Narrowed artery. Light micrograph of a section through an artery that is partially blocked by an atheroma (light pink, botom). Atheroma is the deposition of fatty material on the inner walls of an artery. This can greatly reduce the size of the lumen (inner space, white) which causes a reduction in blood flow (ischaemia) to the tissue supplied by the artery. If the atheroma blocked an artery leading to the heart it would cause a heart attack.