Facial flushing is a protective reaction of an organism to heat. When the air temperature is high, there is intense return the "extra" heat in the body through the skin into the environment. Heat the internal organs are transported to the skin with streams of blood. Obviously, the more people will get rid of the heat, the more it will be carried by the blood to the skin. In the heat of the blood vessels expand considerably. This allows you to pass through them more blood than usual. And the face has a lot of superficial blood vessels from increasing blood flow and its redness.