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Blocked coronary artery. Computer coloured light micrograph of a cross-section through an occluded (blocked) human coronary artery. Heart tissue is coloured purple. The coronary artery (red) lies on the surface of the heart. The outer wall of the artery (dark red) is of equal thickness. Internal to this is a deposit of fibrous plaque (orange, centre left) blocking a large part of the artery. The lumen (inner channel, brown) of the artery is severely narrowed, with some blood seen flowing through it. Plaque deposition in the arteries in atherosclerosis is typically caused by a high fat diet and smoking. This may cause clots to form resulting in heart attack or stroke. (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)