Artery walls are made up of several layers. When the inner lining (endothelium) of an artery becomes damaged (e.g. by high blood pressure) white blood cells and lipids from the blood accumulate to aggregate together under the lining of the artery, forming fatty streaks. Over time these components accumulate, along with connective tissue and harden forming a fibrous plaque, known as an atheroma. Atheromas build up under the endothelium and as a result push it out into the lumen, partially blocking the lumen, restricting the volume of blood that can flow through it in a certain time period, hence blood pressure increases further.