According to HowStuffWorks, blushing is an automatic response to embarrassment and self-consciousness. Embarrassment causes a release of adrenaline hormones that cause blood vessels to dilate. These dilated blood vessels affect the veins in the face, and the increased blood flow causes a reddened appearance.
HowStuffWorks explains that blushing is controlled by the system that produces fight-or-flight responses, such as dilated pupils, an increased heart rate and rapid breathing. While these reactions are triggered by the nervous system, HowStuffWorks considers the psychology behind blushing difficult to understand. It is also not understood why the veins in the face are the only veins in the body that react to dilated blood vessels.