Normally, the walls of an artery are smooth, allowing blood to flow unimpeded. Atherosclerosis is when harmful material collects on the wall of an artery. This material includes fat, cholesterol, and other substances.
Eventually, the material builds up and a plaque is formed, narrowing the artery. When the build-up is severe, a clot could block the vessel completely.
Update Date 2/24/2016
Updated by: Michael A. Chen, MD, PhD, Associate Professor of Medicine, Division of Cardiology, Harborview Medical Center, University of Washington Medical School, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.