For a person to flush is to become markedly red in the face and often other areas of the skin, from various physiological conditions. Flushing is generally distinguished, despite a close physiological relation between them, from blushing, which is milder, generally restricted to the face, cheeks or ears, and generally assumed to reflect embarrassment. Flushing is also a cardinal symptom of carcinoid syndrome—the syndrome that results from hormones (often serotonin or histamine) being secreted into systemic circulation.
Commonly referred to as the sex flush, vasocongestion (increased blood flow) of the skin can occur during all four phases of the human sexual response cycle. Studies show that the sex flush occurs in approximately 50–75% of females and 25% of males, yet not consistently. The sex flush tends to occur more often under warmer conditions and may not appear at all under cooler temperatures.