clusters at branch points, curves, and anywhere blood flow becomes turbulent rather than smooth. In straight sections of an artery, blood flows in an orderly pattern and exerts a steady, protective force on the vessel wall. At bends and forks, though, the flow becomes chaotic, with swirling eddies and areas of very low pressure against the wall.
This disturbed flow activates the cells lining the artery, pushing them toward an inflammatory state and making them more permeable. Smooth, steady blood flow at normal pressure (roughly 15 to 30 dynes per square centimeter) actively protects the endothelium. Turbulent or oscillating flow, common at the outer walls of arterial branches and the inner curves of twisting vessels, does the opposite. That’s why the coronary arteries, the carotid arteries at the neck, and the abdominal aorta are common sites for plaque buildup.
How Cholesterol Gets Trapped Inside the Wall
Once the artery lining is compromised, LDL cholesterol particles pass through the widened gaps and lodge in the tissue beneath. LDL in your bloodstream is relatively harmless while it’s circulating normally. The trouble begins when it gets stuck inside the artery wall, because it undergoes a chemical change called oxidation. Oxidized LDL is essentially rancid cholesterol, and the body treats it as a threat.
Oxidized LDL triggers a cascade of signals. It attracts a type of white blood cell called monocytes, which squeeze through the damaged lining and enter the artery wall. Once inside, these monocytes mature into macrophages, immune cells whose job is to engulf and destroy foreign material. The macrophages recognize oxidized LDL through special receptors on their surface and begin consuming it.
Foam Cells: The Core of Early Plaque
Here’s where the process goes wrong. Normal cells have a feedback mechanism that tells them to stop taking in cholesterol when they’ve had enough. Macrophages using their scavenger receptors don’t have that off switch. They keep gorging on oxidized LDL until they’re bloated with fat droplets, transforming into what pathologists call foam cells. Under a microscope, they look foamy or bubbly because of all the stored cholesterol.
These foam cells are the building blocks of early plaque. A cluster of foam cells in the artery wall is called a fatty streak, and it’s the earliest visible sign of atherosclerosis. Fatty streaks have been found in the aortas of infants less than one year old in autopsy studies, and by the teenage years they’re extremely common. In the Bogalusa Heart Study, which examined 204 young people aged 2 to 39, about 50% of those under 15 already had fatty streaks in their coronary arteries. By ages 21 to 39, that figure rose to 85%.
Fatty streaks alone don’t block blood flow or cause symptoms. But they set the stage for everything that follows.
How Fatty Streaks Become Mature Plaques
As foam cells accumulate and eventually die, they release their cholesterol payload into the surrounding tissue, forming a pool of fatty debris called a lipid core. Meanwhile, the ongoing inflammation recruits more immune cells and triggers the release of molecules that sustain the cycle. Inflammatory proteins like CRP (C-reactive protein) have been found embedded directly in human plaques, where they appear to worsen endothelial damage and promote further LDL oxidation.
In response to this growing mess, smooth muscle cells from the deeper layers of the artery wall migrate inward and begin producing a tough structural protein called collagen. They weave a fibrous cap over the lipid core, essentially building a scar over the fatty deposit. A mature atherosclerotic plaque, according to the American Heart Association, is made up of cholesterol, fatty substances, cellular waste products, calcium, and fibrin (a clotting material).
This fibrous cap is both a help and a risk. It walls off the dangerous lipid core from the bloodstream, preventing clots. But the plaque continues to grow from within, and the cap can thin over time as inflammation eats away at it.
Inflammation Keeps the Process Going
Atherosclerosis is fundamentally a disease of chronic inflammation, not just cholesterol accumulation. The damaged endothelium, the oxidized LDL, and the foam cells all produce inflammatory signals that recruit still more immune cells, creating a self-reinforcing loop.
CRP plays a particularly interesting dual role. Elevated CRP levels in the blood are a well-established marker of cardiovascular risk, but the protein also appears to directly accelerate plaque growth. In lab studies, CRP increases the number of receptors that pull oxidized LDL into cells, reduces nitric oxide production, and promotes the sticking of white blood cells to the artery wall. It has been found sitting alongside cholesterol deposits deep inside human plaques, co-located with immune complexes near areas of fat accumulation.
Decades of Silent Growth
One of the most important things to understand about plaque buildup is how long it takes to cause noticeable problems. The process typically unfolds over 20 to 40 years before a person experiences chest pain or has a heart attack. An intravascular ultrasound study of otherwise healthy heart donors found coronary atherosclerosis in 17% of those under 20, 37% of those in their twenties, 60% in their thirties, 71% in their forties, and 85% of those 50 and older.
For much of this timeline, arteries compensate by expanding outward, a phenomenon called positive remodeling. The plaque grows into the vessel wall rather than inward toward the blood flow channel, so the artery can accommodate significant disease without narrowing. This is why many heart attacks occur at sites that weren’t causing any symptoms and wouldn’t have shown up as blockages on a stress test.
A coronary artery calcium (CAC) scan can detect calcified plaque before symptoms develop. The results are reported as an Agatston score: a score of zero indicates very low risk, 1 to 99 mildly increased risk, 100 to 299 moderately increased risk, and 300 or above moderate to severe risk.
Stable Plaque vs. Vulnerable Plaque
Not all plaques are equally dangerous. A stable plaque has a thick fibrous cap, a relatively small lipid core, and plenty of smooth muscle cells maintaining the cap’s structural integrity. It may narrow the artery and cause predictable symptoms like chest pain during exercise, but it’s less likely to cause a sudden heart attack.
A vulnerable plaque is a different situation entirely. It has a thin fibrous cap (less than 100 micrometers thick), a large lipid core taking up more than 40% of its volume, heavy infiltration by inflammatory macrophages, and relatively few smooth muscle cells to maintain the cap. These plaques are prone to rupture. When the cap breaks open, the lipid core is exposed to flowing blood, which triggers an immediate clot that can block the artery within minutes.
The lesion responsible for a heart attack is often only mildly narrowing the artery beforehand. It’s the rupture and the resulting clot, not the degree of blockage, that causes the acute event. This is why someone can have a “clean” stress test and still suffer a heart attack weeks later: the culprit plaque wasn’t large enough to restrict flow, but it was inflamed, thin-capped, and ready to break.
What Accelerates the Process
Several factors speed up every stage of plaque formation. High LDL cholesterol increases the amount of raw material available to infiltrate the artery wall. High blood pressure physically stresses the endothelium, particularly at branch points where turbulent flow is already a problem. Smoking introduces oxidative stress that damages the endothelial lining and reduces nitric oxide production. Diabetes accelerates the process through a combination of high blood sugar, insulin resistance, and chronic low-grade inflammation.
These risk factors don’t act in isolation. They amplify each other. A person with both high LDL and high blood pressure accumulates plaque faster than someone with either risk factor alone, because one provides more cholesterol to infiltrate the wall while the other creates more entry points for it to get in.