A basic knowledge of how a normal erection occurs is necessary to better understand the causes and treatments of erectile dysfunction . The penis is a highly vascular organ that fills with blood in the excited state to produce an erection. The penile artery brings blood to the penis and the dorsal penile veins allow blood return to the body. The penis is composed of three chambers, the paired corpra cavernosa and the corpus spongiosum. The corpus spongiosum contains the urethra or "urine" channel on the underside of the penis. The corpus cavernosum is the erectile body responsible for providing rigidity to the penis. It contains "spongy" vascular tissue surrounded by a tough elastic covering (tunica albuginea).

When a man is aroused a signal from the brain travels down the spinal cord and along the pelvic nerves to the penis. These nerves which travel alongside the rectum and prostate terminate at the penile artery and "spongy " tissue. Chemical signals from the nerves cause the arteries to enlarge and allow increased blood flow into the penis. The spaces in the spongy tissue fill with blood and the penis begins to elongate (figure below).

When the elastic covering of the corpra is stretched to its limit, the intra-penile blood pressure increases and the penis becomes rigid. The penile veins become compressed against the tunical albuginea, effectively "trapping" the blood and enabling the erection to be maintained. For all of these coordinated events to occur, a sufficient amount of the male hormone testosterone must be present.

From this highly abbreviated description of erectile physiology one can see that any problem effecting the brain, spinal cord, erectile nerves, penile artery, "spongy" tissue, penile veins or testosterone production can influence a man's ability to have a normal erection.