Atherosclerosis develops when cholesterol-rich particles burrow into artery walls, triggering a chain reaction of inflammation, immune cell buildup, and plaque formation that gradually narrows or blocks blood flow to the heart. It isn’t caused by a single factor. It results from the interaction of high cholesterol, chronic inflammation, blood flow patterns, metabolic damage, and genetics, often building silently over decades before symptoms appear.
How Artery Damage Starts
The inner lining of your arteries, called the endothelium, is a single-cell-thick barrier that controls which substances pass into the artery wall. It also produces nitric oxide, a molecule that keeps blood vessels relaxed and open, prevents blood cells from clumping, and discourages inflammation. When the endothelium is functioning well, it acts as a protective shield.
The earliest sign of atherosclerosis is a drop in nitric oxide production or activity. Without enough nitric oxide, arteries lose their ability to relax properly, become more permeable, and start attracting immune cells to their surface. Several things cause this breakdown: high levels of oxidized LDL cholesterol, high blood pressure, smoking, high blood sugar, and chronic inflammation all damage the endothelium in overlapping ways. Oxidized LDL, for example, directly shuts down the enzyme that produces nitric oxide. Free radicals generated by oxidative stress can destroy nitric oxide before it even has a chance to work.
Once the endothelium is compromised, the artery wall becomes sticky and leaky. Platelets begin to aggregate, immune cells latch onto the surface, and inflammatory signaling molecules flood the area. This is the starting point for plaque.
Cholesterol Buildup and Foam Cells
LDL cholesterol particles are small enough to slip through a damaged endothelium and lodge in the artery wall beneath it. Once trapped there, they become chemically modified, primarily through oxidation. These oxidized LDL particles are the real troublemakers: they send out inflammatory signals that recruit immune cells called monocytes from the bloodstream.
Monocytes squeeze through the endothelium into the artery wall and transform into macrophages, a type of immune cell designed to engulf and destroy foreign material. Macrophages recognize oxidized LDL through specialized receptors on their surface and begin consuming it aggressively. Unlike the body’s normal cholesterol-handling systems, these receptors have no off switch. The macrophages keep gorging on cholesterol until they’re bloated with fat droplets, taking on a bubbly appearance under a microscope. These engorged cells are called foam cells, and clusters of them form the fatty streaks that represent the earliest visible stage of atherosclerosis.
Those fatty streaks can appear surprisingly early in life. Pathological studies have found them in the aortas of children and adolescents. Not all fatty streaks progress to dangerous plaques, but as extracellular lipid accumulates, more macrophages infiltrate the area, and the lesion gradually thickens into a more complex structure.
The Role of Chronic Inflammation
Atherosclerosis is fundamentally an inflammatory disease, not just a plumbing problem. Inflammation drives every stage, from the initial endothelial damage to the final event that triggers a heart attack.
C-reactive protein (CRP), a widely used blood marker for inflammation, isn’t just a bystander. It actively worsens the disease process. CRP inhibits nitric oxide production in artery walls, increases the stickiness of the endothelium so more immune cells attach, and promotes the recruitment of monocytes into existing plaques. It also activates the complement system, a branch of the immune response that amplifies local inflammation and tissue damage. The result is a self-reinforcing cycle: damaged arteries attract immune cells, which release inflammatory signals, which attract more immune cells and cause more damage.
This is why conditions that produce low-grade, body-wide inflammation, such as obesity, autoimmune diseases, chronic infections, and even gum disease, are linked to higher rates of atherosclerosis. The artery wall becomes a focal point for inflammation that may have originated elsewhere in the body.
Why Plaques Form in Specific Locations
If atherosclerosis were caused by cholesterol alone, plaques would coat arteries uniformly. Instead, they cluster at predictable spots: where arteries branch, curve, or narrow. The carotid bifurcation in the neck is a classic example.
The reason is blood flow. In straight sections of arteries, blood moves steadily and exerts a consistent, gentle shear force on the endothelium. This steady flow actually promotes nitric oxide production and keeps the lining healthy. At branch points and curves, flow becomes disturbed, turbulent, or oscillating. These chaotic patterns reduce the protective shear stress on the vessel wall and make the endothelium more vulnerable to inflammation and LDL infiltration. Each person’s unique anatomy creates a slightly different map of vulnerable zones, which is one reason two people with identical cholesterol levels can develop plaques in different locations.
How High Blood Sugar Accelerates Damage
Persistently elevated blood sugar causes a specific type of chemical damage. Glucose molecules react with proteins in the artery wall, particularly collagen, to form compounds called advanced glycation end products (AGEs). These reactions don’t require an enzyme; they happen spontaneously when sugar levels stay high over time.
AGEs cross-link collagen fibers in the vessel wall, essentially gluing them together in abnormal configurations. This makes arteries stiffer and less elastic. AGEs also trigger inflammation and oxidative stress in the artery wall, compounding the damage from other risk factors. This is a major reason why people with diabetes develop atherosclerosis faster and more severely than those with normal blood sugar, even when their cholesterol levels are well controlled.
Lipoprotein(a): A Genetic Wild Card
Most people are familiar with LDL cholesterol as a risk factor, but a related particle called lipoprotein(a), or Lp(a), is an independent and largely genetic contributor to atherosclerosis that standard cholesterol tests often miss. Up to 90% of the variation in Lp(a) levels between people is determined by genetics, specifically by variations in a single gene. Diet and exercise have minimal effect on Lp(a) levels.
Lp(a) promotes atherosclerosis through multiple pathways. It disrupts the structural integrity of endothelial cells, making artery walls more permeable to cholesterol and immune cells. It carries a high load of oxidized fat molecules that generate free radicals in the vessel wall, damaging both cells and DNA. Once inside the artery wall, Lp(a) undergoes oxidation and is taken up by macrophages through the same receptors that consume oxidized LDL, driving foam cell formation. It also stimulates the smooth muscle cells in artery walls to multiply and migrate, which contributes to plaque growth and even calcification.
Roughly one in five people has elevated Lp(a), and because it’s genetically determined, it can cause significant atherosclerosis in people who appear to have no other risk factors.
From Stable Plaque to Heart Attack
A plaque can sit in an artery wall for years, gradually narrowing the vessel without causing symptoms. What makes atherosclerosis dangerous isn’t just the narrowing. It’s what happens when a plaque becomes unstable.
Mature plaques have a core of dead foam cells, cholesterol crystals, and cellular debris, covered by a fibrous cap made of collagen and smooth muscle cells. When inflammation is intense, immune cells in the plaque release enzymes that digest the fibrous cap, thinning it. If the cap tears open, the cholesterol-rich core is exposed to the bloodstream. Blood immediately forms a clot at the rupture site, and that clot can partially or completely block the artery. This is the mechanism behind most heart attacks.
Not all dangerous events involve rupture, though. In plaque erosion, a blood clot forms on the surface of an intact plaque where the endothelial lining has worn away. Plaque rupture is associated with significantly higher levels of inflammation throughout the coronary arteries compared to erosion, which helps explain why aggressive inflammation control matters for prevention. Both mechanisms can produce the same outcome: a sudden blockage that cuts off blood flow to part of the heart muscle.
The Major Modifiable Risk Factors
The causes described above don’t operate in isolation. They’re driven by a set of overlapping risk factors, most of which are modifiable:
- High LDL cholesterol provides the raw material that infiltrates artery walls. Current guidelines target LDL below 70 mg/dL for people at high risk and below 55 mg/dL for those at very high risk, such as people who have already had a heart attack or stroke.
- High blood pressure physically stresses the endothelium, especially at arterial branch points, and accelerates the infiltration of LDL into the vessel wall.
- Smoking floods the bloodstream with free radicals that destroy nitric oxide, damage the endothelium, and promote clotting.
- Diabetes and insulin resistance drive AGE formation, increase inflammation, and worsen the cholesterol profile by raising small, dense LDL particles.
- Obesity, particularly visceral fat around the organs, produces a steady stream of inflammatory molecules that keep the endothelium in a state of chronic low-grade activation.
- Physical inactivity reduces the body’s ability to clear cholesterol from the bloodstream and worsens insulin sensitivity.
Age, male sex, and family history are the major non-modifiable risk factors. Atherosclerosis progresses with time in nearly everyone, but the speed of that progression varies enormously depending on how many modifiable factors are present. A person with well-controlled cholesterol, normal blood pressure, and no smoking may develop only mild plaque over a lifetime, while someone with multiple uncontrolled risk factors can have advanced disease by their 40s.