Question: Why do we blush?

Heather McKee answered on 23 Jun 2010:

Blushing and embarassment obviously go hand in hand. Blushing from embarrassment is governed by the same system that activates your fight-or-flight response: the sympathetic nervous system. This system is involuntary in nature, meaning you don’t actually have to think about the process to carry it out. When you’re embarrassed, your body releases adrenaline. This hormone acts as a natural stimulant and has an array of effects on your body that are all part of the fight-or-flight response. Adrenaline speeds up your breathing and heart rate to prepare you to run from danger. It causes your pupils to grow bigger to allow you to take in as much visual information as possible. It slows down your digestive process so that the energy can be redirected to your muscles. All of these effects account for the jolt you feel when you find yourself 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 the 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 you’re embarrassed. In other words, adrenaline causes more local blood flow in your cheeks. Hence you pull a ‘redner’ or blush!