Arteries are delicate and complex structures. They are constantly expanding and contracting in response to chemical cues and the body's moment-to-moment needs. When you exercise, for instance, arteries dilate to allow more oxygen-rich blood to reach your heart and muscles. In cold weather, meanwhile, arteries in the extremities constrict to conserve heat in the body's core.
Atherosclerosis, a build-up of fatty plaque due to poor diet and lack of exercise, can damage arteries from your head to your toes. It can make them stiff and slow to respond to cues for more oxygen, and lead to strokes,
Cholesterol, a waxy substance known as a lipid, is vital to the body. It isn't soluble in blood, however, so to get where it needs to go it fuses with proteins to form lipoproteins (shown here as white globs).
There are two types of lipoprotein: high-density (HDL) and low-density (LDL). HDL is known as "good cholesterol" because it removes cholesterol from the bloodstream and deposits it in the liver for removal. LDL, known as "bad cholesterol," does the opposite.
A lack of exercise and foods high in saturated fat, trans fat, or dietary cholesterol (which is found in eggs and meat) can cause LDL levels in the bloodstream to rise to unsafe levels.
When too much LDL circulates in the blood, it starts to seep into the artery wall (shown here as white splotches).
The presence of the LDL in the artery wall triggers inflammation. This causes white blood cells—the foot soldiers of the immune system—to rush to the spot, where they, too, enter the artery wall.
Within the artery wall, the white blood cells link up with LDL to form something known as "foam cells." This process causes more inflammation, which in turn attracts more white blood cells, thereby accelerating the accumulation of foam cells.
Calcium accumulates in some plaques, but not others. (Certain heart tests look specifically for calcium deposits in the arteries.)
The accumulating foam cells, together with other cells, form a plaque along the artery wall. The plaque gradually grows, narrowing the artery.
The plaque makes the artery wall stiff and less flexible (which is why atherosclerosis is sometimes called a "hardening" of the arteries). This prevents the artery from expanding and contracting, which, along with a reduction in blood flow, can cause symptoms such as chest pain, leg pain, and erectile dysfunction.
There are two kinds of plaque. "Stable" plaques have a thick cap made up of white blood cells and smooth muscle cells. "Unstable" plaques contain more fat, have a much thinner cap, and are far more likely to rupture.
If the lipids in an unstable plaque continue to grow, they can cause the thin cap over the plaque to break open. When this happens, a blood clot can form on the plaque's surface.
If the rupture is small, the blood clot will plug it and the plaque will gradually heal. In some cases, however, the resulting clot is so large that it blocks the entire artery. Tissue can begin to die due to lack of oxygen. If this occurs in a coronary artery, it's called a heart attack; in the brain, it's a stroke. (About 80% of strokes are due to a clot-related drop in blood flow.)