Basic anatomy of an artery (top picture). The intima is the site of atherosclerosis development (bottom picture). Atherosclerosis is the process that allows for the buildup of substances within the walls of arteries. This buildup or plaque consists mostly of lipid, protein, and calcium in conjunction with an excessive presence of macrophages and smooth muscle cells. The lipid is mostly cholesterol derived from LDLs while the protein is largely connective tissue proteins, such as collagen. Injury to the wall of an artery allows LDL and monocytes access to the intima. Monocytes become macrophages and smooth muscle cells also migrate into the intima. Both macrophages and smooth muscle cells engulf LDL, especially after it is oxidized. Smooth muscle cells produce connective tissue proteins and calcium becomes deposited in the intima as well.