Fluorescent light micrograph of an atheroma (fatty) plaque from an artery. The colours of the plaque correspond to its pH (key at bottom left). The lower, or more acidic, the plaque's pH, the more likely it is to rupture, which can cause a blood clot. If this happens in a coronary artery it will cause a heart attack; if it happens in an artery supplying the brain it will cause a stroke.