Blushing is a natural response to sudden self consciousness, usually caused by embarrassment. Blushing is initiated by the same system that activates your "fight or flight" response: the sympathetic nervous system. This system is involuntary, so you have no control over when blushing happens.
When embarrassed you release adrenaline. This hormone acts as a natural stimulant and has many effects on your body which are part of the "fight or flight" response. Adrenaline causes your breathing and heart rate to speed up, your pupils to dilate, and it slows down your digestive system so that energy can be redirected to your muscles. All of these effects and more account for the jolt you feel when embarrassed. "Adrenaline also causes your blood vessels to dilate (called vasodilation), in order to improve blood flow and oxygen delivery. This is the case with blushing. The veins in your face respond to a signal from the chemical transmitter adenylyl cyclase, which tells the veins to allow adrenaline to do its magic. As a result, the veins in your face dilate, allowing more blood to flow through them than usual, creating the reddened appearance that tells others your embarrassed. In other words, adrenaline causes more local blood flow in your cheeks." (Clark 2013). This reaction however is strange because veins elsewhere in your body have little response to adrenaline. So, blushing is quite the phenomanon!
Learn more about the sympathetic nervous system on the "Nervous System" page!
