“Normal Physiologic  Mechanism Leading to Erection”

   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 (cGMP), 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.