Histology of atheroma (plaque caused by atherosclerosis) showing pink-stained fibrous areas and pale lipid-rich areas of plaque. The media is thinned beneath the plaque. Atherosclerosis is the thickening and hardening of the arterial wall due to a buildup of plaque in the artery. As the artery walls thicken, blood flow is reduced and the arteries may eventually become blocked.