Priapism is a prolonged and painful erection that can last from several hours up to a few days.
The priapismic erection is NOT associated with sexual thoughts or sexual activity. What happens is that blood flows into the penis, but is unable to drain as it would in a normally flaccid penis. Considering that the penis provides little room for blood to circulate, the blood becomes stagnant after a while, acidifies, and loses oxygen. Without oxygen, the red blood cells become stiff, and even less able to squeeze their way out of the penis. Very few cases of priapism are without an obvious cause.