To better understand how erectile dysfunction occurs, it's important to understand the anatomy of the penis and how it naturally functions.
There are five stages involved in the erection:
Stage 1 - Filling with blood:
As a result of sexual and/or psychological stimulation, impulses from the brain and nerves around the penis cause the smooth muscles to relax, allowing blood to fill the corporal bodies.
Stage 2 - Partial Erection:
Penile arteries expand with the increased blood flow, which is needed to elongate the penis.
Stage 3 - Full Erection:
The increased volume of blood in the penis is prevented from draining by increased pressure, which compresses the penile veins, expanding the penis to full erection.
Stage 4 - Rigid Erection:
Maximum rigidity is attained. The glans and spongiosum (tip and body of the penis) enlarge until penile veins are fully compressed, increasing engorgement and maintaining maximum penile rigidity. Orgasm and ejaculation then occur.
Stage 5 - Return to Flaccidity:
After orgasm and ejaculation, muscle contractions result in increased blood outflow from the penis, decreasing its length and girth until flaccid.