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Cholesterol. Computer graphic of part of a cholesterol molecule. Pink spheres represent carbon atoms, yellow spheres represent hydrogen and the lines represent the bonds between them. Cholesterol is a lipid, a type of fatty acid. It is an important constituent of cells, playing a crucial role in the synthesis of hormones and bile salts. It also helps to transport fats in the bloodstream to tissues throughout the body. High levels of cholesterol in the blood are thought to increase the risk of atherosclerosis, a condition in which fatty deposits (plaques) build up on the walls of arteries. Atherosclerosis is the principal cause of heart attack and stroke. (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA)