Why do people blush when they become embarrased?

To blush is to display a marked redness of one's face; the term is seldom applied except when the redness is construed as a result of embarrassment, shame, or modesty. Blushing is generally distinguished, despite a close physiological relation, from flushing, which is more intensive and extends over more of the body, and seldom has a mental source. If redness persists for abnormal amounts of time after blushing, then it may be considered an early sign of rosacea. A medical condition known as Idiopathic craniofacial erythema exists, in which the sufferer blushes strongly with little or no provocation.