"Vasocongestion" is the medical term for "blue balls”.
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Vasocongestion is the swelling of bodily tissues caused by increased vascular blood flow and a localized increase in blood pressure. Typical causes of vasocongestion in humans includes menstruation, sexual arousal, REM sleep, strong emotions, illnesses and allergic reactions.
Vasocongestion is essential for sexual procreation in mammals since this is the force that causes the hardening of a penis during an erection. The same force leads to the hardening of the clitoris and vaginal lubrication during sexual arousal.
Other forms of vasocongestion during human sexual activity include sex flush and the swelling of nipples in both men and women. Unpleasant side effects of sex-related vasocongestion may lead to the cramp-like pain of "blue balls" in males and an unpleasant pelvic "heaviness" in women.
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