An erection occurs when the mind gets a sign that intimate action is taking place or thoughts spring up in the mind about sexually stimulating activities. The mind so sends more blood to the penis. This extra blood builds up in the spongy cells of the penis and causes it to get hard in preparation for insertion into a vagina.

Erection is caused by an increased flowing of blood to the penis and by a powerful reduction in the outflow. Muscles around the penis loosen in order to permit the blood into the corpora cavernosa and corpus spongiosum, while contraction of new muscles reduces the outflow. As the tubular structures take complete with blood, the penis expands in both length and girth.

The sympathetic nervous system is responsible for the failure to achieve erection under conditions of stress. This system interferes with the signals sent by the parasympathetic nervous system, which is actually responsible for activating the erections.

The inability to have an erection suitable for intercourse is called as impotence. It can be a result of an injury secondary to radiation therapy, surgical resection of the prostate, hormonal deprivation therapy, or other aspects of neurological, vascular or disease processes. It can also be caused by inadequate nutrition, chronic stress and prescription drugs.

A less known fact is that erections can occur even after death, due to increased pressure in the penis from sinking fluids or the accumulation of gases.