Thus, about 30 percent of the penis is buried in the pelvis behind the pubic bone. They join in the midline for about 70 percent of their length - that part of the penis that extends from the body - and continue separately behind the pubic bone where they are anchored to the underside of the pelvic bony structure, the ischiopubic ramus. A pair of spongy cylinders, the corpora cavernosa, are located side-by-side on top. The penis is composed of three erectile cylinders .

Continued stimulation keeps the process going and maintains the erection. The arterial blood is thus trapped, making the penis very hard and very erect. Meanwhile, veins that normally drain blood from the penis are flattened by the expanding volume of blood into the erection chambers. This molecule tells the smooth muscle tissue to relax, which allows more arterial blood to flow into the penis and the capacity of the penis to dramatically increase in volume. Normally, the smooth muscle tissue keeps both arteries and spaces constricted, but the nerve impulses send a primary messenger called nitric oxide (NO). There are arteries and spaces called sinusoids within these cylinders, and smooth muscle tissue surrounds the arteries and spaces.

The nerve impulses trigger an event involving muscle tissue and blood vessels in the two large erectile cylinders, the corpus cavernosa. The aroused brain sends signals down the spinal column to the penile nerves (which may also be directly stimulated by manual stroking of the genitalia or with the act of sexual intercourse itself). Erotic fantasies or sensory inputs such as the touch, scent, sound, or sight of a partner are channeled into the control center in the brain. This simple tube encloses the urethra and at its tip forms the glans penis, commonly referred to as the "head" of the penis. A single corpus spongiosum is located below the junction of the two corpora cavernosa . This is important in that it allows blood to pass freely from one corporal body to another. Where the corpora cavernosa merge at about the level of the pubic bone, the midline surface between the two cylinders forms an incomplete septum. This bony anchor is very important for normal penile function.