The process starts with the brain registering a sexual stimulus. The brain then orchestrates a cascade of nerve signals that run through the spinal cord to the penis. Here, the cavernosal nerves release nitric oxide, a gas that quickly penetrates the smooth muscle cells that form the penis' spongy tissue. The nitric oxide causes the cells to produce a substance called the cyclic guanine mono phosphate (c GMP), which has an expansive, relaxing effect.

The arteries feeding the penis expand and the smooth muscle relaxes, opening up tiny sacs in the erectile tissue that then swell with blood. The two main chambers of the penis, the corpus cavernosum, fill and the organ becomes so engorged that the veins that usually drain away blood are pressed shut against the tough outer membrane of the erectile chambers. The result is erection. Arteries [top] and veins [bottom] penetrate the long, filled carities running the length of the penis-the corpora cavernosa and the corpus spongiosum. Erection occurs when relaxed muscles allow the corpora cavernosa to fill with excess blood fed by the arteries, while drainage of blood through the veins is blocked.