The penis is made up of three cylindrical bodies, the corpus spongiosum (spongy body) â€” which contains the urethra and includes the glans (head) of the penis â€” and two corpora cavernosa (erectile bodies), that extend from within the body out to the end of the penis to support erection. Blood enters the corpora cavernosa through the central arteries.
As the arteries relax, the thousands of tiny caverns, or spaces, inside these cylinders fill with blood. Blood floods the penis through two central arteries, which run through the corpora cavernosa and branch off into smaller arteries. The amount of blood in the penis increases sixfold during an erection. The blood filling the corpora cavernosa compresses and then closes off the openings to the veins that normally drain blood away from the penis. In essence, the blood becomes trapped, maintaining the erection.