Where is source of a man's hard on?

If you mean an erection of the penis, this due to the fact that blood engorges the penis, means it goes into the penis but prevented from leaving. This causes a temporary erection. This is one reason that men with heart disease or high blood pressure have difficulty getting or maintaining an erection. When the man loses the erection, whether through masturbation, sexual intercourse, or cold shower, the blood slowly leaves the penis. An erection that lasts longer than 4 hours or becomes painful is a medical emergency.